continuous purge gas flow in flare network
DESCRIPTION
Why Purge Gas is required in Flare SystemsTRANSCRIPT
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Continuouspurgegasflowinflarenetwork
Flare/Ventnetworkinoilrefineries,oilandgasprocessingplants,allthereliefdevicesthatcanemithydrocarbonsareconnectedtoaflare/ventnetwork.Theflare/ventnetworkendsupinaflareorventstackwhichcontinuouslyemitsthehydrocarbonstoatmosphere.Aventstackdoesnotburnthehydrocarbonsandsimplydisposesthemtotheatmosphereatasafelocation.Hencegasflowemittedbyaventstackistypicallysmall.Whenthegasflowsintobecontinuouslydisposedarequitehigh,thegasneedstobecombustedbeforedisposingittotheatmosphere.Thisconsumptionisachievedinaflarestack.Thehydrocarbonsarecontinuouslycombustedattheflarestacktipbeforedisposingtoatmosphere.
AirIngressTypicallytheflare/ventnetworkalongwiththeventKOdrumoperatesatalowpressureclosetoatmosphericpressure.Thispressureisactuallythebuiltupbackpressureduetothecontinuousflaredorpurgedgasflowintheventstack.Pressureattheventstacktipisatmosphericandbackpressureintheknockoutdrumisatmosphericpressureplusthefrictionalpressuredropfromcontinuousventing/purgingofthegas.
Iftheflowofgastovent/flarestackstopsforsomereason,thereisapossibilityofairingressintotheflarestackandintotheventKOdrum,ventnetwork.Thiscanresultinanexplosivemixtureofairandhydrocarbonsinthevent/flarenetwork,whichcanbecatastrophic.
ContinuouspurgingofhydrocarbongastoavoidairingressOneofthewaystoavoidairingressintotheflarestack,ventKOdrum,flarenetworkandsubsequentcatastrophicconsequences,istocontinuouslypurgeasmallflowrateofhydrocarbongases.Thiscontinuoushydrocarbongaspurgeflowintheflarenetworkhelpstobuildupsomepositivebackpressureattheventknockoutdrum.ThisisthenormaloperatingpressurefortheventKOdrumwhennoothersources(reliefvalves,blowdownvalvesetc.)areemittinganyreliefflow.ThuscontinuouspurgegasflowmaintainsasmallpositivebackpressureintheventKOdrum,ventheaderandhelpstokeeptheairout.
Itisimportantforthesourceofthecontinuouspurgegasflowtobelocatedatthefarthestendoftheflare/ventnetworkfromtheflarestack.Thiswaytheconstantpositivebackpressurecanbefeltthroughouttheflarenetwork,atalltherelievingsources.
APIstandard521describestheequationtodetermineminimumpurgegasflowrequirementforaflarestackofgivendiameterandgivencompositionofthecontinuouspurgegasflow.
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Anotherwaystoavoidingressintotheflarestack,flareKOdrum,flarenetworkareliquidseal,flamearrestersetc.Althoughflamearrestersarenotfrequentlyusedduetopossibilityofobstructionintheflarestack.