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.

Anotherwaystoavoidingressintotheflarestack,flareKOdrum,flarenetworkareliquidseal,flamearrestersetc.Althoughflamearrestersarenotfrequentlyusedduetopossibilityofobstructionintheflarestack.