Flame arresters. Preventing the spread of fires and explosions in equipment that contains flammable gases and vapours FLAME ARRESTERS.

Transcription

1 Health ad Safety Flame arresters Prevetig the spread of fires ad explosios i equipmet that cotais flammable gases ad vapours FLAME ARRESTERS This is a free-to-dowload, web-friedly versio of HSG158 (First editio, published 1996). This versio has bee adapted for olie use from HSE s curret prited versio. You ca buy the book at ad most good bookshops. ISBN Price 8.50 PREVENTING THE SPREAD OF FIRES AND EXPLOSIONS IN EQUIPMENT THAT CONTAINS FLAMMABLE GASES AND VAPOURS This book provides guidace o how flame arresters ca help prevet the spread of fires ad explosios from pipes ad vessels cotaiig flammable vapour or gas. It describes the differet categories ad types of flame arrester, the criteria for selectio, ad maiteace requiremets. The guidace is aimed primarily at maagers, supervisors ad techical workers resposible for the safety of plat cotaiig flammable vapour or gas. It may also be of iterest to trade orgaisatios or associatios who may wish to use the guidace as a basis for more specific guidace for their ow members. HSE Books Page 1 of 23

2 Health ad Safety Crow copyright 2008 First published 1996 Reprited 2008 ISBN All rights reserved. No part of this publicatio may be reproduced, stored i a retrieval system, or trasmitted i ay form or by ay meas (electroic, mechaical, photocopyig, recordig or otherwise) without the prior writte permissio of the copyright ower. Applicatios for reproductio should be made i writig to: The Office of Public Sector Iformatio, Iformatio Policy Team, Kew, Richmod, Surrey TW9 4DU or licesig@opsi.gov.uk This guidace is issued by the Health ad Safety. Followig the guidace is ot compulsory ad you are free to take other actio. But if you do follow the guidace you will ormally be doig eough to comply with the law. Health ad safety ispectors seek to secure compliace with the law ad may refer to this guidace as illustratig good practice. Page 2 of 23

3 Health ad Safety Cotets Itroductio 4 Legal requiremets 6 Selectig a flame arrester 7 Maitaiig the performace of flame arresters 12 Appedices 14 1 Legal requiremets 14 2 Types of flame arrester ad alteratives 16 3 Stadards for flame arresters 19 4 Glossary of terms 20 5 Refereces 22 Further iformatio 23 Flame arresters Page 3 of 23

4 Health ad Safety Itroductio 1 This booklet provides iformatio ad advice o how flame arresters ca reduce risks by prevetig the spread of fire ad explosio i vessels ad pipes cotaiig flammable atmospheres. 2 Flame arresters are commoly located i vet pipes from storage taks, i pipes carryig vapours or gases to flare stacks, icierators ad furaces, ad i vapour recovery ad balacig systems. They act as a type of barrier betwee pieces of equipmet, prevetig the spread of flame. Typical locatios for flame arresters are illustrated i Figure 1. 3 Flame arresters are used where there are permaet sources of igitio such as icierators ad flare stacks to prevet flashback if a flammable atmosphere develops i the vapour feedlies. They are also used to protect plat ad equipmet, cotaiig flammable vapours from potetial sources of igitio. Ay piece of equipmet or process which is capable of producig sparks, flames or hot surfaces is a potetial source of igitio. Where vapours are veted to atmosphere, they may be igited by lightig or other exteral sources of igitio. Flame arresters are used to prevet flashback from the atmosphere ito the equipmet. Figure 1 Typical locatios for flame arresters Flame arresters Page 4 of 23

5 Health ad Safety 4 Most flame arresters cosist of a elemet ad its housig (Figure 2). The elemet is a permeable matrix made up of arrow passages or opeigs. Gas or vapour ca flow through these apertures. I the evet of a fire or explosio, if burig gas eters the matrix it is subdivided ad quickly cooled o the cold surface of the elemet. The flame is extiguished ad the gases are cooled sufficietly to prevet re-igitio as they emerge from the elemet. 5 This booklet describes the differet categories ad types of flame arrester, the criteria for selectio ad the maiteace requiremets. It replaces HSE s booklet HSG11, Flame arresters ad explosio relief. It is aimed at maagers, supervisors ad techical workers resposible for the safety of plat cotaiig flammable vapour or gas. 6 Flame arresters are also used for weldig ad cuttig equipmet, ad for diesel egies used i potetially flammable atmospheres. These uses are outside the scope of this booklet. Iformatio is available i other publicatios, 1,2 if required. 7 Where a British Stadard is quoted, ay other atioal or iteratioal stadard that provides a equivalet level of safety is acceptable. Harmoised Europea Stadards that bear the preface BS EN may supersede some British Stadards, ad these are equally acceptable, whe published. Figure 2 Sketch of flame arrester Flame arresters Page 5 of 23

6 Health ad Safety Legal requiremets 8 The Dagerous Substaces ad Explosives Atmospheres Regulatios (DSEAR) require that precautios should be take to reduce the risk of fires ad explosios, where flammable liquids or gases are stored or processed. The publicatio Desig of plat, equipmet ad workplaces 3 advises o the use of mitigatio measures such as flame arresters to prevet the propagatio of fires ad explosios i itercoected plat ad equipmet. It also advises that vet pipes from plat cotaiig flammable, highly flammable or extremely flammable liquids, required for ormal operatio, should discharge to a safe place ad ormally be provided with a flame arrester. 9 There are also geeral duties uder health ad safety law which are relevat. The three mai pieces of legislatio are outlied below. Further iformatio is give i Appedix 1. A glossary of terms is provided i Appedix The Health ad Safety at Work etc Act requires employers to provide ad maitai safe systems of work. They are also required to take all reasoable precautios to esure the health ad safety of employees ad aybody else who could be affected by the work activity. Employees ad the self-employed also have a legal duty to take care of their ow ad other people s health ad safety. 11 Uder the Maagemet of Health ad Safety at Work Regulatios 1999, 5,6 every employer has a duty to carry out a assessmet of the risks to the health ad safety of employees ad of ayoe who may be affected by the work activity. This is so that the ecessary prevetive ad protective measures ca be applied. 12 The Provisio ad Use of Work Equipmet Regulatios aim to esure safe work equipmet is provided ad is safely used. They iclude geeral duties coverig the selectio of suitable equipmet, maiteace, iformatio, istructios ad traiig. Flame arresters Page 6 of 23

7 Health ad Safety Selectig a flame arrester 13 A flame arrester is a protective device; it will ot prevet a icidet from happeig. Measures should be take, where practicable, to prevet the flammable atmospheres formig by usig cotaimet, process cotrol or vetilatio. Igitio sources should be cotrolled. 14 Whe decidig whether a flame arrester may be eeded, a assessmet should be carried out to idetify: where flammable atmospheres may arise; where there are igitio sources or potetial igitio sources; whether fire ca spread betwee pieces of equipmet; the cosequeces of a igitio. 15 Flame arresters are desiged, tested ad istalled to meet particular operatig coditios. If there is a mismatch betwee the arrester ad its eviromet, it may fail. To esure that a effective flame arrester is selected, it is essetial that the coditios i which it will be used are specified carefully ad accurately. This sectio provides iformatio o the parameters which eed to be cosidered whe selectig a flame arrester. 16 There are may differet desigs of flame arresters ad may differet maufacturers. Appedix 2 describes the various types of flame arrester available ad outlies their rage of applicatio. Some alteratives to flame arresters are also metioed. 17 Geerally, ew flame arresters should meet the requiremets of the Europea Stadard BS EN 12874: 2001 Flame arresters. Performace requiremets, test methods ad limits for use. 8 Flame arresters already i use may have bee supplied to other atioal ad iteratioal stadards which provide a equivalet level of safety. 9 Appedix 3 provides details of these stadards. 18 If there is more tha oe igitio source, a arrester should be selected to meet the most severe coditios predicted. Depedig o the circumstaces, it may be ecessary to use more tha oe arrester or a arrester that is effective i both directios. 19 Although flame arresters prevet fires ad explosios propagatig through plat, they do ot provide explosio relief. It may be ecessary to provide pressure relief 10 or stregthe equipmet. Thermocouples may also be fitted to arresters to detect overheatig ad trigger automatic shut-off valves. Flame arresters Page 7 of 23

8 Health ad Safety 20 The factors to be cosidered whe specifyig a flame arrester iclude the: category; ru-up distace; vapour or gas ivolved; temperature ad pressure; acceptable pressure drop across the arrester; possibility of blockage or cotamiatio; arragemets for cleaig ad maiteace; materials of costructio; possibility of stabilised burig o the elemet; possibility of oxyge erichmet. Each of these factors is ow described i more detail. Category 21 There are three categories of flame arrester: a ed-of-lie deflagratio arrester; a i-lie deflagratio arrester; ad a detoatio arrester. Typical locatios for i-lie ad ed-of-lie arresters are show i Figure 1, page Ed-of-lie deflagratio arresters are istalled at the ed of vet pipes o process vessels, storage taks ad trasport cotaiers. If the veted vapours are igited, perhaps by lightig, the arrester will prevet the fire spreadig from the atmosphere to the iside of the vessel. 23 I-lie deflagratio arresters are mouted i pipelies to prevet a flame beig trasmitted alog the pipe. They are placed as close as possible to ay likely igitio source to avoid detoatios. Typical applicatios iclude protectio for the fuel/air supply to a burer, or use i a vet where there is a legth of pipe betwee the arrester ad the poit of discharge to the atmosphere. 24 Detoatio arresters are used as i-lie arresters where a detoatio is possible. For example, they may be used to protect taks, vessels or process pipes cotaiig flammable vapour from potetial igitio sources such as fas, compressors, icierators ad flare stacks. 25 The terms deflagratio ad detoatio relate to flame speed. If a flame travels at less tha the speed of soud it is called a deflagratio. If it travels above the speed of soud it is termed a detoatio ad is accompaied by high pressure effects. As a flame travels alog a pipe it accelerates rapidly (Figure 3). If the pipe is log eough the flame will evetually reach detoatio coditio with a speed of about 2000 m/s ad explosio pressure i excess of 20 bar. The rate of acceleratio of the flame is also icreased by beds, T-juctios, obstacles ad chages i the cross sectio of the pipe. Flame arresters Page 8 of 23

9 Health ad Safety Figure 3 Graph represetig acceleratio of flame to detoatio coditios 26 Deflagratio arresters are desiged to stop deflagratios, but are geerally ot strog eough to withstad a detoatio. Detoatio arresters have cosiderable stregth ad are ormally desiged to stop both deflagratios ad detoatios. The most severe evet that a arrester may face is a overdrive detoatio. This may happe durig a trasitio from a fast deflagratio to a detoatio. Uder such circumstaces the detoatio becomes ustable ad achieves much higher flame speeds ad explosio pressures tha a stable detoatio, placig a greater demad o the flame arrester. If a overdrive detoatio is possible, a detoatio arrester which provides protectio agaist both stable ad overdrive detoatios will be required. Ru-up distace 27 The ru-up distace is the legth of pipework betwee the potetial igitio source ad the arrester. Flame speed icreases with ru-up distace as show i Figure 3 ad as described i paragraphs 21 to 26. Cosequetly, flame arresters are fitted to miimise ru-up distaces. This meas miimisig the legth of pipework betwee the possible igitio source ad the flame arrester. For example, ed-of-lie arresters for pipe lies vetig to atmosphere are fitted as close to the ed of the vet lie as possible, to provide protectio agaist igitio sources from outside such as lightig. 28 A maximum ru-up distace is specified for each deflagratio elemet, based o the results of practical tests. If the ru-up distace is exceeded there is a dager of the arrester ot beig able to stop the flame or withstad the overpressure. 29 Beds, obstructios ad rough surfaces cause turbulece ad acceleratio of the flame. The icrease i flame speed is difficult to predict depedig o may factors such as the ru-up distace to the bed, the legth of pipe after the bed ad the umber of beds. Deflagratio arresters oly provide protectio agaist flames travellig at subsoic speeds ad at low explosio pressures. If there is ay doubt about the effectiveess of a deflagratio arrester, a detoatio arrester should be used. Flame arresters Page 9 of 23

10 Health ad Safety Vapour or gas 30 Flame arresters are classified accordig to gas groupigs. 11,12 There are four gas groups: I, IIA, IIB, IIC. Methae is the oly gas i Group I. It is usually combied with Group IIA which covers commoly used gases such as propae or petroleum products. Group IIB covers more reactive gases such as ethylee. Group IIC covers the most reactive gases such as hydroge ad acetylee. Uder idetical coditios of pipe legth ad diameter the flame speeds that result at the flame arrester icrease progressively from gases i Group I to Group IIC. 31 If more tha oe gas or vapour is beig used, select a flame arrester suitable for the most reactive gas. The temperature ad pressure of the gas or vapour 32 The specificatio of a flame arrester also depeds o operatig temperatures ad pressures. Icreases i temperature or pressure above those specified by the supplier may result i deflagratio arresters failig. A similar failure could result if the arrester is too close to a burer flame or other hot surface ad overheats. I those situatios a detoatio arrester may be more appropriate. 33 I additio, beds ad obstructios dowstream of the arrester may also cause a icrease i pressure at the arrester ad impair its effectiveess. The pressure drop across the arrester 34 The mai disadvatage of flame arresters is that because they have a fie structure they itroduce a resistace to flow. Equipmet dowstream may be uable to operate or may operate erratically if the pressure drop is too high. The resistace to flow ca be miimised by usig a arrester elemet ad associated housig that has a diameter greater tha the pipe diameter. However, if the expasio piece leadig to the arrester housig is too diverget it may lead to icreased turbulece ad therefore accelerated flames. 35 As flame arresters reduce the flow rate of gases ad vapours i pipes they are ot fitted i emergecy vetig systems uless the rate of vetig is sufficiet to compesate for the presece of the arrester. If there is a cocer about the possibility of igitio i emergecy vet systems it may be better to provide a iert gas purge. Blockage or cotamiatio 36 Materials that polymerise or eed heatig to maitai them i the vapour state may solidify i the flame arrester, causig blockages. It is geerally ot appropriate to use flame arresters with these substaces. Other methods such as iertig or explosio suppressio may be more suitable. 37 Blockage may also be caused by codesate ad particulate matter. Some flame arresters are desiged or ca be modified to icorporate codesate drais. These will eed emptyig at appropriate itervals. Ed-of-lie arresters may be fitted with a weather cowl to protect agaist rai ad airbore dust. Flame arresters Page 10 of 23

11 Health ad Safety 38 It may be useful to moitor ad record the pressure differetial across the arrester as this will give a early idicatio of cotamiatio or blockage. It is importat that the moitorig device does ot provide a flame path aroud the arrester. 39 If a pre-filter is used upstream of the arrester, it may avoid or reduce blockage. Trace heatig may also help i this respect. Some cautio may be eeded to esure that these measures do ot impair the effectiveess of the arrester. Cleaig ad maiteace 40 Whe selectig the locatio for a arrester, it is importat to cosider its accessibility for cleaig ad maiteace purposes. Some arresters are desiged to allow easy removal ad maiteace. Materials of costructio 41 Flame arrester elemets are ormally made of stailess steel but are also available i specialised materials such as hastelloy, moel or ickel. The housigs are usually made of carbo steel. It is importat that the elemets of the flame arrester are compatible with the operatig eviromet to limit corrosio, blockage or other damage. Stabilised burig 42 Stabilised burig is whe a flame is ot extiguished but cotiues to bur at the flame arrester elemet. I this situatio the flame arrester may overheat util it is o loger able to prevet the flame from propagatig through to the other side. 43 If stabilised burig is a possibility, this may be detected usig a thermocouple liked to a alarm system which may also trigger automatic cut-off of the vapour or gas supply. The flame arrester should have a safe burig time greater tha the respose time of the emergecy shut-dow system. The safe burig time is the period a flame arrester ca edure stabilised burig without flame trasmissio. 44 If, for operatioal reasos, it is ot desirable to stop the flow, flame arresters with miimum safe burig times of up to two hours are available. Methods to quech the flame such as ijectig steam through a ilet port may be required. Oxyge erichmet 45 Gases or vapours mixed with oxyge or oxyge eriched air are more reactive tha mixtures with air ad are more likely to detoate. This eeds to be take ito accout whe specifyig a flame arrester. Flame arrester specificatio sheet 46 The supplier will provide a specificatio or iformatio sheet givig a full descriptio of the flame arrester, icludig its recommeded applicatio, ay limitatios, istallatio ad maiteace procedures. Flame arresters Page 11 of 23

12 Health ad Safety Maitaiig the performace of flame arresters Ispectio ad maiteace 47 Health ad safety law 4 requires that plat ad equipmet is maitaied i a safe coditio. It is essetial that flame arresters are well maitaied. They should be covered by a programme of regular ispectio ad maiteace so that they cotiue to be effective. They should be ispected for damage or distortio followig ay kow or suspected deflagratio or detoatio. 48 Keep records of the specificatio of each arrester, its locatio ad its ispectio ad maiteace history. Iclude relevat iformatio i maiteace ad operatig procedures. 49 The mai maiteace problem is that flame arresters behave as filters. They have a tedecy to block up, so require regular cleaig. Moitorig ad recordig the pressure drop across the elemet will idicate foulig of the arrester ad a suitable cleaig frequecy ca be determied. Alteratively, it may be possible to determie such frequecy by visual ispectio, particularly durig the iitial period of use. 50 Suitable cleaig methods iclude the use of solvets, water, steam, compressed air or ultrasoics. Care is eeded to esure that the cleaig method is compatible with the arrester i terms of the materials of costructio ad its overall robustess. Flame arresters should ot be cleaed by pushig sharp objects ito them because this will damage the flame arrester elemet. 51 If the elemet is damaged, its effectiveess will be impaired ad a replacemet may be ecessary. Carefully esure that the replacemet is of the right specificatio ad that it is fitted correctly, i lie with the maufacturer s recommedatios. Elemets are ofte marked with the specificatio umbers, which may be used as a further check. The effectiveess of a flame arrester may be impaired or destroyed if: it is icorrectly istalled; it is icorrectly assembled; the wrog compoets are used; it is damaged or distorted; it is blocked or cotamiated; process coditios are chaged; plat modificatios are made. Flame arresters Page 12 of 23

13 Health ad Safety Plat ad process modificatios 52 Plat ad process modificatios ca reduce the effectiveess of the flame arrester. Check the flame arrester specificatio to see whether the arrester eeds to be chaged. 53 There may also be a impact o the maiteace requiremets for the arrester. Review the frequecy of ispectio, cleaig ad maiteace after such modificatios. Traiig 54 The provisio of adequate traiig is a requiremet of several pieces of legislatio. 1,5,7 It is essetial that all workers ivolved with the plat desig, operatio ad maiteace are aware of the fuctio of flame arresters ad the importace of maitaiig them i a clea, udamaged coditio. A typical traiig programme will iclude the: importace of correct assembly ad istallatio; effects of damage ad distortio; effects of blockage or cotamiatio; effects of chagig process coditios or makig plat modificatios. Flame arresters Page 13 of 23

14 Health ad Safety Appedix 1 Legal requiremets Health ad Safety at Work etc Act 1974 (HSW Act) 1 The HSW Act is cocered with securig the health, safety ad welfare of people at work, ad with protectig people who are ot at work from risks to their health ad safety arisig from work activities. The HSW Act ad its relevat statutory provisios are used to cotrol the keepig ad use of explosive or highly flammable substaces ad are cocered with precautios agaist the outbreak of fire i all work activities. The HSW Act is eforced either by HSE or by local authorities, as determied by the Health ad Safety (Eforcig Authority) Regulatios Further advice o these matters is obtaiable from HSE s local area offices or the local authority. The Maagemet of Health ad Safety at Work Regulatios These Regulatios require all employers ad the self-employed to assess the risks to workers ad others who may be affected by their work activities so that they ca decide upo the appropriate measures that eed to be take to fulfil their statutory obligatios. These Regulatios also require a assessmet to determie the requiremets for appropriate health ad safety arragemets, health surveillace, emergecy plaig, provisio of iformatio ad traiig. A Approved Code of Practice Maagemet of health ad safety at work 5 gives guidace o the provisios of these Regulatios. A HSE leaflet Five steps to risk assessmet 6 gives simple geeral advice o the steps ivolved i the risk assessmet process. The Dagerous Substaces ad Explosives Atmospheres Regulatios These Regulatios are cocered with protectio agaist risks from fire, explosio ad similar evets arisig from dagerous substaces used or preset i the workplace. The publicatio Desig of plat, equipmet ad workplaces 3 advises o the use of mitigatio measures such as flame arresters to prevet the propagatio of fires ad explosios i itercoected plat ad equipmet. It also advises that vet pipes from plat cotaiig flammable, highly flammable or extremely flammable liquids, required for ormal operatio, should discharge to a safe place ad ormally be provided with a flame arrester. Flame arresters Page 14 of 23

15 Health ad Safety The Provisio ad Use of Work Equipmet Regulatios Uder these Regulatios employers must esure that: suitable equipmet is provided for the jobs ivolved; iformatio ad istructio are adequate; equipmet is maitaied i good workig order ad repair; traiig is provided for operators ad supervisors; equipmet is safeguarded to prevet risks from mechaical ad other specific hazards; equipmet is provided with appropriate ad effective cotrols; maiteace is carried out safely. Guidace o these Regulatios is available i a HSE publicatio, Safe use of work equipmet. Provisio ad Use of Work Equipmet Regulatios Equipmet ad Protective Systems Iteded for Use i Potetially Explosive Atmospheres Regulatios These Regulatios 14 describe measures to prevet igitio by equipmet ad apply both to electrical ad o-electrical equipmet ad protective systems. They apply to all equipmet iteded for use i potetially explosive atmospheres. Flame arresters Page 15 of 23

16 Health ad Safety Appedix 2 Types of flame arrester ad alteratives Flame arrester elemets may be costructed i differet ways. The majority have a solid matrix but some desigs use water or other liquids. This sectio gives a brief descriptio of the differet costructio types curretly available. Other methods to prevet flame propagatio such as fast actig valves ad suppressio are also outlied. Crimped metal Crimped metal arresters may be maufactured from alterate layers of crimped ad ucrimped metal ribbo to produce a rage of differet sized triagular cells, accordig to the degree of hazard. They are robust ad ca be maufactured to withi close toleraces. Because usually less tha 25% of the face of the arrester is obstructed by the ribbo, the resistace to the flow of gas is miimised. They are supplied as both deflagratio arresters ad detoatio arresters. Wire gauze ad gauze packs These arresters are i the form of sigle gauzes or a series or pack of gauzes. They are maufactured i a way which esures that the aperture size is carefully cotrolled. Sigle gauzes are geerally used oly o the eds of vet pipes such as for udergroud petrol taks. Gauze packs are robust with efficiet flame quechig properties ad flow capabilities. They are geerally supplied as deflagratio arresters oly. Compressed or kitted wire These arresters are i the form of wire or a strip of wire mesh woud oto a roll ad compressed ito a cylidrical housig. They have a radom structure with apertures that deped o the amout of compressio applied. They are geerally supplied as deflagratio arresters oly. Note: The above drawigs are ot to scale Flame arresters Page 16 of 23

17 Health ad Safety Perforated plate The elemet cosists of oe or more layers of metal plates perforated with circular holes. They may have greater mechaical stregth tha elemets of similar costructio type but the proportio of free area for gas flow may be lower. Sitered disc These cotai discs or cyliders of sitered metal or other material. The apertures ca be made very small so these arresters are able to quech very violet explosios provided that they are give sufficiet mechaical stregth. However, these arresters have a high resistace to gas flow ad are therefore ormally used where flow resistace is uimportat ad the gases are clea. They are ofte used to protect gases discharged from storage cyliders ad i some types of flammable gas aalysers ad detectors. Metal foam Flame arresters ca be made from a metal foam produced by platig a polyurethae foam. Elemets ca be made i various pore sizes ad are capable of meetig the most severe coditios. They also have low resistace to gas flow. Parallel plate These arresters are costructed of uperforated metal plates or rigs arraged edgewise to the gas flow. They are separated from each other by a small spacer ad ca be made i a rage of sizes. They are very robust ad are able to withstad the most severe coditios. They are easily dismatled for cleaig ad cosequetly are used i the exhaust systems of iteral combustio egies. Oe type used i this situatio is partially self-cleaig so ca be used for loger periods without removal. Note: The above drawigs are ot to scale Flame arresters Page 17 of 23

18 Health ad Safety Packed tower, metal spheres ad pebble box These arresters cosist of a tower or other cotaier filled with small pebbles, Raschig rigs, metal spheres or some other pellet type material. The size of the apertures depeds o the sizes of the pellets which are packed tightly withi the cotaier to prevet movemet. They ca be made to withstad severe explosios. They ca be easily assembled ad dismatled for cleaig purposes but may have a high resistace to flow. Hydraulic or liquid seal I this type of arrester, the gas stream is passed through a liquid (ofte water) which breaks up the gas ito discrete bubbles. Costat moitorig is required to esure that liquid levels are maitaied liquid may be lost through evaporatio or leakage. Also, the gas is wetted by the liquid, so problems may arise dowstream of codesatio or corrosio. Alteratives The followig are alteratives to the static flame arresters already described. They are sometimes referred to as dyamic flame arresters, as they have movig parts. High velocity vet valve These may be fitted to the outlet of vet pipes o storage taks or marie cargo takers for petroleum products. The valve eeds a positive pressure to ope ad the emergig jet of vapour produced durig operatio is at high velocity so that if it igites the high velocity prevets the flame propagatig past the valve. Fast actig valve/slam shut valve Suppressio This may be mouted i a pipelie. As the flame travels dow the pipelie it is detected by a sesor that seds a sigal, to close the valve before the flame reaches it. This system does ot ivolve a pressure drop. A flame sesor may also be used to trigger a suppressio or iertig system which the extiguishes the flame. Suppressats may be used with fast actig valves. Note: The above drawigs are ot to scale Flame arresters Page 18 of 23

19 Health ad Safety Appedix 3 Stadards for flame arresters 1 The Europea Stadard BS EN 12874: 2001 Flame arresters. Performace requiremets, test methods ad limits for use cotais procedures for testig the effectiveess of flame arresters. 8 This supersedes the British Stadard BS 7244:1990 Flame arresters for geeral use which is curretly obsolescet ad may be withdraw. 9 Other stadards which may be ecoutered iclude those developed by the Iteratioal Maritime Orgaisatio 15 ad the Uited States Coastguard. 16 Flame arresters Page 19 of 23

20 Health ad Safety Appedix 4 Glossary of terms deflagratio: Explosio propagatio at subsoic velocity which may be cofied (propagatig withi a eclosure with a speed iflueced by the eclosure), or ucofied (propagatig freely through a uobstructed atmosphere). deflagratio flame arrester: A flame arrester desiged to prevet a deflagratio from beig trasmitted. detoatio: Explosio propagatig at or above soic velocity ad characterised by a shock wave coupled to a combustio wave. detoatio (stable): A detoatio travellig with a velocity equal to the velocity of soud i the burt gas (Chapma-Jouguet velocity). detoatio (ustable or overdrive): I some circumstaces, such as durig the trasitio from a fast deflagratio to a detoatio, the detoatio may become ustable so that higher velocities ad pressures are achieved compared to a stable detoatio. This type of flame is kow as a overdrive detoatio, ad it places a greater demad o the performace of a flame arrester. detoatio flame arrester: A flame arrester desiged to prevet a detoatio from beig trasmitted. edurace burig: This is the property of a flame arrester to edure stabilised burig without flame trasmissio, uder specified test coditios. explosio: Abrupt oxidatio reactio producig a icrease i temperature or pressure, or both simultaeously. flame arrester: A device to be fitted to the opeig of a eclosure or to the coectig pipework of a system of eclosures ad whose iteded fuctio is to allow flow but prevet flames from beig trasmitted. flame arrester elemet: The portio of a flame arrester whose mai fuctio is to prevet flame trasmissio. flame arrester housig: The portio of a flame arrester whose mai fuctio is to provide a suitable eclosure for the flame arrester elemet, ad to allow mechaical coectios to other systems. flammable atmosphere: A cocetratio of flammable gas or vapour i air that falls betwee the upper ad lower explosio limits. It may also be referred to as a explosive mixture or a flammable cocetratio. flare stack: A flare system is used to bur uwated flammable gases i a safe maer. Discharge of the combustio products to atmosphere is via a flare stack. hazard: Somethig that has potetial to cause harm to people, property or the eviromet. iertig: To reder icapable of combustio. lower explosio limit (LEL): The miimum cocetratio of gas or vapour i air below which the propagatio of a flame will ot occur i the presece of a igitio source. It may also be referred to as the lower flammable limit or the lower explosive limit. Flame arresters Page 20 of 23

21 Health ad Safety risk: The likelihood that a hazard will lead to harm withi a specified period or i specified circumstaces. risk assessmet: The process of idetifyig the hazards preset (whether arisig from work or other factors) ad the people likely to be affected by those hazards. It also ivolves a evaluatio of the extet of the risks ivolved, takig ito accout precautios which have already bee take. ru-up distace: The distace betwee the potetial igitio source ad the flame arrester elemet. A maximum ru-up distace is specified for deflagratio arresters. The presece of beds ad obstructios ca be cosidered to icrease the ru-up distace. safe burig time: The period of stabilised burig without flames beig trasmitted. stabilised burig: Steady burig of a flame stabilised at or close to the flame arrester elemet. upper explosive limit (UEL): The maximum cocetratio of gas or vapour i air above which the propagatio of a flame will ot occur i the presece of a igitio source. It may also be referred to as the upper flammable limit or the upper explosive limit. Flame arresters Page 21 of 23

22 Health ad Safety Appedix 5 Refereces 1 BS EN 730-1: 2002 Gas weldig equipmet. Safety devices. Icorporatig a flame (flashback) arrester British Stadards Istitutio 2 Lift trucks i potetially flammable atmospheres HSG113 HSE Books 1996 ISBN Desig of plat, equipmet ad workplaces. Dagerous Substaces ad Explosives Atmospheres Regulatios Approved Code of Practice ad guidace L134 HSE Books 2003 ISBN Health ad Safety at Work etc Act 1974 (C.37) The Statioery Office 1974 ISBN Maagemet of health ad safety at work. Maagemet of Health ad Safety at Work Regulatios Approved Code of Practice ad guidace L21 HSE Books 2000 ISBN Five steps to risk assessmet INDG163 HSE Books 2006 (also available i priced packs, ISBN ) 7 Safe use of work equipmet. Provisio ad Use of Work Equipmet Regulatios Approved Code of Practice ad guidace L22 HSE Books 2008 ISBN BS EN 12874: 2001 Flame arresters. Performace requiremets, test methods ad limits for use British Stadards Istitutio 9 BS 7244: 1990 Specificatio for flame arresters for geeral use British Stadards Istitutio 10 Explosio protectio by deflagratio vetig Natioal Fire Protectio Associatio NFPA 68: BS EN : 2006 Electrical apparatus for explosive gas atmospheres: Geeral requiremets British Stadards Istitutio 12 BS EN : 2004 Electrical apparatus for explosive gas atmospheres. Flameproof eclosures d British Stadards Istitutio 13 Health ad Safety (Eforcig Authority) Regulatios 1989 SI 1989/1903 The Statioery Office 1989 ISBN Equipmet ad Protective Systems Iteded for Use i Potetially Explosive Atmospheres (Amedmet) Regulatios 2005 SI 2005/830 The Statioery Office 2005 ISBN Revised stadards for the desig, testig ad locatio of devices to prevet the passage of flames ito cargo taks i takers MSC/circ December 1994 Iteratioal Maritime Orgaisatio 16 Marie vapour cotrol systems: fial rule Vol 55, No 120, p Uited States Coast Guard Federal Register Flame arresters Page 22 of 23

23 Health ad Safety Further iformatio For iformatio about health ad safety rig HSE s Ifolie Tel: Fax: Textphoe: hse.ifolie@atbrit.com or write to HSE Iformatio Services, Caerphilly Busiess Park, Caerphilly CF83 3GG. HSE priced ad free publicatios are available by mail order from HSE Books, PO Box 1999, Sudbury, Suffolk CO10 2WA Tel: Fax: Website: (HSE priced publicatios are also available from bookshops ad free leaflets ca be dowloaded from HSE s website: British Stadards ca be obtaied i PDF or hard copy formats from the BSI olie shop: or by cotactig BSI Customer Services for hard copies oly Tel: cservices@bsigroup.com. The Statioery Office publicatios are available from The Statioery Office, PO Box 29, Norwich NR3 1GN Tel: Fax: customer.services@tso.co.uk Website: (They are also available from bookshops.) Statutory Istrumets ca be viewed free of charge at Published by HSE 09/09 Page 23 of 23