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April 2017

The evolution of the Ex-proof flame path

Aswritteninpreviousdissertation,themostcriticalpartofaflameproofenclosure'Exd'istheflamepath.Although the Standards aredivided intoNational standards (CEI EN60079- ...), Community standards (EN60079-...)andinternationalstandards(IEC60079-...),whichinfactarethesame,wewillquotebelowonlythereference60079-...The reference standard, 60079-1 "Explosive atmospheres Part 1: Equipmentprotectedby explosionproofenclosures‘d’,hasbeenupdatedsothatinthisdiscussionwewillanalysethechangesrelatingtotheuseoftheflamepath.Thisistheoldestmethodofprotectionthatexists,thefirstusedandstilloneofthesafestbecauseitisbasedonaverysimpletechnologyand,therefore,hardlyfallible.Thismethod,designedforprotection insystemswheremaybeanexplosiveatmosphere informofgas, isbasedontheassumptionthatit’simpossibletopreventagastopenetrateeverywhere.Nogasketwilleverbeabletopreventtheentryofagasinanenclosure.Therefore,ifanexplosiveatmospherepenetratesintoanenclosureproducingatrigger,forexampleasparkbetweentwoelectricalcontacts,theexplosionoccurs,butit remains confined within the enclosure and does not allow the spread of flame to the surroundingatmosphere,thuscausingadevastatingexplosion.Toensurethis,enclosuresmustbeconstructedwithamechanicalstrengthsuchastocontaintheoverpressurecausedbytheexplosionandtoallowtheescapeoffluegases.Thisisthemissionoftheflamepathwhichistheinterfacebetweentwopartsofanenclosure,forexamplethebodyandthelid.Itallowsthegasestoexittheenclosureandtocooldownduringthepassage,sothat theyareno longerabletotrigger theoutsideatmosphere.Forthisreason,theflamepathmustbesufficientlylongandwithanintersticeenoughnarrowtoguaranteethecoolingofthefluegases.Flamepath,whetherofpermanentclosedtypeorofthetypeintendedtobeopenedoccasionallyforoperationand/ormaintenanceoperations,mustmeet,intheabsenceofpressure,therequirementsofstandard60079-1standardandconsideringthatthattheyhavetobesizedtosupportthemechanicaleffortresultingfromtheinternaldeflagrationphase.Theflamepathmaybeofdifferenttypes,dependingonthenatureofthegasandthevolumeofthehousingand,consequently,theywillhaveasizeandshapedependingonthetypeofgaspresentintheenvironmentofinstallationoftheexplosion-proofequipment.

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TobetterexemplifyhowmanyandwhichtypesofflamepathsareprovidedfortheStandard60079-1,wewillgiveabriefdescription,consideringthelimitationoftheiruseinaccordancewithwhatprescribedinTables2and3ofthisStandard,whichforconveniencewereportherebelow.Table2:MinimumwidthoftheflamepathandmaximumgapforenclosuresofGroupI,IIAandIIB

Note1 For the determination of the maximum distance, the constructive values must be rounded in

accordancewithISO80000-1(3)Standard.Note2 Inthiseditionof60079-1Standard,twonewcolumnswereintroducedtodividethe"V"volume.This

subdivisionhasbeenmadetointroducemaximumdimensionsofthegapforcylindricalflangedjointsorspigotoneswithaminimumlength"L"of9.5mm,asitwasnon-existentinthepreviousversionofthestandard.Inparticular,ithasbeenintroducedthe"0:08"valueforthegroupIIAandIIB,with"V"volume2000<V≤5750Vand>5750.

Note3 Thesegapandvolumemaximumvaluesassociated,arebasedonhistoricalUSdata,forClassI,Division1,accordingtothedocumentationfromANSI/UL1203(4).

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Table3:MinimumwidthofthejointandmaximumgapforenclosuresofGroupIIC

Note: Forthedeterminationofthemaximumdistance,theconstructivevaluesmustberoundedinaccordance

withISO80000-1Standard.a Flangedjointispermittedforexplosivemixturesofacetyleneandaironlyiftheycomplywithpara5.2.7

ofthestandard60079-1.b Themaximumgapofthecylindricalportionisincreasedto0.20mmif"f"<0.5mmc Themaximumgapofthecylindricalportionisincreasedto0.25mmif"f"<0.5mm

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Now,weanalysethevarioustypesofflamepathconsideredinthe60079-1standardcurrentlyinforce.

1. Threadedjoints

ThreadedjointsshallbecomplyingwiththerequirementsofTables4and5of60079-1standard.Theyaredividedintotwotypessuchas:

a. Cylindricaljoints(refertoTable4of60079-1Standard),theymusthavealargerpitchthanorequal

to0.7mm,whereas if thestep isgreater than2mm,specialprecautionsmaybenecessaryasagreaternumberoffullthreadsinorderthatthesethreadsmeetthetestofnon-transmissionofaninternalexplosion,asprescribedinthetestsofnon-transmissionofaninternalignition(seepara15.3ofthestandard);furthermore,theshapeofthethreadandthequalityofworkmanshipmayhavetheaverageorfinetolerance,accordingtoISO965-1andISO965-3standardsor,iftheydonotcomplywiththesestandardsotherthreadsareallowed,inrelationtothethreadshapeandthequalityofprocessing,ifitisexceededtheflame-prooftestalwaysprescribedinpara15.3ofthestandard,whenthelengthofthethreadedjointisspecifiedbythemanufacturer;ItisreducedasspecifiedinTable9oftheStandard;theymusthaveatleast5fullthreads;theymusthavealengthofthreadgreaterorequalto5mmforvolumelessthanorequalto100cm3orgreaterthanorequalto8mmforvolumehigherto100cm3.

b. Taperedjoints(refertoTable5of60079-1Standard),theymusthaveatleast5threadsofthesame

size,bothontheexternalandinternalside;threadmustmeettheNPTrequirements,accordingtoANSI/ASMEB1.20.1andmustensureafirmcoupling;accessorieswithmalethreadwithanedgeoranthreadinterruptionmustbeprovidedwithaneffectivelengthofthethreadnotlessthanthe"L2"and a length dimension of not less than "L4" between the face of the edge and the end of theaccessorythread;theinternalthreadsmustbecontrolledusingabuffercalibre,fortheverificationoftheL1measurement.

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2. Non-threadedjointsThenon-threadedjointsshallbecomplyingwiththerequirementsofTables2and3ofthe60079-1standard,reportedaboveandtheyconsistof:

a. Spigotjoints.Forthedeterminationofthe"L"widthofspigotjoints,oneofthefollowingshallbe

takenintoaccount:. boththecylindricalpartandtheplanepart.Inthiscase,thegapshallnowhereexceedthe

maximumvaluesshowninTables2and3(seeFigure1a). thecylindricalpartonly(seeFigure1b).Inthiscase,theplanepartneednottocomplywiththe

requirementsofTable2and3.

Figure1a:Cylindricalpartandplanepart Figure1b:Cylindricalpartonly

KeyL=c+d(I,IIAIIB,IIC) d ≥0.50L(IIC)

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c ≥6.0mm(IIC)

≥3.0mm(i,IIA,IIB)

f ≤1.0mm(I,IIA,IIB,IIC)

b. Conicaljoints.Whenjointsincludeconicalsurfaces,thewidthofthejointandthegapperpendiculartothejointsurfaceshallcomplywiththerelevantvaluesofTable2andTable3above.Thegapshallbeuniformthroughtheconicalpart.ForelectricalequipmentofGroupIIC,theconeangleshallnotexceed5°.

Note:Theconeangleistheanglebetweenthemajoraxisoftheconeandthesurfaceofthecone.c. Jointswithpartialcylindricalsurfaces(notpermittedforGroupIIC).Thereshallnotbeintentional

gapbetweenthetwoparts.ThewidthofthejointshallcomplywiththerequirementsoftheTable2above.Thediametersof thecylindrical surfacesof the twoparts forming the flameproof jointand theirtolerancesshallensurethecompliancewiththerelevantrequirementsforthegapofacylindricaljointasperTable2above.

d. Flangedjointsforacetyleneatmospheres.Flangedjointsarepermittedforelectricalequipmentof

Group IIC intended for use in explosive gas atmospheres containing acetylene providing all theconditionrequiredattheparagraph5.2.7ofthe60079-1Standard(gapI:≤0.04mm;widthL≥9.5mmandvolume≤500cm3).

e. Serratedjoints.Theydon’tneedtocomplywiththerequirementsofTable2andtable3abovebut

shallhaveatleastfivefullthreads,apitchgreaterthanorequalto1.25mmandanincludedangleof60°(±5°).Thisserratedjointsshallonlybeusedforjointsthatarefixedduringoperation.Serratedjointsshallsatisfythetestrequirementsof15.3ofthe60079-1Standard.

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KeyY ≥5T

Testlength:Y/1.5

T ≥1.25mm

α = 60° (± 5°)

f. Multi-stepjoints.Theyconsistofnotlessthan3adjacentsegmentswherethepathchangesdirection

notlessthantwotimesby90°±5°.Multi-stepjointdoesn’tneedtocomplywiththerequirementsofTable2orTable3above,butshallsatisfy the test requirements of 15.3 of 60079-1 Standard,with the test length of each segmentreducedtonotmorethan75%ofthemanufacturer’sspecifiedminimumlength.Forflameproofenclosuresthatincorporatemulti-stepjoints,theequipmentcertificatenumbershallinclude the“X” suffix inaccordancewith themarking requirementsof60079-0Standardand thespecificconditionsofuselistedonthecertificateshalldetailoneofthefollowing:- Dimensionsoftheflameproofjointsshallbedetailed;or- Specificdrawingwithdetailsofthedimensionsoftheflameproofjoints;or- Specific guidance on how to contact the original manufacturer for information about the

dimensionsoftheflameproofjoints;or- Specificindicationthattheflameproofjointscan’tberepaired.

KeyH=3adjacentelements a =90°±5°

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3. HolesinjointsurfacesWhereaplanejointortheplanepartorapartialcylindricalsurfaceofajointisinterruptedbyholesintendedforthepassageofthreadedpartsfortheassemblingofaflameproofenclosure,thedistance“l”totheedgeoftheholeshallbeequaltoorgreaterthan6mmwhenthewidthofjoint“L”islessthan12.5mm;8mmwhenthewidthofjoint“L”isequaltoorgreaterthan12.5mmbutlessthan25mm;9mmwhenthewidthofjoint“L”isequaltoorgreaterthan25mm.

Thedistance“l”isdeterminedasfollows.- Flangedjointswithholesoutsidetheenclosure

Figure3 Figure4

Thedistance“l”ismeasuredbetweeneachholeandtheinsideoftheenclosure

- Flangedjointswithholesinsidetheenclosure

Figure5

Thedistance“l”ismeasuredbetweeneachholeandtheoutsideoftheenclosure

- Spigotjointsinwhichtotheedgesoftheholes,thejointconsistofacylindricalpartandaplanepart.

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Figure6

Thedistance“l”isdefinedasthesumofthewidth“a”ofthecylindricalpartandthewidth“b”oftheplanepart,if“f”islessthanorequalto1mmandifthegapofthecylindricalpartislessthanorequalto0.2mmforelectricalequipmentofGroupsIandIIA,or0.15mmforelectricalequipmentofGroupIIB,or0.1mmforelectricalequipmentofGroupIIC(reducedgap)orthewidth“b”oftheplanepartalone,ifeitheroftheabovementionedconditionsarenotmet.

- Spigotjointswhere,totheedgesoftheholes,thejointconsistoftheplanepart,inspiteofplanejointsarepermitted(seeparagraph5.2.7of60079-1Standard).

Figure7 Figure8

Thedistance“l”isthewidthoftheplanepartbetweentheinsideoftheenclosureandahole,whentheholeisoutsidetheenclosure(Figure7),orbetweenaholeandtheoutsideoftheenclosurewheretheholeisinsidetheenclosure(Figure8).

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4.Flamepathsonrotatingmachines

On‘Exd’rotatingelectricalmachines,it’snecessarytoprovideaflamepathonthepropellershaft.Flamepathspositionedinsuchplacesmustnotbesubjecttowear.Theymustbecompletelyfreeandindependentfrom the other structures that support the propeller shaft allowing the rotation. Any bearings, seals,lubricationchannels,etc.mustbeoutsideofthelengthoftheflamepathanditmustnotbeinterrupt.

5.Gaskets(includingO-rings)

Ifagasketofcompressibleorelasticmaterialisused,forexample,toprotectagainsttheingressofmoistureor dust or against leakage of a liquid, it shall be applied as a supplement and not considered in thedeterminationofthewidthoftheflameproofjointnorinterruptit.

6.Roughness

Thesurfacesofjointsshallbesuchthattheiraverageroughness“Ra”doesnotexceed6,3µm.Note:averageroughnessisderivedfromISO468.Determinationcanbemadebyvisualcomparisontoareferenceplate.Todaytheflamepathsurfaces,thankstomachiningcarriedoutbymodernmachinetoolswithnumericalcontrol,reachdegreesofroughnessenormouslylowerand,therefore,moresafe.

7.Jointsprotectedagainstcorrosion

Thesurfaceoftheflamepathcanbeprotectedagainstcorrosionbutisnotadmittedtheircoatingwithpaintsorfinishesbasedpowder.Othertypesofcoatingscanbeused,ifitisprovedthatthematerialusedandtheprocedurestoapplyitdoesnotadverselyaffectthecouplingofflamepathproperties.Itispossibletoapplyananti-corrosiongreasebeforeassembly.Thefat,whenapplied,mustbeoftypenon-hardeningwithaging,itmustnotcontainsolventswhichevaporateandmustnotcausecorrosionofthejoint surfaces and the verification of its suitability must be in accordance with the manufacturer'sspecificationsfat.Weconcluderecallingthattoensurethesafetyoftheenclosure,itisappropriatetocheckthattheflamepathsarenotobstructedand that theyarenot in closeproximity to solidobjects suchaspipes,walls,trellisesorevenotherenclosures,whichmayimpedetheproperexitofgasesfuels.Inthisregard,pleaserefertothemountingandinstallationrequirementscontainedinthe60079-14standard.