Vapor Laden Atmospheres

ELECTRICAL SAFETY IN FLAMMABLE GAS/VAPOR LADEN ATMOSPHERES

by

W. O. E. Korver Jet Propulsion Laboratory Pasadena, California

NOYES PUBLICATIONS WILLIAM ANDREW PUBLISHING Norwich, New York, U.S.A.

Copyright © 2001 by Noyes Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission in writing from the Publisher. Library of Congress Catalog Card Number: 00-108104 ISBN: 0-8155-1449-2 Printed in the United States Published in the United States of America by Noyes Publications / William Andrew Publishing 13 Eaton Avenue Norwich, NY 1381 1-800-932-7045 www.williamandrew.com www.knovel.com 10 9 8 7 6 5 4 3 2 1

Library of Congress Cataloging-in-Publication Data Korver, W. O. E. Electrical Safety in Flammable Gas/Vapor Laden Atmospheres / by W. O. E. Korver p. cm. Includes bibliographical references. ISBN 0-8155-1449-2

00-108104 CIP

NOTICE To the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such information. This book is intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Publisher. Final determination of the suitability of any information or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. We recommend that anyone intending to rely on any recommendation of materials or procedures mentioned in this publication should satisfy himself as to such suitability, and that he can meet all applicable safety and health standards.

Preface

The purpose of this publication is to make readers aware of the explosion danger that may exist when they are involved in the use of flammable gases and liquids that are stored, processed, or transported in facilities with electrical wiring and equipment. Compliance with the electrical power recommendations in here will essentially provide a safe environment which is a fundamental prerequisite in controlling injuries and damage to properties. One intent of this publication is to provide an in-depth understanding of the factors that influence the classification of a hazardous location. One factor, in combination with one or more other factors, will have an impact on the level of danger and its hazardous boundaries. These factors and their influences are explained in detail in this publication, and once their impact is understood, the classification of a hazardous location becomes a straightforward procedure. The purpose of classification of a hazardous location is to provide safety for personnel and equipment. Another intent of this book is to achieve an electrical installation that will provide an acceptable level of safety for personnel and equipment at the lowest possible cost. To accomplish this, it is necessary to analyze in detail the environmental conditions of the location and the characteristics of the source of hazard. The engineer who is involved in preparing the area classification must understand all of the details that will impact on his decision to classify the area Division 1, Division 2, or nonhazardous. Without a knowledge of the environmental conditions and the characteristics of the source of hazard, he, most certainly, will give the location a safety level much too high, which is not economically justifiable, or a level too low, which is unsafe. It is this approach that must be avoided.

v

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Preface

In nine out of ten cases, a hazardous location is classified much too conservatively. The reasons for this conservative approach are a lack of knowledge and a misunderstanding of the actual concept of safety and danger. In the majority of cases, hazardous areas are classified Division 1 when the location could have been classified Division 2, and areas which are classified Division 2 could have been classified nonhazardous. In other cases, the location is classified nonhazardous when it should have been classified Division 1 or Division 2. It must be kept in mind that a location classified Division 1 requires explosion-proof equipment, which ranges in price from two to four times the cost of general-purpose electrical equipment, some of which are allowed in Division 2 locations. Therefore, it is important to strive to achieve a classification of a lower yet acceptable level of safety, which is commensurate with an acceptable risk and reduces the cost of electrical installations. To establish such a point, it is necessary to evaluate the characteristics of the flammable products, along with the conditions under which the product must operate. By listing this information on appropriate forms, the evaluation of the degree of hazard and its boundaries can be correctly performed, and, as a result, the proper electrical equipment can be selected under the provisions of the NEC. A total of 126 tables and illustrations have been developed to assist the engineer in establishing the degree of danger and its boundaries for locations with flammable products. This publication is divided into three parts with an appendix. Part 1 discusses the flammable and combustible principles of hazardous products and other pertinent information associated with an area classification. Part 2 discusses the environmental conditions in hazardous locations. A number of specific illustrations are included in this section. Part 3 discusses the application procedure for classifying NEC Class I locations. Examples are also included in this section. Following these sections is an appendix listing properties of flammable liquids, gases, and vapors. The application of the information explained herein is mainly for flammable liquids, vapors, and gases that are processed, handled, stored, and/or transported. A small portion of this publication explains the classification of coal handling facilities. Where there may still be cases that are difficult to solve, sound engineering judgment should be applied in compliance with the content of this publication.

October 2000 Pasadena, California

JMR- 2-Jul-01

W. O. E. Korver

Contents

List of Figures, Tables, and Reports ........................................................... xv Figures ........................................................................................ xv Tables ........................................................................................ xx Reports ..................................................................................... xxiii

Part 1 Fundamentals 1

Flammable and Combustible Principles of Hazardous Products ........ 3 A. GENERAL ........................................................................................ 3 B. FLAMMABLE AND COMBUSTIBLE LIQUIDS ............................ 3 B.1. Classification of Flammable and Combustible Liquids ............. 3 B.2. Flash Point ............................................................................... 4 B.3. Ignition Temperature ............................................................... 4 B.4. Vapor Density .......................................................................... 6 B.5. Explosion Range of Flammable Gas or Vapor .......................... 6 B.6. Explosion Hazard as a Function of Temperature and System Pressure ................................................................ 8 B.7. Extent of Hazard as a Function of Molecular Weight ............. 10 B.8. Vapor Traveling Distance ...................................................... 11 B.9. Vapor Volume of Flammable Mixtures .................................. 12

vii

viii

2

Contents B.10. Liquefied Petroleum Gases .................................................... B.11. Grouping of Flammable and Combustible Products ................ C. COMBUSTIBLE COKE AND COAL DUST .................................. D. FIRE AND EXPLOSION HAZARDS IN NEC CLASS I LOCATIONS .................................................................................. D.1. General .................................................................................. D.2. Principles of Fires and Explosions .......................................... D.3. Early and Remote Permanent Non-Electric Ignition Sources .. D.4. Ignition Sources ..................................................................... D.5. Causes of Fires and Explosions ..............................................

13 13 14

Classifying Sources of Hazard............................................................. A. SOURCES OF HAZARD ................................................................ A.1. Static and Dynamic Sources of Hazard ................................... A.2. Open and Closed Sources of Hazard....................................... A.3. Mini, Small, and Large Sources of Hazard ............................. A.4. Determining Small and Large Dynamic-Type Sources of Hazard .................................................................. B. WHEN A LOCATION IS HAZARDOUS ....................................... C. SAFETY VERSUS HAZARD IN NEC CLASS I LOCATIONS ...... D. REQUIREMENT FOR NEC CLASS I, DIVISION 1, AND DIVISION 2 LOCATIONS .................................................... D.1. General Requirements ............................................................ a. “Dangerous” Locations .................................................... b. “Remotely Dangerous” Locations .................................... D.2. Specific Requirements for Classifying a Location as Division 1 .......................................................................... a. Open Sources of Hazard .................................................. b. Closed Sources of Hazard Frequently Leaking or Opened ........................................................................ c. Closed Sources of Hazard Not Sufficiently Ventilated ..... d. Increasing Wear ............................................................... e. Simultaneous Failures ...................................................... f. Early Ignition .................................................................. D.3. Special Requirements for Classifying a Location as Division 2 .......................................................................... a. Sufficiently Ventilated Closed Sources of Hazard ............ b. Failure of Process and Electrical Equipment .................... c. Adjacent Locations .......................................................... D.4. Summary of Conditions for which NEC Class I Locations must be Classified .................................................................. D.5. Classification by European Standard IEC ............................... a. General ............................................................................ b. Definition of IEC Zones ...................................................

20 20 20 21 21

14 14 15 17 19 19

23 24 28 29 29 29 30 30 30 31 31 31 32 32 33 33 33 34 36 42 42 43

Contents

3

4

ix

c. Types of Electrical Equipment ......................................... 44 d. Grouping According to IEC ............................................. 45 The Extent of Explosion Danger for NEC Class I Locations ............. 46 A. GENERAL ...................................................................................... 46 B. THE DIMENSIONAL OUTLINE OF A DIVISION 1 AND DIVISION 2 ZONE ......................................................................... 47 C. QUANTITY OF FLAMMABLE SUBSTANCES VERSUS EXTENT OF EXPLOSION DANGER ............................................ 49 D. FACTORS INFLUENCING QUANTITIES OF FLAMMABLE GASES AND VAPORS .................................................................. 51 E. EARLY AND REMOTE PERMANENT IGNITION SOURCES ..... 77 F. THE EXTENT OF EXPLOSION DANGER FOR CLASS II FLAMMABLE PRODUCTS ........................................................... 78 G. TRANSITION ZONES FOR NEC CLASS I LOCATIONS ............. 81 H. ADDITIONAL DANGER ZONES .................................................. 85 H.1. General Requirements ............................................................ 85 H.2. Additional Danger Zones for Heavier-Than-Air Products ....... 86 H.3. Additional Danger Zones for Lighter-Than-Air Products ........ 91 H.4. Safe Distances for Lighter-Than-Air Products ........................ 91 H.5. Safe Distances for Heavier-Than-Air Products ....................... 91 I. DANGER ZONES ABOVE GROUND ........................................... 92 J. CLASSIFICATION OF SOURCES OF HAZARD IN PUMP STATIONS OCCUPYING 50, 75, OR 100% FLOOR SPACE ........ 92 K. FUME HOODS ............................................................................. 100 K.1. General ................................................................................ 100 K.2. Process Areas in Compliance with Figure 1-10A .................. 100 K.3. Laboratory-type Fume Hood Enclosures in Compliance with Figures 1-10B and 1-10C ............................................. 102 L. STORAGE AND DISPENSING OF FLAMMABLE LIQUIDS ..... 107 L.1. Storage and Dispensing Areas .............................................. 107 L.2. Suitable and Non-Suitable Containers .................................. 108 L.3. Storage Rooms Inside a Building—Classification and Ventilation Requirements ..................................................... 109 L.4. Dispensing in Storage Areas Inside a Building in Rooms Without External Walls ........................................................ 110 L.5. Dispensing in Cut-Off Rooms Inside a Building ................... 111 L.6. Liquid Warehouses .............................................................. 111 L.7. Drum Filling Stations ........................................................... 112 L.8. Storage Cabinets .................................................................. 113 M. SEGREGATION ........................................................................... 113 Spatial Considerations ....................................................................... 115 A. INDOOR AND OUTDOOR LOCATIONS ................................... 115 B. ROOFED SPACES IN HAZARDOUS AREAS ............................. 118

x

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Contents C. NONHAZARDOUS SPACES ABOVE OR BELOW HAZARDOUS AREAS ................................................................. D. SPACES GIVING ACCESS TO HAZARDOUS AREAS .............. The Degree of Explosion Danger for NEC Class II Locations ......... A. GENERAL .................................................................................... A.1. Division 1 Locations ............................................................ A.2. Division 2 Locations ............................................................ B. DETAILED REQUIREMENTS FOR NEC CLASS II, DIVISION 1 LOCATIONS ........................................................... C. DETAILED REQUIREMENTS FOR NEC CLASS II, DIVISION 2 LOCATIONS ........................................................... D. THE DEGREE OF EXPLOSION DANGER IN FOSSIL POWER PLANTS ......................................................................... D.1. Coal Fuel Unloading Systems .............................................. D.2. Coal Pulverizers ................................................................... D.3. Crusher Houses ....................................................................

123 125 130 130 130 131 131 132 133 133 135 139

Ventilation Requirements .................................................................. A. GENERAL .................................................................................... B. NATURAL VENTILATION ......................................................... C. MECHANICAL VENTILATION .................................................. C.1. Pressure Fans ....................................................................... C.2. Suction Fans ........................................................................ D. APPROXIMATE LOCATION OF MECHANICAL VENTILATION ............................................................................ E. CANOPY FUME HOODS ............................................................ F. DEMARCATION LINE ................................................................ G. LOW AND HIGH INTEGRITY SEAL CONNECTORS ............... H. SAFEGUARDS ............................................................................. I. WIRING DIAGRAMS FOR SAFEGUARDS ................................ a. Manual Starting ............................................................. b. Manually Stopping ........................................................ Standby Mode ............................................................................... a. Mechanical Failure of Fan F1 ........................................ b. Power Failure in Circuit #1 ............................................

141 141 144 147 147 148

Electrical Equipment for NEC Class I Locations ............................. A. GENERAL .................................................................................... B. ELECTRICAL EQUIPMENT REQUIRED FOR DIVISION 1 LOCATION .................................................................................. C. ELECTRICAL EQUIPMENT REQUIRED FOR DIVISION 2 LOCATION .................................................................................. D. INTRINSICALLY SAFE ELECTRICAL EQUIPMENT ...............

177 177

158 164 165 167 169 170 171 173 173 173 173

178 179 180

Contents

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8

E. MARKING OF ELECTRICAL EQUIPMENT .............................. F. CONSTRUCTION OF EXPLOSION-PROOF ENCLOSURES ..... G. GROUPING OF ELECTRICAL EQUIPMENT ............................. Electrical Equipment for NEC Class II, Group F Locations ........... A. GENERAL .................................................................................... B. CLASS II, DIVISION 1 LOCATIONS .......................................... C. CLASS II, DIVISION 2 LOCATIONS ..........................................

181 181 182 185 185 186 186

9

Intrinsically Safe Equipment and Wiring ......................................... 188

10

Installation of Electrical Instruments in Hazardous Locations ....... A. TYPE Z PURGING ....................................................................... B. TYPE Y PURGING ...................................................................... C. TYPE X PURGING ......................................................................

11

Hydrogen Gas .................................................................................... 194

12

Cathodic Protection ........................................................................... 196

13

Static Electricity ................................................................................ 198

14

Groundings of Tanks, Pipelines, and Tank Cars .............................. 201

15

Grounding Requirements for Electrical Equipment ........................ A. GENERAL .................................................................................... B. INTERNAL AND EXTERNAL GROUNDING CONDUCTORS ............................................................................ C. SUPPLEMENTARY GROUNDING SYSTEMS ...........................

206 206

16

Application of Seals in NEC Class I Locations ................................. A. GENERAL .................................................................................... B. CLASS I, DIVISION 1 LOCATIONS ........................................... C. CLASS I, DIVISION 2 LOCATIONS ...........................................

215 215 228 229

17

Application of Seals in NEC Class II Locations ................................ 231

191 192 192 193

208 211

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Contents

Part 2 Application of Fundamentals 18

Environmental Conditions in NEC Class I Hazardous Locations ... 235

19

General Requirements for Group A ................................................. A. GENERAL REQUIREMENTS FOR GROUP A ........................... Classified Division 2 ..................................................................... Classified Nonhazardous ............................................................... Storage in the Basement ................................................................

20

General Requirements for Group B ................................................. 264

21

General Requirements for Group C ................................................. 278

22

General Requirements for Group D ................................................. 304

23

General Requirements for Group E ................................................. 310

24

General Requirements for Group F .................................................. 314

25

General Requirements for Group G ................................................. 322

26

General Requirements for Group H ................................................. 329

27

General Requirements for Group I ................................................... 341

28

General Requirements for Group J .................................................. 349

29

General Requirements for Group K ................................................. 369

249 249 259 260 260

Contents

xiii

Part 3 Examples 30

Application Procedure for Classifying NEC Class I Locations ........ A. GENERAL .................................................................................... B. STEPS TO BE FOLLOWED FOR CLASSIFYING A HAZARDOUS LOCATION .......................................................... C. EXAMPLES ................................................................................. Example 1. Pump Station .............................................................. Example 2. Holding Basin ............................................................. Example 3. Crude Oil-Fired Power Plant ....................................... Classification of Area 1........................................................ Classification of Area 2........................................................ Classification of Area 3........................................................ Example 4. Platform Reactor .........................................................

379 379 380 381 388 390 392 395 395 401 405

Appendix: Properties of Flammable Liquids, Gases, and Vapors ........... 409 Definitions .................................................................................................. 417 Bibliography .............................................................................................. 426 Index ......................................................................................................... 428

List of Figures, Tables, and Reports

Figures Figure 1-1A.

Explosive range of NEC Class I Flammable Products. ............. 7

Figure 1-1B.

Flammable gas concentration in terms of percentage gas in air. .............................................................................. 8

Figure 1-2.

Pump sizes versus hazardous boundary sizes (outdoors). ........ 25

Figure 1-3.

Pump sizes versus hazardous boundary sizes (indoors, sufficiently ventilated). .......................................... 26

Figure 1-4A.

Boundaries for NEC Class I locations. ................................... 47

Figure 1-4B.

Boundaries for IEC Class I locations. .................................... 49

Figure 1-5.

Non-electrical permanent ignition source. .............................. 79

Figure 1-6.

Temperature versus vapor pressure for flammable liquids. ..... 82

Figure 1-7.

Transition zones for NEC Class I locations. ........................... 83

Figure 1-8.

Additional danger zones. (A–D) ............................................. 87

Figure 1-8.

Additional danger zones. (E–H) ............................................. 88

Figure 1-8.

Additional danger zones and safe distances. (I–J) .................. 89

Figure 1-8.

Additional danger zones and safe distances. (K) ..................... 90

Figure 1-9.

Classification of small sources of hazard occupying 50, 75, or 100% floor space. ........................................................... 98

Figure 1-10A. Fume hood for process equipment. ...................................... 101 Figure 1-10B. Location of UEL with respect to air flow. ............................ 103 xv

xvi

List of Figures, Tables, and Reports

Figure 1-10C. Fume hood enclosure. .......................................................... 104 Figure 1-11.

Segregation. ......................................................................... 114

Figure 1-12.

Indoor and outdoor locations................................................ 117

Figure 1-13.

Spaces in hazardous locations. ............................................. 120

Figure 1-14.

Spaces adjacent to hazardous locations. ............................... 128

Figure 1-14A. Spaces adjacent to hazardous locations. ............................... 129 Figure 1-15.

Coal fuel unloading system. ................................................. 134

Figure 1-16.

Water spraying for unloading station. .................................. 134

Figure 1-17.

Direct-firing pulverizing system. ......................................... 137

Figure 1-18.

Pulverizer. ........................................................................... 138

Figure 1-19.

Crusher house. ..................................................................... 140

Figure 1-19A. Explosion force magnitude versus air flow rate. ................... 142 Figure 1-20.

Approximate location of mechanical ventilation. (A–D) ....... 160

Figure 1-20.

Approximate location of mechanical ventilation. (E–H) ....... 161

Figure 1-20.

Approximate location of mechanical ventilation. (I–K) ........ 162

Figure 1-21A. Demarcation line X-X. ......................................................... 167 Figure 1-21B. High and low integrity seal connectors. ............................... 169 Figure 1-22A. Wiring diagrams for safeguards. .......................................... 172 Figure 1-22B. Wiring diagrams for type “A” safeguard. ...................... 175-176 Figure 1-23.

Design criteria for constructing explosion-proof enclosures. ........................................................................ 183

Figure 1-24.

Grounding and bonding at shipping wharves and loading racks. ................................................................... 204

Figure 1-24A. Grounding and bonding of drums and cans. ......................... 205 Figure 1-25.

Supplementary grounding of electrical equipment. .............. 209

Figure 1-26.

Incorrect grounding method. ................................................ 212

Figure 1-27.

Internal/external ground with supplementary ground. .......... 213

Figure 1-28.

Sealing fittings. .................................................................... 216

Figure 1-29.

Applications of sealing fittings. (A and B) ........................... 219

Figure 1-29.

Applications of sealing fittings. (C and D) ........................... 220

Figure 1-29.

Applications of sealing fittings. (E and F) ........................... 221

Figure 1-29.

Applications of sealing fittings. (G and H) ........................... 222

Figure 1-29.

Applications of sealing fittings. (I) ....................................... 223

Figure 1-29.

Applications of sealing fittings. (J and K) ............................ 224

Figure 1-29.

Applications of sealing fittings. (L and M) ........................... 225

List of Figures, Tables, and Reports

xvii

Figure 1-29.

Applications of sealing fittings. (N and O) ........................... 226

Figure 1-29.

Applications of sealing fittings. (P and Q) ........................... 227

Figure 1-30.

Dust prevention for NEC Class II location. .......................... 232

Figure A-1.

Pumphouse with small pumps (0–51 hp) handling Class I flammable liquid (sufficiently ventilated location). ........... 250

Figure A-2.

Pumphouse with small pumps (60–201 hp) handling Class I flammable liquid (sufficiently ventilated location). ........... 252

Figure A-3.

Pumphouse with main pumps handling flammable liquid at low or moderate pressure (sufficiently ventilated location).254

Figure A-4.

Pumphouse with main pumps handling volatile liquid at high pressure (sufficiently ventilated location). ........................ 256

Figure A-5.

Detached liquid warehouse (adequately ventilated location). 257

Figure A-6.

Storage room for flammable liquid inside a building, ventilated. ......................................................................... 259

Figure A-7.

Piping system with screwed fittings, bolted flanges, valves and meters, operating at moderate pressure (sufficiently ventilated location). .......................................................... 261

Figure A-8.

Piping system with screwed fittings, bolted flanges, valves and meters, operating at moderate pressure (sufficiently ventilated location). .......................................................... 262

Figure B-1.

Process plant handling flammable liquid at moderate pressure (insufficiently ventilated location). ..................... 265

Figure B-2.

Process plant indoors handling volatile flammable liquid at high pressure (insufficiently ventilated location). .......... 266

Figure B-3.

Pumphouse with small pumps handling Class I flammable liquid (insufficiently ventilated location). ......................... 268

Figure B-4.

Pumphouse with small pumps handling Class I flammable liquid (insufficiently ventilated location). ......................... 271

Figure B-5.

Pumphouse with main pumps handling flammable liquid at moderate pressure (insufficiently ventilated location). ...... 274

Figure B-6.

Pumphouse with main pumps handling flammable liquid (insufficiently ventilated location). ................................... 276

Figure C-1.

Process plant in building handling flammable liquid at moderate pressure. ............................................................ 279

Figure C-2.

Process plant outdoors handling volatile flammable liquid at high pressure. ................................................................ 281

xviii List of Figures, Tables, and Reports Figure C-3.

Pumping well (outdoor location). ......................................... 283

Figure C-4.

Auxiliary pump handling flammable liquid at low or moderate pressure (outdoor location). .............................................. 284

Figure C-5.

Auxiliary pump handling flammable liquid at low, moderate, or high pressure (outdoor location). .................................. 286

Figure C-6.

Main pump handling flammable liquid (outdoor location). .. 287

Figure C-7.

Storage tanks for crude oil (outdoor location). ..................... 289

Figure C-8.

Pits with, or without sources of hazard (outdoor location) ... .290

Figure C-9.

Regularly worked on valve (outdoor location). .................... 292

Figure C-10.

Pump handling flammable liquid at high pressure (outdoor location). .......................................................................... 293

Figure C-11.

Remote permanent ignition source in outdoor location. ........ 296

Figure C-12.

Marine terminal handling flammable liquids. ....................... 298

Figure C-13.

Control room in Div. 2 hazardous location. .......................... 299

Figure C-14.

Control room in Div. 2 hazardous location. .......................... 302

Figure D-1.

Drum filling station for flammable liquid (sufficiently ventilated location). .......................................................... 305

Figure D-2.

Process equipment producing flammable vapors (sufficiently ventilated by fume hood). ................................................. 307

Figure D-3.

Spray room (sufficiently ventilated). .................................... 309

Figure E-1.

Separate dispensing area (insufficiently ventilated location). 311

Figure E-2.

Process plant handling flammable liquid (insufficiently ventilated indoor location). ............................................... 312

Figure F-1.

Open tank, or tank with open hatch, or open tank with stirrer or centrifuge. .......................................................... 315

Figure F-2.

Small storage tank (outdoor location). ................................. 316

Figure F-3.

Large storage tank with fixed roof for crude oil (outdoor location). ............................................................ 317

Figure F-4.

Large impounding basin for oil/water (outdoor location). .... 319

Figure F-5.

Control room adjacent to a hazardous area. .......................... 320

Figure G-1.

Compressor station handling flammable gas with vapor density below 0.75 (sufficiently ventilated location). ........ 323

Figure G-2.

Control room in gas compressor station (with vapor density below 0.75) (sufficiently ventilated control room). ........... 324

List of Figures, Tables, and Reports

xix

Figure G-3.

Storage and chemical process area (sufficiently ventilated indoor locations). .............................................................. 326

Figure H-1.

Compressor station handling flammable gas at low or moderate pressure (upper part of building insufficiently ventilated). ....................................................................... 330

Figure H-2.

Compressor station handling flammable gas with vapor densities below 0.75 (insufficiently ventilated building). .. 331

Figure H-3.

Control room above gas compressor station (insufficiently ventilated gas station). ...................................................... 333

Figure H-4.

Control room adjacent to Div. 1 hazardous area (insufficiently ventilated control room). ............................ 334

Figure H-5.

Control room in gas compressor station (sufficiently ventilated control room). .................................................. 335

Figure H-6.

Control room in gas compressor station (sufficiently ventilated control room). .................................................. 337

Figure H-7.

Compressor station handling flammable gas (insufficiently ventilated location). .......................................................... 339

Figure I-1.

Compressor station handling flammable gases outdoors. ..... 342

Figure I-2.

Compressor station handling flammable gases outdoors. ..... 343

Figure I-3.

Storage cylinders for gaseous hydrogen located outdoors. .... 344

Figure I-4.

Compressor station without walls handling flammable gas at low or moderate pressure (sufficiently ventilated gas station). ...................................................................... 345

Figure I-5.

Control room below Div. 2 hazardous area (insufficiently ventilated control room). .................................................. 347

Figure J-1.

Pits in outdoor, Div. 2 locations. .......................................... 350

Figure J-2.

Loading/unloading platform, outdoors for flammable liquid. 351

Figure J-3.

Pumphouse handling liquid petroleum gas at moderate pressure. ........................................................................... 352

Figure J-4.

Safe distances between ignition source and dot and nondot cylinders filled with LPG. ................................................. 354

Figure J-5.

Access to a Div. 2 hazardous boundary. ............................... 355

Figure J-6.

Piping system with screwed fittings, valve, etc., for indoor gaseous systems of less than 400 CF. ................................ 356

xx

List of Figures, Tables, and Reports

Figure J-7.

Brushing and nonbrushing ventilating air. ............................ 359

Figure J-8.

Process equipment producing flammable gases (sufficiently ventilated by fume hood). ................................................. 361

Figure J-9.

Safe distances for flammable gas released to the outdoors. .. 363

Figure J-10A. One-quarter inch pipe fittings for H2 gases (must be ventilated). ......................................................... 364 Figure J-10B.

One-quarter inch pipe fittings for H2 gases (needs no ventilation). ...................................................... 365

Figure J-11.

Safe distances for hydrogen gas outdoors. ............................ 366

Figure J-12.

Fume hood enclosure (sufficiently ventilated). ..................... 367

Figure K-1.

Piping system with screwed fittings, flanges, valves, etc., for indoor locations containing flammable liquid. ............. 370

Figure K-2.

Piping system with screwed fittings, flanges, valves, etc., for outdoor locations containing flammable liquid. ........... 372

Figure K-3.

Pumphouses with small pumps (60–201 hp) handling Class I flammable liquids at high pressure (sufficiently and insufficiently ventilated location). .................................... 373

Figure 3-1.

Pump station outdoors. ........................................................ 390

Figure 3-2.

Pipelines run from the tank farm via the pump house to Boilers 1 and 2. ..............................................................393

Figure 3-3.

Tank farm. ............................................................................396

Figure 3-4.

Forwarding pumphouse. ........................................................397

Figure 3-5.

Forwarding pump house with economically justified installation which is commensurate with an acceptable level of risk. .......................................................................400

Figure 3-6.

Boiler platform. .....................................................................402

Figure 3-7.

Classification of Areas 1, 2, and 3. ....................................... 404

Figure 3-8.

Reactor. ............................................................................... 408

Tables Table 1-0.

Classification of Liquids .......................................................... 5

Table 1-1.

Vapor Pressure versus Vapor Traveling Distances ................... 9

Table 1-2.

Molecular Weight versus Vapor Traveling Distances ............. 10

List of Figures, Tables, and Reports

xxi

Table 1-3.

Summary of Conditions for which NEC Class I Locations Must Be Classified (1–15) ............................. 37–39

Table 1-4.

Summary of Specific Conditions Influencing the Degree and Extent of Hazard........................................................... 56

Table 1-4A.

Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations ........................... 57

Table 1-4B.

Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations ........................ 58

Table 1-4C.

Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations ................... 59–60

Table 1-4D.

Degree and Extent of Danger Area for Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations ........................... 61

Table 1-4E.

Degree and Extent of Danger Area for Open Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations ....................... 62

Table 1-4F.

Degree and Extent of Danger Area for Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations ........................ 63

Table 1-4G.

Degree and Extent of Danger Area for Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations .......................... 64

Table 1-4H.

Degree and Extent of Danger Area for Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations ....................... 65

Table 1-4I.

Degree and Extent of Danger Area for Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations ........................ 66

Table 1-4J.

Degree and Extent of Danger Area for Open or Closed Sources of Hazard with Heavier or Lighter Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations ....................................................... 67–68

Table 1-4K.

Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations ............ 69

Table 1-5.

Sources of Hazard for Lighter and Heavier Than Air Flammable Substances (1–43) ....................................... 70–75

xxii List of Figures, Tables, and Reports Table 1-6.

Critical Conditions and Severity Factors ................................ 95

Table 1-7.

Roofed Spaces in Hazardous Locations ................................ 122

Table 1-8.

Nonhazardous Spaces Above or Below Hazardous Areas ..... 124

Table 1-9.

Wind Conditions .................................................................. 146

Table 2-1.

Eleven Different Combinations of Different Conditions....... 236

Table 2-2A.

Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations .......................................................................... 237

Table 2-2B.

Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations .......................................................................... 238

Table 2-2C.

Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations ................................................................... 239–240

Table 2-2D.

Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations .......................................................................... 241

Table 2-2E.

Open Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations .......................................................................... 241

Table 2-2F.

Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations .......................................................................... 242

Table 2-2G.

Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations .......................................................................... 242

Table 2-2H.

Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations .......................................................................... 243

Table 2-2I.

Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations ...... 244

Table 2-2J.

Closed or Open Sources of Hazard with Heavier or Lighter Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations .................................. 245

Table 2-2K.

Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations .......................................................................... 245

List of Figures, Tables, and Reports

xxiii

Table App-1

Classification of Severity Factors......................................... 410

Table App-2

Properties of Flammable Liquids, Gases, and Solids ..... 411–416

Reports Cover Sheet

Data On ............................................................................... 383 Introduction ......................................................................... 384 Definition ............................................................................ 385 Form A ................................................................................ 386 Form B ................................................................................ 387

Report: 71591 Rev. 0 .................................................................................. 394 Rev. 1 .................................................................................. 399 Form B: Forwarding Pump House (Fig. 3-5) ........................ 400 Form B: Boiler Platform (Fig. 3-6) ...................................... 402 Report: 81391 Form A, Rev. 0 .................................................................... 406 Form B: Reactor (Fig. 3-8) .................................................. 408

Part 1

Fundamentals

Chapter 1 Flammable and Combustible Principles of Hazardous Products

A.

GENERAL

A hazardous product, as described herein, is a product that has the capability of being easily ignited. The hazardous product may consist of flammable liquid, flammable gas, or combustible dusts. When these vapors, gases, or dusts are in the right proportion with air, they will explode when ignited with a sufficient amount of heat. Sources of sufficient heat shall be mainly considered as coming from electrical equipment, although in rare cases sources that are not electrical are also considered.

B.

FLAMMABLE AND COMBUSTIBLE LIQUIDS

B.1. Classification of Flammable and Combustible Liquids According to the NFPA 30 standard title “Flammable and Combustible Liquids Code,” flammable and combustible liquids are grouped into three classes: Class I, Class II, and Class III. These classes refer to the flammability class of a flammable product. A Class I liquid is a flammable liquid with a closed-cup flash point below 100°F at a vapor pressure not exceeding 40 psi. A Class II liquid is a combustible liquid with a closed-cup flash point at or above 100°F but below 140°F. A Class III liquid is a combustible liquid with a closed-cup flash point at or above 140°F.

3

4

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Class I substances, when released to the atmosphere in large quantities, may produce large volumes of vapors, especially when the more volatile types (such as propane, propylene, ethane, ethylene, butane, etc.) with vapor pressure in excess of 40 psi are released. These types of flammable substances should be treated very conservatively since they may cover large areas before reaching safe concentrations. The extent of the hazardous area may, therefore, be considerably large and may reach up to 100 feet in radius. Class II liquids will produce vapors in their explosion range closer to their point of release when heated above their flash point. Some of the kerosene and diesel fuels, for example, are not much of a hazard because they produce very small quantities of vapor when heated above their flash points. When these liquids are heated above their flash point, the extent of the hazardous area is much smaller than for Class I liquids. Class III liquids are generally not considered for an area classification because the extent of the hazardous area is small and only close to the point of release. When a hazardous area must be classified, ranging from about 10–100 feet for Class I liquids and from 5 to about 25 feet for Class II liquids, the extent of the hazardous area for Class III liquids is less than 5 feet. See Table 1-0 for a list of flammables and combustibles, which are classified in accordance with their flash points.

B.2. Flash Point The flash point of a flammable or combustible liquid is a condition in which sufficient amounts of vapors are produced by the liquid to form an ignitable mixture with air or oxygen. The flash point is measured by a standard ASTM in which the temperature of the liquid is slowly increased and by periodically exposing the vapor space above the liquid to an ignition source. When the vapor first flashes or burns, the temperature of the liquid is called the flash point. In other words, the flash point is the minimum temperature at which the volatile liquid gives off vapor in sufficient quantity to form an ignitable mixture with air near the surface of the liquid. Flammable vapors may also be present at temperatures below the flash point since evaporation will also take place below the flash point. These vapor concentrations with temperatures below the flash point are below the lean limit and will not ignite, and therefore are not considered dangerous. Flash point is usually a few degrees below the LEL.

B.3. Ignition Temperature The ignition temperature is the minimum temperature necessary to ignite a combustible mixture, thus causing an explosion or fire.

Table 1-0. Classification of Liquids

Class IA

Class IB

Class IC

(Flash points

(Flash points

(Flash points

below 73°F)

below 73°F)

at or above

(Boiling points

(Boiling points at

below 100°F)

or above 100°F)

Class II

Class IIIA

Class IIIB

(Flash points

(Flash points

(Flash points

at or above

at or above

at or above

73°F, but

100°F, but

140°F, but

200°F)

below 100°F)

below 40°F)

below 200°F)

Acetaldehyde,

Acetate, Ethyl,

Amyl Alcohol,

Acetic Acid,

Aniline,

Benzyl Alcohol,

Ethyl Cloride,

Acetone, Benzene,

Isobutyl Alcohol,

Alcohol,

Benzal

Clycerine,

Ethyl Ether,

Alcohol, Gasoline,

Ketone, Styrene,

Fuel Oil,

Butyl,

Ethylene,

Formate, Pentane,

Alcohol, Octane,

Methyl Isobutyl,

Glacial,

Cellosolve,

Glycol,

and Propylene,

Carbon Disulfide,

O-xylene and

Hexyl,

Dehyde,

Lubricating Oil,

Isoprene, Methyl

Cyclohexane, Ethyl

Turpentine

Kerosene,

Nitro Benzene

Transformer Oil,

Oxide

Heptane, Methyl,

N-decane,

Toluene and

and Stoddard

Gasoline

Solvent

Crude oil is classified as a Class I flammable liquid, but since it is a mixture of highly varied hydrocarbons it is not listed.

and Tubine

6

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

B.4. Vapor Density Vapor density is the weight of a volume of vapor or gas without the presence of air compared with the weight of an equal volume of dry air at the same pressure and temperature. A figure less than 0.75 indicates a vapor is lighter than air, and a figure that is greater than 0.75 is heavier than air. Between heavier- and lighterthan-air densities, there is a gray area where the gases and vapors seem to be undecided on which way to go. Gases and vapors that have densities between 0.75 and 1.0 may travel along the floor first before rising. For example, if ventilating air does not instantly catch an airborne gas or vapor with a density between 0.75 and 1.0, it may act as a heavier-than-air flammable gas or vapor. If ventilating air instantly catches it, it will behave as a lighter-than-air flammable gas or vapor. Vapor density is calculated by dividing the molecular weight of a vapor by 29, where 29 is the composite molecular weight of air. The molecular weight of naptha petroleum, for example, is 72.5 and its vapor density is 72.5/29 = 2.5. Heavier-than-air gases or vapors will travel along the floor, covering large horizontal areas when the temperature is at or above flash point. Lighter-than-air gases or vapors have the tendency of rising, thereby covering only small horizontal areas when the temperature is at or above flash point.

B.5. Explosion Range of Flammable Gas or Vapor The presence of a flammable gas or vapor in the air is not sufficient to cause an explosion. An explosion will occur only when the gas or vapor has mixed with air or oxygen in a ratio in which the gas or vapor concentration is within certain limits. These limits are known as the lower (LEL) and upper (UEL) explosion limits and are expressed in terms of percentage by volume of gas or vapor in air. Between the explosive limits, the range is known as the explosive range. This range may vary from a few percent to 100% as shown in Fig. 1-1A. Below the LEL, the mixture is too lean for combustion, because there are insufficient gas or vapor molecules. Above the UEL, the mixture is too rich for combustion because there are too many gas or vapor molecules. However, within the LEL and UEL range, combustion is possible and the flame will spread throughout the mixture when it is ignited. This is known as flame propagation, and if the flame propagation is very rapid, it is popularly called an explosion. For example, the LEL of hydrogen gas is 4%, and the UEL is 75%. When the mixture of gas-air contains a concentration of gas of less than 4%, the resulting mixture is too lean (not enough fuel) for combustion. Should the mixture contain more than 75% of gas in air, the resulting mixture is too rich (too much fuel and not enough oxygen) for combustion. The mixture can only cause an explosion if the gas concentration is in between 4% and 75%, as shown in Fig. 1-1B. The maximum rate of explosive pressure developed by hydrogen gas is at a point within the explosion range, specifically at a point between 4% and 75%.

Chapter 1: Introduction and Overview

7

Volatile liquid fuels slowly vaporizing into the air present a different case. Initially, the small fuel vapor concentration will be below the lean limit. At this point, the mixture cannot be ignited. As the vaporization progresses with time, the fuel vapor concentration in the air will reach the lean limit, at which point combustion is possible. When the lean limit is exceeded, and if the mixture is ignited, the flame will propagate through the mixture. The ignitable limits are based on normal atmospheric temperature and pressure. There may be variations in the explosive limits at temperatures and pressures above or below normal. An increase in temperature of the mixture will cause the flammable range to shift downwards and a decrease in temperature will shift it upwards. Under favorable conditions, an electrical spark will ignite a flammable mixture. The minimum amount of energy required to ignite a flammable mixture varies with the type of fuel. The minimum arcing energy to ignite hydrocarbon-air mixtures varies from 0.017–0.3 millijoules. Hydrogen gas, for example, can be ignited by 0.017 millijoules.

Figure 1-1A. Explosive range of NEC Class I Flammable Products.

8

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-1B. Flammable gas concentration in terms of percentage gas in air.

B.6. Explosion Hazard as a Function of Temperature and System Pressure In classifying a hazardous location, it is necessary to consider the temperature and the pressure in the system of the process equipment containing a flammable liquid. Temperature and pressure have a great impact on the quantity of flammable vapors released to the atmosphere. System temperature and system pressure are closely related: the higher the liquid temperature, the higher the pressure in the system. The larger the quantity of vapors a flammable liquid is capable of releasing to the atmosphere, the greater the hazard. A hazardous condition exists when the temperature is above flash point. Systems at temperatures below flash point are not considered hazardous.

Chapter 1: Introduction and Overview

9

When a temperature above flash point is applied to a flammable liquid in a closed containment, a pressure increase in the system is developed. The higher the pressure in the containment, the greater the possibility of a rupture. If the closed containment is under pressure and ruptures, flammable liquid will start to evaporate. The evaporation rate of the liquid is a function of the vapor pressure and the temperature and the liquid discharge rate. The liquid discharge rate, in turn, is a function of the system pressure and the size of the rupture opening. Systems under pressure are expressed in terms of low, moderate, and high. Low pressure is considered to be less than 100 psi; moderate pressure ranges from 100–500 psi; and high pressure is above 500 psi. Generally, Class I, Class II, and Class III liquids have different evaporation rates because of their different vapor pressures. For a given temperature, the vapor pressure of a Class I liquid will be higher than that of Class II and Class III liquids, and the vapor pressure for a Class II liquid will be higher than that of Class III liquids. Consequently, these vapors will cover different floor distances when released to the atmosphere. Vapors from a Class I liquid will spread out farther than the vapors from Class II and Class III liquids, and the vapors of a Class III liquid will stay closer to the point of release. These conditions are illustrated in the example Table 1-1, where heavier-than-air vapors are released to the atmosphere from a liquid spill. Class I and Class II flammable liquids are considered dangerous when temperatures are above flash point. The danger of a Class III flammable liquid is only considered at the surface of the liquid when the temperature is above flash point and, therefore, these vapors will not render any significant hazard. Flammable liquids are also capable of evaporation at temperatures below flash point. These vapors, however, are not considered explosive. The temperatures of the surrounding air may provide an additional limit to the vapor travel distance. If the temperature of the surrounding air is lower than the temperature of the liquid, the vapors will cool and form a mist, thereby reducing the vapor traveling distance.

Table 1-1. Vapor Pressure versus Vapor Traveling Distances

Liquid Class

Flash Point, °F

Liquid Temp. °F

Vapor Pressure, ATM

Vapor Traveling Distance

I

50

200

0.45

Large

II

100

200

0.12

Small

III

140

200

0.048

Minimal

10

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

B.7. Extent of Hazard as a Function of Molecular Weight It is expected that the vapor traveling distances are equal when the vapor pressures are kept the same. This is not necessarily true because of differences in molecular weights. When the vapor pressures of different classes of vapors are kept the same, as shown in Table 1-2, vapors from a Class I liquid will generally cover a larger area than the vapors from Class II and Class III liquids. Vapors from a Class II liquid will cover a larger area than the vapors of a Class III liquid. The difference in areas covered is due to varying molecular weights. For example, if the molecular weights of Class I vapors are lighter than Class II vapors and Class III vapors, and the molecular weight of Class II vapors are lighter than Class III vapors, the vapor traveling distances are not the same. When the vapor pressure is kept the same, vapors from a lighter Class I liquid will generally cover a larger area than the vapors from heavier Class II and Class III liquids, and vapors from a lighter Class II liquid will cover a larger area than the vapors from a heavier Class III liquid. This is shown for heavier-than-air vapors in Table 1-2.

Table 1-2. Molecular Weight versus Vapor Traveling Distances

Liquid

Flash

Liquid

Vapor

Molecular

Vapor

Class

Point,

Temp.

Pressure,

Weight

Traveling

ºF

ºF

ATM

Distance

I

50

200

0.45

Minimal

Large

II

100

200

0.45

Small

Small

III

140

200

0.45

Large

Minimal

Table 1-2 shows that if the vapor pressure is the same, but the molecular weights of the vapors vary, vapor distances will also vary. Since the density of a vapor is the weight of a volume of pure vapor compared to the weight of an equal volume of dry air at the same pressure and temperature, the molecular weight of the vapor is calculated by multiplying the vapor density with the molecular weight of air. The molecular weight of air is 29. Heavier molecular weights have high boiling points. This in turn causes a high flash point, which produces a low vapor pressure resulting in low volatility. In contrast, lighter molecular weights have low boiling points. This in turn causes a low flash point that produces a high vapor pressure resulting in high volatility.

Chapter 1: Introduction and Overview

11

B.8. Vapor Traveling Distance The point at which a flammable gas or vapor will reach a nonhazardous concentration is determined by the distance the gas or vapor must travel to reach the nonhazardous concentration. Once a flammable gas or vapor is released to the atmosphere, it will mix with air rapidly. In the initial stage of release, when a gas is close to its source of release, its concentration is too rich to form an explosive mixture. That is because a gas starts from a 100% concentration. A vapor, on the other hand, is too lean to form an explosive mixture. That is because a vapor starts from a 0% concentration. As air moves the gas or vapor away from its point of release, the gas and vapor will both reach their second stage in which they will mix with air sufficiently to form an explosive mixture. As the gas and vapor continue to move away from their point of origin, they both will enter their third stage in which they will be diluted by air to the extent that their concentrations become too lean to form an explosive mixture. A flammable gas or vapor released to the atmosphere must travel through these two stages before they reach the nonhazardous concentration of the third stage. A flammable gas is normally instantly airborne. Vapors may not be airborne instantly. Flammable liquid must be spilled first before vapors will exist in the air. The evaporation rate of the liquid is primarily a function of temperature. The higher the temperature above flash point, the higher the rate of evaporation. Due to differences in vapor densities, not all flammable liquids being discharged to the atmosphere will have the same evaporation rate. The volume of vapor produced from a given quantity of liquid is a function of the vapor pressure and the specific gravity of the liquid. The volume of vapor produced from a given quantity of flammable liquid varies inversely with the vapor pressure of the liquid. The distance at which a flammable gas or vapor must travel to reach nonhazardous concentrations also varies in inverse proportion to its LEL. At a given emission rate, the gas or vapor in the atmosphere may travel a long distance before reaching a nonhazardous concentration. This is when the LEL is low or when the quantity of flammable gas or vapor released to the atmosphere is large. The traveling distance is shorter at higher LELs or when small quantities of flammable gases or vapors at the same given emission rate are released. Wind velocity also has a great impact on the traveling distance of a flammable gas or vapor. The lower the wind velocity, the greater the traveling distance, the longer the gas or vapor remains ignited, and the more dangerous the location. High wind velocities will reduce the gas or vapor traveling distances because of their faster dilution. Wind-still conditions will also add to larger hazardous areas. Summaries of the major conditions that will have an impact on the traveling distance of a flammable gas or vapor are as follows. A. Long Horizontal Traveling Distances 1. High pressure or high process temperature in the system.

12

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres 2. Large rupture opening causing a high flow rate of flammable substances. 3. Low molecular weight. 4. The release of large quantities of flammable gases or vapors. 5. Low LEL. 6. Low wind velocity without cross winds and a flat terrain. 7. Heavier-than-air gases or vapors. B. Short Horizontal Traveling Distances 1. Low pressure or low process temperature in the system. 2. Small rupture opening causing a low flow rate of flammable substances. 3. High molecular weight. 4. The release of small quantities of flammable gases or vapors. 5. High LEL. 6. High wind velocity with cross winds and obstructed terrain. 7. Lighter-than-air gases or vapors.

B.9. Vapor Volume of Flammable Mixtures Sometimes it is convenient to calculate the volume of air required providing dilution of a flammable vapor to prevent the formation of an ignitable mixture. The volume of air can be calculated when the quantity of solvent is known. The volume of vapor produced from one gallon of solvent can be calculated from the specific gravity of the liquid and the vapor density as follows:

Va =

Eq. (1) where:

8.33 × Sp. Gr. 0.075 × VD × LEL × C

Va

= Cubic feet of vapor from 1 gal of liquid

8.33

= Weight of 1 gal of water in lbs

0.075

= Weight of 1 ft3 of air in lbs at 70°F

VD

= Vapor density of solvent (air = 1.0)

Sp. Gr. = Specific gravity of solvent (water = 1.0) C

= Constant for LEL of solvent vapor-air mixture LEL decreases at elevated temperatures: C = 1.0 for temperatures up to 250°F C = 0.7 for temperatures above 250°F

LEL

= Lower explosive limit of the flammable product

Chapter 1: Introduction and Overview

13

If the vapor density is not known, it can be calculated from the molecular weight as indicated in Sec. B.4.

B.10. Liquefied Petroleum Gases Liquefied petroleum (LP) gases liquefy under low pressure and evaporate when the pressure is removed. The escape of liquefied gas into the atmosphere will normally result in an instantaneous evaporation. The potential fire hazard of LP gas vapor is comparable with the potential fire hazard of manufactured gas or natural gas, except that the vapors of LP gas are heavier-than-air. All LP gases are required to be odorized by the addition of a warning agent.

B.11. Grouping of Flammable and Combustible Products The National Electrical Code (NEC) divides explosion hazards in three classes: Class I, Class II, and Class III. These class designations are not to be confused with the flammability class of flammable liquids. Flammable liquid has the same class designation as the class designation of the NEC. The classes of explosion hazard as described by the NEC represent hazardous locations. These hazardous locations are areas in which sources of hazards are capable of producing flammable liquids, flammable vapors, gases, combustible dust, or flying fibers that may cause explosive mixtures. They are grouped as follows: 1. NEC Class I. Locations where flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures. 2. NEC Class II. Locations that are hazardous because of the presence of combustible dust. 3. NEC Class III. Locations that are hazardous because of the presence of easily ignitable fibers or flyings in sufficient quantities to produce ignitable mixtures, but where such fibers or flyings are not likely to be in suspension in air. This publication is primarily concerned with NEC Class I locations, however, it also includes some information for NEC Class II locations. NEC Class III locations are sparsely considered herein. Each flammable vapor, gas, or combustible dust is capable of producing maximum pressure when exploding. By grouping the various flammable substances according to their explosion characteristics, the maximum explosion pressure can be divided into groups. The NEC lists these flammable products in various groups. For NEC Class I locations, the flammable substances are listed in-groups A, B, C, and D. For NEC Class II locations, the NEC lists combustible dusts in-groups E, F, and G. In this publication, only coal dust (Group F of NEC Class II locations) is considered.

14

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

The grouping is extremely important when it comes to selecting explosion-proof and dust-ignition proof equipment, because of the different strengths of the explosion proof equipment.

C.

COMBUSTIBLE COKE AND COAL DUST

Combustible coke and coal dust are hazardous when exceeding their lower explosive limits. Whereas explosions are eliminated for flammable liquids by gas or vapor concentrations below the LEL and above the UEL, this cannot be applied to combustible dust. An explosion results when the dust cloud has a concentration above the LEL. Any suspension of combustible dust above the LEL must be regarded as hazardous because there is no corresponding upper explosive limit. The LEL is the minimum concentration of combustible dust in the suspending medium that will propagate a flame and may be as little as 0.05 oz. per cubic foot of air. The explosion intensity depends on the rate of pressure rise, which determines the speed at which an explosion is propagated through the suspended dust. The speed of propagation may approach detonation with high rates of pressure rise. Dust accumulation may also be hazardous because it may be thrown in the air by movements and explode if ignited. Electrical failure such as a burned-out motor winding or a phase to ground fault are some of the sources of ignition energy that may cause an explosion of coal dust. Proper prevention of the accumulation of coal dust will greatly limit the possibility of explosion hazard. The installation of proper electrical equipment will also minimize the possibility of an explosion hazard. The simplest way of eliminating an explosion hazard is by dust suppression. Dust suppression may consist of an automatically operated water spraying system that keeps solid fuel constantly wet.

D.

FIRE AND EXPLOSION HAZARDS IN NEC CLASS I LOCATIONS

D.1. General In order to provide safety for property and personnel in NEC Class I locations, it is important to be aware of the nature of fires and explosions associated with flammable products. If their characteristics are fully understood, the knowledge can be applied to reduce the hazardous conditions in NEC Class I locations. The risk of explosion is frequently present when flammable gases or volatile liquids are used in chemical process plants. Once a flammable gas or a flammable

Chapter 1: Introduction and Overview

15

liquid is capable of escaping to the atmosphere from its confinement, the gas or the vapor from the liquid will rapidly become an ignitable mixture that will explode upon contact with a source of ignition. However, not all mixtures that are ignitable will produce an explosion, some of them will produce only a flame. When the ignitable mixture is entering its explosion range, the mixture becomes explosion prone. Only a very short time is required for the gas or vapor to enter its explosion range. The rate at which a flammable gas or vapor will mix with air depends on its molecular weight. Normally, heavy gases will diffuse slower than light gases and flammable liquids must first evaporate before they can mix with air. Diffusion rates in still air, where no drafts or convection currents are present, are the minimum rates at which gases or vapors will mix with air. Any movement of air tends to increase the rate of mixing. The difference between fires and explosions lie primarily in the rate at which their energy is released. An explosion normally has a rapid rate of energy release, and a fire may have either a rapid or slow rate of energy release. For example, a slow rate of energy release is when a flame requires several minutes to consume gasoline in liquid form. A rapid rate of energy release is when the same amount of gasoline is consumed instantly on contact with a source of ignition, after the gasoline has vaporized and mixed with air. Fast spreading fires have a rapid rates of energy release. If the rapid release should take place in the atmosphere, the result is a flash fire, popularly called an explosion. However, if a massive release of energy occurs in the atmosphere, it is possible for the air or surrounding buildings to comprise sufficient confinements to lead to a type of explosion known as an unconfined explosion. If the rapid release of energy is confined to prevent dissipation, or if the pressure resulting from the energy builds up to a point where the confinement will or can burst, the rapid energy release is a true explosion known as a confined explosion. The violence of an explosion depends on the nature and quantity of the fuel and the energy of the ignition source. However, it depends mainly on the ratio to air.

D.2. Principles of Fires and Explosions Since the violence of gas or vapor explosions are characterized by the release of a considerable amount of energy, their behavior is a result of rapid oxidation. A flammable gas or a flammable vapor will release considerable amounts of energy if combined with an oxidizing agent in the proper proportions. A flammable gas in the proper ratio with an oxidizing agent can easily be ignited to produce an explosion, whereas a flammable liquid cannot be ignited. For a flammable liquid to produce an explosion, it is necessary for the liquid to evaporate first. This vapor can be ignited with a source of ignition of sufficient energy, which is equal to or exceeding the ignition temperature of the flammable product of the vapor, and if the vapor is in the proper ratio with an oxidizing agent.

16

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

The most common oxidizing agent is the oxygen in the air. Air contains 21% oxygen. The remaining elements are 79% nitrogen, plus some small amounts of carbon dioxide and other gases. Under normal conditions, the oxygen contents of air are more than adequate to support an explosion or fire. Nitrogen plays no part in the combustion process. When an ignition source of sufficient energy is applied to a flammable gas or vapor-air mixture, oxidation of the flammable substance will take place. The speed at which this oxidation occurs depends on the ratio of the gas or vapor-air mixture. When the gas or vapor concentration in the mixture is lower or higher than the optimum mixture, the oxidation of the flammable substance is slow and the heat produced during the oxidation reaction may require several minutes to consume the gas or vapor fuel. When the concentration of the gas or vapor with oxygen is in the correct proportion (i.e., within its explosion range), the oxidation is rapid and, therefore, the heat produced will be capable of consuming the mixture instantly. (For example, see Fig. 1-1B.) When a gas or vapor concentration is within its explosion range, in a proportion with air that will produce rapid oxidation, the heat produced during the oxidation process will spread rapidly through the entire vapor-air mixture. The heat produced during oxidation will start at the ignition source and spread rapidly away from the source, layer by layer, with each layer consisting of a mixture of gas or vapor and oxygen molecules. In slow motion, the oxidation process will first occur at the source of ignition. The heat from the source of ignition will combine the gas or vapor molecules with the oxygen molecules. The reaction of molecules produces heat. This heat will consume the gas or vapor-air mixture in the first layer and act as an igniter for the next layer, surrounding the first layer closest to the ignition source. The heat of the igniter will combine the molecules of the gas or vapor and oxygen of the second layer. The heat produced as a result of this reaction will consume the gas or vapor-air mixture in the second layer and act as an igniter for the surrounding third layer. This process will continue and repeat itself until the heat consumes the last layer. The entire process will take place in a fraction of a second. The fast spreading heat as a result of the rapid oxidation is called flame propagation. The speed at which the flame propagates through the mixture is a measure for the rate of release of energy of an explosion. The violence of the explosion is measured by this rate of energy release. Flame propagation will only occur when the flammable mixture has a concentration within its explosion range. If the flammable mixture reaches a point where it can be ignited (for example at the LEL), the concentration of the flammable substance in percent-by-volume in air will roughly correspond to the flash point of the flammable substance. The fire point is usually a few degrees above the flash point. The speed at which the flame will spread through the mixture at the LEL is almost zero. Consequently, the release of energy at this point is also almost zero.

Chapter 1: Introduction and Overview

17

If the concentration is increased, it becomes easier to ignite the mixture, and the propagation and release of energy becomes progressively more violent until a maximum is reached. This maximum is at the knee of the explosion curve which will produce the most intense combustion capable of that particular gas or vapor. (See Fig. 1-1B.) A further increase in concentration will result in a gradual decrease in the violence of propagation and the release of energy until a point is reached where the mixture no longer will propagate a flame, but will burn at the point of ignition. This point is called the UEL. Flame propagation does not occur when the flammable mixture is below the LEL or above the UEL. When the concentration of the flammable substance is below the LEL, the mixture is too lean for propagation and may only burn at the source of ignition. Should the mixture contain a concentration which is above the UEL, the mixture is too rich for propagation. Mixtures that are too rich will not support combustion nor will they catch fire. The explosive limits are based upon normal atmospheric temperatures and pressure. There may be considerable variations in explosive limits at pressures or temperatures above or below normal. Increases in temperature cause the LEL to drop. The following summarizes the three basic requirements that must be satisfied for an explosion or fire to occur. 1. A flammable gas or vapor must be present in the atmosphere. 2. The flammable gas or vapor must have mixed with air in proportions that allow the mixture to become ignitable. 3. An ignition of sufficient energy having a temperature equal to or in excess of the ignition temperature of the flammable substance must be present. To prevent an explosion or fire in the atmosphere, it is necessary to do one or more of the following. 1. Keep the concentration of the gas or vapor below the LEL by sufficient air. 2. Remove the source of ignition. 3. Reduce the energy of the source of ignition to safe levels. 4. Confine the explosion or fire.

D.3. Early and Remote Permanent Non-Electric Ignition Sources As a general rule, a flammable gas or vapor will become explosive as soon as it is released to the atmosphere. There are conditions, however, where these gases and vapors in the air are prevented from becoming explosive. They will not become explosive when they are instantly ignited by an ignition source of sufficient energy

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

before they enter their explosion range. Such a condition is called “early ignition.” Early ignition will be possible only when a source of heat is present continuously, and when the heat source is above the ignition temperature of the flammable product at the point of release. As mentioned before, a flammable gas or vapor will not become explosive at the lower and upper limit of its explosive range. At these limits, only a flame will occur when the flammable gas or vapor is ignited. For example, hydrogen liquid escaping from its confinement will be prevented from becoming explosive if the process temperature is above the ignition temperature of the flammable product. Once the hydrogen liquid is released into the atmosphere, it will instantly evaporate to a 100% gas. From the very first moment, the process temperature is not capable of igniting the gas because of its rich concentration. But, as the gas becomes diluted by air, its concentration will reach a point where it can be ignited by the process temperature. At this point, the flammable gas has reached its upper explosive limit, which for hydrogen liquid is 75%. At 75% gas concentration, the process temperature is capable of igniting the hydrogen gas and will make the gas burn so that a flame will appear at the point of release. The flame will consume the gas mixture and prevent it from entering its explosive range, thereby preventing the gas from becoming explosive. At this point the ignition may cause a popping sound. If the process temperature is lower than the ignition temperature, the mixture will not be ignited at the upper explosive limit, allowing the gas to further mix with air and enter its explosive range. Two examples of conditions under which the flammable gas will not enter its explosive range are at a process tank that is heated above the ignition temperature of the flammable substance, and at a pump driven by a highly pressurized steam turbine which has a temperature above the ignition temperature. Highly pressurized steam turbines may have high internal temperatures, with some lower temperatures at the outer surface of the insulation surrounding the turbine and piping system. A failure of the pump, or a breakdown of its gasket may instantly splash the flammable liquid from the pump onto the hot surface of the steam turbine, resulting in the burning of the liquid. This prevents the liquid from evaporating and entering its explosion range. However, when the pump is some distance away from the steam turbine, thus preventing the escaping liquid from splashing on the hot surface, the liquid will begin evaporation immediately. The evaporating liquid will mix with the surrounding air, allowing the vapor to enter its explosive range. When the vapor then reaches the hot surface of the turbine, it will be ignited and propagate the flame through the mixture, resulting in a flash fire. This is called “ignition by a remote permanent ignition source.” Therefore, it can be concluded that there exists as much danger with a non-electrical source of ignition that is continuously present, as when a flammable gas or vapor in the atmosphere is continuously present. It is also clear that a source of hazard involved in “early ignition” is less dangerous, than when involved with a “remote permanent ignition source” which is more dangerous. For both conditions, the areas must be classified, but only if electrical equipment is also within the danger area.

Chapter 1: Introduction and Overview

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D.4. Ignition Sources The principle sources of ignition include flames, static and friction sparks, electrical arcs or sparks, and hot surfaces. Flames are unfailing sources of ignition. Flames must be capable of heating a vapor to its ignition temperature in the presence of air, in order to be a source of ignition. Static and friction sparks must have sufficient intensity and be of sufficient duration to ignite a flammable gas or vapor-air mixture. Electrical sparks are well above flame temperatures and will usually ignite flammable mixtures if of sufficient duration. Hot surfaces are sources of ignition if large enough, and hot enough. The smaller the heated surface, the hotter it must be to ignite a flammable mixture. The larger the heated surface, in relation to the mixture, the more rapidly ignition will take place and the lower the temperature needed for ignition.

D.5. Causes of Fires and Explosions The main causes of fires and explosions in NEC Class I locations are ruptures. Small ruptures are quite likely to occur and could cause great risk unless they are discovered quickly and repaired rapidly. If the rupture gives rise to a gas or vapor cloud which is allowed to grow for a period of time before being ignited, the volume of gas or vapor generated can be considerable and may result in flash fires or unconfined explosions in the open air when ignited. Ruptures are normally limited to joints, fittings, gaskets, or seals, resulting in leakage of the flammable gas or vapor or liquid to the atmosphere. Less common are ruptures of the entire process equipment. The size of a rupture opening is a function of the pressure in the system and will vary from small to large depending on its withstanding capability, material quality and workmanship. The quantity of the flammable gas or vapor released from a given rupture opening is in direct proportion with the pressure in the system and the size of the rupture opening. The rupture opening in a large fitting may have a much greater size than the largest rupture opening in a small fitting. The quantity of a flammable gas or vapor released from these openings are accordingly large and small under its given pressure. Since the size of the rupture opening is limited by the size of the fitting, the quantity of flammable gas or vapor released to the atmosphere is limited accordingly.

Chapter 2 Classifying Sources of Hazard

A.

SOURCES OF HAZARD

A.1. Static and Dynamic Sources of Hazard Sources of hazard are divided into two major types—static and dynamic. Rotating equipment belongs to the dynamic-type, and non-rotating equipment belongs to the static type. It is important to know to which type a particular source of hazard belongs, because of their different rates of wear. A dynamic-type source of hazard, such as a pump or any similar rotating equipment, has a greater rate of wear than a static-type source of hazard, such as a storage tank, drum, valve, flange, screwed fitting, etc. This is because dynamictype sources of hazard have high revolving parts. Static-type sources of hazard have no wear or low wear. A valve, for example, has a low rate of wear because of its slow rotating parts. A storage tank with a floating roof also has a low rate of wear because of its slow moving parts. Screwed fittings and non-welded pipe flanges are also considered to have a low rate of wear. However, a storage tank with a fixed roof or a drum or container is considered to have no wear. Wear in static-type sources of hazard can generally be neglected provided their operating conditions are normal. However, under excessive or abnormal operating conditions, wear in static-type sources of hazard is accelerated causing a breakdown sooner. For example, a static-type source of hazard which is frequently operated, such as a valve, flange, or screwed fitting, and which is regularly worked on, has a greater rate of wear. Connections to a fill pipe for a truck-loading vehicle, for example, may be subject to a greater rate of wear. 20

Chapter 2: Classifying Sources of Hazard

21

If a greater rate of wear is expected, the classification for the equipment is required to be more conservative, particularly when static-type sources of hazard are involved. For example, the degree of danger for a static-type source of hazard, which under normal operating conditions is provided with a small Div. 2 zone, must be changed to a Div. 1 zone if the source of hazard is frequently operated or regularly worked on. A change in classification is generally required if the danger zone is small and the sources of hazard are of the static type. For dynamic-type sources of hazard a change in classification is generally not necessary because the classification for dynamic-type sources of hazard are normally more conservative by virtue of the higher rate of wear caused by the fast rotating elements. The rate of breakdown is normally accelerated when temperatures and pressures in the system are elevated. The higher the temperature and pressure, the greater the probability of breakdown.

A.2. Open and Closed Sources of Hazard Sources of hazard are either open or closed. Their open or closed conditions are referred to as their “operating mode.” Systems containing open sources of hazard are generally not pressurized. Systems containing closed sources of hazard, on the other hand, may operate under low, moderate, or high pressure. It is important to consider pressure in a closed system, because it tends to produce a larger quantity of flammable material to the atmosphere under accidental failure, than when there is low or no pressure. Pressure in the system, therefore, should be taken into consideration when determining the extent of the danger zone. However, the extent of the danger zone should not be based on the system pressure alone. Pressure in the system should always be considered in combination with the vapor density of the flammable product involved, the size of the source of hazard, its location, and whether the location is sufficiently ventilated or not. For example, a “small” dynamic-type source of hazard in an “outdoor” location, processing a flammable product with “heavier-than-air” vapors may require a 10, 15, or 25 feet horizontal boundary when the pressure is low, moderate, or high. A boundary of 50 or 100 feet is required when the dynamic-type source of hazard is “large.” The question, however, is where to draw the line between “small” and “large” dynamic-type sources of hazards. What is “small” and what is “large?” These questions can be answered by referring to Sec. A.4 in this chapter.

A.3. Mini, Small, and Large Sources of Hazard Each dynamic and static-type source of hazard is also subdivided into three broad sizes: mini, small, and large. The word “mini” is generally associated with static-type sources of hazard, which have a physical outline smaller than a small source of hazard. A screwed connection in a piping system, bleeders, or a meter,

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

for example, are generally mini sources of hazard. A valve and flange on the other hand, or similar devices, can be mini, small, or large. Ball valves, gate valves, block valves, butterfly valves, and associated pipe flanges, for example, have physical dimensions that range from very small to very large. These types of sources of hazard are considered “mini” if their sizes are 6" and below, “small” if their sizes are between 6" and 14", and “large” if their sizes are 14" and above. A large storage tank is considered a “small” source of hazard. The large surface of the liquid in the tank could lead to the conclusion that the tank is a large source of hazard requiring a large hazardous area. This is not so. Tanks with fixed roofs are normally provided with atmospheric vents located on top of the tank roof, through which flammable vapors are released into the atmosphere. These quantities of vapors are small because the vents are small and the temperature in the tank can be low. As a result of these considerations, the small vent must be seen as the actual source of hazard, and not the large tank itself. Since the vent is small and capable of releasing only small quantities of flammable vapors because of the low temperature, the required hazardous area must also be small. In addition, the vapor release occurs high in the air and allows these vapors to disperse rapidly into low concentrations. This then supports the requirement for a small hazardous area. Considering all these aspects, the conclusion is that the vent is considered a “small” source of hazard requiring only a small danger zone around the vent pipe. If, however, the flammable vapors are heavier than air, they come down along the shell of the tank. Therefore, the tank shell is also given a classification, but only if the vapors released are heavier than air. As an additional safety precaution, the top of the tank is also given a classification. Dikes for containing the flammable liquid in case of a possible leak normally surround large storage tanks. The horizontal area from the tank up to the dikes must, therefore, also be classified. Another large static-type source of hazard, which is considered a small source of hazard, is a storage tank with a floating roof. Since quantities of flammable vapors escaping from these tanks are also small and also disperse rapidly high into the air, they also need a small hazardous area on top of the tank roof. Therefore, this tank is also considered a “small” source of hazard. However, since the floating roof is subjected to mechanical wear, the hazardous area on top of the roof requires a more conservative classification than the top of a fixed roof tank. Pressurized storage tanks, which are provided with pressure relief valves also, need a small hazardous area. The pressure relief valve is a “mini” source of hazard and is also considered the actual source of hazard, since the relief valve is capable of releasing only a small and limited stream of flammable vapors for a short duration. Thus, only a small hazardous circular zone is required around the relief valve. Impounding basins may also be considered as a mini source of hazard, because of their small release of flammable gases of vapors to the atmosphere. The flammable liquids, in impounding basins, are generally mixed with a great deal of

Chapter 2: Classifying Sources of Hazard

23

water and as a result, they release very small amounts of flammable vapors to the atmosphere. Locations with piping systems, which are all welded, operating at any pressure without valves, screwed fittings, bolted flanges, and meters need no classification. However, if the piping system includes valves, screwed fittings, bolted flanges, and meters operating at moderate or high pressure, classification of the location is necessary.

A.4. Determining Small and Large Dynamic-Type Sources of Hazard It is rather difficult to establish the practical dividing line between a small source of hazard and a large source of hazard, especially when the source of hazard is of the dynamic-type, such as a pump. Pumps range in size from very small to very large. Between these limits, there are a number of different sizes that are also small and large. Since the quantity of a flammable gas or vapor released from a pump has an impact on the size of the hazardous location, it is essential to establish the size of the pump in terms of small and large. An acceptable approach for defining the size of a pump, is when it is related to the size of its associated electric driver. As in electrical power systems, a range in motor horsepower is assigned a given voltage. The same motor horsepower range could be applied for defining whether the pump is large or small. Low voltage motors are assigned a horsepower rating up to 200 hp and are normally considered to belong to a group of small motors. Medium voltage motors are assigned a horsepower rating above 200 hp and are generally considered to be in a group of large motors. Pumps driven by electric motors rated 200 hp and less, therefore, could also be considered small. Pumps driven by motors rated above 200 hp could be considered large. When this logic is adopted as a basis for determining a large or small pump, the same logic can be applied to the quantity of flammable gases for vapors in relation to the size of the pump and the size of the hazardous location. Since the boundary size of a hazardous area is a function of the traveling distance of a flammable gas or vapor in the air, and the traveling distance in turn is a function of the quantity of flammable gas or vapor released by the pump, then the quantities of flammable gas or vapor released to the atmosphere is a function of the size of the pump. Small quantities of flammable gases or vapors released by a small pump under failure would, therefore, require a small hazardous area, and large quantities from large pumps would require large hazardous areas. Since small hazardous areas have a maximum radius of 25 ft and less, and large hazardous areas have a radius of 50–100 ft, it is appropriate to assign, for outdoor locations, a maximum of 25 ft boundary for small pumps with drivers rated 200 hp and less, and a 50–100 ft boundary for large pumps with drivers rated above 200 hp. It is also appropriate for indoor locations to assign a maximum of 50-foot boundary for small pumps with

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

drivers rated 200 hp or less and a 50- or 100-foot boundary for large pumps with drivers rated above 200 hp. The boundary sizes should not be based on the size of the driver alone, but also on the vapor density of the flammable product involved and the pressure in the pump. However, since small boundaries have a maximum size of 50 ft and less, pumps with sizes between 0 and 51 hp are considered too small to be associated with a 50 ft boundary. For example a heavier than air flammable vapor or gas leaking from a 50 hp pump may travel less than 10 ft before reaching safe concentrations. A 50 ft boundary for such a short traveling distance will be far out of proportion. Because of this, the range of horsepower of the pumps should therefore be subdivided in two groups to make up for the smaller traveling distance: one for 0–51 hp and one for 60–201 hp. Taking into account the fact that system pressure plays an important role in sizing the boundary, the boundary outdoors for the lowest group of pumps, from 0–51 hp, should have a maximum radius of 10 or 15 ft and for indoor locations a maximum radius of 25 ft. Above 51 hp up to 201 hp, the boundary should have a radius of 25 ft for outdoors and 50 ft for indoors. This way, a practical method is established for determining the boundary size for a given small pump operating at a given system pressure. The two groups of pumps with ratings expressed in horsepower are listed in Figs. 1-2 and 1-3 Since indoor locations generally require larger boundaries than outdoor locations, the 50 ft boundary for small pumps in indoor locations are subdivided into 50 ft and 25 ft. Figure 1-2 is for pumps in outdoor locations, and Fig. 1-3 is for pumps in sufficiently ventilated indoor locations. When a number of small pumps should occupy a large floor space, such as a pump station, the entire pump station could be considered as one large source of hazard. Although it is not likely that two or more pumps will fail at the same time, the chances that more than one pump can fail is much greater than when there is only one pump. Consequently, a greater amount of flammable substances could be released to the atmosphere from more than one pump, than would be released from only one pump. For this reason, the entire pump station containing small pumps could be considered as one large source of hazard.

B.

WHEN A LOCATION IS HAZARDOUS

For a location to become hazardous, it is necessary that a flammable gas or vapor is, or may be, present in the air in sufficient quantities and has mixed with air in a ratio that allows the flammable gas or vapor mixture to burn, explode, or to produce a flash fire when ignited. A location, therefore, must be considered hazardous when these conditions will or may occur. As long as these conditions could exist because of a rupture, breakdown, leakage, or malfunction of the process equipment, the location must be considered hazardous. Only if these conditions never will exist can the location be considered not to be hazardous.

Chapter 2: Classifying Sources of Hazard

Figure 1-2. Pump sizes versus hazardous boundary sizes (outdoors).

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-3. Pump sizes versus hazardous boundary sizes (indoors, sufficuently ventilated).

Chapter 2: Classifying Sources of Hazard

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The size of the hazardous location is a function of the quantity of the flammable gas or vapor released to the atmosphere. Large quantities of flammable gases or vapors released to the atmosphere require large hazardous areas. This is because large quantities of flammable gases or vapors will require long traveling distances to reach safe concentrations. Small quantities of flammable gases or vapors require short traveling distances because they reach safe concentrations much faster. The area that must be considered hazardous is also a function of the density of the flammable gases or vapors. If heavier-than-air flammable gases or vapors are released to the atmosphere, their traveling distance is longer than if lighterthan-air flammable gases or vapors are released. The traveling distances of the lighter-than-air flammable gases or vapors are much shorter because they have the tendency of rising quickly after they have been released to the atmosphere. Therefore, the larger the traveling distance required by a flammable substance, the larger the size of the area that must be considered hazardous. Also, the larger the traveling distance of a flammable substance, the greater the chances for them to be ignited by electrical equipment. The fact that a large area can house a greater number of electrical equipment, which are considered as prime potential sources of ignition, makes the location basically more dangerous than smaller areas. The first requirement for a location to be classified is that the temperature, ambient or process temperature of the flammable product, is above flash point. If the temperature is below flash point, the location is not hazardous, and consequently, classification of the location is not necessary. In addition to the first requirement, it is also necessary that electrical equipment be present in the hazardous location. Since electrical equipment is a very effective source of ignition and generally is more effective than any other non-electrical source of ignition, classification of a location must be associated with the presence of electrical equipment. High temperatures, arcs, or sparks are frequently produced by electrical equipment under normal and abnormal operating conditions. Less frequent is the presence of sparks of static electricity, because of their remote occurrences as a result of proper grounding. Therefore, static electricity is generally not considered a valid source of ignition when classifying a location as hazardous. Ignition sources with temperatures in excess of the ignition temperature of flammable products produced by non-electrical equipment are considered valid sources of ignition. These sources of ignition in a location require that the location be classified if the location is also provided with electrical equipment. Classification of a hazardous location is also required if electrical equipment is purged or is intrinsically safe. Although intrinsically safe electrical equipment is not capable of ignition, their presence in the location is a result of the classification of the hazardous location.

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

C.

SAFETY VERSUS HAZARD IN NEC CLASS I LOCATIONS

In order to make a correct evaluation of safety of a NEC Class I location, it is necessary to understand the nature of safety and the hazard that exists in Class I locations. If their natures are understood, a level of safety can be established for the location that will either provide a condition of security and freedom from injury, or a condition in which the safety will commensurate with risk. There are two levels of safety that can exist in a hazardous location. If a level of security and freedom from injury is provided, the location is considered “safe.” If the safety must be commensurate with risk, the level of safety is called an “acceptable level of safety at the lowest possible cost.” There are also two levels of hazard in an NEC Class I location. The locations can be “hazardous” or “remotely hazardous.” However, safety and hazard are not related to each other. It is not necessarily true that maximum hazard reduces the safety to a lower level and vice versa. The location can be safe or acceptably safe independent of the levels of hazard. They both can have maximum and minimum levels at the same time. Safety is related to the type of electrical equipment that is selected for the hazardous location and is entirely based on whether or not the electrical equipment in the location is capable of igniting a flammable substance in the atmosphere. A location is considered safe when arcs or sparks from electrical equipment are not capable of igniting a flammable substance in the atmosphere. On the other hand, a location is acceptably safe when ignition of a flammable substance in the atmosphere can, but not necessarily will occur by arcs or sparks from electrical equipment. Some types of electrical equipment used in hazardous locations are designed so that they will prevent their arcs or sparks from igniting a flammable substance when the electrical equipment is surrounded by it. For example, suitable explosion-proof electrical equipment is not capable of igniting a surrounding flammable substance, nor will the explosion-proof electrical equipment burst under explosion pressure if an explosion should occur in the enclosure, nor will it propagate the explosion flame to the atmosphere. Because of these favorable features, the application of explosion-proof electrical equipment in hazardous locations makes the location safe for personnel and equipment. Maximum safety in the hazardous location is, therefore, entirely based on the presence of explosion-proof electrical equipment. Non-explosion-proof electrical equipment does not have these favorable features and when used in hazardous locations, safety is reduced to a lower level. The levels of hazard are determined without considering electrical equipment. Hazard in a NEC Class I location is only considered with regard to a flammable substance that is or may be present in the atmosphere. The levels of hazard in NEC Class I locations are expressed as a dangerous condition and a remotely dangerous condition and are defined by Article 500 of the NEC as Div. 1

Chapter 2: Classifying Sources of Hazard

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and Div. 2. Electrical equipment for the hazardous location is selected after the level of hazard of the location is determined. This will lead to a specific safety in the location (i.e., a safe condition or an acceptably safe condition). The level of hazard is a measure for the type of electrical equipment to be used; and the type of electrical equipment, in turn, is a measure for the level of safety. If the hazard is Div. 1, explosion-proof electrical equipment must be selected which provides maximum safety for the location. If the hazard is Div. 2, non-explosion-proof electrical equipment is selected which provides an acceptable level of safety, at the lowest possible cost. Summarizing the levels of safety and hazard in a hazardous location: 1. A hazardous location that is classified Div. 1 or Div. 2 is considered “safe” if the electrical equipment used in the location is explosion proof. Purged and pressurized enclosures are equivalent to explosionproof enclosures, except their cost and maintenance are normally much higher than for explosion-proof enclosures. 2. A hazardous location that is classified Div. 2 is considered “acceptable safe” if the electrical equipment used is suitable for an NEC Class I, Div. 2 location. 3. A hazardous location that is classified Div. 1 is considered “unsafe” if the electrical equipment used is non-explosion proof. 4. A hazardous location that is classified Div. 2 is also unsafe if the electrical equipment is not suitable for an NEC Class I, Div. 2 location.

D.

REQUIREMENT FOR NEC CLASS I, DIVISION 1, AND DIVISION 2 LOCATIONS

NEC Class I locations are those in which flammable liquid, gases, or vapors in the location are processed, handled, stored, or used. Such a location is considered “dangerous” or “remotely dangerous.”

D.1. General Requirements a. “Dangerous” Locations A location must be considered “dangerous” if ignitable concentrations of flammable gases or vapors can or may exist in the air continuously or frequently. A location becomes dangerous if the source of hazard in the location is:

30

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres • Open • Closed, but frequently leaking or opened • Closed, but not sufficiently ventilated • Subjected to increasing wear • Causing a simultaneous failure of electrical equipment when it breaks down • Capable of early ignition

Because of the above conditions, the location is considered dangerous and therefore, must be classified Div. 1.

b. “Remotely Dangerous” Locations A location must be considered “remotely dangerous” if the flammable gases or vapors exist in the air only “occasionally.” A location becomes remotely dangerous if the source of hazard in the location is: • Closed, but not frequently leaking or opened • Sufficiently ventilated • Not subjected to increasing wear • Does not cause a simultaneous failure of electrical equipment when it breaks down • Not capable of early ignition • Adjacent to a Div. 1 location and the Div. 1 location produces ignitable gases or vapors which occasionally communicate with the adjacent location unless communication is prevented by positive pressure ventilation and a suitable safeguard Because of the above conditions, the location is considered remotely dangerous and therefore it must be classified Div. 2.

D.2. Specific Requirements for Classifying a Location as Division 1 a. Open Sources of Hazard If a source of hazard is open, ignitable concentrations of vapors will exist in the air under normal operating conditions. Open sources of hazard will normally produce a continuous flow of small flammable vapors to the atmosphere. The quantity of flammable vapors from open sources of hazard is generally small because of the pressure in the system that is either non-existent or considered low. For example, the rate of release of flammable vapors to the atmosphere from small open vats or large storage tanks with vents is normally small. It appears, therefore, that the explosion danger in the location is small also. That is not so. The location must be considered highly dangerous because of the continuous presence of the

Chapter 2: Classifying Sources of Hazard

31

flammable vapors in the air and because of this, the location must be classified Div. 1, although the extent of the Div. 1 area may be small.

b. Closed Sources of Hazard Frequently Leaking or Opened A location is also considered highly dangerous if ignitable concentrations of gases or vapors frequently exist in a location. Flammable gases or vapors are only frequently released to the atmosphere if a closed source of hazard is frequently or continuously leaking or frequently opened because of repairs and maintenance. With frequent leaking which requires frequent repair and maintenance, the quantity of flammable gases or vapors released to the atmosphere is considered comparable to the quantity released from open sources of hazard and as such the location is required to be classified Div. 1. Some examples of sources of hazard that may produce highly dangerous conditions when leaking or being frequently repaired or maintained are: • Gas generators • Equipment in gas manufacturing plants • Compressors for flammable gas or pumps for volatile flammable liquid • Flanges, seals, connections in piping systems

c. Closed Sources of Hazard Not Sufficiently Ventilated The reason for applying ventilation in a hazardous location is to dilute and disperse flammable gases or vapors released into the air. But the main purpose of ventilation is to prevent accumulation and to remove the flammable material from the location. If removal is properly accomplished, accumulation is prevented. Under such a condition, the location may have a classification lower than Div. 1. Flammable gases or vapors in a hazardous location cannot be removed successfully and accumulation cannot be prevented if the location lacks ventilation or if the location is not sufficiently ventilated. Such a location must be classified Div. 1.

d. Increasing Wear If the process equipment is regularly operated or worked on, it means that mechanical wear will increase. Increase in wear is normally caused by excessive operations. High temperatures and high pressure generally accelerate the rate of wear in the system. Increasing wear will reduce life expectancy and eventually cause leakage and/or breakdown of the equipment components. For example, valves being used for a prolonged period of time in a piping system for which classification is required, may start leaking when they are subjected to excessive opening and closing cycles. The same is true for piping systems with threaded fittings which are frequently screwed tight and unscrewed for the transfer of flammable liquid from a fixed container to a movable container such as in

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

loading docks. Under these adverse operating conditions, it is necessary that the sources of hazard be provided with a Div. 1 classification even when they are sufficiently ventilated. Leakage or failures are normally not expected from new equipment. Therefore, for new equipment there is no need for a Div. 1 classification.

e. Simultaneous Failures Another condition which requires a location to be classified Div. 1 is the probability of a simultaneous failure of process and electrical equipment. A rupture of a pump seal, for example, could exert a powerful stream of gas or liquid if the pump is operating at high pressure. If such a stream is directed straight to nearby electrical equipment, the electrical equipment might be severely damaged if not properly protected by a suitable enclosure. If the stream is capable of damaging electrical equipment, then it must be considered capable of damaging the insulation of the electrical equipment. The failure of the insulation, in turn, may produce arcs or sparks which may ignite any flammable gases or vapors released from the rupture. Therefore, if during the design stage of a process system, a simultaneous failure of process and electrical equipment is considered possible, then the location must be classified Div. 1. There are two conditions which point in the direction of a possible simultaneous failure: high system pressure and nearby electrical equipment. However, not all-electrical equipment will be damaged during the failure of the process equipment. Other electrical equipment nearby may not be subjected to the destructive force. If the failure in a particular process equipment is capable of damaging the electrical equipment, then only the area in which the electrical equipment can be damaged must be classified Div. 1. Also, the area between the process equipment causing the damage and the electrical equipment must be classified Div. 1. Since a Div. 1 classification dictates the application of explosion-proof electrical equipment, it follows that only the electrical equipment that could be damaged must be provided with an explosion-proof enclosure and not the electrical equipment outside the destructive area.

f. Early Ignition There are conditions which make a hazardous location free from explosion danger. For example, a hazardous location with a flammable gas or liquid confined in a process vessel is considered free from explosion danger if the gas or liquid is heated above its ignition temperature. If the process vessel should break down, the vessel will release the flammable material to the atmosphere and the process temperature will instantly ignite the escaping gases or vapors. This instant ignition is called “early ignition.” If early ignition does occur, it will occur before the gases or vapors can enter their explosion range. Only local burning will occur at or before the upper explosion limit. It is this condition which makes the location free from explosion danger. The local burning requires that the location be classified Div. 1 and the extent of the Div. 1 area needs only to be small. However, classification is only required if electrical equipment is present.

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33

A much larger Div. 1 area is required, however, if the flammable gases or vapors are not prevented from entering their explosion range. For example, if the ignition source is remotely located from the source of hazard, explosion danger will, therefore, exist between the source of hazard and the remote ignition source. For detailed information on “early and permanent ignition,” see Ch. 1, Sec. D.3; “Early and Remote Permanent Non-Electric Ignition Sources.”

D.3. Special Requirements for Classifying a Location as Division 2 In Sec. D.2, above, conditions were highlighted whereby a location must be classified Div. 1. In this section, the conditions described will allow the location to be classified Div. 2. The concept for classifying a hazardous location remotely dangerous (Div. 2) is based on the reverse of the requirements for classifying a hazardous location dangerous. The most important requirements for classifying a hazardous location remotely dangerous is that the source of hazard is: (1) closed, (2) releases flammable material to the atmosphere only under accidental failure or breakdown of the source of hazard, and (3) is sufficiently ventilated as explained in the following.

a. Sufficiently Ventilated Closed Sources of Hazard As indicated in Sec. D.1.b above, a hazardous location is relatively safe if it is sufficiently ventilated. Therefore, a location can be classified Div. 2 if sufficient ventilation is provided and the flammable gases or vapors are not released continuously or frequently. This means that under the Div. 2 requirement the gases or vapors must only be released occasionally. However, there are situations in which the occasional release of a flammable gas or vapor allows an insufficiently ventilated location also to be classified Div. 2. These are specific exceptions to the above rule. For example, a location with static-type sources of hazard such as valves, screwed fittings, bolted flanges, etc., may be classified Div. 2 without ventilation if the probability factor for the location is not more than 5 Pu. For a probability factor to be not more than 5 Pu, it is necessary that the vapors of the flammable material are heavier than air, the system pressure is low, the floor space occupied by the sources of hazard is not more than 50%, the quantity of flammable materials released is small, and the location is attended by personnel. Only under these conditions is a location without ventilation allowed to be classified Div. 2. If the probability factor is more than 5 Pu and a Div. 2 classification is required, the location must be sufficiently ventilated.

b. Failure of Process and Electrical Equipment If a process equipment breaks down, it can be expected that flammable material will be released to the atmosphere. For electrical equipment, it is expected

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

that when it breaks down, it produces arcs or sparks of sufficient energy to ignite a flammable gas or vapor mixture in the air. If the process equipment alone should break down, the flammable gases or vapors released will not cause a dangerous condition because there are no arcs or sparks from electrical equipment. This condition allows the location to be remotely dangerous. If only electrical equipment breaks down there is no danger either, because there are no flammable gases or vapors in the location that can be ignited. This condition also allows the location to be remotely dangerous. However, if both electrical equipment and process equipment break down at the same time, then the location must be considered dangerous even though it is sufficiently ventilated. This does not imply that the failure of process and electrical equipment cannot happen at the same time. Two failures may occur at the same time as long as one failure does not have a direct impact on the other failure and as such does not affect the integrity of the Div. 2 classification.

c. Adjacent Locations An adjacent location is defined either as an open space which surrounds a Div. 1 area (normally called a “transition zone”), or it can be defined as an enclosed space of limited dimensions located outside the Div. 1 area giving access to the Div. 1 area. A transition zone, which surrounds a Div. 1 area, may be classified Div. 2 as long as ignitable flammable material from the Div. 1 area will communicate with the transition zone occasionally. This type of communication causes the transition zone to become remotely dangerous (Div. 2). Any other type of communication will cause the transition zone to be more susceptible to danger, in which case, the transition zone cannot be classified Div. 2. When a flammable material enters the transition zone, it is not necessarily true that the flammable material will be ignitable. Whether it will be ignitable depends on the traveling distance of the flammable material. The traveling distance is the distance between the point of release and the point at which the flammable material will reach safe concentrations. The distance is influenced by the quantity of material being released and wind conditions. Safe concentration, therefore, may be reached within the Div. 1 area or beyond the Div. 1 area. Whether the flammable material will be ignitable when it enters the transition zone is not the main reason for classifying the transition zone Div. 2. It is the duration it takes for an ignitable flammable material to communicate with the transition zone. For example, if ignitable flammable material enters the transition zone “continuously” or “frequently,” the duration of communication is considered too long for the transition zone to be classified Div. 2. The Div. 2 classification, therefore, depends entirely on an occasional communication of flammable material with the transition zone. The less frequent such an occurrence is, the smaller the possibility flammable materials will become ignited by electrical equipment in the transition zone.

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35

It must be understood that electrical equipment in the transition zone does not necessarily have to be explosion proof. It must be suitable for a Div. 2 environment and as such, arcs, sparks, and sufficient heat are only harmful under fault conditions. An incidental communication reduces the danger of ignition in the transition zone since electrical equipment in the transition zone may not produce arcs, sparks, or sufficient heat at the time ignitable materials enter the transition zone. If, on the other hand, the electrical equipment in the transition zone should produce arcs, sparks, or sufficient heat when the flammable materials enters the transition zone, the flammable material may not be ignitable. These favorable conditions reduce the risk of an explosion considerably. Although there is always the possibility that under an “occasional” entering of ignitable flammable material, the flammable material may come in contact with arcs, sparks, or sufficient heat from electrical equipment in the transition zone, the transition zone can be safely considered remotely dangerous because the encounter will occur only occasionally. The conditions under which an enclosed roofed space must be classified when it is located at the outside of the Div. 1 area, is different from the requirements for the transition zone. For detailed information on this subject, refer to Sec. D in Ch. 4, which highlights the requirements for classifying such a space. An enclosed roofed space can be made free from explosion danger, if flammable gases or vapors are prevented from entering the enclosed space by an adequate ventilation system. For the ventilating system to assure an environment free from explosion danger, it is necessary that the space be provided with a roof, four walls, and a pressure fan. As long as the pressure fan is in operation, the environment in the space remains nonhazardous because flammable gases or vapors are prevented from entering the enclosed space. However, to maintain an environment free from explosion danger, the sole application of a pressure fan is not enough. The pressure fan must be supplemented with a suitable safeguard. (For safeguards refer to Sec. H in Ch. 6.) A suitable safeguard consists of a system which provides an uninterrupted flow of air, or it gives a warning in case of a power outage or ventilation failure. However, a safeguard, which maintains an uninterruptable flow of air, is expensive and should only be applied if the enclosed space, requiring a nonhazardous classification, gives access to a Div. 1 area. An alarm should only be applied if the enclosed space, requiring a nonhazardous classification, gives access to a Div. 2 area. An enclosed space provided with an alarm system can generally not be classified nonhazardous if it gives access to a Div. 1 area. A space with an alarm can only be classified nonhazardous if it gives access to a Div. 2 area, or as indicated in Secs. B and D of Ch. 4. Normally locations classified nonhazardous do not contain sources of hazard. Exceptions to this general rule, however, are locations containing sources of hazard which are ventilated by canopy fume hoods, or locations containing sources of hazard which are partially classified nonhazardous.

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

D.4. Summary of Conditions for which NEC Class I Locations must be Classified There are fifteen different valid cases explained here for which a location is required to be classified. These cases are summarized and illustrated in Table 1-3. Each case in Table 1-3 has a different condition for which classification is required. An open or closed vat containing flammable liquid represents a source of hazard in Table 1-3. Sources of ignition are represented by open flames or flames in an enclosure. When the flame is shown in confinement, the ignition source must be considered to be an electrical device or equipment. When the flame is shown open, the flame represents a non-electrical ignition source with a continuous temperature in excess of the ignition temperature of the flammable product involved. There are two types of non-electrical ignition sources. Both types are “permanent” ignition sources. If the non-electrical ignition source is in the immediate vicinity of the source of hazard, as shown in Item 4 of Table 1-3, a flammable gas or vapor escaping from its confinement will be “early ignited,” thereby preventing the flammable gas or vapor from entering its explosion range. If the permanent ignition source is located at a remote point as shown in Item 5, the escaping flammable gases or vapors will enter their explosion range and will be ignited at the remote point. This condition produces a much greater hazard than with early ignition. Except as shown in Items 1, 6, 9, and 10, the release of a flammable gas or vapor to the atmosphere as a result of a breakdown of process equipment is considered occasional. In items 1, 6, and 9 the release is continuous, and in item 10 the release is frequent. An occasional release of flammable gases or vapors is considered remotely dangerous. A location is considered dangerous when the flammable gases or vapors are released frequently or continuously, even when sufficient ventilation is present. A location is also considered dangerous when flammable gases or vapors in the air will accumulate. If accumulation will occur, the location must be classified Div. 1, except when mini sources of hazard operating at low or moderate pressure are involved, as shown in Items 11 and 12. Accumulation will not occur if a location is sufficiently ventilated, except when the source of hazard is open, accumulation may occur. Sufficient ventilation will also dilute a flammable gas or vapor to a safe concentration. But before reaching a safe concentration, the gas or vapor will first go through its explosion range. All flammable gases or vapors in the location in Table 1-3 will enter into their explosion range, with the exception of Item 4. This flammable gas or vapor at the source of hazard cannot be diluted to a safe concentration. Safe concentrations will be reached some distance away from the source of hazard. This distance is dependent upon the volume and flow rate of the moving air. The greater the volume and flow rate, the smaller the distance.

Table 1-3. Summary of Conditions for which NEC Class I Locations Must Be Classified (1–15)

Chapter 2: Classifying Sources of Hazard 37

38

Table 1-3. (Cont'd.)

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Table 1-3. (Cont'd.)

Chapter 2: Classifying Sources of Hazard

39

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Outdoor areas are considered sufficiently ventilated. Also, indoor locations are sufficiently ventilated when provided with mechanical ventilation. Table 1-3 shows this when the indoor location is provided with a mechanical fan. The conditions shown in Table 1-3 are for flammable substances, which produce heavier-than-air gases or vapors. Table 1-3 may also be used for lighter-than-air gases or vapors. For lighter-than-air products, no accumulation of flammable gases or vapors is expected at ground level. In column (7), reference is made to conditions with flammable products, which are heavier- or lighter-than-air. Column (1) in Table 1-3 gives a clear picture of the conditions under which a particular source of hazard will operate. Column (2) applies to the operating mode of the source of hazard (open or closed). Column (3) pertains to the size of the source of hazard, which is large, small, or mini. Column (4) indicates whether accumulation of a flammable gas or vapor in the air will occur. Column (5) pertains to the classification of the zone or area under consideration. Column (6) indicates the reason for the classification. When a location is classified Div. 1, it means that the Div. 1 area borders directly on the source of hazard. This Div. 1 area can be small or large. If the location is classified Div. 2, it means that this area also borders directly on the source of hazard. This Div. 2 area can also be small or large. Only the closest zone or area, which borders at the source of hazard is listed in column (5). For example, suppose that one of the locations illustrated in column (1) must be classified Div. 1 with a Div. 2 transition zone; then only the Div. 1 classification is listed in column (5) and not the Div. 2 transition zone. Column (7) pertains to the extent of the Div. 1 or Div. 2 danger zone. The extent of the danger zone can be found in Table 1-4. The applicable item numbers in the subtitles of Table 1-4 are shown in column (7). For the capital letters with numbers shown in column (7) of Table 1-3, refer to the illustrations in Part II, “Environmental Conditions in NEC Class I Hazardous Locations.” The following is a brief explanation of the reasons for classifying the locations as shown in Table 1-3. •

Item 1. No accumulation of flammable gases or vapors is expected in a freely ventilated outdoor location. The primary reason for classifying the location as Div. 1 is the continuous release of flammable vapors.

•

Item 2. No accumulation of flammable gases or vapors will occur in this location because of the presence of sufficient ventilation. The primary reason for classifying the location as Div. 2 is the presence of sufficient ventilation.

•

Item 3. This item assumes for this location that if process equipment will rupture and damage nearby electrical equipment, it will produce arcs and sparks. The primary

Chapter 2: Classifying Sources of Hazard reason for classifying this location as Div. 1 is the simultaneous failure of process and electrical equipment. •

Item 4. In this area, there is a non-electrical ignition source continuously present. Since the permanent ignition source is part of the process equipment, early ignition will take place as soon as the flammable gases of vapors escape from their confinement. Because of the early ignition, the flammable gas or vapor will not enter its explosion range. The main reason for classifying the location as Div. 1, is “early ignition.” Only a small Div. 1 zone around the source of hazard is required.

•

Item 5. This location has a permanent ignition source. However, this permanent ignition source is not part of the process equipment. Because of this, flammable gases or vapors in the air are capable of entering their explosion range. No accumulation will take place, because of the presence of sufficient ventilation. The primary reason for classifying this location as Div. 1 is the “permanent” ignition source. A large area must be classified Div. 1.

•

Item 6. The open source of hazard in the enclosed building in this location produces flammable vapors to the atmosphere continuously. Accumulation will occur due to lack of ventilation. The entire indoor location must, therefore, be classified Div. 1.

•

Item 7. Accumulation of flammable gases or vapors will occur in this location because of lack of ventilation. The primary reason for classifying this location as Div. 1 is the lack of ventilation.

•

Item 8. Accumulation of flammable gases or vapors will not occur in this location due to the presence of sufficient ventilation. The primary reason for classifying this location as Div. 2 is the presence of sufficient ventilation.

•

Item 9. Even when the indoor location is sufficiently ventilated, if the source of hazard is open as indicated in this location, the location must be classified Div. 1. For mini sources of hazard, the Div. 1 zone needs only to be small.

•

Item 10. The sources of hazard in this location are regularly operated and/or worked on. The main reason for classifying this location as Div. 1 is because of excessive equipment wear.

41

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres •

Item 11. The indoor location without ventilation does not have to be classified Div. 1 if the source of hazard is “mini” operating at low pressure and the probability factor is not over 5.

•

Item 12. The indoor location can be classified nonhazardous if provided with sufficient ventilation and contains a mini source of hazard which is well maintained and operates at low pressure and the probability factor is not over 5.

•

Item 13. The building in this location receives clean, purged air from a nonhazardous location. The ventilating system in the building is not equipped with a safeguard. The primary reason for classifying this location Div. 2 is lack of a safeguard. Additional reasons for classifying the indoor location as Div. 2 is the occasional presence of a flammable gas or vapor into the air.

•

Item 14. The conditions for the building in this item are the same as for Item 13 except the ventilation system is provided with a suitable safeguard. The primary reason for classifying this location nonhazardous is sufficient ventilation and a suitable safeguard for the ventilation system.

•

Item 15. Because the indoor location does not have access to a hazardous location as shown in this location, the indoor location may be classified nonhazardous without ventilation.

D.5. Classification by European Standard IEC a. General There is a new standard incorporated in the 1999 NE code, the European Standard IEC (International Electro-Technical Commission) for classifying hazardous areas. This IEC Standard offers an expanded method for classifying hazardous areas and is described in the NEC under article 505-9. If an area classification is required, either article 500-7(a) and (b) in the NEC should be applied or article 505-9. It is not the intent that both be applied for the same installation. Where article 500-7 in the NEC requires the application of two classifications, Div. 1 or Div. 2, article 505-9 in the NEC requires that three classifications be applied; Zone-2, Zone-1, and Zone-0.

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43

b. Definition of IEC Zones Zone-2 Locations. The concept of a Zone-2 location is that the flammable liquid, gases, or vapors are confined within closed systems from which they can escape only as a result of failures or breakdowns of the confinement. This feature reduces the danger in the location considerably. The danger is reduced even more when the location is sufficiently ventilated. The purpose of ventilation is to dilute and remove the unexpected release of flammable gases or vapors. If an unexpected release of flammable material in a Zone-2 location takes place, its existence in the air is generally considered short because of the presence of ventilation and because failures and breakdowns of process equipment normally may be rapidly detected. If failures and breakdowns of process equipment in a Zone-2 location do occur, they generally occur as a result of improper and insufficient maintenance. Locations can only become dangerous if the flammable material will exist in the air for long periods of time, because the longer the flammable materials exist in the air the more vulnerable they become to ignition. However, the likelihood that they will exist in the air for long periods of time must be considered remote when the location is sufficiently and continuously ventilated. On the other hand, ventilating equipment may fail, breakdown, or suddenly stop due to loss of power. If this should happen, the location is subjected to danger because airborne flammable materials will not be diluted and removed by ventilating air. During such a condition, process equipment might fail or breakdown and electrical equipment could produce arcs, sparks, or sufficient heat at the same time causing a great potential explosion hazard. However, this type of failure in which three simultaneous failures will take place is rare and must be considered remote. Besides, when arcs, sparks, or sufficient heat are produced, the process equipment does not necessarily have to fail or breakdown at the same time; and if process equipment does fail or breakdown, electrical equipment may not necessarily have to produce arcs, sparks, or sufficient heat either. Comparing the above remote dangerous conditions with other types of danger in which flammable materials are being released frequently, continuously, or for long periods of time, Zone-2 locations are therefore, much safer because they are less dangerous than Zone-1 or Zone-0 locations. Zone-2 locations are also used as transition Zones for Zone-1 locations in which the Zone-2 location is considered a safe haven for flammable material being released in Zone-1 locations. However, sufficient ventilation and the application of suitable safeguards can prevent the communication of flammable material between both locations. Overall, the requirements for a Zone-2 location are the same as for a Div. 2 location. Zone-1 Locations. Zone-1 locations are defined as locations in which ignitable concentrations of flammable gases or vapors are “likely to exist” under normal operating conditions. Comparing this requirement with Div. 1 locations, Div. 1 locations are defined as locations in which the ignitable concentration “can exist” under normal operating conditions. Although the requirements likely to

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

exist and can exist do not positively suggest that flammable gases or vapors will exist in the air under normal operating conditions, it does imply that there is a great potential danger in the location. The requirements for Zone-1 locations are in principle the same as for a Div. 1 location, except for gas generator rooms and portions of gas manufacturing plants. In the fine print for Zone-1 locations, it is clearly indicated that even sufficiently ventilated rooms for gas generators and other portions of gas plants, need to be classified Zone-1. This requirement differs from the requirements in the fine print for Div. 1 locations in which “ventilation” is left out. However, the following passage in both fine prints for gas generator rooms and gas manufacturing plants are the same and read: “where flammable gas may escape.” This is an important statement because the word “escape” implies that the gas generator has a closed system. Therefore, when the gas generator has a closed system and is located in an “adequately” ventilated room, the room needs to be classified Zone-2 and not Zone-1. The statement in the fine print for Zone-1 locations [NEC 505-9(b)] is, therefore, a direct contradiction of the requirements for a Div. 1 classification as highlighted in NEC 500-7(a). Only when no ventilation is present, the location must be classified Zone-1. Bear in mind that the concept of a Zone-2 and Div. 2 location is based on a “closed system” and “sufficient ventilation.” Zone-0 Locations. Zone-0 locations are locations in which ignitable concentrations of flammable gases or vapors are present in the air continuously, or in which ignitable concentrations of flammable gases or vapors are present for long periods of time. These locations are normally associated with the following operations: • Inside vented tanks and vessels (see Fig. 1-4B) • Floating roof tanks • Insufficiently vented spraying enclosures • Pits containing heavier than air flammable material • Interior of an exhaust duct • Inside enclosures not sufficiently ventilated • Other similar enclosures The emphasis for this classification is on “continuous” and “for long periods of time.” In this case, there is no doubt as to whether the location should be considered dangerous or not.

c. Types of Electrical Equipment Only certain types of electrical equipment can be used in hazardous zone locations. Such an equipment must be specifically listed and marked for the location. For Zone-0 locations, equipment shall be listed and marked as suitable

Chapter 2: Classifying Sources of Hazard

45

for the location. Except for intrinsically safe systems, it is not recommended to install electrical equipment and associated wiring in 0-Zones unless the equipment and wiring are essential to the process. For Zone-1 locations, equipment listed and marked as suitable for the location shall be permitted and equipment approved for Zone-0 of the same gas group and with suitable temperature rating. For Zone-2 locations, electrical equipment may be used if listed and marked as suitable for the location. Class I, Div. 1, or Class I, Zone-0, and Zone-1, if of the same gas group and with suitable temperature rating may also be used.

d. Grouping According to IEC The grouping of electrical equipment is as follows: Group II is subdivided into: Group IIA equivalent to NEC Class I, group D. Group IIB equivalent to NEC Class I, group C. Group IIC equivalent to NEC Class I, group A and B. Group I is generally used for gases normally found underground.

Chapter 3 The Extent of Explosion Danger for NEC Class I Locations

A.

GENERAL

The purpose of classifying a hazardous location is to provide an acceptable level of safety against explosion danger for personnel and equipment. This is accomplished not only by establishing the degree of explosion danger as shown in Table 1-3, but also by establishing the extent of the explosion danger, and by furnishing suitable electrical equipment in compliance with the established degree and extent of explosion danger. The determination of the degree of the explosion danger is a rather simple procedure. By applying the guidelines in Ch. 2 and in Table 1-3 in that chapter, the degree of danger for the hazardous location can be established without any difficulty. The determination of the extent of explosion danger, however, is far more complex and requires an in-depth understanding of the nature of the explosion danger. For example, in Item 8 of Table 1-3, a closed source of hazard is located in a sufficiently ventilated building. According to Item 8, the classification of the location must be Div. 2. The question now is, how far does the Div. 2 area extend in horizontal and vertical direction? Should it be 10, 15, 25, 50, or 100 feet, or should the entire indoor location be classified, or only partially classified Div. 2? These questions are addressed in Sec. D of this chapter.

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Chapter 3: Explosion Danger for NEC Class I Locations

B.

47

THE DIMENSIONAL OUTLINE OF A DIVISION 1 AND DIVISION 2 ZONE

As a general rule, the dimensional outline of a Div. 1 and a Div. 2 zone is measured from the outline of the source of hazard in horizontal and vertical directions as shown in Fig. 1-4A. The distance between the outline of the source of hazard and the boundary line is called the extent of danger or simply, the boundary, or boundary distance. The vertical and horizontal distances are expressed in, respectively, “V” and “Ho.” As shown in Fig. 1-4A, the distance “V” starts from the top of the source of hazard in a vertical direction. “Hi” is also a vertical distance measured from the floor or grade. Thus, a boundary with a 5 V, 25 Ho and 3 Hi means a boundary of 5 ft vertical, 25 ft horizontal, and 3 ft high. All “V” and “Ho” dimensions are measured in feet. When “Hi” reads 18 Hi, it means a vertical distance of 18 inches high. This is the only figure, which is related to inches. All other figures are in feet.

Figure 1-4A. Boundaries for NEC Class I locations.

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

There is a great controversy as to whether the vertical height should be 18 in, 2 ft, or 3 ft high. Application Engineers may size the vertical height 2 ft where others may consider an 18 inch height more appropriate, or where 3 ft high is recommended; others may settle for 2 ft height. Because the height is such a great controversy, it is normally left to the discretion of the Application Engineer. He is the one who is directly involved in the type of danger the area will produce. However, to simplify the selection of the boundary height, the following is recommended:

Source

18 inches High

2 ft High

3 ft High

Outdoors Dynamic Static

0–51 hp Any size

60–201 hp and above —

— —

Indoors Dynamic Static

0–51 hp Any size

60–201 hp —

201 hp and above —

The classification of a Class I hazardous location can either be Div. 1 or Div. 2. If a hazardous area is shown double crosshatched, as shown in Fig. 1-4A, it means that the area is considered “dangerous” and therefore, it is classified Div. 1. If the area is shown single crosshatched, it means the area is considered “remotely dangerous” and as such it is classified Div.2. In Fig. 1-4B, the Class I hazardous location is classified in zones in accordance with the European standard IEC (International Electro-Technical Commission). The IEC standard has three hazardous locations that can be related to “extremely dangerous,” “dangerous,” or “remotely dangerous.” A location that is considered “extremely dangerous” is classified as a 0-Zone. In Fig. 1-4B this is shown as the dotted area. As shown in this same figure, the dotted area is inside the ventilated tank location. A location that is considered “dangerous” is classified as a Zone-1. This is shown in Fig. 1-4B as double crosshatched. A location which is considered “remotely dangerous” is a Zone-2 location and shown single crosshatched. The zoning method is described in Ch. 2, Sec. D5.

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Figure 1-4B. Boundaries for IEC Class I locations.

C.

QUANTITY OF FLAMMABLE SUBSTANCES VERSUS EXTENT OF EXPLOSION DANGER

The extent of explosion danger is primarily a function of the quantity of the flammable gas or vapor released to the atmosphere. For open systems, these quantities are influenced by evaporation rates and for closed systems containing flammable liquid, the quantities of vapors released are influenced by discharge and evaporation rates, system pressure, and the size of the rupture opening through which a flammable liquid will escape. Gases from gas leaks are influenced by discharge rates as a function of rupture openings. This in turn is a function of the type and size of the source of hazard, the vapor density, the flammability class of flammable products, and the temperature and pressure in the system. These conditions must be considered before the extent of the explosion danger can be established.

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However, it is not practical to make predictions of the size of the rupture opening through which a flammable gas, vapor, or liquid will escape if the source of hazard breaks down. It requires a great deal of study and analysis, making the evaluation too involved and too cumbersome. It is also not practical to determine the rate of release of the flammable gas, vapor, or liquid, or the diffusion rate of the flammable gas or vapor in the air. It is far more practical to predict a quantity of flammable gas or vapor that a source of hazard of a given size is capable of releasing to the atmosphere when the source of hazard breaks down. It is also practical to establish whether a static-type or dynamic-type source of hazard is involved. The quantities of flammable gases or vapors released to the atmosphere are proportional to the traveling distance of the flammable gas or vapor. The greater the quantity, the longer the traveling distance. The smaller the quantity, the shorter the traveling distance. Before the flammable gas or vapor has reached its maximum traveling distance, there is a point in space at which the flammable gas or vapor will reach a nonhazardous concentration. It is the distance between the source of hazard and this point that determines the extent of the explosion danger. The size of the hazardous area must not be less than this distance. A more in depth discussion on traveling distances can be found under Sec. G, “Transition Zones for NEC Class I Locations.” The quantity of flammable gases or vapors released to the atmosphere and consequently, the traveling distance of the flammable gas or vapor therefore, is a valid basis for determining the extent of the danger area. Large sources of hazard are normally capable of releasing large quantities of flammable gases or vapors, whereas, small and mini sources of hazard are more likely to produce small quantities. To simplify the method for establishing the proper size of a hazardous area, the quantity of the flammable gas, vapor, or liquid released to the atmosphere is taken as the basis for establishing the size of the hazardous area. Consequently, the size of a hazardous area is established by the size of the source of hazard, and since the source of hazard is either large, small, or mini, the quantity of flammable substances released to the atmosphere can only be large or small. Unfortunately, the size of the source of hazard alone cannot be relied upon completely in determining the size of the hazardous area. This is due to the fact that the quantity of the flammable gas or vapor released to the atmosphere is greatly influenced by temperature and pressure in the system, and type of source of hazard. The higher the temperature or pressure in the system, the greater the quantity released. Therefore, small sources of hazard may also produce large quantities of flammable gases or vapors when their temperature or pressure is high, and large sources of hazard may produce small quantities of flammable gases or vapors when their temperature or pressure is low. Consequently, any source of hazard, regardless of its size, that is capable of releasing large quantities of flammable gases or vapors, requires large hazardous areas, whereas, sources of hazard that can release only small quantities of flammable gases or vapors require small hazardous areas.

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The different boundary requirements with respect to the size of the source of hazard clearly indicate that the size of the source alone is not a valid guideline for determining the amount of flammable gases or vapors that the source is capable of releasing. In order to determine whether a source of hazard is capable of releasing small or large quantities of flammable gases or vapors into the atmosphere, it is necessary to take into account several additional factors, which are explained in Sec. D of this chapter. These will affect the relationship between source of hazard size and small or large hazardous areas. It is important to understand, however, that the wording “small” and “large” quantities of flammable gases or vapors released as used above must not be confused with the wording, “in quantities sufficient” as described in Article 500-7 of the NEC which reads: “Class I locations are those in which flammable gases or vapors are or may be present in the air ‘in quantities sufficient’ to produce explosive or ignitable mixtures.” To eliminate confusion between the two wordings, it is important to understand the intent of the wording in the NEC. Misinterpretation of the intent of the wording in the NEC could lead to the conclusion that a flammable gas or vapor in the air is not dangerous if its quantity in the air is small. That is not so. The wording, “in quantities sufficient,” in the NEC is related to the explosion range of the flammable product and the wording “small or large quantities released,” as explained here, to the bulk of the flammable product. A large bulk of flammable gas or vapor in the air, therefore, could be of insufficient quantity if it has not entered its explosion range. On the other hand, a small bulk of flammable gas or vapor in the air will have a sufficient quantity when it has entered its explosion range. Hydrogen gas, for example, has an explosion range between 4 and 75%. When the hydrogen gas in the air has a concentration of less than 4%, the NEC considers it as being of insufficient quantity to produce an explosive mixture. When the flammable gas or vapor is within its explosion range, the NEC considers it to have a concentration of sufficient quantity to produce an explosive mixture. Therefore, for a small or large quantity of flammable gas or vapor to become explosive, it is necessary that its concentration in the air be present in sufficient quantities, which is only possible when it has entered its explosion range.

D.

FACTORS INFLUENCING QUANTITIES OF FLAMMABLE GASES AND VAPORS

Factors that influence the quantity of a flammable gas or vapor released into the air are: • The type and size of the source of hazard • The temperature and the pressure in the system • The flammability class of the flammable product

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The larger the quantity released into the air, the greater the danger in the location. The vapor density of the flammable product mainly dictates the extent of the danger. The vapor density and any one or more of the factors listed above will influence the traveling distance of the flammable gas or vapor in the air, and subsequently the degree and extent of the hazardous area. Sufficient ventilation is another factor which has a great impact on the traveling distance of the flammable gas or vapor and subsequently also on the extent of the hazardous area. The traveling distance is inversely proportional to air velocity. Information on traveling distances of flammable gases and vapors in the air with respect to ventilation is outlined under “Ventilation Requirements” in Ch. 6. An extremely important factor is pressure in the system. Pressure is directly related to failure or breakdown of process equipment. Pressure in the system is defined as “low,” “moderate,” and “high.” The failure of process equipment is also a function of wear, but mainly of pressure in the system. Then they will also have an impact on the extent and degree of the hazardous area. Higher pressure in a system requires a more conservative approach than lower system pressures. For example, mini sources of hazard, such as piping systems including screwed fittings, bolted flanges, valves, and meters located in a sufficiently ventilated indoor area, must be classified Div. 2 if operating at moderate pressure. The extent of the Div. 2 area shall only be a circular zone of 3 ft radius around each individual component of the piping system. The reasons for this classification are the presence of sufficient ventilation and because of the moderate pressure in the system. If the indoor location containing the same piping system is not sufficiently ventilated, then the location must be classified Div. 1 with a 3 ft radius and Div. 2 with a 5 ft radius, 10 ft horizontal and 18 inches high. The same indoor location without sufficient ventilation must be classified Div. 1 with a 5 ft radius and Div. 2 with a 2 ft radius, beyond the Div. 1 radius, 15 ft horizontal and 18 inches high, if the piping system is operating at high pressure. At high pressure the risk of breakdown is much greater, and the fact that there is no sufficient ventilation makes the location more dangerous. In outdoor locations, the individual components of the same piping system must be provided with a Div. 2 circular zone of 3 ft radius, 10 ft horizontal. The reason for this classification is that outdoor locations are allowed to have smaller hazardous boundaries. On the other hand, an all welded piping system without screwed fittings, bolted flanges, valves, and meters operating at any pressure is allowed to be classified nonhazardous. This is because the breakdown of an all welded piping system is considered remote. A well-maintained mini-type piping system with screwed fittings, bolted flanges, valves, and meters operating at low pressure located in sufficiently ventilated indoor areas is also allowed to be classified nonhazardous. The supporting arguments for this classification are: 1) low pressure, 2) sufficient ventilation, 3) only small quantities of flammable material will be released to the atmosphere in case a component should breakdown, and most

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important, 4) the piping system is well maintained. For detailed classification requirements, refer to Part II, Figs. K-1 and K-2. Sources of hazard other than the mini-type (such as sources of hazard that are small and large) are required to have a larger area size. For example, a large pump handling a Class I flammable liquid outdoors operating at high pressure will produce much larger quantities of flammable material to the air in case of breakdown or failure than a mini-type source of hazard. The supporting reasons for this case are: 1) type, because the pump is rotating equipment; 2) size, because it is large; 3) pressure, because it is high; 4) flammable product, because it is Class I. All of these factors support the conclusion that large quantities of vapor, much larger than by mini sources of hazard, will be formed under accidental failure. Thereby, when the flammable product is heavier-than-air, the traveling distance will be long and subsequently a large hazardous area, say of 50 ft horizontal, is required. A large quantity of flammable gases or vapors is associated with boundary sizes of 50 and 100 ft. (See Figs. 1-2 and 1-3.) If the same pump should handle a Class II heavier-than-air flammable product at the same high pressure, then smaller quantities of flammable vapors are expected in the air requiring a smaller hazardous area. This means that a horizontal hazardous area of only approximately 25 ft maximum is needed, even though it is a large source of hazard. Justification for this is that Class II flammable products will cover smaller areas than Class I flammable products. In this example then, the flammability class of the flammable product becomes more important than the size of the pump, the high pressure, and type of the equipment. The question may arise as to what impact the type of source of hazard may have on the classification of the hazardous area. The answer is that the particular kind of source of hazard will indicate how much mechanical wear may be expected, which will then influence the ultimate classification of the hazardous area. If mechanical wear is non-existent or low, then only the size of the source of hazard, the product, and its pressure are relevant factors in classifying the area. But if there is continuous mechanical wear, the type of source of hazard becomes an important matter. Generally, a more conservative approach is required for a high rate of mechanical wear than for a low rate of wear. Therefore, the mechanical wear must also be taken into consideration in establishing the classification and the size of the hazardous area. When a source of hazard is frequently operated or worked on, it is subjected to greater wear, will breakdown sooner, and will also more often release ignitable concentrations of gases or vapors to the atmosphere. This puts the closed mini source of hazard in the same category as an open source of hazard which is normally releasing flammable vapors to the atmosphere continuously. For example, if a mini source of hazard, such as a valve, is operated often or is frequently worked on, the source is considered to have a high rate of wear. In this case, a circular Div. 1 zone with a 3 or 5 ft radius for the mini source of hazard would be required instead of a Div. 2 circular zone.

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For dynamic-type sources of hazard, the required size of the hazardous area is generally larger than for the static-type sources of hazard even when the statictype sources of hazard are larger. The reason for this is that the dynamic-type source of hazard is normally subjected to a higher rate of wear and the static-type is not. However, if the static-type source of hazard has a greater wear, it must be provided with a Div. 1 classification because the equipment may breakdown more often. On the other hand, a dynamic source of hazard having the same wear does not have to have a Div. 1 classification because it may not break down as often. Process equipment, therefore, that might frequently breakdown requires a Div. 1 classification and process equipment that only occasionally breaks down requires a Div. 2 classification. If frequent breakdowns can be expected, the process equipment becomes comparable with an open source of hazard that is normally classified Div. 1. The process equipment of the dynamic-type source of hazard which breaks down only occasionally is not comparable with an open source of hazard and therefore can be classified Div. 2. In view of these considerations, the determination of whether quantities of flammable gases or vapors released into the atmosphere, and whether their associated traveling distances are large or small, becomes more meaningful. However, whether the quantity of the flammable gas or vapor and the traveling distance is large or small, it is still necessary to determine the required dimensions of the size of the hazardous area in order to produce a safe and economical electrical installation. These dimensions must be large enough to cover the entire distances which the flammable gas or vapor must travel to reach a nonhazardous concentration. Since the dimensions of a hazardous area must cover each individual traveling distance, a great number of hazardous area dimensions is needed. It is, however, impractical to provide a great number of hazardous area dimensions for each individual traveling distance. It is more practical to apply a limited number of dimensions that cover a given group of traveling distances of different lengths. Each one of these dimensions must also cover the largest possible distance a Class I flammable product will travel. Therefore, for some flammable products, the size of the hazardous area for a given group of traveling distances is more than ample, and for others they may be just right. The groups of dimensions are broken down in horizontal distances of 100, 50, 25, 15, 10, 5, 3, and less than 3 ft. They are classified Div. 1 or Div. 2 depending on the situation in the location as explained above. Large sources of hazard normally require 100 and 50 ft boundaries. Small sources of hazard normally will require 25, 15, 10, 5 and 3 ft boundaries. Mini sources of hazard are generally in the range between 15 and 3 ft or less. The various groups of dimensions are listed in Tables 1-4 and 1-5 shown in Part 2. All dimensions listed and shown are prepared for Class I flammable products. For area dimensions required for Class II flammable products refer to Sec. F. To maintain conservatism for Class II flammable products, it is best to apply the dimensions for Class I flammable products.

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The question as to how the Div. 2 classification should extend in Item 8 of Table 1-3 can now be answered as follows. The main ingredients shown in Item 8 of Table 1-3 are: 1) a closed operating mode, 2) an indoor location and 3) sufficient ventilation. With these three ingredients known, the size of the hazardous area can be obtained from Items 1–11 in Table 1-4. A number of items in Table 1-4 are for heavier-than-air flammable products, and others are for light-than-air flammable products. If it is assumed that the flammable substances in Item 8 of Table 1-3 are heavier-than-air, then only Item 1 of Table 1-4 will apply. The next important ingredient is to establish the size of the source of hazard and the pressure in the system. With the size of the source of hazard and the pressure in the system known, the size of the hazardous area is narrowed down more to one particular dimension. For example, if the source of hazard is of the dynamic type, if the size of the source is “small” (up to 51 hp), and the pressure in the system is “moderate,” and if flammable gases or vapors which might escape from the enclosed confinement are “heavier than air,” the required dimensions of the Div. 2 hazardous area can be obtained from the subtables listed in Col. (7) of Table 1-4. Only subtable A will apply and from this subtable, only Item 1 which reads “5 V, 25 Ho, 18 Hi” will apply. Because of the strong relationship between the quantity of a flammable gas or vapor released to the atmosphere and the temperature in the system, one method is to divide the temperature into a number of temperatures, each of which can be associated with a given quantity of flammable gases or vapors released into the air. This method, however, is not practical. Since the temperature is in direct proportion to the system pressure, it is much simpler to use the other method which equates temperature to system pressure. Temperature is important if related to dilution of a flammable gas or vapor released into the air. For example, how much dilution is required for a Class II and III flammable gas or vapor released into the air, if the temperature is “above” or “slightly above” flash point. This is explained in detail in Sec. L.4. System pressure is defined as low, moderate, and high. Low pressure in a system is below 100 psi. Moderate pressure is generally 100 psi up to and including 500 psi, and high pressure is above 500 psi. The three pressure categories, low, moderate, and high, are included in the subtables of Table 1-4 labeled “A” through “K.” Table 1-4 has five self-explanatory columns. Column (5) of this table refers to the subtables marked with a letter. For example, if the system operating mode of a source of hazard is “closed” and the source of hazard is handling a “heavier”than-air flammable product “outdoors,” refer to Item 3 of Table 1-4. Column (5) of this table reads the associated subtable for Item 3 of Table 1-4, which for this case is subtable C. In subtable C, there are fifteen different conditions. Only one of the fifteen conditions will apply. The solution of the proper condition is based on the pressure in the system, the size of the source of hazard, the size of the driver if any, and the type of location. Whether or not an additional danger zone must be applied can be found in column (8) of each subtable.

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Table 1-4. Summary of Specific Conditions Influencing the Degree and Extent of Hazard

Table 1-4A. Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations

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Table 1-4B. Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Table 1-4C. Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations

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Table 1-4C. (Cont’d.)

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Table 1-4D. Degree and Extent of Danger Area for Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations

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Table 1-4E. Degree and Extent of Danger Area for Open Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations

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Table 1-4F. Degree and Extent of Danger Area for Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations

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Table 1-4G. Degree and Extent of Danger Area for Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Table 1-4H. Degree and Extent of Danger Area for Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations

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Table 1-4I. Degree and Extent of Danger Area for Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations

Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Table 1-4J. Degree and Extent of Danger Area for Open or Closed Sources of Hazard with Heavier or Lighter Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations

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Table 1-4J. (Cont’d.)

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Table 1-4K. Degree and Extent of Danger Area for Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations

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Table 1-5. Sources of Hazard for Lighter and Heavier Than Air Flammable Substances (1–43)

Chapter 3: Explosion Danger for NEC Class I Locations Table 1-5. (Cont’d.)

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Table 1-5. (Cont’d.)

Chapter 3: Explosion Danger for NEC Class I Locations Table 1-5. (Cont’d.)

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Table 1-5. (Cont’d.)

Chapter 3: Explosion Danger for NEC Class I Locations Table 1-5. (Cont’d.)

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Boundaries in the subtables of Table 1-4 are based on the following major requirements: 1.

Type source of hazard (dynamic or static) (more wear in dynamic, less in static).

2.

Quantity of gases/vapors released (based on emission rates.).

3

Emission rates (based on rupture opening and pressure).

4.

Rupture opening (based on size of source of hazard and pressure).

5.

Extent of ignitable concentration (based on vapor density, mol. weight, and no crosswinds).

6.

Flat terrain (based on minimum obstruction).

Instead of using the boundary dimensions in the subtables, the dimensions in the illustrations in Part 2 (Application of Fundamentals) can be used, or the boundaries in Table 1-5 for less common sources of hazard. The illustrations in Part 2 will show various conditions that may reflect actual situations encountered in chemical and petro-chemical industries. The illustrated boundaries in Part 2 can be applied for gas compressors and for conditions where Class I flammable liquid is used with either lighter or heavier than air vapors. The illustrations may also be used for actual conditions that are similar to the illustrations, or for a Class II flammable liquid with the requirements explained in this chapter. For example, assume a large pump driven by a 250 hp electric motor located outdoors operating under low pressure and transferring flammable liquid with a vapor density of heavier than air. Determine the classification and extent of the hazardous area surrounding the pump. First, find the appropriate picture from Table 1-3. Four items must be considered; size of pump, location, operating mode, and whether accumulation can take place. Only Item 2 in this table complies with the above requirements. Column (5) in Table 1-3 indicates that the condition stated above requires that the classification of the hazardous area be Div. 2. Next establish the extent of the Div. 2 area from Table 1-4. Only Item 3 will be applicable because of the system operating mode, the location, ventilation, and the vapor density of the flammable liquid. For these features, subtable C is recommended. In subtable C, find pump stations (Col. 2) and the size of the pump driver (Cols. 3 and 4). Only Items 6 and 7 will apply. (Since the system pressure is low, refer to Fig. C-6 as indicated by Items 6 and 7 and, in Item 3 of Fig. C-6, read the extent of the hazardous area which is 5 V; 25 Ho, 2 Hi. This boundary complies also with Fig. 1-2, Item D.)

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E.

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EARLY AND REMOTE PERMANENT IGNITION SOURCES

As explained in Sec. D.3 of Ch. 1, early ignition is defined as a condition in which no explosion will occur if an ignitable concentration of flammable gases or vapors in the air comes in contact with a “local” ignition source, with a temperature above the ignition temperature of the flammable product. Also explained in Sec. D.3 is a condition in which an explosion will occur if the ignitable concentration comes in contact with a “remote ignition source” with a temperature above the ignition temperature of the flammable product. Even when the flammable material is confined in a closed system from which it can escape only if the system breaks down, the location cannot be classified Div. 2 if the above conditions exist. If “local” or “remote” ignition sources exist with a temperature above the ignition temperature of the flammable product, the location must be classified Div. 1. These types of ignition sources can be compared with conditions in which flammable gases or vapors are released to the atmosphere continuously or frequently, which, as a result of this condition, require that the location be classified Div. 1. Therefore, a location containing a “local” or “remote” ignition source is also required to be classified Div. 1. The “local” ignition source is required to have a circular zone of 3 ft radius. A Div. 2 transition zone surrounding the Div. 1 zone may be applied but is not necessarily required. When a “remote” ignition source is involved, the area between the source of hazard and the remote ignition source must be classified Div. 1. This is only true if the flammable gases or vapors in the area have not reached safe concentrations before contacting the remote ignition source. If the flammable gas or vapor has reached safe concentrations before contacting the remote permanent ignition source, then the area need not be classified Div. 1. To avoid a possible explosion as a result of a remote ignition source, it is necessary to maintain sufficient distance between the source of hazard and the remote ignition source. But first, what is sufficient distance? Sufficient distance is the proper boundary size for a hazardous location that is selected from Table 1-4, on the basis of: • The type and size of the source of hazard • The pressure in the system • The flammability class and the vapor density of the flammable product For example, if the recommended boundary size for a source of hazard is to be 25 ft long, and the actual distance between the source of hazard and remote ignition source is within the 25 ft boundary, an explosion can be expected if the flammable gas or vapor in the air comes in contact with the remote ignition source. On the other hand, if the actual distance should be longer than 25 ft, no explosion will be expected, in which case the 25 ft area can be considered remotely hazardous.

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The following example will illustrate the conditions mentioned above. Consider, for example, an area with four electric motor driven pumps and one pump driven by a high-pressure steam turbine as shown in Fig. 1-5. All pumps are handling volatile flammable liquid. The high-pressured steam turbine in Fig. 1-5 has a surface temperature in excess of the ignition temperature of the flammable product and is, therefore, considered a “remote permanent ignition source.” The equivalent hp rating of the steam turbine driving pump P1 is 50 hp. Pumps P2 and P3 are small pumps operating at high pressure and driven by 50 hp electric motors. Pumps P4 and P5 are large, operating at moderate pressure and driven by electric motors of 250 hp. Determine for each individual pump the recommended boundary distance. The boundary distance is influenced by the size of the pump and the pressure in the system. Since the pumps are located in an outdoor area, refer to Fig. 1-2 for the required boundary distance. Select in Fig. 1-2 the recommended boundary distances recommended for the particular pump drivers on the basis of size, hp rating of electric motor, and pressure in the system. According to Fig. 12B, the recommended horizontal boundary distance for Pumps P1, P2, and P3 is 15 ft, and for Pumps P4 and P5, 50 ft. Since the permanent ignition source “a” in Fig. 1-5 is within the 15 and 50 ft horizontal boundary distance of Pumps P1, P2, and P4, the area between the ignition source and Pumps P1, P2, and P4 must be classified Div. 1. The area in the opposite direction of Pump P4, also 50 ft long, is allowed to be classified Div. 2. Since the actual distance between P3 and the ignition source “a” is longer than 15 ft, the area between Pump P3 and the ignition source need not be classified Div. 1. However, since Pump P3 is within the 50 ft Div. 1 zone of Pump P4, the area between Pump P3 and the ignition source automatically becomes Div. 1. The required 50 ft horizontal boundary for Pump P5 does not reach the permanent ignition source “a.” Because of this, the area surrounding Pump P5 is not required to be classified Div.1, but must be classified Div. 2 in compliance with Item 3 of Table 1-4, subtable “C,” Item 7 and Item 2 in Fig. C-6. Since Div. 1 zones normally do not directly border to a nonhazardous area, a Div. 2, 5 ft wide transition zone, is required between the Div. 1 zone and the nonhazardous area. However, the breakdown of Pump P1 could cause early ignition of the flammable vapors released. If early ignition is possible, the Div. 1 area surrounding Pump P1 does not have to be 15 ft, but should be reduced to 3 ft without a Div. 2 transition zone.

F.

THE EXTENT OF EXPLOSION DANGER FOR CLASS II FLAMMABLE PRODUCTS

As pointed out previously, the required extent of explosion danger for locations storing, handling, and/or processing Class I flammable products can be obtained directly from Tables 1-4 and 1-5. These tables may also be used for Class II flammable products. However, bear in mind that the recommended hazardous areas in these tables are prepared exclusively for Class I flammable products

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Figure 1-5. Non-electrical permanent ignition source.

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because of their large vapor traveling distances, and not for Class II flammable products, which generally have shorter vapor traveling distances. Although the vapor traveling distance for Class II flammable products are generally shorter, it is very convenient to apply Tables 1-4 and 1-5 for Class II flammable products as well. Greater safety is achieved by applying the larger areas. However, the larger areas for Class II flammable products are not the most economical. For additional feet of area, electrical equipment needs to be adequate. If for economical reasons, or any other reason, smaller hazardous areas for Class II flammable products are preferred. The following steps are to be followed to establish the proper dimensions for these Class II flammable products. First, assume that the location under consideration does not contain a Class II flammable product, but a Class I flammable product. This is necessary to obtain the required area size for the Class I product which later must be reduced to suit the Class II flammable product. Second, the actual operating temperature of the Class II flammable product must be used for the Class I product. This is necessary to obtain vapor pressure of the Class I flammable product which must be used to determine the required area size for the Class II product. Third, find in Fig. 1-6 the vapor pressure for the Class I flammable product based on a 100% flash point, and the actual operating temperature of the Class II flammable product. Figure 1-6 shows flash points, operating temperatures, and vapor pressures for Class I, Class II and Class III flammable products. Class III flammable products, however, are not considered. Next, find in Fig. 1-6, the vapor pressure for the Class II flammable product at its original flash point. Finally, establish the inverse ratio of both vapor pressures. This ratio is a measure for the actual boundary size for the Class II flammable product. For example, assume a closed source of hazard containing a Class II flammable liquid has a flash point of 153°F. Assume that the operating temperature of the Class II flammable product is 200°F. Consider the same source of hazard processing a Class I flammable product also operating at a temperature of 200°F. Find from Table 1-4 or Table 1-5, the required horizontal boundary distance for the Class I flammable product. Say the required boundary distance is 25 ft. From Fig. 1-6, find the vapor pressure for the Class I flammable product at 100°F flash point by starting at the bottom of Fig. 1-6 at 100°F flash point. Go straight up until the vertical line is intersected by the 200°F line and read at the left border line the vapor pressure for the Class I flammable product, which is 0.13 atm. The vapor pressure for the Class II flammable product is found in the same manner, except start in Fig. 1-6 at the original flash point for the Class II flammable product, which is 153°F. Go straight up until the 200°F vertical line intersects and read at the left borderline the vapor pressure for the Class II flammable product which is 0.035 atm. The inverse ratio of the two vapor pressures is 0.27. Multiply the 25 ft boundary for the Class I flammable product by 0.27, which equals to 6.7 ft. Boundary sizes are standardized in 3, 5, 10, 15, 50, and 100 ft. Take the next higher size above 6.76 ft. The boundary size required for the Class II flammable product

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is then 10 feet. Since the source of hazard is closed and if it is assumed to be in a sufficiently ventilated location, the 10 ft size needs only to be classified Div. 2. If cost of the electrical installation for the Div. 2 area is a major consideration, 10 ft is a valid size. If cost has little impact on the hazardous area, then the 25 ft size should be applied.

G.

TRANSITION ZONES FOR NEC CLASS I LOCATIONS

As previously indicated, the size of a hazardous area is established on the basis of the horizontal distance a heavier-than-air flammable gas or vapor must travel to reach safe concentrations. Safe concentrations will be reached at any point on a boundary surrounding a hazardous area. Such a point is shown at “A” in Fig. 1-7. The radius of the boundary is the distance at which a flammable gas or vapor needs to travel to reach safe concentrations. The traveling distance is a function of the quantity of flammable gases or vapors released to the atmosphere. Only two quantities of flammable gases or vapors are important, “small” and “large.” These quantities are mainly a function of the type and size of the source of hazard, the flammable product, the vapor density, the pressure in the system, and whether ventilation is present or not. Large quantities of flammable gases or vapors released to the atmosphere have the tendency to travel long distances before reaching safe concentrations, causing the point of safe concentration to shift farther away from the point of release. Small quantities of flammable gases or vapors in the air have the tendency to travel short distances before reaching safe concentrations. This brings the point of safe concentration closer to the point of release. Therefore, traveling distances may have different sizes before reaching safe concentrations. They may be short or long, depending on the conditions under which a flammable gas or vapor is released to the atmosphere. To apply a hazardous boundary safely, it is necessary that each individual distance a flammable gas or vapor must travel to reach safe concentrations is covered by a matching boundary. Because of the many conditions under which a flammable gas or vapor can be released to the atmosphere, a great number of different matching boundary sizes are required to cover each individual traveling distance. However, too many hazardous boundary sizes will cause confusion and makes the application too elaborate. To eliminate such a confusion, the different traveling distances are divided into separate groups based on specific conditions whereas each group is assigned one specific hazardous boundary size. This reduces the number of boundaries and simplifies the application considerably. Of course the size of the boundary selected for a particular group of traveling distances must match the longest traveling distance in that group. Since each selected boundary will cover a number of actual traveling distances of different sizes, the result is that the boundary, in some cases, may be too wide, and in other cases, just right. To further simplify the application of the boundaries, each

82 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-6. Temperature versus vapor pressure for flammable liquids.

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Figure 1-7. Transition zones for NEC Class I locations.

83

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

boundary radius is assigned one particular size as follows: 3, 5, 10, 15, 20, 25, 50, or 100 ft. The sizes are established by tests, experiments, and analyses, and are listed in columns (6) and (7) of Table 1-4 subtables. Unfortunately, the various sizes as listed in Tables 1-4 subtables may not always cover the longest traveling distance in the group. There are unforeseen environmental conditions in which flammable gases or vapors in the air may reach safe concentrations beyond the recommended sizes (in the subtables) making sizes for the hazardous area in the tables too short. Sometimes the flammable gases or vapors in the air may reach a nonhazardous concentration at a much shorter distance than the shortest traveling distance in the group, making the boundary sizes in the tables much too long. The point at which a flammable gas or vapor will reach a nonhazardous concentration without being influenced by unforeseen conditions is marked in Fig. 1-7 with a letter “A.” If nonhazardous concentrations should be reached before point “A” as a result of unforeseen environmental conditions, the hazardous areas in the tables remain applicable and safe, although they are then oversized. Safety is the key factor in area classification, and, therefore, the sizes in the tables should not be reduced to a smaller dimension if the actual traveling distance is shorter than the sizes shown in Cols. (6) and (7) of the subtables. One of the unforeseen conditions that may cause the flammable gases or vapors to reach safe concentrations well before point “A” is the presence of crosswinds. Crosswinds will shorten the traveling distance considerably, causing the flammable gases or vapors in the air to reach safe concentrations much closer to the point of release. On the other hand, if safe concentrations are reached beyond point “A,” (for example, at point “B” as shown in Fig. 1-7) then the situation becomes critical. Under this condition, the boundary sizes in the subtables are too short and, therefore, must be considered unsafe. Such an unsafe condition may occur if flammable gases or vapors in the air are released continuously in large quantities from open sources of hazard and are slowly moving in one direction by light wind conditions over unobstructed flat terrain without crosswinds, causing the flammable gases or vapors to reach nonhazardous concentrations beyond point “A.” It seems reasonable to cover this unsafe condition by extending the boundary size up to point “B,” as shown in Fig. 1-7, so that the integrity of the danger area can be maintained. This solution however, is impractical and not economical. Expensive electrical equipment is not only required in the area up to point “A,” but also in the area between point “A” and point “B.” Since it is not certain whether a flammable gas or vapor in the air will reach safe concentrations at point “A” or beyond point “A,” it is not recommended to extend the danger area or boundary of the danger area to point “B.” It is far more practical to apply a transition zone beyond point “A” and extend this transition zone to slightly beyond point “B” up to point “C” (as shown in Fig. 1-7) in which the area between point “A” and point “C” is considered to be a safety margin. This transition zone will in fact compensate for possible unforeseen conditions that may force the flammable gases or vapors to reach safe concentrations beyond point “A.”

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With the transition zone, it becomes unimportant whether the flammable gases or vapors in the air will reach safe concentrations beyond point “A” or not. The reason is that if the flammable materials may or may not reach safe concentrations in the transition zone, the communication of flammable materials with the transition zone can be considered as an occasional event. As a result of this condition, the transition zone needs not to be classified Div. 1, but can be classified Div. 2. The possibility of igniting flammable materials in the Div. 2 zone can safely be considered remote. With the lower classification, electrical equipment in the Div. 2 zone does not have to be explosion-proof. Generally, any area surrounding a source of hazard that is required to be classified Div. 1 must be provided with a transition zone. No transition zone is necessary if the source of hazard is surrounded only by a Div. 2 area. This is because of the normally greater size of the Div. 2 area. Sizes of transition zones can be found in column (7) of Table 1-4 subtables.

H.

ADDITIONAL DANGER ZONES

H.1. General Requirements An additional danger zone is an outdoor extension of an inside classified hazardous area in a roofed space. The roofed space may have three, two, one, or no walls. No additional danger zone is required if the space has four walls or if the classified hazardous area inside the space does not extend to the open perimeter of the space. For example, an additional danger zone outdoors is required if the classified hazardous area inside the space ends at the open perimeter of the space or if the classified hazardous area inside the space extends beyond the open perimeter of the space. The purpose of an additional danger zone is to provide a designated safe area outdoors for flammable gases or vapors which inside this space are unable to reach safe concentrations, causing the outdoors to be contaminated. Whether safe concentrations is reached within the space or beyond the open perimeter of the space, normally depends on the quantity of flammable material being released in the space. Small quantities generally allow the flammable material to reach safe concentrations some distance from the perimeter of the space, in which case, no additional danger zone outdoors is required. Large quantities on the other hand, may reach safe concentrations at the perimeter of the space or beyond the open perimeter of the space. Under such a condition, an additional danger zone outdoors at the open perimeter of the space is required. Large quantities of flammable vapors are normally produced by large open sources of hazard or when there is a failure of a large closed source of hazard. Additional danger zones for heavier-than-air flammable products need to have a size of 10 ft wide extending vertically all the way up to the building opening. The 10 ft wide additional danger zone is generally classified Div. 2, although it could also be classified Div. 1. The choice between a Div. 1 or Div. 2

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classification depends on the conditions under which a flammable gas or vapor is released in the space. For example, if small quantities of flammable gases or vapors are released in the space occasionally or frequently, the additional danger zone needs to be classified Div. 2. In this case, natural ventilation is capable of successfully diluting and dispersing the flammable material outdoors and as such the outdoor area is considered a remotely dangerous area. On the other hand, if the flammable materials are released in large quantities and continuously, then the additional danger zone should be classified Div. 1. In this case, natural ventilation may not be successful and the outdoor area must be considered dangerous. An indoor location in which flammable vapors are continuously released in large quantities must be considered as the actual source of hazard as shown in Fig. 1-8 D. If the indoor location is considered as the actual source of hazard, the 10 ft wide danger zone is too small and a larger zone must be applied. This larger zone is obtained from the subtables of Table 1-4. However, since the larger zone in the subtables has a height of not more than 18 in or, 2 ft, the 10 ft wide danger zone must also be applied extending vertically all the way up to the building opening as shown in Figs. 1-8A, and 1-8C.

H.2. Additional Danger Zones for Heavier-Than-Air Products A horizontal danger zone of 10 ft wide extending vertically all the way up to the building opening is considered of sufficient size for heavier-than-air flammable products. However, these types of additional zones are only required under one of the following conditions: 1.

If the indoor location is entirely classified as shown in Figs. 1-8A and 1-8C.

2.

If the indoor location is partially classified and the classification is extended to the building access opening as shown in Fig. 1-8B. (If the classification does not extend to the building access opening no additional danger zone is required.)

3.

If the hazardous boundary, measured from the source of hazard, is exceeding the open building perimeter as shown in Figs. 1-8A and 1-8C.

For example, if according to the subtables of Table 1-4, the required boundary size for a heavier-than-air flammable product needs to be 25 ft wide, while the actual distance between the source of hazard and the building opening is only 12 ft wide, the additional danger zone beyond the building opening must then be 13 ft wide to make up for the required 25 ft boundary. In addition, a 10 ft wide boundary is also necessary as shown in Figs. 1-8A and 1-8C. (The boundary sizes of 25, 50, and 100 feet in Fig. 1-8A are only shown as an example.)

Chapter 3: Explosion Danger for NEC Class I Locations

Figure 1-8. Additional danger zones.

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-8. (Cont’d.)

Chapter 3: Explosion Danger for NEC Class I Locations

Figure 1-8. Additional danger zones and safe distances.

89

90

Figure 1-8. (Cont’d.)

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K

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If the boundary size for the heavier-than-air flammable product, as required by the subtables, should be shorter than the actual distance between the source of hazard and the building opening, then no additional danger zones are necessary. If partially classified indoor locations require additional danger zones, these zones shall not extend vertically all the way up to the top of the building access opening as shown in Fig. 1-8B.

H.3. Additional Danger Zones for Lighter-Than-Air Products The requirements for additional danger zones for lighter-than-air flammable products are somewhat different than for heavier-than-air flammable products. An additional danger zone is only required if the indoor location is entirely classified and not provided with an mechanical fan. The size of the additional danger zone shall either be 10 or 15 ft wide and depends on the pressure in the system. If the system pressure is low or moderate, the size of the additional danger zone shall be 10 ft wide and if the pressure is high it must be 15 ft wide as shown in Figs. 1-8E and 1-8F. Where forced ventilation is provided an additional danger zone is required at the roof exhaust opening that extends vertically to 25 ft above the roof of the building. The 10 or 15 ft additional danger zone at the open building perimeter may be deleted as shown in Fig. 1-8G. The reason for deleting the 10 or 15 ft additional danger zone is that forced ventilation may fail at a time that the source of hazard may not fail, in which case, contamination of the outdoors can be considered remote. A building with a non-vapor-proof roof without roof ventilation needs an additional danger zone of 15 ft wide around the perimeter of the roof, as shown in Fig. 1-8H.

H.4. Safe Distances for Lighter-Than-Air Products The horizontal safe distance between a Div. 2 additional danger zone and the air intake “a” in Fig. 1-8I for an enclosed nonhazardous area shall be 25 ft. The horizontal safe distance between the air intake “a” and the opening “b” of the exhaust pipe as shown in Fig. 1-8J, shall be 50 ft minimum. This horizontal safe distance may be 25 ft minimum if the exhaust pipe opening is extended 15 ft up to point “c.”

H.5. Safe Distances for Heavier-Than-Air Products An enclosed space must be classified if its access opening gives access to an area which is hazardous or if it gives access to an additional danger zone. For example, if an enclosed space has a door opening which gives access to a boardwalk, the space must be classified if the top of the boardwalk is within the hazardous boundary. For detailed information on spaces giving access to hazardous areas, refer to Sec. D, in Ch. 4. On the other hand, if the vertical distance

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between the boundary top and the top of the boardwalk is twice the height of the boundary, (as shown in Fig. 1-8K) then the enclosed space does not have to be classified. In addition, if the side walls of the enclosed space are vapor tight (without access opening and windows or with bolted windows), the enclosed space does not have to be classified if the side wall is less than 10 ft away from the open perimeter of the adjacent building. However, if the side walls are not vapor tight, or if they are provided with openings and non-bolted windows, the enclosed space does not need to be classified if the distance between the side walls and the outer perimeter of the vertical additional danger zone is twice the width of the vertical danger zone.

I.

DANGER ZONES ABOVE GROUND

Flammable gases or vapors released from a source of hazard located outdoors far above ground will usually be diluted more rapidly than when the source of hazard is located close to the ground. There are usually fewer obstructions against air flow higher up in the air than close to the ground and, because of this, the moving air higher up can dilute a flammable gas or vapor much faster than at ground level. The higher the source of hazard is located above ground, the more successful the dilution, and the faster the flammable gas or vapor will reach a safe concentration. The dilution of the flammable gas or vapor is greatly influenced by the velocity of the surrounding air and therefore, the speed and the point at which the flammable gas or vapor will reach a nonhazardous concentration is dependent on this velocity. The hazardous zones required for sources of hazard such as a vent need to be small when located high above ground. Larger zones are necessary when the vents are closer to the ground, or when the vents are connected to relief valves. A vent releasing lighter-than-air vapors high up in the air needs only a small spherical Div. 1 and a larger Div. 2 danger zone. However, when the flammable vapors are heavier-than-air, the Div. 2 danger zone is not spherical, but must extend all the way to the ground. The Div. 1/Div. 2 classification for the vent is necessary because the flammable vapors are released continuously. If flammable gases or vapors are not released continuously, such as with pressure relief valves, a single Div. 2 zone would be satisfactory. For danger zones for vents and relief valves refer to Table 1-5.

J.

CLASSIFICATION OF SOURCES OF HAZARD IN PUMP STATIONS OCCUPYING 50, 75, OR 100% FLOOR SPACE

Pump stations containing sources of hazard are required to be sufficiently ventilated. Ventilation for a pump station is not only necessary for diluting and removing a flammable gas or vapor in the air, but it is also required for economical

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reasons. Cost of an electrical installation, in the pump station for example, is much higher if the pump station is not sufficiently ventilated. Without sufficient ventilation, the pump station must be classified Div. 1; as a result of this classification, the entire electrical installation in the pump station must be made explosion proof. The electrical installation does not have to be explosion proof if the pump station is classified Div. 2 and this can only be accomplished if the pump station is sufficiently ventilated. Pump stations containing sources of hazard are classified in two different ways. The pump station is either classified “entirely” or it is classified only “partially.” A partially classified pump station is defined as a 50% classification, and an entirely classified pump station is defined as a 100% classification. If the pump station is required to be classified 50%, then the space above the 50% danger zone is considered nonhazardous. This of course is only true if the flammable product in the pump station has a vapor density heavier than air. If the vapor density is lighter than air, then the lower part of the enclosed space is considered nonhazardous. The worst condition, however, exists when the flammable products are capable of covering large areas. Since heavier-than-air flammable products normally cover larger areas than lighter-than-air flammable products, only flammable products with vapor densities heavier than air shall be considered for a 50% or 100% classification in this chapter. The classification of a pump station containing a single source of hazard is a fairly simple and straightforward procedure because the dimensional outline of the danger area can be obtained directly from the subtables of Table 1-4. When the pump station contains more than one source of hazard, then its classification is more complex because more than one source of hazard can cause an explosion and the danger becomes greater. Where a single source of hazard is capable of producing one breakdown, a group of sources of hazard may produce multiple breakdowns. The size of the source of hazard and the system pressure play an important role in whether a pump station should be classified 50% or 100%. If large sources of hazard are involved, the classification of the pump station is generally 100% except in cases where the system pressure is low. Since the majority of classifications for large sources of hazard are 100%, in this chapter large sources of hazard shall not be considered. If small sources of hazard are involved, the classification is not a simple matter because the pump stations may have to be classified 100% or 50%. Therefore, only small sources of hazard will be discussed. Pump stations normally contain dynamic type sources of hazard, such as rotating pumps. Pumps are sized in accordance with their electric drivers. Small drivers range in size from 0–201 hp. Thus, a pump which is driven by a 100 hp electric motor is considered to have an equivalent hp size of 100 hp. For classifying a pump station 50% or 100%, the range of 0–201 hp is, as explained in Ch. 2, Sec. A.4, too broad to be of practical value. Since the quantity of flammable gases or vapors, released to the atmosphere, is mainly a function of system pressure and pump size, a practical system can be obtained if the hp range is subdivided and associated with quantity and system pressure. For example, a

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

quantity of flammable gas or vapor released to the atmosphere can be small, medium, or large. (Medium is included to average small and large.) System pressure can be low, moderate or high. If the hp range is associated with these two groups, then a practical workable method is developed that will establish whether the pump station should be classified 50% or 100%. For small quantities of flammable gases or vapors at low or moderate system pressure, the practical hp range as determined in Ch. 2, Sec. A.4 is between 0 and 51 hp. For medium quantities at high system pressure and for low and moderate pressure, the practical hp range is between 0–51 hp and 60–201 hp, respectively. The method for establishing whether a pump station must be classified 50% or 100% is by evaluating five critical conditions under which a group of small pumps in the pump station must operate. These five conditions are listed in Table 1-6, “Critical Conditions and Severity Factors.” Each of the five conditions is assigned one or more numbers called “severity factors.” Each severity factor is an indication of the susceptibility to explosion danger in the location. The greater the severity factor, the greater the explosion danger. The five critical conditions are: A. The percent of floor space occupied by a group of pumps. B. The pressure in the system. C. The quantity of flammable gases or vapors released during a pump or component failure. D. The possibility of accumulation of flammable gases or vapors. E. The volatility of the flammable product. Under item “D,” “accumulation” is associated with a non-ventilated pump station which is “attended” or “not attended.” If attended, it may take a while before an accidental release of flammable liquid is discovered. This condition requires a “1” severity factor. If not attended, it may take a long time before the release is discovered in which case a “2” severity factor is required. To make a proper analysis of the critical conditions above, it is necessary that from each of the conditions a severity factor is selected (such as 1, 2, 3, or 4). The combination of the severity factors will produce a value which determines the susceptibility to explosion danger. This value is called the “probability factor.” The probability factor is derived by multiplying items A and B and adding it to C, D, and E as follows: A × B + C + D + E. A probability factor below 10 indicates that the location is less susceptible to explosion danger and this allows the pump station to be classified for 50%. A factor of 10 and greater indicates that the location is more susceptible to explosion danger, and this will require a 100% classification. How the probability factor is derived can best be explained by the following example: Assume a group of small pumps each having a hp rating below 51 hp. Assume that this group of pumps operates at moderate pressure and occupies 50% of the pump station floor space. According to the severity factors listed

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Table 1-6. Critical Conditions and Severity Factors A. Floor Space Occupied 50 % 75% 100%

Severity Factors

Size of Sources of Hazard Static Dynamic

2 3 4

B. System Pressure Low Moderate High C. Quantity of Release

1 2 3 Severity Factors

Size of Sources of Hazard Static Dynamic

Small

1

mini

(is associated with low/mod pressure for 0–51 hp)

Medium

2

small

(is associated with high pressure for 0–51 hp and low/mod pressure for 60–201 hp)

Large

3

large

(is associated with high pressure for 60–201 hp)

D. Accumulation Attended Unattended

1 2

E. Degree of Hazard Very flammable/very volatile (4) (Class IA flash point below 73°F, boiling point below 100°F)

1

(only when loca tion is not sufficiently ventilated)

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Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

above, the quantity of flammable vapors that could be released to the atmosphere by pumps of less than 51 hp is small requiring a severity factor of 1.0 Pu. A moderate system pressure, requires a severity factor of 2.0 Pu. When the pumps do not occupy more than 50% of the floor space the severity factor is also 2.0 Pu. When these severity factors are combined, the probability of an explosion is: A × B + C = 2 × 2 + 1 = 5 Pu, which is far below 10 Pu. Because the probability factor is far below 10 Pu, it indicates that the explosion danger in the pump station is relatively low and because of this the pump station is allowed to be classified 50%. At 100% occupied floor space and at high system pressure, the probability factor is considerably higher (4 × 3 + 2 = 14 Pu) which requires that the pump station be 100% classified. The question may arise as to what severity factors should be used if the pump station contains a mixture of pumps from both hp groups. For example, when a floor space is occupied by two 50 hp pumps and two 100 hp pumps, which of the two hp ratings will prevail? Since the 100 hp pump can produce twice as much vapor to the atmosphere, the greater individual hp rating will take precedence. Although more than one pump could fail at the same time, it must be assumed that only one pump will fail at a time. Therefore, in this case, the largest pump must prevail. However, if the pump station should contain two rows of three pumps of 50 hp and a single 100 hp (a total capacity of 400 hp), then the smaller pumps must prevail if the system pressure for the 50 hp pump is higher than for the 100 hp pump. For example, if the floor space of the pump station for both groups of pumps is occupied for 50% and the pressure in the 50 hp pump line is high and the pressure in the 100 hp pump line is low, the probability factor for both pumps is established as follows: 50 hp Pumps

100 hp Pumps

Floor Space

= 50%

= 2 Pu

= 50%

= 2 Pu

System Pressure

= High

= 3 Pu

= Low

= 1 Pu

Quantity of Release

= Medium

= 2 Pu

= Medium

= 2 Pu

Probability Factor

=2×3+2

= 8 Pu

2×1+2

= 4 Pu

In this example, the smaller 50 hp pumps will prevail because of the greater probability factor as a result of the higher system pressure. Classification of a pump station can be obtained directly from Fig. 1-9. In this figure, there are six cases in which the location is classified 50% and four cases in which the location is classified 100%. The probability factor for each case is marked in the right hand corner of each location. From Fig. 1-9 it can be seen that a probability factor below 10 is associated with a 50% classification and a probability factor of 10 and above with a 100% classification.

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For example, what illustration in Fig. 1-9 must be applied for a pump station which contains pumps with sizes ranging from 60–201 hp occupying 50% floor space and operating at high pressure? The answer is Item I in Fig. 1-9. This figure shows that the pump station needs only a 50% classification, because of a probability factor of less than 10 Pu. The illustrations in Fig. 1-9 are self-explanatory. Where the pump stations in Fig. 1-9 are considered to have four walls, the walls are shown with heavy solid lines. When the pump stations have three walls, one side of the pump station is shown with a dotted line. Each pump station is marked with a pump hp range, system pressure, and above the hp range, the percentage of floor space occupied is shown. Below the system pressure, it is indicated whether the pump station is 50% or 100% classified. In some cases, as shown in Fig. 1-9, an additional danger zone beyond the perimeter of the pump station is required. The extent of the danger zone for the various types of pump sizes shall be as follows: 25 ft for pumps ranging in hp between 0 and 51 hp, when associated with low, moderate, or high pressure, 25 ft for pumps ranging in hp from 60–201 hp when associated with low or moderate system pressures, and 50 ft when pumps ranging in hp from 60–201 hp are associated with high system pressure. For danger zones refer to Fig. 1-3. If the extent of the danger zone runs beyond the pump station perimeter as shown in Figs. 1-9C, 1-9E, 1-9H, and 1-9J, the danger zone outdoors shall only be applied if the pump stations has 1, 2, 3, or no walls, in which case, the danger zone must be applied at the building opening. No additional danger zone shall be applied if the pump station has four walls. Percent floor space occupied is that portion of the floor that is occupied by electric driven pumps as a group with spaces between the pumps. For example, if 12 pumps with electric drivers are grouped in a block of 3 rows of 4 units, the block will cover one half or 50% of the floor space if the block is located in one half of the pump station floor. However, the same block will cover 100% of the floor space if the block is located in the middle of the pump station floor. The 12 electric driven pumps will also cover 100% of the floor space if they are scattered over the entire floor of the pump station. The location of the fans in the roof in Fig. 1-9 are only to indicate that the pump stations are sufficiently ventilated and because of this ventilation, it is not necessary to bother about accumulation of flammable gases or vapors in the location. Accumulation will only occur if a pump station is not sufficiently ventilated. If the pump station is not sufficiently ventilated, flammable vapors released during a pump station failure will accumulate causing a dangerous condition in the location. If accumulation of flammable vapors will occur, an accumulation factor of 1.0 or 2.0 Pu must be added to the probability factor. For example, if a pump for transporting flammable liquid should fail, or if one or more of the components in the associated piping system should fail, such as, gaskets,

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Figure 1-9. Classification of small sources of hazard occupying 50, 75, or 100% floor space.

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seals, joints, or fittings, flammable liquid will be exposed to the surrounding air. This liquid will give off flammable vapors and will accumulate to dangerous concentrations if the station is not sufficiently ventilated. A liquid exposure is normally visible. However, attending personnel may not instantly discover the failure or rupture of components. This adds up to a more dangerous condition and, because of this, a 1.0 Pu severity factor must be added to the probably factor, if the station is attended and not sufficiently ventilated. If the pump station is unattended, a 2.0 Pu severity factor must be added because the danger of explosion will be greatest. Explosion danger is greater, if gas compressors are involved with densities lighter than air. The gas may also be odorless. A gas leak, therefore, may not be discovered for many hours. If the total probability factor with the accumulation factor does not exceed 10 Pu, the station is allowed to be classified to 50% and if it exceeds 10 Pu it must be entirely classified. Regardless of whether a station is partially or entirely classified, if the station is sufficiently ventilated it can be classified Div. 2. If it is not sufficiently ventilated, the station must be classified Div. 1. An insufficiently ventilated pump station is shown in Fig. B-3, Part 2. In the Appendix tables for “Property of Flammable Liquid, Gases, and Solids,” some of the flammable materials are marked with a number “4.” The number 4 is an indication that these gases or vapors are very flammable and/or volatile. If the source of hazard involved is one of the flammable materials so marked, a severity factor of 1.0 Pu must be added to the probability factor. If the addition results in a 10 Pu probability factor, the location must be entirely classified. The flash point of a flammable material that is marked with the number 4 is below 73°F, its boiling point is below 100°F, and its classification is 1A. Small static-type sources of hazard, such as components in a piping system, may also require a 50 or 100% classification and even a non hazardous classification if the probability factor does not exceed 5 Pu and the components are regularly and well maintained. For example, a piping system occupying less than 50% floor space of a sufficiently ventilated indoor location containing the following mini components: valves, screwed fittings, flanges, etc., operating at moderate pressure, will have the following probability factor: Floor space occupied

= 50%

2 Pu

System pressure

= moderate

2 Pu

Quantity of release

= small

1 Pu

Probability factor

= 2 × 2 + 1 = 5 Pu

Since the probability factor is 5 Pu, the location need not be classified if the piping system is regularly and well maintained. (Refer to Fig. K-1 in Ch. 29 of Part 2.)

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K.

FUME HOODS

K.1. General As a general rule, a roofed 4-wall indoor location is normally classified nonhazardous if it does not contain a source of hazard. As an exception to this rule, there are indoor locations that may be classified nonhazardous if they do contain a small source of hazard. These sources of hazard are either located inside a fume hood enclosure or are located in close proximity underneath a canopy fume hood. As long as the canopy fume hood and the fume hood enclosure is provided with a suitable operating exhaust system, the surrounding spaces in the indoor location are allowed to be classified nonhazardous. The canopy fume hood is shown in Fig. 1-10A. The fume hood enclosure is shown in Fig. 1-10C. The canopy fume hood normally has a limited number of electrical equipment but the fume hood enclosure is sometimes provided with a large number of electrical equipment such as receptacles, lighting fixtures, switches, motor driven pumps, hot plates, stirrers, etc. This electrical equipment is used when testing is required with a flammable product. Fume hoods are normally connected through a duct system to a roof or wall fan. Since gravity-operated dampers in the duct system sometimes may become inoperative due to corrosion, it is preferred that they not be used. All metal parts of the duct system shall also be effectively electrically bonded and grounded to eliminate possible static electrical discharge. Released flammable gases or vapors removed by the fume hood ventilation must be discharged in safe outdoor areas where there are no pockets for accumulation.

K.2. Process Areas in Compliance with Figure 1-10A The inlet of a canopy fume hood must be mounted to any point where flammable material in the air can be expected. Suction air between the source of hazard and the fume hood must be of sufficient force and quantity for removing any flammable material released by the source of hazard. The fume hood duct system shall operate below atmospheric pressure. Lighter- and heavier-than-air gases or vapors in the air need not be diluted to below the LEL if all airborne gases or vapors are caught by the canopy fume hood, if the area between the source of hazard and the fume hood is not a working space for personnel, and personnel are not subjected to any airborne gases or vapors. A lighter-than-air gas or vapor will rise by itself when airborne. Suction air for lighter-than-air flammable material, therefore, needs only to be of sufficient quantity and force to accelerate the speed of the rising gas or vapor particles. Heavier-than-air gases or vapors may need a greater quantity and force of suction air to assure positive removal of the flammable material. If the area between the source of hazard and fume hood is a working space, or if personnel are subjected to any airborne flammable gas or vapor from the source of hazard, it is necessary that the flammable material be diluted to below 1/4 of the LEL.

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If the reduced concentration of the flammable gas or vapor may still cause a health hazard, the dilution of the flammable material must be based on the threshold limit value (TLV) of the flammable product. As an additional safety precaution, an escape opening in the roof of the building is also required if the flammable material is lighter than air. The ventilating system of a canopy fume hood shall be provided with a safeguard that: 1. Prevents the failure of the fume hood suction fan. 2. Is electrically interlocked with the process equipment, so that failure of the fume hood fan will initiate a shut down of the process equipment and operate an alarm. 3. Will initiate only an alarm. For open sources of hazard, the safeguard required is as in number 1 or 2 above. For closed sources of hazard, number 3 must be considered. The area between the source of hazard and fume hood shall be classified as follows: 1. If the source of hazard is open and small: A Div. 1 zone surrounded by a Div. 2 zone. 2. If the source of hazard is open and large: Div. 1. 3. If the source of hazard is closed and small: A Div. 2 zone. 4. If the source of hazard is closed and large: Div. 2.

The fume hood located directly above the source of hazard makes the entire building nonhazardous, except for an area close to the source itself. Figure 1-10A. Fume hood for process equipment.

102 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The location of the exhaust fan in the fume hood duct system shall be as follows: 1. If the flammable material is lighter than air: roof or in the wall below the ceiling. 2. If the flammable material is heavier than air: center of the building wall. The classification and the extent of the danger zone at the outlet shall be as indicated in Table 1-5. Fan outlet motors shall be of spark proof construction and may be of the weatherproof type provided they have an electronic switch instead of the usual centrifugal switch normally furnished for single phase motors.

K.3. Laboratory-type Fume Hood Enclosures in Compliance with Figures 1-10B and 1-10C The classification of a laboratory-type fume hood enclosure is more complex. It mainly depends on how dangerous the working space in the fume hood enclosure is when the inside space is provided with electrical equipment. The type of danger that can be expected in a fume hood enclosure is when flammable gases or vapors are purposely or accidentally released in the fume hood enclosure in the presence of general purpose electrical equipment of the heat producing type. General-purpose electrical equipment of the heat producing type will produce arcs, sparks, or sufficient heat under normal and abnormal operating conditions. If these arcs, sparks, or sufficient heat come in contact with ignitable concentrations of flammable gases or vapors, an explosion will or may occur regardless of whether the fume hood enclosure is ventilated or not. For example, if a flammable gas is purposely or accidentally released inside a sufficiently ventilated fume hood enclosure, the gas will be rapidly diluted by air. The greater the airflow, the closer the explosion range will move towards the point of release and the faster the flammable material will become ignitable. A source of hazard which is brushed by ventilating air, will rapidly enter the explosion range. Under normal operating conditions, arcs and sparks from electrical equipment may not always have sufficient ignition power, but under abnormal operating conditions, the electrical equipment will have a much greater ignition power. Therefore, if electrical equipment of the general-purpose type is located at the point of release, inside the fume hood enclosure a potential source of danger will exist at all times when the fume hood enclosure is ventilated. Under this condition, explosion danger will not exist at a distance from the source of hazard, because at a remote location, the flammable material will be too lean for combustion. Rapidly entering its explosion range will also occur if flammable liquid is transferred from one container to another. For example, if gasoline liquid is poured into a gas can, or as shown in Fig. 1-10B-B, the moving liquid will produce air turbulence around the liquid stream. This turbulence will accelerate the mixing process between vapor and air. In turn, this process will cause the vapor to enter its explosion range

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more rapidly. Whether the liquid is flowing through static air or air is forced into a gas or vapor concentration, in both cases, the vapor concentration will enter its explosion range much faster. If the fume hood enclosure is not ventilated, the flammable material will not be diluted at the point of release and, as a result, the flammable material at the source is prevented from entering its explosion range. Without ventilation, the flammable concentration starts to saturate the air at the source. Its concentration starts to grow and will expand. Eventually the concentration will disperse to the extent that it can be ignited by electrical equipment located at a remote location. Under this condition, there is no potential hazard at the source because the flammable material, not being diluted, will be too rich for combustion. However, a potential hazard will exist at some distance away. This potential hazard will also exist if the flow of air is blocked. A worker standing in front of the fume hood opening, for example, will cause blockage of the airflow and as such may contribute to a rise of explosion danger.

Figure 1-10B. Location of UEL with respect to air flow.

104 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-10C. Fume hood enclosure.

A flammable gas or vapor released into the atmosphere will enter its explosion range much faster if it has a wide explosion range. The wider the explosion range, the closer the UEL of the flammable material moves towards the point of release, and the faster the flammable material becomes ignitable. With forced air, the explosion range moves even closer to the source and the gases or vapors will become ignitable even faster. Hydrogen gas, for example, has a wide explosion range. If airborne, the gas will become ignitable in a very short time, since the UEL of hydrogen gas, being 75%, will be closer to the point of release than a gas which has an UEL lower than 75%. If the hydrogen gas is subjected to airflow, the time at which it becomes ignitable is much shorter. This is clearly shown in Fig.1-10B-A.

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The distance between the source and the point at which the flammable material becomes ignitable is greater when the flammable material has a narrow explosion range. The smaller the explosion range, the farther away the point of ignition. For example, gasoline has a narrow explosion range. For octane 92, the range runs from 7.6–1.5%. Since its upper explosion limit is much lower than for hydrogen gas, the distance between the point of release and the point at which the gasoline becomes ignitable is greater than for hydrogen gas. For gasoline with airflow, this is shown at point “C” and without airflow this is shown at point “D.” Summarizing the various conditions as highlighted above, the following will apply: There is great explosion danger in the fume hood enclosure if there is no airflow or blocked airflow when a flammable material is released and general-purpose electrical equipment of the heat producing type in the fume hood enclosure is located some distance from the point of release. There is a minimal explosion danger in the fume hood enclosure if there is an air flow or blocked air flow when a flammable material is released and general purpose electrical equipment of the heat producing type is located in close proximity with the point of release. However, there are conditions in which no explosion danger will exist at the point of release if sufficient ventilation is present. For example, a hot plate located underneath an open container filled with a flammable liquid is an equipment that is in close proximity with the source of hazard. One reason there is no explosion danger is that the heat rays from the hot plate may not be capable of igniting the flammable material because the temperature of the heat-rays are below the ignition temperature of the flammable material. For example, if the heat-rays are cooled down by ventilating air, the temperature of the heat rays may be lower than the ignition temperature of the flammable material, therefore, ignition cannot take place. Ignition can only take place if the heat-rays are not cooled down, and, therefore, are of sufficient heating capacity or if the heat-rays are cooled down, but the temperature of the heat rays are still higher than the ignition temperature of the flammable material. Since general purpose electrical equipment, of the heat producing type, under normal and abnormal operation, can ignite flammable gases or vapors which are purposely or accidentally released, it is recommended not to use this type of electrical equipment in a fume hood enclosure. However, if it is vital that electrical equipment be used inside the fume hood enclosure, other types of electrical equipment should be considered; such as explosion proof equipment and equipment specifically designed for remote hazardous locations. Which of the two types should be considered as the most suitable depends on the type of classification the fume hood enclosure needs to have. For example, if flammable gases or vapors are continuously present in a fume hood enclosure, the environment in the fume hood enclosure is dangerous and as such it is required to be classified Div. 1. Under this condition, all electrical equipment

106 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres needs to be explosion proof. On the other hand, the flammable materials may be occasionally present, because of malfunction of equipment. In this case, the fume hood must be considered remotely dangerous and should be classified Div. 2. However, a Div. 2 classification can only be applied if flammable liquid gases and vapors are normally confined within closed systems from which they can escape only in case of accidental rupture or breakdown of such systems. Since the utilization of the flammable materials inside the fume hood enclosure does not always comply with this requirement, the Div. 2 classification is not a suitable classification. Classifying the fume hood enclosure nonhazardous is not suitable either since the application of flammable materials in a nonhazardous location, is prohibited. The only and most appropriate classification for a fume hood enclosure, using flammable materials in conjunction with electrical equipment, is a dual classification which consists of Div. 1 and Div. 2. This dual classification will cover open and closed sources of hazard. Open sources of hazard normally have a dual classification that consists of a Div. 1 zone at the opening and a Div. 2 area that surrounds the Div.1 zone. If the source of hazard is closed, then the Div. 1 zone should be omitted. The extent for the Div.1 classification is a function of the size of the source of hazard. Since the sources of hazard in the fume hood enclosure are of the mini type, the Div.1 zone should be small. A Div. 1 circular zone of 6" to 12" around the opening of the source of hazard seems appropriate. Since a Div. 1 zone must be followed by a Div. 2 zone, the entire fume hood enclosure should be classified Div. 2 beyond the Div. 1 zone. The Div. 1/Div. 2 classification, therefore, is the only acceptable classification for a fume hood with electric equipment of the heat producing type. Proper electrical equipment can be selected after the fume hood enclosure has been classified. Arcing devices such as switches and receptacles in a Div. 2 area must be suitable for the Div. 2 location. They shall have a surface temperature not in excess of 80% of the ignition temperature of the flammable product. However, the best and safest solution is not to provide any electrical equipment inside the fume hood enclosure, except for hot plates and stirrers. Electrical equipment should be kept outside the fume hood enclosure as shown in Fig. 1-10C. The use of hot plates and stirrers inside the fume hood enclosure cannot be avoided. Hot plates and stirrers are normally within the Div. 1 zone. Therefore, they must be labeled for a Class I, Div. 1 location and their surface temperature shall not exceed the ignition temperature of the flammable material. OSHA requires that laboratory-type fume hoods be provided with an exhaust system that has an average face velocity of at least 100 linear feet per minute with a minimum of 70 cfm at any point. Thus, a fume hood enclosure with 3 ft high and 4 ft wide opening must have at least an inward flow of 100 linear ft per minute. If the sash of the fume hood is lowered half a foot, the face velocity will rise to (3 × 4/2.5 × 4) 100 = 120 linear ft per minute. Laboratory type fume hoods are normally connected through a duct system to a roof mounted centrifugal exhaust fan. The fan motor should be of the weatherproof type equipped with an electronic switch instead of a centrifugal switch normally provided for single-phase motors. An exhaust outlet stack shall

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extend 7 ft minimum above the roof of the building. For small quantities of flammable gases or vapors released, two circular zones are required around the outlet of the fume hood, one of which is classified Div. 1 with a radius of 3 ft minimum, the second classified Div. 2, should be 2 ft wide surrounding the Div. 1 zone. Safety in the work space in the fume hood enclosure can be considerably improved when it can be arranged in a manner that ventilating air is flowing in a direction from electrical equipment to the source of hazard, as shown in Fig. 1-10B-C and Fig. 1-10C. Safety is considered reduced if ventilating air in the workspace is flowing from the source of hazard to the electrical equipment as shown in Fig. 1-10B-D.

L.

STORAGE AND DISPENSING OF FLAMMABLE LIQUIDS

This section covers the following indoor areas for storing and dispensing flammable liquids in portable containers: 1. A storage room inside a building 2. A dispensing area inside a building 3. A liquid warehouse 4. A drum filling station 5. Storage cabinets 6. Basement

L.1. Storage and Dispensing Areas 1. A storage room inside a building is a room: a) of approximately 500 ft2 with no external walls, or b) a room of 1,000 ft2 or less with one or two external walls called a cut-off room. Both rooms are “inside rooms.” Internal walls must have a minimum fire rating of 2 hrs. 2. A dispensing area inside a building is an “inside room” in which flammable and/or combustible liquid is dispensed. 3. A liquid warehouse is a location in which flammable and combustible liquid is stored in suitable containers. A liquid warehouse is either a separate “attached” building with 4-hr firewalls and a 3-hr automatic closing communicating fire door or it is a building “detached” from other buildings. Generally, liquid warehouses have over 1,000 ft2 of floor area. 4. A drum filling station is either an open space or an enclosed space in which flammable and combustible liquids are transferred from one container to another container. A drum filling station is normally part of a bulk storage plant which receives flammable

108 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres and combustible liquids that are distributed to other locations by tank cars, tank vessels, or pipe lines. 5. A storage cabinet is an enclosure for the purpose of storing containers with flammable or combustible liquids. 6. A basement is a space in a building that has its walls for more than 50% below ground level.

L.2. Suitable and Non-Suitable Containers Flammable and combustible liquids shall be stored in “suitable containers.” For example, a suitable container is a metal drum or a metal portable shipping tank, which meets the requirements of, and contains products authorized by Chap. I, Title 49 of the Code of Federal Regulations (DOT regulation), or it is a nonbreakable durable plastic container that meets the requirements of ANSI and ASTM standards. These containers are so well sealed that they prevent liquid or vapors from escaping. For example, metal drums may store Class I, II, and III liquids and may, for all three classes, have a maximum storage capacity of 60 gal. Portable shipping tanks may have a maximum capacity of 660 gal and may also store Class I, II, and III liquids. However, when they store Class IA liquids, they must be provided with an automatic sprinkler system as an additional safety precaution because of their large storage capacity. Durable plastic containers may store Class III, II, and I liquid if they have limited storage capacity. For example, nonmetal containers, such as approved plastic and polyethylene containers meeting the above regulations, are considered “suitable” for storing flammable and combustible liquid if they do not exceed the following sizes: 1. Approved plastic: 1 gal for Class IA, 5 gal for Class IB, IC, II, and III liquids. 2. Polyethylene: 1 gal for Class IA, 5 gal for Class IB and IC, and 60 gal for Class II and III liquids. There are also “non-suitable” containers that are breakable and cannot withstand rough handling, such as glass containers. Another non-suitable container is a metal container that does not meet DOT regulations. These containers may not be used in liquid warehouses. They may be used in inside rooms if they have limited storage capacity. For example, a glass container must have a storage capacity of 1 pt for Class IA, 1 qt for class IB, 1 gal for Class IC and Class II, and 5 gal for Class III liquids. Metal drums other than DOT drums must have a storage capacity of 1 gal for Class IA, and 5 gal for Class IB, IC, II, and III liquids. Sometimes flammable or combustible liquid of high purity content cannot be stored in metal containers and, therefore, it is necessary to store them in glass containers. Class IA and IB liquids may be stored in glass containers of not more than 1 gal storage capacity if the liquid purity could be affected by storage in metal containers.

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Some of the flammable and combustible liquids are listed below: Class IA:

Ethyl ether, ethyl chloride, isoprene, pentane

Class IB:

Acetone, benzene, ethyl acetate, ethyl alcohol, gasoline, octane, methyl alcohol

Class IC:

Styrene, isobutyl alcohol, turpentine, amyl alcohol

Class II:

Fuel oil, kerosene

Class IIIA: Nitrobenzene, pine oil Class IIIB: Ethylene glycol, glycerin, lubricating oil, transformer oil, vegetable oil The requirements for storing and dispensing flammable liquids are discussed in the following sections.

L.3. Storage Rooms Inside a Building—Classification and Ventilation Requirements “Inside rooms” must be entirely classified Div. 2 if Class I liquids with vapors heavier or lighter than air are stored in containers meeting DOT regulations, or if stored in containers not meeting DOT regulation but of limited storage capacity. Ventilation such as a gravity or mechanical fan is required if a Class I liquid is stored in non-suitable or breakable containers. No minimum exhaust rates are required. Storage of a Class IA liquid is prohibited in portable metal tanks of 660 gal capacity if the location is not provided with an automatic sprinkler system and other fire fighting facilities. Inside rooms are allowed to be classified nonhazardous if Class II and III liquids are stored in suitable and non-breakable containers meeting the requirements of DOT regulations or if the temperature is below flash point. Inside rooms storing Class II and III liquid in containers not meeting DOT regulations or in breakable containers may also be classified nonhazardous if the location is sufficiently ventilated. The type of ventilation shall be the same as for Class I. For heavier-than-air flammable products, the exhaust air shall be taken 12" above the floor; for lighter-than-air flammable products the exhaust air shall be taken 12" below the ceiling; or, the exhaust air shall be taken from both locations when the flammable products are heavier and lighter than air. Storage of a Class I flammable liquid in the basement of inside rooms is prohibited. However, Class II and IIIA liquids in the basement are permitted. Although the flash point of Class II and IIIA liquid is generally above normal ambient temperatures, if these liquids are stored in the basement, the basement must be provided with an automatic sprinkler system and other fire fighting facilities. The basement may be classified nonhazardous if ventilation is present or if temperatures of the Class II and III liquids are below flash point. If temperatures should be higher than flash point and the vapors of the Class II and III liquids are

110 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres heavier than air, the basement must be classified Div. 2. If no ventilation is provided, the basement must be classified Div. 1 if vapors are heavier than air and temperatures are above flash point.

L.4. Dispensing in Storage Areas Inside a Building in Rooms Without External Walls a.

Ventilation Requirements

The following ventilation requirements must be applied. First, storage areas in which dispensing takes place must be ventilated whether dispensing is a principle or incidental activity. Second, for dispensing Class I liquid, the ventilating system must consist of an electrically operated exhaust fan. In addition, the fan must be provided with a type “B” safeguard. Ventilation shall provide air movement across the floor to prevent accumulation. Third, a gravity fan shall be provided for Class II and Class III liquids if transfer or dispensing is incidental. Fourth, if an electric exhaust fan is required, the general rule is that vapors from a Class I liquid and from Class II and III liquids with temperatures above flash point be diluted to below 1/4 of the LEL. However, for Class II and III liquids it is not always necessary to dilute the vapors to below 1/4 of the LEL. For example, where Class II and III liquids are incidentally transferred from one container to another, there is no need for diluting the vapors to below 1/4 of the LEL if the temperature is “slightly above flash point” or if the liquid is incidentally dispensed. Ambient temperatures may vary daily. Therefore, temperatures slightly above flash point may drop to below flash point during the variation of the ambient temperature. This fluctuation in temperature allows the dilution of Class II and Class III liquids to be “slightly below the LEL.” Slightly above flash point is considered to be approximately 125% of flash point temperature. If the temperature is within the 125% range and Class II and III liquids are being dispensed and/ or transferred incidentally, dilution shall be slightly below LEL. Temperatures above 125%, however, will cause the temperature to stay above flash point. With higher temperatures, i.e., above 125%, the dilution of vapors should be more conservative and be below one-fourth of the LEL but not less than 1 CFM per ft2 of floor area for Class I. Dispensing or transferring Class II and Class III liquids on a regular and daily basis requires a dilution of below one-quarter of the LEL. Exhaust air must remain in operation as long as liquid is dispensed or transferred. b.

Classification Requirements

Classification of dispensing areas or areas in which Class I flammable liquids or Class II and Class III flammable liquids with heavier-than-air vapors and temperatures above flash point are transferred from one container to another

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(which are sufficiently ventilated) shall be as follows: the fill opening of each container shall be provided with a Div. 1 zone of 3 ft radius. Beyond the Div. 1 zone, the extent of the Div. 2 area shall be as follows: 1. For a flammable liquid in containers occupying not more than 50% of the floor space, the extent of the Div. 2 area shall be 2 ft wide extending downward to the floor and, from there, 10 ft in a horizontal direction, 18" high. 2. For flammable liquids in containers occupying more than 50% of floor space, the extent of the Div. 2 area shall be up to the walls of the dispensing area. It is not necessary to classify the upper part of the area Div. 2. It needs only to be classified Div. 2 if the same location contains flammable liquids with vapors lighter than air. 3. For Class II and III liquids with temperatures below flash point, the area is unclassified. 4. For lighter-than-air flammable liquids, the required Div. 2 area shall extend upwards in a cone. Some engineering judgment is required to establish the width of the cone at the ceiling because of the deflection of the cone towards the air outlet in the ceiling.

L.5. Dispensing in Cut-Off Rooms Inside a Building Dispensing of Class I and II liquids in cut-off rooms is permitted if the room is not over 1,000 ft2. Classification and ventilation requirements shall be the same as for inside rooms.

L.6. Liquid Warehouses Liquid warehouses may store flammable liquids of all classes if the containers and portable tanks meet DOT regulations and/or ANSI and ASTM standards. Because of its large size, a portable tank (660 gal maximum) may not store Class IA liquids without an automatic sprinkler system and other fire fighting facilities. In basements, a Class I liquid is prohibited. Class II and IIIA liquids may be stored in the basement if automatic sprinkler systems and other fire fighting facilities are provided. The liquid warehouse need not be ventilated if flammable liquid is stored in metal containers meeting DOT regulations or in non-breakable plastic containers meeting the requirements of ANSI and ASTM standards as follows: 1. Metal drums of 60 gal maximum for Class I, II, and III liquids.

112 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres 2. Approved metal portable tanks of 660 gal maximum for all classes except for Class IA unless protected by fire fighting facilities. 3. Approved plastic containers: 1 gal for Class IA, and 5 gal for Class IB, IC, II, and III. 4. Approved polyethylene containers of 1 gal for Class IA, 5 gal for Class IB and IC, and 60 gal for Class II and III. If flammable liquids are not stored in suitable or non-breakable containers, a mechanical or gravity fan is required. For a Class I liquid and/or Class II and III liquids with temperatures above flash point, ventilation shall be the same as for storage rooms inside a building in Part 3. Liquid warehouses need not be classified if Class II and III liquids are stored. However, a Div. 2 classification is required if Class I liquid is stored. A safety zone of 3 ft minimum between an electric arcing device and a container must be maintained as an additional safety precaution. Dispensing of Class I or Class II liquids in liquid warehouses is not permitted unless the dispensing area is cut off from the storage area. Classification and ventilation requirements for dispensing shall be the same as for inside rooms.

L.7. Drum Filling Stations a.

Indoor Filling Stations

Indoor filling stations must be classified and sufficiently ventilated. The classification and ventilating requirements for indoor filling stations shall be the same as for “Dispensing in Areas Inside a Building.” b.

Outdoor Filling Stations

The classification of an outdoor filling station shall be as follows: for a group of containers, each 10 ft or more apart, the fill opening shall be Div. 1 extending 3 ft in all directions. Beyond the Div. 1 zone, the Div. 2 area shall be 2 ft wide extending downward, and 10 ft, 18" high in a horizontal direction. For containers less than 10 ft apart, classification is the same as above except that the 10 ft, 18" horizontal Div. 2 area shall extend from the perimeter of every outer container in the group. The empty spaces between the containers shall also be Div. 2.

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L.8. Storage Cabinets A storage cabinet shall not store more than 120 gal of Class I, Class II, and Class III liquid. Of the 120 gal total, not more than 60 gal may consist of Class I and Class II liquid. Not more than three of these storage cabinets may be located in one area, unless the area is protected by an automatic sprinkler system, in which case, six storage cabinets may be located in one single area.

M.

SEGREGATION

It may also be economically and operationally preferable to segregate certain electrical equipment from a hazardous area. Electric motors may be segregated as shown in Fig. 1-11. Here the driving end of the motor is to be extended through a packing gland in one of the enclosing walls. To prevent the accumulation of flammable vapors or gas within the motor room, the room should be effectively ventilated by clean air or kept under a slight positive air pressure. A gas detector can be installed as an additional safety feature to give visual and/or audible alarm. It is preferable that such a building should not have spaces underneath the floor where heavier-than-air flammable vapors or gases can accumulate. If such areas cannot be eliminated, proper ventilation should be provided, since seepage of vapors or gas may occur through settlement or vibration cracks.

114 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Electrical equipment isolated by being placed outside the hazardous area.

Figure 1-11. Segregation.

Chapter 4 Spatial Considerations

A.

INDOOR AND OUTDOOR LOCATIONS

The terms indoor and outdoor, as described herein are primarily related to whether a location is sufficiently or insufficiently ventilated by natural ventilation. In outdoor locations, natural air movement is present all the time and, generally, natural ventilation is considered to have sufficient capacity to dilute a flammable gas or vapor to below the LEL. In outdoor locations, air moves continuously as a result of wind force. Therefore, flammable gases or vapors in the air outdoors are dispersed, diluted rapidly, and prevented from accumulating. In indoor locations, air movements may not be as favorable as in outdoor locations. Natural ventilation is considered effective in indoor locations only if the airflow is unobstructed when passing through it. Building walls are generally the main cause for the ineffectiveness of natural ventilation. The basic concept of a sufficiently ventilated location is that the location must be substantially open and free from obstruction to the natural passage of air flowing through. Such a location is considered an outdoor location. The location is also considered an outdoor location if it is roofed and provided with one or no walls. For natural ventilation to be effective, the location cannot have more than one wall. For example, wind can easily rotate 90°. When wind initially blows perpendicularly to the outside of a single wall, it is capable of blowing along the inner side of the wall when it rotates 90°. Any 90° change in wind direction will prevent the flammable gas or vapor from accumulating at the inner side of the wall. 115

116 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Therefore, a location is considered outdoors when natural ventilation is restricted to not more than 25% of the building enclosure in passing through the location. This means that the location has no corners in which a flammable gas or vapor can accumulate. A building with two walls opposite each other may also be considered as an outdoor location. This is true as long as both walls do not have a common corner in which a flammable material can accumulate. If both walls are put together in an L-shape, the location cannot be considered an outdoor location. In this case, flammable gases or vapors inside the location may not be dispersed to safe concentrations. The wind may rotate 90° in a clockwise direction causing a flammable gas or vapor to accumulate in the corner of the location. Natural ventilation is also incapable of sufficiently dispersing and diluting a flammable gas of vapor in a building with three or four walls. Natural ventilation, therefore, must be considered incapable of sufficiently dispersing and diluting flammable gases or vapors in buildings with more than one wall and walls in an L-shape. Because of this, buildings with more than one wall and walls in an L-shape are considered indoor locations. The building in Fig. 1-12A has a roof and only one wall. With respect to the wind direction shown, dilution and dispersion of a flammable gas or vapor inside the location could be considered poor. This is not so, as explained above, since wind can pass through the building from any direction other than that shown in Fig. 1-12A. Since one wall causes an obstruction of 25%, the building in Fig. 1-12A is considered an outdoor location. The building in Fig. 1-12B has a roof and two walls. These walls are located opposite each other. With respect to the wind direction as shown in Fig. 1-12B, the building is comparable to a building with only one wall, as shown in Fig. 1-12A. The two walls in this arrangement make it impossible for ignitable gases or vapors to accumulate because there are no corners in which the ignitable material can accumulate. Since the obstruction with respect to air flow from one direction is only 25%, the building with two opposite walls is considered an outdoor location. The building in Fig. 1-12C has two walls and a roof. Since the building has two walls forming an L-shape, the corner of the L-shape may provide an air pocket of still air where flammable gases or vapors can easily accumulate. With respect to the wind direction as shown in Fig. 1-12C, there is not sufficient air movement inside the corner. Because the building has two walls in an L-shape, it has a 50% obstruction that cannot be sufficiently ventilated by natural ventilation. Since natural ventilation is obstructed 50%, the location in Fig. 1-12C must be considered an indoor location. A roofed space with three walls (as shown in Fig. 1-12D) must be considered indoors, because the walls provide 75% obstruction to the passage of natural ventilation. Therefore, this building with three walls, must be considered insufficiently ventilated.

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Figure 1-12. Indoor and outdoor locations.

An enclosed space with a roof and four walls as shown in Fig. 1-12E must also be considered indoors, because it provides 100% obstruction to the passage of natural ventilation, and therefore, it is insufficiently ventilated. An enclosed building, as shown in Fig. 1-12F, may be considered sufficiently ventilated if it is provided with mechanical ventilation that produces sufficient ventilation. Although the building in Fig. 1-12F has 100% obstruction to the passage of natural ventilation, it has zero obstruction with respect to the mechanical ventilation. Since this building has more than one wall with common corners it is an indoor location.

118 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres A roofed space without walls as shown in Fig. 1-12G has 0% obstruction, and, therefore, this location can be considered an outdoor location. Locations that are considered to be outdoors are generally ventilated by natural ventilation. Locations that are considered to be indoors are generally ventilated by mechanical ventilation. Both types of ventilation must be capable to diluting flammable gases or vapors in the air to acceptably low levels. Mechanical ventilation may be considered for outdoor locations when the outdoor location is provided with obstructions other than walls that will restrict the flow of natural ventilation. Generally, mechanical ventilation is considered insufficient for ventilating roofed buildings with two walls in an L-shape. Since the free passage of natural ventilation through a building is expressed in percent obstruction, the percentage in turn dictates whether the location must be considered indoors or outdoors. It is important in area classification to define what is outdoors and what is indoors since it will affect the degree of danger in the location and whether or not mechanical ventilation must be applied. Access openings have no impact on whether the location should be considered indoors or outdoors. Hinged windows, on the other hand, will have an impact on the type of location. A fully opened window of regular size, located in the center of a building wall, with a width of not less than 50% of the width of the building wall, with its sill not over 3 1/2 ft from the finished floor, is considered to produce sufficient natural air movement inside the building. A building with this type of window may be considered to have a 25% obstruction. For a building to have 25% obstruction, a building with two walls in an Lshape must have two windows and a three-wall building must have three windows. Mechanical ventilation, even with a type “A” safeguard, has no impact on the type of location.

B.

ROOFED SPACES IN HAZARDOUS AREAS

Roofed spaces with one or more walls, which do not contain a source of hazard, must be classified, if they are located in a hazardous area. The number of walls influences classification of these spaces. Also, whether or not the spaces are provided with sufficient ventilation and whether the ventilation is equipped with a safeguard that prevents failure of the ventilating system or sounds an alarm when ventilation stops operating, will have an impact on the classification of the space. The classification of the surrounding hazardous area and the type of release of flammable materials in the hazardous area will also have an impact on the classification of the space. Three different types of releases can be expected in the surrounding hazardous area: 1. A continuous release from open sources of hazard in which flammable materials are released continuously under normal operating conditions.

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2. A frequent release from closed sources of hazard as a result of frequent repairs and maintenance of the process equipment. 3. An occasional release from closed sources of hazard when flammable materials escape during a breakdown, malfunction, or leakage of the process equipment. Therefore, in cases where the release of flammable materials in the outdoor hazardous area is continuous or frequent, the general requirement is that the hazardous area be classified Div. 1. If the release is only occasional, the classification of the hazardous area needs to be Div. 2. Generally, small open sources of hazard located outdoors produce small quantities of ignitable concentrations of vapors to the atmosphere. These small quantities are, normally, rapidly diluted to safe concentrations by natural ventilation. As a result of this condition, the required Div. 1 area for the open source of hazard is small normally. The size of the Div. 1 area will generally have a radius of not more than 3 or 5 ft. Because of the small Div. 1 area, the majority of roofed spaces are found in Div. 2 areas and seldom in those small Div. 1 areas. However, if large quantities of flammable vapors are produced, the radius of the Div. 1 area is larger than 3 or 5 ft, and, as such, roofed spaces may be located in Div. 1 areas as shown on Fig. 1-13. A roofed space with one or two walls located in a hazardous area must have the same classification as given to the surrounding hazardous area. For example, a space with one or two walls located in a Div. 1 area must be classified Div. 1 and where the space is located in a Div. 2 area it needs to be classified Div. 2. This is because spaces with one or two walls become part of the hazardous environment. These spaces cannot prevent flammable materials from entering the space even though the space is sufficiently ventilated by natural ventilation. See Items 1 and 5 in Table 1-7. Roofed spaces which have only one or no walls are normally considered sufficiently ventilated by natural ventilation. Because one or no wall spaces are substantially open, they have no air pockets in which flammable gases or vapors can accumulate. Natural ventilation is considered capable of diluting and dispersing any flammable material in this type of space because of the lack of air pockets. However, these spaces allow easy access to any flammable material in the hazardous area. Because of this, these spaces cannot be classified under the same conditions as for three or four-wall spaces. Spaces with two walls in which the walls form an L-shape cannot be sufficiently ventilated by mechanical or natural ventilation. Therefore, these spaces are considered as not sufficiently ventilated. Because they are substantially open, they must be classified Div. 1 if they are located in a Div. 1 area. They also must be classified Div. 1 if they are located in a Div. 2 area, because the L-shape of the space allows flammable gases or vapors to accumulate in the space.

120 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

PLAN

Figure 1-13. Spaces in hazardous locations.

Spaces with two walls located opposite each other must be treated the same as spaces with only one wall. Spaces with more than two walls do not become part of the hazardous area because of their greater number of walls. Ventilation in spaces with more than two walls is generally capable of preventing flammable materials from entering the space. A three-wall space therefore, whether located in a Div. 1 or Div. 2 area, may be classified Div. 2, if it is sufficiently ventilated. See Items 3 and 7 in Table 1-7. A classification lower than Div. 2, i.e., nonhazardous, is not permitted for a three-wall space even though it is sufficiently ventilated. The reason for this requirement is that a three-wall space can easily be contaminated with flammable materials during a ventilation failure and, therefore, it cannot be classified nonhazardous. Since contamination of the space is possible, the environment in the threewall space could be considered dangerous. As such, it appears that the three-wall space should be classified Div. 1 when ventilation failure or outage cannot be avoided. However, if the release of flammable materials in the outdoor Div. 1 area is other than continuous, the three-wall space does not have to be classified Div. 1. In that case, flammable materials may not necessarily exist outdoors when the space has ventilation failure, and when flammable materials do exist outdoors, a ventilation failure may not necessarily occur at the same time. This feature makes the three-wall space remotely dangerous and, as such, this condition allows the three-wall space to be classified Div. 2. See Item 3 and 7 in Table 1-7. The

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environment in a three-wall space is considered dangerous if the release of the flammable material is continuous. In this case, the three-wall space needs to be classified Div. 1 even thought the space may be sufficiently ventilated. See exception in Item 3 in Table 1-7. For a space to qualify for a nonhazardous environment it is mandatory that the space be provided with four vapor tight walls, sufficient ventilation, and most important, a suitable safeguard. Bear in mind that even a four-wall space may not be gas or vapor tight. For example, the access opening in the four walls used by personnel for entering and leaving the enclosed space allows flammable materials to enter the space. Also, non-vapor-tight walls, non-bolted windows, and a roof which is not sealed to the walls will allow flammable materials to enter the space. However, if sufficient ventilation is provided, flammable materials are prevented from entering the space. As mentioned above, ventilation alone is not enough to allow a four-wall space to be classified nonhazardous. With sufficient ventilation, the four-wall space is allowed to be classified Div. 2, and with a suitable safeguard and ventilation, the space can be classified nonhazardous. See Items 4 and 8 of Table 1-7. Switch houses and control rooms are four-wall spaces. When they are located in a hazardous area they require a nonhazardous classification. This nonhazardous classification can only be achieved if the switch house and control room is provided with sufficient pressure ventilation and a type “A” or type “B” safeguard. In Fig. 1-13, Space 1 and Space 2 are both four-wall spaces. They may be classified nonhazardous if they are provided with sufficient ventilation and a suitable safeguard. A type “A” safeguard must be used in Space 1 if the release of flammable materials in the outdoor Div. 1 area is continuous. See Item 4 in Table 1-7. A type “B” safeguard may be used if this space is located in a Div. 2 area, such as Space 2 in Fig. 1-13 (see Item 8 in Table 1.7) or if the release of flammable material is other than “continuous” as indicated by Item 4 in Table 1.7. Although a type “B” safeguard does not prevent the failure of the ventilating system in the space, the fact remains that if the release of flammable materials is other than “continuous,” a ventilation failure may occur at a time that no flammable gases or vapors are present and visa versa. A type “B” safeguard may be used in Space 2. See also Item 8 in Table 1-7. Under such a condition, the application of a type “B” safeguard is justified. See Item 8 in Table 1-7. Another reason for applying a type “B” safeguard is if Space 2 in the hazardous area is vapor tight, causing the space only to give access to the Div. 2 area. To assure a complete nonhazardous environment in Space 2, windows should be bolted and the space should have vapor tight walls and roof. In Fig. 1-13, Space 3 with three walls is partially located in a Div. 1 and a Div. 2 area. In this case, the worst condition must prevail which is that the space gives access to a Div. 1 area. Therefore, Space 3 must be classified Div. 1, if not

122 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres ventilated, and Div. 2 if ventilated. See Items 2 and 3 in Table 1-7. However, Space 3 must also be classified Div. 1, if the release of flammable materials in the Div. 1 area is continuous. See the exception in Item 3 in Table 1-7.

Table 1-7. Roofed Spaces in Hazardous Locations

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C.

123

NONHAZARDOUS SPACES ABOVE OR BELOW HAZARDOUS AREAS

As explained in Sec. B, roofed spaces without a source of hazard located in a hazardous area can be classified nonhazardous if they are provided with four walls, sufficient ventilation, and a suitable safeguard. Also explained in Sec. B are the two types of safeguards that are applicable for a nonhazardous location. One that prevents ventilation from breaking down (type “A”) and one that sounds an alarm when there is a ventilation failure (type “B”). The question is whether nonhazardous classified spaces also need to be provided with a safeguard if they are located above or below a hazardous area. The answer is yes. Their presence and type depends on the vapor density of the flammable material and whether the nonhazardous space with respect to the vapor density is located above or below the hazardous area. Of course, if the floor and ceiling construction of the nonhazardous space above or below the hazardous area is gas and vapor tight, no safeguards are necessary. However, since this is not generally the case, safeguards must be applied because ignitable gases or vapors may enter the nonhazardous space should the ventilation in the nonhazardous space break down. Table 1-8 lists a number of conditions under which a nonhazardous space should be provided with a safeguard. According to Item 1 of Table 1-8, a type “A” safeguard is required if the nonhazardous space is located above a Div. 1 area in which the flammable product is lighter than air. The reason for a type “A” safeguard is the greater danger to which the nonhazardous space is subjected. The type of danger that can be expected in a hazardous location that needs to be classified Div. 1 is as follows: 1. The source of hazard in the Div. 1 location must be open. 2. The source of hazard in the Div. 1 location is closed, but frequently repaired, operated, or worked on. 3. The closed source of hazard in the Div. 1 location is not sufficiently ventilated. Condition 1 and 2 normally occur outdoors. Condition 3 does not occur in the outdoor location because there is sufficient natural ventilation which allows the outdoor hazardous area to be classified Div. 2. On the other hand, conditions 1, 2, and 3 can exist in a Div. 1 location that is enclosed. Whereas, in outdoor locations the danger of condition 1 generally is limited to small Div. 1 areas (except when the release of flammable material is continuous and in large quantities), the danger of condition 1 cannot be limited to small hazardous areas if the hazardous area is enclosed and not ventilated.

124 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table 1-8. Nonhazardous Spaces Above or Below Hazardous Areas

The application of a type “A” safeguard, therefore, is justified if condition 1 applies in a non-ventilated enclosed Div. 1 area. However, if conditions 2 or 3 should exist in the enclosed Div. 1 area, then an exception may be applied which allows the use of a type “B” safeguard instead of a type “A” safeguard. Although a type “A” safeguard provides much greater safety for a nonhazardous space, a type “B” safeguard may be considered if conditions 2 and 3 apply. This consideration is based on the fact that if a flammable gas or vapor is airborne in an enclosed Div. 1 location, the outage or failure of ventilation in the nonhazardous space above or below the hazardous location may not necessarily occur at the same time and visa versa. A nonhazardous space located above a Div. 2 hazardous area requires a type “B” safeguard as shown in Item 5 in Table 1-8. The reason for the type “B” safeguard is the remote danger to which the nonhazardous space is subjected. The remote danger is based on the fact that the enclosed hazardous location below the nonhazardous location is required to be ventilated. Simultaneous failure of two

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ventilating systems is considered quite remote and, since their failure may not occur at the same time as the failure of the source of hazard, the nonhazardous space is relatively safe and, therefore, need only to be provided with a type “B” safeguard. If, on the other hand, the vapor density of a flammable product is greater than 0.75 and the nonhazardous space is located below a Div. 1 or Div. 2 area, the same conditions will apply as when the vapor density is less than 0.75 (see Items 4 and 8 in Table 1-8). Although flammable gases or vapors may enter a nonhazardous space, if the floor and ceiling construction is not gas or vapor tight, safeguards are not necessary (if the vapor density is greater than 0.75, and the nonhazardous space is located above the hazardous area, as shown in Items 2 and 6). Neither sufficient ventilation nor a suitable safeguard is necessary if the vapor density is less than 0.75 and the nonhazardous area is located below the hazardous, as shown in Items 3 and 7.

D.

SPACES GIVING ACCESS TO HAZARDOUS AREAS

Roofed spaces which do not house a source of hazard are not required to be classified, except as follows: if roofed spaces without a source of hazard are located in a hazardous area, or give access to a hazardous area, they must be classified. This section will mainly discuss the classification of roofed spaces with three walls giving access to a hazardous area. The classification of three-wall spaces will depend on the type of release of flammable materials in the outdoor hazardous area. Therefore, it is necessary that first this type of release of flammable materials in the hazardous area be determined before the classification of the three-wall space can be established. For example, the release of flammable materials in an outdoor hazardous area can either be: 1) continuous from open sources of hazard, or 2) frequent from closed source of hazard as a result of repairs and maintenance, or 3) the release can be occasional from closed sources of hazard as a result of breakdowns, malfunctions of equipment, or due to leakage. The size of the hazardous area required for each of the above conditions are dependent on the distance a flammable material in the hazardous area must travel to reach safe concentrations. (The “traveling distance” is the distance between the point at which the flammable material is released and a point at which the flammable material will reach nonhazardous concentrations.) The traveling distance in turn is influenced by the quantity of flammable materials being released and wind conditions. Only small and large quantities of flammable materials being released are considered. Small quantities of flammable materials will generally reach safe concentration at a short distance from the point of release. It is clear, therefore, that the size of the hazardous area will be small if small quantities of flammable material are released. The size must be large if large quantities are released.

126 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Hazardous areas are required to be classified Div. 1 if the release in the Div. 1 area is continuous or frequent and Div. 2, if the release is occasional. Larger quantities will generally reach safe concentration farther away from the point of release; and therefore will cover larger Div. 1 areas. Large open process plants for example, are capable of producing large quantities of flammable materials under normal operating conditions and because of this they require large Div. 1 areas. Each Div. 1 area is normally surrounded by a Div. 2 zone called a “transition zone.” Wind conditions will also influence the size of the hazardous area outdoors. Normally, natural ventilation is capable of rapidly diluting and dispersing flammable materials in the air, and because of this, the size of the hazardous area is required to be smaller than without natural ventilation. Therefore, it is important that wind conditions also be taken into consideration when the size of a Div. 1 area outdoors must be established. Wind conditions will normally cause the flammable materials to reach safe concentrations closer to the point of release. The co-existence of quantity of release and natural ventilation will not only have an impact on the size of the hazardous area, but also on the classification of a three-wall space. For example, if a three-wall space gives access to a Div. 2 transition zone and small quantities of flammable materials are released in the associated Div. 1 area, the basic rule requires that the classification of the threewall space needs to be Div. 1, if the space is not sufficiently ventilated, and Div. 2, if sufficiently ventilated. On the other hand, if large quantities of flammable materials are released, the required Div. 1 area will be larger which causes the three-wall space to give access to the larger Div. 1 area. Also in this case, the basic rule will apply (i.e., the space needs to be classified Div. 1, if it is not sufficiently ventilated and Div. 2, if sufficiently ventilated). However, an exception to the basic rule requires that, although the three-wall space is sufficiently ventilated, the space must be classified Div. 1, if the release of flammable materials in the Div. 1 area is “continuous.” (See Item 3 in Table 1-7.) A continuous release of flammable vapors, for example, may exist in process plants in which a number of small open process equipment is located. Where a single small open process equipment normally will produce small quantities of flammable vapors, process plants with a number of small open process equipment may produce a combined large quantity of flammable material. However, if the release in the Div. 1 area is “frequent,” the basic rule will again apply (i.e., the three-wall space may be classified Div. 2, if it is sufficiently ventilated and Div. 1, if not sufficiently ventilated). These conditions are clearly shown in Fig. 1-14A. For example, small quantities of flammable materials released continuously from small process equipment will reach a nonhazardous concentration at boundary “A” in Fig. 1-14A. Large quantities released continuously from a number of open process equipment will reach a nonhazardous concentration at boundary “B” beyond “A” as shown in Fig. 1-14A. In the first case, the three-wall space gives access to a Div. 2 transition zone and in the second case, the space will give access to an extended Div. 1 area.

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The question may be raised as to why a three-wall space may be classified Div. 2 if the release is frequent. The answer is the different time frames of two different events. For example, during a mechanical ventilation outage in a threewall space, the three-wall space is not subjected to danger if there is no release of flammable materials at the same time and visa versa. Although there is still the possibility that both events could take place at the same time it is considered a remote possibility. This remote possibility allows the space to be classified Div. 2. The above conditions are not considered for spaces with less than three walls. Spaces with less than three walls must have the same classification as the classification of the surrounding hazardous area. This is because these types of spaces are considered part of the hazardous environment. Such spaces therefore, must be classified Div. 1 if they give access to a Div. 1 area and they must be classified Div. 2 if they give access to a Div. 2 area. These spaces are also considered sufficiently ventilated by natural ventilation. In Fig. 1-14 there are eight three-wall spaces shown; Number 1 and Number 2. Only four of the eight spaces contain one or more sources of hazard. Also shown in Fig. 1-14 are two hazardous boundaries; one of 50 ft and one of 100 ft, to which a three-wall space gives access. In Fig.1-14-A, Space 2 needs no classification since it does not give access to a hazardous area. Because this space doesn’t give access to a hazardous area it does not have to be ventilated for it to be classified nonhazardous. In Fig. 1-14-B, Space 2 must be classified Div. 1 for the following two reasons: 1) Space 2 gives access to a hazardous area, and 2) Space 2 is not sufficiently ventilated. In Fig. 1-14-C, Space 2 gives access to a hazardous area. In this case, Space 2 is sufficiently ventilated. And because of the sufficient ventilation, Space 2 may be classified Div. 2 provided the flammable materials are not continuously released. See Items 3 and 7 in Table 1-7. In Fig. 1-14-D, Space 2 may or may not have to be classified. Its classification depends on whether the opening of Space 2 will give access to the hazardous area or not. If Space 2 is provided with Door 1, it must be classified because Door 1 gives access to the hazardous area. In this case, where Space 2 is sufficiently ventilated, it may be classified Div. 2 if the release of flammable materials is not continuous. If Space 2 is provided only with Door 2, it does not have to be classified or ventilated because it does not give access to a hazardous area. There are three conditions under which Space 2 giving access to a hazardous area can be classified nonhazardous: 1. If Space 2 has four vapor tight walls. 2. If Space 2 is sufficiently ventilated. 3. The ventilating system is provided with a suitable safeguard.

128 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-14. Spaces adjacent to hazardous locations.

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Figure 1-14A. Spaces adjacent to hazardous locations.

The type of safeguard to be applied depends on whether the opening of the space will give access to a Div. 1 or Div. 2 hazardous area. If Space 2 gives access to a Div. 1 area, in which the release of flammable material is continuous, the safeguard must be type “A.” If it gives access to a Div. 2 area, or to a Div.1 area in which the release of flammable materials is other than continuous, the safeguard may be type “B.” For types of safeguards, refer to Sec. H in Ch. 6.

Chapter 5 The Degree of Explosion Danger for NEC Class II Locations

A.

GENERAL

Establishing the degree of explosion danger in NEC Class II locations has basically the same concept of classification as for NEC Class I locations. Hazardous areas for Class II locations, as defined by the National Electrical Code, are areas in which sources of hazard exist that may produce combustible dust from products grouped under E, F, and G. This chapter will discuss the explosion hazard of coal and coke dust covered in group F only. Although coal and coke dust may be electrically conductive, this condition is not considered. The NEC also divides the explosion hazard for coal and coke dust into two broad ranges, i.e., Div. 1 which is the dangerous condition, and Div. 2 which is the remotely dangerous condition. The requirements for a Div. 1 and a Div. 2 location freely interpreted from Article 500-8 of the NEC are discussed in the following sections.

A.1. Division 1 Locations A location is considered Div. 1 if: 1. Under normal operating conditions, combustible dust is in the air in sufficient quantities to produce an explosive mixture (i.e., above the LEL). 130

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2. A mechanical failure or abnormal operation of the process equipment might produce an explosive mixture in the atmosphere, and at the same time, produce a source of ignition through simultaneous failure of electrical equipment or the operation of protective devices, or other causes that may initiate ignition sparks.

A.2. Division 2 Locations A location is considered Div. 2 if: 1. Combustible dust will be in suspension in the air under normal operating conditions, but not in quantities sufficient to produce an explosive or ignitable mixture (i.e., too lean). 2. Combustible dust accumulations or deposits are too small to interfere with the normal operation of electrical equipment or other apparatus (i.e., allows sufficient heat dissipation). 3. Combustible dust is in suspension in the air as a result of infrequent malfunction or handling of process equipment, which may interfere with safe heat dissipation of electrical equipment or which may ignite the dust because of abnormal operation or failure of electrical equipment.

B.

DETAILED REQUIREMENTS FOR NEC CLASS II, DIVISION 1 LOCATIONS

In analyzing the requirements for a Div. 1 location, the statements in Items 1 and 2 of Sec. A.1 are quite clear. A location must be considered dangerous when coal or coke dust exists in the atmosphere in sufficient quantities as a result of normal operation. “Under normal operation” means a continuous production of coal or coke dust. For coal or coke dust to have a quantity which is ignitable, there should be at least a quantity of 0.05 oz per cubic foot of air. Any concentration in excess of 0.05 is considered explosive. When these conditions will or can exist, the location must be classified Div. 1. The location must also be classified Div. 1 if ignitable coal or coke dust can be produced as a result of failure or malfunction of the process equipment, and the failure or malfunction of the process equipment can also cause a simultaneous failure of associated electrical equipment which may produce arcs or sparks. If such a condition can exist, the location must be classified Div. 1. If sufficient quantities of coal or coke dust are released to the atmosphere at the same time that a failure of electrical equipment or operation of equipment which produces arcs or sparks may occur, an explosion will most likely result.

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C.

DETAILED REQUIREMENTS FOR NEC CLASS II, DIVISION 2 LOCATIONS

The basic concept of the Div. 2 classification as required by Sec. A.2, Item 1, is the acceptance of coal or coke dust in the atmosphere as long as the concentration is below the lower explosive limit (LEL). If coal or coke dust is below its LEL, it cannot be ignited. This means that the quantity of coal or coke dust per cubic foot of air is so thinly spread that no ignition is possible. It also means that the accumulation or deposits of coal or coke dust may normally be too small to interfere with the operation of electrical equipment or other apparatus as stated in Item 2 of Sec. A.2. However, if that is so, the question may arise as to why then bother with classification of the location. If no danger is expected from the coal or coke dust, then why is it necessary to classify the location Div. 2? The answer to this question is that coal or coke dust concentrations in the air may always grow to larger quantities, which means that the quantity per cubic foot of air may reach 0.05 oz or more. Also, accumulations of coal or coke dust blown in the air by sudden air movement may reach quantities in excess of 0.05 oz per cubic foot of air. These reasons, therefore, justify the classification of the location, and, because of this, dust accumulations from dust thrown in the air as a result of breakdown, malfunction, or handling of process equipment could also be ignited by abnormal operation or failure of electrical or other equipment. In other words, ignition of coal or coke dust could take place within an electric motor if the coal or coke dust is not prevented from entering the electric motor. Additionally, it might interfere with safe heat dissipation and might be ignited by the heat or by sparks caused by an insulation failure of the motor winding. If such a condition can exist, the location, according to Item 3, Sec. A.2, must be classified Div. 2. The following summarizes the requirements for classifying an NEC Class II location Div. 2. If process equipment under normal operating conditions (instantly, or after a given time of operation) (a) is capable of producing quantities of coal or coke dust to the atmosphere which are too small to be ignited, or (b) can have dust accumulations which could be ignited when there is not sufficient heat dissipation, or (c) interferes with the normal operation of electrical or other equipment or by the failure or abnormal operation of electrical or other equipment, the location is required to be classified Div. 2. Combustible dust will not exist in the air when the process equipment is dust tight and will remain dust tight for its entire life. Dust-tight process equipment is supposed to contain dust for as long as it is in operation. Generally, this is not true for the dynamic type of process equipment such as crushers and pulverizers. Eventually, this equipment will start to leak due to wear, vibration, inadequate maintenance, or other causes.

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Dynamic process equipment that is labeled dust tight may only remain dust tight for the first year of operation and may start to leak after that time or may even start leaking sooner. Therefore, although some process equipment is labeled “dust tight,” it may eventually reach a point where it will leak small quantities of dust into the atmosphere. These small quantities may grow to large quantities of dust when there is insufficient maintenance. Therefore, when dealing with dust tight or essentially dust-tight process equipment of the dynamic-type they must be regarded as sources of hazard that are capable of producing coal or coke dust when they operate.

D.

THE DEGREE OF EXPLOSION DANGER IN FOSSIL POWER PLANTS

D.1. Coal Fuel Unloading Systems Where coal is used as fuel for a fossil power plant, the coal fuel unloading system or car dumper is considered the initial stage of the fuel supply. The coal arriving at the power plant in railroad cars is unloaded into large hoppers as shown in Fig. 1-15 and the solid fuel is further transported via a number of belts to its final destination. The fuel unloading process creates large clouds of fuel dust that may result in an explosion when ignited. Since the combustible dust from coal under unloading conditions is exposed to the atmosphere, the entire car dumper location, as shown in Fig. 1-15, must be considered Class II, Div. 1. Therefore, most of the electrical equipment required in the area must be dust ignition proof. The main feature of a dust ignition proof enclosure is that it must exclude ignitable amounts of dust. Therefore, explosion proof enclosures are not suitable for a Class II location. Switches, circuit breakers, motor controllers, etc., intended to interrupt current (arcing devices), must be provided with dust ignition proof enclosures. Electric motors shall be of the dust ignition proof type or be provided with a totally enclosed piped ventilation system approved for Class II locations, meeting temperature limitations as described in the NEC. Lighting fixtures shall also be approved for Class II, Div. 1 locations. However, the classification of the location can be reduced if a suitable dust suppression system can eliminate the production of ignitable dust. The most effective dust suppression system is a water spray that keeps the fuel and dust wet during the dumping process. An effective water spraying system for an unloading station is shown in Fig. 1-16.

134 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-15. Coal fuel unloading system.

Figure 1-16. Water spraying for unloading station.

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To make the dust suppression system most effective, the water spray must operate before the solid fuel is dumped. The dust suppression may consist of an automatically operated water spraying system which keeps the solid fuel constantly wet under unloading conditions. This automatic water spraying system may reduce the classification from a Div. 1 to a Div. 2 location or from Div. 1 to nonhazardous. A Div. 2 location can be obtained if the water spraying system covers approximately 75% of the coal dust. If 100% of the coal dust can be covered by water spraying, the area is considered nonhazardous. However, in addition to the water spraying system, it is mandatory to install suitable safeguards against system failure. If possible, it must prevent the unloading operation from commencing if spray water is not available and it must stop the fuel dumping process when the water supply suddenly stops. If these safeguards are not possible, then alternative safeguards must be considered, such as a suitable alarm system and interlocks that shut off the electrical power supply, since the unloading area with a water spraying system remains dangerous if safeguards are not provided. Safeguards installed in addition to a water spraying system that covers 75% of the dust will reduce the area to a Class II, Div. 2 location as shown in Fig. 1-16. The water spray must come on before the solid fuel is dumped. If the water spray fails to operate, the fuel should not be dumped. The same operating procedure should be followed at other locations of the coal handling system where large clouds of coal or coke dust are exposed to the atmosphere and would create an explosion hazard.

D.2. Coal Pulverizers The coal pulverizing system in a fossil power plant is considered the final stage of the fuel supply. Pulverizing equipment reduces the coal to the required fineness before it enters the boiler furnace. Different types of pulverizing units are used for reducing the coal to the required fineness, such as: 1. The “Impact Mill” which hammers the coal into fine particles by means of hammers mounted on a rotating shaft. 2. The “Ball Mill” where the coal is crushed by heavy steel balls rotating in a drum. Pulverizing units must be designed so that leakage of coal fuel is reduced to a minimum. The most commonly used pulverizer fuel system is the direct fire unit in which the fuel is pulverized near the point of use. At this point, the pulverized coal has reached a mixture of fine particles of coal and is then air swept through pipes to the furnace (see Fig.1-17). Coal dust or the gases released from freshly crushed coal is hazardous when present in the air in sufficient quantity. If a dangerous combustible mixture is created in the atmosphere, an explosion will result if the cloud of dust is ignited

136 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres (but only when it is above the LEL). Newly installed dust-tight pulverizer equipment may or may not allow the escape of combustible coal dust to the atmosphere in quantities sufficient to produce a hazardous condition (which means it is below the LEL). Therefore, it seems that a location in which pulverizer units are used is safe and could be classified nonhazardous because of the lack of explosive coal dust in the atmosphere. This is not so. Even when pulverizer units are considered dust tight, they eventually may begin to leak. As explained before, during the first year of operation the pulverizer equipment may maintain its dust tightness, but in the years following, the equipment may start to leak due to wear and handling. Pipe connections and other points in the pulverizer fuel system may eventually break down and coal dust or gases may escape through cracks, wear, or misalignment. Eventually, coal dust leaking from dust-tight pulverizer units may create a dangerous condition, either by producing ignitable amounts of coal dust in the atmosphere or by coal dust deposits growing to amounts that will interfere with safe operation of electrical equipment (which means insufficient heat dissipation). These coal dust deposits may also be thrown in suspension in the air by sudden air movements forming an ignitable dust cloud. A coal dust cloud becomes ignitable when it has reached its LEL. Proper maintenance and regular housekeeping to reduce the amount of coal dust to below the explosive limit may not be the total answer for keeping the coal dust below the lower explosive limit. An unexpected mechanical failure of process equipment or leakage may also occur after any repair or maintenance cycle. The failure or leakage may last from a few minutes to several hours before it is discovered and action is taken. For these reasons, the location for coal pulverizers cannot be considered safe. In view of this, it is not good practice to classify the location nonhazardous only because the amount of coal dust thrown in the air is too lean. It is much safer to apply a more conservative approach by classifying the location Div. 2 as shown in Fig. 1-18 and in accordance with the requirements of Article 500-8(b) of the NEC which states that even when “combustible dust” is not normally in suspension in the air in quantities sufficient to produce explosive or ignitable mixtures, the location is required to be classified Div. 2. Also, classifying a location Div. 2 is much safer than classifying a location nonhazardous. A Div. 2 classification reduces the possibility of explosion danger because, for example, the arcing devices in a location must be housed in dust-tight enclosures. Arcs or sparks produced by these dust-tight arcing devices are not capable of igniting coal dust when present in the air, nor is coal dust capable of entering the dust-tight enclosures. Since dust-tight enclosures will eliminate explosion danger, the level of safety in the location will rise. General purpose arcing devices which are permitted when the location should be classified nonhazardous are unsafe because they cannot eliminate the explosion danger. Therefore, by classifying the location Div. 2, arcing devices are required to have dust-tight enclosures.

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Figure 1-17. Direct-firing pulverizing system.

Electric motors without arcing devices do not produce arcs or sparks under normal operating conditions. Because of this, they do not have to have dust-tight enclosures as required for arcing devices. However, if these motors should have an insulation failure, an arc or spark resulting from this failure could occur. If the motor is totally enclosed (i.e., without external openings preventing coal dust from entering the motor enclosure), arcs or sparks resulting from winding failure are considered not harmful. Such a motor must be used if the location is classified Div. 2, whereas, when the location is classified nonhazardous, all types of electric motors can be used. Electric motors permitted in a Class II Div. 2 location must be of the totally enclosed fan cooled type, totally enclosed non-fan cooled type, enclosed pipe ventilated type, or be of the dust ignition-proof type, for which the maximum full load external temperature shall not exceed 150°C when operating in free air (not dust blanketed). An exception to the above is that if the authority having jurisdiction believes that accumulation of nonconductive, nonabrasive dust will be moderate, and the machines can easily be reached for routine cleaning and proper regular housekeeping, standard open type machines without arcing devices or with dusttight arcing devices may also be used. Preventive cleaning activities applied to electric motors will reduce the possibility of explosion danger and will not interfere with safe operations.

138 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-18. Pulverizer.

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Therefore, the only good reason for deciding on open machines is if there is a certainty that accumulations of coal dust are moderate and proper and regular housekeeping will be maintained. However, the irony of the exception is that moderate accumulations of coal dust and proper and regular housekeeping can only be determined by actual observation after completion of the design and construction of the plant. If it has been decided during the design stage of the plant to use totally enclosed machines, it is too late after it has been designed and constructed to decide otherwise. Therefore, since it is not known during the design stage of the plant how well the cleaning activities are and how moderate the accumulation of coal dust will be, it is much safer to rely on the totally enclosed machines which give much better protection against the entry of coal dust than on open machines.

D.3. Crusher Houses Coal dust is also produced in reclaim hoppers, storage silos, and crusher houses, and may cause dust explosions if the cloud of dust has a concentration above the LEL. The crusher house is the most potential source of hazard of the three. With sufficient free ventilation inside the crusher house, the area can be classified Div. 2, as shown in Fig. 1-19.

140 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-19. Crusher house.

Chapter 6 Ventilation Requirements

A.

GENERAL

When there is no air movement, a flammable gas or vapor released to the atmosphere will spread in all directions. With air movement, the flammable gas or vapor will move in one particular direction, thereby covering a large distance, when forced by air currents in that direction. As the flammable gas or vapor travels away from the point of release, its ignitable concentration will diminish because of dilution. Eventually it will reach a concentration below the lower explosive limit (LEL). The flow rate of ventilating air will influence the distance and the time at which a flammable gas or vapor will enter its explosion range and reach a nonhazardous concentration. For example, if a flammable gas accidentally escapes from a sufficiently ventilated confinement, its ignitable range will be small and close to the point of release, as shown in Fig. 1-19A-A. If ignition should take place within this range, the explosion force will be small. If the gas escapes from a confinement, which is insufficiently ventilated, the ignitable range will move away from the point of release and be much larger, as shown in Fig. 1-19A-B. In this case, the explosion force is much greater when ignition of the flammable gas or vapor in the ignition range takes place. The

141

142 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres greater the flow of air, the closer the explosion range moves towards the point of release and the smaller the ignitable concentration. The smaller the flow of air, the farther the explosion range will move away, and the greater the explosion force. An airborne flammable gas or vapor will also enter its explosion range much faster if its ignition range has a high upper limit. It will take longer to enter the ignition range if the ignition range has a low upper limit.

Figure 1-19A. Explosion force magnitude versus air flow rate.

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If electrical equipment of the heat producing type is located in close proximity with a sufficiently ventilated source of hazard, an explosion hazard will exist at the source of hazard and not at some distance away. In the case where the containment is sufficiently air brushed (as shown in Fig. 1-19A-A), the concentration at the remote location is too lean for combustion. If electrical equipment is located at a distance from a containment which is not sufficiently ventilated, an explosion hazard will exist at the remote location and not at the containment. In the case where the containment is not sufficiently air brushed (as shown in Fig. 1-19A-B), the concentration at the containment is too rich for combustion. Sufficient air brushing is a condition in which the air is moving in sufficient quantities in one direction over and along side the source of hazard and is capable of diluting the flammable material in the air to below the LEL. The purpose of ventilation is to reduce the danger in a hazardous location to a lower level or to prevent a location in a hazardous area from becoming hazardous. There are two types of ventilation that are normally used for reducing the danger level in a location—natural ventilation and mechanical ventilation. Both types of ventilation must have sufficient capacity to dilute a flammable gas or vapor in the air to acceptably low concentrations. An acceptably low concentration occurs when ventilation dilutes a flammable gas or vapor to below the LEL. The selection between the two different types of ventilation generally depends on whether the ventilation is applied indoors or outdoors. The general practice is to use natural ventilation for outdoor locations, and to use mechanical ventilation for indoor locations. In providing natural or mechanical ventilation, it is important to consider the presence of obstructions. To compensate for obstructions in an indoor location, the mechanical ventilation output must be increased. For an outdoor location, the size of the hazardous area should be made larger. There are two different types of mechanical ventilation that can be used to reduce the danger level in an indoor location. One type is a “pressure fan” which pressurizes an enclosed space. The other type is an “exhaust or suction fan” which causes a negative airflow in an enclosed location. The pressure fan is normally used for a roofed space with four walls (without a source of hazard) that are required to have a nonhazardous classification. Sometimes, the pressure fan may be used for a roofed space with a source of hazard if the space has only three walls, or if the space has three walls and is without a source of hazard. Locations which have three walls cannot be made nonhazardous. The suction fan is normally used for a location with three or four walls which contains a source of hazard in order to reduce the danger level in the hazardous location to Div. 2. The vapor density of a flammable material normally plays an important role in determining the location of a suction fan. Therefore, it is crucial to know whether the flammable gas or vapor is heavier or lighter than air. A vapor density greater than 1.0 is defined as heavier than air and a vapor density of less than 0.75 is defined as lighter than air. However, a vapor density between 0.75 and 1.0 is not necessarily lighter than air, it can also be considered as heavier than air. The reason is that flammable gases or vapors with vapor densities between 0.75 and 1.0 are

144 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres usually unstable in the air. If released from their confinement, they may not rise instantly as they would if the vapor density was smaller than 0.75. Gases and vapors with densities between 0.75 and 1.0 may move over the floor first before rising. During their initial presence in the air, and when they move over the floor, the gas or vapor acts as if it is heavier than air, and when the gases or vapors begin to rise they act as lighter than air. This behavior might take place if the diluting air is not brushing the source of hazard. If the ventilating air is not brushing the source of hazard, a flammable gas or vapor with a vapor density between 0.75 and 1.0 must be considered as heavier than air. On the other hand, if the airflow is brushing the source of hazard, the gas or vapor is considered lighter than air (in this case, the gas or vapor will be caught instantly and sucked away by the moving air). As a result of these conditions, a flammable gas or vapor with a vapor density between 0.75 and 1.0 released into the air, which is not instantly caught by the moving ventilating air, will require a larger hazardous area. Acetylene gas is the only known Class I flammable material with a vapor density between 0.75 and 1.0. All other Class I flammable gases or vapors have densities less than 0.75 or greater than 0.75. The range between 0.75 and 1.0, therefore, can be deleted for any Class I gas or vapor other than acetylene. Consequently, the division between “lighter” and “heavier” than air can be conveniently drawn at 0.75. Any Class I gas or vapor with a density greater than 0.75, therefore, must be considered as heavier than air and with a vapor density below 0.75, lighter than air.

B.

NATURAL VENTILATION

The general concept for natural ventilation is that natural ventilation is considered capable of diluting flammable gases or vapors in the air to safe concentrations. Based on this favorable feature, most outdoor hazardous locations can be classified Div. 2. Wind conditions have a great impact on the traveling distance of a flammable gas or vapor in the air. The impact depends entirely upon wind velocity. The traveling distance of a flammable gas or vapor (i.e., the horizontal distance between the point at which the flammable material is airborne and the point at which the flammable material will reach a safe concentration) is inversely proportional to wind velocity. The lower the wind velocity, the greater the traveling distance and the longer it takes before an ignitable concentration of gases or vapors is diluted to safe concentrations. The higher the wind velocity, the shorter the traveling distance and the faster the dilution. In between both conditions, the distance of the traveling gas or vapor in the air will vary from small to large and this, in turn, will influence the point at which the flammable substance will reach a nonhazardous concentration.

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Although the actual traveling distance of a flammable gas or vapor in the air can be calculated, a quick solution cannot be obtained due to the complexity of the calculation. Factors such as emission rates, wind velocities, crosswinds, and terrain conditions must be known before the calculation can be applied. The emission rate is a function of pressure in the system and the size of the rupture opening. Emission rates therefore must be assumed. Crosswinds may or may not exist and their existence and speeds are difficult to predict. Calculations of the gas or vapor traveling distance based on these unknown factors therefore become far from simple. Wind conditions, on the other hand, are more realistic in the application, provided their behaviors are known. However, wind conditions alone are not sufficient to determine the traveling distance of a flammable gas or vapor in the air. Some of the wind behaviors are shown in Table 1-9. For given quantities of flammable gases or vapors in the air, “moderate” wind conditions tend to shorten the traveling distance of a flammable substance in the air. “Light” and “very light” wind conditions on the other hand will allow the traveling distance to be longer for the same given quantity of flammable gases or vapors in the air. Between “light” and “moderate” wind conditions, the traveling distance tends to be more dependent on the emission rate and the quantity of the flammable gas or vapor in the air. Under ideal conditions, when winds are “light” or “moderate” and are blowing over flat terrain without crosswinds, it is possible to determine the traveling distance of a flammable gas or vapor in the air fairly accurately. Unfortunately, the actual conditions in outdoor locations are usually not ideal because of the existence of crosswinds and wind changes. When only a steady wind velocity should exist, a flammable gas or vapor in the air will travel a certain distance before reaching safe concentrations. The traveling distance will be longer if wind changes direction. Crosswinds will make the distance shorter. As a result of these conditions, safe concentrations will be reached farther away from the source or at a much shorter distance (i.e., closer to the source of hazard). Therefore, the length of the traveling distance outdoors is dictated by these conditions. Since the changes in wind direction and the existence of crosswinds are generally unknown, they are normally not considered. Only steady state wind velocities are considered in combination with other factors to determine the traveling distance of a flammable gas or vapor in the air. These other factors are: 1) the size of the source of hazard, 2) the system pressure, 3) the operating mode, and most important, 4) whether the quantity of the flammable gas or vapor released to the atmosphere is small or large. Steady state wind conditions can be obtained from Table 1-9. This type of information will greatly simplify the determination of the traveling distance and that in turn will simplify the selection of a suitable boundary size. The application of the additional factors is explained in the following example. Assume a small closed source of hazard breaks down when it is processing a flammable substance under low pressure. What is the traveling distance of the

146 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres escaping vapor expressed in terms of “short” or “long,” when wind conditions in that area are unknown? And, what is the required size of the hazardous boundary? First, determine the possible size of the rupture opening in terms of “small” and “large.” The rupture opening cannot be large but must be considered to be small because of the following. The source of hazard is small and the pressure in the system is low. Therefore, the rupture opening is most likely small. With the small opening and low pressure, the emission rate is also considered low. In view of these considerations, it is concluded that small quantities of flammable vapors are released to the atmosphere. Next, consider two wind conditions from Table 1-9. Since actual wind conditions are unknown, consider only the worst wind condition without wind changes and crosswinds. Select two conditions, light winds with speeds of 4–7 miles per hour, and moderate winds with speeds of 13–18 miles per hour. As stated before, light winds will allow longer traveling distances than moderate winds. Consider, of the two, the worst possible wind condition, which is the light wind. Light winds allow a flammable gas or vapor to travel a longer distance. This longer distance, however, is defeated by the low emission rate and the quantity of the flammable vapors in the air which is expected to be small. A low emission rate and small quantities of flammable vapors in the air allow faster dilution than when the emission rate is high and the quantity of the flammable vapors in the air is large. Faster dilution means that the point of safe concentration is close to the point of release. As a result of this condition, the actual traveling distance of the flammable vapor is short. To complete the evaluation, another factor must also be known. That is, what type of source of hazard is involved? Is the source of hazard of the static or dynamic type? The type plays an important role in establishing the size of the hazardous area because it will influence the extent of the hazardous area.

Table 1-9. Wind Conditions Type of Wind

Miles per Hour

Wind Effects Observed on Land

Very Light

1–3

Direction of wind shown by smoke drift, but not by wind vanes

Light

4–7

Leaves rustle, ordinary vane moved by wind

Gentle

8–12

Leaves and small twigs in motion, wind extends light flag.

Moderate

13–18

Raises dust, loose paper, small branches.

Fresh

19–24

Small trees in leaf begin to sway, crested wavelets form on inland waters.

Compiled by U. S. Weather Bureau

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147

For example, if the source of hazard should consist of a storage container that leaks flammable liquid, the vapor from the liquid may travel less than 3 ft before reaching safe concentrations. On the other hand, rotary equipment that operates under pressure may leak a flammable vapor that travels more than 3 ft before reaching safe concentrations. Under the first condition, the boundary size needs to be at least 3 ft and under the second condition, it needs to be at least 5 ft or even 10 ft. Fortunately, it is not important to know the actual traveling distance. In evaluating a particular situation, it is only important to determine the type and size of the source of hazard, its operating mode, pressure in the system, flammability class, and vapor density. It is not necessary to determine vapor traveling distances or wind velocities. All that is required for determining the size of the boundary for a particular situation are the features mentioned above and whether ventilation is present in sufficient quantity. With this type of information, the required boundary size can be obtained directly from the subtables in Table 1-4.

C.

MECHANICAL VENTILATION

C.1. Pressure Fans A pressure fan is normally required for a location that is totally enclosed and does not contain a source of hazard. If the location is totally enclosed, the pressure in the enclosed space will prevent flammable gases or vapors from entering the space. Such a location is allowed to be classified nonhazardous if provided with a suitable safeguard. A pressure fan can only be applied if the air intake for the totally enclosed space is located in a nonhazardous area. The air pressure is considered “sufficient” if flammable vapors or gases are prevented from entering the enclosed space. This is accomplished when the pressure fan for the totally enclosed space produces an even pressure of not less than 0.1 inch of water above atmospheric pressure but not more than 0.25 inch of water with all openings closed. A pressure greater than 0.25 inch of water may make it difficult to open doors. To make the pressurized system reliable, safeguards must be provided in addition to the pressure fan. The normal practice is to use pressure fans for control rooms and switch-rooms, or other 4-wall locations where a nonhazardous classification must be maintained. Normally, locations containing sources of hazard are not classified nonhazardous. On the other hand, enclosed locations containing sources of hazard are allowed to be classified nonhazardous if the sources of hazard are applied with canopy fume hoods (as shown in Figs. 1-10-A and 1-10-C). Enclosed locations containing small sources of hazard, which have a probability factor below 10, are allowed to be classified partially nonhazardous. (For probability factors see Table 1-6.)

148 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

C.2. Suction Fans While the pressure fan will prevent flammable gases or vapors from entering an enclosed space, the purpose of the suction fan is to dilute and remove the flammable gases and vapors from the space. The discharge from the location shall be to a safe exterior location without recirculation of the exhaust air. The fan capacity must be such that the moving air is capable of diluting the flammable gases or vapors in the air to sufficiently low levels. For locations containing flammable liquids operating at temperatures above flash point, suction ventilation shall be considered sufficient if vapor air mixtures are diluted to concentrations below one-quarter of the LEL of the flammable material. This is the general rule for all classes of flammable liquid with temperatures above flash point. However, there is a difference in dilution requirements for flammable materials which are continuously released from open process equipment or which are incidentally or accidentally released from closed process equipment. The general rule for open process equipment is to dilute flammable vapors to below one-quarter of the LEL. For closed process equipment, the dilution must be greater than below one-quarter of the LEL. Normally, the release of flammable materials from open process equipment takes place in enclosures or chambers in which the emitted flammable concentrations are prevented from causing a health hazard to personnel. In these cases, it is necessary that the rate or vapor generation in the enclosure be established on the basis of actual evaporation data. The incidental release of flammable materials from closed process equipment normally occurs during maintenance and repair cycles while the accidental escape of the flammable material takes place during a failure of the process equipment. The magnitude of failure in closed process equipment normally depends on the pressure and size of the process equipment. The probability that mechanical failure may occur is much greater if the system is under pressure and temperatures are high. Under these conditions, the emission rate is also much higher. Most types of failures are small and are normally the result of failures of flange-gaskets, valve packing, pump seals, and process drains. Magnitudes of leaks of any one of them are generally expressed in gallons per minute or pounds per minute. Thereby, it is assumed that the evaporation of flammable liquid is instant and 100%. Although the preferred rule is that all closed process equipment with flammable liquids with temperatures above flash point be diluted to below one-quarter of the LEL, it is not necessarily required for Class II and III flammable liquids. For example, storage rooms in which Class II and III liquids are occasionally transferred from one container to another or are incidentally dispensed, need not have a dilution below onequarter of the LEL. The vapors released from these liquids may be diluted to slightly below the LEL which ranges approximately up to 90% of the LEL. The less stringent requirement for a Class II and III liquid is based on conditions of low system pressure, low temperature, and short operating times.

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149

Fumes released under these conditions are less dangerous because they are “incidental,” small, and dilute rapidly. However, the question may arise as to what difference it makes when the dilution is slightly below the LEL or below 25% of the LEL as far as explosion danger is concerned. The only advantage for requiring a dilution of flammable gases or vapors to below one-quarter of the LEL is that a 25% margin is capable of compensating for unfavorable environmental conditions such as some loss in air flow and reduction in dilution. Under favorable environmental conditions, a fan built with a 25% safety margin (i.e., a fan with 4 times greater output), will produce a dilution of flammable gases or vapors in the air to below one-quarter of the LEL. Under unfavorable environmental conditions such a fan will still be capable of producing sufficient dilution, that is, between the LEL and one-quarter of the LEL, but not below one-quarter of the LEL. This performance cannot be expected from a fan that is not provided with a 25% safety margin. When a 25% safety margin is not applied, the fan is built to produce a dilution slightly below the LEL. However, such a fan may not be capable of diluting the flammable gas or vapor to slightly below the LEL when the conditions are not favorable. For example, when there are too many obstructions in the path between the fan and the air intake opening, or when the path is too far away from the source of hazard, or when the fan is not properly located with respect to the location of the air intake opening, some loss of air flow will occur. This loss is compensated for when the fan is provided with a 25% margin. Fans with 25% built-in safety margins are more costly, but their higher costs are worth spending when flammable products are involved. Factors such as atmospheric pressure and elevated temperatures will also have an impact on the dilution of a flammable gas or vapor. For example, at elevated temperatures, the LEL of the flammable product decreases. This decrease makes the LEL of the flammable product smaller. As a result of this, a fan built without a safety margin will not be capable of diluting a flammable gas or vapor below the LEL because the point of safe dilution will be above the smaller LEL. However, a fan with a 25% safety margin is still capable of diluting the flammable gases or vapors to below the smaller LEL. Since elevated temperatures will lower the LEL, it is necessary to correct the volume of air. The temperature borderline for the correction is 250°F. Any temperature above 250°F requires a correction factor for the volume of air produced. For example, if the required volume of air for reaching a nonhazardous concentration to slightly below the LEL is 1.0 Pu CFM for temperatures of 250°F or less, a correction factor of 0.7 must be applied for any temperature above 250°F. The required volume of air is then, 1.0 ⇒ 0.7 = 1.43 CFM and not 1.0 CFM. The density of air also plays an important role in calculating the required volume of air. If the density is lowered, a correction factor must be applied. The weight of 1.0 ft3 of air is 0.075 lbs at 70°F. If the temperature is in excess of 70°F, the density of air changes and a correction factor must be applied to the volume of

150 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres air in addition to the 0.7 correction factor. For example, if the actual process temperature is 350°F, the weight of air per cubic foot is less and needs to be corrected. If the weight of air per cubic foot at 70°F is 1.0 Pu, then at 350°F temperature, it is 0.654 Pu. The correction factor required is then 1 ÷ 0.654 = 1.528. This correction factor can also be calculated by the following equation:

Eq. (1)

460 + 350 = 1.528 460 + 70

The volume of air required for diluting a particular flammable vapor from an open process equipment is calculated as in the following example. Assume a process takes place in an enclosed chamber operating at 335°F, which will emit 30 gal of flammable liquid per hour. From the 30 gal of liquid per hour, 80% per gallon will evaporate in 60 min. Dilution ventilation is required for removing and reducing the flammable vapor concentration to below 25% of the LEL. The effective quantity of liquid which will evaporate, reduces to 24 gal per hour. If it is also assumed that the explosion limits of the flammable vapor in air ranges from 1.25–10% at 70°F with a ratio of 98.75% of air and 1.25% of vapor, the vapor-air mixture is too lean to produce an explosion. The volume of vapor produced from 1 gal of solvent can be calculated from the specific gravity of the liquid and the vapor density of the vapor as follows. The cubic feet of vapor from 1 gal of liquid is:

8.33 × Sp.Gr. 0.075 × VD × LEL × C

Eq. (2)

Va =

where

Va = vapor released in cubic feet per gallon of solvent per hour 8.33 = the equivalent weight of 1 gal of water in lbs 0.075 = the weight of 1 ft3 of air in lbs at 70°F ambient temperature VD = vapor density of the solvent Sp. Gr. = the specific gravity of the solvent C = the correction factors for the LEL of the solvent vapor at elevated temperature (C = 1 for temperatures up to 250°F and C = 0.7 for above 250°F).

The density of air at 70°F is 29.92 inches of mercury atmosphere pressure and 50% relative humidity. If the specific gravity of the liquid is assumed to be 0.72 and the vapor density is 8 (which is the relative weight of a volume of vapor to the weight of an equal volume of air under the same conditions), the cubic feet of vapor from 1.0 gal of solvent is:

Chapter 6: Ventilation Requirements

Va =

Eq. (3)

151

8.33 × 0.72 = 10 cubic feet of vapor per hour 0.075 × 8

Since there are 24 effective gallons of liquid released per hour, 24 gal will produce 240 ft3 of vapor per hour or 4 ft3 per minute. The volume of air required to dilute the vapor to slightly below the LEL is 4 (100 ÷ 1.25) = 320 ft3 of air per minute. To reduce the level to below one-quarter of the LEL, the volume of air required is 4 × 320 = 1280 ft3 of air per minute. To compensate for the higher operating temperature, the following equation for correcting the air density will apply:

C=

Eq. (4) where

(460 + T 2 ) (460 + T 1)

T1

is 70°F ambient temperature

T2

is the actual dilution air temperature

If it is assumed that the vapor will not cool off and remain at a temperature of 335°F, the correction factor for air is:

Eq. (5)

C=

(460 + 335) = 1.5 (460 + 70 )

Since the temperature is above 250°F, the correction factor for the LEL is 0.7. Total correction factor is then equal to 2.143, and the total volume of air required is 2.143 × 1280 = 2742 ft3 of air per minute. If all elements are included in one equation, the outcome is also:

Eq. (6)

8.33 / 60 × 0.72 × 24 × 100 × 1.5 = 2,742 cubic feet per minute 0.075 × 8 × 1.25 × 0.25 × 0.7

As mentioned earlier, the actual concept of safeguarding building rooms and pump stations against fire and explosion hazard is quite different if the flammable materials are released incidentally from closed systems or when they accidentally escape from closed systems in building rooms or pump stations. In those cases, the emphasis should not be on the 25% rule, but on the actual ventilation-capture velocity. This capture velocity will require a more conservative approach for safeguarding rooms and pump stations against fire and explosion hazards. However, this conservatism applies only to heavier-than-air flammable gases and vapors and not to flammable materials which are lighter than air. Also crucial for building rooms and pump stations when the escaping flammable material is heavier than air, the location of the suction fan should be

152 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres placed so that it provides sufficient air movement across the floor to prevent accumulation of ignitable flammable gases or vapors. The operation of the suction fan shall be continuous although the flammable material is only released occasionally or accidentally. Therefore, Class I flammable liquids (operating above flash point) being incidentally or accidentally released to the atmosphere from closed process equipment, require a suction ventilation in building rooms and pump stations which is much greater than would be required for diluting vapor air mixture below one-quarter of the LEL. This conservative requirement applies also to Class II and III liquids, if their temperatures are far above flash point. The reason for the conservatism is that suction fans in building rooms and pump stations may produce too small quantities of suction air. It may not always be feasible to obtain sufficient air velocity along the source of hazard if the 25% requirement is considered for a possible accidental release of heavier-than-air gases and vapors. Remember that the point at which a flammable gas or vapor normally will reach sufficiently low levels is a function of quantity of airborne flammable material, its emission rate, and the flow rate of ventilating air. As stated before, the distance at which a flammable gas or vapor must travel to reach a nonhazardous concentration is inversely proportional to the flow rate of ventilating air. The greater this flow rate, the shorter the distance between the source of hazard and the point at which the gas or vapor mixture will reach a nonhazardous concentration. During the design stage of a ventilating system for a building room and pump station, three factors will generally play an important role in determining exhaust air capacity: 1.

Cost of the ventilating system.

2.

The magnitude of failure of process equipment or its components which must be guessed.

3.

The amount of air velocity required to prevent accumulation of the flammable material.

For example, the size of an exhaust fan for a 3-wall or with louvers on the 4th wall, unattended, 30 ft long, 20 ft wide and 13 ft high room needs to be 200 CFM, if a pump driven by a general purpose electric motor in the room leaks 1.85 gal/hr of ethyl ether liquid which is 0.03 gallons per minute. The exhaust fan capacity is calculated with the following equation: Eq. (7) where

Va =

8.33 × Sp. Gr. × gal/min × 100 0.075 ×VD × LEL × 0.25

Sp. Gr. = 0.71 VD = 2.56 LEL = 1.9 0.25 = 1/4 of LEL 100 = percentage air

Chapter 6: Ventilation Requirements

Eq. (8)

Va =

153

8.33 × 0.71 × 1.85 / 60 × 100 = 200 CFM 0.075 × 2.56 × 1.9 × 0.25

The air velocity “V” between make-up air inlet and exhaust air outlet is

Eq. (9)

V =

Va 200 = = 0 .7 7 feet per minute A 13 × 20

which is far too low to have sufficient capture velocity. The air velocity in the room between inlet and outlet, which is 30 ft apart, should be at least 2.5" or 0.208 ft/sec. If the 0.208 ft/sec velocity is applied (which is 12.5 ft/min), there will be sufficient capture velocity for preventing accumulation of ignitable flammable contaminants in the room. Although it is always desirable to obtain a gas or vapor concentration below one-quarter of the LEL for heavier-than-air gases or vapors, more important than this is the prevention of accumulation of an ignitable flammable material in the room. This prevention is accomplished if a minimum of 12.5 ft/min velocity is maintained at all times. The actual suction fan generation in the example should not be 200 CFM, but: 12.5 × 200 = day 3200 CFM 0.77

Consequently, to maintain a 12.5 ft/min flow rate, the fan generation for the room is far more expensive than if only the 25% rule is applied. In view of this, other means should be considered to reduce the cost of the ventilating system and still maintain sufficient airflow along the source of hazard and floor. An explosion-proof electric driver replacing the general-purpose driver could reduce the cost of the ventilating system considerably. The fan could even be eliminated all together because of the change in classification in the room (from Div. 2 to Div. 1). However, another more appropriate solution is to apply a smaller room height than the application of the actual room height of 13 ft in the example. Bear in mind that this smaller height is only applicable if flammable gases or vapors are heavier than air. It does not apply to flammable materials which are lighter than air. Since the vapor density of ethyl ether is 2.56, its fugitive vapors will move along the floor of the room and not rise to the upper region of the room. Because of this favorable condition, the 13 ft height of the room in the example should not be used for calculating air velocity. Instead, a 4 ft room height, or any suitable height, is recommended in the calculation instead of the 13 ft height. However, this procedure is recommended only as long as the inlets and outlets are located near the floor, normally 12" above finished floor.

154 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres It follows then that the required fan capacity can be reduced to 12.5 (4 × 20) = 1,000 CFM. This is equivalent to Eq. (10)

1000 × 60 = 25 air changes per hour 4 × 20 × 30

A fan capacity of 200 CFM for the subject example is considered not to have sufficient capture velocity. A 3,200 CFM fan capacity may be too expensive and certainly not economically justified. A 1,000 CFM fan capacity is just right. It still provides ample dilution below the LEL of the flammable material and also prevents accumulation. Without one-quarter of the LEL, Va = 50 CFM. For 1,000 CFM, the dilution below the LEL = 50/1,000 × 100 = 5%. To produce 1,000 CFM with 5% LEL, substitute 0.25 in Eq. (7) with 0.05. Raising the fan capacity from 200 CFM to 1,000 CFM, which is 5 times higher, provides sufficient ventilation for the subject room of 30 × 20 × 13 ft. One should not go lower than 5 times the initial calculated suction fan capacity, although the adopted method is 2(¼) LEL which requires 800 CFM. The suction fan should be installed on the roof of the building with a stack 10 ft high and a 14" vertical duct along the wall in the room. Since the dilution is below 10% LEL, no sprinklers are required in the duct system. Assuming a static pressure of 3/4 inches of water gauge, the electric motor for the fan should be rated 1/2 hp. A 2 hp fan motor and 22" duct for the same static pressure is required for 3,200 CFM. However, since air velocity decreases inversely with the square of the distance, it is important to establish the distance x between source of hazard and 14" duct if a flow rate of 12.5 FPM and 1,000 CFM must be maintained. If V = 12.5 FPM, x is the square root of 1,000 divided by 12.5 π = 5 ft. If x is 15 ft, Va needs to be 12.5 π (15)2 = 8831 CFM. Both calculations are based on 1/4 of a full sphere area with its center point as the air vent inlet. Leak detection equipment may be considered in the room as an additional safety precaution. A practical substitute, which is sometimes used, is to apply a minimum ventilation rate of 1 CFM for 1 ft2 of floor area. According to this principle, the fan generation for the example should be 20 × 30 = 600 CFM (which is more than 200 CFM, but less than the recommended ventilation rate). Keep in mind that ventilation rates based on this principle may not be sufficient or may not be economically justified. Guessing how much leakage can occur during a breakdown becomes insignificant, if the 12.5 ft/min velocity requirement is used. Also, the facts that during a leak of flammable material, the general-purpose electric driver may not breakdown at the same time and, if the electric driver should break down, there may be no leaks, are features that reduce explosion hazard considerably. It is this favorable concept that makes the explosion hazard so conveniently remote. Explosion hazard can even be further reduced if the ventilating air flows in a direction from ignition source towards process equipment and not visa versa. However,

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155

since the probability of an explosion in such a location is conveniently remote, it makes little difference where the ignition source is located with respect to the source of hazard. Suction ventilation does not always have to dilute a flammable gas or vapor to below the LEL if the vapor density of the gas of vapor is less than 0.75. A gas or vapor with a density below 0.75 will rise quickly by itself when airborne. Economically, it might be desirable to have just enough ventilation to accelerate the upward flow of the gas or vapor when airborne. This is particularly true for building rooms with canopy fume hoods. Even an opening in the ceiling/roof may be sufficient if the ceiling area is small and low. However, having only a ceiling/ roof opening has some disadvantages. The removal of lighter-than-air fugitive gases or vapors from the room is rather slow and may introduce accumulation at the ceiling. Forced, low-capacity ventilation, therefore, is more appropriate and safer than just an opening in the ceiling/roof. If the electric driver of the suction fan is located in the air stream in which lighter-than-air flammable gas will be present, the driver should be explosion proof. If the electric driver is not located in the air stream, but outside the fan housing on the roof of a building, the driver does not need to be explosion-proof. However, a vertical stack of a minimum of 7 ft in height may be required as indicated in Fig. 1-8 in Part 1. The fan blades and its enclosure should be of sparkproof construction. Although aluminum impellers are widely used, it is known that ignition may take place if the impellers inadvertently come in contact with rusted steel members of the fan housing. Although the explosion hazard can be reduced for a building room by diluting a heavier-than-air flammable gas or vapor concentration to below 5% of the LEL, the concentration may still cause a health hazard for personnel in the location. To prevent personnel from exposure to the toxicity of a contaminant in the air, the contaminant must be diluted even more. Therefore, it may be necessary to dilute a toxic gas or vapor concentration to far below 5% of the LEL. When a health hazard could exist in a room, the safety margin should be below the threshold limit value (TLV) of the toxic material. Threshold limit values are safe concentrations of airborne toxic particles. They are listed under time weighted average concentration (TWA) for a normal eight hour workday. Threshold limit value’s are generally safe for personnel who may be continuously exposed to airborne toxic particles, workday after workday, without adverse health effects. The data available on TLV’s is established by experimental human and animal studies. However, there are conditions in which individuals who are hyper susceptible to some chemicals, are not adequately protected from adverse health effects when toxic concentrations are at or below the TLV. Threshold limit value’s are established in terms of parts per million (PPM) and are expressed in mg/m3 of volume of air at 25°C.

156 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Eq. (11)

TLV in mg / m 3 =

TLV × gram mol. weights 24.45

where 24.45 is molar volume in liters. In order to successfully apply the principles of the TLV, it is important that the location of the air supply and the exhaust outlets be selected in a manner such that air passes through the zone of contamination. To obtain effective safety for personnel, it is necessary that they remain between the air supply and the source of contamination. Electrical equipment, if in close proximity with the source of hazard, should also preferably be located between the source of hazard and the air supply. It is necessary that ventilating air moves to the electric equipment first before reaching the source of hazard. It makes no difference whether the operating mode of the source of hazard is closed or open (see Fig. 1-20-A). For example, assume that dilution ventilation is required for a small closed process system in an indoor location. The flammable liquid that must be processed in the system is ethyl chloride. It has the following characteristics: Flash point

= –58ºF

Mol.Wt.

Ignition Temp

= 966ºF

TLV

= 100 PPM

Vapor Density = 2.22

Flam.Class

= IA

Specific Gravity = 0.9

Amb.Temp

= 80ºF

NEC Class

= I, Group D

Flam. Range

Boiling Point

= 54ºF

Health Fac. = 1 (Col. (6), Table App. 2)

= 3.8~15.4%

= 64.38

Ethyl chloride is toxic and harmful if absorbed through the skin or when inhaled. The calculated dilution in the working space must stay within practical and economical limits. First, calculate the required dilution to below 25% of the LEL for fire and explosion. Then, calculate for health to see the difference in airflow capacity. The most difficult part of the calculation is to determine the emission rate. This should be determined on the basis of equipment size and the pressure in the system. In this example, assume that if a leak should occur in the process equipment, it is expected that (in view of the size of the source of hazard and the system pressure) the source of hazard is capable of leaking 1.875 gal of liquid per hour or 0.03125 gal/min. For fire and explosion, the dilution shall be: Eq. (12)

Va1 =

8.33 × Sp.Gr. × LR 8.33 × 0.9 × 0.03125 = = 1.434 CFM 0.0736 × VD 0.0736 × 2.22

1.434 ×

100 × 4 = 151 CFM 3.8

where LR = leakage rate in gal/min, weight of air = 0.0736 at 80°F, and 4 = 25% safety margin for LEL.

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157

For health, the dilution shall be: Eq. (13)

Va2 = 1.434 ×

106 × (K = 2 ) = say 20,000 CFM 100

The K-factor depends on the speed of evaporation and volume of vapors per minute produced. It also depends on how efficiently the vapors will mix with diluting air. Low speed evaporation and/or small volumes produced require a K-factor of 1–5. High speed evaporation and/or large volumes produced require a K-factor of 6–10. K-factors can also be related to actual toxicity as follows: Toxicity

Health Factor

K-factor

Minimal Moderate Heavy

1 2–3 4

1–2 3–8 9–10

In both cases, the exhaust system is required to operate continuously. Where large quantities of air are exhausted, the cost of energy to condition the air can be very high. The airflow rate could also provide an intolerable working condition for the worker in the indoor area. For example, if the 20,000 CFM discharge of air is being used for a room of 30 ft wide, 50 ft long, and 8 ft high, the flow rate in the room is then: Eq. (14)

20,000 = 83 feet per minute (FPM ) 240

and the number of air changes per hour required is

Eq. (15)

20,000 × 60 = 100 12,000

The question is whether the 20,000 CFM discharge rate will cause discomfort to working personnel. This depends on the working conditions in the room. For example, when worker activity is light, the continuous air flow may cause discomfort to the workers. However, much higher velocities may be required when higher working activities are necessary. For example, where contaminants have a higher degree of toxicity, much higher air velocities are necessary. Bear in mind that an airflow that feels refreshing in hot weather may not feel comfortable in the winter. Care must be taken to insure that air movement over a person is kept within acceptable limits. Therefore, sometimes it is wiser and more economical to use a chemical fume hood enclosure or local spot exhaustion when highly toxic materials are being used. Local exhaust ventilation is normally required close to the point of release. Capture and control of a contaminant are

158 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres achieved by inward airflow in the hood. One type of hood is the canopy fume hood, located above a working place. For additional information on canopy fume hoods, refer to Sec. E in this chapter and Sec. K in Ch. 3.

D.

APPROXIMATE LOCATION OF MECHANICAL VENTILATION

It is important to establish the approximate location of mechanical ventilation. The location of a pressure fan in an enclosed space is normally not critical. Generally, a pressure fan can be installed at any point in an enclosed space since the function of the pressure fan is to pressurize the location. The location requirement of a suction fan is more critical. If the suction fan is properly located, its airflow will dilute and quickly remove the contaminant from the location. The installation of the suction fan must be efficient and within acceptable economical limits. The ideal airflow passes over and along side the source of hazard. This is called air brushing. However, air brushing is not always feasible if the suction fan must be located in a building wall. To establish the proper location for a suction fan is important and, therefore, should be given great consideration with respect to the type and location of the source of hazard. The basic requirement for establishing the location for a suction fan is the vapor density of the flammable material. If a flammable gas or vapor in an enclosed space is heavier than air, a suction fan located in the roof of the enclosed space has little or no effect on diluting and removing the flammable contaminant from the location. A flammable gas or vapor lighter than air is also not efficiently diluted and removed if the suction fan is located in the wall 12" from the floor. Therefore, it is necessary to establish the location of the suction fan on the basis of the vapor density of the flammable material. In Fig. 1-20-B, the removal of airborne contaminants is considered ineffective as a result of the location of the exhaust fan (which does not correspond with the vapor density of the flammable material). The airflow shown in Fig. 1-20-B is insufficiently strong (with respect to the location of the source of hazard) to capture and quickly remove airborne flammable contaminants released from the source of hazard. However, small air currents will exist near the source of hazard. These air currents may be just strong enough to move the contaminants towards the “still air region” as shown in Fig.1-20-B. If the flammable material moves in this direction, accumulation of ignitable flammable material will still exist and will cause an increase of explosion danger. The longer accumulation exists, the greater the flammable cloud will grow and the greater the explosion danger. The cloud will expand as long as flammable materials are being released into the air. Above the cloud, air currents are stronger. Eventually, the stronger air currents will move the cloud slowly towards the exhaust fan.

Chapter 6: Ventilation Requirements

159

In Fig. 1-20-C, the flammable material being released is lighter than air. Although the source of hazard in Fig. 1-20-C is air brushed, some of the flammable material may escape to the ceiling where it will accumulate. Some of the reasons for this insufficient airflow over the source of hazard are: obstruction between the source of hazard and air outlet; a much lighter than air vapor density in combination with insufficient airflow; and improper location of the exhaust fan with respect to the vapor density of the flammable material. Therefore, the location of the exhaust fan in Fig. 1-20-D is more effective. Here, the contaminant in the air will be effectively captured and quickly removed. Since there are only two ranges of densities (a vapor density below 0.75 and a density above 0.75), the suction fan should be located in compliance with these two densities. If the suction fan is to dilute and remove a flammable material with a density of less than 0.75, it must be located in the roof or in the wall directly underneath the ceiling as shown in Fig. 1-20-D. On the other hand, if the fan is used for a flammable material with a density greater than 0.75, it must be located as shown in Figs. 1-20-E or F. An alternate solution, in the center of the wall, is shown in Fig. 1-20-G, although this solution may cause accumulation. However, if the vapor density of the flammable material is greater than 0.75, but less than one, it is generally safer to have the suction fan located half way up the wall. For a vapor density greater than one, it is safer to have the outlet closer to the floor. Normally, it is too cumbersome to locate the actual fan in the wall 12" from the floor. The recommended practice is to put the fan on the roof of the building and connect the fan to an air duct with louvers 12" from the floor as shown in Fig. 1-20-F. If the duct outlet cannot be located in this manner, installing the suction fan half way in the wall as shown in Fig.1-20-G is recommended. In Figs. 1-20-E and F, the source of hazard is in the direct path of the suction fan airflow. Because of this, the source of hazard is considered brushed by air. On the other hand, if the source of hazard is not in the path of the ventilating air (as shown in Fig. 1-20-B), the source of hazard is not brushed by air. In the first case where the source of hazard is in the path of the airflow, the actual traveling distance between the source and the point at which the flammable material will reach safe concentrations will be short. In this case, safe concentration will be reached close to the source. In the second case where the source of hazard is not in the path of the airflow, the traveling distance will be longer and will cause accumulation of the flammable material. When the source of hazard is brushed by air as shown in Figs. 1-20-E and F, the required boundary will be short because of the smaller traveling distance. If the source is not brushed by air as shown in Fig. 1-20-G, the required boundary must be longer because of the longer traveling distance. The size of the boundary can be found in the subtables of Table 1-4 under the heading “sufficiently ventilated indoor location.” However, the boundary for the conditions as shown in Fig. 1-20B needs to be much greater. In this case, the boundary recommended in the subtables of Table 1-4 is considered too small. Where the boundary selected from

160 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres the subtables is considered too small, it is necessary to compensate for the size. A larger boundary size than listed in these tables must be applied. Larger boundary sizes are 5, 10, 15, 20, and 30 ft respectively for 3, 5, 10, 15, and 25 ft standard sizes listed in the subtables of Table 1-4.

Figure 1-20. Approximate location of mechanical ventilation.

Chapter 6: Ventilation Requirements

Figure 1-20. (Cont’d.)

161

162 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-20. (Cont’d.)

Chapter 6: Ventilation Requirements

163

The selected locations for the suction fans may not always be the best solution if ducts, pipes, or other unavoidable items obstruct the airflow. In these cases, it is necessary to use judgment with consideration given to all factors discussed herein. The correct location for a suction fan may also not be the total answer for solving airflow problems if the location needs to be cooled or when mini sources of hazard are involved. For locations containing mini sources of hazard, an intrinsically safe or explosion-proof gas detector could be used instead of a continuously operating suction fan. The gas detector should be set at below 25% of the LEL and must activate a standby suction fan and an alarm only when airborne gas is detected. However, where normally a sufficiently ventilated area containing mini sources of hazard may be classified Div. 2 (if operation of the fan must depend on the gas detector), it is necessary that part of the location (in particular an area surrounding the mini sources of hazard) be classified Div.1. It may not always be possible to locate a suction fan in the wall of a 4-wall building because of adjacent rooms or an external building attached to the walls. In these cases, where the suction fan is to be used for flammable materials having vapor densities greater than 0.75 which cannot be located in any of the four walls, it should be located in the roof as an alternate solution. This arrangement is more costly since the suction fan must be connected to a duct system. If the building has three walls, the suction fan must be located in the wall opposite the open perimeter of the 3-wall building. Where a suction fan cannot be located in the wall opposite the open perimeter of the 3-wall building (because of adjacent buildings or attached rooms), a roof-mounted pressure fan may be used as an alternate solution (as shown in Fig. 1-20-H). However, this arrangement is very unusual for gases or vapors with densities greater than 0.75. A much better arrangement is to use a suction fan in the roof with a duct system along the wall opposite the open perimeter of the building instead of a pressure fan. It must be borne in mind that the mechanical ventilation as shown in the illustrations in Part 2 “Application of Fundamentals” are not intended to show the required location of the mechanical fan. These locations merely indicate the presence of forced ventilation. They do not have any bearing on their actual location. The actual location for the mechanical ventilation in the illustrations should be obtained from Fig. 1-20 and as explained herein. For example, in Fig. A-8 in Part 2, sources of hazard are located in a 4-wall building. According to Fig. A-8, the vapor density of the flammable product is heavier than air, which means that the vapor density is greater than 0.75. For the enclosed location to be classified Div. 2 with area sizes as shown in Fig. A-8, forced ventilation is necessary. This is shown in Fig. A-8 by means of a mechanical fan in the roof. Because the 4-wall building contains sources of hazard and because the vapor density is greater than 0.75, the mechanical fan must be of the suction type and its location should be as indicated in Fig. 1-20-E or F and not as shown in Fig. A-8. The replacement air, which enters the location, must be at least equal to the volume of air sucked out of the location. Multiple inlet points are usually the best

164 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres way to provide uniformity of make-up air. During winter, when the warmer air is removed from the location, it is best to mix the cooler make-up air with the warmer air in the location.

E.

CANOPY FUME HOODS

Small process systems mounted on elevated platforms or tables processing a mixture of lighter-than-air gas in a piping system, normally require canopy type fume hoods. Due to the great number of connections in the piping system, lighterthan-air gases may, as a result of breakdown of the pipe connections, be released accidentally to the atmosphere. The canopy fume hood should pick up these gases. The canopy fume hood is required to capture all flammable gases that are accidentally emitted from the process system. Airflow towards the hood opening must be of sufficient capacity to maintain control of airborne contaminants. It must also prevent contamination of adjacent areas. The flow rate of inward air need not dilute the flammable material to below the LEL. The flow rate must be enough to draw in all airborne flammable particles, so that the contaminant will not communicate with adjacent areas. Because of high losses in the capture velocity of open type canopy fume hoods, these hoods are not recommended for toxic gases which are used by personnel who are required to work in close proximity with this toxic material. Side panels are normally required to improve the inward airflow to the hood and to eliminate accumulation of toxic material in the breathing zone. For example, the hood in Fig. 1-20-I is not considered capable of sufficiently removing toxic material out of the breathing zone. This is because the hood is not equipped with vertical side panels. Therefore, a toxic contaminant may enter the breathing zone. The four open sides along the fume hood allow false air to be drawn in thereby reducing the capture velocity in the breathing zone. The type of arrangement as shown in Fig. 1-20-I is not recommended when toxic materials are being used. The arrangement in Fig. 1-20-J is better, although false air will also enter the fume hood. In this arrangement, air moves from operator to fume hood keeping the breathing zone clear from toxic contaminants. The arrangement shown in Fig. 1-20-K where airborne contaminants are completely enclosed and false air is blocked out from entering the fume hood keeping the breathing zone clear from toxic contaminants is preferable. At a given discharge rate, the flow rate will increase if the fume hood is provided with side panels—for example, if a canopy fume hood (as shown in Fig. 1-20-I) is sized 4 by 6 ft, equipped with three side panels, and located three feet above a workplace. The required discharge rate at the suction end needs to be 15 CFM (if an inward air flow of 1.0 ft/min must be maintained). With two side panels, the approximate discharge rate should be 30 CFM. With no side panels, the discharge rate should be 84 CFM (if an inward airflow of 1.0 Pu should be

Chapter 6: Ventilation Requirements

165

maintained). At a fixed discharge rate, the inward airflow rate will increase in accordance with the increasing number of vertical side panels. Bear in mind, the distance between the point of release and opening of the hood varies with the square of the distance.

F.

DEMARCATION LINE

A demarcation line is exclusively used for mini sources of hazard associated with lighter-than-air gases such as hydrogen gas, or a process gas with more than 30% hydrogen, or when it consists of any other gas of equivalent hazard. The demarcation line is an imaginary line that is drawn horizontally underneath the source of hazard. It should only be applied for small process plants associated with Class I flammable gases that are processed, handled, and/or transmitted. The purpose of the demarcation line is to divide the hazardous area that surrounds mini sources of hazard into two zones of different sizes. One zone is small and located below the demarcation line. The other zone is large and located above the demarcation line. The reason for the different sizes is because hydrogen gas will instantly rise once the gas is released into the air. The gas below the demarcation line, therefore, needs a small classified area. Above the demarcation line, it needs a much larger classified area. The gas is normally stored in a single container with a gas content of not more than 400 cf (cf = cubic feet of gas at 14.71 psia and 70°F). Safety dictates that the container be located in a ventilated area, preferably outdoors. The actual process system is normally located indoors. In most cases, the piping system for transporting the gas from the container outdoors to the process plant indoors has a size not greater than 1/4". The components in the piping system indoors may consist of one or more mini valves, manifolds, pressure reducers, gauges, etc. If these components are connected in the piping system by means of “low integrity connectors,” they are considered as the actual sources of hazard and therefore the components are required to be sufficiently ventilated. It is mandatory that a canopy fume hood or electric exhaust fan be used in the roof of the location. A roof opening in lieu of an electric suction fan is not permitted unless a canopy fume hood with forced ventilation is used directly above the mini sources of hazard (in which case a roof opening also must be applied). As a general rule for Class I flammable gases or vapors, dilution of lighterthan-air gases by suction ventilation shall be below one-quarter of the LEL. However, for a 1/4" piping system with mini sources of hazard, small quantities of the lighter-than-air gases need not be diluted to below one-quarter of the LEL. Since hydrogen gas, if airborne, will rise quickly, it is not necessary to dilute the gas to below one-quarter of the LEL. This is true for canopy fume hoods located directly above the source of hazard. Some suction air is necessary to accelerate the

166 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres rising hydrogen gas to the canopy fume hood. A canopy fume hood located directly above the source of hazard needs only a low velocity of air to assist the upward movement of the airborne hydrogen gas. Above the canopy fume hood, it is sufficient to have a ventilating opening in the building ceiling and roof as an additional safety precaution. Indoor locations with low ceilings not using canopy fume hoods may be provided with an electrically operated fan to accelerate upward movement of airborne gases. Gas detection for activating the operation of an exhaust fan shall not be used in lieu of a permanently operating electrical exhaust fan. If a roof exhaust fan is used, the gas must exit in a nonhazardous area via a roof mounted vent-stack of at least 7 ft high. The application of ventilating air is not required if all components in the piping system are provided with “high integrity fittings” and are pressure tested. If the ventilation is such that each of the mini sources of hazard is brushed by ventilating air as indicated in Fig. 1-21-A, the airflow will oppose the flow of gas if it escapes underneath a source of hazard. Under this condition, it is virtually impossible for a gas escaping from underneath a source of hazard to be ignited by an ignition source, if the ignition source is 3 ft away from the source of hazard. If brushing does not occur, the gas escaping from underneath the source of hazard will not be opposed and therefore, may contaminate the area beyond the 3 ft distance (in which case, the safe distance below the source of hazard should be at least 5 ft or be extended to the finished floor). Brushing is movement over and along side the source of hazard (for additional information refer to Fig. J-6, Ch. 28, Part 2). Pressure in the system also plays an important role on the size of the safe distance, particularly when the gas escapes from underneath the source of hazard. A low pressure in the source of hazard being brushed by ventilating air requires a safe distance, below the source of hazard, of 3 ft. If the source of hazard is not sufficiently brushed by ventilating air, or if the system pressure is moderate or high, the safe distance must be 5 ft, or the safe distance should be extended to the floor. Above the demarcation line, the danger zone is a vertical cone which is required to have a minimum safe distance of 15 ft. The width of the cone is normally much smaller than 15 ft. This width is a function of the speed at which the gas is forced upwards. Sometimes it is rather difficult to determine the width of the cone and whether electrical equipment above the demarcation line is located in the cone. In the cases where it is difficult to determine whether an electrical equipment will fall in the cone or not, it is safer to consider the electrical equipment in the cone. Equipment A in Fig. 1-21A must be suitable for a Div. 2 location because the electrical equipment is in the cone. Equipment B is located outside the cone but since it is close to the cone it is safer to consider it in the cone. Equipment C and D below the demarcation line may be of the general-purpose type since they are below the 3 or 5 ft boundary, and if sufficient air-brushing is present. Equipment E must be explosion-proof because it is within the 3 ft danger zone. Any electrical equipment of the heat producing type located within a distance of 3 ft from the

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source of hazard must be of the explosion-proof type regardless of sufficient airbrushing. Equipment F which is too far from the hazardous cone may also be of the general-purpose type. The extent of the danger zone for a single noncapped gas container located outdoors in a freely ventilated location is not the same as for the components in the piping system indoors. Because of the high pressure in the container (2000 psi and more), a Div. 2 zone of 5 ft radius is required below the demarcation line and also a Div. 2 cone of 15 ft minimum length is required above the demarcation line.

Figure 1-21A. Demarcation line X-X.

G.

LOW AND HIGH INTEGRITY SEAL CONNECTORS

Pressurized gas systems, as explained in Sec. F, can cause serious injuries and extensive damage if the system is not carefully assembled and does not operate in strict conformance with approved operating procedures. The greatest risk of failure is when the system is under high pressure. Nothing is more likely to cause

168 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres an accident than over pressurizing the system. Pressure relief valves can prevent over pressurization. Also, a seemingly insignificant modification of the system could result in a disaster. An accident-producing situation may exist when a fitting in the system looks like it can hold the pressure while it cannot. Reliable manufacturing methods, quality control, and careful assembly of the components can prevent pressure accidents. The components in the system normally consist of pressure reducers, valves, manifolds, and gauges (see Fig. 1-21B). These components are usually connected in piping systems by means of “low or high integrity seal connectors.” As an example, for hydrogen gas “low integrity seal connectors” are considered unreliable because they may cause leakage. Vibration, temperature fluctuation, or remake of the connectors are usually the main reasons for leakage. “High integrity seal connectors,” on the other hand, are considered leak free because of their quality construction. Low integrity connectors, are not considered leak free. High integrity seal connectors are considered leak free if they are: 1) provided with metal gaskets of proper material, 2) are welded into the piping system, 3) provided with proper workmanship (seal beads are not rotated against the gaskets), and 4) a new metal gasket is used every time the connector is loosened and retightened. Valves and manifolds of high quality are considered sources of hazard if they are provided with low integrity seal connectors or if they are systematically opened and closed at short intervals. As explained in Sec. F, for a pressurized gas system equipped with low integrity seal connectors, it is mandatory that sufficient ventilation be applied so that the possibility of an explosion is reduced. Since airborne hydrogen gases will rise quickly, it is not necessary to dilute them to below one-quarter of the LEL. However, all emitted gas particles must be captured by the ventilating system and only low velocity air is needed to assist the upward movement of the airborne gas. During processing of the hydrogen gases in the piping system, it is necessary that the ventilation is continuously running. If the process of the gas is not attended, a type “B” safeguard may be considered as an additional safety protection for loss of ventilation. Safeguards need not to be considered if the process system is attended. As indicated in Sec. F, no ventilation and classification of the location is required if the system is provided with high integrity seal connectors welded into the piping system, and, preferably, gauges in the system are omitted. Gauges are usually the weakest part of a compressed gas system. All components in the piping system should be thoroughly helium leak-tested directly after assembly. If located indoors, gas cylinders placed too close to a heater or furnace may cause a disaster because the system pressure will build up and may cause failure, or the pressure may trigger a relief valve to vent flammable gases. The preferred location of gas cylinders for mini sources of hazard in a 1/4" piping system is a free-ventilated outdoor location where pressure may be stable.

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Figure 1-21B. High and low integrity seal connectors.

H.

SAFEGUARDS

Safeguards are required because mechanical ventilation can break down. If the mechanical ventilation breaks down, the location becomes instantly hazardous. Therefore, safeguards are required to either prevent the failure of the ventilating system or to warn of ventilation loss. Safeguard failure is not considered because of low wear and because they normally are powered from different

170 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres circuits than the ventilating system. They are normally necessary for enclosed spaces, which require a nonhazardous classification and which do not contain a source of hazard. Normally, no safeguards are required for locations that contain sources of hazard. However, if a greater sense of security is required, safeguards are applied for some dispensing areas and for locations which are classified partially nonhazardous. Generally, only the upper part of the partially classified location is classified nonhazardous. This is true only if the vapor density of the flammable material in the location is heavier than air. Safeguards are also required for spaces which do require canopy fume hoods for small process areas, and spaces that are located above or below a hazardous area. They are generally required if the space, such as a room with a roof, has four walls and is provided with sufficient ventilation to obtain a nonhazardous environment in the space. There are two types of safeguards available: type “A” and type “B.” The type “A” safeguard is a redundant ventilating system that operates on loss of ventilation. This type is, or may be, provided with an audible and visible alarm system. The type “B” safeguard is only an audible and visible alarm system that operates on loss of ventilation. The loss of airflow must be detected by an air vane or by a differential air pressure control sensor that must operate the required safeguard. A type “A” safeguard is required if a nonhazardous space is located in a Div. 1 area or gives access to a Div. 1 area. The basis for this requirement is the continuous presence of flammable gases or vapors in the surrounding Div.1 area. For detailed information, refer to Secs. B and D in Ch. 4. A type “B” safeguard is required if the nonhazardous space is located in a Div. 2 area or gives access to a Div. 2 area. The basis for this requirement is the occasional presence of flammable gases or vapors in the Div. 2 area as a result of failure or rupture of the source of hazard in the Div. 2 area (or as indicated in Sec. D of Ch. 4). However, although the general rule requires that a space within a Div. 1 area or one that gives access to a Div. 1 area must be provided with a type “A” safeguard. A type “B” safeguard may, for economical reasons, be used if flammable gases or vapors are released frequently. The basis for this is that when there is a release of flammable material, there may be no breakdown of the ventilating system at the same time, and if there is a breakdown of the ventilating system, there may be no release of flammable material.

I.

WIRING DIAGRAMS FOR SAFEGUARDS

Two simplified wiring diagrams are shown in Fig. 1-22A, one for a type “A” safeguard, and one for a type “B” safeguard. The wiring diagram for the type “B” safeguard is simple and straightforward. When switch S is closed to energize fan F, the alarm will go off momentarily. The alarm is controlled by an air switch AS,

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an air vane normally located in the air duct. Once the fan F operates, the airflow will force the contacts of air switch AS to open. If a failure of the fan F occurs, switch AS will close its contacts and set off the alarm. Opening switch S can silence the alarm. For the alarm to operate on loss of power, each circuit breaker would best be connected to its own power source. The wiring diagram for a type “A” safeguard is more complex. As shown in the wiring diagram of Circuit #1, fan F1 (the running fan) is powered by one power source while the redundant fan F2 in Circuit #2 is powered from another source. Both fans are equipped with back draft dampers and must operate under the following conditions: 1. The circuits must allow each fan to operate independently. 2. Under airflow failure of the running fan F1, the alarm in Circuit #2 must go off and start fan F2. 3. Under loss of power in Circuit #1, the alarm must also go off and start the redundant fan F2. The following is a detailed description of the operation of fans F1 and F2 for safeguard “A” as shown in Fig. 1-22A.

a. Manual Starting Circuit #1 To start fan F1, depress its associated start button. When this start button is depressed, relay R1 will be energized. It remains energized as long as the start button is depressed. This relay will not lock in when energized. Because of this, relay R1 will only momentarily operate its contacts in Circuit #1 and Circuit #2. In Circuit #1, it energizes relay R2 which locks it in place. This causes the continuous operation of fan F1. As a result of the operation of Fan F1, air switch AS in Circuit #2 will open its contacts. This switch remains open as long as there is airflow from fan F1. If there is no airflow from fan F1, the contacts of switch AS are closed (switch AS is located in the air duct of fan F1). In Circuit #2, contacts R1 will energize relay R3 which locks it in place. This causes the standby lamp L to light. The other contact R1 in Circuit #2 opens which prevents fan F2 from operating. It is important to know that contact R1 in Circuit #2 is a “switch over switch” or a three-way contact switch. Fan F2 will not operate as long as relay R2 in Circuit #1 is energized.

Circuit #2 To start fan F2, depress its associated start button. This will operate relays R3 and R4. Both relays will lock in place. Relay R3 will cause the continuous operation of fan F2 and the standby light L. Relay R4 will open the circuit for the

172 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres TDC relay thereby disabling alarm AL and Circuit #1. If fan F2 is manually started, alarm AL is prevented from operating. Relay R4 will energize the flasher FL in Circuit #2 indicating that the alarm is disabled. Contact R2 between A and B in Circuit #2 is not required if an AS switch is used. Only when a DPS device is used, is a R2 contact necessary.

Figure 1-22A. Wiring diagrams for safeguards.

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b. Manually Stopping Circuit #1 & #2 To stop fan F1, depress the stop button in Circuit #1. Relay R2 in Circuit #1 will de-energize causing fan F1 to stop. At the same time, contacts R2 in Circuit #2 will close causing the operation of fan F2. Contact R3 in Circuit #2 was closed due to start up of fan F1. To prevent operation of fan F2, first press the stop button in Circuit #2 before stopping fan F1. Depressing the stop button in Circuit #2 will deenergize relay R3 and the standby pilot light, and prevent operation of fan F2. Depressing the stop button in Circuit #2 will prevent fan F2 from operating if the stop button in Circuit #1 is depressed afterwards.

Standby Mode a. Mechanical Failure of Fan F1 For Circuit #2 in Fig. 1-22A to be in the standby mode, relay R3 and the associated pilot light must be energized. This is accomplished automatically by depressing the start button in Circuit #1 to start fan F1. If a mechanical failure of fan F1 should occur while in operation, relay TDC will be energized as a result of the closure of switch AS. Switch AS will close due to loss of airflow. In this stage, contact R2 in Circuit #2 remains open as long as relay R2 in Circuit #1 is energized (contacts R2 between A and B are not required with AS). After some time delay, relay TDC will sound the alarm and open Circuit #1. The opening of Circuit #1 causes the closure of contacts R2 in Circuit #2, which causes fan F2 to operate. Since switch AS does not operate under airflow from fan F2, the alarm will continue to operate. Operating the start/silencer button in Circuit #2 will silence the alarm, which in turn activates relay R4 and the flasher. The flasher is a reminder that the alarm is being disabled. To re-energize Circuit #1, it is necessary to first de-energize relay R4 in Circuit #2. This is accomplished by depressing the stop button in Circuit #2.

b. Power Failure in Circuit #1 If a loss of power should occur in Circuit #1 in Fig. 1-22-A during the operation of fan F1, Circuit #1 will be de-energized causing the instant closure of contact R2 in Circuit #2. (Contact R3 in Circuit #2 was closed during the startup of fan F1.) The closure of contact R2 in Circuit #2 causes fan F2 to operate. If air switch AS in Circuit #2 has entered its rest mode (because it is without airflow from fan F1), it energizes relay TDC in Circuit #2. After some time delay, the TDC relay will initiate alarm AL. Fan F2 remains in operation until it is manually shut down by depressing the stop button in Circuit #2. Depressing the “start/silence” button in Circuit #2 will silence the alarm. Depressing the “stop” button in Circuit

174 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres #2 will also silence the alarm, but this causes fan F2 to stop and will also cause the disabling of the standby mode. If fan F2 should remain in operation, but the alarm stopped, only the silence button should be used. By depressing the silence button, relay R4 will be energized causing the shutdown of Circuit #1. If power to Circuit #1 is restored, Circuit #1 cannot be re-energized as long as relay R4 and/or TDC is energized. If a differential pressure switch DPS should be used instead of air vane AS, it is necessary to slightly modify Circuit #2. The modification consists of an addition of one NC contact (R2) which must be wired in parallel with the DPS switch between points A and B in Circuit #2. This contact R2 is necessary to override the opening mode of the DPS switch when fan F2 is operating. A DPS switch is not normally located in an air duct but in the room and therefore will operate at any time when fan F1 or fan F2 is energized. Without the additional contact R2 between points A and B in Circuit #2, the following will occur: a power failure in Circuit #1 will switch off fan F1 and start fan F2 (both fans have back draft dampers). This results in a momentary drop of air pressure in the room. This momentary drop in air pressure causes the DPS switch in Circuit #2 to close its contacts momentarily, which subsequently energizes the TDC relay. Since the drop in air pressure is only momentarily, the alarm may not sound at all depending on the setting of relay TDC. If the DPS switch opens its contacts after the TDC relay has timed out, the alarm will sound only for a short period of time. The purpose of contact R2 between points A and B is that it overrides the opening mode of the DPS switch during depressurization of the room. Contact R2 therefore will assure that the alarm will sound for a prolonged period of time until it is manually silenced. The wiring diagram in Fig. 1-22A shows both fans F1 and F2 and the control circuit for one and the same voltage level. If, however, a different voltage level is required for the control circuit, or if the hp ratings of the fans are too high for a 120volt circuit, the wiring diagram in Fig. 1-22B should be used. In this wiring diagram, the control circuits are branched off from a control-transformer in the “combination starters” for fans F1 and F2. A 24-volt AC control voltage could be used to operate the control circuits, while the power source for the fans could be 208 V or 480 V. In this schematic, a mechanical auxiliary contact C2, in the combination starter for the standby fan F2, could be wired in parallel with the normally closed contacts R1 and R2 in Circuit #2 (as shown in Fig. 1-22B). However, the use of this contact is optional. If wired in Circuit #2, it will prevent the interruption of currentflow to fan F2 if the start-button in Circuit #1 is depressed after return of voltage. If contact C2 is not wired in Circuit #1, the current flow to fan F2 will be interrupted if the start button in Circuit #1 is depressed. However, for interrupting the current flow to fan F2, it is necessary that relay R4 is not energized and the

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Figure 1-22B. Wiring diagrams for type “A” safeguard.

176 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-22B. (Cont’d.)

Chapter 7 Electrical Equipment for NEC Class I Locations

A.

GENERAL

Maximum safety can be obtained in Div. 1 and Div. 2 hazardous areas when enclosures of electrical equipment for the areas are purged, pressurized, or are explosion proof. Without this type of enclosure, safety in a Div. 1 area is nonexistent. In a Div. 2 area, safety will exist, but only at a reduced level. The basis for the reduced safety level in the Div. 2 hazardous area is the remote possibility of a simultaneous failure of electrical equipment and process equipment. The risk that both equipment will breakdown at the same time is sufficiently remote to consider the Div. 2 area as an “acceptably safe” environment. The question, however, is why bother with non-purged, non-pressurized, and non-explosion-proof electrical equipment when the application of the purged, pressurized, and explosion-proof electrical equipment will provide the location with greater safety? The reason for this is the high cost of the equipment. Explosion-proof electrical equipment, for example, can cost two to four times more than non-explosion-proof electrical equipment. Therefore, it is more economical to accept a lower level of safety for the Div. 2 area with electrical equipment at low cost.

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178 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

B.

ELECTRICAL EQUIPMENT REQUIRED FOR DIVISION 1 LOCATION

Electrical equipment for a Div. 1 hazardous location must be approved for NEC Class I, Div. 1. This means that the enclosures for the electrical equipment must either be explosion proof, purged, or pressurized. For a great number of electrical equipment, it is far more practical, and economical, when individual components of the electrical system are of the explosion-proof type, rather than having the enclosures purged or pressurized. Approved for NEC Class I, Div. 1 also means that the electrical equipment is not permitted to have a temperature exceeding the ignition temperature of the involved flammable product. Non-arcing type devices of the heat-producing type such as transformers, relay coils, resistors, electric motors, etc., must also be approved for NEC Class I and have a surface temperature not exceeding the ignition temperature of the flammable product. Temperatures that shall not exceed the ignition temperature of a flammable product include temperatures from overload and locked rotor conditions from electrical motors in a NEC Class I, Div. 1 location. There are four different types of electrical motors that may be used in an NEC Class I, Div. 1 location: 1. An explosion-proof electrical motor approved for the location with temperature restrictions as indicated above. 2. TEFC electric motors with positive pressure ventilation and a surface temperature not exceeding 80% of the ignition temperature of the flammable product involved. 3. A TE inert gas-filled motor, also with a surface temperature not exceeding 80% of the ignition temperature. 4. A non-explosion-proof electric motor designed to be submerged in oil, in which the oil is flammable only when vaporized and mixed with air or in a gas with a pressure greater than the atmospheric pressure and which is flammable when mixed with air. The electric motors of Items 2 and 3 must be provided with safeguards that automatically shut down the motors or are provided with a suitable alarm system if temperatures are exceeding the temperature limits as designed for the electric motors. An explosion-proof equipment enclosure is designed to withstand a given explosion force. Therefore, not all explosion-proof enclosures have the same physical strength. The difference in strength is marked on the enclosure with a group letter A, B, C, D, etc. Each individual letter represents explosion strength, which coincides with the explosion force of the flammable material that is also grouped under the same letter.

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C.

179

ELECTRICAL EQUIPMENT REQUIRED FOR DIVISION 2 LOCATION

If the location is classified Div. 2, the electrical equipment for the location does not have to be approved for NEC Class I. For example, arcing devices such as fuses, switches, circuit breakers, controllers, push button stations, etc., do not have to be approved for a NEC Class I location if their contacts are immersed in oil or enclosed in a hermetically sealed chamber. The current interrupting contacts of non-explosion-proof electrical equipment that is within hermetically sealed chambers do not allow flammable gases or vapors to enter the sealed chambers. If the current interrupting contacts are immersed in oil, the oil level must be a minimum of 2" above the contacts for power and 1" for control. The design of these types of equipment is so that under normal operating conditions, both the hermetically sealed and oil immersed contacts are not capable of igniting a flammable gas or vapor in the air. These types of electrical equipment are permitted to use generalpurpose enclosures. Non-arcing type devices (but devices of the heat producing type such as resistors, control transformers, fixed lighting fixtures, etc.) are also permitted in general-purpose enclosures or may be of the general purpose type if their temperatures do not exceed 80% of the ignition temperature of the flammable product involved. Arcing devices, such as circuit breakers and fuses for protection of lighting circuits and isolating switches, may also be installed in general-purpose enclosures if they are not intended to interrupt currents by manual operation. The probability that the fuses and circuit breakers will operate as a result of a fault current at the same time that a hazardous condition exists must be considered remote. Because of this, the enclosures of these types of arcing devices do not have to be explosion proof. Rotating electrical machinery in Div. 2 locations do not have to be approved for Class I locations, unless they do contain arcing devices. Non-explosion-proof electric motors, and even open type motors, are permitted in a Div. 2 location as long as they are not capable of igniting a flammable gas or vapor in the atmosphere under normal operating conditions. The application of non-explosion-proof electrical motors without arcing devices in a Div. 2 location is generally considered safe. The safety is based on the fact that the motor, when it fails, may not fail at the same time as the failure of process equipment, and vica versa. The chances that a non-explosion-proof electrical motor will fail simultaneously with the process equipment are considered remote. However, there are conditions in which the non-explosion-proof electrical motor in a Div. 2 location may become unsafe without its failure. For example, if the motor, under normal operating conditions, should operate at too high a temperature, the motor may become a source of ignition. The majority of electric motors are provided with Class B insulation. However, electric motors may also be provided with Class F or Class H insulation, allowing the motor to operate at a

180 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres higher temperature. If motors with Class F or Class H insulation are operating at a higher temperature and should be overloaded for too long a period of time (because the overload relays are set too high, or when these motors have a long starting time, or are started a number of times in quick succession), excessive heat will develop in the motor. The temperature may rise considerably and approach, or exceed, the ignition temperature of the flammable product. If the electric motor should reach these elevated temperatures when a flammable gas or vapor is in the air, and the ignition temperature of the flammable product is below the elevated temperature of the motor, an explosion may result. To avoid these possible conditions, it is necessary that during the purchase of the electric motor, the motor manufacturer be informed about the involved flammable products. It is vital that the motor manufacturer knows the explosion features of the flammable product if he is to furnish a trouble-free motor. A trouble-free motor is designed to produce a motor starting time which includes at least a safety margin of 20% in temperature and time. For example, if the electric motor is used for pumping jet fuel JP-5 which has an ignition temperature of 204°C and requires 6.0 sec to accelerate from standstill to full speed at rated voltage, the temperature in the motor under locked-rotor condition shall not exceed 0.8 × 204 = 163.2°C in not less that 1.25 × 6.0 = 7.5 sec. Generally, the time for the motor under locked-rotor conditions to reach 80% of the ignition temperature is much longer than is required by the 20% margin. However, if the actual time should be close to the 20% margin, the permitted temperature in the motor should also be checked against this locked-rotor condition at reduced voltage. Unfortunately, not all over-current relays can be used for protection of electric motors in a Div. 2 location. For example, induction disk-type over-current relays do not provide adequate protection for electric motors in hazardous locations. Frequent starting of a motor, for example, will not produce a temperature rise in the induction disk-type relay. Therefore, only thermal-type over-current relays with a characteristic similar to that of the motor heating curve are recommended for motors in hazardous locations. Care must be taken in setting the overcurrent relay so that it does not operate prematurely under the manufacturers allowable starts. Non-explosion-proof motors that are selected on the basis of temperature restrictions will guarantee a higher degree of safety in the location than without temperature restrictions, provided proper overload relays with proper settings are used for motor protection.

D.

INTRINSICALLY SAFE ELECTRICAL EQUIPMENT

Intrinsically safe electrical equipment and wiring may be used in a Div. 1 and Div. 2 hazardous location as long as they are approved for the location. An approved intrinsically safe system is not capable of igniting a flammable gas or vapor mixed with air. In view of this, enclosures for intrinsically safe equipment

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and wiring do not have to be explosion proof, purged, or pressurized. For additional information, see ANSI/RP 12.6, 1995.

E.

MARKING OF ELECTRICAL EQUIPMENT

Electrical equipment approved for NEC Class I, and Class II, Div. 1 locations shall be marked to show the NEC Class (Class I, Class II, etc.), the grouping of the flammable product (Group A, B, C, D, E, F, etc.), and the operating temperature of the electrical equipment based on a 104°F ambient temperature. The temperature shall be shown as an identification number, which is listed in Table 500-5(d) of the NEC. This temperature marking shall not exceed the ignition temperature or the involved flammable gases or vapors. Electrical equipment that does not have to be marked is as follows: 1. If the equipment is of the non-heat-producing type, such as boxes, conduits, fittings, etc. This type of equipment does not need to have a temperature marking. 2. If the equipment is of the heat-producing type, but has a temperature not exceeding 212°F. This type of equipment does not need to have a temperature marking. 3. A fixed lighting fixture which is marked for use in a Class I, Div. 2 or Class II, Div. 2 location does not need to have a “group” marking. 4. A fixed general-purpose type of equipment which is approved for a Class I, Div. 2 location does not need to be marked with a class, division, group, and/or temperature. 5. Fixed dust-tight equipment suitable for Class II, Div. 2 locations needs no class, division, group, and/or operating temperature marking. For example, a general-purpose induction motor not being equipped with arcing or heat-producing devices does not have to be marked when located in a Div. 2 area. Locked-rotor and overload conditions are generally not considered as heat producing features that require temperature or group markings as long as the electric motor is only used and designed for the Div. 2 location. However, there are conditions in which the motor needs to be marked to show its maximum operating temperature, as explained in Sec. C.

F.

CONSTRUCTION OF EXPLOSION-PROOF ENCLOSURES

The construction of explosion-proof enclosures will prevent the flame of an explosion inside an enclosure from propagating to the outside of the enclosure. This unique feature makes the hazardous location equipped with explosion-proof

182 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres electrical equipment completely safe. Explosion-proof enclosures must also withstand the pressure of the internal explosion. Most manufacturers will follow the UL Standards for explosion-proof electrical equipment, which have been developed on the basis of extensive laboratory tests. The UL is a non-profit organization sponsored by the American Insurance Association. Its purpose is to determine the safety of equipment and materials for use in hazardous locations and to establish standards for the industry. The UL will subject the explosion-proof electrical equipment and its materials to a series of tests with specific gas or vapor mixtures over a range of flammable or explosive concentrations from which the maximum explosion pressure that may occur in the enclosure is determined. The clearance and the width of the joints of the equipment are tested to make sure that the flame or sparks in the enclosure are prevented from passing through to ignite the surrounding atmosphere (as shown in Fig. 1-23). Explosion-proof electrical equipment must also operate at a low enough temperature so that the atmosphere around it will not be ignited. Explosion-proof enclosures are not supposed to be gas tight, nor are gaskets permitted between the joints to make the enclosure watertight since this would ruin its explosion-proof properties (except for ring-type gaskets that will be blown out under explosion pressure). Because of this, the enclosures cannot be prevented from breathing and when installed in an atmosphere saturated with a flammable vapor mixture, this vapor mixture may, in time, enter the enclosure. For this reason, the joint clearance and the width are of critical importance. Larger clearances would require a larger joint width. A threaded joint shall be made up with at least five threads, fully engaged, and the threads shall not exceed 20 threads/inch. The strength of the explosion-proof equipment depends upon the group in which the enclosure is to be located.

G.

GROUPING OF ELECTRICAL EQUIPMENT

Grouping of electrical equipment is only required when the equipment needs to be explosion proof. Equipment that is not required to be explosion proof needs no grouping. The basis for grouping of electrical equipment is the explosion pressure that is created by the flammable product. Each flammable product has its own specific explosion pressure. For example, acetylene can produce a maximum pressure of 1,140 lbs/in2 (psi), hydrogen—845 lbs psi, diethyl ether—200 lbs psi, and gasoline—160 lbs psi. By grouping the various flammable products according to their explosion characteristics, the maximum explosion pressure can also be grouped. The NEC recognizes four groups of explosion pressures for Class I flammable products as shown in Table 1-1-A in Ch. 1. The strength of the explosionproof equipment depends upon the group in which the enclosure is located. This means that an enclosure must be designed to withstand the maximum pressure of

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an internal explosion of a particular group without bursting and without loosening the joints in the enclosure. The enclosure shall also withstand, without rupture or permanent distortion, a hydrostatic test of four times the maximum internal explosion pressure developed during the explosion test. Such hydrostatic tests may be omitted where acceptable calculations indicate a safety factor of five for the enclosing parts and bolts, based on the maximum pressure and the tensile strength of the materials. In addition, joints must be wide enough and clearances small enough so that flames will be quenched and will not propagate from the interior of the enclosure to the atmosphere surrounding it.

Figure 1-23. Design criteria for constructing explosion-proof enclosures.

184 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres As a result of these requirements, an explosion-proof enclosure that is designed for a particular group may not be capable of withstanding the explosion pressure of another group. For example, suppose an explosion-proof enclosure designed for Group D will be used for a Group B atmosphere. The maximum explosion pressure that can occur in a Group D enclosure is from gasoline which can develop 160 lbs psi. The enclosures for Group D must withstand 4 × 160 = 640 lbs psi. If the B atmosphere should contain hydrogen, the enclosure is subject to an explosion pressure of 845 lbs psi when the hydrogen entering the enclosure would be ignited. The enclosure, then functioning as a hand grenade, could do considerable damage. In view of the above, it is not recommended that an explosion-proof enclosure for a given group be used in an atmosphere of another group, unless the explosion pressure of the group in the atmosphere is less than that of the enclosure, or an explosive mixture is not capable of entering an explosion-proof enclosure. If Group B enclosures are not commercially available, Group C enclosures may be used for gases or vapors of equivalent hazard to hydrogen such as ethylene oxide, propylene oxide, and acrylene. Group D enclosures may be used for butylene provided both ends of the enclosures are sealed by sealing fittings of a size 1/2" or larger. Whether an explosive mixture is capable of entering an enclosure depends mainly on the operating conditions of the flammable substance. These operating conditions must be established when an area classification is required. Much careful research has been done in various countries on the relationship of turbulence and pressure buildup in electrical equipment. The explosion pressure is greatly influenced by the turbulence of the explosive mixture, resulting in a higher explosion pressure. The speed at which the pressure is developed tends to increase with the increase of turbulence. The internal explosion pressure of motors when they are running is often more than twice as great as when they are stationary because of the turbulence created by the moving rotor and internal fan. Ignition of the flammable gas at one end of the motor (i.e., the fan end) forces unexploded gas to the opposite end, causing an increase in pressure prior to ignition. A similar condition would occur if two explosion-proof boxes connected by a short piece of conduit were considered. If the gases in one box are ignited, there is a rise in pressure in the other box caused by the compression of these gases. Excessively high explosion pressure can develop in long conduit runs. Therefore, it is good practice to place a seal in the conduit run every 50 ft to limit the explosion pressure.

Chapter 8 Electrical Equipment for NEC Class II, Group F Locations

A.

GENERAL

Electrical equipment in an NEC Class II, Group F location must either be installed in dust-ignition-proof or dust-tight enclosures or in enclosures that minimize the entrance of dust. Dust-ignition-proof enclosures are normally used in NEC Class II, Div. 1 locations. Dust-tight enclosures or enclosures that minimize the entrance of dust are normally used in NEC Class II, Div. 2 locations. Division 2 locations are not permitted for electrically conductive dust having a resistance of less than 100 megohm centimeters. Therefore, these dusts are not considered. Most coal, coke, or carbon black dusts in Group F have resistances greater than 100 megohm centimeters and are considered not to be electrically conductive. Dust-ignition-proof enclosures will exclude the entrance of dust and also prevent arcs or sparks from igniting combustible dust in the air surrounding the enclosure. Dust-ignition-proof enclosures allow electrical equipment in the enclosure to operate at full capacity without developing surface temperatures that can cause ignition of exterior accumulation of specific combustible dust. Explosionproof electrical equipment is not acceptable in NEC Class II locations unless approved for the location. Only a few electrical items for NEC Class II, Group F locations are highlighted herein to provide basic information on the type of enclosures to be used in Div. 1 and Div. 2 locations.

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186 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

B.

CLASS II, DIVISION 1 LOCATIONS

Arcing devices such as switches, circuit breakers, fuses, push-button stations, relays, etc., must be enclosed in dust-ignition-proof enclosures when the electrical equipment is to be located in NEC Class II, Div. 1 locations. Electrical equipment of the non-arcing type (such as isolating switches containing no fuses and which are not intended to interrupt current, and where the dust is not conductive), shall be provided with tight metal enclosures that minimize the entrance of dust and no openings that allow the escape of sparks or burning material. Electrical rotating equipment in NEC Class II, Div. 1 locations shall be approved for Class II, Div. 1 locations and must be of the dust-ignition-proof type or be of the totally enclosed pipe ventilated type with temperatures not exceeding 150°C when operating normally and temperatures not exceeding 200°C when operating under overload condition. The wiring method for an NEC Class II, Div. 1 location shall consist of fittings and boxes that are designed to minimize the entrance of dust and are provided with threaded bosses for connections to threaded rigid metal conduits or Type MI cable with approved terminal fittings or dust-tight flexible connections approved for the location. The fittings and boxes shall have no opening through which dust might enter or through which sparks or burning material might escape. Fittings and boxes in which taps, joints, or terminal connections are made shall be approved for NEC Class II locations.

C.

CLASS II, DIVISION 2 LOCATIONS

In NEC Class II, Div. 2 locations, dust-tight enclosures are required for arcing devices. Devices not intended to interrupt current, such as isolating switches, may be installed in enclosures that minimize the entrance of dust. The wiring method for NEC Class II, Div. 2 locations, including wire ways, fittings, and boxes, those provided with terminals, splices, and taps, also requires a design that minimizes the entrance of dust. Electrical rotating equipment for NEC Class II, Div.2 locations shall be: totally enclosed non-ventilated; totally enclosed fan-cooled; totally enclosed pipe ventilated; or dust-ignition-proof for which the maximum full load current external temperatures shall not exceed 150°C when operating under normal operation in free air and the enclosures are without dust blankets, and 200°C when operating under overload condition. Standard open type motors without arcing devices may be used in NEC Class II, Div. 2 locations if dust accumulation is light, the motor can easily be cleaned and has a surface temperature not exceeding 150°C under normal operation. Standard open motors may be used with arcing devices in dusttight enclosures. If dust accumulation is heavy, any type of electrical motor may be used in separate dust-free rooms.

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A cloud of coal dust, for example, can be ignited by a temperature of 610°C, but a layer of coal dust can be ignited by 170°C, which is less than the maximum operating temperature of 180°C of a motor which is provided with Class H insulation. Dust blankets on the motor housing will increase the motor surface temperature. Depending on the thickness of the blanket, the motor surface temperature may exceed the ignition temperature of a coal dust layer, especially when the motor operates at elevated temperatures because of its higher class of insulation.

Chapter 9 Intrinsically Safe Equipment and Wiring

There is an increasing demand for intrinsically safe equipment for signaling and automatic process control in oil refining and other industries. Foreign countries have conducted a considerable amount of investigation (in particular, Great Britain which has initiated a comprehensive research effort). Where it would be impracticable to employ expensive, explosion-proof equipment, the principle of intrinsic safety could be applied to great advantage and at reduced costs. Intrinsic safety is restricted to electrical apparatus and circuits in which the output or consumption of energy is small. The principle of it depends upon a number of factors such as the supply voltage, resistance, capacitance, inductance, the manner in which the circuit is broken, the material, the shape of the contacts, and the type of gas or vapor. The goal of using intrinsic safety in electrical equipment is to enable such equipment to be used without risk of igniting any flammable gas or vapor which may be present. Electric ignition energy is expressed in joules or millijoules. 1 joule = 1 watt-second 1 millijoule = 1/1000 of a joule In a capacitance circuit, the capacitive energy is: A = 1/2 E 2C In an inductive circuit, the electromagnetic energy is: A = 1/2 I 2L

188

Chapter 9: Intrinsically Safe Equipment and Wiring where A C E L I

is is is is is

189

expressed in joules the effective capacitance in farads the voltage in volts the effective inductance in Henrys the inductive current in amps

The available spark energy should be limited to less than the required ignition energy for the specific flammable liquid or gas under consideration. Minimum ignition energies range from 0.01 to approximately 0.3 millijoules for unsaturated hydrocarbons. The minimum ignition energy, which may be released for a certain flammable mixture, should be controlled by laboratory procedures. Flammable gases and vapors, when mixed with air or oxygen in suitable proportions, can be ignited by comparatively weak electrical sparking such as may be produced by circuits of low inductance or by discharging of a capacitor. While the spark produced by the discharge of a capacitor having an energy content of less than one millijoule may cause ignition, under other conditions, a greater energy can be released in a spark without giving rise to ignition. Equipment and associated wiring approved as intrinsically safe may be installed in any hazardous location without fulfilling the requirement of explosionproof equipment. The intrinsically safe wiring shall be kept separate from nonintrinsically safe wiring, whether installed in a hazardous or nonhazardous location. In a nonhazardous location, intrinsically safe wiring may occupy the same enclosure or cable tray if there is a minimum two inch separation between the two, and both types of wiring are tied down. Inside a panel located in a nonhazardous area, the intrinsically safe wiring shall be segregated from the non-intrinsically safe wiring. In hazardous locations, the intrinsically safe wiring may use any wiring method provided it complies with the following requirements: 1. Different intrinsically safe circuits shall be in separate cables or shall be separated from each other by circuits within a grounded metal shield or by minimum insulation thickness of not less than 0.01 inch and suitable for the maximum temperature. 2. Intrinsically safe circuits shall not be placed in any raceway, cable tray, or cable with circuits of any non-intrinsically safe circuits unless the intrinsically safe circuits are separated from non-intrinsically safe circuits by at least 2 inches. 3. Conduit used for intrinsically safe circuits shall be sealed to prevent flammable mixtures from being transmitted from one location to another. If a cable core should be able to transmit flammable mixtures from one location to another, the cable should also be sealed.

190 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres 4. Intrinsically safe circuits shall be identified or color-coded in a manner that does not duplicate the color of other circuits. 5. Terminal strips shall be provided with barriers between the terminals for different intrinsically safe circuits. 6. Different intrinsically safe systems shall be adequately separated. 7. Intrinsically safe apparatus may be used with general-purpose enclosures provided the enclosures are suitable for the hazardous locations. Thermocouples and pressure transducers are normally not considered intrinsically safe. Although their energy consumption is in the micro-amps and millivolt range, they could, under fault condition, generate sufficient energy to ignite a surrounding flammable gas or vapor. It is not so much their energy consumption that may cause arcing or sparking, but their circuiting from associated amplifiers and power supplies that may generate high-energy arcing or sparking under fault conditions. Additional information regarding intrinsically safe equipment and wiring can be obtained from ANSI/UL 913-1997, Intrinsically Safe Apparatus and Associated Apparatus for use in Class I, II, and III, Div. 1 Hazardous (Classified) Locations.

Chapter 10 Installation of Electrical Instruments in Hazardous Locations

Electrical equipment in hazardous locations cannot always be installed in explosion-proof enclosures because of cost, space limitation, and installation complexity. For simple electrical instruments, the cost of explosion-proof enclosures can be substantial. A suitable practice to reduce cost and space and to simplify installation is to install ordinary electrical instruments in an air or inert gas purged general-purpose enclosure. Air or inert gas purging enclosures are suitable for Div. 1 and Div. 2 hazardous locations. The addition of air or inert gas (nitrogen) into the general-purpose enclosure at sufficient flow will prevent the entry of a hazardous vapor. The general-purpose enclosure should have a size of 2 ft deep, 2 ft high, and 2.5 ft wide or any other combination of dimensions not to exceed 10 ft3. If air for purging is used, the compressor intake air must come from a nonhazardous location, and must be of top quality. Plant compressed air is usually not suitable. Instrument air that passes through hazardous locations should be avoided as much as possible. The Instrument Society of America has issued a booklet entitled “Recommended Practice for Instrument Purging for Reduction of Hazardous Area Classification.” It gives detailed information of the requirements of the various types of purging for instruments. Detailed information on purging can also be found in NFPA 496, the standard for purged and pressurized enclosures for electrical equipment. Purging for enclosures as recommended by the Instrument Society of America and NFPA 496 is classified into three types: Z, Y, and X purging. Type Z 191

192 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres purging will reduce the classification of the area within an enclosure from Div. 2 to nonhazardous. Type Y purging will reduce the classification of the area within an enclosure from Div. 1 to Div. 2. Type X purging will reduce the classification of the area within the enclosure from Div. 1 to nonhazardous.

A.

TYPE Z PURGING

The following is required to reduce the classification of the area within an enclosure from Div. 2 to nonhazardous: 1. Pressure control valves are required to prevent power from being applied before the enclosures have been purged. 2. Before power is turned on, the enclosure must have been purged by a minimum of four enclosure volumes of purge air, thereby maintaining an internal enclosure pressure of not less than 0.1 inch of water. 3. When power is on, the enclosure must be maintained under a positive pressure of not less than 0.1 inch of water. 4. Safety interlocks to remove power upon failure of purging supply are not required. However, an alarm for purging system failure suitable for the location must be provided. 5. Under normal operation, no external enclosure temperature over 80% of the ignition temperature of the gas or vapor under consideration shall exist under normal operating conditions with 100% voltage applied to the instruments. 6. A warning nameplate in red must be mounted on the instrument and must be visible before the enclosure is opened. The warning nameplate shall state that the enclosure shall not be opened before power to all devices has been removed and that the enclosure has been purged for X minutes as recommended by the manufacturer.

B.

TYPE Y PURGING

The following is required to reduce the classification of the area within an enclosure from Div. 1 to Div. 2: 1. All requirements for Type Z purging will also apply for Type Y. 2. In addition, all equipment shall conform to the requirements for Div. 2 locations.

Chapter 10: Installation of Electrical Instruments

C.

193

TYPE X PURGING

The following is required to reduce the classification of the area within an enclosure from Div. 1 to non-hazardous. 1. Installation of enclosures with Type X purging must be avoided where possible. It is not considered good practice to install electrical equipment in connection with Type X purging. 2. In this system, a device must be incorporated to automatically remove all potentials from all circuits upon failure of purging supply. 3. A door switch must be provided to automatically remove potential from all circuits within the enclosure if the enclosure can be readily opened without the use of a key or tools. 4. A timing device must be incorporated to prevent power from being applied until sufficient time has elapsed to permit ten enclosure volumes of purge gas to have passed through the enclosure while maintaining an internal air pressure of at least 0.1 inch of water. 5. The maximum operating temperature shall be the same as for Type Z purging. Exact and detailed information as to the type of electrical equipment and the volume of purged gas can be obtained from NFPA 496 and the booklet ISA-S-12-4, sponsored by the Committee on Instrumentation for Hazardous Areas.

Chapter 11 Hydrogen Gas

Hydrogen gas is colorless and odorless, but it is highly flammable with a high rate of flame propagation. Hydrogen gas is produced by catalytic process or by electrolytic decomposition of water separating it into hydrogen and oxygen. Oxygen will not burn, but is a strong supporter of combustion. It is essential to keep hydrogen gas and oxygen or air separated since a small amount of these two gases will produce an explosive mixture. The explosion range of hydrogen gas is 4–75%, and its ignition temperature is 932°F. If ignited, hydrogen gas burns instantly upon contact with air. When not mixed with air, hydrogen gas is not explosive. Hydrogen gas is used in many ways. For example, it may be used as: 1) as a cooling medium for large generators, because it is an excellent heat conductor, and 2) in chemical process plants. In electrical machines, it is important to maintain hydrogen gas above atmospheric pressure in order to avoid mixing it with air. Hydrogen gas should be admitted to rotating equipment only after air has been removed completely by inert gas. Although hydrogen gas leaks are potentially dangerous, the odorless gas may escape entirely unnoticed because of its diffusion rate. For cooling generators, it is recommended that hydrogen gas cylinders be located in well-ventilated locations, separated from the generator. Where hydrogen gas-cooled generators are protected by a reliable source of carbon dioxide gas, the area around the generator may be considered nonhazardous. Adequate safeguards are required to ensure prompt operation of the carbon dioxide gas because the discharge of large amounts of carbon dioxide may create hazards to personnel. 194

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Large quantities of hydrogen gas are generally stored in a number of small metal cylinders. Hydrogen gas in quantities above 400 cf are normally stored in outdoor locations or separate detached buildings. [CF is cubic feet at 14.7 psia (101 kpa) and 70°F.] In these outdoor and indoor locations, a minimum safe clearance of 15 or 25 ft must be maintained between any general purpose electrical equipment and storage containers. Fifteen feet is considered sufficient if the ventilating air is not obstructed and 25 ft when the ventilating air is obstructed. If the location is sufficiently ventilated, the area within this boundary must be suitable for a Div. 2 location. However, any electrical equipment located within 3 ft of a hydrogen gas container or system must be explosion proof and labeled Class I, Group B. Group C equipment may be used for gases of equivalent hazard such as ethylene oxide, propylene oxide, and acrolein provided that the conduits to the equipment are sealed by conduit seals. Hydrogen gas containers should not be considered as the actual source of hazard. Only devices such as pressure reducers, pressure relief valves, manifolds, and valves connected to the piping system with low integrity seal connectors are normally the actual sources of hazard (refer to Fig. J-10A, Ch. 28 in Part 2). In small process areas, the quantity of hydrogen gas is normally less than 400 cf. The hazardous boundaries required for these areas are usually much smaller than 15 or 25 ft. Only when the process area is: 1) located indoors, 2) the ventilating system consists of an electrically-operated exhaust fan, and 3) the ventilating air moves from air inlet to the electrical equipment first and then to the source of hazard, may a smaller boundary be used. It the ventilating air flows in this sequential order, the hazardous boundary below the source of hazard may be reduced to 3 or 5 ft. Above the source of hazard, the boundary must be much larger than 3 or 5 ft.

Chapter 12 Cathodic Protection

Cathodic protection used in hazardous locations to prevent corrosion in tanks, pipelines, and other associated equipment has proven to be unsafe with respect to sparking and provides a source of danger in the neighborhood of flammable vapors. Therefore, care should be exercised when using installations with cathodic protection in hazardous locations. Dangerous sparking may be produced when flexible conductive hoses are connected to or disconnected from tankers, road vehicles, or rail car loading platforms. It is recommended that each item of equipment under cathodic protection and located in hazardous areas be double grounded. The transformers of the rectifier unit used in cathodic protection should have a grounded shielding between the primary and secondary windings. The dc power supply from the rectifier should be a two-wired ungrounded system controlled by a double pole disconnecting switch. One particular case where protection against corrosion may be considered a source of explosion danger follows. Assume that a storage tank containing a flammable liquid must be protected against corrosion caused by the continuous presence of water at the bottom of the tank. Assume also that the tank is protected by a forced current drainage system such as a rectifier that must deliver a continuous current to the tank. A dangerous condition will exist if it is required to disconnect the piping system from the tank while the current from the rectifier is 196

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still flowing. When the piping system is disconnected from the tank, a residue of flammable liquid will run out of the piping system and an arc will appear at the point where the piping is to be separated from the tank. For an explosion to occur, it is necessary that the liquid spill be given sufficient time to evaporate and be present at the instant that the vapor has entered its explosion range. At the initial separation of the pipe, the spill of liquid will start to evaporate while the arc has ceased to exist. Therefore, during the first instant of pipe separation, there may be no explosion danger. However, a few moments later when the liquid spill is in its evaporation stage (and subsequently entering its explosion range), a dangerous condition will exist. Under this condition, if the pipe and the tank are momentarily rejoined and separated again, an arc will occur at the point of separation at a time when a highly explosive cloud of flammable vapor is developing. As a result of the simultaneous presence of both occurrences, an explosion will result. Since the presence of an arc is considered to be at the pipe flanges, the dangerous arc can be eliminated when the pipe and flanges are insulated at a minimum of two locations at the tank, and at the other end of the pipe.

Chapter 13 Static Electricity

Some understanding of static electricity is necessary to make the reader aware of the explosion hazard that may exist in a hazardous location due to static electricity. A condensed discussion of the nature and origin of static electricity and the recommendation for it is included. The information contained in this chapter shall be used to evaluate existing installations and to reduce hazardous conditions for new installations. Static electricity is generated, for example, when: 1. A belt is running over a pulley. 2. Low conducting liquids are flowing through a pipe or hose. 3. Liquid is agitated in a tank. 4. Liquid is transferred from one container to another. 5. There is turbulence of flammable liquid in a tank or splashing during filling. 6. A gas containing particles of foreign material rubs against the surface of a solid. Belts made of any insulating material running at any speed except slow will generate high quantities of static electricity. Flat belts will produce considerable amounts of static electricity, while static charges from V-belts seem to be minimal. Using conductive pulleys can reduce charges on belts. This will allow the charge to drain through the metallic shaft and pedestal to ground. In dry, hazardous locations, it is recommended that belts be eliminated.

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Chapter 13: Static Electricity

199

A liquid of low conductivity pumped through a pipe or hose will produce static charges. The static charges will increase when the liquid is pumped through the pipe or hose at a high velocity or when a filter is placed in the pipe. Tank vehicles in dry seasons sometimes accumulate static charges because they are isolated from the ground by their rubber tires. The static charges are promoted either by filling fuel or by the tires in contact with the road when driving. Drag chains for removing static charges have proven to be inadequate. Especially in dry seasons when a tank truck is being loaded, a static charge may develop between the filling pipe and the tank truck. To prevent the possibility of a spark, the tank truck should be bonded to the loading pipe. Spark promoters also initiate the generation of static electricity. Spark promoters are unbonded conductive objects in a tank. For example, a gauge pipe projecting in the tank space could be a spark promoter. Between the pipe and the surface of the rising liquid there is a gap that may be bridged by static sparking. A solution to avoid static splashing is to extend the pipe to the bottom of the tank where it can make physical contact. Splashing and turbulence of the liquid entering the tank during open dome loading may cause electrostatic generation. Therefore, the opening of the fill pipe should be extended to the bottom of the tank. An oversized tee should be provided at the opening of the fill pipe and make physical contact with the bottom of the tank to prevent the pipe from lifting during the initial flow of liquid. Bottom loading tends to reduce the generation of static charges (except in the initial stage of bottom loading, it may produce higher liquid surface voltages). The human body may accumulate a dangerous static charge by the rubbing of shoes over floor coverings, particularly in dry weather. Therefore, rubber boots or rubber soled shoes are not recommended in dry locations where flammable vapors or gases are handled or processed. The basic concept of static electricity is the generation of an electrical charge after two bodies have been brought into close physical contact and then separated. When two bodies, particularly when they are of different materials, are brought into close physical contact, a redistribution of electrons in the bodies will occur causing the bodies to produce an attractive force. When the bodies are separated, a counter force is created and appears as energy between the bodies causing them to be charged. One body will become positively charged because it is left with an excess of protons. The other body will become negatively charged because it is left with an excess of electrons. The charges appear as a potential difference or a voltage between the two bodies. The voltage so obtained is the result of numerous physical contacts and separations. When the total number of electrons and protons of the bodies are equal, the bodies are neutralized or uncharged. The charge on a body is trapped when the body is non-conductive. The charge is also trapped when a conductive body is in intimate contact with a nonconductive body. If no conductive path is available between the bodies, the bodies remain charged. The potential between the bodies will increase with the increase

200 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres of separation and may reach several thousand volts. With a conductive path present, the electrical charges on the bodies will reunite and neutralize instantly. Bodies with no or low conductivity can be charged by an external force which causes one or more of the electrons to transfer from one place to another. For example, when glass is rubbed with silk, the friction transfers some of the free electrons from the glass to the silk, causing the glass to become positively charged and the silk negatively charged. The separated charges will try to recombine. If they are both conductive or if there is a conductive path between them, recombining is easy. If both bodies are non-conductive or if one conductive body is in close contact with a non-conductive body, recombining is difficult and recombination takes place either by a conductive path or spark. The conductivity of a body is basically a function of the number of free-floating electrons wandering between the atoms of the body. The nucleus of an atom may be made up of several protons and electrons and may be surrounded by large or small numbers of free-floating electrons. If the atoms have numerous free-floating electrons wandering between them, the body will have excellent conductivity. If the atoms have only a few free wandering electrons, the body has poor conductivity. Examples of high conductivity substances are crude oil, fuel oil, bitumen (asphalt), and other black oils. Examples of poor conductivity substances are plastic, rubber, resins, paraffins, glass, gases, and most of the petroleum oils such as gasoline, kerosene, naphtha, jet fuel, diesel oil, gas oil, benzene, and other white oils. Crude oil, therefore, would not be considered a hazard with respect to static electricity, but the other products that are not black oils would be considered hazards. The ability of a liquid to hold a charge is expressed in resistivity Ohm cm. A liquid with a resistivity greater than 10,000 megohm centimeters (1010 Ohm cm) may hold a charge, but when below this value, the liquid tends not to hold the charge. If the body has sufficient energy, a spark may occur. If the spark occurs in the presence of a flammable vapor-air mixture, ignition may result. The ability of a spark to produce ignition is mainly a function of its energy. The possibility of a spark is reduced when the loading product consists of crude oil or Class II and Class III flammable liquids and, in addition, when the tank has never held any Class I products (switch loading), or when the piping system forms a closed system with the tank. To comply with a closed connection, it is necessary that the pipes are connected to the tank before loading and disconnected from the tank after loading. If the tank should previously have held another liquid and should be loaded with a liquid of different vapor pressure, an ignition could develop. The conclusion gained from the foregoing is that all equipment used in a hazardous location should be adequately bonded and grounded. Bonding and grounding is the best and simplest way known for the rapid reduction of accumulated static charges.

Chapter 14 Groundings of Tanks, Pipelines, and Tank Cars

Potential differences between two objects will be eliminated by bonding them together by grounding one of them so that the static charges can drain away as fast as they are produced, or by increasing the relative humidity. When the free charges collect on the grounded substance, they flow to the earth and are neutralized; therefore, the possibility of sparks is zero. A resistance to ground up to 1,000,000 ohms is sufficient to prevent the electric charges from building up to a sparking potential. A film of moisture provides a suitable path to ground and also increases the conductivity. A humidity of between 60% and 70% will usually prevent the accumulation of static electricity. The shell of tanks intended to contain products that can produce a dangerous atmosphere should be permanently grounded. Tanks that do not contain flammable products but are located in a hazardous area should also be permanently and effectively grounded. The tanks should be grounded with ground anodes that are independently connected to the shell of the tank and spaced symmetrically around the tank. The resistance of these anodes in earth can be reduced by surrounding them by a backfill such as graphite, coke, or carbon. Increasing the length of the diameter of the anode will reduce its resistance to earth. Ground wires are to be thermally welded or bolted to the tank shells. To avoid corrosion, each earth connection should be installed not less than 18 inches above ground level.

201

202 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres If the inner walls of the tanks are provided with a thick coating of insulating material, the tank must be protected either by a metal surface on the coating or with a grounded earth plate in contact with the liquid. These measures should keep the contents at zero potential and reduce the accumulation of static electricity. With a thin epoxy coating, the static potential difference between the tank wall and the liquid is negligible and is, therefore, not considered hazardous. The possibility of a spark between the liquid surface and the tank wall is related to the static generating qualities of the product. When a tank is being filled, friction between the liquid and the feed pipe could cause the pipe to become charged, especially when the liquid is agitated in the tank. However, bonding and grounding may not be the total answer for a tank, if it is being filled with a low conductivity liquid. The low conductivity liquid becomes charged during the filling because of the friction between the liquid and the feed pipe. Therefore, in addition to the bonding and grounding, time must be allowed for the liquid to relax (to allow the liquid charge to stabilize and dissipate). It is safe practice to introduce a relaxation time of about 60 seconds or to attain a reduced flow rate. Normally, storage tanks are self-protecting and do not require lightning protection when the bottom of the tank is metal and in direct contact with earth of good conductivity. No additional grounding is then required. Metallic tanks with fixed or floating roofs have proven to remain in good condition and are well protected from damage from direct lightning strikes when all tank components are properly bonded and grounded. For tanks with floating roofs, the roof should be bonded to the shell of the tank. Floating internal blankets should also be effectively connected to the tank shell. Where metallic tanks are not in direct contact with earth or where the tanks are resting on an oil prepared foundation and are isolated from the associated pipelines, the normal grounding of the tank shell will suffice. No special grounding requirements are necessary for metal pipelines when they are electrically continuous and, therefore, part of the installation considered as properly grounded. However, non-conducting pipelines may become highly charged because of the movement of liquid or other substances flowing through them. Therefore, these pipes should be grounded by means of a ground wire wrapped around the pipe or by increasing the conductivity of the substance flowing through it (i.e., by using anti-static additives in the liquids). All pipelines terminating at a platform should be permanently and effectively grounded. Hoses are to be electrically continuous and the pipeline to the nozzle, including any swivel joints. When an open filling hatch is used, the hose should be lowered in the tank until the nozzle touches the bottom of the tank. Hoses that are semi-conducting can be considered electrically continuous. The pipelines should be made electrically continuous with the platform framework, which is to be directly grounded, preferably to a grounding grid. Bonds or jumpers are not required around flexible joints or swivel joints.

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The platform should be provided with an adequate flexible grounding cable for connection to a road vehicle prior to any loading or unloading procedure. A grounding clamp for the rail car is not necessary since the grounding of this car is obtained by contact of the wheels with the rails, which are bonded to the platform structure. It is recommended that a road vehicle that carries petroleum products be grounded in a safe area to release the electrostatic charge before it enters a platform. In addition, insulating couplings should be placed in the fill pipe below this bond connection or in the rail joints of the spur tracks to eliminate stray currents. Stray currents may flow in the pipeline or in the rails and may produce an arc when tank car connections are broken. The insulating joints should not be bridged by a tank car during filling with flammable liquids. Hoses that are used for filling containers, other than those mentioned above, should also be of the semiconducting type or be provided with an externally mounted, continuous, metal bonding wire that is visible to the operator. Bond wires should preferably be uninsulated for visual inspection of their continuation. The bonding and grounding of tank cars and unloading platforms are shown in Fig. 1-24. The truck and tank cars in Fig. 1-24 are shown for open dome top loading. Bonding or grounding for motor vehicles is not required during delivery of gasoline from service stations. Tests and experiences indicate that no ignition hazard is created during fueling operations. That is because the nozzle makes contact with the fill pipe before the liquid is being loaded. Prior to loading, the charge has equal quantities of positive and negative ions making the charge in the liquid neutral. During loading, a charge is forced upon the uncharged body of the gas tank. During this stage, sparking may occur, if the nozzle is not in contact with the fill pipe. Charging of the gas tank will increase as a result of splashing and turbulence of the liquid until such a time that they are equal in magnitude as the charge in the liquid. Immediately, the charge in the gas tank starts to separate into positive and negative ions. Positive ions may settle in the inner tank wall, but then negative ions will settle on the outer tank wall or settling will occur vise versa. No sparking will occur as long as there is a bond between nozzle and fill pipe. As soon as the vehicle moves, the charge in the outside of the tank wall will drain off, due to the rolling movements of the rubber tires over the pavement. The charge in the inside of the tank will stay until such time that positive and negative ions will neutralize each other. For bonding drums and cans refer to Fig. 1-24A.

204 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-24. Grounding and bonding at shipping wharves and loading racks.

Chapter 14: Grounding of Tanks, Pipelines, and Tank Cars

Figure 1-24A. Grounding and bonding of drums and cans.

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Chapter 15 Grounding Requirements for Electrical Equipment

A.

GENERAL

In a hazardous location, grounding of an electrical power system and bonding of enclosures for circuits and electrical equipment in the power system is essential. Power systems which are not grounded are highly susceptible to over-voltage. If the neutral of a power transformer is not grounded to earth, the power distribution system, connected to the power transformer, is highly susceptible to severe overvoltages during a phase to ground fault. Bear in mind that if a neutral system is not purposely grounded to earth, the neutral is not truly divorced from earth because it is still capacitively coupled to earth. Every element of the power system contains capacitance to ground no matter how small. This capacitance constitutes an inherent capacitive impedance between circuit-elements of the power system and earth. However, the connection of a “resistance” or “capacitance” between circuit elements and earth, accidentally or purposely applied, does not produce overvoltages. Only the connection of a “reactance” to earth is responsible for overvoltages, such as a starter coil or motor winding or any other inductive circuit element. If an inductive reactance should accidentally be connected to earth, the reactance will be in series with the charging capacitive reactance of the power system. As a result of this condition, the accidental connection of an inductive reactance to ground may be the cause of serious over-voltages in the power system.

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If the inductive reactance of one or more circuit elements to earth and the total charging capacitive reactance of the power system to earth are equal, the voltage across both reactances are also equal and may become extremely high (as much as 10 times normal or more). It is the ratio of the inductive reactance to earth to the total capacitive reactance of the power system to earth which controls the degree of the over-voltage. The highest over-voltage will occur when there is a large charging capacitance in the system and both reactances are the same. Therefore, if the neutral of a power system is not purposely grounded to earth, it must be recognized that a phase to ground fault may produce serious overvoltages. The danger of the over-voltage is that it puts the insulation of circuits and electrical equipment under too much stress (which will cause the insulation to breakdown). If the insulation breaks down, as a result of too much stress, a current will flow from the point of failure to earth accompanied by arcs or sparks. These arcs and/or sparks will increase the explosion danger in the hazardous location. Dangerous arcs or sparks as a result of ungrounded neutrals can be completely eliminated by suppressing the over-voltage with a relatively high resistance connected between the electrical system neutral and earth. A ground resistor of about the same ohmic value as the total charging capacitive reactance to earth, is generally sufficient to completely eliminate a dangerous voltage stress, which consequently reduces insulation failures. However, arcs and/or sparks can still occur if the power system is properly grounded (i.e., solidly or by low or high resistance), because the insulation of the power system is still under voltage stress, although to a much lesser extent. Therefore, in a grounded power system arcs and/ or sparks will occur if the insulation breaks down due to damage, or if worn out, or squeezed, or if the grounding path to earth lacks sufficiently continuity. For example, arcing or sparking may occur along the conduit path of an electric motor when the conduit lacks sufficient continuity. The electric motor shown in Fig. 1-25, for example, receives its power through wiring enclosed in a metal conduit. If this metal conduit should be the sole external ground return path, current will flow along this path as a result of insulation failure when the motor enclosure becomes unintentionally energized. During the flow of the fault current, a substantial potential difference could exist between the motor housing and earth. If the external return path lacks sufficient continuity, sparks or arcs will occur at the location where continuity is lacking. If there should be a separation in the conduit run to the motor, a potential difference caused by an insulation failure will appear at the separated elements and arcs or sparks will be produced at this location. A small, non-visible separation could exist, for example, in the conduit union marked with an a in Fig. 1-25. This separation could easily exist if the two parts of the union are not completely tightened, or when the union is not free from dirt, grease, or corrosion. When an arc or spark does appear in the separated elements of the union (and most likely they will under sufficient voltage stress), they can easily ignite any flammable gas or vapor in the immediate vicinity of the electric motor.

208 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Arcs or sparks may also be produced between the two metal pipes at location b, as shown in Fig. 1-25. One pipe is in direct contact with the electric motor enclosure, and the other pipe is in direct contact with earth. Both pipes are separated from each other by a gap of high resistance. During an insulation failure, the gap allows a potential difference to exist between the pipes. Arcs or sparks are generally of sufficient energy to initiate an explosion when the faulty electrical equipment is inductive and surrounded by a flammable gas or vapor. The minimum sparking energy required to ignite hydrocarbon-air mixtures ranges from approximately 0.017–0.3 millijoules. A hydrogen gas-air mixture, for example, can be ignited by a spark with an energy as low as 0.017 millijoules (one joule is electrical energy in terms of 1.0 volt × 1.0 amp/sec). When the neutral of the power system is grounded, dangerous arcs or sparks can also be completely eliminated by applying two grounds: 1) an internal or external grounding conductor running in close proximity with the phase conductors between the electrical equipment housing and the neutral of the power supplying equipment; and 2) by bonding the housing of the electrical equipment to a supplementary grounding system which also must be connected to the grounded neutral of the power supply equipment. This supplementary grounding system has a dual function. It eliminates arcs or sparks and it eliminates shock hazards when a fault current is flowing as a result of an insulation failure. The supplementary ground alone, without an internal or external grounding conductor, is not permitted. The power supplying conduit to the electric motor as shown in Fig. 1-25, which is partially buried in earth, is generally inadequate functioning as a supplementary ground. Proper grounding of circuits and electric equipment in a hazardous location is of vital importance. The recommended grounding practice for a hazardous location is not only grounding of the system neutral, at the power source, but also by using an external or internal grounding conductor in combination with a supplementary grounding system.

B.

INTERNAL AND EXTERNAL GROUNDING CONDUCTORS

There are two types of grounding conductors: an internal grounding conductor and an external grounding conductor. Both types are required for carrying phase to ground fault currents from an unintentionally energized circuit or equipment enclosure to the neutral of the electrical power source. An internal grounding conductor may consist of a copper wire, solid or stranded, insulated or bare. An external grounding conductor usually consists of rigid metal conduit, electrical metallic tubing, flexible metal conduit approved for the purpose, a cable tray, armor of type AC cables, or other raceway approved for carrying ground fault currents. Both types of grounding conductors are applied for bonding and grounding enclosures for circuits and electrical equipment. Only one of the types is normally used for grounding purposes.

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Figure 1-25. Supplementary grounding of electrical equipment.

If an internal grounding conductor is used for grounding electrical equipment, it is normally green colored, insulated or bare. External grounding conductors consisting of rigid metal conduits are usually less reliable than internal copper grounding conductors. The reason for this is that conduit joints may be of poor workmanship causing high resistance or prevent continuity as explained before. Therefore, external grounding conductors may allow arcs or sparks to occur under fault conditions. Arcing may start between the threads of the joints at certain current levels when the joints are not completely tightened or when they are not sufficiently clean. These joints may not only produce arcs or sparks but also a stream of molten metal during heavy fault conditions. External grounding conductors consisting of aluminum conduits are more suitable for a fault return path because the probability of producing arcs between the threads is much less. The reason is that the softer the material, the more the threads tend to deform. Use of aluminum will ensure a better

210 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres electrical continuity when the couplings are tightened. However, external grounding conductors and fittings made of aluminum shall not be used in earth or concrete when subject to corrosive conditions. Where general-purpose enclosures are used in hazardous locations, the ground return path may become even more unreliable because metal threaded conduits may be used in conjunction with ordinary locknuts and bushings. Ordinary locknuts may be used, but only if bonding jumpers are applied between the enclosure and the raceway. Bonding jumpers could be deleted if both locknuts are of the carving type. These locknuts, when applied to rigid steel conduits entering a general-purpose enclosure, will carve into the metal of the enclosure and will provide a low impedance between raceway and enclosure. However, the application of carvingtype locknuts must be considered unreliable because their application depends entirely on proper workmanship. When this is lacking, arcing and sparking may occur between raceway and enclosure under fault condition. Bonding jumpers may be deleted if the bushings are of the bonding type in which a jumper must be applied between the bushing and a ground terminal in general-purpose enclosures. An external grounding conductor, for example, is the vertical conduit to the electric motor in Fig. 1-25. Whether an internal or external grounding conductor is used, it is required to run in close proximity with its phase conductors. This is to minimize the impedance of the ground return path. The impedance of the phase conductors and of the grounding conductors depends greatly on the size of the conductors but mostly on the distance between the individual conductors. A low impedance of the ground return path is important because it allows fast tripping of the over-current devices under fault condition and it will shorten the life span of arcs or sparks if they do appear under a phase to ground fault. The tripping time of over-current devices is dependent on the magnitude of the phase to ground fault current which in turn is a function of the impedance of the ground fault return path. If the impedance of the grounding conductor is high, the higher impedance will reduce the fault current to a lower magnitude resulting in a longer tripping time. For example, the arrangement in Fig. 1-26, is in violation of the requirements for fast tripping. The fault current in the lamp is required to follow the same route as the current in the supply conductors. Instead, the fault current will flow through the metallic return path as shown by arrows in Fig. 1-26. Since the metallic return path has a much higher impedance, the magnitude of the fault current will be smaller, resulting in a longer tripping time. With the equipment ground not kept physically close to the supply conductors, the impedance of the fault circuit will have a greater inductive reactance and a greater AC resistance due to a smaller mutual cancellation of the magnetic fields around the conductors. This will result in a greater voltage to ground while the circuit over-current devices will operate more slowly because of the smaller current. Therefore, the steel framework of a building that is constructed without regard for a low impedance for the flow of fault current, does not comply with the fast tripping requirements when it is used as the sole grounding conductor.

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Internal and external grounding conductors are shown in Fig. 1-27. The power supplying equipment shown in Fig. 1-27 represents a “service supplied AC system” which requires two system grounds where the “service applied AC system” is located inside a building, and the “power supplying equipment” outside the building. The main service equipment in the building must be grounded and one additional ground connection must be made at the power supplying equipment as shown in Fig. 1-27. This additional ground connection shall not be made on high impedance grounding. Electrical energy is normally provided by the Utility Company, but can also be provided by a separately derived system. The neutral of a separately derived system is allowed to be connected to a grounding electrode at any point between the power supplying equipment and the first service disconnecting means. The electrode grounding conductor must be connected at the same point as the grounding conductor is connected. For example, if the electrode conductor is connected to a stud in the transformer, the neutral of the transformer must also be connected to the same stud. On the other hand, if the electrode conductor is connected to a stud in the main service equipment, the neutral of the transformer must also be connected to the same stud. Since two grounding connections are made in Fig. 1-27 (one ground at the power supplying equipment and one ground at the main service disconnecting means), the power supplying equipment is not considered a separately derived power source. A separately derived power source is a generator, converter, or transformer including a solidly connected grounded conductor without direct electrical connection to another grounded system.

C.

SUPPLEMENTARY GROUNDING SYSTEMS

The basic concept for applying supplementary grounding in a hazardous location is to reduce the potential differences between the electrical equipment and earth during a phase to ground fault. Reducing the potential differences is accomplished by bonding the enclosures of circuits and electrical equipment to the supplementary ground system by means of a bonding jumper (c), as shown in Fig. 1-25. A supplementary ground system may consist of the following: 1. A ground grid system of copper conductors buried in earth 2½ ft or more deep, each conductor not smaller than 1/0 AWG. 2. A single bare copper conductor sized 1/0 AWG minimum, buried in earth at least 2½ ft deep and looped around the electrical equipment. 3. The metal frame of a medium-sized building with the building columns thermally welded to a copper grounding conductor looped around the building. The ground loop is required to be buried a minimum of 18" below the finished grade. If a water pipe is

212 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres available, the loop must be connected to the water pipe, if it is of sufficient length (10 ft minimum and electrically continuous). Underground metal gas pipes and aluminum electrodes are prohibited. 4. The metal frame of a large building provided with a network of copper conductors underneath the foundation of the building, and the grounding network supplemented by galvanized or coppercoated grounding rods of at least 8 ft in length and 3/4" in diameter.

Figure 1-26. Incorrect grounding method.

Chapter 15: Grounding Requirements for Electrical Equipment 213

Figure 1-27. Internal/external ground with supplementary ground.

214 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Electrical equipment located in these buildings must be grounded to the building structure by means of ground leads or by bolting or welding the electrical equipment to the steel frame of the building. There are practical limits which will determine the minimum and maximum size of the supplementary grounding system. For mechanical strength, the buried conductors shall not be smaller than 1/0 AWG but it is not necessary to exceed 500 MCM. A supplementary grounding system is not permitted to be used in lieu of internal or external grounding conductors. They may only be used for supplementary protection. Where a metal sleeve is used for protection of the grounding conductor to the supplementary ground system, the sleeve must be bonded at both ends to the grounding conductor. The supplementary ground system must also be connected to the neutral of the power supplying equipment as shown in Fig. 1-27. The size of a grounding conductor can be determined on the basis of currentflow through the grounding conductor, the duration of the current-flow and the initial conductor temperature and the maximum conductor temperature rise. The initial conductor temperature should be based on ambient temperature during summer time. The duration of current flow will vary with the type of tripping requirements in the over-current devices protecting the phase conductors. The size of a grounding conductor can be determined with the following equation: CM = Is × t × 11 where Is is the short circuit current flowing through the grounding conductor to earth, t is the duration of the current flow, 11 is a factor based on an initial conductor temperature of 20ºC and a final temperature of 250ºC. This factor is slightly greater than 11 if the initial conductor temperature is in excess of 20ºC. For example, assume that the initial grounding conductor temperature is 20ºC, the final temperature 250ºC, the short circuit current through the grounding conductor to earth 31,000 A and the current flow duration 5½ cycles when the phase wires are protected by a circuit breaker. Assume a circuit breaker parting time of 2½ cycles and 2½ cycles for the arcing time and ½ cycle for the relay time (which totals to 5½ cycles or 0.0917 sec). The size of the grounding conductor in circular mills is then: 31,000 × 0.0917 × 11 = 103,270 CM. The next larger standard conductor size is: 105,600 CM or 1/0 AWG. As in this case, if the grounding conductor is 1/0 AWG, then the loop or ground grid should also have a size of 1/0 AWG.

Chapter 16 Application of Seals in NEC Class I Locations

A.

GENERAL

To maintain a safe environment in a hazardous location, it is important that conduit systems are provided with suitable seals. The recommended practice for locating sealing fittings in electrical conduit installations is based on the requirements outlined in Article 501-5 of the NEC. Sealing fittings in conduits are to prevent the movement of gases and vapors from a hazardous to a remote hazardous location, or from a hazardous to a nonhazardous location. They are also to prevent an explosion from traveling from one portion of the conduit system to another portion of the conduit system, thereby reducing the build-up of high pressures. When seals are not used, an explosion inside an explosion-proof enclosure will travel rapidly through the conduit system, building up a pressure that could exceed the strength of the conduit. Three types of well-known sealing fittings are shown in Fig. 1-28. They are to be applied as follows. Seals are required in conduits in a Div. 1 or Div. 2 location under any one of the five following conditions: 1. If the conduit of any size enters an enclosure that is explosion proof in which the enclosure contains arcing devices (such as a switch, circuit breaker, fuse, receptacle, controller, or starter) capable of igniting a flammable gas or vapor, or in which the 215

216 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres enclosure contains a device that operates hot enough to ignite a flammable gas or vapor (such as resistors, relay coils, electric motors, etc.). 2. If the conduit has a size of 2" or larger and is entering an explosion-proof enclosure for terminals, splices, or taps. 3. If the conduit has a size of 2" or larger and is entering an explosion-proof enclosure which is factory sealed. Generally no seal is required if the equipment is “factory sealed,” unless, for example, a conduit of 2" or larger terminates in an electric motor, an additional seal is required. Factory sealed equipment must be marked as such.

Figure 1-28. Sealing fittings.

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For the above three items, the seal shall be placed within 18" of the enclosure. 4. In a conduit-run passing from a Div. 1 location to a Div. 2 or nonhazardous location. 5. In a conduit-run passing from a Div. 2 location to a nonhazardous location. For Items 1, 2, and 3, the conduit between the seal and enclosure may be provided with approved fittings. These approved fittings are explosion-proof unions, couplings, elbows, and conduit bodies similar to T, L, or other type bodies. According to the NEC, they may be installed in the conduit between the seal and the enclosure. They are permitted because the internal volume of the fittings are sufficiently small to prevent accumulation of dangerous concentrations of flammable gases or vapors. However, good practice dictates that only the union, coupling, and elbow between the seal and the explosion-proof enclosure be applied, not capped conduit bodies from which the cover can be permanently removed allowing flammable material easy access to the explosion-proof enclosure. None of the above conduit fittings may be installed in the conduit between the seal and the point where the conduit leaves the Div. 1 or Div. 2 location to a nonhazardous location (as pointed out in Items 4 and 5). The NEC allows a seal on either side of the boundary for the conduits in Items 4 and 5. Such an application in which only one seal is used is considered safe if a solid wall separates two different locations. In this case, it makes no difference whether the seal in the penetrating conduit is located on one side of the wall or the other side of the wall as long as there are no fittings in the penetrating conduit between the seal and the wall. Where the conduit does not penetrate a dividing solid wall, the application of a seal on either side of the boundary may not be safe. For example, if the conduit does not penetrate a dividing wall but leaves the hazardous area through the floor instead, a seal located on either side of the boundary must be considered unsafe. A heavier-than-air flammable gas or vapor could by-pass the seal located above or directly below the floor when it enters a conduit joint under the floor via a crack in the floor. This could be disastrous when the conduit under the floor penetrates a wall to a nonhazardous location. Therefore, if the flammable material in the hazardous area above the floor is heavier than air, two seals must be applied, one in the conduit above the floor and one beyond the conduit joint under the floor. Both seals will prevent a flammable gas or vapor from spreading out when it enters the conduit joint. However, if the flammable product in the hazardous area is lighter than air (i.e., having a vapor density of below 0.75), then only one seal is required in the conduit. In this case, it makes no difference whether the seal is located in one or the other location. Also, under these conditions, no fittings, unions, couplings, elbows, boxes, etc. may be placed in the conduit between the seal and the point where the conduit leaves the area.

218 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The borderline between a Div. 1 area and a Div. 2 area or between a Div. 2 area and a nonhazardous area does not always have to be separated by a wall. It can also be a fictitious borderline in the open. If such a borderline does exist, a seal on either side of the boundary is not required if the Div. 1 area is relatively small with respect to the Div. 2 area or if the Div. 2 area is relatively small with respect to the nonhazardous area. Seals in Div. 1 and Div. 2 locations are not required in conduits if: 1. The conduits are entering non-explosion-proof enclosures. 2. Explosion-proof enclosures containing arcing devices with their sparking contacts immersed in oil or in hermetically sealed chambers are connected to conduits with sizes of 1½ inches and smaller. 3. Explosion-proof enclosures provided with internal factory seals are connected to conduits with sizes of 1½ inches and smaller. Factory sealed devices shall be marked as such. 4. Explosion-proof enclosures containing terminals, splices, or taps are connected to conduits with sizes of 1½ inches and smaller. 5. The conduits in the hazardous locations have an entire unbroken length (without fittings). Both ends of the conduits are extending into a nonhazardous location and no fittings are present in the extensions within 12 inches from the hazardous location (see Fig. 1-29-M). Cables which are capable of transmitting flammable material are comparable with conduits. Therefore, the sealing requirements for conduits are also applicable to these types of cables. Even if the cable terminals are above ground in a short conduit sleeve, a seal must be applied in the sleeve if a seal also would have been required in a comparable conduit run. If the cable is not capable of transmitting flammable material, a seal in the conduit sleeve has no effect on the transmission of flammable material. The reason that a seal must be applied in the sleeve (if a seal should be required in a conduit run) is not because flammable material must be prevented from moving from one location to another location. Its purpose is to confine and restrict the explosion danger to the local enclosure. The following section is a detailed explanation of the location of the sealing fitting in conduits under various conditions (refer to Fig. 1-29).

Chapter 16: Application of Seals in NEC Class I Locations

Figure 1-29. Applications of sealing fittings.

219

220 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-29. (Cont’d.)

Chapter 16: Application of Seals in NEC Class I Locations

Figure 1-29. (Cont’d.)

221

222 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-29. (Cont’d.)

Chapter 16: Application of Seals in NEC Class I Locations

Figure 1-29. (Cont’d.)

223

224 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-29. (Cont’d.)

Chapter 16: Application of Seals in NEC Class I Locations

Figure 1-29. (Cont’d.)

225

226 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 1-29. (Cont’d.)

Chapter 16: Application of Seals in NEC Class I Locations

Figure 1-29. (Cont’d.)

227

228 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

B.

CLASS I, DIVISION 1 LOCATIONS

A sealing fitting which is required in a conduit of any size entering an explosion-proof enclosure capable of producing arcs, sparks, or high temperatures under normal operating conditions is shown in Fig. 1-29-A. Some NEC articles shown in Fig. 1-29 refer to the 1996 Code. Good practice is to allow only explosion-proof unions, couplings, and elbows between the sealing fitting and the enclosure. Explosion-proof enclosures containing terminals, splices, or taps need to have a sealing fitting of 2" or larger in the conduit as shown in Fig. 1-29-B. Two separate explosion-proof enclosures together may have one seal as shown in Fig. 1-29-C if both enclosures are not more than 36" apart. However, if the explosion-proof enclosures are more than 36" apart, as shown in Fig. 1-29-E, then two seals are required within 18" of each enclosure. In Fig. 1-29-D, there are two explosion-proof enclosures shown, one enclosure contains arcing devices with sparking contacts, the other contains arcing devices without sparking contacts because they are located in a hermetically sealed chamber. If the explosion-proof enclosure containing arcing devices with contacts within a hermetically sealed chamber (or if these contacts are immersed under 2 inches of oil for power or under 1 inch of oil for controls), a seal in the conduit is only required if the conduit entering the explosion-proof enclosure is 2 inches or larger. An example of a hermetically sealed chamber is a glass tube (as shown in Fig. 1-29-I) that will prevent the igniting of a flammable gas or vapor in air. Contacts immersed in oil will prevent ignition of a flammable gas or vapor in the air. Ignition is only possible if the glass tube breaks or if the oil drains away, thereby exposing the contacts to the air. The conduit entering the explosion-proof enclosure with the arcing devices not hermetically sealed, as shown in Fig. 1-29-D, needs a seal for any size conduit entering the enclosure. A sealing fitting is required in each conduit run leaving a Class I, Div. 1 location, as shown in Fig. 1-29-E. The sealing fitting may be located on either side of the boundary of the location as explained in Sec. A of this chapter. Where explosion-proof unions, couplings, boxes, or fittings are permitted between the seal and the explosion-proof enclosure, none of these fittings are permitted between the sealing fitting and the point at which the conduit leaves the Div. 1 location, as shown in Figs. 1-29-E and 1-29-I. It is good practice to seal a conduit above ground in a hazardous area every 50 ft. Sealing fittings are also required in a conduit entering an explosion-proof terminal box for an electric motor. Even though the terminal box is sealed by the manufacturer, if the conduit entering the terminal box is 2 inches or larger, an additional sealing fitting is required as shown in Fig. 1-29-F. A multi-conductor cable, which is not capable of transmitting a flammable gas or vapor through its core, entering an explosion-proof enclosure, requires a seal in the conduit (as shown in Fig. 1-29-G) if:

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1. The enclosure contains arcing devices. 2. If the arcing devices are hermetically sealed in a chamber and the conduit entering the explosion-proof enclosure is 2 inches or larger. 3. If the contacts of the arcing devices are immersed under 2 inches of oil minimum for power and 1 inch minimum for control and the conduit to the explosion-proof enclosure is 2 inches or larger. If a multi-conductor cable, with gas- and vapor-tight sheath in conduit entering an explosion-proof enclosure, is capable of transmitting flammable gases or vapors through the cable core, the method of sealing is different than shown in Fig. 1-29-G. The jacket of the cable and the other coverings must be partially removed from the cable so that the sealing compound can surround the individual insulated conductors in the seal, as shown in Fig. 1-29-H. The multi-conductor cable in Fig. 1-29-H shall be sealed as required by Fig. 1-29-G.

C.

CLASS I, DIVISION 2 LOCATIONS

Arcing devices of the sparking type in a Div. 2 location also require explosion-proof enclosures. Conduits of any size entering these explosion-proof enclosures must be provided with sealing fittings, as shown in Fig. 1-29-J. However, as for Div. 1 locations, if the contacts of the arcing devices are located in hermetically sealed chambers or if the contacts of the arcing devices are immersed under a minimum of 2 inches of oil for power, or under a minimum of 1 inch of oil for controls, a sealing fitting is not required if the conduit entering the explosionproof enclosure is 1½ inches or smaller (as shown in Fig. 1-29-I). If the enclosure for the arcing devices in hermetically sealed chambers or immersed in oil is of the general purpose type, no seals are required (as shown in Fig. 1-29-K). The enclosure of an oil immersed starter in a Div. 2 location need not be explosion proof because the contacts of the starter are immersed in oil. Since the enclosure is not explosion proof, no sealing fittings are required. However, if the enclosure of the oil immersed starter is required to be explosion proof, a sealing fitting is required in the conduit entering the starter if the size of the conduit is 2 inches or larger. A sealing fitting is required in each conduit run leaving the Class I, Div. 2 location (as shown in Fig. 1-29-L). The sealing fitting may be located on either side of the boundary of the location. In addition, rigid steel conduit or threaded steel intermediate conduit must be used between the sealing fitting and the point at which the conduit leaves the Div. 2 area. No seals are required on either side of the boundaries if an entire unbroken conduit length in the hazardous area and within a length of 12 inches beyond the hazardous area does not contain a union, coupling, box, or fitting (as shown in Fig. 1-29-M).

230 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Cables in conduit which are not capable of transmitting flammable gases or vapors through their core require a sealing fitting in the conduit of any size entering an explosion-proof enclosure if the enclosure contains arcing devices of the sparking type, as shown in Fig. 1-29-N. A sealing fitting is also required if the enclosure contains arcing devices which are in a hermetically sealed chamber and the conduit entering the enclosure is 2" or larger, or if the contacts of the arcing devices are under a minimum of 2" of oil for power and a minimum of 1" for control and the conduit entering the enclosure is 2" or larger. However, a cable in a conduit which is capable of transmitting flammable gases or vapors through the cable core requires that each individual conductor be sealed if a seal in the conduit is required. If a seal is required, the jacket and other coverings of the cable must be partially removed so that each individual conductor in the sealing fitting can be surrounded by the sealing compound (as shown in Fig. 1-29-O). The seal will prevent flammable material from moving through the conduit and the cable from one location to another location. In Div. 2 locations, multi-conductor cables do not necessarily have to be terminated in explosion-proof enclosures. Enclosures for terminating cables may be of the general-purpose type if the enclosures are not provided with arcing devices. Multi-conductor cables in conduit which are not capable of transmitting a flammable gas or vapor do not have to be sealed if the enclosure is of the general purpose type, as shown in Fig. 1-29-P. Multi-conductor cables, which are capable of transmitting gases or vapors through the cable core, are normally cables with a continuous gas/vapor-tight sheath. If these cables are capable of transmitting flammable gases or vapors through their core, but are entering a general-purpose enclosure, a seal is not required except when they leave the Div. 2 hazardous area (in which case a seal must be placed at the border of the Div. 2 area and nonhazardous area). However, if the enclosure in which the cable must be terminated is required to be explosion proof, a seal is required in the conduit for the cable (in accordance with the requirements of Figs. 1-29-N and 1-29-O). A seal is not required if the enclosure need not be explosion proof as shown in Fig. 1-29-Q. Cables in a hazardous location approved for direct burial do not necessarily have to be protected in the ground by a raceway. Only the part of the cable which emerges from the ground must be protected by a raceway. The sealing requirements for these cables in conduit sleeves shall be the same as for cables not approved for direct burial. Splices, taps, and terminals in a Div. 2 location normally do not require an explosion-proof enclosure, and, therefore, no seal is necessary. However, if an explosion-proof enclosure should be required for splices, taps, and terminals in a Div. 2 location, a seal is also necessary. Boxes, fittings, and conduit joints are not normally required to be explosion proof in a Div. 2 location. In Div. 1 locations, boxes, fittings, and conduit joints shall be of metal, threaded, and explosion proof. Threaded joints must be made up with a minimum of five threads.

Chapter 17 Application of Seals in NEC Class II Locations

Sealing fittings in a conduit are only required in Class II hazardous locations if the conduit enters a dust-ignition-proof enclosure that is connected to a nondust-ignition-proof enclosure. Seals or other means of protection must be provided in the conduit to prevent dust from entering the dust-ignition-proof enclosure. When a conduit is entering the dust-ignition-proof enclosure from the top, dust can be prevented from entering the enclosure when provided with a sealing fitting as shown in Fig. 1-30-a. Any sealing fitting designed for a Class I location can be used in a Class II location. Dust can also be prevented from entering a dust-ignition-proof enclosure with other suitable means such as sufficient conduit length. When the conduit is entering the enclosure from the side, the conduit must be at least 10 ft long (as shown in Fig. 1-30-b). If the conduit enters the enclosure from the bottom, the conduit must be at least 5 ft long as shown in Fig. 1-30-c. No seals are necessary with conduits 5 and 10 ft long. The 5 ft and 10 ft requirements are based on the traveling capability of dust in conduits. It has been established that a straight conduit of 10 ft long connected to the side of an ignition-proof enclosure and a straight conduit of 5 ft long connected to the bottom of an ignition-proof enclosure will prevent dust from entering the enclosure. Any conduit length entering an enclosure from the side or bottom which is shorter than 5 ft or 10 ft must be considered unsafe, since dust is considered capable of entering the enclosure because of the shorter traveling distance. 231

232 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Unions, boxes, condulets, etc., in the 5 ft or 10 ft length of conduit are considered to reduce the safe length of the 5 ft and 10 ft. A union that is not made wrench tight may allow the entrance of dust. From the outside, no one can see whether the union is made wrench tight, and, therefore, this fitting is not as reliable as a tapered conduit coupling or an elbow that does not allow the entrance of dust, even when not made wrench tight. The purpose of condulets and small boxes is to have access to the conduit system. If they are opened during inspection or maintenance, the cover plates of the condulets and small boxes may not be reinstalled properly or even be forgotten, allowing dust to enter the fittings. Therefore, it is a safe practice not to allow any condulets, boxes, unions, or other similar fittings in the 5 ft and 10 ft safe conduit runs. If the conduits of 5 ft and 10 ft length should require a union, condulet, box, or similar device, it is recommended to place a sealing fitting between these fittings and the dust-ignition-proof enclosure (as shown in Fig. 1-30-d). In Class II locations, sealing fittings do not have to be explosion proof. When sealing fittings are required, any of those designed for Class I locations can be used.

Figure 1-30. Dust prevention for NEC Class II location.

Part 2

Application of Fundamentals

233

Chapter 18 Environmental Conditions in NEC Class I Hazardous Locations

Part 2 covers a great number of illustrations which can be applied directly to a hazardous area under consideration to establish the degree and extent of the hazard in the location. The various illustrations are documented in tables and represent a number of cases which normally will exist in petrochemical and chemical plants for Class I flammable products. The degree and extent of hazard shown in the illustrations are developed on the basis of four specific conditions as follows: 1. An open or closed source of hazard. 2. A heavier- or lighter-than-air flammable product. 3. A source of hazard in an indoor or outdoor location. 4. A location sufficiently or insufficiently ventilated. The above conditions allow a number of different combinations which are compiled in Table 2-1. Each combination of conditions in Table 2-1, is shown in bold letters and is identified by a code letter. Each code letter represents a specific condition under which a source of hazard can operate. For example, code letter “A” in Table 2-1 represents a condition in which a closed source of hazard handling a heavier-than-air flammable product is located in an indoor location which is sufficiently ventilated. The code letter and each combination of conditions are shown in Tables 2-2. There are eleven Tables 2-2.

235

236 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table 2-1. Eleven Different Combinations of Major Conditions A

CLOSED sources of hazard with HEAVIER-than-air gases or vapors in SUFFICIENTLY ventilated INDOOR locations.

B

CLOSED sources of hazard with HEAVIER-than-air gases or vapors in INSUFFICIENTLY ventilated INDOOR locations.

C

CLOSED sources of hazard with HEAVIER-than-air gases or vapors in SUFFICIENTLY ventilated OUTDOOR locations.

D

OPEN sources of hazard with HEAVIER-than-air gases or vapors in SUFFICIENTLY ventilated INDOOR locations.

E

OPEN sources of hazard with HEAVIER-than-air gases or vapors in INSUFFICIENTLY ventilated INDOOR locations.

F

OPEN sources of hazard with HEAVIER-than-air gases or vapors in SUFFICIENTLY ventilated OUTDOOR locations.

G

CLOSED sources of hazard with LIGHTER-than-air gases or vapors in SUFFICIENTLY ventilated INDOOR locations.

H

CLOSED sources of hazard with LIGHTER-than-air gases or vapors in INSUFFICIENTLY ventilated INDOOR locations.

I

CLOSED sources of hazard with LIGHTER-than-air gases or vapors in SUFFICIENTLY ventilated OUTDOOR locations.

J

CLOSED or OPEN sources of hazard with HEAVIER- or LIGHTERthan-air gases or vapors in SUFFICIENTLY or INSUFFICIENTLY ventilated locations

K

CLOSED sources of hazard with HEAVIER-than-air gases or vapors in SUFFICIENTLY or INSUFFICIENTLY ventilated locations.

Each Table 2-2 has three vertical columns indicating the following specific features under which a source of hazard will operate: 1. Type of location. 2. Size of source of hazard. 3. System pressure. Each of the three features in combination with the four specific conditions in the title block of Table 2-2 will result in a great number of different cases. These cases are represented by a number of illustrations and are grouped under the same code letter as in Table 2-2. The cases are primarily related to Class I flammable products although some cases will also cover Class II and Class III flammable products.

Chapter 18: Environmental Conditions

237

Table 2-2A. Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations

238 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table 2-2B. Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations

Chapter 18: Environmental Conditions

239

Table 2-2C. Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations

240 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table 2-2C. (Cont’d.)

Chapter 18: Environmental Conditions

241

Table 2-2D. Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations

Table 2-2E. Open Sources of Hazard with Heavier Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations

242 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table 2-2F. Open Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations

Table 2-2G. Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Indoor Locations

Chapter 18: Environmental Conditions

243

Table 2-2H. Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Insufficiently Ventilated Indoor Locations

244 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table 2-2I. Closed Sources of Hazard with Lighter Than Air Gases or Vapors in Sufficiently Ventilated Outdoor Locations

Chapter 18: Environmental Conditions

245

Table 2-2J. Closed or Open Sources of Hazard with Heavier or Lighter Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations

Table 2-2K. Closed Sources of Hazard with Heavier Than Air Gases or Vapors in Sufficiently or Insufficiently Ventilated Locations

246 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The illustrations are identified as follows. In the top of the illustration, the vapor density of the flammable product is identified as heavier or lighter than air and whether the source of hazard has an open or closed operating mode. At the bottom of the illustration, the type of source of hazard is identified and whether the hazardous location for the source of hazard is sufficiently or insufficiently ventilated. Specific environmental conditions and the degree and extent of the hazard are also shown in the illustrations. If the source of hazard in the illustrations is shown in an enclosed location, the location is considered indoors. If the source of hazard is shown in a nonenclosed location, the location is considered outdoors. If the location in the illustration is shown with a roof opening or mechanical fan, it must be understood that the location is sufficiently ventilated. If the location is non-enclosed, it must also be understood that the location is sufficiently ventilated by natural ventilation. If the enclosed location is shown without a roof opening or mechanical fan, it is understood that the location is insufficiently ventilated. Sufficiently ventilated location means a ventilation system that is satisfactory and in compliance with the ventilation requirements as explained in Ch. 6 of Part 1. For mechanical ventilation to operate in a building satisfactorily, its proper location must be determined on the basis of the actual location of doors, windows, louvers, and similar openings. This may require that the mechanical ventilation be placed in the wall or in the roof of the building opposite and as far away as possible from these openings. Bear in mind that the mechanical ventilation or vent as shown in the roof or wall of buildings in the illustrations does not intend to indicate its actual location. These buildings merely indicate the presence of sufficient ventilation. Actual locations for mechanical ventilation can be found in Ch. 6 of Part 1. To assure that mechanical ventilation provides sufficient ventilation, it is necessary that the ventilation is capable of diluting a flammable gas or vapor in the air to safe concentrations as explained in Ch. 6 of Part 1. Sometimes it is necessary to provide the mechanical ventilation with a safeguard. For example, a location adjacent to or within a hazardous area, which is required to be classified nonhazardous, could be considered unsafe if not provided with a suitable safeguard (even when there is ventilation which is capable of diluting the flammable gas or vapor in the air to safe concentrations). These requirements are also explained in Ch. 6 of Part 1. All specific data shown in bold letters in the title block of Tables 2-2 are related to the source of hazard. To establish what illustrations in Part 2 should be used for a particular hazardous location under consideration, start with Table 2-1. From Table 2-1, determine the four basic requirements under which the source of hazard must operate, such as: 1. Is the system operating mode open or closed? 2. Is the flammable product heavier or lighter than air? 3. Is the source of hazard located indoors or outdoors? 4. Is the location sufficiently or insufficiently ventilated?

Chapter 18: Environmental Conditions

247

For example, assume that the four basic requirements for the source of hazard read as follows: 1. System operating mode

=

Closed

2. Vapor density

=

Heavier than air

3. Location

=

Indoors

4. Ventilation

=

Insufficient

To establish into which category this condition falls, refer to Table 2-1 and find this combination under a code letter. For the above example, the combination is covered by code letter “B.” Next, refer to Tables 2-2 and find the table with this same code letter (for this case, Table 2-2J does not apply). Establish in Table 2-2B the appropriate figure number for the applicable illustration by establishing the three specific requirements for the location of the source of hazard. For this example, assume the following three specific requirements: 1. Type of location

=

Pump station

2. Size of the source of hazard

=

Large

3. System pressure

=

High

The applicable illustration which matches the above requirements in Table 2-2B is narrowed down to one particular illustration which is number B-5. Bear in mind that when the ambient or process temperature for the flammable product is below flash point, the location need not be classified. When the temperature is equal or exceeding the flash point of the flammable product, then of course, the location is considered hazardous and classification of the location is necessary. Next, consider another example which is not directly related to the source of hazard, but is a control room located adjacent to a hazardous area. To determine the classification for the control room, it is necessary to first determine the type of area where the control room is located or that is adjacent to it. Follow the preceding steps to determine the four basic requirements: 1. System operating mode =

Closed

2. Vapor density

=

Lighter than air

3. Location

=

Indoors

4. Ventilation

=

Insufficient

Select the applicable code letter from Table 2-1, which in this case is letter “H.” In this example, size of the source of hazard and system pressure is irrelevant. Next, refer to Table 2-2H and determine the applicable figure number. In Table 22H, there are four control rooms listed: Figs. H-3, H-4, H-5, and H-6. As shown in the illustrations, three control rooms are located adjacent to a Div. 1 location (Figs. H-4, H-5, and H-6), and one control room (Fig. H-3) is above a Div. 1 area. To establish which illustration will apply, consider the following three items:

248 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres 1. Type of location under consideration

=

Control room

2. Location of control room

=

Adjacent to Div. 1 area

3. Access to the adjacent hazardous area

=

No

Since the control room is located adjacent to a hazardous area, Fig. H-3, does not apply. The fact that the control room has no access to a hazardous area means that only one illustration will apply: Fig. H-4. As shown in Fig. H-4, the control room is nonhazardous without being ventilated, because walls are vapor tight. A brief explanation of each individual illustration is given in the following chapters. Each explanation includes answers to questions which have led to the classification and extent of the hazard for the location involved. In the explanation each question is preceded by a capital letter and a number. They are duplicated from Form “A” in Part 3.

Chapter 19 General Requirements for Group A

A.

GENERAL REQUIREMENTS FOR GROUP A

Group A represents closed sources of hazard that contain Class I flammable products with heavier-than-air gases or vapors located indoors that are sufficiently ventilated and which are classified in accordance with the information in Part 1. Figure A-1. The major requirements for classifying the location as shown in Fig. A-1 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

pump house

E1 - type of ventilation

=

exhaust

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

249

250 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure A-1. Pumphouse with small pumps (0–51 hp) handling Class I flammable liquid (sufficiently ventilated location).

Chapter 19: General Requirements for Group A

251

Items pertaining to the extent of danger: A1 - type of source of hazard

=

pump

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

moderate

D6 - floor space occupied

=

50%

F5 - pump driver and size

=

50 hp

A sufficiently ventilated pump station which contains small sources of hazard may be classified Div. 2 for 50% or 100%. The choice between both types of classifications is a function of the magnitude of the explosion danger. This magnitude of explosion danger is expressed in a probability factor which, as explained in Sec. J of Ch. 3, is below 10 or 10 and above. If the probability factor is less than 10, the explosion danger is considered small and the pump station should be classified for 50%. If the probability factor is 10 or more, the explosion danger is considered large and requires a 100% classification. As explained in Sec. J of Ch. 3, the probability factor is expressed in Pu value and depends on four major conditions: 1) pump size, 2) floor space occupied, 3) pressure in the system, and 4) whether flammable vapors could accumulate in the pump station when there is a pump failure. Since the pump station in Fig. A-1 is sufficiently ventilated, accumulation of flammable vapors is not an issue. Based on the information given in the data above, the probability factor for the pump station in Fig A-1 is determined as follows: a—

Floor space occupied =

50%

=

2 Pu

b—

System pressure

=

moderate

=

2 Pu

c—

Quantity of release

=

small

=

1 Pu

The probability factor for the above conditions is: a × b + c or 2 × 2 + 1 = 5 Pu. Since the probability factor is 5 Pu, the classification of the pump station in Fig. A-1 needs to be 50% Div. 2. The boundary dimensions for pumps rates 50 hp and below, operating at moderate system pressure shall be 5 V, 25 Ho and 18 Hi as shown in Fig. A-1. In view of this dimension and the fact that the floor space is 50% occupied, no additional danger zone is necessary. The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4A

Item 8 1

Figure 1-3

Item F

252 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure A-2. Pumphouse with small pumps (60–201 hp) handling Class I flammable liquid (sufficiently ventilated location).

Figure A-2. The major requirements for classifying the location as shown in Fig. A-2 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

above

=

Chapter 19: General Requirements for Group A D3 - how is location considered

=

253

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

pump house

E1 - type of ventilation

=

exhaust

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

pump

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

high

D6 - floor space occupied

=

50%

F5 - pump driver and size

=

60 hp

Items pertaining to the extent of danger:

A sufficiently ventilated pump station which contains small sources of hazard may be classified Div. 2 for 50% or 100%. The choice between both types of classifications is a function of the magnitude of the explosion danger. This magnitude of explosion danger is expressed in a probability factor which, as explained in Sec. J of Ch. 3, is below 10 or 10 and above. If the probability factor is less than 10, the explosion danger is considered small and the pump station should be classified for 50%. If the probability factor is 10 or more, the explosion danger is considered large and requires a 100% classification. As explained in Sec. J of Ch. 3, the probability factor is expressed in Pu value and depends on four major conditions: 1) pump size, 2) floor space occupied, 3) pressure in the system, and 4) whether flammable vapors could accumulate in the pump station when there is a pump failure. Since the pump station in Fig. A-2 is sufficiently ventilated, accumulation of flammable vapors is not an issue. Based on the information given in the data above, the probability factor for the pump station in Fig. A-2 is determined as follows: a—

Floor space occupied =

50%

=

2 Pu

b—

System pressure

=

high

=

3 Pu

c—

Quantity of release

=

large

=

3 Pu

The probability factor for the above conditions is: a × b + c or 2 × 3 + 3 = 9 Pu. Since the probability factor is 9 Pu, the classification of the pump station in Fig. A-2 needs to be 50% Div. 2. The boundary dimensions for pumps ranging from 60 hp and up, operating at high system pressure, shall be 5 V, 50 Ho, and 2 Hi as shown in Fig. A-2. In view of this dimension and the fact that the floor space is 50% occupied, an additional danger zone is necessary.

254 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4A

Item 8 2

Figure 1-3 1-9

Item H I

Figure A-3. Pumphouse with main pumps handling flammable liquid at low or moderate pressure (sufficiently ventilated location).

Figure A-3. The major requirements for classifying the location as shown in Fig. A-3 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

4

D5 - type of location

pump house

=

Chapter 19: General Requirements for Group A E1 - type of ventilation

=

exhaust

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low to moderate

F5 - pump driver and size

=

elect. motor, 201 hp and up

255

Items pertaining to the extent of danger:

The basis for entirely classifying the pump station is the large size of the source of hazard. The reason for classifying the pump station Div. 2 is the presence of sufficient ventilation. The extent of the Div. 2 area shall be 50 ft horizontal because of Items A2, B1, B5, B6, and C5. Since the Div. 2 classification extends to the perimeter of the pump station, an additional danger zone 10 ft wide is required at any opening of the pump station such as a door opening, non-bolted windows, louvers for inlet air, or any other openings which may release flammable gases or vapors to the outdoors under ventilation failure. In addition to the 10 ft wide additional danger zone, the horizontal boundary must extend to its full length beyond any opening of the pump station if the boundary extends to the outdoors. The additional danger zone and the boundary outdoors also may be classified Div. 2. The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4A

Item 8 3

Figure 1-3 1-8

Item I C

Figure A-4. The major requirements for classifying the location as shown in Fig. A-4 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

highly-volatile liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

pump house

=

256 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres E1 - type of ventilation

=

exhaust

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

high

F5 - pump driver and size

=

elect. motor, 201 hp and up

Items pertaining to the extent of danger:

Figure A-4. Pumphouse with main pumps handling volatile liquid at high pressure (sufficiently ventilated location).

The basis for entirely classifying the pump station in Fig. A-4 is the large size of the source of hazard. The reason for classifying the pump station Div. 2 is the presence of sufficient ventilation. The extent of the Div. 2 area shall be 100 ft horizontal because of Items A2, B1, B5, B6, and C5. Since the Div. 2 classification extends to the perimeter of the pump station, an additional danger zone 10 ft wide

Chapter 19: General Requirements for Group A

257

is required at any opening of the pump station such as a door opening, non-bolted windows, louvers for inlet air, or any other opening which may release flammable gases or vapors to the outdoors under ventilation failure. However, for proper ventilation, air louvers at the right side of the building in Fig. A-4 must be eliminated. Air needs to flow in one direction, from inlet to outlet. In addition to the 10 ft wide additional danger zone, the horizontal boundary must extend to its full length beyond any opening of the pump station if the boundary extends to the outdoors. The additional danger zone and the boundary outdoors may also be classified Div. 2. The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4A

Item 8 4

Figure 1-3 1-8

Item J C

Figure A-5. Detached liquid warehouse (adequately ventilated location).

Figure A-5. The major requirements for classifying the location as shown in Fig. A-5 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

258 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres C1 - system operating mode

=

closed

C2 - equipment regularly worked on

=

no

C5 - system pressure

=

low

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above and below

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

2

D5 - type of location E1 - type of ventilation F1 - release of flammable gas/vapor

= = =

liquid warehouse natural or electric accidentally

A1 - type of source of hazard

=

drums, containers

A2 - size of source of hazard

=

small

B5 - flammability class

=

I, II, and III

B6 - vapor density

=

heavier and lighter than air

Items pertaining to the extent of danger:

The liquid warehouse shown in Fig. A-5 is a separate building for storing flammable liquid. Liquid warehouses may store Class I, II, and III flammable liquids if the containers and portable tanks meet DOT regulations and/or ANSI and ASTM standards. However, Class IA flammable liquids may not be stored in portable tanks (660 gal maximum) without automatic fire fighting facilities, and Class I liquid is prohibited in the basement. Class II and III liquids may be stored in the basement if the basement is provided with an automatic sprinkler system and other fire fighting facilities. Suitable fire extinguishers and or hose lines shall be installed throughout the building. Dispensing of Class I and II liquids is not permitted unless the dispensing area is cut off from the storage area and suitably ventilated. However, an arcing device within 3 ft of any dispensing nozzle shall be provided with an enclosure suitable for a Class I, Div. 1 location. Liquid warehouses need not be ventilated provided flammable liquid is stored in metal containers meeting DOT regulations or in non-breakable plastic containers meeting the requirements of ANSI and ASTM standards as follows: 1. Metal drums of 60 gal maximum for Class I, II, and III liquid. 2. Approved metal portable tanks of 660 gal maximum for all classes except Class IA unless protected by fire fighting facilities. 3. Approved plastic containers of 1 gal for Class IA and 5 gal for Class IB, IC, II, and III. 4. Approved polyethylene containers of 1 gal for Class IA, 5 gal for Class IB and IC, and 60 gal for Class II and III.

Chapter 19: General Requirements for Group A

259

However, if flammable liquids are not stored in suitable containers a gravity or mechanical fan is required. A Div. 2 classification is required for Class I liquid. No classifications are required for Class II and III liquids. The classification and extent of hazard for Class II and Class III liquid is in compliance with Table 1-4, Item 1, see subtable A, Item 5. The enclosed room in Fig. A-6 is a room without external walls inside a building for storing flammable liquid.

Figure A-6. Storage room for flammable liquid inside a building, ventilated.

Classified Division 2 The inside storage room must be entirely classified Div. 2 if the room stores Class I flammable liquids with vapors heavier or lighter than air in containers meeting DOT regulations or if stored in containers not meeting DOT regulations, but of limited storage capacity as follows:

260 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres 1. Metal DOT type containers of 60 gallon maximum. 2. Metal non-DOT type containers of 1 gallon for Class IA and 5 gallon for IB and IC. 3. Glass containers of 1 pint for Class IA, 1 quart for Class IB, and 1 gallon for Class IC (Class IA and Class IB liquids may be stored in glass containers of not more than 1 gallon capacity if the required liquid purity would be affected by metal containers). 4. Approved plastic containers of 1 gallon for Class IA and 5 gallon for Class IB and IC. 5. Safety cans of 2 gallon for Class IA and 5 gallon for Class IB and IC. 6. Polyethylene DOT type containers of 1 gallon for Class IA and 5 gallon for Class IB and IC. 7. Approved metal portable tanks of 660 gallon maximum for all classes except for Class IA unless provided with sprinkler system. Ventilation, such as a gravity or mechanical fan, must be provided if liquids are stored in non-suitable or breakable containers.

Classified Nonhazardous The enclosed room may be classified nonhazardous without ventilation if Class II and III flammable liquids are stored in suitable and non-breakable containers. The room may also be classified nonhazardous if the liquids are stored in non-suitable and breakable containers, provided the storage room is ventilated.

Storage in the Basement The storage of a Class I flammable liquid in the basement is prohibited. The storage of Class II and III liquids is permitted if the basement is provided with automatic sprinkler systems and other fire-fighting facilities. The classification and extent of hazard for the storage area are in compliance with Table 1-4, subtable A, Item 6. Figure A-7. The major requirements for classifying the location as shown in Fig. A-7 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

C2 - equipment regularly worked on

=

no

D1 - location with/without source of hazard =

with

D2 - above or below grade

above

=

Chapter 19: General Requirements for Group A D3 - how is location considered

=

261

indoors

D4 - number of vapor tight walls of location =

4

D6 - floor space occupied

=

not more than 50%

E1 - type of ventilation

=

exhaust fan

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

piping system

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

moderate

Items pertaining to the extent of danger:

Figure A-7. Piping system with screwed fittings, bolted flanges, valves and meters, operating at moderate pressure (sufficiently ventilated location).

262 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The location in Fig. A-7 contains a piping system with a number of large sources of hazard. Each individual component of the piping system is allowed to be classified Div. 2 because of: 1. Its closed operating mode (Item C1). 2. The occasional release of flammable vapors in case the components should break down (Item F1). 3. The sufficient ventilation in the location (Items E1 and E3). The extent of the Div. 2 area shall be as follows. Since the individual components of the piping system are large (Item A2), and are operating at moderate pressure (Item C5), and not more than 50% floor space is occupied (Item D6), the extent of the danger zone shall be 10 ft vertical, 25 ft horizontal, and 18" high as shown in Fig. A-7. If the piping system is operating at high pressure or if more than 50% of the floor space is occupied, the entire location must be classified Div. 2. The classification and extent of hazard for the location is in compliance with Table 1-4, subtable A, Item 7, and with Fig. K-1.

Figure A-8. Piping system with screwed fittings, bolted flanges, valves and meters, operating at moderate pressure (sufficiently ventilated location).

Chapter 19: General Requirements for Group A

263

Figure A-8. The major requirements for classifying the location as shown in Fig. A-8 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

C2 - equipment regularly worked on

=

no

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

4

D6 - floor space occupied

=

not more than 50%

E1 - type of ventilation

=

exhaust fan

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

piping system

A2 - size of source of hazard

=

mini

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

moderate

Items pertaining to the extent of danger:

The location in Fig. A-8 contains a piping system with a number of mini sources of hazard. Each individual component of the piping system is allowed to be classified Div. 2 because of: 1) its closed operating mode (Item C1); 2) the occasional release of flammable vapors in case the components should breakdown (Item F1); and 3) the sufficient ventilation in the location (Items E1 and E3). The extent of the Div. 2 area shall be as follows. Since the individual components of the piping system are of the mini type (Item A2), and are operating at moderate pressure (Item C), and not more than 50% of the floor space is occupied (Item D6), the extent of the danger zone shall be 3 ft horizontal and 3 ft vertical. If the distance between the source of hazard and the floor is 3 ft, or slightly over 3 ft, the danger zone shall be extended all the way to the ground as shown in Fig. A-8. If the piping system is operating at high pressure and does not occupy more than 50% of the floor space, the extent of the Div. 2 area shall be 5 Ra, 15 Ho, 18" Hi. If the piping system is operating at high pressure and occupying more than 50% floor space, the entire location must be classified Div. 2. The location does not have to be classified if the probability factor is 5 or less and the mini sources of hazard are well maintained. The classification and extent of hazard for the location are in compliance with Table 1-4, subtable A, Item 8, and with Fig. K-1.

Chapter 20 General Requirements for Group B

Group B represents closed sources of hazard that contain Class I flammable products with heavier-than-air gases or vapors located indoors that are not ventilated, or are insufficiently ventilated, and which are classified in accordance with the information in Part 1. Figure B-1. The major requirements for classifying the location as shown in Fig. B-1 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

process plant

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

occasional

264

Chapter 20: General Requirements for Group B

265

Items pertaining to the extent of danger: A1 - type of source of hazard

=

process plant

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

moderate

D6 - floor space occupied

=

100%

Figure B-1. Process plant handling flammable liquid at moderate pressure (insufficiently ventilated location).

As indicated in Fig. B-1, the operating mode of the sources of hazard in the indoor location is closed (Item C1). The release of flammable material to the atmosphere, therefore, is only occasional (Item F1). Since the indoor location is not ventilated (Item E1), it is necessary that the location be classified Div. 1. Because the indoor location contains a large process plant (Items A1 and A2), operating at moderate pressure (Item C5), it is necessary that the entire indoor location be classified Div. 1. The extent of the Div. 1 area as measured from the source of hazard in both vertical and horizontal directions (Items A2, B5, B6, and

266 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres D5) shall be 25 and 50 ft respectively. As shown in Fig. B-1, an additional danger zone 10 ft wide must be applied beyond the building opening. This additional danger zone is required because 1) the indoor location is entirely classified, or 2) the distance between the source of hazard and the opening of the building is the same as the required 50 ft horizontal boundary. The additional danger zone shall be classified Div. 2 because the release of flammable gases or vapors to the outdoors, in case of an accidental rupture, is only occasional, and, therefore, they are considered rapidly dispersed and diluted by natural ventilation. The classification and extent of hazard for the process plant is in compliance with the following: Table 1-3 1-4B

Item 7 1

Figure B-2. Process plant indoors handling volatile flammable liquid at high pressure (insufficiently ventilated location).

Chapter 20: General Requirements for Group B

267

Figure B-2. The major requirements for classifying the location as shown in Fig. B-2 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

volatile liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

process plant

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

process plant

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

high

D6 - floor space occupied

=

100%

Items pertaining to the extent of danger:

As indicated by Item C1, the operating mode of the source of hazard is closed. Therefore, the release of flammable material to the atmosphere can only occur occasionally (Item F1). Since the indoor location lacks sufficient ventilation (Item E1), the location must be classified Div. 1. In this case, the entire indoor location must be classified Div. 1 because: 1. the source of hazard (Item A2) is large 2. the flammable liquid is highly volatile (Item B1) 3. the pressure in the system (Item C5) is high The extent of the Div. 1 area as measured from the outline of the source of hazard in both horizontal and vertical directions (as a result of Items A2, B1, B5, B6, and D5) shall be 100 ft and 25 ft respectively and outdoors 2 ft high. Although the 100 ft hazardous boundary is extending beyond the open perimeter of the building, an additional danger zone 10 ft wide extending vertically up to the roof of the building must also be applied. Because the source of hazard has a closed operating mode, the boundary and additional danger zone outdoors may be classified Div. 2. The reason for the Div. 2 classification is 1) the release of flammable gases or vapors to the outdoors is considered an occasional event, and 2) the occasional presence of flammable gases or vapors outdoors is rapidly dispersed and diluted by natural ventilation. Since the

268 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres roof of the building is not vapor tight, a danger zone 10 ft wide on top of the roof should be applied as an additional safety precaution. This is only required for an indoor location that is entirely classified. The classification and extent of hazard for the process plant are in compliance with the following: Table 1-3 1-4B

Item 7 2

Figure B-3. Pumphouse with small pumps handling Class I flammable liquid (insufficiently ventilated location).

Chapter 20: General Requirements for Group B

269

Figure B-3. The major requirements for classifying the location as shown in Fig. B-3 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

pump house

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

moderate

D6 - floor space occupied

=

50%

F5 - pump driver and size

=

elect. motor, 60–201 hp

Items pertaining to the extent of danger:

Normally a pump station that is not sufficiently ventilated is required to have a 100% Div. 1 classification. However, in some cases, half of the pump station may be classified Div. 1 and half may be classified Div. 2. The Div. 2 classification, in this case, is the transition zone. The choice between a 100% and a 50% Div. 1 classification depends on whether the explosion hazard in the pump station is large or small. As explained in Sec. J of Ch. 3, the extent of explosion danger is expressed in a “probability factor.” This probability factor is expressed in a Pu value and is either 10 and above or below 10 Pu. If the probability factor should be below 10 Pu, the explosion danger is considered small and the pump station is allowed to be classified 50% Div. 1. If the probability factor is 10 Pu or more, the explosion danger is considered large. This requires that the pump station be classified 100% Div. 1. Because the pump station, in this case, lacks sufficient ventilation, the emphasis is on the Div. 1 classification. If there is no Div. 1 classification because the pump station is sufficiently ventilated, then the emphasis, of course, is on the Div. 2 classification. The probability factor is a function of the following four major conditions: 1. pump size 2. floor space occupied by the pumps

270 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres 3. pressure in the system 4. whether flammable vapors are likely to accumulate in the pump station in case one or more pumps should break down Since the pump station in Fig. B-3 is not sufficiently ventilated, accumulation of flammable vapors must be expected in case of a breakdown. If the pump station is assumed to be unattended and since the pumps are small and operating at moderate pressure, the explosion danger in the pump station is considered small. Based on this information, the following probability factor is derived as follows: a—

Floor space occupied =

50%

=

2 Pu

b—

System pressure

=

moderate

=

2 Pu

c—

Quantity of release

=

medium

=

2 Pu

d—

Personnel in location =

unattended = 2 Pu (accumulation fac.)

The probability factor for the above condition is: a × b + c + d or 2 × 2 + 2 + 2 = 8 Pu. Since the probability factor is below 10 Pu, the Div. 1 classification of the pump station needs to be only 50%. Beyond the Div. 1 zone, the entire location shall be classified Div. 2 as shown in Fig. B-3. The boundary dimensions for pumps rated between 60 and 201 hp operating at moderate system pressure shall be 5 V, 25 Ho, and 2 Hi as shown in Fig. B-3. If flammable vapors should be released in the pump station as a result of a breakdown or failure of one or more pumps, ignitable vapors will also contaminate the surrounding outdoor area. If the outdoor area can be contaminated, an additional danger zone must be provided adjacent to the building opening (but only because the 25 Ho boundary for the pumps reaches the building opening or if the boundary should extend beyond the building opening). The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4B

Item 7 3

Figure 1-3

Item G

Figure B-4. The major requirements for classifying the location as shown in Fig. B-4 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

above

=

Chapter 20: General Requirements for Group B D3 - how is location considered

=

271

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

pump house

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

occasional

Figure B-4. Pumphouse with small pumps handling Class I flammable liquid (insufficiently ventilated location).

272 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Items pertaining to the extent of danger: A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low for 50 hp, moderate for 150 hp

D6 - floor space occupied

=

75% = 50 hp, 100% = 150 hp

F5 - pump driver number and size

=

8 of 50 hp, 2 of 150 hp

Normally a pump station that is not sufficiently ventilated is required to have a 100% Div. 1 classification. However, in some cases, half of the pump station may be classified Div. 1 and half may be classified Div. 2. The Div. 2 classification, in this case, is the transition zone. The choice between a 100% and a 50% classification depends on whether the explosion hazard in the pump station is large or small. As explained in Sec. J of Ch. 3, the extent of the explosion danger is expressed in a “probability factor.” This probability factor is expressed in a Pu value and is either 10 Pu and above or below 10 Pu. If the probability factor should be below 10 Pu, the explosion danger is considered small and the pump station is allowed to be classified 50% Div. 1. If the probability factor is 10 or more, the explosion danger is considered large. This requires that the pump station be classified 100% Div. 1. Because the pump station, in this case, lacks sufficient ventilation, the emphasis is on the Div. 1 classification. If there is no Div. 1 classification because the pump station is sufficiently ventilated, then the emphasis, of course, is on the Div. 2 classification. The probability factor is dependent on the following four major conditions: 1) pump size, 2) floor space occupied by the pumps, 3) pressure in the system, and 4) whether flammable vapors are likely to accumulate in the pump station in case one or more pumps should break down. Since the pump station in Fig. B-4 is not sufficiently ventilated, accumulation of flammable vapors must be expected in case of breakdown. If the pump station is assumed to be unattended, the explosion danger in the pump station is considered as follows: a — Floor space occupied = 75%, 100%

= 3 Pu, 4 Pu

b — System pressure

= low for 50 hp = 1 Pu moderate for 150 hp = 2 Pu

c — Quantity of release

= small for 50 hp = 1 Pu medium for 150 hp = 2 Pu

d — Personnel in location = unattended

= 2 Pu

Chapter 20: General Requirements for Group B

273

For the 50 hp pumps, the probability factor is: 3 × 1 + 1 + 2 = 6 Pu. For the two 150 hp pumps, the probability factor is: 4 × 2 + 2 + 2 = 12 Pu. Only the worst condition shall apply (i.e., the 150 hp pumps which occupy 100% floor space including the 50 hp pumps). The probability factor for the worst condition is 12 Pu, and, therefore, the explosion danger in the pump station is large. Subsequently, the pump station needs to be classified 100% Div. 1 (see Fig. B-4, Item 3). The boundary dimensions for the 150 hp pumps in the pump station operating at moderate pressure shall be 5V, 25 Ho. The boundary shall be 50 Ho if the system pressure is high. Flammable vapors released to the atmosphere as a result of a breakdown or failure of a pump must be assumed to contaminate the surrounding outdoor area. This assumption is based on the 100% Div. 1 classification. Since the breakdown or failure of a pump may cause flammable vapors to contaminate the surrounding outdoor area, it is necessary that an additional danger zone 10 ft wide be provided at the entrance of the 3-wall building. If the 25 or 50 Ho boundary should extend beyond the 10 ft wide danger zone, it is necessary that the balance of the 25 Ho or 50 Ho boundary be applied outdoors also. Because outdoor locations are normally sufficiently ventilated, the additional danger zone and the boundary extension outdoors need only be classified Div. 2. The classification and extent of hazard for the pump station is in compliance with the following: Table 1-3 1-4B

Item 7 4

Figure 1-8 1-3

Item A F, G for low and mod. pressure

Figure B-5. The major requirements for classifying the location as shown in Fig. B-5 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid (highly volatile)

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

pump house

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

occasional

=

pumps

Items pertaining to the extent of danger: A1 - type of source of hazard

274 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

moderate

F5 - pump driver and size

=

201 hp and up

Figure B-5. Pumphouse with main pumps handling flammable liquid at moderate pressure (insufficiently ventilated location).

Since pumps are normally closed (Item C1), a failure or breakdown of a pump will cause flammable material to be released to the atmosphere only occasionally. Because of this occasional release, the pump house can be classified Div. 2 if it is sufficiently ventilated. If the pump station is not sufficiently ventilated as indicated by Item E1, the pump station must be classified Div. 1. Because of the presence of large pumps in the pump station (Items A2 and B1) and because of the moderate pressure in the system (Item C5), it is necessary that the entire pump station be classified Div. 1. The extent of the hazardous boundary for each individual pump shall be 25 ft vertical, 50 ft horizontal, and 2 ft high outdoors, and 3 ft high indoors. The boundary size shall be increased from 50 ft to 100 ft if the flammable material being pumped is highly volatile and under high pressure. As shown in Fig. B-5, an

Chapter 20: General Requirements for Group B

275

additional danger zone 10 ft wide is applied beyond the building opening. There are three conditions under which an additional danger zone must be applied: 1. if the indoor location is entirely classified 2. if the 50 ft boundary is as long as the distance between the pump and building opening 3. if the boundary extends beyond the building opening The additional danger zone outdoors and the boundary extending beyond the building opening do not have to be classified Div. 1. They can be classified Div. 2 because 1) the release of flammable material to the outdoors is only occasional, and 2) most importantly, there is sufficient ventilation outdoors to dilute and disperse the flammable materials rapidly. These two conditions allow the danger zone and boundary outdoors to be classified Div. 2. Probability factors are not applicable to large sources of hazard because their locations are generally 100% classified. Even when the boundary for the large sources of hazard is shorter than the actual distance between the source of hazard and the perimeter of the pump station, the pump station needs to be 100% classified. This is true if the system pressure is high or moderate. Pump stations are normally built to allow electrically driven pumps to occupy not less than 75% of the floor space. Based on the above conditions, the lowest probability factor for the pump station will be 10 Pu or more. The probability factor would be less than 10 Pu if the system pressure is low. This is the only exception that allows the pump station not to have a 100% classification. The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4B

Item 7 5, 6

Figure 1-3 1-8

Item I/J A

Figure B-6. The major requirements for classifying the location as shown in Fig. B-6 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid (highly volatile)

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

pump house

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

occasional

276 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Items pertaining to the extent of danger: A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low

F5 - pump driver and size

=

201 hp and up

Figure B-6. Pumphouse with main pumps handling flammable liquid (insufficiently ventilated location).

Chapter 20: General Requirements for Group B

277

Since pumps are normally closed (Item C1), a failure or breakdown of a pump will cause flammable material to be released to the atmosphere only occasionally. Because of this occasional release, the pump house can be classified Div. 2 if it is sufficiently ventilated. If the pump station is not sufficiently ventilated (as shown in Fig. B-6 and as indicated by Item E1), the pump station must be classified Div. 1. However, since the system pressure for the source of hazard shown in Fig. B-6 is low (Item C5), the pump station does not have to be entirely classified. This is the only exception under which a pump station containing large sources of hazard does not have to be classified for 100%. For system pressures other than low, it is necessary that the location be 100% classified. This is shown in the table of Fig. B-6. This 100% classification is based on a floor space that is not less than 75% occupied. Pump stations for large sources of hazard are generally built for a group of electrically driven pumps that normally will occupy not less than 75% floor space. Because the majority of pump stations containing large dynamic-type sources of hazard are 75% or 100% occupied, no probability factors need to be applied to establish whether a location should be classified 100% or 50%. As indicated above, the only case in which a pump station containing large sources of hazard must be classified 50% is when the system pressure is low, as shown in Item 4 of the table in Fig. B-6. However, if large electrically driven pumps operating at low pressure occupy more than 50% of the floor space, the entire pump station must also be classified. As shown in Fig. B-6, no 10 ft wide additional danger zone is required. There are three conditions under which an additional danger zone must be applied: 1. if the indoor location is 100% classified 2. if the boundary for the source of hazard is as long as the distance between the pump and building opening 3. if the boundary extends beyond the building opening Since none of the three conditions above apply to Fig. B-6, no additional danger zone is required. The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4B

Item 7 7

Figure 1-3

Item I

Chapter 21 General Requirements for Group C

Group C represents closed sources of hazard that contain Class I flammable products with heavier-than-air gases or vapors located outdoors which are sufficiently ventilated and which are classified in accordance with the information in Part 1. Figure C-1. The major requirements for classifying the location as shown in Fig. C-1 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D4 - number of vapor tight walls of location =

none

D5 - type of location

=

open building

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

E4 - percent obstruction

=

none

F1 - release of flammable gas/vapor

=

occasional

278

Chapter 21: General Requirements for Group C

279

Items pertaining to the extent of danger: A1 - type of source of hazard

=

process plant

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

moderate

Figure C-1. Process plant in building handling flammable liquid at moderate pressure.

The process plant shown in Fig. C-1 is located in a building which, because of its construction, is defined as “outdoors” (Item D3). Such a building is considered sufficiently ventilated by natural ventilation (Items E1 and E3). As a result of the sufficient ventilation and because the source of hazard has a closed operating mode (Item C1), in which flammable gases or vapors are expected to be released to

280 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres the atmosphere only occasionally (Item F1), the process plant may be classified Div. 2 as shown. The extent of the Div. 2 area is a function of the following items: 1. pressure in the system (Item C5) 2. size of the source of hazard (Item A2) 3. vapor density 4. flammability class of the flammable product (Items B5 and B6) In view of these items, the extent of the Div. 2 area shall be 25 ft vertical and 50 ft horizontal as shown in Fig. C-1. No additional danger zones are required left and right of the building openings for the following reasons. On the right side, the 50 ft boundary does not reach the building perimeter. On the left side, the vertical distance of the boundary is considered sufficiently large to replace the additional danger zone. The classification and extent of hazard for the location are in compliance with the following: Table 1-3 1-4C

Item 2 1

Figure C-2. The major requirements for classifying both locations as shown in Fig. C-2 are as follows: Items pertaining to the degree of danger: Process Plant B1 - type of product

=

highly-volatile liquid

C1 - system operating mode

=

closed

D2 - above or below grade

=

above

D3 - location of source of hazard

=

outdoors

E3 - amount of ventilation

=

sufficient

E4 - percent obstruction

=

25%

F1 - release of flammable gas/vapor

=

occasional (large quantities)

A1 - type of source of hazard

=

process plant

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

high

Items pertaining to the extent of danger:

Building Adjacent to Div. 2 Area D1 - building with/without source of hazard =

without

D4 - number of vapor tight walls of building =

3

Figure C-2. Process plant outdoors handling volatile flammable liquid at high pressure.

281

282 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres D5 - type of building

=

open building

E1 - type of ventilation in building

=

none

The process plant shown in Fig. C-2 is located outdoors. An outdoor location is considered sufficiently ventilated by natural ventilation even when the process plant is 25% obstructed. As a result of the sufficient ventilation of the process plant and because the source of hazard has a closed operating mode (Item C1) in which flammable gases or vapors are expected to be released to the atmosphere only occasionally (Item F1), the process plant may be classified Div. 2. The extent of the Div. 2 area is a function of the following items: 1. pressure in the system (Item C5) 2. size of the source of hazard (Item A2) 3. vapor density and flammability class of the flammable product (Items B5 and B6) 4. most importantly, the highly-volatile flammable liquid (Item B1) and the large quantities that can be released (Item F1) In view of these four items, the extent of the Div. 2 area shall be 25 ft vertical and 100 ft horizontal in which the horizontal extension of the hazardous area shall be 2 ft high. Although the 3-wall building in Fig. C-2, located adjacent to the hazardous area, does not contain a source of hazard, it must be classified Div. 1 for the following reasons: 1. the building is not ventilated 2. most importantly, the door opening of the building gives access to a Div. 2 zone The building could easily be classified nonhazardous if the door opening is deleted so that the building does not give access to a hazardous area. The building with its door opening giving access to a hazardous area can also be classified nonhazardous if it has four vapor-tight walls, is sufficiently ventilated, and is provided with a type “B” safeguard. The classification and extent of hazard for the location are in compliance with the following: Table 1-3 1-4C 1-7

Item 2 1 6 (building gives access to Div. 2 zone)

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283

Figure C-3. Pumping well (outdoor location).

Figure C-3. Figure C-3 represents a pumping well located outdoors, pumping flammable liquid. A pumping well with drivers not exceeding 50 hp is considered a small source of hazard. Since the pumping well is located in a freely ventilated area, the extent of danger from the outline of the pumping well need not be more than 10 ft horizontal, 18 inches high, and 5 ft vertical above the pump. Since the location is freely ventilated, the area within the hazardous boundary shall be classified Div. 2. The classification and extent of hazard for the location is in compliance with the following: Table 1-3 1-4C

Item 2 2

Figure 1-2

Item A

Figure C-4. The major requirements for classifying the location as shown in Fig. C-4 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

284 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D5 - type of location

=

pump station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low or moderate

F5 - pump driver and size

=

up to 51 hp

Items pertaining to the extent of danger:

Figure C-4. Auxiliary pump handling flammable liquid at low or moderate pressure (outdoor location).

Chapter 21: General Requirements for Group C

285

Figure C-4 represents an open pump station located outdoors with auxiliary pumps ranging from 0–51 hp pumping flammable liquid under low or moderate pressure. Since the pumps are located outdoors (Item D3), the pump station is considered sufficiently ventilated (Items E1 and E3). If flammable vapors should escape from the pumps as a result of a failure or breakdown of the pump, the release of the flammable vapors to the atmosphere is expected to be only occasional (Item F1). In view of this and the fact that the outdoor location is sufficiently ventilated, the location for the pumps can be classified Div. 2. The extent of the Div. 2 area in horizontal and vertical directions as measured from the outline of the pumps is a function of the following items: 1) the vapor density and flammability class of the flammable product (Items B5 and B6), 2) the system pressure (Item C5), and 3) the type and size of the source of hazard (Items A1 and A2). Because of these items, the size of the hazardous area shall be 3 ft vertical, 10 ft horizontal, and 18 inches high. The same pumps operating at high pressure require a 15 ft horizontal boundary. The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4C 1-4C

Item 2 3 4

Figure 1-2 1-2

Item A (Low or Moderate) B (High)

Figure C-5. The major requirements for classifying the location as shown in Fig. C-5 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D5 - type of location

=

pump station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low, moderate, or high

F5 - pump driver and size

=

60–201 hp

Items pertaining to the extent of danger:

286 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure C-5. Auxiliary pump handling flammable liquid at low, moderate, or high pressure (outdoor location).

Figure C-5 represents an open pump station located outdoors with auxiliary pumps ranging from 60–201 hp pumping flammable liquid under low, moderate, or high pressure. Since the pumps are located outdoors (Item D3), the pump station is considered sufficiently ventilated (Items E1 and E3). If flammable vapors should escape from the pumps as a result of a failure or breakdown of the pump, the release of the flammable vapors is expected to be only occasional (Item F1). In view of this and the fact that the outdoor location is sufficiently ventilated, the location for the pumps can be classified Div. 2. The extent of the Div. 2 area in horizontal and vertical directions as measured from the outline of the pumps is a function of the following items: 1. the vapor density and flammability class of the flammable product (Items B5 and B6) 2. the system pressure (Item C5) 3. the type and size of the source of hazard (Items A1 and A2) Because of these items, the size of the hazardous area shall be 5 ft vertical, 25 ft horizontal, and 2 ft high. The classification and extent of hazard for the pump station are in compliance with the following: Table 1-3 1-4C

Item 2 5

Figure 1-2

Item C

Chapter 21: General Requirements for Group C

287

Figure C-6. The major requirements for classifying the location as shown in Fig. C-6 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D5 - type of location

=

pump station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

Figure C-6. Main pump handling flammable liquid (outdoor location).

288 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Items pertaining to the extent of danger: A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low, moderate, or high

F5 - pump driver and size

=

above 200 hp

Figure C-6 represents an open pump station located outdoors with main pumps pumping flammable liquid under low, moderate, or high pressure. Since the pumps are located outdoors (Item D3), the open pump station is considered sufficiently ventilated (Items E1 and E3). If flammable vapors should escape from the pumps as a result of a failure or breakdown of one or more pumps, the release of the flammable vapors is expected to be only occasional (Item F1). In view of this and the fact that the outdoor location is sufficiently ventilated, the location for the pumps can be classified Div. 2. The extent of the Div. 2 area in horizontal and vertical directions as measured from the outline of the pumps as shown in Fig. C-6 is a function of the following items: 1. the vapor density and flammability class of the flammable product (Items B5 and B6) 2. the system pressure (Item C5) 3. the type and size of the source of hazard (Items A1 and A2) As shown in the table, there is a distinct difference between “highlyvolatile” liquid and “flammable” liquid. A highly-volatile liquid is more dangerous because it may produce large volumes of gases. Highly-volatile liquids include butane, ethane, ethylene, propane, and propylene. Gases or vapors designated as flammable are less dangerous because they are released more slowly. Both types have an impact on the extent of danger. The classification and extent of hazard for pumps operating at moderate pressure are in compliance with the following: Table 1-3 1-4C

Item 2 6, 7

Figure 1-2

Item D, E

Figure C-7. Figure C-7 represents two large storage tanks for storing flammable liquids; one with a fixed roof and the other with a floating roof. Both storage tanks appear to be large-closed sources of hazard. The fixed-roof tank is provided with a vapor exhaust vent, the floating-roof tank is not. The exhaust vent is considered to be the actual source of hazard because it continuously releases flammable vapors to the atmosphere. Since the vent is small and open, the fixedroof tank is considered a small source of hazard and since it is open, the fixed-roof tank does not belong in Group C, but in Group F.

Chapter 21: General Requirements for Group C

289

Figure C-7. Storage tanks for crude oil (outdoor location).

The floating-roof tank is considered a small-closed source of hazard. Because of possible leakage between the tank body and the roof, the top of the tank is considered to be the actual source of hazard. Therefore, the entire top requires a 10 ft, Div. 1 zone. In addition, a 10 ft wide, Div. 2 zone is required around the tank shell. This 10 ft Div. 2 zone shall extend up to the dikes surrounding the tank as shown in Fig. C-7. The classification and extent of hazard for the storage tanks are in compliance with the following: Table 1-4C

Item 8

290 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure C-8. Pits with, or without sources of hazard (outdoor location).

Figure C-8. Figure C-8 represents pits, below grade, in outdoors hazardous and nonhazardous locations. Because a pit below grade can easily pick up heavierthan-air flammable gases or vapors, the pit in Fig. C-8A, which is located in a Div. 2 area, must be classified Div. 1 (if the flammable gases or vapors above the pit are heavier than air). If the same pit in the hazardous area is equipped with forced ventilation and provides clean air as shown in Fig. C-8B, the pit may be classified Div. 2. The pit in Fig. C-8B may be classified nonhazardous if:

Chapter 21: General Requirements for Group C

291

1. The pit is sufficiently ventilated from a source of clean air. 2. The fan motor has no arcing devices. 3. It is provided with a type “B” safeguard. 4. The area above the pit is classified Div. 2. If the pit below grade is not located in a hazardous area and not forced ventilated, but equipped with pipes, valves, flanges, fittings, etc., which handle a flammable product of which the gases or vapors are heavier than air, the pit must be classified Div. 1 (as shown in Fig. C-8C). The classification and extent of hazard in the pits are in compliance with the following: Table 1-4C

Item 9

Figure C-9. The major requirements for classifying the location as shown in Fig. C-9 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

closed

C2 - equipment regularly worked on

=

yes

C5 - system pressure

=

low, moderate, or high

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

valves

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

Items pertaining to the extent of danger:

Figure C-9 represents an outdoor location with a piping system equipped with a small source of hazard handling flammable liquid. In Fig. C-9, the small source of hazard is a valve. The classification of the valve is normally Div. 2 with a 3 ft radius if the valve is operating at low pressure. At moderate pressure, the Div. 2 circular zone should have a radius of 3 ft, 10 ft horizontal, and 18 inches high. At high pressure, the Div. 2 circular zone must have a radius of 5 ft, 15 ft horizontal, and 18 inches high. If the valve is regularly operated or worked on, the classification of the valve shall be as shown in Fig. C-9. The same classification shown in

292 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Fig. C-9 is also applicable for an indoor location when the valve is regularly worked on or operated. The classification and extent of hazard for this location are in compliance with the following: Table 1-3 1-4C

Item 10 9

Figure C-9. Regularly worked on valve (outdoor location).

Chapter 21: General Requirements for Group C

293

Figure C-10. Pump handling flammable liquid at high pressure (outdoor location). (A and B indicate wind directions.)

Figure C-10. The major requirements for classifying the location as shown in Fig. C-10 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

butane

B2 - flash point

=

gas

B3 - ignition temperature

=

550 × F

B4 - explosion range

=

1.6–8.4%

C1 - system operating mode

=

closed

C7 - pressure/temperature of driver

=

450 psi/750°F

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D5 - type of location

=

pump station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

=

pump

Items pertaining to the extent of danger: A1 - type of source of hazard

294 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

2.0

C5 - system pressure

=

high

F4 - is flammable product early ignited

=

yes

F5 - pump driver and size

=

steam turbine (up to 51 hp)

Figure C-10 represents an open pump station located outdoors with small steam-driven pumps pumping butane at high pressure. Since the pumps are located outdoors (Item D3), the pump station is considered sufficiently ventilated (Items E1 and E3). If flammable vapors should escape as a result of a failure or breakdown of one of the pumps, the release of the flammable vapors to the atmosphere is expected to be only occasional (Item F1). In view of this and the fact that the outdoor location is sufficiently ventilated, the location for the pumps could be classified Div. 2. The Div. 2 classification is valid if the ignition temperature of the flammable product is higher than the surface temperature of the steam turbine. However, since the ignition temperature indicated for this example is lower than the surface temperature of the steam turbine (Items B3 and C7), the Div. 2 classification is not valid. If flammable gases should be released to the atmosphere because of a pump failure, the failure may result in an early ignition. Under the conditions shown in Fig. C-10, early ignition may or may not take place, depending on wind conditions. If the wind moves in direction A as shown in Fig. C-10, flammable gases will not come into contact with the hot surface of the steam turbine. Under this condition, the gases will have sufficient time to reach and enter their explosion range, and because of this, the Div. 2 classification is valid. However, if the wind moves in the opposite direction (direction B), early ignition may take place resulting in local burning of the gas mixture. Local burning will occur if the gas has reached, but has not entered, its explosive range and makes contact with the hot surface of the turbine. Local burning, in turn, will prevent the gas from entering the explosive range. Since this condition could exist, it is necessary that the source of hazard be provided with a Div. 1 zone of 3 ft radius and an additional Div. 2 zone of 2 ft vertical, 15 ft horizontal, and 18 inches high (as indicated in Fig. C-10). However, since wind conditions may change, it is necessary to consider only the worst condition for which the location is required to be classified as indicated by Fig. C-10. Early ignition will take place if the external temperature is higher than the ignition temperatures of the flammable product. The classification and extent of hazard for this location are in compliance with the following: Table Item Figure Item 1-3 4 1-4C 11 1-2 B

Chapter 21: General Requirements for Group C 295

Figure C-11. Remote permanent ignition source in outdoor location.

296 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure C-11. The major requirements for classifying the location as shown in Fig. C-11 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

B3 - ignition temperature

=

pump 1 below skin temp. of steam turbine; pump 2 above skin temp. of steam turbine

C1 - system operating mode

=

closed

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D5 - type of location

=

pump station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

pumps

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

P1 = High, P2 = Mod.

F4 - will flammable material ignite early

=

no

F5 - pump driver and size

=

steam turbine (50 hp) elect. driver (60–201 hp)

Items pertaining to the extent of danger:

Figure C-11 represents a small open pump station with two pumps, Pump 1 and Pump 2. Both pumps are handling flammable liquids. Pump 1 is driven by an electric motor, Pump 2 by a steam turbine. The steam turbine is considered a “permanent” ignition source for Pump 1 because the surface temperature of the steam turbine is above the ignition temperature of the flammable material in Pump 1. The electric motor of Pump 1 has a hp rating between 60 and 201 hp. The steam turbine has an equivalent hp rating of 50 hp. Normally, an area containing a source of hazard may be classified Div. 2 if the source of hazard has a closed operating mode and is sufficiently ventilated. Since Pump 1 is small and operates at high pressure, the required danger zone is 5 ft vertical, 25 ft horizontal, and 18 inches high. However, since the ignition temperature of the flammable product in Pump 1 is lower than the skin temperature of the steam turbine, the danger zone between Pump 1 and the turbine may not

Chapter 21: General Requirements for Group C

297

be classified Div. 2. It must be classified Div. 1. The change in classification is only valid as long as the steam turbine is located within the 25 ft horizontal boundary that is required for Pump 1. If the horizontal distance between Pump 1 and the steam turbine is longer than the required 25 ft, the area between Pump 1 and the steam turbine does not have to be classified Div. 1. The Div. 1 area shall be provided with a 2 ft, Div. 2 transition zone. In addition, Pump 1 must also be provided with a 25 ft, 2 ft high Div. 2 danger zone in the opposite direction of the Div. 1 zone, as shown in Fig. C-11. If a breakdown of Pump 1 should occur under the conditions stated above, flammable vapors would be exposed to the atmosphere. Before the flammable vapors reach the remote turbine ignition source, they may have entered their explosion range and become ignitable. An explosion will follow once the ignitable vapors reach the hot surface of the steam turbine. The classification and extent of hazard for this location are in compliance with the following: Table 1-3 1-4C

Item 5 12

Figure 1-2 1-5

Item C

Figure C-12. Figure C-12 represents a marine terminal for loading and unloading flammable liquids. The sources of hazard consist of manifolds and hose connections. Since there is sufficient ventilation around the marine terminal, the terminal should be classified Div. 2. The extent of the Div. 2 area as measured from each individual source of hazard is 25 ft horizontal and 25 ft vertical. The Div. 2 area shall also extend 25 ft vertical from the highest lift point of the hose or knee of the loading arm. On the pier, the Div. 2 area shall extend towards the shoreline 2 ft high and 15 ft horizontal beyond the high tide water line as shown in the “Plan” of Fig. C-12. On both sides of the main terminal, perpendicular to the pier, the Div. 2 area shall extend 50 ft in a horizontal direction. If the manifolds and/or hose couplings on the pier are regularly operated or worked on, a Div. 1 zone of 5 ft radius is required around each fitting. The area below the pier up to the water line shall also be classified Div. 2. The classification and extent of hazard for this location are in compliance with the following: Table 1-4C

Item 13

298 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure C-12. Marine terminal handling flammable liquids.

Figure C-13. Control room in Div. 2 hazardous location.

299

300 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure C-13. The major requirements for classifying the location as shown in Fig. C-13 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

highly-volatile liquid

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

control room without, process plant with

D2 - above or below grade

=

above

D3 - how is location considered

=

control room indoors, process plant outdoors

D4 - number of vapor tight walls of location =

4 for control room

D5 - type of location

=

control room adjacent to hazardous area

E1 - type of ventilation

=

none in control room, natural for process plant

E3 - amount of ventilation

=

sufficient in process plant

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

process plant

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

high

Items pertaining to the extent of danger:

The location in Fig. C-13 represents two buildings, a roofed space with a single wall containing a process plant and, adjacent to this building, a control room with four vapor-tight walls. The process plant is classified Div. 2, and the control room is classified as nonhazardous. The reason for classifying the process plant Div. 2 is as follows: 1. The operating mode of the source of hazard is closed. Because of this, a failure or breakdown of the source of hazard will cause flammable material to escape to the atmosphere only occasionally. 2. The building has only one wall and, therefore, is considered sufficiently ventilated by natural ventilation. In view of these two items, the process plant can be classified Div. 2. The extent of the Div. 2 area shall be 25 ft vertical, 100 ft horizontal, and 2 ft high. The reasons for this extension are as follows:

Chapter 21: General Requirements for Group C

301

1. highly-volatile flammable liquid 2. large size of the source of hazard 3. Class I flammability 4. heavier-than-air vapor density 5. high system pressure In addition, it is necessary that the building be provided with 10 ft wide additional danger zones. The reasons for classifying the control room nonhazardous are as follows: 1. Four vapor-tight walls. 2. The building opening does not give access to a Div. 2 hazardous boundary. The door opening is outside of the hazardous boundary. In view of these two conditions, the control room does not have to be classified or ventilated. The classification and extent of hazard for this location are in compliance with the following: Item Figure Item Table 1-4C 14 1-14 A Figure C-14. The major requirements for classifying the location as shown in Fig. C-14 are as follows: Items pertaining to the degree of danger: B1 - type of product = C1 - system operating mode = D1 - location with/without source of hazard = D2 - above or below grade D3 - how is location considered

= =

D4 - number of vapor tight walls of location =

highly-volatile liquid closed process plant with, control room without above control room indoors, process plant outdoors 1 for process plant, 4 for control room

D5 - type of location

=

control room adjacent to hazardous area

E1 - type of ventilation

=

pressure in control room, natural for process plant

E3 - amount of ventilation

=

sufficient in process plant and control room

E5 - type of safeguards

=

type “B” for control room

F1 - release of flammable gas/vapor

=

occasional

Figure C-14. Control room in Div. 2 hazardous location.

Chapter 21: General Requirements for Group C

303

Items pertaining to the extent of danger: A1 - type of source of hazard

=

process plant

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

high

The location in Fig. C-14 represents two buildings, a roofed space with a single wall containing a process plant and, adjacent to this building, a control room with four, vapor-tight walls. The process plant is classified Div. 2, and the control room is classified nonhazardous. The reasons for classifying the process plant Div. 2 are as follows: 1. The operating mode of the source of hazard is closed. Because of this, a failure or breakdown of the source of hazard will cause flammable material to escape to the atmosphere only occasionally. 2. The building has only one wall and, therefore, is considered sufficiently ventilated by natural ventilation. In view of these two items, the process plant can be classified Div. 2. The extent of the Div. 2 area shall be 25 ft vertical, 100 ft horizontal, and 2 ft high. The reasons for this extension are as follows: 1. highly-volatile flammable liquid 2. large size of the source of hazard 3. Class I flammability 4. heavier-than-air vapor density 5. high system pressure In addition, it is necessary that the building be provided with 10 ft wide additional danger zones. The reasons for classifying the control room nonhazardous are as follows: 1. Four vapor-tight walls. 2. The building opening does give access to a Div. 2 hazardous boundary, but the building is sufficiently ventilated. 3. The ventilation is provided with a type “B” safeguard. In view of these three conditions, the control room can be classified nonhazardous. The classification and extent of hazard for this location are in compliance with the following: Table 1-3 1-4C 1-7

Item 14 15 8

Figure 1-14

Item D

Chapter 22 General Requirements for Group D

Group D represents open sources of hazard that contain Class I flammable products with heavier-than-air gases or vapors located indoors that are sufficiently ventilated and which are classified in accordance with the information in Part 1. Figure D-1. The major requirements for classifying the location as shown in Fig. D-1 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

open

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

4

D5 - type of location

=

drum filling station

E1 - type of ventilation

=

electric exhaust fan

E5 - type of safeguard

=

“B” if required

F1 - release of flammable gas/vapor

=

frequent

304

Chapter 22: General Requirements for Group D

305

Figure D-1. Drum filling station for flammable liquid (sufficiently ventilated location).

Items pertaining to the extent of danger: A1 - type of source of hazard

=

drums

A2 - size of source of hazard

=

small

B6 - vapor density

=

greater than 1.0

C5 - system pressure

=

low

Figure D-1 represents an indoor drum filling station that normally is part of a bulk storage plant. A bulk storage plant receives flammable liquid, which is distributed by tank cars, loading vessels, or pipelines. The intent of the drum filling station is to transfer flammable liquid from one container to another. For transferring Class I liquid, or Class II and III liquids with temperatures above flash point,

306 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres the ventilating system must consist of a gravity fan or an electric suction fan. An electric suction fan is required for a Class I liquid. The suction fan must be provided with a type “B” safeguard. For Class I liquids, dilution of flammable vapors shall be below 1/4 of the LEL, but not less than 1 CFM per ft2 of floor area. Where drum filling is a daily principal activity, dilution for Class II and Class III liquids with temperatures above flash point shall be below 1/4 of the LEL. Where drum filling is an incidental activity for Class II and III liquids or having temperatures slightly above flash point, dilution shall be slightly below LEL. The classification of the location shall be as follows: the fill opening of each container shall be provided with a Div. 1 zone of 3 ft radius. Beyond the Div. 1 zone, a Div. 2 area is required. The extent of the Div. 2 area shall be as follows: 1. For individual containers as shown in Fig. D-1, 2 ft wide extending downward to the floor, 10 ft horizontal, and 18 inches high. 2. For containers occupying not more than 50% of the floor space, the 10 ft wide, 18 inches high area shall be beyond the perimeter of the occupied space. 3. For containers occupying more than 50% of the floor space, the Div. 2 area shall extend up to the walls of the filling station. (Since vapors are heavier than air, there is no need for classifying the entire location.) 4. For Class II and Class III flammable liquids with temperatures below flash point, the location shall be unclassified. The classification and extent of hazard for the filling station in Fig. D-1 are in compliance with the following: Table 1-4D

Item 1

Figure D-2. The major requirements for classifying the location as shown in Fig. D-2 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

open

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

4

D5 - type of location

=

process plant

E1 - type of ventilation

=

fume hood

E5 - safeguards

=

yes

F1 - release of flammable gas/vapor

=

continuous

Chapter 22: General Requirements for Group D

307

Items pertaining to the extent of danger: A2 - size of source of hazard

=

small or large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low

Figure D-2. Process equipment producing flammable vapors (sufficiently ventilated by fume hood).

Figure D-2 represents part of a process plant located indoors, handling heavier-than-air flammable liquids. The building, as shown in Fig. D-2, may be classified nonhazardous if the source of hazard is provided with a fume hood. Since the source of hazard is open, the area between the source of hazard and the fume hood shall be classified Div. 1. A Div. 2 transition zone around the Div. 1 zone is not required because the flow of air is towards the inlet of the fume hood. The inlet of the fume hood must be as close as possible to any point where the escape of flammable vapors may be expected. The whole exhaust duct system should operate below atmospheric pressure with the exhaust fan located in the wall outside the boundary of the nonhazardous location. To maintain a nonhazardous environment around the source of hazard, the exhaust fan must be provided with a type “A” safeguard.

308 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Flammable vapors released by the source of hazard do not have to be diluted to below the LEL or below 1/4 of this LEL. However, suction air must be of sufficient quantity to force any flammable vapor in the air to enter the fume hood. At the same given exhaust air, the capture velocity will increase 25% if the fume hood is provided with flanges or opposite-side panels. At the outlet of the suction fan, the degree and extent of the danger zone shall be as follows. For large sources of hazard, the danger zone around the outlet shall be Div. 1 with a 5 ft radius, and beyond the Div. 1 zone, a Div. 2 zone of 2 ft wide. The 5 ft radius shall extend downwards to the floor and 25 ft horizontally as shown in Fig. D-2. For small sources of hazard, the Div. 1 danger zone shall have a 3 ft radius and beyond the Div. 1 zone, a Div. 2 zone of 2 ft wide. The 2 ft wide zone shall extend downwards to the floor and 10 ft horizontally, as shown in Fig. D-2. For lighter-than-air flammable materials refer to Fig. J-8 in Ch. 28. The classification and extent of hazard for the location are in compliance with the following: Table 1-4D

Item 2

Figure D-3. Figure D-3 represents a paint spraying facility using flammable and combustible liquids such as glycol ether, toluene, xylene, butyl acetate, acetone, hexane, methyl-ethyl-ketone, etc. All flammables have vapors which are heavier than air. Because the vapors from these liquids are very explosive, the entire spraying area must be classified Class I, Div. 1. The spraying equipment shall be interlocked with the ventilating system preventing the spraying equipment from operating. The ventilating system shall consist of a dual ventilating system; filtered pressure ventilation with a heater and an exhaust fan located in the opposite direction. In addition, a water-wash system shall be present to reduce residues from entering the exhaust duct. All electrical equipment of the heat-producing type shall be installed outside the spraying area. This is to prevent temperature-sensitive residues from being ignited when they are deposited on the electrical equipment. Illumination for the spraying area shall be located above wired, tempered, or translucent panels that must seal off the spraying area from the lighting compartment. When combustible powders are used, the classification of the spraying area shall be Class II, Div. 1. Walls and ceiling of the spraying area shall be substantially constructed of noncombustible materials. The classification and extent of hazard for the location are in compliance with the following: Table 1-4D

Item 3

Chapter 22: General Requirements for Group D 309

Figure D-3. Spray room (sufficiently ventilated).

Chapter 23 General Requirements for Group E

Group E represents open sources of hazard that contain Class I flammable products with heavier-than-air gases or vapors located indoors that are insufficiently ventilated and which are classified in accordance with the information in Part 1. Figure E-1. The major requirements for classifying the location as shown in Fig. E-1 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

open

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

4

D5 - type of location

=

dispensing area

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

frequent

310

Chapter 23: General Requirements for Group E

311

Items pertaining to the extent of danger: A1 - type of source of hazard

=

containers

A2 - size of source of hazard

=

small

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low

Figure E-1. Separate dispensing area (insufficiently ventilated location).

Figure E-1 represents a dispensing area located indoors for Class I liquid, and Class II and III liquids with temperatures above flash point. A dispensing area may be located in a liquid warehouse if completely isolated from the storage area. It may be a separate building as shown in Fig. E-1 or it may be a part of a bulk storage plant. 1. If the dispensing area is sufficiently ventilated, each container shall be provided with a Div. 1 zone of 3 ft radius. Beyond the Div. 1 zone, the extent of the Div. 2 zone shall be as follows:

312 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres A. For individual containers—2 ft wide area extending downward, 10 ft horizontal, and 18 inches high. B. For a floor space occupied not more than 50%—10 ft horizontal and 18 inches high from the perimeter of the occupied floor space. C. For a floor space occupied more than 50%—up to the walls of the dispensing area. D. For all three conditions, electric equipment and wiring located within 3 ft of dispensing nozzles shall be suitable for Div. 1. 2. If the area is not sufficiently ventilated for containers with Class I liquid, or Class II and III liquids with temperatures above flash point (as shown in Fig. E-1), the Div. 1 zone shall have a 5 ft radius. Beyond the Div. 1 zone, the Div. 2 zone shall be as in Item C above for floor spaces occuping more than 50%. Non-ventilated dispensing areas or areas in which flammable liquid is transferred without ventilation are considered unsafe and, therefore, are not acceptable. Ventilation requirements shall be as in Fig. D-1. The classification and extent of hazard for the location are in compliance with the following: Table 1-4E

Item 1

Figure E-2. Process plant handling flammable liquid (insufficiently ventilated indoor location).

Chapter 23: General Requirements for Group E

313

Figure E-2. The major requirements for classifying the location as shown in Fig. E-2 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

open

C4 - process temperature

=

above flash point

C5 - system pressure

=

low

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

process plant

E1 - type of ventilation

=

none

F1 - release of flammable gas/vapor

=

continuous

A1 - type of source of hazard

=

process equipment

A2 - size of source of hazard

=

large

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

Items pertaining to the extent of danger:

Figure E-2 represents a process plant with an open system located in a 3-wall building, handling Class I flammable liquid. Because the process plant is large and open, and not sufficiently ventilated, the entire indoor area must be classified Div. 1. Since flammable vapors are released to the outdoors in large quantities and continuously, the building becomes the actual source of hazard. In view of this, it is necessary that the hazardous boundary for the source of hazard be applied to the building, which requires that the outdoors be classified Div. 1 also. For a Class I flammable material, the boundary size shall be 25 V, 100 Ho, and 2 Hi. This boundary shall be measured from the building opening. For a Class II flammable material, the boundary shall also be Div. 1, but is allowed to be smaller. Natural ventilation is considered not capable of sufficiently diluting and dispersing the Class I flammable material to acceptably low levels within the boundary size when the quantities of flammable material are large and continuous. The classification and extent of hazard for the location are in compliance with the following: Table 1-4E

Item 2

Figure 1-8

Item D

Chapter 24 General Requirements for Group F

Group F represents open sources of hazard that contain Class I flammable products with heavier-than-air gases or vapors located outdoors that are sufficiently ventilated and which are classified in accordance with the information in Part 1. Figure F-1. The major requirements for classifying the location as shown in Fig. F-1 are as follows: Figure F-1 represents an open tank, stirrer, centrifuge, or a tank with an open hatch for flammable liquids. As shown in Fig. F-1, the open tank must have a dual classification that is dictated by the following items: A2 - size of source of hazard

=

small

B6 - vapor density

=

heavier than air

C1 - system operating mode

=

open

C5 - system pressure

=

low

D2 - above or below grade

=

above

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapors

=

continuous

314

Chapter 24: General Requirements for Group F

315

Figure F-1. Open tank, tank with open hatch, or open tank with stirrer or centrifuge.

In analyzing the above items, the following shall apply. The classification of the area for the source of hazard must be Div. 1/Div. 2 because of Items C1 and F1. Any electrical equipment within the Div. 1 area must be suitable for this area. In view of Items A2, B6, and C5, the extent of the Div. 1 area shall be small (5 ft radius). Because of Items A2, C5, E1, and E3, the transition zone shall also be small (3 V, 25 Ho, and 18 Hi). The classification and extent of hazard for the location are in compliance with the following: Table 1-4F

Item 1

316 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure F-2. Small storage tank (outdoor location).

Figure F-2. The major requirements for classifying the location as shown in Fig. F-2 are as follows: Figure F-2 represents a small storage tank for flammable liquids. Although the tank itself appears to be a large source of hazard, the explosion danger from the tank is considered relatively small and

Chapter 24: General Requirements for Group F

317

is mainly concentrated at the pipe vent. Since the flammable liquid in the tank is at atmospheric pressure, the pipe vent will only release small quantities of flammable vapors to the atmosphere, although there may be a considerable amount of vapor within the tank. Since the storage tank is of the static type and provided with an open vent, only a small, 5 ft, Div. 1 circular zone is required around the vent opening. In addition, a 10 ft wide, Div. 2 zone is required around the tank as shown in Fig. F-2. The classification and extent of hazard for the location are in compliance with the following: Table Item 1-4F 2

Figure F-3. Large storage tank with fixed roof for crude oil (outdoor location).

318 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure F-3. The major requirements for classifying the location as shown in Fig. F-3 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable liquid

C1 - system operating mode

=

open

C3 - ambient temperature

=

above flash point

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D5 - type of location

=

tank farm

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

continuous

A1 - type of source of hazard

=

storage tank

A2 - size of source of hazard

=

small

B5 - flammability class

=

I

B6 - vapor density

=

heavier than air

C5 - system pressure

=

low

Items pertaining to the extent of danger:

Figure F-3 represents a large storage tank for flammable liquids. If a flammable liquid enters the tank during loading, the liquid will start to evaporate and mix with air. The vapor-air mixture will consist of three layers. If the vapors are heavier than air, the first layer immediately above the liquid will have the greatest vapor concentration. The upper, or third layer, has the smallest vapor concentration. The first vapor layer is too rich and the third layer is too lean to form an explosive mixture. The second layer between the first and third layer, however, will be within the explosion range. As loading progresses, evaporation will continue so that the vapor layers become broader as they move up in the tank with the surface of the liquid. Eventually, the second vapor layer will reach the vent pipe and spill over to the atmosphere. Only in the final stage of loading does an ignitable vapor mixture leave the tank. Since the ignitable mixture leaves the vent pipe high up in the air, the vapors will disperse rapidly to low concentrations and because the vapors are heavier than air, they will move downward along the tank shell. Depending upon wind conditions, the vapor layers may reach safe concentrations before touchdown. After the loading has been completed, the vapors in the tank become more homogeneous. When the tank is being discharged, the ignitable mixture at the pipe vent will disappear. Air will flow through the vent in the tank forcing the explosion hazard at the vent to disappear. If the tank is discharged too quickly, turbulence will occur in the vapor concentration in the tank allowing the vapor to mix with air

Chapter 24: General Requirements for Group F

319

rapidly. Because of the turbulence, the entire tank atmosphere above the flammable liquid may become ignitable. However, this condition has no bearing in the area classification. The area classification is only required for the area external to the tank and is based on ignitable vapors in the atmosphere. Because the tank vent is open, the area around the vent must be classified Div. 1. A transition zone surrounding the Div. 1 zone must be classified Div. 2. Although the storage tank is large, the actual source of hazard is small. This small source of hazard is the open vent on the tank. The extent of the Div. 1 zone shall have a 5 ft radius, because of the small size of the vent (Item A2), and the low pressure in the tank (Item C5). The transition zone beyond the Div. 1 zone shall also have a 5 ft radius. Along the shell of the tank, an additional 10 ft wide zone shall be provided which must extend all the way up to the dikes. The extension shall have a height the same as the height of the dikes. The classification and extent of hazard for the storage tank are in compliance with the following: Table 1-4F

Item 2

Figure F-4. Large impounding basin for oil/water (outdoor location).

Figure F-4. The major requirements for classifying the location as shown in Fig. F-4 are as follows: Figure F-4 represents an open impounding basin in which Class I flammable liquids (slop oil) are ditched. Since the impounding basin is located in a freely ventilated open area, it makes sense to classify the entire location Div. 2. However, since the impounding

320 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres basin is open, the location cannot be classified Div. 2, but must be classified Div. 1. Additonally, the impounding basin has a large size, therefore, it also seems justified to classify a zone adjacent to the impounding basin Div. 1. Although this conclusion seems to be valid, the conclusion is incorrect. The fact is that slop oil consists of a mixture of flammable and nonflammable liquids. Most of the slop oil consists of water. Only a small portion of the slop oil consists of a flammable liquid that is able to give off flammable vapors. Taking into account the fact that these small quantities of flammable vapors are being diluted rapidly by natural ventilation, the danger of the impounding basin must be considered small and remote. Although the impounding basin itself is large, the actual danger comes from the flammable vapors. Since their quantities are considered very small, the impounding basin is considered a small or mini source of hazard. In view of these considerations, the area surrounding the impounding basin is not required to be classified Div. 1. When dealing with very small quantities of flammable gases or vapors in an area that is sufficiently ventilated, the location is considered remotely dangerous, requiring a Div. 2 classification. Since very small amounts of flammable vapors will cover little areas, the extent of the Div. 2 area should not be more than 15 ft horizontal and 18 inches high. The classification and extent of hazard for the location are in compliance with the following: Table 1-4F

Figure F-5. Control room adjacent to a hazardous area.

Item 3

Chapter 24: General Requirements for Group F

321

Figure F-5. The major requirements for classifying the location as shown in Fig. F-5 are as follows: Figure F-5 represents a control room that is adjacent to a hazardous area. The source of hazard in the hazardous area is an open container or sump for oil and other flammable liquids. Because the sump is open and contains a flammable product, an area above the surface of the flammable liquid must be classified Div. 1. Since the flammable liquid is not under pressure, the Div. 1 area does not have to extend beyond the finished grade. A Div. 2 transition zone is necessary, however. This Div. 2 zone shall extend 10 ft both vertically and horizontally. The control room adjacent to the Div. 2 area does not have to be classified provided the wall of the control room adjacent to the hazardous area is vapor tight. Since the door opening of the control room does not give access to a hazardous area, the control room can be considered nonhazardous without ventilation. The classification and extent of hazard for the location are in compliance with the following: Table 1-4F

Item 4

Figure 1-14

Item A

Chapter 25 General Requirements for Group G

Group G represents closed sources of hazard that contain Class I flammable products with lighter-than-air gases or vapors located indoors that are sufficiently ventilated and which are classified in accordance with the information in Part 1. Figure G-1. The major requirements for classifying the location as shown in Fig. G-1 are as follows: Items pertaining to the degree of danger: B1 - type of product = C1 - system operating mode = D1 - location with/without source of hazard = D2 - above or below grade =

flammable gas closed with above

D3 - how is location considered = D4 - number of vapor tight walls of location = D5 - type of location =

indoors 3 compressor station

E1 - type of ventilation E3 - amount of ventilation F1 - release of flammable gas/vapor

= = =

mechanical sufficient occasional

A1 - type of source of hazard

=

gas compressor

A2 - size of source of hazard

=

small or large

B6 - vapor density

=

below 0.75

Items pertaining to the extent of danger:

322

Chapter 25: General Requirements for Group G

323

Figure G-1. Compressor station handling flammable gas with vapor density below 0.75 (sufficiently ventilated location).

Figure G-1 represents a 3-wall, sufficiently ventilated building for gas compressors handling flammable gases with vapor densities below 0.75. The entire station must be classified as a result of the 10 Pu probability factor. Since there is sufficient ventilation, the compressor station is allowed to be classified Div. 2. At the roof, the Div. 2 area shall extend 25 ft vertically and 15 ft horizontally. Because of the presence of forced ventilation, no additional danger zone is required at the access opening of the compressor station. The classification and extent of hazard for the location are in compliance with the following: Table 1-4G

Item 1

Figure 1-8

Item G

324 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure G-2. Control room in gas compressor station (with vapor density below 0.75) (sufficiently ventilated control room).

Figure G-2. The major requirements for classifying the location as shown in Fig. G-2 are as follows: Compressor Station Items pertaining to the degree of danger: B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

without

D4 - number of vapor tight walls of location =

3

E1 - type of ventilation

=

roof opening

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

Chapter 25: General Requirements for Group G

325

Items pertaining to the extent of danger: A1 - type of source of hazard = gas compressor B6 - vapor density = below 0.75 Control Room Items pertaining to the degree of danger: D3 - how is location considered = indoors D4 - number of vapor tight walls of location = 4 E1 - type of ventilation = pressure fan E5 - safeguards required = yes The major requirements for both locations as shown in Fig. G-2 are as follows. Figure G-2 represents a station for gas compressors handling lighter-thanair flammable products and a control room which gives access to the compressor station. The requirements for classifying the compressor station in Fig. G-2 are the same as for the compressor station in Fig. G-1, except, in Fig. G-2, the location must be provided with an additional danger zone. The additional danger zone is required because the station is provided with a roof opening. The roof opening provides an air draft in the station. Because this draft will exist intermittently, an additional danger zone must be provided. Possible lack of chimney effect may cause airborne gases to contaminate the outdoors. When the compressors operate at high pressure, the additional danger zone must be 15 ft wide. At moderate or low pressure, the additional danger zone may be 10 ft wide. The additional danger zone must extend 25 ft vertically above the roof as shown in Fig. G-2. The control room can be classified nonhazardous under the following conditions: 1. The control room is provided with a pressure fan. 2. The pressure fan produces sufficient ventilation. 3. The control room is provided with a type “B” safeguard. 4. The air intake to the control room is located in a nonhazardous area. A type “B” safeguard is required because the control room gives access to a Div. 2 area. The vertical inlet pipe must either be as long as A or as short as B. A long inlet pipe A is not recommended if flammable gases can be sucked in by the inlet pipe. This condition is possible under unfavorable wind conditions (for example, when the direction of the wind is towards intake pipe A). Under this condition, the shorter pipe B is recommended. The classification and extent of hazard for the location are in compliance with the following: Table 1-3 1-4G 1-7

Item 14 2 8

326 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure G-3. Storage and chemical process area (sufficiently ventilated indoor locations).

Chapter 25: General Requirements for Group G

327

Figure G-3. Figure G-3 represents two different indoor locations, one for storing lighter-than-air gases (Fig. G-3-1) and one for processing, handling, and transmitting lighter-than-air gases (Fig. G-3-2). The lighter-than-air gas in both locations consist of hydrogen gas or a lighter-than-air gas of equivalent hazard. The content of hydrogen gas in the storage area is over 400 cf and is stored in more than one container. In the process area, the total gas content is less than 400 cf and is stored in a single container (cf = cubic ft of gas at 14.71 psia and 70°F). A single capped container with a content of less than 400 CF is generally not considered a source of hazard. A number of capped containers in storage must be considered sources of hazard. A single noncapped container connected to a piping system, usually 1/4" in size, and the components in the piping system, such as pressure reducers, valves, manifolds, screwed fittings, etc., are considered sources of hazard, if the connections are of low integrity. For low integrity connections see Ch. 6, Sec. G. Both locations require ventilation. Generally, dilution of airborne flammable materials is required to be below 1/4" of the LEL. However, for the storage and process areas in Fig. G-3, this rule does not apply. The storage area may be ventilated by an electrically operated roof fan, a gravity fan, or by an opening in the roof. A roof opening generally requires the air inlet and outlet to have a minimum ventilation opening of one square foot per 1,000 cubic feet of room volume. For the process area, normally attended by personnel for processing, handling, and transmitting flammable gas, the ventilation must consist of an electrically operated exhaust roof fan, or closer to the sources of hazard, a canopy fume hood, which is the preferred ventilation method. For detailed information on fume hoods refer to Fig. J-6 in Ch. 28, Part 2. However, if high-integrity pipe connectors are used, no ventilation is necessary, except as an additional safety precaution, an opening in the roof above the process equipment may be considered. Although the noncapped container in Fig. G-3-2 is shown indoors, the preferred location of the container is outdoors. The classification of the process area is different than for the storage area. For the process area, the area below the demarcation line X–X shall be Div. 2 with a radius of 3 or 5 ft. If the sources of hazard in the process area are brushed by ventilating air or the pressure in the system is low, the danger zone below the demarcation line shall be 3 ft. However, if the source of hazard is not brushed by ventilating air or if the system pressure is moderate or high, the danger zone below the demarcation line shall be 5 ft or extend to the floor. Above the demarcation line, the area within the hazardous cone and the area in the corner adjacent to the cone, which is outside the airstream, shall be classified Div. 2 as shown in Fig. G-3-2. For demarcation lines refer to Fig. 1-21A. This type of classification is only required if the individual components in the piping system are connected with low-integrity seal connectors. The classification for a single noncapped container located outdoors is a 5 ft, Div. 2 radius below the demarcation line and

328 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres a 15 ft, Div. 2 cone above the demarcation line. Classification of the process area is not required if high-integrity seal connectors are used. The storage area storing more than one container needs to be classified as Div. 2. The type of electrical equipment in the storage area shall be suitable for a Div. 2 location. Electrical equipment, of the heat-producing type, in the process area in which low-integrity connections and low-quality pipe components are used, shall be as follows. Equipment A shall be suitable for Div. 2. Equipment B shall be general purpose. Equipment C, within a 3 ft radius, shall be explosion proof, and equipment D shall also be general purpose. The classification and extent of hazard for the location are in compliance with the following: Table 1-4G

Item 3

Chapter 26 General Requirements for Group H

Group H represents closed sources of hazard that contain Class I flammable products with lighter-than-air gases or vapors located indoors that are insufficiently ventilated and which are classified in accordance with the information in Part 1. Figure H-1. The major requirements for classifying the location as shown in Fig. H-1 are as follows: Items pertaining to the degree of danger: A2 - size of source of hazard

=

small

B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

partially enclosed

D5 - type of location

=

compressor station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

insufficient

F1 - release of flammable gas/vapor

=

occasional

329

330 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Items pertaining to the extent of danger: A1 - type of source of hazard

=

gas compressor

B6 - vapor density

=

lighter than air

C5 - system pressure

=

low or moderate

F5 - pump driver and size

=

electric, 60–201 hp

F7 - probability factor

=

6 Pu

Figure H-1. Compressor station handling flammable gas at low or moderate pressure (upper part of building insufficiently ventilated).

Figure H-1 represents a gas compressor shelter which has a roof supported by four walls. Four vertical beams support the four walls in turn. Since the shelter in Fig. H-1 is partially enclosed, the vapor density of the flammable product will determine whether the shelter is an indoor or outdoor location. The flammable gas in Fig. H-1 is lighter than air, therefore, the flammable gases could accumulate in

Chapter 26: General Requirements for Group H

331

the upper part of the shelter. The possibility of accumulation in the upper part of the shelter determines that the shelter is an indoor location. If the flammable gases should be heavier than air, then the lower part of the shelter should prevail which makes the shelter an outdoor location since flammable gases cannot accumulate. Because flammable gases could accumulate in the upper part of the shelter, the upper part must be classified Div. 1. Since there are no walls in the lower part of the shelter, the lower part is considered sufficiently ventilated, allowing the lower part to be classified Div. 2. Because the probability factor is less than 10 Pu, the extent of the Div. 2 area shall be 15 ft horizontally and all the way up to the Div. 1 area. An additional Div. 2 area extending 15 ft vertically is also required above the non-vapor tight roof of the shelter. The upper part of the shelter is enclosed and the lower part is open, consequently an additional Div. 2 danger area extending 10 ft horizontally is also required beyond the solid walls of the upper part of the shelter. The classification and extent of hazard for the location are in compliance with the following: Table 1-4H

Item 1

Figure 1-8

Item H

Figure H-2. Compressor station handling flammable gas with vapor densities below 0.75 (insufficiently ventilated building).

332 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure H-2. The major requirements for classifying the location as shown in Fig. H-2 are as follows: Items pertaining to the degree of danger: A2 - size of source of hazard

=

small

B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

3

D5 - type of location

=

compressor station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

insufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

compressor

B6 - vapor density

=

below 0.75

C5 - system pressure

=

high

F5 - pump driver and size

=

electric, 60–201 hp

F7 - probability factor

=

11 Pu

Items pertaining to the extent of danger:

Figure H-2 represents a 3-wall compressor shelter with a vapor-tight roof. The compressor shelter is insufficiently ventilated and therefore it is necessary that the shelter be classified Div. 1. Since the probability factor for the location is more than 10 Pu, it is necessary that the entire shelter be classified Div. 1. An additional danger zone must be provided at the opening of the shelter extending 15 ft vertically above the shelter roof. The width of the zone is a function of the pressure of the system. If the pressure is low or moderate, the additional danger zone at the building opening shall be 10 ft wide; if the pressure is high, it must be 15 ft wide. The classification and extent of hazard for the location are in compliance with the following: Table 1-4H

Item 2

Figure 1-8

Item E

Figure H-3. Figure H-3 represents a control room and an adjacent area both located above a gas compressor station. The vapor density of the flammable product in the gas compressor station is below 0.75. Because the gas compressor station lacks sufficient ventilation it must be classified Div. 1. The location of pressure fan 2 is considered safe, however, the location of pressure fan 1 is

Chapter 26: General Requirements for Group H

333

considered unsafe because it is within the 25 ft danger zone. Since the intake opening for the adjacent location is within the 25 ft danger zone, flammable gases, if escaping from the Div. 1 area, could be sucked in by fan 1. This is the reason for not classifying the adjacent location nonhazardous, nor is it recommended to have a pressure fan.

Figure H-3. Control room above gas compressor station (insufficiently ventilated gas station).

Because there is a door opening from the adjacent location to the control room, the control room cannot be classified nonhazardous either. For the control room and the adjacent location to be classified nonhazardous, it is necessary that pressure fan 1 be relocated outside the 25 ft danger zone. However, even with a relocated pressure fan, if the floor above the gas station is not vapor-tight, both the adjacent location and control room need a type “A” safeguard as a result of the compressor station being classified Div. 1. But since the compressor system has a

334 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres closed operating mode, from which only flammable gases can occasionally escape, a type “B” safeguard may be used instead. The classification and extent of hazard for the location are in compliance with the following: Table 1-4H 1-8

Item 3 1

Figure 1-8

Item I

Figure H-4. Control room adjacent to Div. 1 hazardous area (insufficiently ventilated control room).

Figure H-4. The major requirements for classifying the location as shown in Fig. H-4 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - control room with/without source of hazard

=

without

D3 - how is location considered

=

indoors

D4 - number of walls of control room

=

4

D5 - type of location

=

control room

E1 - type of ventilation in control room

=

none

F1 - release of flammable gas/vapor

=

occasional

Chapter 26: General Requirements for Group H

335

Items pertaining to the extent of danger: A1 - type of source of hazard

=

compressors

B6 - vapor density

=

lighter than air

Figure H-4 represents a control room that is separated from a Div. 1 area by means of a vapor-tight wall. The control room is allowed to be classified nonhazardous without ventilation or safeguards since there is no direct access from the control room to the Div. 1 area indoors nor to the hazardous area outdoors. If flammable gases from the Div. 1 area should be capable of reaching the access opening of the control room (that is, when the distance between both openings is shorter than 25 or 50 ft), the control room is not permitted to be classified nonhazardous. The size of the electric driver is 60 hp or above, requiring 25 ft at moderate pressure and 50 ft at high pressure. The classification and extent of hazard for the locations are in compliance with the following: Table 1-4H

Item 4

Figure 1-14 1-3

Item A G, H

Figure H-5. Control room in gas compressor station (sufficiently ventilated control room).

336 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure H-5. The major requirements for classifying the location as shown in Fig. H-5 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - control room with/without source of hazard

=

without

D3 - how is location considered

=

indoors

D4 - number of walls of control room

=

4

D5 - type of location

=

control room

E1 - type of ventilation in control room

=

pressure fan

E3 - amount of ventilation in control room

=

sufficient

E5 - type of safeguards in control room

=

none

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

compressors

B6 - vapor density

=

below 0.75

C5 - system pressure

=

high

F5 - pump driver and size

=

electric, 60 hp and above

Items pertaining to the extent of danger:

Figure H-5 represents a control room which gives access to a Div. 1 hazardous area. Although the control room is provided with sufficient ventilation from a source outside the hazardous area, the control room cannot be classified nonhazardous, but must be classified Div. 2. There are three reasons for classifying the control room Div. 2: 1. The control room is sufficiently ventilated 2. The control room gives access to a Div. 1 area 3. The control room is not provided with a suitable safeguard Refer to Ch. 4, Sec. B, for additional information why this control room needs to be classified Div. 2. Since the flammable product in the Div. 1 hazardous area has a vapor density below 0.75 and is operating at high pressure, an additional danger zone 15 ft wide must be provided at the opening of the compressor station. This additional danger zone needs to be classified only Div. 2. The classification and extent of hazard for the location are in compliance with the following: Table 1-3 1-4H 1-7

Item 13 5 3

Figure 1-8

Item E

Chapter 26: General Requirements for Group H

337

Figure H-6. Control room in gas compressor station (sufficiently ventilated control room).

Figure H-6. Figure H-6 represents two locations: an indoor, hazardous area in which gas compressors and associated process equipment are located, and a control room which gives access to the hazardous area. As shown in Fig. H-6, the control room is classified nonhazardous and the hazardous area, Div. 1. The basis for this classification is as follows. If it lacks sufficient ventilation, there are three conditions under which an indoor hazardous location must be classified Div. 1: 1. flammable gases are continuously released from the process equipment under normal operating conditions 2. if the release of flammable gases is frequent because of repairs and maintenance of the process equipment 3. if the flammable gases are released only occasionally as a result of failure or rupture of the process equipment Since the process equipment has a closed operating mode, the release of flammable gases cannot be continuous under normal operation. There is also no indication in Fig. H-6 that the release of flammable gas is frequent. However, an occasional release is considered possible because the process equipment can leak,

338 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres fail, or rupture during its operation. Although all three conditions require that the hazardous area be classified Div. 1 because of lack of ventilation, the occasional release is the least dangerous because such a release may occur only once in a while. The control room may be classified nonhazardous under the following three conditions: 1. if it has four vapor-tight walls 2. if it is sufficiently ventilated 3. if it is provided with a suitable safeguard The general rule for safeguards is that a type “A” safeguard must be applied if the control room gives access to a Div. 1 area. However, there are exceptions to this rule. The rule allows the use of the less expensive type “B” safeguard if the release of flammable gases is other than continuous. This exception is based on the fact that if a release is other than continuous, the other release must be considered in conjunction with a possible failure or outage of the ventilating system in the control room. For example, if an occasional release of flammable gases should occur, there may not necessarily be a failure or outage of the ventilating system. If on the other hand, there is a ventilation failure or outage, there may be no release of flammable gases. Such a condition allows the use of the much less expensive type “B” safeguard. This consideration will also apply to frequent releases. Since the source of hazard is operating at moderate pressure, the additional danger zone at the opening of the enclosed Div. 1 area shall only be 10 ft wide. The additional danger zone need only be classified Div. 2. The classification and extent of hazard for the location are in compliance with the following: Table 1-3 1-4H 1-7

Item 14 6 4

Figure 1-8

Item E

Figure H-7. Conservative classification of a compressor station requires that the entire station be classified Div. 1 if it is not sufficiently ventilated. For large compressors (or for a number of small compressors which occupy more than 50% of the floor space), this is a valid classification; but for a single small compressor (or for a number of small compressors which occupy not more than 50% of the floor space), this is not a valid classification. Where to precisely draw the line between an entire or partial classification is a matter of: 1. size and number of compressors occupying the floor space 2. whether the station is attended 3. whether the flammable gas is “very flammable”

Chapter 26: General Requirements for Group H 4. the pressure in the system 5. quantity of vapors released to the atmosphere in case of an accidental rupture 6. whether the flammable gas must be considered “heavier” or “lighter” than air

Figure H-7. Compressor station handling flammable gas (insufficiently ventilated location).

339

340 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres By evaluating these conditions, the susceptibility of explosion danger in the station becomes evident. This can be expressed in a probability factor. The application of a probability factor is explained in Ch. 3, Sec. J. A probability factor of 10 or more requires that the entire station is classified and probability factors of less than 10 Pu require that the station be only partially classified. Whether a flammable material is “very flammable” can be obtained from the tables in the Appendix. Only when marked with a “4” is the material in the appendix considered very flammable. In Fig. H-7, the compressors shown are not over 50 hp. They also occupy not more than 50% of the floor space and the gas is not “very flammable.” The station in Fig. H-7 is attended. If the compressor station does not occupy more than 50% of the floor space, it requires a severity factor of 2 Pu. With high pressure in the system, the severity factor is 3 Pu. The quantity of release for compressors below 51 hp requires a severity factor of 2 Pu and if personnel attend the compressor station the accumulation factor is 1.0 Pu. The failure of a compressor and/or its associated components in the piping system cannot be resolved as quickly as the failure of a pumping system processing flammable liquid. This is due to the fact that flammable liquid is generally visible when exposed to the surrounding atmosphere. The gas may not be visible and may also be odorless. With a high system pressure, the probability factor is: 2 × 3 + 2 + 1 = 9 Pu, as shown in Fig. H-7. This indicates that the station does not have to be entirely classified Div. 1. If the gas is not very flammable a horizontal distance of 15 ft from the source of hazard is considered sufficient, and, since the station is not ventilated and has four walls, a Div. 2 transition zone is also required in addition to the Div. 1 zone. The classification and extent of hazard for the location are in compliance with the following: Table 1-4H

Item 7

Chapter 27 General Requirements for Group I

Group I represents closed sources of hazard that contain Class I flammable products with lighter-than-air gases or vapors located outdoors that are sufficiently ventilated and which are classified in accordance with the information in Part 1. Figure I-1. The major requirements for classifying the location as shown in Fig. I-1 are as follows: Items pertaining to the degree of danger: B1 - type of product = C1 - system operating mode = D1 - location with/without source of hazard = D2 - above or below grade = D3 - how is location considered = D5 E1 E3 F1 -

type of location type of ventilation amount of ventilation release of flammable gas/vapor

341

= = = =

flammable gas closed with above outdoors compressor station natural sufficient occasional

342 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Items pertaining to the extent of danger: A1 - type of source of hazard

=

compressor

B6 - vapor density

=

lighter than air

F5 - pump driver and size

=

as indicated in Fig. I-1

Figure I-1. Compressor station handling flammable gases outdoors.

Figure I-1 represents a gas compressor located outdoors pumping flammable gas. Since the gas compressor is located outdoors (Item D3), the location is considered sufficiently ventilated by natural ventilation (Items E1 and E3). Because the gas compressor has a closed system and is located outdoors, the location for the gas compressor can be classified Div. 2. Since the flammable material in the gas compressor is lighter than air, the extent of the hazard horizontally need only to be as indicated in Fig. I-1. The classification and extent of hazard for the location are in compliance with the following: For low or moderate pressure For high pressure

Table 1-4I 1-4I

Item 1 2

Fig 1-2 1-2

Item A B

Chapter 27: General Requirements for Group I

343

Figure I-2. The major requirements for classifying the location as shown in Fig. I-2 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D5 - type of location

=

compressor station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

compressor

B6 - vapor density

=

lighter than air

F5 - pump driver and size

=

as indicated in Fig. I-2

Items pertaining to the extent of danger:

Figure I-2. Compressor station handling flammable gases outdoors.

344 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure I-2 represents a gas compressor located outdoors pumping flammable gas. Since the gas compressor is located outdoors (Item D3), the location is considered sufficiently ventilated by natural ventilation (Items E1 and E3). Because the gas compressor has a closed system and is located outdoors, the location for the gas compressor can be classified Div. 2. The flammable material in the gas compressor is lighter than air, therefore, the extent of the hazard horizontally need only to be as indicated in Fig. I-2. The classification and extent of hazard for the location are in compliance with the following: For low or moderate pressure For high pressure

Table 1-4I 1-4I

Item 3 4

Fig 1-2 1-2

Figure I-3. Storage cylinders for gaseous hydrogen located outdoors.

Item C C

Chapter 27: General Requirements for Group I

345

Figure I-3. Figure I-3 represents an outdoor storage area for storing more than 400 cf of hydrogen gas (cf = cubic feet of gas at 14.71 psia and 70°F). The general safety requirement for hydrogen gas is to maintain a minimum safe clearance of 15 or 25 ft around the storage cylinders. The 25 ft clearance is required if ventilating air is obstructed. Electrical equipment may be of the general-purpose type if it is located outside the 15 or 25 ft boundary. Within the 15 or 25 ft boundary, the electrical equipment is required to be suitable for a Class I, Div. 2 location. The storage cylinders for the hydrogen gas are not to be considered as the actual source of hazard. Valves, screwed fittings, and gauges are normally the actual source of hazard. All cylinders must be capped when in storage. As an additional safety precaution, storage cylinders for hydrogen gas shall not be located underneath power lines or below piping systems containing flammable and combustible liquids. Where flammable and combustible liquids are involved, a minimum horizontal safe distance of 25 ft must be maintained between the two. The classification and extent of hazard for the location are in compliance with the following: Table 1-4I

Item 5

Figure I-4. Compressor station without walls handling flammable gas at low or moderate pressure (sufficiently ventilated gas station).

346 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure I-4. The major requirements for classifying the location as shown in Fig. I-4 are as follows: Items pertaining to the degree of danger: A2 - size of source of hazard

=

small

B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

outdoors

D4 - number of vapor tight walls of location =

none, except upper part

D5 - type of location

=

compressor station

E1 - type of ventilation

=

natural

E3 - amount of ventilation

=

sufficient

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

compressor

B6 - vapor density

=

lighter than air

C5 - system pressure

=

low or moderate

F5 - pump driver and size

=

electric, 0–51 hp

F7 - probability factor

=

5 Pu

Items pertaining to the extent of danger:

Figure I-4 represents a shelter for gas compressors operating at low or moderate pressure. Since the shelter virtually has no walls, it is considered an outdoor location (Item D3). If the gas compressor breaks down, flammable gases will escape from the gas compressor and since the gases are lighter than air (Item B6), the gas will rise to the roof of the shelter. Because the roof of the shelter is open, no accumulation of flammable gases will occur under the roof of the shelter. Because the area under the roof is sufficiently ventilated, it may be classified Div. 2. The extent of the Div. 2 hazard above the roof shall be 15 ft horizontal and 25 ft vertical. At the source of hazard, the hazard shall also extend 15 ft horizontally. No additional danger zones are necessary since the 15 ft horizontal distance is shorter than the distance between the source of hazard and the outer perimeter of the building. Only when the location in the shelter is entirely classified is an additional danger zone required. The classification and extent of hazard for the location are in compliance with the following: Table 1-4I

Item 6

Chapter 27: General Requirements for Group I

347

Figure I-5. Control room below Div. 2 hazardous area (insufficiently ventilated control room).

Figure I-5. The major requirements for classifying the location as shown in Fig. I-5 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - control room with/without source of hazard

=

without

348 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres D3 - how is location considered

=

indoors

D4 - number of walls of control room

=

4

D5 - type of location

=

control room

E1 - type of ventilation in control room

=

none

F1 - release of flammable gas/vapor

=

occasional

A1 - type of source of hazard

=

gas compressor

B6 - vapor density

=

below 0.75

Items pertaining to the extent of danger:

Figure I-5 represents a control room located under a compressor station that is classified Div. 2. The control room in Fig. I-5 is without ventilation and does not have to be classified. The reason for this is that the control room does not give access to a hazardous area, but, most importantly, the vapor density of the flammable product in the compressor station is below 0.75. A vapor density below 0.75 will cause a flammable gas in the air to rise quickly. These gases in the air, therefore, cannot enter the control room and in view of this, the control room is allowed to be classified nonhazardous without ventilation. The classification and extent of hazard for the location are in compliance with the following: Table 1-4I 1-8

Item 7 7

Chapter 28 General Requirements for Group J

Group J represents open or closed sources of hazard that contain Class I flammable products with heavier- or lighter-than-air gases or vapors located in sufficiently or insufficiently ventilated locations and which are classified in accordance with the information in Part 1. Figure J-1. Figure J-1 represents two pits below finished grade, located outdoors, adjacent to a source of hazard. The pits must be classified as follows. If a pit is located in a hazardous area in which flammable gases or vapors released to the atmosphere have a vapor density greater than 0.75, the pit must be classified Div. 1 as shown in Fig. J-1A. The reason for this classification is that with a vapor density greater than 0.75, the gas or vapor will accumulate in the pit. The pit within the hazardous area must be classified nonhazardous if the flammable gases or vapors in the hazardous area have a vapor density below 0.75 as shown in Fig. J-1B. The reason for this classification is that airborne flammable gas or vapors with vapor densities below 0.75 will quickly rise when exposed to the open and, therefore, will not accumulate in the pit.

349

350 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The classification and extent of hazard for the location are in compliance with the following: Table 1-4J

Figure J-1. Pits in outdoor, Div. 2 locations.

Item 1 and 2

Chapter 28: General Requirements for Group J

351

Figure J-2. Loading/unloading platform, outdoors for flammable liquid.

Figure J-2. Figure J-2 represents a loading platform for handling flammable liquids with heavier-than-air gases or vapors. A loading platform is normally part of a bulk storage plant that receives flammable and combustible liquid to be distributed to other locations by tank cars, tank vessels, or pipelines. The loading of the flammable product is either at the top or bottom of the tank car, with or without an atmospheric vent or recovery system. The degree and extent of the hazard are as indicated in Columns (5) and (6) of the table in Fig. J-2. The classification and extent of hazard for the location are in compliance with the following: Table 1-4J

Item 3

352 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure J-3. Pumphouse handling liquid petroleum gas at moderate pressure.

Figure J-3. Figure J-3 represents a building that is divided into two separate areas. One area is a pump house handling liquefied petroleum gas, and the other area is an adjacent location. Liquid petroleum gas (LP gas) includes a material with a vapor pressure equivalent to that of propane that is mainly made up of the following hydrocarbons: propane, propylene, butane, or butylene. A pump house which is not sufficiently ventilated, as shown in Fig. J-3A, must be classified Div. 1.

Chapter 28: General Requirements for Group J

353

A pump house, which is sufficiently ventilated as shown in Fig. J-3B and J-3C, is allowed to be classified Div. 2. According to Fig. 1-3 (Item G), the horizontal boundary for small pumps in the range of 60–201 hp, operating at moderate pressure, needs to be 25 ft long. The pumps in Fig. J-3 occupy 100% of the pump house floor space. As shown in Fig. J-3, 10 ft of the 25 ft boundary reaches the open perimeter of the pump house, only 15 ft extends beyond the open building perimeter. Since the hazardous area indoors reaches the open perimeter of the building, it is necessary that an additional danger zone 10 ft wide beyond the open perimeter of the building also be present. Since only 5 ft of the 15 ft boundary extends beyond the 10 ft wide additional danger zone, the 5 ft extension may, from a practical stand point, be extended vertically all the way up to the open perimeter of the building as shown in Fig. J-3. The adjacent location does not need an additional danger zone because the 25 ft boundary does not reach the open perimeter of the adjacent building. The location adjacent to the pump house does not have to be classified if the common wall between the pump house and the adjacent location is vapor tight and the distance between the opening of the adjacent location and the pump house opening is greater than 25 ft. A common wall which is not vapor tight requires that the adjacent location be classified as follows: 1. Div. 1, if the adjacent location is not sufficiently ventilated and the pump house is classified Div. 1 or Div. 2. 2. Div. 2, if the adjacent location is sufficiently ventilated and the pump house is classified Div. 1 or Div. 2. (See Fig. J-3C.) 3. Nonhazardous, if the adjacent location is not sufficiently ventilated, but is provided with vapor-tight walls and does not give access to an outdoor hazardous area. The classification and extent of hazard for the pump house and adjacent location are in compliance with the following: Item Figure Item Table 1-4J 4 and 5 1-14 A, B, and C 1-7 2, 3, and 8 1-3 G Figure J-4. Figure J-4 represents minimum requirements for safe distances between containers filled with liquefied petroleum gas (LPG) and an electrical ignition source. The minimum safe distance between a container filled with LPG and an ignition source, a central air conditioning (AC) unit or window-type AC unit, is 5 ft if the container is of the DOT type. The minimum safe distance must increase to 10 ft if the DOT type containers are locally filled. The minimum safe distance should be 15 ft if the containers are of the non-DOT type. The safe distances between containers for flammable products and electrical ignition sources are in compliance with the following: Table 1-4J

Item 6

354 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure J-4. Safe distances between ignition source and DOT and non-DOT cylinders filled with LPG.

Figure J-5. Figure J-5 represents a 2-wall building with sources of hazard and two, 4-wall spaces without sources of hazard. The two 4-wall spaces are marked A and B and are located within the hazardous boundary of the sources of hazard in the 2-wall building. The door opening of space A is located at grade level and the opening of space B gives access to an elevated walk-board located above the hazardous boundary. The hazardous boundary from the sources of hazard extends to the outdoors, horizontally to left and right as shown in Fig. J-5. Both boundaries are classified Div. 2. Since the door opening of space A gives direct access to the hazardous boundary, space A must be classified Div. 2, if space A is sufficiently ventilated, and Div. 1 if it is without ventilation. Space A may be classified nonhazardous if it is ventilated and provided with a type “B” safeguard. Space B need not be classified nor ventilated if the distance between the bottom of its door opening and the finished floor outdoors is twice the height of the 18" high hazardous boundary, and if the horizontal distances between side wall and perimeter of the 2-wall building is twice the width of the “Additional Danger Zones.” Any distance smaller than twice the size of the boundary or less than twice the additional danger zone makes the space unsafe unless the side walls are vapor tight. If the side walls are vapor tight, but provided with non-bolted windows, the space becomes unsafe unless twice the horizontal distance is maintained.

355

Figure J-5. Access to a Div. 2 hazardous boundary.

356 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The classification of both enclosed spaces A and B is in compliance with the following: Table 1-4J 1-7

Item 7 and 8 6 and 7

Figure 1-14

Item B and C

Figure J-6. Piping system with screwed fittings, valve, etc., for indoor gaseous systems of less than 400 CF.

Chapter 28: General Requirements for Group J

357

Figure J-6. Figure J-6 provides requirements for hazardous boundaries for small process areas associated with lighter-than-air, Class I flammable gases. The lighter-than-air gases are processed and handled by operating personnel. The gases are transported from a single gas container, normally located outdoors, to the process area, located indoors, via a piping system that is sized not greater than ¼". The components in the piping system are of the mini type and consist of a pressure reducer, valves, manifolds, gauges, and pipe connectors. The gas container is pressurized between 2,000 and 3,000 psi and its gas content is less than 400 cf (cf = cubic feet of gas at 14.71 psia and 70°F). The pressure reducer is normally located at the gas container outside the process area and reduces the pressure in the piping system inside the process area to a desired lower pressure. The components in the piping system indoors are considered sources of hazard as follows: 1. Valves and manifolds, if they are connected to the piping system with low integrity seal connectors (see Ch. 6, Sec. G). 2. Gauges, regardless of how they are connected to the piping system. The hazardous boundary for the components is listed in Fig. J-6. The sizes of the boundaries are selected on the basis of the following requirements: 1. pressure in the piping system 2. air brushing of the sources of hazard 3. location of electrical equipment with respect to the location of the source of hazard These requirements are shown in the diagram in Fig. J-6. The correct application of the diagram is to turn it 90° counterclockwise. Since the flammable material is lighter than air, the air stream of the ventilating system must move upwards to a suction fan in the roof of the indoor location or to a preferred canopy fume hood above the process equipment. When leakage occurs in the piping components only small quantities of gas are expected to be released. If the process area is located in a relatively large room, the recommended ventilation is a canopy type fume hood located above the process equipment. With a canopy fume hood, dilution below one-fourth of LEL is not considered. Since gases with vapor densities of 0.75 or less in the air will rise quickly, suction air in the fume hood needs only to be of sufficient force to accelerate the upward movement of the gas. This requirement is acceptable if the gas at any pressure escapes above the demarcation line. This is also acceptable if the gas escapes at low pressure below the demarcation line. In addition, it is also necessary that all ignitable gas particles in the air are caught by the fume hood. However, the force of suction air may not be sufficient if the gas escapes below the demarcation line at moderate or high pressure.

358 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres As shown in the diagram, a mini source of hazard is located between two electric devices, A and B. Also shown are two air flows: one that is brushing the mini source of hazard, and the other that is not. When a lighter-than-air flammable material is airborne, it will rise, it will not move in the opposite direction. If the gas is forced out by system pressure, the gas escaping from underneath a source of hazard will move downwards before rising. For example, when a leak occurs underneath the source of hazard, the pressure in the system forces the gas down before it can rise. However, if the source of hazard is brushed by air, which is generally an ideal condition, the downward movement of the gas is greatly negated by the rising air, if there is sufficient lift. If the pressure in the system is low and the rising air, under ideal conditions, counteracts the downward movement of the gas, and air first reaches an electric device before reaching the source of hazard, a safety zone of 3 ft below the source of hazard is considered ample. Three feet is considered sufficient to allow the gas at low pressure to move down before it is caught by rising air. Therefore, any electric device, such as device A in the diagram may be of the general-purpose type, if it is sufficiently below the 3 ft boundary. However, if the system pressure is moderate or high, the downward movement of the gas is much greater and, therefore, a longer radius should be applied. The radius should be 5 ft instead of 3 ft. It is anticipated that the demarcation line will not be higher than 5 ft from the finished floor. Also, if ventilating air is not brushing the source of hazard, a radius of 5 ft or up to the finished floor is required. Any electric device or equipment of the heat-producing type located between the 3 ft and 5 ft radius must be suitable for a Div. 2 location. Sufficiently, below 5 ft, the electric device may be of the general-purpose type. Electrical equipment within 3 ft of the source of hazard must be explosion proof (for explosion-proof construction, see Ch. 7, Sec. F). It must be understood that canopy fume hoods generally do not have sufficient capture velocity due to false air entering the fume hood. For example, a canopy fume hood 3 ft above process equipment may produce sufficient suction air and brushing capability, if the suction fan produces 3000 CFM. The same canopy fume hood with 3 side panels may need only 765 CFM. A much larger than 5 ft hazardous area must be applied above the source of hazard if ventilating air reaches the source of hazard first before reaching the electric device. In this case, the boundary above the source of hazard must extend at least 15 ft. Above the source of hazard, the hazardous area is a cone from which the centerline can be easily determined. The width of the cone, however, is not easily determined because it is a function of the quantity of gas being released and the speed at which the gas rises. The greater the speed of the gas, the smaller the width of the cone required. Any electrical equipment in the cone must be suitable for a Div. 2 location. Outside the cone, electrical equipment can be of the generalpurpose type. The division between a 3 or 5 ft danger zone and an opposite zone of a minimum of 15 ft length is called a “demarcation line.” (For an explanation of demarcation line, see Ch. 6., Sec. F.)

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As indicated in the table, the greater the systems pressure, the more conservative the area classification. However, the extent of the hazardous area below the demarcation line is limited by the presence of the floor. When a pressure reducer is applied, a less conservative classification may be used. However, if the pressure reducer shares the same location as the components with the lower pressure, the classification should be based on the higher pressure. If the pressure reducer is not in the same location, the classification should be based on the lower pressure. The classification of the location is in compliance with the following: Table 1-4J

Figure J-7. Brushing and nonbrushing ventilating air.

Item 9

360 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure J-7. Figure J-7 represents two sufficiently ventilated enclosed process areas, (1) and (2), in which lighter-than-air flammable gases are processed. The gases are processed via a one-quarter inch piping system from a single container to a process tank. The gas content in the container is less than 400 cf (cf = cubic feet of gas at 14.7 psia and 70°F). The piping system contains the following mini-type components: a pressure reducer located at the container outdoors, gauges, and other components located indoors. In Fig. J-7, all components are connected to the piping system with low integrity seal connectors (for low integrity seal connectors, refer to Ch. 6, Sec. G). Because of the low integrity seal connectors, the components are considered sources of hazard. Gauges are considered sources of hazard by themselves. Both process areas (1) and (2) are identical except for the locations of the air inlets. In process area (1), the air inlet is located in the right building wall and in building (2) in the left wall. As a result of the different air inlet locations, the sources of hazard in building (1) are brushed by ventilating air and in building (2) they are not. Since the vapor density of the flammable gas is below 0.75, the flammable gas does not necessarily have to be diluted to below ¼ of the LEL. Airborne gas with a vapor density below 0.75 will rise quickly by itself. When a canopy-type fume hood is being used, which is the preferred ventilation method, ventilating air needs to be of sufficient quantity to support the upward flow of the gas, which requires that all escaping gases are caught by the canopy fume hood. Because the sources of hazard in both process areas are of the mini-type and ventilated, each component can be provided with a small danger zone that needs to be classified Div. 2. The radius of the danger zones shall be selected from a table in Fig. J-6. In building (1) the radius under the demarcation line shall be 3 ft in accordance with Item 1, Column (4) for a system pressure that is low. For building (2) the radius shall be 5 ft or up to the finished floor for a moderate pressure. In process area (1), the source of hazard is brushed by ventilating air, and in process area (2), it is not. Ventilation and classification can be eliminated if the mini-components are connected with high-integrity seal connectors to the piping system. Electrical equipment A in building (1) can be of the general-purpose type since it is well below the demarcation line and outside the 3 ft boundary. Electrical equipment A in building (2) is also below the demarcation line but at the boundary of the 5 ft danger zone and therefore must have an enclosure that is suitable for a Div. 2 location. Electrical equipment B in building (1) can be of the generalpurpose type since it is sufficiently away from the hazardous cone. If not, as in building (2), equipment B must be suitable for a Div. 2 location. In building (1), the cone is narrow as a result of air brushing the source of hazard. In building (2), the cone is wide because the source of hazard is not brushed by ventilating air and requires that the electrical equipment be suitable for a Div. 2 location. The electrical equipment C and D in both buildings may be of the general-purpose type since they are far away from the hazardous cones. All electrical equipment shown in Fig. J-7 are of the heat-producing type.

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The classification of the location is in compliance with the following: Table 1-4J

Item 10

Figure J-8. Process equipment producing flammable gases (sufficiently ventilated by fume hood).

Figure J-8. The major requirements for classifying the location as shown in Fig. J-8 are as follows: Items pertaining to the degree of danger: B1 - type of product

=

flammable gas

C1 - system operating mode

=

closed

D1 - location with/without source of hazard =

with

D2 - above or below grade

=

above

D3 - how is location considered

=

indoors

D4 - number of vapor tight walls of location =

4

362 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres D5 - type of location

=

process plant

E1 - type of ventilation

=

fume hood

E5 - safeguards

=

yes

F1 - release of flammable gas/vapor

=

occasional

A2 - size of source of hazard

=

small or large

B5 - flammability class

=

I

B6 - vapor density

=

lighter than air

C5 - system pressure

=

low, moderate, or high

Items pertaining to the extent of danger:

Figure J-8 represents a part of a process plant located indoors, handling lighter-than-air flammable gases. The building as shown in Fig. J-8 may be classified nonhazardous if the source of hazard is provided with a canopy type fume hood. Since the source of hazard is closed, the area between the source of hazard and the canopy type fume hood shall be classified Div. 2. The inlet of the fume hood must be as close as possible to any point where the escape of flammable vapors may be expected. The whole exhaust duct system should operate below atmospheric pressure with the exhaust fan located in the roof. To maintain a nonhazardous environment around the source of hazard, the exhaust fan must be provided with an alarm system that operates when there is a suction fan failure. With a canopy-type fume hood, the lighter-than-air gases released by the source of hazard do not have to be diluted to below 1/4 of the LEL of the flammable gas. A lighter-than-air gas will rise quickly by itself when airborne. Therefore, ventilating air needs only to be of sufficient capacity to support the upward flow of the gas. In addition, the fume hood shall catch all gas particles released into the air. An exhaust opening in the roof is recommended as an additional safety precaution. At the outlet of the suction fan, the degree and extent of the danger zone shall be as follows. For large, closed sources of hazard, the danger zone around the outlet shall be Div. 2 with a 5 ft radius. For small, closed sources of hazard, the danger zone shall have a 3 ft radius. For heavier-than-air flammable material refer to Fig. D-2 in Ch. 22. The classification of the location is in compliance with the following: Table 1-4J

Item 11

Figure J-9. Figure J-9 provides safe clearances between buildings and intake openings when lighter-than-air, Class I flammable gases need to be released to the atmosphere. As shown in Fig. J-9, the gases are produced in a process area on the lower floor of multi-story building (A). Removal of the gases in the process area to the outdoors in the process area is either by a canopy-type fume hood connected to a wall opening or a suction fan in the wall. The flammable gases may

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be vented through the wall if the right wall above the process area is vapor tight. A vapor-tight wall with bolted windows is also considered vapor tight. If the right wall above the process area is not vapor tight or if it is provided with non-bolted windows or openings, the suction fan for the process area must be provided with a vertical riser that extends 7 ft minimum above the roof of building (A). The 7 ft riser must also extend above the roofline of neighboring buildings. Without a riser, the minimum horizontal distance between building (A) and the neighboring building (B) shall be 25 ft, provided the neighboring wall is vapor tight. A smaller clearance than 25 ft may be applied, if the vapor-tight neighboring wall has a minimum fire resistance of two hours. If the neighboring wall is not vapor tight or if provided with an air inlet, as shown in Fig. J-9, the horizontal distance between the suction fan without a stack and the neighboring wall shall be 50 ft. If the vertical distance on the roof top of building (A) is 7 ft minimum, the horizontal distance between riser and pressure fan shall be 50 ft minimum. This horizontal distance may be 25 ft if the riser is extended vertically to a 15 ft minimum. Clearances are in compliance with the following subtable in Table 1-4: Subtable J

Item 12

Fig 1-8

Figure J-9. Safe distances for flammable gas released to the outdoors.

Item J

364 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure J-10. The size of a piping system normally used for small content or hydrogen gas in a small process plant is generally sized 1/4". The components in the piping system, such as pressure reducers, valves, manifolds, and gauges, are normally connected in the piping system by means of seal connectors. These connectors may consist either of low-integrity or high-integrity seal connectors. If the piping system is equipped with low-integrity seal connectors, the system is considered unreliable since it may cause leakage. On the other hand, high-integrity seal connectors are usually considered leak free because of their high-quality construction. Low-integrity seal connectors, therefore, are considered sources of hazard and high-integrity seal connectors are not. (For seal connectors see Ch. 6, Sec. G.)

Figure J-10A. One-quarter inch pipe fittings for H2 gases (must be ventilated).

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Figure J-10B. One-quarter inch pipe fittings for H2 gases (needs no ventilation).

Gauges by themselves are also sources of hazard. They are the weakest component in the system because the pressure-sensing element in the gauge is known to fail. Valves and manifolds of high quality are considered sources of hazard only if they are connected in the piping system with low-integrity seal connectors or if they are systematically opened and closed at short intervals, causing excessive wear. For a pressurized gas system, equipped with low-integrity seal connectors, it is recommended that sufficient ventilation be applied (as shown in Fig. J-10A) in accordance with the requirements of Fig. J-6. Classification of the location shall also be in compliance with Fig. J-6. Ventilation and classification of the location may be omitted if the piping system is provided only with high-integrity seal connectors welded into the piping system, and gauges in the piping system are preferably omitted (as shown in Fig. J-10B).

366 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Hydrogen gas containers and pressure reducers are normally not provided with high-integrity seal connectors. Therefore, they should be located outdoors unless sufficient ventilation is applied when they are located indoors. Indoors, the piping system and components are considered leak free as long as they are provided with high-integrity seal connectors, gauges are preferably omitted, and the system is thoroughly leak tested. The classification and extent of hazard for the location are in compliance with the following: Table 1-4J

Item 13

Figure J-11. Safe distances for hydrogen gas outdoors.

Figure J-11. Figure J-11 covers safe clearances for air vents in connection with the possible presence of hydrogen gas in the air. Also covered in Fig. J-11 are safe clearances for public assemblies, cars, and welding and cutting areas. Safe horizontal distances between containers filled with hydrogen gas and electric driven suction fans should not be less than 50 ft and 25 ft for welding and cutting areas, and 15 ft for electrical arcing devices. As shown in Fig. J-11, there are three exhaust systems, #1, #2, and #3. A distance of 50 ft between air inlet #1 and the hydrogen gas containers must be considered unsafe because a hydrogen gas leak

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367

will be easily sucked into the building. However, if vent #3 is an air outlet, it is also unsafe during a gas leak if the vent driver breaks down. Air outlet #3 is safe only if it is provided with a type “A” safeguard. Clearances are in compliance with the following subtable of Table 1-4: Subtable J

Item 14

Figure J-12. Fume hood enclosure (sufficiently ventilated).

Figure J-12. Figure J-12 represents a laboratory-type fume hood that normally is used for testing of small process systems. Inside the fume hood enclosure, the process system may purposely or accidentally release flammable gases or vapor. By itself, this does not cause a dangerous condition. Dangers will exist if the release of flammable gases or vapors will occur during operation of

368 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres general-purpose electrical equipment in the fume hood enclosure. For example, switches, circuit breakers, receptacles, heaters, lighting fixtures, etc., may be used inside the fume hood enclosure. Operation of this type of electrical equipment will produce arcs, sparks, and sufficient heat to make the working environment in the fume hood enclosure unsafe. The working environment in a fume hood enclosure is considered unsafe if the electrical devices are of the general-purpose type and are being used in conjunction with flammable materials. For whatever reason, if electrical equipment must be used inside the fume hood enclosure, the use of general-purpose electrical equipment should be avoided because, as explained above, its operation in conjunction with flammable material is not safe. General purpose electrical equipment produces not only arcs, sparks, and sufficient heat under normal operating conditions, but also under fault conditions. That makes them more dangerous. The working space in the fume hood enclosure becomes even more dangerous if operation of the electrical equipment with simultaneous release of flammable materials will occur at a time that there is an outage of ventilation. It is not uncommon that the flow of ventilating air stops due to a malfunction of the ventilating system or because of a power failure. Laboratory fume hoods are normally not provided with safeguards to warn against ventilation outage. Therefore, if electrical equipment inside the enclosure is required, explosion-proof type or equipment suitable for a Div. 2 environment is recommended. In such cases, where electrical equipment must share the workspace with flammable materials, the fume hood enclosure should be classified in accordance with Fig. 1-10C, in Part 1. Laboratory fume hoods have been improved over the years. Most of them are provided with electrical equipment mounted externally on side panels of the fume hood enclosure (see Fig. J-12). In this case, the electrical equipment does not have to be other than the general-purpose type. Although this equipment does produce arcs, sparks, and sufficient heat under normal and abnormal operating conditions, the equipment is not considered an explosion hazard during its operation. The reason is ventilating air entering the fume hood enclosure will first reach the externally located electrical equipment and then the source of hazard inside the enclosure. As a result of this airflow, ventilating air forces flammable gases or vapors that may be released inside the fume hood enclosure away from the electrical equipment. This then eliminates the possibility of an explosion. For example, see Fig. 1-10B, in Part 1 (Items C and D). The location of lighting fixtures inside the working space has been improved on current models also. In current models, lighting fixtures are mounted above the working space. A translucent barrier between the fixtures and the working space prevents gases or vapors from entering the lighting compartment, thereby eliminating possible ignition of flammable materials in the working space. Figure J-12 is in compliance with the requirements in Ch. 3, Sec. K.3.

Chapter 29 General Requirements for Group K

Group K represents closed sources of hazard that contain Class I flammable products with heavier-than-air gases or vapors located in sufficiently or insufficiently ventilated locations which are classified in accordance with the information in Part 1. Figure K-1. Figure K-1 represents components in a piping system for heavier-than-air flammable material operating at low, moderate, or high pressures located indoors which are either sufficiently or insufficiently ventilated. The classification and extent of the danger zone for the components in Fig. K-1 are functions of: 1. size of the components 2. their operating modes 3. the pressure in the piping system 4. whether the location is sufficiently ventilated or not For classification requirements for the components refer to the table in Fig. K-1. In this table, there are two classifications shown: a location entirely classified and a location partially classified. Some of the partially classified locations are provided with an asterisk. This means that the location must be entirely classified if the piping system with components occupy more than 50% of the floor space. If 369

370 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres they do occupy more than 50% of the floor space, the components are considered to occupy 100% of the floor space. For example, the location for mini sources of hazard in Item 3 of Column (1) in Fig. K-1 is marked with an asterisk. When these sources of hazard occupy more than 50% of the floor space, the location is considered 100% occupied and the asterisk indicates that it is required that the entire location be classified.

Figure K-1. Piping system with screwed fittings, flanges, valves, etc., for indoor locations containing flammable liquid.

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As indicated in the table, the greater the system pressure, the more conservative the area classification. When a pressure-reducer is applied, a less conservative classification may be used. However, if the pressure-reducer shares the same location as the components with the lower pressure, the classification should be based on the higher pressure. If the pressure-reducer is not in the same location, the classification should be based on the lower pressure. If the flammable product is volatile or very flammable, a more conservative classification is required. To establish whether these locations require a more conservative classification, it is necessary that a severity factor of 1.0 Pu be added to the probability factor that must first be established for the location. For example, if the sources of hazard in Item 4, Column (1) occupy 100% floor space, the probability factor for that location is 9 Pu. The addition of a 1.0 Pu severity factor for very flammable material makes the probability factor 10 Pu. This 10 Pu factor requires that the location be entirely classified Div. 2. As another example, the probability factor for Item 5, Column (2) is 6 Pu when the location is unattended and less than 50% floor space is occupied. The addition of a 1.0 Pu severity factor for a very flammable product brings the probability factor to 7 Pu. In this case, where the probability factor is less than 10, the location need not be entirely classified. The classification in Item 5, Col. (2), complies with Table 1-3, Item 11, in Ch. 2, if location is attended. However, the boundary recommended for the sources of hazard in Item 5, Column (2) is required to be of a larger size when the flammable product is volatile or very flammable. For standard boundary sizes of 3, 5, 10, and 15 ft, increase the size to 5, 10, 15, and 20 ft respectively. For volatile and very flammable products refer to the tables in the Appendix. Add only a 1.0 Pu severity factor if the flammable product being used in the location is very volatile or very flammable (in the appendix tables, it is marked with a “4”). For information on how to determine a probability factor, refer to Ch. 3, Sec. J. If the location needs to be sufficiently ventilated, it is necessary to determine the suction fan location. Refer to Ch. 6, Sec. D, for approximate locations of suction fans. Some of the locations in Fig. K-1 do not require classification if the location is: 1. sufficiently ventilated 2. the process equipment well maintained 3. the probability factor is 5 Pu or less Locations which comply with these three conditions are marked in the table with “NC” which means “non-classified.” For example, the location in Item 4, Column (1) needs not be classified if: 1. the sources of hazard do not occupy more than 50% of the floor space 2. the location is sufficiently ventilated 3. the sources of hazard are well maintained 4. the probability factor for the above conditions is not more than 5 Pu

372 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres There is a distinct difference between a non-classified location and a nonhazardous location. A non-classified location is a location with one or more sources of hazard but which is considered free from explosion danger because of tight inspections and maintenance procedures, and sufficient ventilation. A nonhazardous location is a location free from explosion danger because it is provided with sufficient ventilation and suitable safeguards and generally without a source of hazard. Although there is a distinct difference between both locations, a location that needs no classification is, in fact, a nonhazardous location. Therefore, a nonclassified location can also be considered a nonhazardous location. “Well maintained” is defined as a maintenance procedure in which the probability of leakage or failure of a component in a piping system is almost nonexistent. This is normally accomplished by frequent inspections. Lack of frequent inspections requires that the location be classified. Column (1) and Column (2) of Item 5 in the table in Fig. K-1 is marked with a ≠. This means that the classification shown in Column (1) and Column (2) of Item 5 is valid only as long as the probability factor for the location does not exceed 5 Pu. If the probability factor is higher than 5 Pu, then the classification for the location in Item 5 shall be as in Item 3 of Column (1) for mini sources of hazard, and Item 3 of Column (2) for small sources of hazard. To maintain a probability factor of 5 Pu, the piping system cannot carry Class IA liquid and also the location must be attended. The classification of the location is in compliance with the following: Table 1-4K

Item 1

Figure K-2. Piping system with screwed fittings, flanges, valves, etc., for outdoor locations containing flammable liquid.

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373

Figure K-2. Figure K-2 represents a piping system operating at low, moderate, or high pressure located outdoors. The classification and the extent of classification for the piping system is a function of: 1. the size of the source of hazard 2. the operating mode of the source of hazard 3. the vapor density of the flammable product 4. pressure in the system The classification of the location is in compliance with the following: Table 1-4K

Item 2

Figure K-3. Pumphouses with small pumps (60–201 hp) handling Class I flammable liquids at high pressure (sufficiently and insufficiently ventilated location).

374 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Figure K-3. Figure K-3 represents two pump houses: station 1 and station 2. Station 1 has one large area, station 2 has its large area divided into two smaller areas by a vapor-tight wall separating the pumps from their electric drivers. The sizes of the pumps are small and their electric drivers are rated between 60–201 hp. All pumps are processing flammable liquid at high pressure. Their vapors are heavier than air. In both pump stations, the pumps do not occupy more than 50% floor space. Only pump station 1 is provided with an exhaust fan. Pump station 1 shall be classified as follows: Floor space occupied

=

50%

=

2 Pu

System pressure

=

High

=

3 Pu

Quantity of release

=

large

=

3 Pu

The probability factor for pump station 1 is as follows: 2 × 3 + 3 = 9 Pu. Since the probability factor is below 10 Pu, pump station 1 does not need to be entirely classified. Refer to Item I in Fig. 1-9 for this type of classification. The degree of hazard can be obtained from Table 1-3, Item 8. This table indicates that the hazardous area should be classified Div. 2. The extent of the hazardous area can be obtained from Fig. 1-3, Item H, in Part 1, that indicates that its horizontal distance needs to be 50 ft. The height of the 50 ft danger zone can be obtained from the table in Ch. 3, Sec. B, which for 60–201 hp electric drivers located indoors is 2 ft. However, only on the right side of pump station 1 does the danger zone extend beyond the building opening by 25 ft. This 25 ft danger zone needs also to be 2 ft high. According to Fig. 1-8, Item C, in Part 1, an additional danger zone 10 ft wide must also be applied beyond both access openings of pump station 1. Pump station 2 needs no classification. The left room of pump station 2 needs no classification because there is no source of hazard that can produce flammable material. The right room also needs no classification because there is no electrical equipment. The electric driver in pump station 2 is separated from the source of hazard by a vapor-tight wall. Classification is only required if a source of hazard shares its location with electrical equipment. Since this is not the case in either room of pump station 2, no classification is necessary. Ventilation in both rooms is also not required. However, if for any reason the pump room cannot be sealed off from the driver’s room, a pressure fan should be installed in the driver’s room to prevent flammable vapors from seeping through the shaft packing glands. Pump station 1 has a lighting fixture outdoors above each access opening. Since both fixtures are located above a danger zone, it is necessary that the fixtures be of the totally enclosed type. Pumpstation 2 also has outdoor lighting fixtures above its access openings. The question is whether the right outdoor fixture also needs to be of the totally enclosed type. This depends on whether the fixture is located above a danger zone. For pump station 1, this is a clear-cut case. Both lighting fixtures need to be of the totally enclosed type because they are located above a hazardous area. For pump station 2, it appears that the fixture at the right access opening does not need to be the totally enclosed type.

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Although the pump room of pump station 2 does not need to be classified because there is no electrical equipment present, the pump room contains pumps which are capable of releasing flammable materials during failure, breakdown, or malfunction of the pump. These flammable materials may, according to Fig. 1-3, Item H, cover a horizontal area of 50 ft before reaching a nonhazardous concentration. Because of this possibility, 25 ft of the outdoor area beyond the access opening of the pump room is considered hazardous. This area needs to be classified Div. 2 as shown in Fig. K-3B. In this case, classification of the outdoor area is justified because of the presence of electrical equipment. The lighting fixture located above the right access opening of the pump room, therefore, is considered to be above a hazardous area and needs to be the totally enclosed type. An additional danger zone 10 ft wide is also required at the pump room of pump station 2. The classification and extent of hazard for both pump stations are in compliance with: Figure 1-3 1-8

Item H C

Table 1-4

Subtable K

Item 3

Part 3

Examples

377

Chapter 30 Application Procedure for Classifying NEC Class I Locations

A.

GENERAL

The intent of the classification of a hazardous location is to provide safety for personnel and equipment. The intent also is to achieve an economical electrical installation which will provide an acceptable level of safety for personnel and equipment at the lowest possible cost. To accomplish this goal, it is necessary to analyze, in detail, the environmental conditions of the location and the characteristics of the source of hazard. Not only must the presence of hazard be considered, but also the size and type of the source of hazard, flammability class, operating conditions, and its location with respect to the area that must be classified. The engineer who is involved in preparing the area classification must understand all of the details which will have an impact on his decision to classify the area Div. 1, Div. 2, or nonhazardous. Without the knowledge of the environmental conditions and the characteristics of the source of hazard, the location will most certainly be given a safety level too low or much too high, which is not economically justified. It is this approach which must be avoided. Therefore, it is recommended that anyone involved in an area classification approach it scientifically in accordance

379

380 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres with the requirements as highlighted in Part 1. The engineer should read Part 1 entirely, or, at least in part, before applying this chapter. In nine out of ten cases, a hazardous location is classified much too conservatively. The reasons for this conservative approach are, generally, a lack of knowledge and a misunderstanding of the actual concept of safety and danger. In the majority of cases, hazardous areas are classified Div. 1 when the location could have been classified Div. 2, and areas which are classified Div. 2 could have been classified nonhazardous. It must be kept in mind that when a location is classified Div. 1, explosion-proof electrical equipment is required. This explosion-proof electrical equipment will range in price from two to four times the cost of generalpurpose electrical equipment (some of which are allowed in Div. 2 locations). Therefore, it is important to strive for a classification of a lower, but acceptable, level of safety which is commensurate with an acceptable risk, and reduces the cost of the electrical installation. Consequently, it can be assumed that, from an economical standpoint, it is well worth the time and effort to approach the classification scientifically so that the areas under consideration can be classified at a lower, but acceptable, level of safety. When involved in area classification, first determine whether classification is necessary. Process equipment which contains a flammable gas or flammable liquid is not always hazardous. The hazard depends entirely on whether the ambient or process temperature is, will, or can rise above the flash point of the flammable product. If the temperature will remain below the flash point, the location cannot be hazardous and classification is not required. Classification is also not necessary when there is no electrical installation. Classification of a location is only required if the product is flammable and the process or ambient temperature of the flammable product is equal to or above flash point, and if the hazardous location is provided with electrical equipment. Even though the temperature of the flammable product is equal to or above flash point, the location does not need to be classified if there is no electrical equipment in the location. These conditions must be analyzed first before proceeding with the classification of a location.

B.

STEPS TO BE FOLLOWED FOR CLASSIFYING A HAZARDOUS LOCATION

The initial step in classifying a hazardous location is to mark and number on a plot plan drawing all of the areas which are considered hazardous. The next step is to fill out a report which is to accompany the plot plan drawing as a complete package. The report includes two forms, “A” and “B,” which must be used for each individual area that is considered hazardous. Sample blanks of this report and these forms are shown herein and consist of a face sheet, an introduction sheet, a definitions sheet for Form “A,” and Forms “A” and “B.” Characteristics and

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operating conditions of the flammable product and the environmental conditions of the areas are to be shown on Form “A.” The degree and extent of danger are to be shown on Form “B.” For example, if the entire hazardous location should consist of three hazardous locations, each location on the plot plan is to be numbered in sequential order (1, 2, 3, etc.). Each numbered area will be assigned a Form “A” and a Form “B.” It is important to mention that when Form “A” is being filled out, it is vital that assistance of the process engineer is obtained. Only the process engineer is capable of providing the correct characteristics and operating conditions for the flammable product in question. When Form “A” has been filled out for each individual area, the degree and extent of the danger must be outlined on Form “B” for each individual hazardous location. After having filled out Form “A,” the degree and extent of hazard must be established. The degree of danger is obtained from Tables 1-3, 1-4, and 1-5. The extent of danger is obtained from Tables 1-4 and 1-5. When the degree and extent of hazard are established for each hazardous location, the final step is to establish the proper electrical equipment and wiring for the hazardous locations in compliance with the requirements of Article 500 of the NE Code. The determination of the degree and extent of hazard and how to apply Forms “A” and “B” are shown in the following examples.

C.

EXAMPLES

There are a total of four examples which range from simple to complex situations. Each example is discussed in detail and will clearly illustrate how to establish the degree and the extent of danger for a particular hazardous location. The degree and the extent of danger as listed in Form “A” are grouped separately as follows: Items pertaining to the degree of danger: B1 - type of product B2 - flash point C1 - system operating mode C3 - ambient temperature C4 - process temperature D1 - location with/without source of hazard D2 - above or below grade D3 - How location is considered D4 - number of vapor tight walls of location D5 - type of location E1 - type of ventilation

382 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres E2 - velocity of natural ventilation E3 - amount of ventilation E4 - percent obstruction F1 - release of flammable gas/vapor Items pertaining to the extent of danger: A1 - type of source of hazard A2 - size of source of hazard B5 - flammability class B6 - vapor density C5 - system pressure D6 - floor space occupied F5 - pump driver and size F6 - probability factor Before starting with the actual evaluation of a particular area, first determine whether there is a hazardous condition by comparing the ambient temperature (C3) or the process temperature (C4) with the flash point (B2). If the ambient or process temperature is higher than the flash point temperature, there is a hazardous condition; if not, there is no hazardous condition. Next, consider all information in Form “A” which is related to the degree of hazard. A brief rundown of some of the items listed will be discussed. Start with the system operating mode (C1). If it reads “open,” the location or part of the location needs to be classified Div. 1 regardless of the presence of sufficient ventilation in E1, E2, and E3. If it is “closed,” it is not certain whether the location should be classified Div. 1 or Div. 2. To determine this, additional information must be evaluated. If the system operating mode is “closed,” it means that the release of flammable gases or vapors to the atmosphere can only be “occasional,” which could mean that the area is remotely dangerous, and, therefore, could be classified Div. 2. A Div. 2 classification is valid if the items in “E” indicate that there is sufficient ventilation. A Div. 2 classification is also valid if Item F1 reads “occasional.” If Item D3 should read indoors, then Item E1 should not read natural ventilation, especially when Item D4 reads 4. A location with 4, 3, or even 2 walls in an L-shape is an indoor location which produces sufficient obstruction against the passage of natural ventilation. Therefore, if Item E1 should read natural ventilation, and Item D4 reads 4, the location cannot be classified Div. 2, but must be classified Div. 1. This is true for small and large sources of hazard. This may not be true for mini sources of hazard. Therefore, it is necessary to determine from Item A2 the size of the source of hazard. A 4-wall indoor location can be classified Div. 2 if it is sufficiently ventilated in accordance with Item E3. If the indoor location is sufficiently ventilated, Item E1 should read mechanical ventilation.

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Report for compiling characteristics of flammable product, its operation, and environmental conditions.

384 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

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385

386 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

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388 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres To determine the extent of the danger area, it is necessary to evaluate the remaining information in Form “A.” If Item B6 reads heavier than air, the flammable gases or vapors, once they are released to the atmosphere, could cover large floor areas. Certainty about large floor areas must be obtained from Item B5. Heavier-than-air, Class I, flammable gases or vapors will generally cover large areas. The quantity of flammable gases or vapors released to the atmosphere will also determine whether a large area will be covered by the flammable gases or vapors. Quantities of flammable gases or vapors are related to Item A2 (the size of the source of hazard). In turn, this size will influence the size of the danger area. To obtain certainty in whether the danger area should be small or large, establish the probability of breakdown of the source of hazard. This is found by reading Item C5, the system pressure. The system pressure is either low, moderate, or high. If Item A1 reads “dynamic,” the chances of an accident are much greater than if Item A1 reads “static.” If the probability of an accident from static type sources of hazard is qualified as “low,” then the probability of an accident from dynamic-type sources of hazard under the same operating conditions is qualified as moderate because of the greater wear and tear in dynamic-type sources of hazard. If C5 reads high, the probability of an accident is much greater. This greater probability will influence the size of the hazardous area. The size of the hazardous area is established on the basis of how much area will be covered by a flammable gas or vapor during its escape from its containment before reaching safe concentrations. The answer to this is obtained from Items A2, B5, B6, and C5 or F6. Item F6 is the probability factor and includes the size of the source of hazard, the pressure in the system, the quantity of release of the flammable material, the degree of hazard, the percent floor space occupied, and whether the hazardous location is attended or not. Item F6 will generally provide information as to whether the hazardous location should be classified entirely or partially (100 or 50%). The brief explanation above is only to indicate how the classification of a particular hazardous area should be performed. There are, of course, a number of other item combinations which allow the location to be classified Div. 1, Div. 2, or nonhazardous.

Example 1. Pump Station Assume that the degree and extent of danger must be determined for a pump station with electric motor-driven pumps. Before classifying the location, first establish whether the location is really hazardous. Answers given to the questions listed below will determine this. B1 - What is the type of product?

=

Naphtha, petroleum

B2 - What is the flash point?

=

–18°C

C4 - What is the process temperature?

=

38°C

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Since the flammable product has a process temperature above its flash point as indicated by Items B2 and C4, the location is hazardous. As explained before, classification of the hazardous location is only required if the location is provided with an electrical installation. Since the pumps are driven by electric drivers, the hazardous location is required to be classified. The next step is to determine the degree of danger of the hazardous location which is established by giving answers to the following questions, for example: C1 - What is the system operating mode?

=

closed

D3 - How is the location considered?

=

outdoors

D4 - Number of vapor tight walls?

=

none

D5 - Type of location?

=

open pump station

E1 - Type of ventilation?

=

natural

E2 - Velocity of natural ventilation?

=

4 mi./hr

E3 - Amount of ventilation?

=

sufficient

F1 - How flammable gas/vapor is released? =

accidental

From answering the above questions, the following can be concluded. Since the source of hazard has a closed system as indicated by Items C1 and F1, only an occasional release of flammable vapors can be expected. Because the pump station has no walls (Items D4 and D5), natural ventilation is capable of sufficiently diluting an airborne vapor. Natural ventilation which moves with a speed of 4 mph is considered sufficient to prevent accumulation of flammable vapors in the pump station. In view of Items C1, D3, D4, D5, E1, E3, and F1, it is possible to determine the proper classification of the pump station. Refer to Table 1-3 in Part 1 and compare the above items with the conditions listed in Table 1-3. Determine the correct classification by finding the proper illustration. Only Item 2 of Table 1-3 will apply. Item 2 says that the pump station must be classified Div. 2. The next step is to determine the extent of the danger zone. This is established by answering the following questions: A1 - What is the type of the source of hazard?

=

dynamic

A2 - What is the size of the source of hazard?

=

large

B5 - What is the flammability class?

=

Class I

B6 - What is the vapor density?

=

2.5

C5 - What is the system pressure?

=

moderate

F5 - What is the size of the pump drivers?

=

250 hp

The extent of the danger area for heavier-than-air vapors can generally be divided into three broad horizontal distances: 10 ft, 25 ft, and 50 ft, where the 10 and 25 ft distances normally would be applied for small pumps and the 50 ft for large pumps. Since the pumps are large (above 201 hp, Item F5), the vapor density of the flammable product is greater than 0.75 (Item B6), and the flammability class

390 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres of the flammable product is Class I as indicated by Item B5, the selection of a 50 ft distance appears to be correct. The 50 ft boundary is supported by the fact that large quantities of flammable vapors can be released to the atmosphere during a failure because of 1) the flammability class (Item B5), and 2) the vapor density (Item B6), 3) the type of the source of hazard (Items A1), and 4) its size (Item F5). Therefore, the 50 ft horizontal distance is in compliance with Fig. 1-2D and Table 1-4, Item 3, subtable C, Item 7, and Fig. 3-1.

Figure 3-1. Pump station outdoors.

Example 2. Holding Basin The classification of an open holding basin is required for a petrochemical plant in which small amounts of Class I flammable liquid (slop oil) will be ditched. The flammable product will have the following properties and operating conditions which are marked with the same item numbers as listed in Form “A:” A1 -

type of the source of hazard

=

holding basin

A2 -

size of the source of hazard

=

large

B1 -

type of product

=

slop oil

Chapter 30: Application Procedures B2 -

flash point

=

–7 to 30°C

B5 -

flammability class

=

Class I

B6 -

vapor density

=

>0.75

B7 -

NEC Class I group

=

D

C1 -

system operating mode

=

open

C3 -

ambient temperature

=

35°C

D1 -

location with/without source of hazard

=

with

D2 -

above or below grade

=

below

D3 -

location is considered

=

outdoors

E1 -

type of ventilation

=

natural

E2 -

velocity of natural ventilation

=

moderate

E3 -

amount of ventilation

=

sufficient

E4 -

percent of obstruction

=

none

391

Illumination and receptacles are installed around the holding basin. In this example, the classification of the location requires a more in-depth analysis. First, determine whether there is a hazardous condition by comparing Item B2 with Item C3. Since the ambient temperature is higher than the flash point of the flammable product, there is a hazardous condition. Next, consider the degree of danger in the holding basin by reviewing Items B6, C1, and D2. Since the holding basin is open, a continuous production of flammable gases and vapors is released to the atmosphere which requires that the holding basin be classified Div. 1. Because of the continuous release of flammable gases and vapors, the flammable material will also travel beyond the perimeter of the holding basin. The basin is a large source of hazard, therefore, it is concluded that large quantities of flammable gases and vapors are released (although natural ventilation is present as indicated by Items E1, E2, and E3). Natural ventilation may not be capable of sufficiently diluting and dispersing the flammable gases and vapors at the basin and beyond the perimeter of the holding basin. If that should be the case, the area beyond the holding basin must also be classified Div. 1. The next step is to determine the extent of the Div. 1 area. The key items for determining the extent of the danger area are Items A2, B5, and B6. These items indicate that the size of the source of hazard is large, the flammability class of the flammable product is Class I, and the vapor density is heavier than air. All of these items lead to the conclusion that the extent of the danger area beyond the holding basin should be at least 50 ft. Fortunately, this is not so, because one very important factor has been overlooked and that is “water.” Slop oil contains a great deal of water, both drain water and rain water. In spite of the fact that the holding basin is open and large, it does not represent real danger. The reason for this is that the actual flammable liquid concentration per square foot of basin area is very small. Therefore, the

392 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres quantity of flammable gases and vapors given off is also very small. Taking into account the fact that natural ventilation is capable of sufficiently dispersing and diluting small quantities of flammable vapors, it must be concluded that the flammable gases and vapors in the basin will rapidly reach concentrations below the LEL. In view of these conditions, neither the holding basin nor the area outside the holding basin have to be classified Div. 1. They both can be safely classified Div. 2 and the extent of the danger area beyond the holding basin shall be only 15 ft. Item A2, therefore, should read “small” instead of “large.” This puts the size of the danger zone in a bracket which is smaller than 50 ft (refer to Table 1-4, subtable F, Item 3).

Example 3. Crude Oil-Fired Power Plant The area to be classified in this example is a crude oil-fired power plant consisting of a tank farm, pump house, and two boilers. Pipelines run from the tank farm via the pump house to Boilers 1 and 2 (see Fig. 3-2). The first step in evaluating the power plant is to divide the area into three locations and number them as follows: Tank farm Fuel oil pump house Boilers 1 and 2

= = =

Area 1 Area 2 Area 3

Areas 1, 2, and 3 have the following operating and environmental conditions: Area 1. The tanks in Area 1 will store petroleum crude. Each tank will have a floating roof. Both tanks are separated from each other by a dike that also extends around each tank. The tank farm, located in a freely ventilated location is considered sufficiently ventilated by natural ventilation. Area 2. The fuel oil pump house in Area 2 is divided into two locations by means of a solid fireproof wall. The larger area of the two contains fuel-forwarding pumps driven by electric motors rated 300 hp each. The smaller area is a heater house for the crude oil and is equipped with motor-operated valves and controls. Both areas have open, opposite sides. At the north side of the pump house building, there is a switch house containing switch-gear for the fuel forwarding pumps and MOV’s. The wall between the switch house and the pump house is also fireproof. Area 3. The boiler area in Area 3 is completely enclosed and ventilated by a pressure fan. The air input is slightly higher than the air output. The oil burners with the remaining part of the firing system are completely enclosed. The flammable substance used for the oil-firing system is as follows: Product Properties Substance

=

Petroleum crude

API gravity

=

0.83–0.89 at 60°F

Chapter 30: Application Procedures Vapor density

=

>0.75

Explosive limits

=

2.2–9.5% by volume

Flash point

=

20–90°F

Ignition temperature

=

897°F

Process temperature

=

Above flash point

Ambient temperature

=

15–90°F

393

All of the applicable information must be incorporated in one report that must be given a number, for example, #71591.

Figure 3-2. Pipelines run from the tank farm via the pump house to Boilers 1 and 2.

394 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Characteristics of flammable product, its operation, and environmental conditions shown for crudefired power plant.

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Classification of Area 1 As explained before, it is best to divide the data in Form “A” into two groups, one for establishing the degree of danger, the other for establishing the extent of danger. However, for tank farms, the division of data is not necessary because of the simplicity of the case. The classification of a tank farm is generally straight-forward so that a detailed analysis is not required. The degree and extent of danger is normally obtained from Table 1-4, Item 3, subtable C, Item 8. Since the storage tank has a floating roof, the entire top of the storage tank must be classified Div. 1 which must extend upwards 10 ft. The sides of the tank shall be classified Div. 2 which must extend 10 ft horizontally. A Div. 2 area shall also extend horizontally from the tank shell up to the dikes. With the degree and extent of danger known, the classification of Area 1 is completed and can now be incorporated in Form “B” of Report #71591 as shown in Fig. 3-3.

Classification of Area 2 The classification of Area 2 requires a more in-depth analysis. Therefore, it is recommended to use Form “A” in Report #71591 for all applicable data and divide the data for the pump station into two separate groups. One group consists of items that determine the degree of danger (Div. 1 or Div. 2), and the other group consists of items that determine the extent of the danger. Items in Form “A” which are involved in the degree of danger are marked with an “X,” and items in Form “A,” which are related to the extent of danger, are marked with a dot. First, start with Item C1 in Form “A” to establish the degree of danger in the pump station. According to Item C1, the pump station should be classified Div. 2 because all sources of hazard have a closed operating mode. This conclusion is supported by Item F1 which says that flammable vapors can only be released occasionally. Since the pump station has three walls as indicated by Item D4, the opening of the pump station is exposed to natural ventilation outdoors. This situation also seems to favor a Div. 2 classification. However, if Items D3, E3, and E4 are taken into account, the pump station cannot be considered sufficiently ventilated by natural ventilation. As explained in Part 1, a 3-wall location will produce an obstruction of 75% to the free passage of natural ventilation. Such a location, therefore, cannot be considered sufficiently ventilated by natural ventilation. In view of this, Items E3 and E4 will prevail, and, as a result, the pump station cannot be classified Div. 2 (because it is lacking sufficient ventilation). The pump station can only be classified Div. 1 in compliance with Item 7 in Table 1-3.

396 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 3-3. Tank farm.

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The extent of the Div. 1 area is established by how much floor space an ignitable vapor will cover if it is released to the atmosphere. The size of the floor area can be determined by considering Items A2, B5, and B6 in Form “A.” For Area 2, each of these items indicate that the flammable vapors will cover large floor areas. Item A2 indicates this because of the large size, B5 because of the flammability class, and B6 because of the heavier-than-air flammable vapors. The extent of the Div. 1 area can be found in Table 1-4. But first, Items C1, D3, E3, and B6 in Form “A” must match one of the requirements in Table 1-4. Only Item 2 in Table 1-4 is a perfect match. Item 2 refers to subtable B. It is this table that should be used for selecting the actual size of the required hazardous area. However, subtable B lists five pump stations each with a different boundary size. The question now is which of the five boundary sizes must be applied for this example. To find the correct answer, it is necessary to consider the system pressure and the size of the pumps. Refer to Form “A” of Report #71591 and read the system pressure from Item C5 and the size of the pumps from Item F5. Use this information in subtable B to select the correct hazardous area size which is Item 5. According to column (8) in subtable B, an additional danger zone of 10 ft wide is also required (as shown in Fig. 3-4).

Figure 3-4. Forwarding pumphouse.

398 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres The heater house with three walls containing motor-operated valves also needs to be classified. Because the heater house contains small sources of hazard, which occupy 75% of the floor space, a small Div. 1 zone is required around these small sources of hazard. Beyond the Div. 1 area, a large Div. 2 area is required. The reason for the Div.1 area and the larger Div. 2 area is lack of sufficient ventilation. The classification of the heater house can also be established by using the data in Table 1-6 and Fig. 1-9 as follows. Motor-operated valves occupying 75% of floor space require a 3 Pu severity factor. The moderate pressure calls for a 2 Pu severity factor and the release of flammable material is 2 Pu. Since the heater house is not ventilated and not attended, another 2 Pu should be added. The probability factor, therefore, is: 3 × 2 + 2 + 2 = 10 Pu which requires a 100% classification. As a result of this classification, all electrical equipment located in the Div. 1 area must be explosion proof. Material cost of large explosion-proof pump drivers will be excessively high. These pump drivers should either have an explosion-proof enclosure or be totally enclosed and provided with a positive pressure ventilation, or filled with inert gas. In this example, the electrical installation is far from economical and safety for personnel is unnecessarily high, and therefore, violates the principles of an economical classification. As explained in the introduction, the whole purpose of a classification is to find a point of balance between cost and safety in which both are satisfied, and which provides an economical electrical installation commensurate with an acceptable level of risk. To achieve such a balance, a simple modification of the pump station and the heater house is required. Both locations must be provided with a fourth wall and louvers. In addition, each location must be provided with an electrically-operated suction fan which provides sufficient ventilation. This allows both locations to be classified Div. 2, and it allows the locations to have non-explosion-proof electrical equipment. The suction fan must be located at the most effective place where ventilating air must move from the louvers in one side of the location to the other side. Crude oil vapors are normally heavier than air. Therefore, the suction fan must be located in the wall 12" from the floor or on the roof with a duct system with an inlet near the floor. For exact location of suction fans, refer to Part 1, Ch. 6, Sec. C-2. Since now both locations have four walls, the additional danger zones are not necessary. This new concept of building arrangement requires that Form “A” of Report #71591 be revised to reflect the lower desired classification. To obtain the Div. 2 classification, the following items must be changed: D4 -

number of vertical walls

Div. 1 3

Div. 2 4

E1 -

type of ventilation

natural

exhaust fan

E3 -

amount of ventilation

insufficient

sufficient

E4 -

percent obstruction

75%

none

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400 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 3-5. Forwarding pump house with economically justified installation which is commensurate with an acceptable level of risk.

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With sufficient mechanical ventilation and no obstruction, the pump house and heater house can safely be classified Div. 2. Form “B” should now be revised accordingly as shown in Fig. 3-5. (See Report # 71591, Rev. 1.) Bear in mind that the location of the suction fan shown in Fig. 3-5 is shown for simplicity and is not the actual location of the fan. The probability factor for the heater house will change also. The 2.0 Pu severity factor which initially was required for a nonventilated location which is not attended will be deleted. Because of this, the probability factor changes from 10 Pu to 8 Pu.

Classification of Area 3 The operating platforms for the furnaces in the boiler areas are, in fact, the only hazardous locations. The sources of hazard at these platforms normally consist of individual components in the fuel line such as valves, flanges, screwed connections, fittings, controls, etc. Normally, these components are small and operate at high temperatures and high pressures. Because of the elevated temperatures and pressures, frequent leakage can be expected from these components. Both boilers and furnaces with the associated platforms are completely enclosed and pressure ventilated. This pressure ventilation is designed to produce an over-pressure of less than 1%. This less than 1% over-pressure is considered insufficient ventilation for the operating platforms. Pressure ventilation is generally not recommended for removing, diluting, and dispersing flammable gas or vapor concentrations. Since ventilation for the platforms is considered insufficient and since the flammable vapors at the platforms are expected to be frequently present, the platform must be classified Div. 1 as shown in Fig. 3-6. The extent of the Div. 1 area is a function of the pressure in the system and the lack of sufficient ventilation. It is also a function of the size of the sources of hazard; how much space the sources of hazard will occupy, and whether the location is attended or not. An evaluation and combination of each of the above items will determine how probable is the explosion danger in the hazardous location. The probability of explosion, in turn, will determine whether the location must be partially or entirely classified. A probability factor of 10 Pu or more requires that the entire platform be classified Div. 1, while a probability factor of less than 10 Pu requires that the platform be partially classified Div. 1. For example, the probability factor for the platform in Fig. 3-6 is above 10 Pu if the conditions given below apply. A. B. C. D.

Floor space occupied System pressure Quantity of vapors released Accumulation

= = = =

100% high medium unattended

Severity Factor 4 3 2 2

402 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 3-6. Boiler platform.

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With a probability factor of A × B + C + D = 16 Pu, the platform must be entirely classified. However, the source of hazard, which is not the drip pan but only the oil burner, does not occupy 100% of the platform floor space. The probability factor, therefore, cannot be 16 Pu, but much lower—7 Pu. (If the floor space is less than 50%, a severity factor of 1.0 Pu is considered.) This lower probability factor will, of course, change the Div. 1 classification from “entirely” to “partially” classified. However, in spite of the fact that the probability factor for the platform is reduced to 7 Pu, the Div. 1 zone may have a size which is almost equal to the size of the platform. If this is the case, the Div. 1 zone should be extended to the outer perimeter of the platform as shown in Fig. 3-6. This classification complies with Item I (2) of Fig. K-1, Ch. 29, Part 2. Since the location is not sufficiently ventilated, accumulation of vapors will occur. However, if the location is attended, the danger in the location is reduced because a failure of the source of hazard will be spotted instantly and repair can be applied immediately. Therefore, the explosion danger will increase only 1 point if an insufficiently ventilated location is attended. If the location is not attended, accumulation of flammable vapors will occur and may spread over the entire location. Explosion danger is, therefore, doubled. Form “A” for Area 3 is identical to Form “A” for Area 2, except in Form “A” for Area 3, a number of items must be changed to comply with the conditions stated above. They are as follows: A1 - Type of the source of hazard?

=

Valves, controls, fittings, etc.

A2 - Size of the source of hazard?

=

Small

C2 - Regularly worked on?

=

No

C5 - System pressure?

=

High

D5 - Type of location?

=

Burner platform

E1 - Type of ventilation?

=

Pressure

E3 - Amount of ventilation?

=

Insufficient

F1-

=

Frequent

How the flammable vapors are released?

Crude oil vapors will descend to the bottom of the boiler area since these vapors are heavier than air. They will accumulate at the bottom floor and cause a dangerous condition. Therefore, the bottom area underneath the platform must also be classified Div. 1. The piping system from the tank farm to the forwarding pump house and from the pump house to the boilers is not shown, but should be classified in accordance with Table 1-4, Item 11, subtable K, Item 2. Safety requires that the outer surface temperature of the furnace will be maintained below the ignition temperature of the flammable product. Safety also requires that continuous ignition and complete burning of the fuel in the furnace occur before the fuel enters its explosion range. All three areas, Nos. 1, 2, and 3, are classified as shown in Fig. 3-7.

Figure 3-7. Classification of Areas 1, 2, and 3.

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Example 4. Platform Reactor In this example, the area that requires classification contains a reactor that consists of a three-stage vessel mounted in an upright position to which a number of pipes and flanges are connected at the body, top, and bottom. The reactor contains a hydrogen gas (H2) that is used for chemical processing. The characteristics of the hydrogen gas and the environmental conditions of the reactor are listed in the attached Form “A,” #81391. Besides hydrogen gas, the reactor also contains some other minor flammable products. These products, however, are not considered because of their small quantities. The area classification will only involve hydrogen gas. Hydrogen gas is very explosive and, once mixed with air in the proper proportions, it can easily be ignited by an ignition source of low energy. If a failure of the reactor should occur, it is expected that the gaskets of the pipe flanges will fail rather than the three-stage vessel itself. For the hydrogen gas to form an ignitable mixture, it is necessary that the gas, after escaping from its confinement, be given sufficient time to become ignitable. This is accomplished when the gas in the atmosphere is diluted by the surrounding air to concentrations that allow the gas to enter its explosion range. If the gas mixture within the explosive range is not ignited, the air will continue to dilute the gas, and, eventually, the gas will reach a safe concentration that is not ignitable. This safe concentration takes place at a point remote from where the hydrogen gas is released to the atmosphere. The actual time required for the gas to reach its highest explosion point and the point at which it reaches a nonhazardous concentration is very short. It is a function of quantity of gas and its rate of release to the atmosphere. As the gas continues to escape at the same rate, the quantity of the gas becomes larger. This requires a longer distance for the gas to dilute to a safe concentration. The longer the gas is permitted to escape, the greater the risk of an explosion. Any electrical equipment within the perimeter of the ignitable gas concentration is a potential source of ignition. As before, the following items in Form “A” (#81391) are applied to determine the degree of hazard in the location: Item C1, closed system; Item C2, equipment not regularly worked or operated on; Items D1/3, outdoor location; Items E1/2, moderate natural ventilation; Item E3, sufficient ventilation; Item E4, no obstructions; Item F1, occasional release of flammable gas; and Item F2, no simultaneous failure with electrical equipment. It seems that these items, in combination with each other, allow the location to be classified Div. 2. The most favorable items for allowing the location to be classified Div. 2 are a closed system, an outdoor location, sufficient ventilation, and an occasional release of flammable gases.

406 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Characteristics of flammable product, its operation, and environmental conditions for platform reactor.

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407

The extent of the Div. 2 hazard is based on the size of the source of hazard (Item A2) and the vapor density (Item B6). Generally, the extent of hazard for lighter-than-air gases with densities below 0.75 are 15 ft in a horizontal direction when unobstructed. Since the hydrogen gas density is below 0.75 and leakage from the pipe flanges will most probably produce small quantities of gas, the extent of hazard in the horizontal direction should not be more than 15 ft when the natural air flow is unobstructed. Any electrical equipment within the 15 ft distance is a potential source of ignition. Electrical equipment located beyond the 15 ft perimeter is not potentially dangerous since the gas concentration at the perimeter will have diluted to safe concentrations. The basic concept for the Div. 2 classification is the remote possibility of a simultaneous failure of process equipment and electrical equipment and the presence of sufficient ventilation. However, the above conditions are only valid if the process temperature is below the ignition temperature of the hydrogen gas. Since, according to Item C4, the process temperature is higher than the ignition temperature, the requirements for classifying the location Div. 2 will not apply. The higher process temperature will have sufficient ignition power to ignite the hydrogen gas in an early stage whenever it escapes from its confinement. Justification for not classifying the location Div. 2, therefore, are Items B3 (ignition temperature), C4 (process temperature), and F3 (which indicates that the ignition source of sufficient energy is always present). Therefore, the higher process temperature will cause early ignition. This early ignition will play an important role in the classification of the location. The hydrogen gas, when escaping from its confinement, will initially be 100%, and eventually will become ignitable at its upper explosion limit. Within this range, ignition of the gas-air mixture will occur if the mixture consists of 75% gas and 25% oxygen. A local flame will be produced which will not propagate through the mixture. Since the flame will consume the gas-air mixture between 100% and 75%, air will not dilute the gas any further. This prevents the gas from entering its explosion range. If the gas cannot enter its explosive range, there is no explosion hazard. Since the hydrogen gas is consumed by the flame, an explosive mixture is prevented from traveling away from the source of hazard. If the explosive mixture does not travel, the extent of hazard is small, and, therefore, the danger area should be limited to a few feet from the source of hazard. Within a few feet from the source of hazard, the location is dangerous and should be classified Div. 1, but only if electrical equipment is present. The extent of the Div. 1 area should not be more than 3 ft vertical and horizontal. Since there is no ignitable gas mixture beyond the 3 ft boundary, an additional Div. 2 area is not required. The area beyond the 3 ft perimeter can, therefore, be safely classified nonhazardous as shown in Fig. 3-8. Bear in mind that the 3 ft radius is based on the fact that the flammable gas, when escaping from its confinement, is unable to enter its explosion range. Because of this, it is not necessary to extend the radius to a larger size. Form “A,” in particular Item A2, must be revised to suit the above conditions. Item A2 should read “mini” or “small” instead of “large.”

408 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres

Figure 3-8. Reactor.

Appendix Properties of Flammable Liquids, Gases, and Vapors

For evaluating the degree of hazard in a location that contains a flammable product, it is necessary to know the values of the flash point, ignition temperature, and the severity of explosion danger of the flammable product. Without these elements, the degree of hazard cannot be determined. To simplify the preparation of Form “A,” which is required to establish the degree of hazard, tables have been prepared that list these elements for a great number of flammable products. The severity of explosion danger for some of the flammable products is marked with “3” and “4” in column (6) of the tables in the appendix. Flammable products marked with “4” are designated as Class IA. They are extremely flammable and very volatile and, therefore, extremely hazardous. Their flash points are also below 73°F and their boiling points are below 100°F. Products marked with “3” are less flammable and less volatile, but still hazardous. Their flash points are below 100°F, but their boiling points are at or above 100°F. These products are designated as Class IB. Class IC flammable products are also marked with “3.” Their flash points are at or above 73°F, but below 100°F. The following table lists severity factors with respect to classification of flammable liquid.

409

410 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table App-1. Classification of Severity Factors

SEVERITY FACTORS 4

3

Class 1A Liquid

Class 1B Liquid

2 Class 1C Liquid

Class II Liquid

1 Class IIIA Liquid

Class IIIB Liquid

SEVERITY FACTORS DEFINED AS FOLLOWS: 4

3

2

1

0

Materials that will rapidly or completely vaporize at atmospheric pressure and normal ambient temperature, or that are readily dispersed in air and that will burn readily

Liquids and solids that can be ignited under almost all ambient temperature conditions

Liquids that must be moderately heated or exploded to relatively high ambient temperatures before ignition can occur

Materials that must be preheated before ignition can occur

Materials that will not burn

Appendix: Properties of Flammable Liquids, Gases, and Vapors Table App-2. Properties of Flammable Liquids, Gases, and Solids

411

412 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table App-2. (Cont’d.)

Appendix: Properties of Flammable Liquids, Gases, and Vapors Table App-2. (Cont’d.)

413

414 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table App-2. (Cont’d.)

Appendix: Properties of Flammable Liquids, Gases, and Vapors Table App-2. (Cont’d.)

415

416 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Table App-2. (Cont’d.)

Definitions

Access Opening

An opening of a three-wall building or a hinged door in a vapor-tight wall.

ANSI

American National Standard Institute

Air Brushing

Diluting air flowing in close proximity over and along-side the source of hazard instantly catching airborne gases or vapors.

Attended

One or more persons occasionally present for a short or longer time during a 24 hr period for checking and maintaining satisfactory operation.

AWG

American Wire Gauge

Basement

A building floor 50% or more below ground level to which access for fire fighters is restricted.

Capture Velocity

Airflow able to seize contaminated air.

Classification

An area which is classified according to the degree of its hazardous condition.

417

418 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Class I Location

Area in which flammable gases or vapors are or may be present in sufficient quantities to produce ignitable mixture(s).

Class II Location

Area in which combustible dust is present.

Combustion

An oxidizing agent, a combustible material, and an ignition source are essential for combustion. In the burning of most substances, the actual combustion takes place only after a liquid fuel has been vaporized or decomposed by heat to produce a gas or vapor. The combustible material must be heated to its ignition temperature before it will burn.

Demarcation Line

An imaginary line drawn horizontally underneath a mini static-type source of hazard which is associated with Class I lighter-than-air flammable products. Its purpose is to divide the danger area surrounding the source of hazard into two zones and to indicate the extent of danger in them.

Diffusion

Rate at which two gases mix depending upon their molecular weight and the condition of the atmosphere in which the mixing takes place.

Division 1 Location

A hazardous area with a dangerous atmosphere.

Division 2 Location

A hazardous area with a remote dangerous atmosphere because of the remote possibility of a simultaneous failure of process and electrical equipment.

Dust/IgnitionProof Enclosure

An enclosure which prevents arcs, sparks, or heat liberated inside the enclosure from igniting specified ignitable dust on the outside of the enclosure.

Early Ignition

A process in which a permanent ignition source ignites a flammable gas or vapor at its UEL once it is airborne without causing flame propagation.

Equipment Suitable for a Division 2 Location

Non-explosion-proof electrical equipment with arcing contacts sealed from a hazardous environment or without arcing contacts but capable of operating at a reduced temperature.

Definitions

419

Evaporation Rate

The rate of change from a liquid state to a vapor state at a temperature below the boiling point. It is the vapor from the evaporation of a flammable liquid when exposed to air or under the influence of heat, rather than the liquid itself, which burns or explodes when mixed with air in certain proportions in the presence of a source of ignition.

Explosions

Characterized by the release of energy and a large scale of rapid expansion. The release of energy is generated by rapid oxidation. The difference between a fire and an explosion lies in the rate at which energy is released. In a confined space, it is known as an explosion; in the open, it is known as a flash fire. The word “explosion” used in this manuscript means rapid expansion in a confined or unconfined space. An explosion can only occur when the flammable gas or vapor concentration is within its explosion range (between its LEL and UEL).

Explosion Range

A quantity of flammable gas or vapor mixed with air within certain boundaries which allows the mixture to be explosive and to propagate the explosion flame. Beyond these boundaries, which are known as the lower explosion limit (LEL) and the upper explosion limit (UEL), the mixture is not explosive and will not propagate the explosion flame. At the LEL or UEL, the mixture burns when ignited causing insignificant flame propagation. Between the two limits, there is a point at which flame propagation is the greatest.

Explosion Prone Area

An area in which systems containing flammable materials are located in the presence of electrical equipment. (An area is not considered explosion prone if the electrical equipment is unable to produce arcs, sparks, or sufficient heat outside its enclosure.)

Explosion Proof Equipment

An apparatus enclosed in a reinforced housing which can withstand the pressure of an internal explosion, prevent the ignition of a flammable gas or vapor mixture surrounding the housing, and operates at an external temperature that is unable to ignite the surrounding flammable gas or vapor.

420 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Flammable Liquid

Flammable and combustible liquids are divided into three classes: Class I (Flash points below 100°F at a vapor pressure not exceeding 40 psi), Class II (Flash points at or above 100°F and below 140°F), Class III (Flash points at or above 140°F). Combustible liquid, hereinafter, is called flammable liquid.

Flash Fire

An unconfined large-scale rapid expansion of energy.

Flash Point

The flash point of a liquid is the lowest temperature at which the liquid gives off vapor in sufficient concentration to produce a flammable mixture with air near the surface of the liquid. The flash point is usually a few degrees below the LEL.

Fume Hood Enclosure

Three sides, top, and bottom enclosed with movable sash, designed to draw-in air by mechanical ventilation.

Handled

The management and control of equipment containing flammable materials.

Ignition Temperature

The minimum temperature necessary to ignite a combustible mixture, thus causing an explosion or fire.

Indoor Locations

An area insufficiently ventilated by natural ventilation. Insufficiently ventilated means it is substantially closed, resisting the free passage of natural air. A location resists the free passage of air if it is roofed and provided with 4, 3, or 2 L-shaped, vapor-tight walls.

Intrinsic Safety

Intrinsically safe equipment and wiring, in which electrical sparking may occur under normal and abnormal conditions, but the sparking is incapable of causing ignition of a specific hazardous atmospheric mixture.

ISA

Instrument Society of America

LEL

The lower explosive limit of a flammable product.

Megohm

Million ohms

Definitions

421

NEC

National Electrical Code

NFPA

National Fire Protection Association

Operating Mode

The condition under which a source of hazard must operate. The source of hazard may have an open or a closed operating mode. If the source of hazard has an open operating mode, it is capable of releasing a flammable substance to the atmosphere continuously. If it has a closed operating mode, it may occasionally release a flammable substance into the atmosphere because of a rupture or breakdown of the source of hazard, or it may release flammable gases or vapors to the atmosphere as a result of frequent repairs and maintenance.

Outdoor Locations

A location sufficiently ventilated by natural ventilation. A location is sufficiently ventilated when it is substantially open and allows free passage of air. Such a location may be roofed and provided with one or two vapor-tight walls located opposite from each other.

Oxidation

The combination of a flammable material with air. The concentration of the oxidizing agent, such as oxygen. The oxidation reaction determines whether ignition and combustion can occur.

Permanent Ignition Source

A source of heat with a temperature in excess of the ignition temperature of the flammable product which is continuously present.

Probability Factor

A factor used to determine the susceptibility of explosion danger in a hazardous location. A factor of 10 Pu or more is considered highly susceptible and a factor of less than 10 Pu is considered less susceptible to explosion danger.

Process

A series of operations.

Propagation of Flame

The spread of flame from layer to layer independent of the source of ignition. A gas or vapor mixed with air in proportions below the LEL or above the UEL of flammability may burn at the source of ignition without propagating away from the source of ignition.

422 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Quantity of Release

The quantity of a flammable material released depends on the size of the source of hazard and system pressure. The rate of release is directly proportional to the size and pressure in the system.

RP

Reproduced Practices

Safe Concentration

There are three stages a flammable gas or vapor in the air must go through before reaching a safe concentration. First, a flammable gas must consist of a 100% concentration when released to the air; next, it enters its explosion range; and, finally, it reaches a concentration below the LEL. The total time frame at which the flammable gas goes through these three stages depends on the volume and flow rate of the moving air. Therefore, every time a flammable gas is released to the atmosphere, it enters its explosion range before reaching a safe concentration. A vapor from a flammable liquid also goes through three stages before reaching safe concentrations but it starts below the LEL.

Safeguard

A type “A” redundant mechanical ventilating system which operates upon failure of a ventilating system, or a type “B” alarm system which operates upon failure of the ventilating system.

Segregation

Isolating the electrical part of the equipment and placing it outside the hazardous area.

Slightly Above Flashpoint

Up to 125% of flash point.

Slightly Below LEL

Dilution of approximately 90% of LEL.

Source of Hazard

Process equipment which is either open or closed and is of the dynamic- or static-type. A storage tank is a static-type, and a pump is a dynamic-type source of hazard. Sources of hazard are divided into three categories: mini (valves, bolted flanges, manifolds, etc.), small (pumps), and large (pumps, tanks, etc.).

Definitions

423

Sufficient Quantity

A flammable gas or vapor present in the air in its explosion range forming an ignitable mixture.

Sufficient Ventilation

A moving air of sufficient flow rate and volume for reducing a given quantity of flammable gases or vapors in the air to safe concentrations. Suction ventilation is considered to produce sufficient air flow if vapor-air mixtures are diluted to concentrations below the LEL. Pressure ventilation is considered sufficient if the air pressure produced is slightly above atmospheric pressure. A minimum of 0.1 inch of water above atmospheric pressure, but not more than 0.25 inches of water is sufficient to maintain a safe nonhazardous environment. Atmospheric pressure is 14.7 psi = 407.484 inches of water (1 psi = 2.31 ft of water). A pressure in excess of 0.25 inches of water may make it difficult to open access doors.

System Pressure

Values expressed as low (less than 100 psi), moderate (from 100 to 500 psi), and high (above 500 psi).

TE

Totally Enclosed

TEFC

Totally Enclosed Fan Cooled

Temperature Slightly Above Flashpoint

A temperature of up to 125% of the flash point.

TLV

Threshold limit value of air-borne toxic material without adverse effect to workers on a daily basis.

Traveling Distance

Traveling distance of a flammable gas or vapor is the horizontal distance between the point at which the flammable material will be airborne and the point at which the flammable material will reach a concentration below the LEL.

Twilight Zone

An intermediate zone between a Division I area and a nonhazardous area.

424 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Used

The utilization of flammable material.

Vapor Tight Walls

Walls that are impervious. These walls may be provided with access openings such as hinged doors and/or bolted windows.

Vapor Density

The weight of a gas or vapor without air compared to an equal weight of air. A value of a gas or vapor less that 1.0 indicates that the vapor is lighter than air. A value greater than 1.0 indicates that the vapor is heavier than air. For values between 0.75 and 1.0, the vapor may be unstable and may be either lighter or heavier than air. Therefore, the density of a gas or vapor greater than 0.75 is considered as heavier than air and a density below 0.75 is considered lighter than air.

Vapor Pressure

The pressure exerted by a flammable liquid measured in pounds per square inch (psi) absolute.

Vapor Tight

A building sealed so that vapor can not enter or escape.

Vapor Traveling Distance

The point in space at which a flammable gas or vapor must travel to reach a nonhazardous concentration.

Very Volatile or Flammable

A Class IA liquid with flash point below 73°F and boiling point less than 100°F such as: ethyl ether, ethyl chloride, isoprene, methyl oxide, pentane, and propylene oxide. These liquids are extremely flammable and have a severity factor of “4.”

Volatile Liquids

Liquids with low boiling points causing a rapid transfer to a gaseous state or vapor when released to the atmosphere (e.g., butane, ethane, ethylene, propane, and propylene).

UEL

The upper explosive limit of a flammable product.

Definitions

425

Zone-0

An area in which a flammable gas or vapor is present in the air continuously or for long periods of time.

Zone-1

An area in which a flammable gas or vapor may be present in the air under normal operating conditions.

Zone-2

An area in which a flammable gas or vapor may be present in the air occasionally.

Bibliography

1. Classification of Locations for Electrical Installations in Petroleum Refineries, API Recommended Practice 500A, 4 th edition, (Jan. 1982); American Petroleum Institute, Washington, DC (reaffirmed Dec. 1987) 2. Recommended Practice for Classification of Locations for Electrical Installations at Drilling Pipe and Production Facilities on Land and in Marine Fixed and Mobile Platforms, API Recommended Practice 500B (RP500B), 3rd edition, American Petroleum Institute, Washington, DC (Oct. 1, 1987) 3. Classification of Locations for Electrical Installations at Pipeline Transportation Facilities, API Recommended Practice 500C, 2nd edition, American Petroleum Institute, Washington, DC (July 1984) 4. Electrical Installation in Petroleum Process Plants, API Recommended Practice 540-91, 3rd edition, American Petroleum Institute, Washington, DC 5. National Fire Protection Association, National Fire Code, NFPA 30, Flammable and Combustible Liquids Code, 2:30-1–36-89, Quincy, MA (1996) 6. National Fire Protection Association, National Fire Code, NFPA 36, Standards for Solvent Extraction Plants, 2:36-1–36-25, Quincy, MA (1997) 7. National Fire Protection Association, National Fire Code, NFPA 50A, Standards for Gaseous Hydrogen Systems at Consumer Sites, 2:50A-1–50A-10, Quincy, MA (1994) 8. National Fire Protection Association, National Fire Code, NFPA 58, Standards for the Storage and Handling of Liquefied Petroleum Gases, 3:58-1–58-94, Quincy, MA (1995)

426

Bibliography

427

9. National Fire Protection Association, National Fire Code, NFPA 59, Standards for the Storage and Handling of Liquefied Petroleum Gases at Utility Gas Plants, 3:59-1–59-36, Quincy, MA (1995) 10. National Fire Protection Association, National Fire Code, NFPA 59A, Standards for the Production, Storage, and Handling of Liquefied Natural Gases, 2:59A-1–59A-40, Quincy, MA (1996) 11. National Fire Protection Association, National Fire Code, NFPA 69, Standard on Explosion Prevention Systems, 3:69-1–69-33, Quincy, MA (1997) 12. National Fire Protection Association, National Fire Code, NFPA 45, Standard on Fire Protection for Laboratories Using Chemicals, 2:45-1–45-47, Quincy, MA (1996) 13. National Fire Protection Association, National Fire Code, NFPA 70, National Electrical Code, 3:70-1–70-444, Quincy, MA (1996) 14. National Fire Protection Association, National Fire Code, NFPA 90A, Installation of Air Conditioning and Ventilating Systems, 4:90A-1–90A-20, Quincy, MA (1996) 15. National Fire Protection Association, National Fire Code, NFPA 91, Standard for the Installation of Blower and Exhaust Systems for Air Conveying of Materials, 4:91-1–91-14, Quincy, MA (1995) 16. National Fire Protection Association, National Fire Code, NFPA 321, Standard on Basic Classification of Flammable and Combustible Liquids, 6:321-1–321-4, Quincy, MA (1990) 17. National Fire Protection Association, National Fire Code, NFPA 120, Standards for Coal Preparation Plants, 5:120-1–120-14, Quincy, MA (1994) 18. National Fire Protection Association, National Fire Code, NFPA 496, Standard for Purged and Pressurized Enclosure for Electrical Equipment, 7:496-1–496-19, Quincy, MA (1993) 19. National Fire Protection Association, National Fire Code, NFPA 325M, Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids, 10:325M-1–325M-95, Quincy, MA (1990) 20. National Fire Protection Association, National Fire Code, NFPA 497A, Recommended Practice for Classification of Class I Hazardous (Classified) Locations for Electrical Installation in Chemical Process Areas, 11:497A-1–497A-36, Quincy, MA (1992) 21. National Fire Protection Association, National Fire Code, NFPA 77, Recommended Practice on Static Electricity, 10:77-1–77-29, Quincy, MA (1993) 22. Protection Against Ignition Arising Out of Static, Lightning and Stray Currents, API Recommended Practice 2003, 4 th edition, American Petroleum Institute, Washington, DC (Mar. 1982) 23. Recommended Practice Installation of Intrinsically Safe Systems for Hazardous (Classified) Locations, ANSI/ISA-RP 12.6, Instrument Society of America, NC (approved Nov. 1987) 24. Industrial Ventilation, 23rd edition, a manual of recommended practice, Committee of Industrial Ventilation, Lansing, MI (1998)

Index

Acceptable level of safety 28 Acceptable risk 380 Acceptably safe location 28 Access openings 118, 417 Accumulation 31, 36, 40, 41, 94, 97, 110, 158 vapors 403 Acetylene gas 144 Additional danger zones 85, 86, 91 conditions 86 heavier-than-air products 86 lighter-than-air products 91 Adjacent location definition 34 Adverse health effects 155 Adverse operating conditions 32 Air currents 158 inlet 153, 360 intake 149 movement 141 oxygen content 16 stream 357 supply 156 switch 170 vane 170 vents 366 volume 149, 150

Air brushing 143, 158, 417 sufficient 143 Air density 149 correcting for temperature 151 correction factor 150 Air pocket 116, 119 Air pressure drop 174 sufficient 147 Air purging enclosure 191 Air velocity 152 Airborne gases 11 Airflow 157, 166, 358 insufficient 159 rate 157 suction fan 158 Alarm system 35, 135, 170 Ambient temperatures 110 Anodes 201 ANSI 417 Application Engineer 48 Arcing 258 Arcing devices 133, 179, 186, 229, 230 dust-tight 136 Arcs 27, 207 Area classification 118, 380 ASTM 4 Atmospheric pressure 149

428

Index Attended 417 Available spark energy 189 AWG 417

Ball Mill 135 Basement 108, 417 non-hazardous 109 Belts static electricity 198 Boilers 392 Boiling point 10 Bond wires 203 Bonding 200, 201, 202 jumper 210, 211 Bonds 202 Borderline 218 Bottom loading 199 Boundary 47, 76, 159 application 81 distance 47, 78 height selection 48 pump dimensions 251 radius 81 requirements 51, 76 size 24, 77, 80, 84, 86, 145 sized by group 81 Breakdown 21, 53, 54, 93, 154, 297, 388 Brushing 166, 357, 358 Bulk storage plant 305 Bushings bonding type 210 Butylene 352

Cable 218, 230 conduit 230 core 189 direct burial 230 terminals 218 tray 189 Canopy fume hood 100, 147, 164, 357 inlet 100 safeguards 101, 170 side panels 164 Capacitance circuit 188 to ground 206

429

Capacitive energy 188 Capture velocity 151, 164, 308, 417 sufficient 153 Car dumper 133 Carbon dioxide 16 Cars 366 Catalytic process 194 Cathodic protection 196 Ceiling opening 155 Centrifuge 314 Chemical fume hood enclosure 157 Chemical process plants 14 Class I liquids hazardous area 4 Class I location 418. See also Electrical equipment Class II liquids explosion range 4 Class II location 132, 418. See also Electrical equipment Div. 1 186 Class III liquids hazardous range 4 Classification 153, 417 Classification requirements dispensing areas 110 indoor filling stations 112 inside rooms 109 Classified partially 147 Closed connection 200 Closed operating mode 55 Closed process equipment dilution requirements 148 Closed sources of hazard 21 not ventilated 31 safeguards 101 Closed system 44, 77 operating mode 55 Coal dust 14, 131, 187 Coal fuel unloading system 133 Coal pulverizer 135 location 136 Code of Federal Regulations 108 Coke dust 131 Combination starters 174 Combustible coke 14 Combustible dust 3, 13, 130, 131, 136

430 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Combustible liquid 3, 4 Combustion 6, 418 Committee on Instrumentation for Hazardous Areas 193 Communication flammable material 85 Components mini-type 360 Compressor intake air 191 shelter 332 station 323, 338 Concentration 103 ignitability 7 ignitable 29 nonhazardous 144, 152 Conditions ideal 145 Conductive path 200 Conductive pulleys 198 Conductivity 200 Conductors buried 214 Conduit 189 fittings 217 length 231 penetrating 217 rigid steel 229 seals 228 sleeve seal 218 system 215 Condulets 232 Cone 327, 358 Connections low integrity 327 Conservative approach 380 Contact switch three-way 171 Contacts immersed in oil 228 Containers non-suitable 260 Containment 143 Contamination 91 zone 156 Continuity 207 Continuous release 36, 40, 121, 126 Control circuit 174

Control room 121, 147, 300, 303, 321, 325, 332, 333, 335, 336, 337, 348 classification 247 nonhazardous conditions 325 Control sensor differential air pressure 170 Control-transformer 174 Cooling generators 194 Copper conductors 212 Corrosion explosion danger 196 Critical conditions 94, 158 Crosswinds 84, 145 Crude oil 392 vapors 403 Crusher houses 132, 139 Cubic feet of vapor 327 Current 368 interrupting contacts 179 Cut-off room 107 Cutting areas 366

Dampers 100 Danger 52, 102. See also Degree of danger classified Div. I 123 Danger area 50. See also Extent of danger Danger zone 21, 40, 86, 97, 102, 270, 325, 336, 360 above ground 92 additional 55, 275, 277, 280 extent 389 vertical cone 166 Dangerous classification 9 condition 130 Dangerous location 29, 34, 36, 43, 48 Degree of danger 21, 118, 249–347, 381 Demarcation line 165, 327, 358, 418 above 166 below 165 Density of air 150

Index Detonation 14 Differential pressure switch 174 Diffusion 418 rate 15, 50, 194 Dilution 55, 92, 116, 141, 146, 149, 165, 327 ethyl chloride 156 requirements 148 slightly below LEL 149 ventilation 156 Dimensions 76 Class II flammable products 80 groups 54 Direct fire unit 135 Discharge rate 49, 157, 164 suction fan 148 Dispensing flammable liquids 107 in cut-off rooms 111 liquid warehouse 112 principal activity 110 storage areas 110 Dispensing area 107, 311 safeguard 170 Dispersion 116 Dividing solid wall 217 Division 1 hazard 29 location 130, 418 zone 47 Division 2 area 111 classification 33 location 131, 230, 418 Door switch 193 Double crosshatched 48 Double grounded 196 Drag chains 199 Drum filling stations 107, 112, 305 Duct outlet 159 Duct system 163 Dust 13 accumulation 14, 132 cloud 14 suppression 14, 133 Dust blankets 187

431

Dust-ignition proof equipment 14. See also Electrical equipment Dust-tight enclosure 185. See also Electrical equipment Dust/ignition-proof enclosure 231, 418. See also Electrical equipment Dynamic hazard sources 20

Early ignition 18, 77, 78, 294, 407, 418 Electric driver 155 explosion proof 153 Electric ignition energy 188 Electric motors enclosed fan cooled 137 Electric suction fan 306 Electrical energy 211 Electrical equipment 156, 308, 328, 360, 374 Div. 1 location 178 Div. 2 locations 179 dust ignition proof 133 enclosures 177 explosion-proof 28, 32, 177 fume hood 106, 368 grouping 45, 182 hazardous locations 191 heat producing 102, 143 intrinsically safe 27, 180 marking 181 NEC Class I location 177 NEC Class II 185 non-arcing 186 non-explosion-proof 28 not marked 181 presence in hazardous areas 27 temperature marking 181 types for hazardous zones 44 Electrical ignition source 353 Electrical installation 54, 93, 389 Electrical machines hydrogen gas 194 Electrical motors non-explosion-proof 179 types 178

432 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Electrical power system grounding 206 Electrical rotating equipment 186 Electrical sparks 19 eliminating arcs or sparks 208 Electrolytic decomposition 194 Electromagnetic energy 188 Elevated walk-board 354 Emission rate 11, 145, 146, 148 boundary requirement 76 Enclosed space 91, 117 roofed 35 Enclosure 360 bonding 206 dust-ignition-proof 185, 231 dust-tight 136, 185 volume 192 Energy release rate 15 Environmental conditions 246, 379 Epoxy coating 202 Ethyl chloride 156 European Standard IEC 42 Evaporation 157 Evaporation rate 9, 11, 49, 419 Event time frames 127 Exhaust air 109, 308 outlet 153, 156 Exhaust duct system 307 Exhaust fan 110, 143 capacity 152 centrifugal 106 effective location 159 electric 165 ineffective 158 Exhaust rates 109 Exhaust system 100, 157 face velocity 106 Explosion 6, 15, 215, 419 causes 19 characteristics 13 coke or coal dust 131 combustible dust 14 confined 15 curve 17 features 180 force 141, 178

hazard 143 internal 183 lower limit 6 prevention 17 range 105, 141 requirements 17 strength 178 unconfined 15 upper limit 6 Explosion danger 32, 46, 50, 99, 102, 105, 130, 158, 251, 269, 272, 340, 409 determining degree 46 extent 49 probability factor 94 Explosion hazard coal and/or coke dust 130, 135 NEC classes 13 static electricity 198 Explosion pressure 182, 184 due to compression 184 groups 182 Explosion prone area 419 Explosion proof 230 construction 358 electrical equipment 28, 32 enclosures 133, 181, 182, 228, 229 equipment 105, 419 Explosion range 4, 6, 16, 18, 32, 36, 41, 51, 102, 104, 419 Hydrogen gas 51 Explosive limit 17. See also Lower explosive limit Explosive mixture 11 Explosive pressure 6 Extent of danger 47, 251–348, 362, 381, 382, 388 External enclosure temperature 192 External grounding conductor 208 External return path 207 Extremely dangerous location 48

Failure magnitude 148 Failure of process equipment 33 False air 164

Index Fan blades 155 built-in safety margin 149 capacity 154 failure 362 generation 153 location 163 manual starting 171 manual stop 173 motor 154 outlet motors 102 output 149 type A 171 Fast spreading fires 15 Fault current 210 phase to ground 207 Filling station outdoor 112 Fire door 107 Fire hazard LP gas 13 Fires and explosions causes 19 differences 15 Firewalls 107 Fittings 19, 327, 345 approved 217 high integrity 364 threaded 31 Fixed roof tank 288 Flame propagation 6, 16, 17 Flames 19 Flammability class 3, 285. See also Liquid: flammability classes related to hazardous area 53 Flammable cloud 158 Flammable gas 3, 11 heavier-than-air 6 large quantities 81 lighter-than-air 6 small quantity 81 Flammable liquid 3, 107, 288, 420 storing 107 Flammable material 99 dilution 100 large quantities 86 quantity 85 release 148

Flammable mixture 22 Flammable product bulk 51 fire and explosion characteristics heavier-than-air 85 NEC groups 13 vapor density 52 Flammable substances 55 Flammable vapors 4 large quantity 85 Flash fire 15, 419, 420 Flash point 4, 8, 10, 16, 27, 80, 380, 409, 420 temperatures 109 Flasher 173 Flat belts 198 Flat terrain boundary requirement 76 Flexible joints 202 Floating roof tank 288 Floor distance 9 Forced ventilation 91 Free-floating electrons 200 Frequent release 36, 126 Friction sparks 19 Fugitive vapors 153 Fume hood 147, 307 duct system 102 flanges 308 laboratory type 106, 367 Fume hood enclosure 100, 420 classification 106 electrical equipment 106 explosion danger 105 laboratory-type 102 not ventilated 103 safety 107 Furnaces 401

Gas compressor 323, 325, 337, 342, 344 shelter 330, 346 station 332 Gas cylinders 168 Gas detection 166 Gas detector 113, 163 Gas generator 44

433

14

434 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Gas mixture ratio 16 Gas tank 203 charging 203 Gauge pipe 199 Gauges 168, 365 General-purpose induction motor 181 purged enclosure 191 Glass containers 108 Gravity fan 110, 327 Ground anodes 201 Ground grid system 211 Ground leads 214 Ground loop 211 Ground resistor 207 Ground return path 210 Ground wires 201 Grounded earth plate 202 Grounded shielding 196 Grounding 200, 201, 202 clamp 203 grid 202 practice 208 requirements 202 Grounding conductor electrode 211 external 209 internal 208, 209 size 214 types 208 Group A requirements 249 Group B requirements 264 Group C requirements 278 Group D requirements 304 Group E requirements 310 Group F requirements 314 Group G requirements 322 Group H requirements 329 Group I requirements 341

Group J requirements 349 Group K requirements 369 Grouping 13

Handled 420 Hazard 133. See also Source of hazard closed source 31 levels 28 open sources 30 source size 21 sources 20 Hazardous area 48, 53, 118 boundary 23, 127, 146, 357 Class II defined 130 classification 76 classified 126 classifying 42 determining classification 76 determining the size 50 recommended 78 remote 28, 77 size 23, 50, 81, 125 Hazardous areas large 23 Hazardous cone 327 Hazardous location 8, 28, 48, 389 cathodic protection 196 classification 27, 379 classifying 46 definition 24 density of gas 27 enclosed 124 intrinsically safe wiring 189 pump size 23 size 27 steps to classifying 380 Hazardous product 3 Health hazard 101, 155 Helium leak tested 168 Hermetically sealed chamber 179, 228 High conductivity substances 200 High impedance grounding 211 High resistance 208 Hinged windows 118 Holding basin 390

Index Hood opening airflow 164 Hoppers 133 Horizontal boundary 296 recommended distance 78 Hoses grounding 202 Hot plates 106 Hot surfaces 19 HP ranges 174 Human body static charges 199 Hydrogen gas 194, 405 airborne 168 containers 195, 366 explosion range 104 leaks 194 storage 195 uses 194 Hydrostatic test internal explosion 183

IEC 48 Groups 45 Zones 43 Ignitable concentration 141 boundary requirement 76 Ignitable gases and vapors 15 Ignitable mixture 318 Ignitable range 141 Igniter 16 Ignition 105, 200 energy 14 permanent 33, 36, 41, 296 power 102 temperature 4, 15, 18, 78, 178, 294, 409, 420 Ignition source 16, 19, 27, 36, 155, 166, 179 Illumination 308 Impact mill 135 Impedance 210 Impellers 155 Impounding basin 22, 319 Incidental communication 35 Indoor location 55, 115, 117, 420. See also Location

435

Inductive circuit 188 Inductive reactance 207 Inert gas purging enclosures 191 Initial conductor temperature 214 Inlet pipe 325 Inspections 372 Instrument Society of America (ISA) 191, 420 Insufficient ventilation 31 Insulating couplings 203 Insulating joints 203 Insulating material 202 Insulation 180 breakdown 207 failure 137, 207 Intake openings 362 Internal blankets 202 Internal explosion 182, 183. See also Explosion Internal grounding conductor 208 Intrinsic safety 420 factors 188 Intrinsically safe apparatus 190 circuits 189, 190 equipment 188, 189 Inverse ratio 80 Isolating switches 186

Joint clearance 182 Joints 183, 209 Jumpers 202

Large motors 23 Leakage 154, 168 Lean limit 4 LEL 4, 6, 132, 141, 421 Levels of hazard 28, 29 Lighting fixtures 368 Liquefied petroleum gases 13 Liquid discharge rate 9 flammability classes 3 flammable and combustible 3, 288 fuels 7 highly volatile 288

436 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Liquid petroleum gas 352, 353 Liquid warehouses 107, 111, 258 Loading platform 351 Local burning 294 Local ignition sources 77 Local spot exhaustion 157 Location cooled 163 indoor and outdoor 115, 116, 118, 158, 420, 421 non-classified 372 nonhazardous 147 sufficiently ventilated 115 ventilation 158, 246 Location classification 40 Division 1 40 requirements 36 Locked-rotor condition 180 Locknuts 210 Low concentration, acceptable 143 Low pressure 52, 55 Low voltage motors 23 Lower explosion limit 6. See also Explosion Lower explosive limit 132, 136, 141, 421 LP gas (LPG) 352, 353 odorized 13

Maintenance 136 procedure 372 Marine terminal 297 Mechanical auxiliary contact 174 Mechanical failure 131 Mechanical fan 163 location 163 Mechanical ventilation 117, 246 location 158 outdoor locations 118 types 143 Mechanical wear 53 Megohm 421 Metal bonding wire 203 Metal pipelines grounding 202 Metal sleeve 214 Metallic tanks 202

Mini source of hazard. See Source of hazard Mini sources of hazard 21, 42, 52, 53 boundary requirements 54 Mini-type piping system 52 Minimum ignition energy 189 Minimum sparking energy 208 Moderate pressure 52, 55 Molecular weight 10, 15 Motor trouble-free 180 Motor sizes 23 Motor vehicles grounding 203 Multi-conductor cables 228 Div. 2 locations 230 Multi-story building 362

Narrow explosion range 105 National Electrical Code 13, 421 National Fire Protection Association 421 Natural ventilation 116, 119, 126, 143, 144, 395 effective 115 NC contact 174 NEC 13, 421 Class I location 14, 28, 29 NEC Class II, Div. 1 location wiring method 186 NEC Class II, Div. 2 locations 186 classification 132 requirements 132 wiring methods 186 Negative ions 203 Neutralization 199 Non-arcing type devices 178, 179 Non-conductive body 199 Non-electric ignition source 17, 36, 41. See also Ignition source Non-explosion proof electrical equipment 28 Non-suitable containers 108 Nonhazardous 52, 99 area 78 classification 127 concentration 11, 84, 92 environment 121

Index location 35, 42, 135 space 120, 121, 123, 124 Nonmetal containers conditions for being “suitable” Normal operation 131 Nozzle 203

108

Obstructions 143, 149 Occasional communication 34, 35 Occasional event 85 Occasional release 36 Oil 321 refining 188 Open dome top loading 203 Open machines 139 Open process equipment 148 plants 126 Open sources of hazard 21, 30, 41, 106 outdoors 119 safeguards 101 Open tank 314 Operating conditions area classification 184 Operating mode 21, 421 closed 395 Operating temperature 80 Outage 124 Outdoor areas 40 Outdoor locations 115, 421 Over-current relays induction disk-type 180 Over-voltage 206, 207 Oxidation 421 process 16 rapid 15 Oxidizing agent 16 Oxygen 4, 16

Packing gland 113 Paint spraying facility 308 Parts per million (ppm) 155 Percent floor space occupied 97 Permanent ignition source 33, 36, 41, 421

Petrochemical plant 390 Phase conductors 210 Pipe flanges 197, 405 Pipe vent 317, 318 Piped ventilation system 133 Pipelines grounding 263 Piping system 22, 23, 165, 291, 360, 364, 369, 373 large sources of hazard 262 mini sources of hazard 263 Pits 290, 349 Plastic containers 108 Platform grounding 203 reactor 405 Plot plan drawing 380 Point of release 34, 142 Point of safe concentration 81 Poor conductivity substances 200 Popping 18 Portable container 107 Portable shipping tanks 108 Positive ions 203 Potentials 193 Power failure 173 Power plant crude oil fired 392 Power supplying equipment 211 Power system grounded neutral 208 neutral 207 Pressure 3, 21, 52, 166. See also System pressure buildup 184 control valves 192 effect on hazard area 50 low/mod/high 21 positive 192 reducer 357, 359, 366 relief valve 168 rise 14 of system 9 transducers 190 ventilation 401 Pressure fan 35, 143, 147, 374 location 158 roof mounted 163 Pressurized gas systems 167, 365

437

438 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Pressurized system 21 reliable 147 Preventive cleaning activities 137 Probability 388 Probability factor 94, 96, 99, 251, 269, 270, 272, 275, 371, 374, 388, 421 conditions 251, 253, 269 Process 422 Process areas enclosed 360 Process Engineer 381 Process equipment 40, 54, 337, 380 breakdowns 36, 43 dust tight 132 malfunction 131 Process plant 279, 300, 303, 307, 313, 362 Process tank 18 Process temperature 18, 407 Process vessel 32 Propagation 14 of flame 422 Proper condition 55 Propylene oxide 352 Pu value 251 Public assemblies 366 Pulverizer 132 equipment 136 fuel system 135 Pulverizing units types 135 Pump 23 boundary dimensions 273 failure 273, 277 hazardous boundary 274 large 274 probability factor 273 Pump house 352, 374, 392 Pump station 92, 251, 285, 286, 288, 294, 296, 388, 389 attended 94 classification 93, 94, 97 containing sources of hazards 93 entirely classified 93 explosion danger 272 fire safeguard 151

not attended 94 not ventilated 269 partially classified 93 probability factor 275 unattended 99 Pumping well 283 Purging type 191 type X 193 type Y 192 type Z 192

Quantity of a flammable gas factors influencing 51 Quantity of gases boundary requirement 76 Quantity of release 422

Raceway 189, 210, 230 Rate of breakdown 21 Rate of release 50 Rate of wear 20 Reactance inductive 206 Reclaim hoppers 139 Rectifier unit 196 Relative humidity 201 Relatively safe location 33 Relaxation time 202 Release continuous 118 frequent 119 occasional 119 types 118 Release of flammable gas/vapor 12, 22 Relief valve 22, 92, 168 Remote danger 124 Remote hazardous area 77. See also Hazardous area Remote ignition source 77, 85 permanent 18, 33, 78 Remote possibility 127 Remotely dangerous 36, 320 conditions 43, 130 locations 29, 30, 33, 34, 35, 48

Index space 120 transition zone 34 Remotely hazardous location 28. See also Hazardous area Replacement air 163 Reproduced practices 422 Resistance 185, 206 to ground 201 Resistivity 200 Ring-type gaskets 182 Roof fan 327 Roof opening 155 Roofed space 85, 118, 123, 300, 303 classified 125 hazardous area 119 natural ventilation 119 Rotary equipment 147 Rotating electrical machinery 179 Rotating/nonrotating equipment 20 RP 422 Rupture 9, 19, 40, 49 Rupture opening 19, 145, 146 boundary requirement 76

Safe area 85 Safe clearance 362 Safe concentrations 36, 77, 81, 84, 92, 422 Safe conduit runs 232 Safe distances 166 heavier-than-air products 91 lighter-than-air products 91 Safe environment 215 Safe location 28 Safeguard 35, 42, 43, 101, 110, 121, 123, 124, 125, 129, 135, 147, 151, 168, 246, 422 conditions for nonhazardous space 123 electric motors 178 hydrogen gas 194 non-hazardous location 123 type A 118, 307 type A conditions 171 type A fans 171 type A requirement 170

439

type B 306 type B requirement 170 types 170 wiring diagrams 170 wiring type A 171 wiring type B 170 Safety 28 interlocks 192 levels 28 margin 84 related to hazard 28 zone 177 Screwed connections 23 Seal connectors 168, 364 high integrity 168 Seal for pump 32 Sealed 374 Sealing fitting 215, 228, 229, 231 types 215 Seals 215 not required 218 Security 170 Segregation 422 electrical equipment 113 Separately derived system 211 Service applied AC system 211 Service supplied AC system 211 Settlement 113 Severity factors 94, 96, 99, 371 Shell of tanks 201 Simultaneous failure 32, 41 conditions 32 Single bare copper conductor 211 Single crosshatched 48 Slightly above flashpoint 422 Slightly below LEL 423 Slop oil 319, 390 Small hazardous areas 22, 23 Small motors 23 Small process areas 195, 357 Source of hazard 29, 35, 36, 41, 50, 77, 423 and classification of hazard area 53 boundary requirement 76 boundary requirements 54 closed 249 closed location 246 combinations of conditions 236 determining 23

440 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres dynamic 21 dynamic type 54, 388 establishing size 55 examples 31 large 22, 40, 53 location 163 location requirements 247 mini 21, 53, 163 open 36 operating features 236 requirements 247 size 93 sized for classification 106 small 22, 23, 40, 53, 54 static 21, 54 type and size 146, 147, 246 Space classification 118, 120 not sufficiently ventilated 119 three-wall 120 Spark 200, 207 promoters 199 Sparking potential 201 Sparks 27, 40 electric 19, 28 Standard conductor size 214 Standby mode 173, 174 Start/silencer button 173 Starter oil immersed 229 Static charges produced by liquid 199 Static electrical discharge 100 Static electricity 27, 198 basic concept 199 generation 198 Static hazard sources 20. See also Source of hazard Static potential 202 Static pressure 154 Static sparks 19 Static splashing 199 Steam turbine 18, 296 Stirrers 106, 314 Storage Class I flammable liquid 109 flammable liquid 112 Storage area 345

Storage cabinet 108, 113 Storage cylinders 345 Storage rooms 107 Class II and III liquids 148 inside a building 109 Storage silos 139 Storage tank 288, 316, 318 pressurized 22 static electricity 202 Stray currents 203 Suction air 100 Suction fan 143, 148, 159, 362 capacity 154 duct system 163 installation 154 location 151, 158, 163 louvers 159 placement 143 vertical riser 363 Sufficient distance criteria 77 defined 77 Sufficient quantity 423 Sufficient ventilation 33, 41, 52, 55, 117, 121, 125, 127, 423 Sufficiently ventilated 40, 382 definition 246 Suitable containers 108 Supplementary ground 208 Supplementary ground system 214 components 211 Supply conductors 210 Suspension 136 Switch houses 121 Switch loading 200 Switch-over switch 171 Switch-rooms 147 Swivel joints 202 System operating mode 76 System pressure 49, 55, 91, 93, 94, 277, 371, 423 related to temperature 55 System temperature 8, 11, 21

Tank farm 392, 395 Tank vessels 351 Tanks shell 201

Index Tapered conduit coupling 232 TE inert gas filled motor 178 TEFC electric motors 178 Temperature borderline 149 effect on flammable range 7 effect on hazard area 50 operating 151 related to quantity 55 slightly above flashpoint 423 of system 8 Terminal strips 190 Terminating cables 230 Terrain conditions 145 Thermocouples 190 Threaded joints 230 Three-wall space classification 126, 127 classified 125 Threshold limit value (TLV) 101, 155, 420 Time weighted average concentration 155 Timing device 193 Total capacitive reactance 207 Toxic concentrations 155 Toxic gases 164 Toxicity 155, 157 Transformers 196 Transition zone 77, 78, 84, 85, 126, 269, 272, 297, 307, 319 Traveling capability, dust 231 Traveling distance 27, 34, 50, 52, 54, 84, 125, 147, 159, 423 calculation 145 conditions 11 determining factors 145 dimensions to cover 54 groups 81 outdoors 145 sizes 81 wind conditions 144 Tripping time 210 Turbulence 102, 184 TWA 155

441

Twilight zone 424

UEL 6, 17, 424 UL Standards 182 Union 232 Unsafe condition 84 Upper explosion limit 6, 424. See also Explosion Used 424

V-belts 198 Valves 31 small source of hazard 291 Vapor density 424 pressure 424 release 22 traveling distance 9, 11, 424 volume 12 Vapor concentration 4 Vapor density 6, 11, 13, 21, 24, 76, 93, 123, 125, 153, 155, 158, 246, 285 definition 143 range 159 unstable 143 Vapor generation 148 Vapor pressures 9, 10, 80 inverse ratio 80 Vapor tight 92, 424 walls 424 Vapor traveling distance Class II flammable products 80 Velocity 151 Vent 92, 166 Ventilated 371 insufficiently 30, 44 location 33 sufficiently 30, 36 Ventilating air 166, 195 Ventilating equipment breakdown 43 Ventilating requirements indoor filling stations 112

442 Electrical Safety in Flammable Gas/Vapor Laden Atmospheres Ventilating system 308 design stage 152 redundant 170 Ventilation 31, 36, 40, 43, 44, 86, 97, 109, 166, 246, 327, 357, 374, 382. See also Sufficient ventilation application 143 dilution 150 failure 124, 255 forced 163 forced low capacity 155 hydrogen gas 195 loss 170 minimum rate 154 natural 115 natural and mechanical 118 outage 368 pumpstation 92 purpose 143 rate 154 suction 148, 152, 155 two wall spaces 120 types 143 Ventilation requirements 110, 168 inside rooms 109 Ventilation system 35 Ventilation-capture velocity 151 Vertical cone 166 distance 91 height 48 Very volatile 424 or flammable 422 Vibration cracks 113 Volatile 99, 371 liquids 424 Volatility 10

Warning nameplate 192 Water spray 135 Water spraying system automatic 135 Wear 20, 31, 41 Welding areas 366 Wind conditions 125, 144, 145, 146, 294

Wind direction 145 Wind velocity 11, 144, 145 Winding failure 137 Worst condition 121

Zone-0 425 locations 44 Zone-1 425 locations 43 Zone-2 425