FMDS0549

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September 1987

Revised January 2000

Page 1 of 11

GAS AND VAPOR DETECTORS AND ANALYSIS SYSTEMS

Table of ContentsPage

1.0 SCOPE ................................................................................................................................................... 2

1.1 Changes .......................................................................................................................................... 2

2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 2

2.1 Introduction ...................................................................................................................................... 2

2.2 Electrical .......................................................................................................................................... 2

2.2.1 Power Supplies ..................................................................................................................... 2

2.3 Equipment and Processes .............................................................................................................. 2

2.3.1 Alarms and Supervisory Devices .......................................................................................... 2

2.4 Operation and Maintenance ............................................................................................................ 3

2.4.1 Initial Tests and Acceptance .................................................................................................. 3

2.4.2 Maintenance and Testing Systems ....................................................................................... 32.5 Training ............................................................................................................................................ 3

3.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................... 3

3.1 General ............................................................................................................................................ 3

4.0 REFERENCES ....................................................................................................................................... 4

4.1 FM Global ........................................................................................................................................ 4

APPENDIX A GLOSSARY OF TERMS ....................................................................................................... 4

APPENDIX B DOCUMENT REVISION HISTORY ....................................................................................... 4

APPENDIX C SUPPLEMENTARY INFORMATION ..................................................................................... 4

C.1 Description ...................................................................................................................................... 4

C.2 Application ....................................................................................................................................... 6

C.3 Operation ........................................................................................................................................ 6

C.3.1 Thermal Conductivity-Combustibles ..................................................................................... 7

C.3.2 Thermal Type Paramagnetic-Oxygen ................................................................................... 7

C.3.3 Non-dispersive Infrared ......................................................................................................... 7

C.3.4 Ultraviolet (uv) Absorption ..................................................................................................... 8

C.3.5 Zirconium Oxide Cell-Oxygen ............................................................................................... 9

C.3.6 Electrochemical Method—Micro-Fuel Cell-Oxygen ............................................................ 10

C.3.7 Gas Chromatography ......................................................................................................... 10

C.3.8 Flame Ionization-Total Hydrocarbons ................................................................................. 11

List of FiguresFig. 1. Thermal conductivity. .......................................................................................................................... 7

Fig. 2. Thermal paramagnetic. ....................................................................................................................... 8

Fig. 3. Non-dispersive infrared. ...................................................................................................................... 8

Fig. 4. Ultraviolet absorption. ......................................................................................................................... 9

Fig. 5. Zirconium oxide cell. ........................................................................................................................... 9

Fig. 6. Electromechanical-micro-fuel cell. .................................................................................................... 10Fig. 7. Gas chromatography. ........................................................................................................................ 10

Fig. 8. Flame ionization. ............................................................................................................................... 11

FM GlobalProperty Loss Prevention Data Sheets 5-49

©1987 Factory Mutual Insurance Company. All rights reserved. No part of this document may be reproduced,stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical,photocopying, recording, or otherwise, without written permission of Factory Mutual Insurance Company.

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1.0 SCOPE

The basic methods of detecting gases or vapors and the general aspects of detection analysis systems are

discussed. References are made to data sheets concerning specific occupancies and hazards where theapplication of such detectors are recommended.

1.1 Changes

January 2000. This revision of the document has been reorganized to provide a consistent format.

2.0 LOSS PREVENTION RECOMMENDATIONS

2.1 Introduction

The application recommendation for specific occupancies may be found in the data sheets concerning the

hazard and occupancy (see Section 4.0, References).

Factory Mutual Research Approved equipment and devices are preferred by FM Global, and should be used

when available and suitable for the application. ‘‘Approved’’ means equipment tested by the Factory Mutual

Research Corporation and listed in the Factory Mutual Research Approval Guide .

Because of processes and arrangement of many of the installations, Factory Mutual Research Approved

equipment may not be available to meet specific installation arrangements or particular field operating

conditions. In these cases, the equipment chosen should be from a reliable manufacturer and have a proven

satisfactory field experience.

2.2 Electrical

2.2.1 Power Supplies

2.2.1.1 The power supply for electrical detection and any actuation devices should be independent of the

supply for the equipment and the hazard area. Where this is not practical, use pneumatic or mechanical

devices or provide an emergency battery-powered supply with automatic switchover if the primary supply fails.

The batteries should have the capacity to operate the system for at least 24 hours.2.2.1.2 For critical and important occupancies, an alternate power supply should be provided for any electri-

cally operated detection and actuation system. An emergency battery-powered supply, with automatic

switchover if the primary supply fails, is an acceptable alternate power supply. The electric power supply

should not be exposed by the hazards in the protected area.

2.2.1.3 Wiring, cables and tubing should be located and protected to avoid mechanical damage. Tubing in

vulnerable locations should be in conduit or equivalent. Conduit is not needed for short lengths of cables or

tubing near detectors and controls.

2.3 Equipment and Processes

2.3.1 Alarms and Supervisory Devices

2.3.1.1 An audible alarm should be provided to sound when the system operates and continue until reset

manually.

2.3.1.2 Where a system is provided for valuable and important structures, equipment or contents such as

chemical processing equipment, multi-zone oven or a remote processing or pumping station, the detection

devices and circuits should be supervised. Trouble alarms distinctive from the operation alarms should be

provided to sound, preferably at a constantly attended location.

5-49 Gas and Vapor Detectors and Analysis SystemsPage 2 FM Global Property Loss Prevention Data Sheets

©1987 Factory Mutual I nsurance Company. All rights reserved.

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2.4 Operation and Maintenance

2.4.1 Initial Tests and Acceptance

2.4.1.1 The installer or manufacturer’s representative should make turnover tests of the completed installa-

tion. These tests should be witnessed by the purchaser and can be used to train selected plant personnel.

Turnover tests should determine that the system has been properly installed and will operate as intended.

Specifically, the tests should check the operation of all components. Records should be retained indicat-

ing the test results for future reference.

2.4.1.2 All controls should be properly identified as to function, area controlled, and operating instructions.

2.4.2 Maintenance and Testing Systems

2.4.2.1 Systems should be maintained in operating condition at all times and restored to service promptly

after any impairment. A program of scheduled inspections, tests and maintenance is essential and should

include the following:

1. Provide regular maintenance in accordance with manufacturer’s instructions and as determined neces-

sary by the experience derived from the operation of the detector and the associated equipment.

2. Maintain records of periodic performance calibration and maintenance checks. The records should be

reviewed by supervisory management whose responsibility includes both safety and production.

3. The periodic inspections and tests should be done in accordance with manufacturer’s guidelines. These

should include but not be limited to the following:

a) Check calibration, zero and span adjustments (at least weekly). A known calibration gas as specified

by the manufacturer should be used.

b) Check the operation of any controls and alarms at the designated set point (at least monthly).

c) Check the electrical connections and electronics at detectors (remote), and for the controls and alarms

(at least monthly).

d) Check sample lines for leaks, obstructed filters and flame arrestors, proper flow and condensate (atleast monthly).

e) Check primary and secondary power supplies (batteries) for adequate power and reliability (at least

monthly).

f) Check equipment or location which might contain flammable or otherwise hazardous atmosphere. Test

and verify that the atmosphere is suitable before entering or making repairs and tests.

2.5 Training

2.5.1 Train instrument personnel and equipment operators in the proper operation and specific function of

the detectors and the associated system. Operating instructions should be available for ready reference.

3.0 SUPPORT FOR RECOMMENDATIONS

3.1 General

System inspection maintenance and testing are outlined because operational reliability is of major importance

in providing early warning of impending hazardous conditions and in taking corrective action.

Gas and Vapor Detectors and Analysis Systems 5-49FM Global Property Loss Prevention Data Sheets Page 3


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4.0 REFERENCES

4.1 FM Global

Data Sheet 6-2/12-63, Pulverized Coal-Fired Boilers .

Data Sheet 6-4/12-69, Oil and Gas-Fired Single Burner Boilers .

Data Sheet 6-5/12-70, Oil or Gas Fired Multiple Burner Boilers .

Data Sheet 6-7/12-7, Fluidized Bed Combustors and Boilers.

Data Sheet 6-8/12-68, Combustion Air Heaters.

Data Sheet 6-9, Industrial Ovens and Dryers.

Data Sheet 6-10, Process Furnaces.

Data Sheet 6-11, Fume Incinerators.

Data Sheet 6-13/12-13, Waste Fuel Fired Boilers.

Data Sheet 6-17/13-20, Rotary Kilns and Dryers.

Data Sheet 6-21/12-21, Chemical Recovery Boilers.

Data Sheet 6-23/17-15, Black-Liquor Recovery Boilers-Direct Contact Evaporators.

Data Sheet 6-24/13-21, Coal Pulverizers and Pulverizing System.

Data Sheet 7-2, Waste Solvent Recovery.

Data Sheet 7-13, Mechanical Refrigeration.

Data Sheet 7-34, Electrolytic Chlorine Processes.

Data Sheet 7-45, Instrumentation and Control in Safety Applications.

Data Sheet 7-46/17-11, Chemical Reactors and Reactions.

Data Sheet 7-52, Oxygen.

Data Sheet 7-59, Inerting and Purging of Tanks, Process Vessels and Equipment.

Data Sheet 7-73, Dust Collectors and Collection Systems.

Data Sheet 7-95, Compressors.

APPENDIX A GLOSSARY OF TERMS

Gas detector: device that responds to a specific concentration or range of concentrations of gaseous or

vaporous substances or compounds. (See Section C.1 Description)

APPENDIX B DOCUMENT REVISION HISTORY

This document does not have any revision history.

APPENDIX C SUPPLEMENTARY INFORMATION

C.1 Description

Gas detectors or analyzers give a response to a specific concentration or range of concentrations of gaseous

or vaporous substances or compounds. These substances may be detected in the ‘‘normal’’ atmosphere

in or around process or experimental equipment. Some devices are designed to detect specific gases in low

oxygen or oxygen-free atmospheres. The detector and associated system may analyze continuous samples

or an isolated sample.



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Most detectors have meters for visual indication of the concentration, usually expressed in parts per million

or percentage. These may also have set points with electrical contacts which permit signaling when the set

point or limit has been reached.

Common uses include in-line process control, fire detection or explosion prevention and detection of poten-

tial atmosphere or process contaminants. These devices have the capability for continuous or repetitive

measurement of specific gas or vapor concentration in the atmosphere. The detector may be incorporated

into a system to control the concentration of the gas or vapors. Alarm and corrective action can be initiated

when the concentration point or concentration range is insufficient or is exceeded. A continuous recording

device may be incorporated to print a record of the gas concentration.

The detector elements may be located either remote from the sampling point or at the sampling point. The

in-site detectors simplify the sampling method but erosion, corrosion and temperature of the gas stream can

present problems. The environment around the installation outside the sampled gas stream must be consid-

ered for protection of the associated mechanical, electronic and electrical equipment. For a remote detec-

tor element, a forced flow is frequently used to transport the sample to the detector element. This usually

complicates the system by requiring a pump-blower arrangement. As soon as the sample leaves its origi-

nal environment enroute to the sensor, care must be taken to prevent detrimental alteration of the sample.

Condensation, particulate matter and contamination are common problems.

Some detectors use a media which is sensitive to a specific substance. The media compound may change

color or composition when exposed to a certain gas or vapor. These may be used for ‘‘grab sample’’ instan-

taneous tests or accumulative sample long term exposures for time weighted averages. Most detectors of this

type require that the sensitized material be discarded after use, although some substances may be regen-

erated for repeated use.

Some detection and measuring methods are only suitable for laboratory conditions. The processing method,

equipment size, processing time, sensitivity of the equipment to temperature, atmosphere corrosion and

vibration are all restrictive factors. In some instances, the same sample processing method may be used

in the laboratory and the field with either portable or fixed equipment. The accuracy and sensitivity may vary

with the analysis method, the detector design, and the sample collecting method.

The required accuracy and range of the detectors will vary with the application and the detector’s design.

A cold storage food warehouse with an ammonia refrigeration system could need a detector for concentra-tions in the range of 5 to 25 ppm and above. A flammable vapor detector for a coil coater drier would measure

concentration in percent of the lower flammable limit or between 0 and about 5000 ppm depending upon the

solvent.

Industrial equipment is expected to have an accuracy of ±1% and a reproducibility of ±3% of full scale

concentration or 10% of actual gas-vapor concentrations.

Gas detectors are used in control systems of the relatively unsophisticated hard wired electromagnetic relay

type. There is a trend to incorporate gas detectors with a microprocessor based, closed loop analytical con-

trol for process analysis and control feedback. The control gas detector and a dedicated safety limit gas

detector may be similar devices, but interfaced into different control systems. Each is important to the proper

continuous operation of a system. The control analyzer is in continuous service or is available during the

process as a production control device. The safety limit analyzer must also be constantly in service ready

to signal actuation of the emergency operations, shutdown, bypass and alarm. This actuation would occur

when the hazardous limits were approached due to the failure, malfunction or control inability of the process

analyzer and associated controls.

Because of the degree of reliance upon the dedicated safety limit gas detection, it is important that the sys-

tem have adequate reliability in circuitry and power supplies. In some instances, the safety limit gas detec-

tor system may need to be entirely independent of the process control microprocessor and its system.



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C.2 Application

An isolated sample taken manually would be a simple application procedure. This method would measure

the concentration at a specific time and with the conditions of the sample and its environment at that time.Repetitive sampling during a test cycle or production process is a procedure commonly used for verifying

the adequacy of safety ventilation for a solvent evaporation oven or the concentration of a fire extinguish-

ing or an inerting agent.

A slightly more complex system would be a single detector continuously sampling a specific area. This can

be limited to a type of detector with the capability of continuously analyzing the compound specified.

For sampling multiple points, two methods are available:

1. A multiplicity of detectors each at a sample point transmitting to a central controller. The controller may

query the status of each detector in a rotational basis or it may respond to a signal for a condition varying from

a present set point.

2. A single detector with a multiplicity of sample draw tubes working on a rotational basis to analyze a sample

from each tube. This method usually requires several minutes to draw and process samples. In some

instances, response time could be excessive. A hazardous condition could occur in the processing equip-ment before the detector could sense the increased concentration.

High dust concentrations in the sampled atmosphere may make the continuous and reliable operation of a

detector nearly impossible. In these instances, the location of the sampling probe may be located down-

stream of a dust removal device, such as an electrostatic precipitator. This method has proven effective on

black liquor recovery boiler applications.

Some of the Data Sheets which mention the application of gas-vapor detectors and analyzers are listed in

Section 4.0, References.

C.3 Operation

The property of the gas or vapor component being measured and the operating principle of some commonly

employed detectors may be classified or grouped as follows:

1. Thermal properties.

2. Electrical or magnetic properties.

3. Radiant energy properties.

4. Chemical properties.

5. Combinations of the above.

a) Electrochemical.

b) Chromatographic.

c) Flame ionization.

The basic principles by which some detector types operate are described as follows:

Chemiluminescence — NO, NO2

, NOx

(NO + NO2

) and Ozone

When the sample containing nitric oxide (NO) is combined with ozone (03

) the reaction releases light energy

in measurable wave lengths, 6000A (600 mm) and above. The light intensity is proportional to the nitric oxide

(NO) concentrations. When a thermal catalytic NOx

converter is used in conjunction with the chemilumi-

nescence NO analyzer, the oxides of nitrogen are converted to NO and the analyzer can determine the total

NOx

concentration. With the conversion of the NOx

, N02

can be determined by the difference.



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C.3.1 Thermal Conductivity-Combustibles

The flammable gas samples and air are passed over a tungsten or platinum alloy filament. This catalyst is

heated to a low temperature which promotes an oxidation reaction. The temperature increase due to the reac-tion is proportional to the combustibles in the sample. The increased conductivity of the filament due to tem-

perature increase is proportional and is measured by a Wheatstone bridge circuit. Filament materials may

become corroded, coated or poisoned by presence of silicon, lead or phosphorous compounds in the gases.

C.3.2 Thermal Type Paramagnetic-Oxygen

Magnetic fields attract oxygen. Using this effect, the oxygen content of a gas can be measured by passing

the gas through two parallel passages which have an interconnecting crossover. When an energized elec-

tromagnetic is positioned at one end of the crossover, flow through the crossover will occur. This ‘‘magnetic

breeze’’ is proportional to the oxygen content. The breeze can be measured by its cooling effect on a heated

wire placed in the crossover and connected to a Wheatstone bridge.

C.3.3 Non-dispersive Infrared Energy paths of separate infrared sources pass respectively through sample and reference cells. The energy

sources are chopped to allow energy to pass through these cells alternately, with the presence of an absorb-

ing gas in the sample cell. The microphone-diaphragm chamber heats up. Movement of the diaphragm

changes the microphone capacity thereby generating an electrical output signal. The signal produced by the

imbalance between the sample and reference cells is a function of gas concentration in the sample cell.

Solid state detectors are becoming more common.

Fig. 1. Thermal conductivity.



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C.3.4 Ultraviolet (uv) Absorption

Ultraviolet absorption analyses are used to determine the concentration of a component in a gas mixturebased on its specific wave-length absorption characteristics in the uv region. Components suited for uv

absorption analysis techniques include chlorine, hydrogen, hydrogen sulfide, sulfur dioxide, nitrogen oxides,

and many ignitable solvents. Use of monochromatic radiation is essential to measurements of the specific

absorption by these components. Monochromators may employ prism or grating systems, or optical uv

interference filters.

The following diagram illustrates simultaneous analysis of two components in a gas mixture passing through

a flue gas stack by use of a grating monochromator arrangement. Various wavelengths of radiation from a

uv source passing through the stack are absorbed according to the kind and amounts of gas components in

Fig. 2. Thermal paramagnetic.

Fig. 3. Non-dispersive infrared.



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the mixture. The radiation reaching a mirror is reflected onto a grating and dispersed according to wave-

length. Finite wavelength ranges corresponding to absorption characteristics are measured by a slit and

phototube detector arrangement. The proportionality of this absorption output measured against a reference

radiation output yields the concentration of the gas being analyzed.

C.3.5 Zirconium Oxide Cell-Oxygen

Zirconium oxide becomes a conductor of oxygen ions (O2-) at relatively high temperatures. At these

temperatures, measurable signals are produced in a zirconium oxide cell in response to an oxygen partial

pressure differential. The response to the oxygen concentration is logarithmic having the greatest sensitivity

at the lower concentration level. The platinum heating element is integral to temperature sensors in main-

taining constant temperature of the cell which is sensitive to temperature change. Platinum gauze or porous

coating electrodes on inner and outer cell faces serve as conductors.

Fig. 4. Ultraviolet absorption.

Fig. 5. Zirconium oxide cell.



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C.3.6 Electrochemical Method—Micro-Fuel Cell-Oxygen

The oxidation-reduction cell has a membrane permeable to oxygen. Oxygen from the sample diffuses through

the membrane into the cell where it is reduced on the surface of the cathode. Oxidation occurs on the anodegenerating a voltage proportional to the oxygen concentration.

C.3.7 Gas Chromatography

A gaseous sample introduced into the chromatograph is moved by a carrier gas (mobile phase) through a

tube (column) packed or lined with solid or liquid materials either separately or in combination (stationary

phase). The individual gaseous components of a gas mixture are separated according to differences in their

physical behavior in the stationary phase which cause them to be eluted from the column more or less sepa-

rated in time. The eluted components are then monitored by a detector whose signal displays a series ofpeaks corresponding to the respective gas components (chromatogram). From the chromatogram, two types

of peak data yielding qualitative and quantitative information may be obtained: emergence or retention time,

and integral characteristics. The retention time for each peak is used to identify its gas component, and the

integral characteristics of each component’s peak is used to determine its relative or absolute amount.

Fig. 6. Electromechanical-micro-fuel cell.

Fig. 7. Gas chromatography.



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C.3.8 Flame Ionization-Total Hydrocarbons

The sample gas is fed into a hydrogen fueled flame ionization chamber where the gases are burned. The

conductivity of the resulting ionized gases, positively charged carbon ions, are measured. The conductivityis proportional to the carbon concentration in the sample. Below the lower flammable limits the conductivity is

nearly a direct ratio to the combustible concentration or percentile of the lower flammable limit.

FM Engr. Comm. July 1987

Fig. 8. Flame ionization.


©1987 Factory Mutual I nsurance Company All rights reserved

FMDS0549

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