April 2017
The evolution of the Ex-proof flame path
Aswritteninpreviousdissertation,themostcriticalpartofaflameproofenclosure'Exd'istheflamepath.
Although the Standards are divided into National standards (CEI EN 60079- ...), Community standards (EN
60079-...)andinternationalstandards(IEC60079-...),whichinfactarethesame,wewillquotebelowonly
thereference60079-...
The reference standard, 60079-1 "Explosive atmospheres Part 1: Equipment protected by explosion proof
enclosures‘d’,hasbeenupdatedsothatinthisdiscussionwewillanalysethechangesrelatingtotheuseof
theflamepath.
Thisistheoldestmethodofprotectionthatexists,thefirstusedandstilloneofthesafestbecauseitisbased
onaverysimpletechnologyand,therefore,hardlyfallible.
Thismethod,designedforprotectioninsystemswheremaybeanexplosiveatmosphereinformofgas,is
basedontheassumptionthatit’simpossibletopreventagastopenetrateeverywhere.Nogasketwilleverbe
abletopreventtheentryofagasinanenclosure.Therefore,ifanexplosiveatmospherepenetratesintoan
enclosureproducingatrigger,forexampleasparkbetweentwoelectricalcontacts,theexplosionoccurs,but
it remains confined within the enclosure and does not allow the spread of flame to the surrounding
atmosphere,thuscausingadevastatingexplosion.
Toensurethis,enclosuresmustbeconstructedwithamechanicalstrengthsuchastocontaintheoverpressure
causedbytheexplosionandtoallowtheescapeoffluegases.Thisisthemissionoftheflamepathwhichis
theinterfacebetweentwopartsofanenclosure,forexamplethebodyandthelid.Itallowsthegasestoexit
theenclosureandtocooldownduringthepassage,sothattheyarenolongerabletotriggertheoutside
atmosphere.Forthisreason,theflamepathmustbesufficientlylongandwithanintersticeenoughnarrowto
guaranteethecoolingofthefluegases.
Flamepath,whetherofpermanentclosedtypeorofthetypeintendedtobeopenedoccasionallyforoperation
and/ormaintenanceoperations,mustmeet,intheabsenceofpressure,therequirementsofstandard600791standardandconsideringthatthattheyhavetobesizedtosupportthemechanicaleffortresultingfromthe
internaldeflagrationphase.
Theflamepathmaybeofdifferenttypes,dependingonthenatureofthegasandthevolumeofthehousing
and,consequently,theywillhaveasizeandshapedependingonthetypeofgaspresentintheenvironment
ofinstallationoftheexplosion-proofequipment.
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TobetterexemplifyhowmanyandwhichtypesofflamepathsareprovidedfortheStandard60079-1,wewill
giveabriefdescription,consideringthelimitationoftheiruseinaccordancewithwhatprescribedinTables2
and3ofthisStandard,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
subdivisionhasbeenmadetointroducemaximumdimensionsofthegapforcylindricalflangedjoints
orspigotoneswithaminimumlength"L"of9.5mm,asitwasnon-existentinthepreviousversionof
thestandard.Inparticular,ithasbeenintroducedthe"0:08"valueforthegroupIIAandIIB,with"V"
volume2000<V≤5750Vand>5750.
Note3 Thesegapandvolumemaximumvaluesassociated,arebasedonhistoricalUSdata,forClassI,Division
1,accordingtothedocumentationfromANSI/UL1203(4).
2
Table3:MinimumwidthofthejointandmaximumgapforenclosuresofGroupIIC
Note: Forthedeterminationofthemaximumdistance,theconstructivevaluesmustberoundedinaccordance
withISO80000-1Standard.
a
Flangedjointispermittedforexplosivemixturesofacetyleneandaironlyiftheycomplywithpara5.2.7
ofthestandard60079-1.
b
Themaximumgapofthecylindricalportionisincreasedto0.20mmif"f"<0.5mm
c
Themaximumgapofthecylindricalportionisincreasedto0.25mmif"f"<0.5mm
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Now,weanalysethevarioustypesofflamepathconsideredinthe60079-1standardcurrentlyinforce.
1. Threadedjoints
ThreadedjointsshallbecomplyingwiththerequirementsofTables4and5of60079-1standard.Theyare
dividedintotwotypessuchas:
a. Cylindricaljoints(refertoTable4of60079-1Standard),theymusthavealargerpitchthanorequal
to 0.7 mm, whereas if the step is greater than 2 mm, special precautions may be necessary as a
greaternumberoffullthreadsinorderthatthesethreadsmeetthetestofnon-transmissionofan
internalexplosion,asprescribedinthetestsofnon-transmissionofaninternalignition(seepara15.3
ofthestandard);furthermore,theshapeofthethreadandthequalityofworkmanshipmayhavethe
averageorfinetolerance,accordingtoISO965-1andISO965-3standardsor,iftheydonotcomply
withthesestandardsotherthreadsareallowed,inrelationtothethreadshapeandthequalityof
processing,ifitisexceededtheflame-prooftestalwaysprescribedinpara15.3ofthestandard,when
thelengthofthethreadedjointisspecifiedbythemanufacturer;ItisreducedasspecifiedinTable9
oftheStandard;theymusthaveatleast5fullthreads;theymusthavealengthofthreadgreateror
equalto5mmforvolumelessthanorequalto100cm3orgreaterthanorequalto8mmforvolume
higherto100cm3.
b. Taperedjoints(refertoTable5of60079-1Standard),theymusthaveatleast5threadsofthesame
size,bothontheexternalandinternalside;threadmustmeettheNPTrequirements,accordingto
ANSI/ASMEB1.20.1andmustensureafirmcoupling;accessorieswithmalethreadwithanedgeor
anthreadinterruptionmustbeprovidedwithaneffectivelengthofthethreadnotlessthanthe"L2"
and a length dimension of not less than "L4" between the face of the edge and the end of the
accessorythread;theinternalthreadsmustbecontrolledusingabuffercalibre,fortheverification
oftheL1measurement.
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2. Non-threadedjoints
Thenon-threadedjointsshallbecomplyingwiththerequirementsofTables2and3ofthe60079-1standard,
reportedaboveandtheyconsistof:
a. Spigotjoints.Forthedeterminationofthe"L"widthofspigotjoints,oneofthefollowingshallbe
takenintoaccount:
. boththecylindricalpartandtheplanepart.Inthiscase,thegapshallnowhereexceedthe
maximumvaluesshowninTables2and3(seeFigure1a)
. thecylindricalpartonly(seeFigure1b).Inthiscase,theplanepartneednottocomplywiththe
requirementsofTable2and3.
Figure1a:Cylindricalpartandplanepart
Figure1b:Cylindricalpartonly
Key
L=c+d(I,IIAIIB,IIC)
d ≥0.50L(IIC)
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c ≥6.0mm(IIC)
f
≤1.0mm(I,IIA,IIB,IIC)
≥3.0mm(i,IIA,IIB)
b. Conicaljoints.Whenjointsincludeconicalsurfaces,thewidthofthejointandthegapperpendicular
tothejointsurfaceshallcomplywiththerelevantvaluesofTable2andTable3above.Thegapshall
beuniformthroughtheconicalpart.ForelectricalequipmentofGroupIIC,theconeangleshallnot
exceed5°.
Note:Theconeangleistheanglebetweenthemajoraxisoftheconeandthesurfaceofthecone.
c. Jointswithpartialcylindricalsurfaces(notpermittedforGroupIIC).Thereshallnotbeintentional
gapbetweenthetwoparts.ThewidthofthejointshallcomplywiththerequirementsoftheTable2
above.
The diameters of the cylindrical surfaces of the two parts forming the flameproof joint and their
tolerancesshallensurethecompliancewiththerelevantrequirementsforthegapofacylindrical
jointasperTable2above.
d. Flangedjointsforacetyleneatmospheres.Flangedjointsarepermittedforelectricalequipmentof
Group IIC intended for use in explosive gas atmospheres containing acetylene providing all the
conditionrequiredattheparagraph5.2.7ofthe60079-1Standard(gapI:≤0.04mm;widthL≥9.5
mmandvolume≤500cm3).
e. Serratedjoints.Theydon’tneedtocomplywiththerequirementsofTable2andtable3abovebut
shallhaveatleastfivefullthreads,apitchgreaterthanorequalto1.25mmandanincludedangleof
60°(±5°).Thisserratedjointsshallonlybeusedforjointsthatarefixedduringoperation.
Serratedjointsshallsatisfythetestrequirementsof15.3ofthe60079-1Standard.
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Key
Y ≥5T
T ≥1.25mm
Testlength:Y/1.5
α = 60° (± 5°)
f. Multi-stepjoints.Theyconsistofnotlessthan3adjacentsegmentswherethepathchangesdirection
notlessthantwotimesby90°±5°.
Multi-stepjointdoesn’tneedtocomplywiththerequirementsofTable2orTable3above,butshall
satisfy the test requirements of 15.3 of 60079-1 Standard, with the test length of each segment
reducedtonotmorethan75%ofthemanufacturer’sspecifiedminimumlength.
Forflameproofenclosuresthatincorporatemulti-stepjoints,theequipmentcertificatenumbershall
include the “X” suffix in accordance with the marking requirements of 60079-0 Standard and the
specificconditionsofuselistedonthecertificateshalldetailoneofthefollowing:
- Dimensionsoftheflameproofjointsshallbedetailed;or
- Specificdrawingwithdetailsofthedimensionsoftheflameproofjoints;or
- Specific guidance on how to contact the original manufacturer for information about the
dimensionsoftheflameproofjoints;or
- Specificindicationthattheflameproofjointscan’tberepaired.
Key
H=3adjacentelements
a =90°±5°
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3. Holesinjointsurfaces
Whereaplanejointortheplanepartorapartialcylindricalsurfaceofajointisinterruptedbyholesintended
forthepassageofthreadedpartsfortheassemblingofaflameproofenclosure,thedistance“l”totheedgeof
theholeshallbeequaltoorgreaterthan6mmwhenthewidthofjoint“L”islessthan12.5mm;8mmwhen
thewidthofjoint“L”isequaltoorgreaterthan12.5mmbutlessthan25mm;9mmwhenthewidthofjoint
“L”isequaltoorgreaterthan25mm.
Thedistance“l”isdeterminedasfollows.
- Flangedjointswithholesoutsidetheenclosure
Figure3
Figure4
Thedistance“l”ismeasuredbetweeneachholeandtheinsideoftheenclosure
- Flangedjointswithholesinsidetheenclosure
Figure5
Thedistance“l”ismeasuredbetweeneachholeandtheoutsideoftheenclosure
- Spigotjointsinwhichtotheedgesoftheholes,thejointconsistofacylindricalpartandaplane
part.
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Figure6
Thedistance“l”isdefinedasthesumofthewidth“a”ofthecylindricalpartandthewidth“b”ofthe
planepart,if“f”islessthanorequalto1mmandifthegapofthecylindricalpartislessthanorequal
to0.2mmforelectricalequipmentofGroupsIandIIA,or0.15mmforelectricalequipmentofGroup
IIB,or0.1mmforelectricalequipmentofGroupIIC(reducedgap)orthewidth“b”oftheplanepart
alone,ifeitheroftheabovementionedconditionsarenotmet.
Spigotjointswhere,totheedgesoftheholes,thejointconsistoftheplanepart,inspiteof
planejointsarepermitted(seeparagraph5.2.7of60079-1Standard).
Figure7
Figure8
Thedistance“l”isthewidthoftheplanepartbetweentheinsideoftheenclosureandahole,when
theholeisoutsidetheenclosure(Figure7),orbetweenaholeandtheoutsideoftheenclosurewhere
theholeisinsidetheenclosure(Figure8).
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4.Flamepathsonrotatingmachines
On‘Exd’rotatingelectricalmachines,it’snecessarytoprovideaflamepathonthepropellershaft.Flame
pathspositionedinsuchplacesmustnotbesubjecttowear.Theymustbecompletelyfreeandindependent
from the other structures that support the propeller shaft allowing the rotation. Any bearings, seals,
lubricationchannels,etc.mustbeoutsideofthelengthoftheflamepathanditmustnotbeinterrupt.
5.Gaskets(includingO-rings)
Ifagasketofcompressibleorelasticmaterialisused,forexample,toprotectagainsttheingressofmoisture
or dust or against leakage of a liquid, it shall be applied as a supplement and not considered in the
determinationofthewidthoftheflameproofjointnorinterruptit.
6.Roughness
Thesurfacesofjointsshallbesuchthattheiraverageroughness“Ra”doesnotexceed6,3µm.
Note:averageroughnessisderivedfromISO468.Determinationcanbemadebyvisualcomparisontoa
referenceplate.Todaytheflamepathsurfaces,thankstomachiningcarriedoutbymodernmachinetools
withnumericalcontrol,reachdegreesofroughnessenormouslylowerand,therefore,moresafe.
7.Jointsprotectedagainstcorrosion
Thesurfaceoftheflamepathcanbeprotectedagainstcorrosionbutisnotadmittedtheircoatingwith
paintsorfinishesbasedpowder.
Othertypesofcoatingscanbeused,ifitisprovedthatthematerialusedandtheprocedurestoapplyit
doesnotadverselyaffectthecouplingofflamepathproperties.
Itispossibletoapplyananti-corrosiongreasebeforeassembly.Thefat,whenapplied,mustbeoftypenonhardeningwithaging,itmustnotcontainsolventswhichevaporateandmustnotcausecorrosionofthe
joint surfaces and the verification of its suitability must be in accordance with the manufacturer's
specificationsfat.
Weconcluderecallingthattoensurethesafetyoftheenclosure,itisappropriatetocheckthattheflame
paths are not obstructed and that they are not in close proximity to solid objects such as pipes, walls,
trellisesorevenotherenclosures,whichmayimpedetheproperexitofgasesfuels.Inthisregard,please
refertothemountingandinstallationrequirementscontainedinthe60079-14standard.
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