Flame Arresters in Piping Systems

API RECOMMENDED PRACTICE 2028THIRD EDITION, FEBRUARY 2002

Flame Arresters in Piping Systems

Safety & Fire Protection

API RECOMMENDED PRACTICE 2028THIRD EDITION, FEBRUARY 2002

SPECIAL NOTES

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Copyright © 2002 American Petroleum Institute

FOREWORD

This recommended practice was prepared under the direction of the API Safety and FireProtection Subcommittee. This third edition of API 2028

Flame Arresters in Piping Systems

has been extensively rewritten and updated from the previous edition. Appendices to the doc-ument are intended to provide additional supplementary information.

This guide was prepared to help provide a basic understanding of ßame arresters used inpiping systems. The information presented is based primarily upon experience in the petro-leum industry. It is not intended to exclude or limit the use of other approaches of compara-ble merit. Because of the special nature of ßame arresters, especially those used fordetonation protection, this recommended practice strongly encourages dialogue with theequipment supplier and the use of sound engineering judgement in ßame arrester selectionand application.

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CONTENTS

Page

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Concept of Hazard vs. Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Retroactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 REFERENCED PUBLICATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3 DEFINITIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

4 COMBUSTION AND FLAME PROPAGATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.2 Combustion Rates and MESG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.3 Deßagration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44.4 Detonation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5 FLAME ARRESTER FUNCTION AND CONCERNS FOR USEIN PIPING SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45.1 Flame Arrester Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45.2 Pressure Concerns and Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45.3 Potential Effects of Installation Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.4 Flame Arresters Not Using Metal Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.5 Pyrophoric Iron SulÞde Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65.6 Unilateral and Bilateral Flame Arresters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

6 LIMITATIONS OF FLAME ARRESTERS ON TANK VENTS. . . . . . . . . . . . . . . . . 6

7 FLAME ARRESTER TESTING AND CERTIFICATION . . . . . . . . . . . . . . . . . . . . . 77.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77.2 Deßagration and Detonation Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77.3 Flame Retention Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77.4 SigniÞcance of MESG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77.5 Use of Established Test Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

8 SUMMARY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

APPENDIX A BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9APPENDIX B GASES OR VAPORS WITH A MAXIMUM EXPERIMENTAL

SAFE GAP (MESG) < 0.90 MM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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Flame Arresters in Piping Systems

SECTION 1—INTRODUCTION

1.1 PURPOSE

This recommended practice is intended to inform industryabout limitations of ßame arresters installed in piping systems.Concerns about potential environmental effects of hydrocar-bon and chemical vapor emissions have led to regulationsrequiring the installation of vapor control systems. In theUnited States, for marine transfer of oil or hazardous materi-als, United States Coast Guard regulations require installationof ßame arresters (suitable to interrupt a detonation) in vaporcontrol piping. These USCG regulations speciÞcally direct (indetail) where to install these ßame detonation arresters in thevapor control systems. An independent laboratory must testdetonation arresters installed to meet these regulations.

The diversity of commercial ßame arresters can lead to theinstallation of these arresters in piping systems where the con-ditions within the piping may be signiÞcantly different fromthe conditions for which they were designed, or tested andlisted by testing laboratories. Under certain conditions, ßamespropagating through piping systems can reach velocities andpressures at which detonation can occur. Unless a ßamearrester has been designed and tested for a detonation, it maynot stop the progression of a combustion wave in the piping.Guidance is provided concerning the important factorsinvolved in the selection, installation and maintenance ofappropriate ßame arresters. The intent is to assist the user ofthis recommended practice in developing the awareness ofreview needs, and to encourage discussions with ßame arrestermanufacturers regarding speciÞc applications and test results.

1.2 SCOPE

The scope of this recommended practice is the use and lim-itations of ßame arresters installed in piping systems in thepetroleum and petrochemical industries. It provides a general

overview of ßame arresters currently in use and some poten-tial concerns or limitations. Applicable combustion and ßamepropagation parameters are discussed including the distinc-tion between arresting ßames versus arresting detonations.

This recommended practice is neither a design manual nora regulatory compliance document. It does provide referenceto more detailed technical discussions of ßame arresters andcombustion. Various standards, codes, and regulations arenoted in the Section 2 references and in the Appendix A Bib-liography.

1.3 CONCEPT OF HAZARD VS. RISK

Hazards are properties of materials with the inherent abilityto cause harm. Flammability, toxicity, corrosivity, stored chem-ical or mechanical energy all are hazards associated with vari-ous industrial materials. Risk requires exposure. Theßammability of a material transported in piping is an inherenthazard, but becomes a risk only when having access to an oxi-dizer and being exposed to an ignition mechanism. There is norisk of ignition when there is no potential for those exposures.Determining the level of risk involves estimating the probabil-ity and severity of exposure conditions that could lead to harm.

1.4 RETROACTIVITY

Any provisions in this recommended practice related todesign are intended for reference when designing new facili-ties or when considering major revisions or expansions. It isnot intended that any recommendations in this recommendedpractice be applied retroactively to existing facilities unlessdeemed appropriate based on facility review. Each facilitymust make their own determination regarding how to complywith any applicable regulations.

SECTION 2—REFERENCED PUBLICATIONS

The most recent edition or revision of each of the followingstandards, codes, and regulations are cited in this recom-mended practice. Additional references not speciÞcally citedin this document are listed in the Bibliography, Appendix A.

API

1

Std 2000

Venting Atmospheric and Low-PressureStorage Tanks

RP 2210

Flame Arresters for Vents of Tanks StoringPetroleum Products

ASTM

2

F 1273

Standard SpeciÞcation for Tank VentFlame Arresters

1

www.api.org

2

American Society for Testing and Materials, 100 Barr HarborDrive, West Conshohocken, Pennsylvania, USA 19428.www.astm.org

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CEN

3

EN 12874

Flame Arresters, Performance Require-ments, Test Methods and Limits for Use

CGA

4

G-1.2

Acetylene Metering and PipelineTransmission

FM

5

Approval Guide, A Guide to Equipment,Materials & Services Approved by FactoryMutual Research for Property Conservation

¥ÒFlame Arresters for Gas Piping SystemsÓ¥ÒFlame Arresters, Dry TypeÓ¥ÒFlame Arresters, Hydraulic TypeÓ¥ÒDetonation Flame Arresters for Flamma-

ble Vapor Piping SystemsÓ¥ÒFlame Arresters for Storage Tank Vent

PipesÓ

IEC

6

IEC 79-1A

First Supplement to Publication 79-1, Elec-trical apparatus for explosive gasatmospheres, Part 1: Construction and testof ßameproof enclosures of electricalapparatus

Appendix D: Method of test for ascertain-ment of maximum experimental safe gap

NFPA

7

30

Flammable & Combustible Liquids Code

69

Standard on Explosion Prevention Systems

OSHA

8

1910.106 Subpart HÑ

Hazardous Materials

UL

9

UL 525

Standard for Safety for Flame Arresters

UL Gas and Oil Equipment Directory

USCG

10

33

CFR

154

Facilities Transferring Oil or HazardousMaterial in Bulk

¥Subpart E, Vapor Control Systems

¥Appendix A to Part 154ÑGuidelines forDetonation Flame Arresters

¥Appendix B to Part 154ÑStandard Speci-Þcation for Tank Vent Flame Arresters

SECTION 3—DEFINITIONS

3.1 autoignition temperature:

The minimum tempera-ture at which a material will ignite with self-sustained combus-tion without an external source of ignition (such as a spark orßame).

3.2 deflagration:

A combustion wave that propagatessubsonically (as measured at the pressure and temperature ofthe ßame front) by the transfer of heat and active chemicalspecies to the unburned gas ahead of the ßame front.

3.3 detonation:

A reaction in a combustion wave propa-gating at sonic or supersonic velocity (as measured at the pres-sure and temperature of the ßame front). A detonation is stablewhen it has a velocity equal to the speed of sound in the burntgas or may be unstable (overdriven) with a higher velocity andpressure.

3.4 explosion:

A rapid release of energy (such as burn-ing) which produces a pressure wave.

3.5 hazard:

An inherent chemical or physical propertywith the potential to do harm (ßammability, toxicity, corrosiv-ity, stored chemical or mechanical energy).

3.6 inerted:

(For vessels under U.S. Coast Guard regula-tions.) Means the oxygen content of the vapor space in a tankvesselÕs cargo tank is reduced to 8% by volume or less, inaccordance with the inert gas requirements of 46

CFR

32.53or 46

CFR

153.500.

3.7 maximum experimental safe gap (MESG):

TheMESG is the maximum clearance between two parallel metalsurfaces that has been found, under speciÞed test conditions,to prevent an explosion in a test chamber from being propa-gated to a secondary chamber containing the same gas or

3

European Committee for Standardization, rue de Stassart 36, B-1050 Brussels, Belgium. www.cenorm.be

4

Compressed Gas Association, Inc., Fifth Floor, 4221 Walney Road,Chantilly, Virginia 20151-2923. www.cganet.com

5

Factory Mutual Insurance Company, 22055 Network Place, Chi-cago, Illinois 60673-1220. www.fmglobal.com

6

International Electrotechnical Commission, 3 rue de Varemb�, Casepostale 131, 1211 Geneva 20, Switzerland. www.iec.ch

7

National Fire Protection Association, Batterymarch Park, Quincy,Massachusetts 02269. www.nfpa.org

8

U.S. Department of Labor, Occupational Safety and Health Admin-istration, 200 Constitution Avenue, N.W., Washington, D.C. 20210.OSHA regulations are posted on, and can be downloaded from, theOSHA web site. www.osha.gov

9

Underwriters Laboratories, Inc., 333 PÞngsten Road, Northbrook,Illinois 60062. www.ul.com

10

United States Coast Guard, U.S. Department of Transportation.www.uscg.mil. The Code of Federal Regulations is available fromthe U.S. Government Printing OfÞce, Washington, D.C. 20402.

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vapor at the same concentration. The MESG factor wasdeveloped for designing electrical equipment for use in haz-ardous atmospheres.

3.8 pyrophoric:

Iron sulÞde or carbonaceous materialswhich, when exposed to air, can oxidize and heat, providing asource of ignition if a ßammable vapor/air mixture is present.

3.9 risk:

The probability of exposure to a hazard whichcould result in harm to personnel, property, the environmentor the general public.

3.10 risk assessment:

The identiÞcation and analysis,either qualitative or quantitative, of the likelihood and out-come of speciÞc events or scenarios with judgements of prob-ability and consequences.

3.11 risk-based analysis:

A review of potential needsbased on a risk assessment

3.12 self-ignition temperature:

See autoignition tem-perature.

SECTION 4—COMBUSTION AND FLAME PROPAGATION

4.1 GENERAL

This discussion of the combustion of gases or vaporsemphasizes combustion phenomena in piping. This back-ground focuses on understanding the functioning and poten-tial problems when ßame arresters are used in piping systems.A ßame arrester may not function or provide the desired pro-tection if it has not been designed for (or tested at) conditionsappropriate for the process in which it is to be installed (pres-sure, temperature, and fuel type).

For combustion to occur, the gas or vapor must be mixedwith an oxidizer and must be within the ßammable limits forthe mixture. Typically, the oxidizer is the oxygen contained inair. Combustion within piping can occur even if the amount ofoxygen within the piping is signiÞcantly below the normal20.8% concentration of oxygen in air. It is a typical reÞneryand chemical plant safe operating practice to maintain processpiping at or below an oxygen concentration of 5%. The UnitedStates Coast Guard (USCG) regulations for marine transfervapor collection systems specify that when analyzers arerequired to be used, the process shall be shut down if the oxy-gen concentration increases to 8% or greater. As pressuresincrease, the level of oxygen required to have a combustiblemixture decreases. And, as the temperature of a ßammable gas-eous mixture is increased, the ßammable limits increase orwiden. So, at elevated temperatures and pressures, a combus-tion reaction will be initiated more easily, and the reaction willproceed faster.

Combustion reactions involving hydrocarbons or othercombustible gases or vapors in an 100% oxygen environmentare rapidly (explosively) accelerated. The presence of oxidiz-ing agents, such as chlorine, ßuorine, nitrate salts, perchloratesalts, or peroxides, in a process stream can allow combustionto occur in the absence of oxygen or air. Unless conÞrmed bymanufacturerÕs tests, a ßame arrester may not have beendesigned for or tested for use in these special circumstances.

Industry studies have documented many accidents where asigniÞcant contributing cause of the accident was the failureto maintain a piping system free of oxygen. This should berecognized during the process design, start-up, operation,

shutdown and during maintenance activities requiring theopening of process piping or equipment.

Flame arrestersshould not be used as a substitute for proper process designand operation.

4.2 COMBUSTION RATES AND MESG

The combustion reaction rate for some particular gases orvapors, such as acetylene, hydrogen, or oleÞnic hydrocar-bons, is signiÞcantly accelerated over rates for normal hydro-carbons. Specialty ßame arresters offered to quench ßamesfor such sensitive materials should be conÞrmed by manufac-turerÕs tests for the speciÞc type of service, material, tempera-ture, pressure and piping conÞguration.

The Maximum Experimental Safe Gap (MESG) conceptwas developed for designing electrical equipment for use inhazardous atmospheres. MESG is deÞned as the maximumclearance between two parallel metal surfaces that has beenfound, under speciÞed test conditions, to prevent an explosionin a test chamber from being propagated to a secondarychamber containing the same gas or vapor at the same con-centration. Some standards-making organizations and regula-tory authorities have utilized a MESG threshold of 0.90 mmbelow which special testing of a ßame arrester is required. Alist of hydrocarbon and chemical gases or vapors which havebeen identiÞed as having a MESG below 0.90 mm is pro-vided in Appendix B along with some typical hydrocarbonsand alcohols for comparison.

Since the MESG is a factor developed for the design ofelectrical equipment in hazardous atmospheres (see IEC 79-1A), it does indicate gases and vapors with high combustionrates. However, it should not be used as the only determiningfactor when evaluating the suitability of a ßame arrester. Themolecular structure of a gas may also indicate that a morerapid combustion reaction is possible, such as with reactivemolecules containing double or triple bonds, or moleculescontaining oxygen or another oxidizer, and nitrates. Givenenough turbulence generation, it is possible for the combus-tion reaction rate of a gas or vapor with a MESG higher than0.90 mm to be accelerated enough so that a detonation can

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occur. Research organizations have documented that it is notpossible to characterize the potential for the occurrence of arapid combustion reaction with a single physical parameter.

4.3 DEFLAGRATION

Flames propagating through piping systems are capable ofreaching extremely high speeds. Initially, ßames travel at aburning velocity of a few feet per second. This is the laminarßame speed typically tabulated in handbooks. The ßame frontcan be accelerated by turbulence induced in the unburnedmixture ahead of the ßame, by the combustion wave itself, orcan result from factors such as pipe wall roughness or turbu-lence-producing Þttings and bends. A particularly strong igni-tion source can ÒimmediatelyÓ initiate a combustion reactionwith greater than normal initial turbulence. Increased turbu-lence also can be generated as the pressure of a processincreases. It is possible for the ßame front to accelerate bothupstream and downstream of the original direction of ßow.Flame fronts in piping can readily reach a velocity of severalhundred feet per second. As long as the ßame front propa-gates in the unburned mixture at a velocity less than the speedof sound, it is characterized as a deßagration. All ßamearresters should be designed to interrupt a deßagration.

4.4 DETONATION

If a pipe is long enough, or if enough turbulence is gener-ated, a ßame front many accelerate to the point where a deto-nation occurs. Detonations travel at or above the speed ofsound (which is a function of the density of the mixturewithin the piping), and typically reach speeds of several thou-sand feet per second. Pressure pulses accompanying the ßamefront may exceed 20 times the initial pressure. Even higherpressures can be generated at:

a. Closed ends and elbows, where reßection occurs, b. The point where a deßagration transforms into a detonation,which is known as an overdriven or unstable detonation, or c. At the termination of a closed system as the mixture ofunburned gases is compressed before the transition to detona-tion, which is known as pressure piling.

Not all ßame arresters are designed to quench and/or with-stand the elevated pressure and impulse of a detonation. USCGregulations require use of detonation-type ßame arresters whenthose regulations require ßame arresters in vapor collectionsystems. The potential for a detonation to occur is difÞcult topredict except in controlled laboratory settings. Transforma-tion of a ßame front from a deßagrating to a detonating com-bustion wave is more probable if the combustion reaction isoccurring within a piping system than if in open air.

SECTION 5—FLAME ARRESTER FUNCTION AND CONCERNS FOR USEIN PIPING SYSTEMS

5.1 FLAME ARRESTER FUNCTION

Flame arresters function by interrupting the combustionwave as it progresses through the ßame arrester. Typicalßame arresters accomplish this by quenching the ßame frontusing a heat sink with high surface-to-volume ratio and nar-row passageways, such as a wire screen, woven wire gauze,metal honeycomb, parallel metal plates, or a porous metalplate. The metal absorbs heat from the ßame and quenchesit, thus preventing it from passing through the ßame arrester.Various types of ßame arresters and potential problemsassociated with their use in piping systems are discussed inthe following sections.

5.2 PRESSURE CONCERNS AND MAINTENANCE

High pressures developed in piping, especially during a det-onation, may damage the element in a ßame arrester or evenrupture the housing. Flame arresters should be included inperiodic maintenance checks. After interrupting a ßame front,ßame arrester elements should always be inspected for possibledamage. A ßame arrester should be designed and installed inpiping so that maintenance can be done without the need tocompletely remove the ßame arrester. Some high risk systems

use parallel ßame arresters to enable one at a time to be takenout of service for maintenance. For these systems, the effects ofpiping conÞguration should be evaluated to determine if therehas been an inßuence on ßame speed.

5.2.1

Typical ßame arresters with elements will cause apressure drop. Because of this pressure drop and the high sur-face area of the elements, condensation can readily occur.Gases that have a high carbon content or that can polymerizecan plug the elements. Or, if the gas mixture contains sulfuror hydrogen sulÞde, deposition of sulfur compounds mayoccur. It may be necessary to heat or heat trace the ßamearrester to reduce the potential for condensation, depositionand plugging of the element. Some facilities install pressuregauges upstream and downstream of a ßame arrester to moni-tor changes in pressure drop and facilitate determining if ele-ments have become plugged. Where condensation is aconcern, it may be appropriate to install normally closed,valved drains on the housing of the ßame arrester to enabledraining of accumulated condensed liquids.

The manufacturer should be consulted if it is necessary toheat or heat trace a ßame arrester for the service conditions itwill experience. The test results for the ßame arrester should

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be reviewed to ensure that the speciÞc heating conditions envi-sioned for operation of the ßame arrester will not cause it to beincapable of quenching a ßame front. This review should takeplace before the changed conditions are implemented. Forinstallations under Coast Guard jurisdiction the vapor controlsystem must be separated or insulated from external heatsources to limit vapor control system piping surface tempera-ture to not more than 350¡F (177¡C) during normal operation.

5.3 POTENTIAL EFFECTS OF INSTALLATION GEOMETRY

The geometry, size, and length of piping and piping systemsare important to consider when selecting a ßame arrester. It ispossible that the level of turbulence generated by combinationsof these factors may render a ßame arrester incapable ofquenching a ßame front. Studies have noted that a correlationof the performance of a ßame arrester and piping size is notalways possible. It may be necessary to have tests performedfor the particular size of ßame arrester proposed for use. This isparticularly relevant as piping diameter increases. For pipingsystems, it is advisable to install only ßame arresters that havebeen designed and tested for detonations. In some situations,this is required for regulatory compliance.

Placing two or more ßame arresters in series is not advis-able. It has not been demonstrated that additional protectionwould be provided. If the ßame front propagates through theÞrst ßame arrester, it can be expected to propagate through thesecond ßame arrester.

Pipe diameter going into and out of a ßame arrester shouldbe kept constant. Proprietary testing indicates that changes indiameter can cause ßame fronts to accelerate through anarrester.

5.4 FLAME ARRESTERS NOT USING METAL ELEMENTS

Some standards and testing laboratories have providedfor ßame arresters that have a design that does not use metalelements. Examples are hydraulic (water) type ßame arrest-ers in CEN-EN 12874 and in the FM Approval Guide. Theperformance of these ßame arresters must be demonstratedby testing.

In certain situations, the USCG regulations allow theinstallation of Òwater sealsÓ and quick closing valves formechanical interruption of the ßame path. These devicesmust meet USCG certiÞcation requirements. Demonstrationof the suitability of these devices may require performancetesting of the design. By contrast, emergency shutdownvalves required by the USCG regulations for oil or hazardousmaterial transfer lines and cargo vapor shutoff valves are notintended to act as ßame arresters and are not required to func-tion as quickly.

Flame arresters using devices and techniques other thanmetal elements are available and in use within the hydrocarbon

industry. Some of these ßame arresters have been in service foryears without incident; however, without proof of performanceof the design by testing, it should not be assumed that the ßamearrester will be capable of performing properly. Examples ofßame arresters with designs that do not use metal elements arediscussed in the following sections.

5.4.1 Water Seals

Water (hydraulic) seals are often installed to preventreverse gas ßow. Their design is capable of interrupting aßame front. The gas mixture is bubbled through a reservoirof water (or sometimes another liquid). Passage of the ßamefront is prevented because each gas bubble is isolated by theliquid water. There is no standard design for water seals.Each installation presents a speciÞc design problem involvingthe rate of gas ßow, the depth of the seal, and the size andconÞguration of the vessel containing the water. If composi-tion of the process gas is such that a ßame arrester using ametal element could become frequently plugged, using awater seal may be appropriate. If a water seal is used as aßame arrester, some important design considerations are:

a. The water seal must be capable of withstanding the pres-sures developed. Water seals are typically used immediatelyadjacent to ignition sources such as ßare stacks. In such acase, the water seal would most likely have to withstand onlya deßagration. If a water seal is installed within a closed pip-ing system, it should be designed to withstand a detonation.b. The water must remain in the seal for it to function as aßame arrester. Automatic water level control and low levelalarms are desirable. It is doubtful that it is possible to designfor the water to be retained within the seal in the event of adetonation and its accompanying pressure.c. In cold climates, the water seal must be protected againstfreezing which may require being heated or heat traced. Insome instances, anti-freeze has been added to water, or lowerfreezing point materials (such as glycerine) have been used asthe ßuid for hydraulic seals.

5.4.2 Packed Beds

Vessels with gravel, raschig rings, small pebbles, and otherbulk materials have been used as ßame arresters. CGA G-1.2provides guidelines for acetylene operating within speciÞclow velocity and pressure regimes; liquid wetted packing ispreferred. However, there are no established general designcriteria for using packed beds as in-line ßame arresters. Peri-odic inspection and maintenance of packed beds used asßame arresters would be required.

5.4.3 Velocity-type Seals

Where the ßow of a gas is only in a single direction, it is pos-sible to ensure, by design, that the ßow velocity will never beless than the velocity corresponding to the maximum rate that a

6 API R

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ßame front will propagate in the gas. Velocity type seals aretypically used in ßare stacks, where the ignition source is at theopen end of the pipe. If a velocity type seal is used as a ßamearrester, some important design considerations are:

a. It will only be effective where the ignition source is at theopen end of a pipe.

b. The appropriate velocity must be determined for each gasand pipe diameter.

c. Some means must be provided either to maintain a mini-mum velocity under all conditions or to interrupt the gassupply if the ßow velocity becomes too low.

d. To prevent ßames from heating the arrester, which couldpermit ßames to pass through the arrester, the design mustprovide for some means either to interrupt the gas supply orto extinguish burning within the velocity section of thearrester.

5.4.4 Mechanical Interruption of the Flame Path

Closing a valve in the pipe can prevent ßame passage;however, the valve must be a fast-operating valve. In order toutilize a fast-closing valve as a ßame arrester, sensing devicesmust be installed within the piping system. For this schemeto function as a ßame arrester, the combination of theresponse time of the sensing devices and the valve closuretime must be very fast, on the order of hundreds of millisec-onds. A propagating ßame front would not be stopped if such

sensing devices malfunction or are not properly maintained,including periodic testing.

5.5 PYROPHORIC IRON SULFIDE CONCERNS

If the gas mixture contains sulfur or hydrogen sulÞde, thepotential for formation of pyrophoric iron sulÞde within thepiping should be considered in the design. This formationoccurs only in oxygen deÞcient conditions such as inert atmo-spheres. When exposed to oxygen, pyrophoric iron sulÞdeoxidation can heat gases in piping or act as an ignition source.Also, as was noted in 5.2, sulfur compounds can deposit onßame arrester elements causing restrictions.

5.6 UNILATERAL AND BILATERAL FLAME ARRESTERS

Not all ßame arrester designs will interrupt a ßame fronttraveling in either direction through the ßame arrester. Thosethat can function with gas ßow only in one direction areknown as ÒunilateralÓ type ßame arresters. USCG regula-tions require that ÒbilateralÓ type ßame arresters be used inthe cargo vapor piping of marine transfer facilities. Potentialignition sources along with the process design and piping sys-tem conÞguration should be reviewed in order to determine ifa unilateral type ßame arrester installation is appropriate andprotective. At certain locations, such as at a ßare stack or at acombustion unit, installing a unilateral type ßame arrestermay be sufÞcient.

SECTION 6—LIMITATIONS OF FLAME ARRESTERS ON TANK VENTS

API RP 2210 discusses beneÞts and concerns related toinstallation of ßame arresters on tank vents. The characteris-tics of ßame propagation in piping systems makes the instal-lation of ßame arresters in piping systems fundamentallydifferent from the installation of ßame arresters on tank vents.API RP 2210 establishes that the risk of ignition is very lowon tank vents with a pressure-vacuum (P/V) valve, and thatwhen tank vent systems are equipped with a P/V valve the useof a ßame arrester is not considered necessary.

API Std 2000 and NFPA 30 both state that a ßame arrester isnot considered necessary for use in conjunction with a P/Vvalve where the tank is normally closed except when venting.This is consistent with OSHA requirements in1910.106(b)(2)(iv)(f). Coast Guard regulations provide a simi-lar exemption for tanks equipped with P/V valves. 33

CFR

154.820(h) states ÒExcept for a discharge vent from a vapordestruction unit, each outlet of a vapor control system thatvents to atmosphere and is not isolated with a pressure-vacuumrelief valve must have a ßame arrester located at the outlet.Ó

If tanks equipped with vents without P/V valves are part ofa vapor control system falling under U.S. Coast Guard juris-diction, then the vent may be subject to the requirements of33

CFR

Part 154 Subpart E, Appendix B

Standard SpeciÞca-tion for Tank Vent Flame Arresters.

Most companies accept the premise that a tight steel roofand a P/V valve provide appropriate protection. Any poten-tial additional protection afforded by ßame arresters must bebalanced against practical safety concerns for the necessarymaintenance required to ensure that the required ventingcapacity is maintained to avoid tank damage through theintroduction of a new potential failure mode.

API RP 2210 states that ßame arresters designed for tankvents should not normally be installed within piping systemsor with signiÞcant downstream open piping. In order toensure that a ßame arrester is not being used in conditionsoutside of its design and testing parameters, the manufacturershould always be consulted before installing a tank-ventßame arrester within a piping system.

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SECTION 7—FLAME ARRESTER TESTING AND CERTIFICATION

7.1 GENERAL

Flame arresters installed in piping systems should alwaysbe tested to verify that the design would quench a ßame frontpropagating within a closed piping system. If the test condi-tions are not equivalent to, or representative of, the actual ser-vice conditions of the particular piping system and gas(including the system temperature and pressure), further test-ing is warranted before installing a particular ßame arrester.

7.2 DEFLAGRATION AND DETONATION TESTING

Deßagration tests as well as detonation tests should be per-formed for ßame arresters used in closed piping systems.Experience has shown that some ßame arrester designs willpass a detonation test but will fail a deßagration test. Deßa-gration, detonation, and explosion tests for ßame arresters aretypically performed using propane. However, if the processgas of concern has an accelerated combustion reaction rate,such as acetylene, hydrogen (or hydrogen-containing mix-ture), or oleÞnic hydrocarbons, additional tests using the spe-ciÞc process gas are warranted.

7.3 FLAME RETENTION TESTING

If a ßame is retained on the element of a ßame arrester, theßame arrester may become incapable of preventing passage ofthe ßame through the arrester as a result of the arrester housingand element becoming heated by the ßame. Continuous burnand endurance burn tests typically are performed using gaso-line vapor or n-hexane to determine how the ßame arrester willperform during ßame retention. Arresters tested using thesefuels are probably suitable for use with most common parafÞnor aromatic hydrocarbons. However, if the gas to be trans-ported in the piping supports accelerated rate combustion reac-tions, such as acetylene, hydrogen, or oleÞnic hydrocarbons,additional tests using the speciÞc process gas are warranted.

7.4 SIGNIFICANCE OF MESG

Background for MESG is discussed in Section 4. If theMESG of the gas is less than 0.90 mm., additional tests usingthe speciÞc process gas are warranted. A list of hydrocarbonand chemical gases or vapors which have a MESG less than0.90 mm. is contained in Appendix B. If the process gas con-tains some portion of a gas or vapor which has a MESG less

than 0.90 mm (like hydrogen), it is advisable to conduct addi-tional tests at least to establish the speciÞc MESG for the pro-cess gas mixture. It is advisable not to rely totally upon theMESG value as a determining factor as to the nature of apotential combustion wave within a piping system. Themolecular structure of the gas or vapor can be an importantfactor, particularly if double or triple bonds exist. Turbulencegeneration can cause a combustion reaction to accelerate suchthat a ßame arrester design is challenged beyond its capabilityto interrupt a ßame front and/or a detonation occurs.

7.5 USE OF ESTABLISHED TEST PROCEDURES

The test results for a ßame arrester listed or approved by atesting laboratory should be reviewed to ensure suitability forthe intended use. The distinction between arresting ßamesversus protecting against detonations should be recognized. Ifdetonation tests have not been performed, additional testingmay be warranted prior to utilizing a ßame arrester in a pip-ing system. It is important to understand the tests used.

Flame arresters tested in accordance with USCG require-ments in 33

CFR

Part 154, Appendix A, have been tested fordetonations. However, Appendix B of the same regulationaddresses testing for ßame arresting capability of both in-lineand end-of-line devices, but not detonation. The standardUSCG tests address fuels with an MESG of 0.90 mm orgreater; if the MESG for the fuel of concern is lower than0.90 mm then tests on an equivalent fuel must be run.

Test procedures generally considered suitable for detona-tion ßame arresters installed in piping systems are enumer-ated in 33

CFR

Part 154 Appendix A, and included in UL 525and CEN EN 12874. However, standards such as UL 525may cover tank vent deßagration ßame arresters as well as in-line detonation ßame arresters. So, ßame arresters tested inaccordance with UL 525 or CEN EN 12874 or approved byFM may not have been tested for detonation performance andthe listing and test results for the speciÞc ßame arrestershould be consulted. Flame arresters tested only in accor-dance with ASTM F 1273 or 33

CFR

Part 154, Appendix Bhave not been required to be tested for detonations.

Only tested ßame arrester designs should be used. It is notadvisable to rely upon, or use, untested ßame arrester designsirrespective of regulatory requirements.

SECTION 8—SUMMARY

8.1

Only ßame arresters, the design of which has beentested, should be installed in piping systems. Flame arrestersmust be installed and maintained in the exact mechanical

form in which they were tested. This includes proper mainte-nance of the ßame arrester element, housing, and gaskets.

8 API R

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PRACTICE 2028

8.2 Unless the ßame arrester is installed at the end of a pipeopen to atmosphere, ßame arresters used in piping systemsshould be capable of withstanding and interrupting a propa-gating, detonating ßame front.

8.3 A hazard and risk analysis should be performed beforeutilizing a unilateral type ßame arrester. If enough turbulenceis generated, it is possible for a ßame front within piping topropagate opposite the normal process ßow direction. Instal-lations regulated by the Coast Guard in 33 CFR Section 154,for cargo vapor piping of marine transfer facilities, requirebilateral ßame arresters capable of withstanding a detonation.Other installations, such as at a ßare stack, may not needbilateral performance.

8.4 Listings of ßame arresters provided by testing laborato-ries should be reviewed to determine that the ßame arresterhas been tested for conditions (deßagration or detonation) inwhich it is expected to perform as installed. Actual serviceconditions include:

a. Gas mixture composition.b. Temperature.c. Pressure.d. Flow rate.

e. ConÞguration (in-line or at end of pipe).f. Potential ignition location relative to the ßame arrester.

8.5 A ßame arrester capable of withstanding a ßame burn-ing on the face of its element should be used where such acondition would be expected to occur. If such a conditionmay occur with a ßame arrester which is not capable of with-standing the ßame, provision must be made to prevent, ordetect and suppress, the burning.

8.6 When comparing gases or vapors based on MESG, avalue of < 0.90 mm is used by some standards as the thresh-old for requiring special ßame arrester testing. While no sin-gle parameter can characterize a fuel, one can use as aworking hypothesis that the ßame arrester challenge becomesgreater as the MESG becomes smaller.

8.7 Flame arresters should not be considered the sole meansof protection but should be used as a supplement to proper pro-cess design, other systems, and operational controls.

8.8 Where tank vent systems are equipped with a P/V valvethe use of a ßame arrester is not considered necessary. If ßamearresters are used on tank vents, they should be at the end of thevent piping open to the atmosphere.

9

APPENDIX A—BIBLIOGRAPHY

API11

Std 620 Design and Construction of Large, WeldedLow-Pressure Storage Tanks

Std 650 Welded Steel Tanks for Oil StorageRP 12N Recommended Practice for the Operation,

Maintenance and Testing of Firebox FlameArresters

Publ 758 Section 1 Safety Digest of Lessons Learned, General

Safety Precautions in ReÞningPubl 758 Section 2 Safety Digest of Lessons Learned, Safety in

Unit OperationsPubl 758Section 3 Safety Digest of Lessons Learned, Safe

Operation of AuxiliariesPubl 758Section 4 Safety Digest of Lessons Learned, Safety in

MaintenanceK19900 Special Report on Detonation Arrester

Safety: ÒMitigation of Explosion Hazards ofMarine Vapor Control SystemsÓ, R.E.White and C.J. Oswald, SouthwestResearch Institute, October 1992

AIChE, CCPS12

Guidelines for Engineering Design forProcess Safety

Christian Michelsen Research (CMR)13

Gas Explosion Handbook CMR Research Report No. CMR-93-A5034

FM14

Approval GuideÑA Guide to Equipment,Materials & Services Approved by FactoryMutual Research Corporation for PropertyConservation¥Flame Arresters for Vent Pipes of StorageTanks

¥Detonation Flame Arresters for Flamma-ble Vapor Piping Systems

¥Flame Arresters for Gas Piping Systems¥Flame Arresters, Dry Type¥Flame Arresters, Hydraulic Type

IEC15

IEC 79-20 Electrical apparatus for explosive gasatmospheresPart 20: Data for ßammable gases andvapours, relating to the use of electricalapparatus

NFPA16

Fire Protection Handbook, 18th editionPrinciples of Fire Protection Chemistryand Physics, Raymond Friedman, 1998

UL17

Gas and Oil Equipment Directory

11www.api.org12American Institute of Chemical Engineers, Center for ChemicalProcess Safety, 345 East 47th Street, New York, New York 10017.www.aiche.org/docs/ccps13Christian Michelsen Research, P.O. Box 6031 Postterminalen,N-5892 Bergen, Norway.

14Factory Mutual Insurance Company, 1151 Boston ProvidenceTurnpike, P.O. Box 9102, Norwood, Massachusetts 02062.www.factorymutual.com15International Electrotechnical Commission, 3 rue de Varemb�,Case postale 131, 1211 Geneva 20, Switzerland. www.iec.ch16National Fire Protection Association, 1 Batterymarch Park,Quincy, Massachusetts 02269. www.nfpa.org17Underwriters Laboratories, Inc., 333 PÞngsten Road, Northbrook,Illinois 60062. www.ul.com

11

APPENDIX B—GASES OR VAPORS WITH A MAXIMUM EXPERIMENTAL SAFE GAP (MESG) < 0.90 MM

(From IEC Publications 79-1A and 79-20 & 33 CFR Part 154, Appendix A)lowest value reported shown along with some > 0.90 mm typical hydrocarbons and alcohols for comparison

(Reported values above 0.90 are in parentheses)

Gas or Vapor MESG (mm)

Hydrogen 0.102

HYDROCARBONSAcetylene < 0.025Ethylene 0.711,3-Butadiene 0.792-Butene (isomer not stated) 0.89Ethylene 0.65Methane (1.170)Propane (0.965)Hexane (0.965)Benzene (0.99)Cyclohexane (0.94)Isooctane (1.040)

CHEMICALS PROCESSED IN SOME PETROLEUM FACILITIESHydrogen SulÞde 0.89Ethyl Alcohol (Ethanol) 0.89Carbon Monoxide 0.84Carbon Disulphide 0.20Ethyl Alcohol (1.016)Methyl Alcohol (0.915)

OTHER CHEMICALSAcrolein (Acryl Aldehyde) 0.72Acrylic Acid 0.86Acrylonitrile 0.87Allyl Alcohol 0.84Allyl 2,3-Epoxypropyl Ether 0.70Blue Water Gas (52% H2, 47% CO) 0.203Butanone 0.843-Buten-3-olide 0.84Butyl Acrylate 0.88Butyl 2,3- Epoxypropyl Ether 0.78Butyl Glycolate 0.881-Butyne 0.71Carbon Disulphide 0.201-Choloro-2, 3-Epoxypropane 0.74Coal Gas (57% H2) 0.482Crotonaldehyde 0.81Dibutyl Ether 0.88Di-tert-Butyl Peroxide 0.84Dichlorodiethylsilane 0.451,2-Diethoxyethane (Diethyl Glycol) 0.81Diethyl Carbonate 0.83Diethyl Ether (Ethyl Ether) 0.864

12 API RECOMMENDED PRACTICE 2028

1,2-Dimethoxyethane(Ethylene Glycol Dimethyl Ether)

0.72

Dimethyoxymethane (Methylal) 0.86Dimethyl Ether (Methyl Ether) 0.841,1-Dimethylhydazine 0.85p-Dioxane (Diethylene Dioxide) 0.701,2-Epoxypropene 0.702-Ethoxyethanol(Ethylene Glycol Monoethyl Ether)

0.84

Ethyl Acrylate 0.86Ethyl Nitrate < 0.025Ethylene Oxide 0.592-Ethylhexyl Acetate 0.88Ethyl Propylacrolein (isomer not stated) 0.86Formaldehyde 0.572-Furaldehyde (Furfural) 0.88Furan 0.68Furfuryl Alcohol 0.80Hydrogen Cyanide 0.802-Methoxyethanol 0.85Methyl Acetoacetate 0.85Methyl Acrylate 0.85Methylene Cyclobutane 0.764-Methylene Tetrahydropyran 0.892-Methyl-1-buten-3-yne 0.782-Methylpent-2-enal 0.84A-Methylstyrene 0.88Nitroethane 0.871-Nitropropane 0.84Paraformaldehyde 0.57Phenylacetylene 0.861-Propanol (Propyl Alcohol) 0.89Propionicaldehyde 0.86Propylene Oxide 0.70Prop-2-yn-1-ol 0.58Tetraßuoroethylene 0.60Tetrahydrofuran 0.87Tetrahydrofurfyryl Alcohol 0.85Town Gas (57% H2, 16% CO) 0.531,3,5-Trioxane 0.752-Vinyl Oxyethanol 0.86

Gas or Vapor MESG (mm)

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