Design of Combustible and Toxic Gas Detection Systems

December 2004

Process Industry Practices Process Control

PIP PCCA001 Design of Combustible and Toxic Gas Detection Systems

PURPOSE AND USE OF PROCESS INDUSTRY PRACTICES In an effort to minimize the cost of process industry facilities, this Practice has been prepared from the technical requirements in the existing standards of major industrial users, contractors, or standards organizations. By harmonizing these technical requirements into a single set of Practices, administrative, application, and engineering costs to both the purchaser and the manufacturer should be reduced. While this Practice is expected to incorporate the majority of requirements of most users, individual applications may involve requirements that will be appended to and take precedence over this Practice. Determinations concerning fitness for purpose and particular matters or application of the Practice to particular project or engineering situations should not be made solely on information contained in these materials. The use of trade names from time to time should not be viewed as an expression of preference but rather recognized as normal usage in the trade. Other brands having the same specifications are equally correct and may be substituted for those named. All Practices or guidelines are intended to be consistent with applicable laws and regulations including OSHA requirements. To the extent these Practices or guidelines should conflict with OSHA or other applicable laws or regulations, such laws or regulations must be followed. Consult an appropriate professional before applying or acting on any material contained in or suggested by the Practice.

This Practice is subject to revision at any time by the responsible Function Team and will be reviewed every 5 years. This Practice will be revised, reaffirmed, or withdrawn. Information on whether this Practice has been revised may be found at www.pip.org.

© Process Industry Practices (PIP), Construction Industry Institute, The University of Texas at Austin, 3925 West Braker Lane (R4500), Austin, Texas 78759. PIP member companies and subscribers may copy this Practice for their internal use. Changes, overlays, addenda, or modifications of any kind are not permitted within any PIP Practice without the express written authorization of PIP.

PIP will not consider requests for interpretations (inquiries) for this Practice.

Not printed with State funds

December 2004

Process Industry Practices Process Control

PIP PCCA001 Design of Combustible and Toxic Gas Detection Systems Table of Contents 1. Introduction .................................. 2 1.1 Purpose ............................................. 2 1.2 Scope................................................. 2

2. References ................................... 2 2.1 Process Industry Practices ................ 2 2.2 Industry Codes and Standards .......... 2 2.3 Government Regulations ................... 2

3. Definitions .................................... 3 4. Requirements ............................... 3 4.1 4.2 4.3 4.4 4.5

General Design.................................. 3 Equipment Location ........................... 4 Environment....................................... 4 Calibration and Testing...................... 4 Types of Combustible Gas Detectors.................................... 5 4.6. Toxic Gas Detectors .......................... 6 4.7. Control Modules and Alarms ............. 7

Process Industry Practices

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PIP PCCA001 Combustible and Toxic Gas Detection Systems Criteria

1.

December 2004

Introduction 1.1

Purpose This Practice provides the criteria for engineers for the design of detection and monitoring systems that are installed in plant areas to alert personnel of the presence of combustible or toxic gas.

1.2

Scope The systems addressed in these criteria are for permanently installed, fixed combustible and toxic gas monitors.

2.

References Applicable parts of the following Practices, industry codes and standards, and references shall be considered an integral part of this Practice. The edition in effect on the date of contract award shall be used, except as otherwise noted. Short titles will be used herein where appropriate. 2.1

Process Industry Practices (PIP) – PIP PCCA01D - Data Sheet for Combustible and Toxic Gas Monitors

2.2

Industry Codes and Standards •

American Petroleum Institute (API) – API Publication 2031 - Combustible Gas Detector Systems and Environmental and Operational Factors Influencing Their Performance.

•

The Instrumentation, Systems, and Automation Society (ISA) – ISA S12.13, Part I-1995 - Performance Requirements, Combustible Gas Detectors – ISA S12.13, Part II-1987 - Installation, Operation and Maintenance of Combustible Gas Detection Instruments – ISA 92.0.01 Part I-1998 - Performance Requirements for Toxic Gas-Detection Instruments: Hydrogen Sulfide

•

National Fire Protection Association (NFPA) – NFPA 101 - Life Safety Code – NFPA 325M - Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids for LEL of Gases

2.3

Government Regulations •

U.S. Department of Labor Occupational Safety and Health Administration (OSHA) – Part 1910 - Occupational Safety and Health Standards

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Process Industry Practices

PIP PCCA001 Combustible and Toxic Gas Detection Systems Criteria

December 2004

3.

Definitions LEL: Lower explosive limit, which is the minimum concentration of a gas in a gas/air mixture at which the mixture will explode if exposed to an ignition source owner: The party who owns the facility wherein combustible and toxic gas detection systems will be used. BPCS: basic process control system approved testing laboratories: FM: Factory Mutual UL: Underwriters Laboratory CSA International: formerly Canadian Standards Association NRTL: Nationally Recognized Testing Laboratories

4.

Requirements 4.1

General Design 4.1.1

The detection system shall measure and provide indication and alarms for specified concentrations of toxic gases or combustible gases in air.

4.1.2

The detection system shall be designed to a. Allow for a timely operator response. b. Prevent unauthorized system bypass or alarm reset. c. Initiate appropriate action.

4.1.3

The owner shall specify on PIP PCCA01D the location, number of detectors, and alarm limits for each detector.

4.1.4

The manufacturer’s requirements shall be followed as a minimum.

4.1.5

The system shall be composed of one or more detectors connected to a control unit or logic device as specified on PIP PCCA01D.

4.1.6

The control unit or logic device shall provide audible and visual alarms in the field and in attended control buildings.

4.1.7

The monitor and detector shall be approved for the electrical area classification where it will be installed and shall be approved by UL, FM, CSA International, or other recognized NRTL.

4.1.8

Refer to the following industry standards for additional information on system requirements: a. ISA S12.13, Part I-1995 b. ISA S12.13, Part II-1987 c. ISA 92.0.01 Part I-1998

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PIP PCCA001 Combustible and Toxic Gas Detection Systems Criteria

December 2004

d. API Publication 2031 e. OSHA Part 1910 f.

NFPA 325M

g. NFPA 101 4.2

4.3

4.4

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Equipment Location 4.2.1

Detector location shall be appropriate to detect the presence of specified combustible or toxic gases (i.e., lighter or heavier than air) in the area in which the detector is installed.

4.2.2

Point detectors shall normally be located adjacent to identifiable, single-spot, potential release locations having a significant risk of toxic or combustible gas leak (such as pump and gas compressor seals, valves, etc.).

4.2.3

Detectors shall not be located in areas having excessive vibrations.

4.2.4

Detectors shall be protected from direct sprays of oils and other liquids.

4.2.5

Prevailing wind direction shall be considered when detector location is selected.

4.2.6

If gas detection is required at the inlet of air ducts, pressure and airflow across the sensor shall be within manufacturer recommendations.

Environment 4.3.1

Each part of the detection system, alarms, and associated electronic circuits shall be suitable for the environment in which the part will be installed.

4.3.2

Detectors shall have automatic temperature compensation for ambient temperature and humidity changes.

4.3.3

As specified on the PIP PCCA01D data sheet, detectors shall be provided with guards for protection from mechanical damage, rain, water wash, strong wind, dust, and sand if required by site conditions. Sunshades shall also be furnished if required by site conditions.

4.3.4

Radio frequencies shall not interfere with any device in the system.

4.3.5

Detector assemblies shall not be painted or otherwise covered, which would prevent gas from diffusing into the sensor.

Calibration and Testing 4.4.1

Detectors shall be calibrated for the specified gas being monitored.

4.4.2

Detectors shall be designed to allow accurate, effective testing and calibration at regular periods.

4.4.3

Detectors shall be readily accessible for calibration and ease of replacement. If available, plug-in sensor heads shall be used.

4.4.4

Detectors shall be designed for testing without disabling the entire system or causing shutdown of equipment.

Process Industry Practices

PIP PCCA001 Combustible and Toxic Gas Detection Systems Criteria

December 2004

4.4.5 4.5

An alarm shall activate in a continuously occupied area when a detector is removed from service for any reason.

Types of Combustible Gas Detectors 4.5.1

General 4.5.1.1 The owner shall specify on PIP PCCA01D which type of combustible gas detector shall be used. 4.5.1.2 If more than one combustible gas is present at a particular location, the detector shall be calibrated for the hardest-to-detect (least sensitive) component gas as specified by owner. 4.5.1.3 Transmitter ranges and meter scales for combustible gas indication shall be 0-100% LEL (refer to NFPA 325M). 4.5.1.4 The use of point type or open-path detectors shall require the approval of the owner. 4.5.1.5 Each sensor shall be calibrated using a gas that will actuate the highhigh alarm level of the analyzer to verify analyzer and alarm operation. 4.5.1.6 Calibration frequency of the sensor shall be as specified by the manufacturer or as stated by the owner’s applicable standard.

4.5.2

Catalytic Bead Detectors 4.5.2.1 Applications and Restrictions

1. Catalytic bead types shall not be used where silicones or other compounds containing fluorine, chlorine, bromine, and iodine and heavy metals such as tetraethyl lead may be present. Comment: Sulfur compounds present in the area will shorten the life of the detector. 2. Catalytic bead type detectors need oxygen to operate properly and shall not be used in an atmosphere where no oxygen is present. 3. If multiple gases may be present, the gas of primary interest shall be specified, and the detector shall be calibrated for that gas. Comment: Easily oxidized fuels such as hydrogen may present a special case; for these gases, see manufacturer’s guidelines. 4. Continuous exposure to the target gas shall be avoided because it will consume the cell and cause premature failure of the sensor. 5. The detector manufacturer shall be required to furnish a conversion table or chart that converts the LEL of other gases to the LEL of the gas for which the detector is calibrated.

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PIP PCCA001 Combustible and Toxic Gas Detection Systems Criteria

December 2004

Comment: Catalytic bead type detectors will detect all combustible gases, not just the one for which the detector is calibrated. 6. The detector manufacturer shall be required to furnish a conversion table that converts from percentage of LEL to percentage of concentration in air by volume. 4.5.3

Infrared (IR) Detectors 4.5.3.1 Point Type IR Detectors

Point type detectors shall be specified for the following applications: a. Locations having inert gas backgrounds Comment: IR detectors can be used in locations that have inert gas backgrounds because such detectors do not require oxygen to operate. b. Locations having constant background of gases Comment: IR detectors are not poisoned by a constant background of gases and do not respond to hydrogen. c. If specific gases require detection Comment: IR detectors can be tuned to detect somewhat specific gases because they measure different hydrocarbons at specific wavelengths. 4.5.4

Open Path IR Detectors 1. The installation of an open path type IR systems shall not be installed without prior approval of the owner. 2. Open path IR detectors shall not be used in congested areas or in areas of high traffic. 3. The IR source and reflector shall be installed on a rigid structure to prevent movement caused by ambient atmospheric changes.

4.6.

Toxic Gas Detectors 4.6.1

The owner shall specify on PIP PCCA01D whether a toxic gas detector(s) shall be installed by a project and which gas the detector shall check.

4.6.2

Transmitter range and meter scales for toxic gas analyzers shall comply with owner’s specification.

4.6.3

The toxic gas detector shall be calibrated in accordance with the detector manufacturer’s procedure and shall meet all specified requirements. Comment:

4.6.4

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The life span of some toxic gas detectors will be shortened considerably if exposed to a constant background of the toxic gas being measured.

The detector “zero” shall be checked using a toxic-free gas.

Process Industry Practices

PIP PCCA001 Combustible and Toxic Gas Detection Systems Criteria

December 2004

4.7.

Control Modules and Alarms 4.7.1

General 4.7.1.1 The owner shall specify on PIP PCCA01D the control and logic philosophy for toxic or combustible analyzers. 4.7.1.2 The logic shall be designed to be fail-safe. 4.7.1.3 Normally open (shelf state) contacts shall be closed during normal operation and shall open on alarm. 4.7.1.4 If the system control is microprocessor based, it shall be field programmable and be a fully addressable system. 4.7.1.5 Programmed information shall be stored in non-volatile memory. 4.7.1.6 Unless each device can supply individual alarms, toxic and combustible gas control units shall not be installed together in the same rack. 4.7.1.7 Silence, trouble, test, and acknowledge functions shall be included in the design of the logic as specified by the owner. 4.7.1.8 A system alarm shall be activated upon the failure of any sensor.

4.7.2

Multi-Channel Control Units 4.7.2.1 Multi-channel control units shall have individual alarms for each separate channel. 4.7.2.2 When control modules are used, each module shall have two adjustable (0-100%) alarms for high and high-high levels. 4.7.2.3 The alarm functions shall be visibly displayed from the front of the control module by indicating lamps. 4.7.2.4 Each control module shall incorporate self-checking circuitry so that if a component fails, a common “failure” light on the control module shall illuminate to indicate system malfunction. 4.7.2.5 Each channel shall have adjustments for span, zero, and alarm set points and shall be capable of being individually adjusted without affecting any other channel.

4.7.3

Field-Mounted Control Units 4.7.3.1 Each unit shall have independent alarm adjustments for high and high-high levels. 4.7.3.2 If field-mounted control units are to be two-wire type transmitters, alarms shall be initiated in the appropriate logic solver. 4.7.3.3 Field-mounted control units shall have self-diagnostic features and shall generate a failure alarm upon detection of loss of a correctly functioning system.

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PIP PCCA001 Combustible and Toxic Gas Detection Systems Criteria

4.7.4

December 2004

Indication and Field-Mounted Units 4.7.4.1 Control units and field-mounted units shall incorporate an integral digital readout device to indicate the level of gas concentration for each channel. 4.7.4.2 The system shall be designed for field indication of visual and/or audible alarms for any sensor that is in alarm condition as specified by owner.

4.7.5

System Alarm Design Strategy 4.7.5.1 The design of the systems shall enable quick identification of any specific location at the facility that is being monitored. 4.7.5.2 The failure of any component in the system shall generate a malfunction alarm. 4.7.5.3 Each combustible gas or toxic gas detector shall have individual alarms that indicate a high and high-high alarm when a leak is detected. 4.7.5.4 Gas detector alarm types (i.e., audible, colored beacon) shall be as specified by the owner on PIP PCCA01D. Comment: Even within the same company different locations may have different requirements and strategies for types of alarms to be installed. 4.7.5.5 Combustible or toxic gas detection system alarms shall be selflatching and shall reset manually. 4.7.5.6 The operational function shall be documented (logic diagrams, true table, P&ID), clearly identifying which detector activates which beacon or horn.

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Process Industry Practices