Prog. Energy Combust. Sci., Vol. 2, pp. 129-141, 1976. Pergamon Press. Printed in Great Britain FLARING IN THE ENERGY INDUSTRY T. A. BRZUSTOWSKI Thermal Engineering Group, Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3GI Abstraet--Flaring is the combustion process used for the safe disposal of large quantities of flammable gases and vapours in the petroleum industry. This paper is a critical review of the technology of flaring and of the state of knowledge on which design information can be based. Discussion includes the length and shape of the flame on an elevated flare, its radiation field, as well as noise and air pollution from flares. NOTATIONS released to the flare, and various kinds of noise-- CL = lean limit concentration of flare gas in air, volume particularly the noise of steam jets and low-frequency fraction vibrations of the flare system. d = diameter A thorough understanding of all these phenomena F = fraction of heat release radiated by the flame is now required for the design of modern flare systems, which are generally larger and, at the same time, subject Fr -~ gd~U~ \P~-Pf/( Pl t densityFr°ude number for a flame of constant to much stricter regulations than their forerunners. L = flame length measured from the nozzle along the flame The design guidelines which have evolved over the years axis from experience with smaller flares must be put on a M =- pu 2 momentum flux firm basis, and reliable methods for extrapolation in = molecular weight both size and performance must be developed. ~h = mass flow p = density The purpose of this paper is to assist this develop- R =-MJM~ ment in two ways. It is written both to acquaint s = curvilinear coordinate along flame axis from the flare combustion specialists with the particular combustion tip problems encountered in flaring, and to introduce S =- s/(djR ~) engineers who specify and design flare systems to the u = velocity x = horizontal coordinate downwind from the flare tip research results which bear on these combustion X = mole fraction problems. A critical description of a number of generic X =- x/(d~R ½) and proprietary designs of flares which are available z = vertical coordinate upward from the flare tip from the principal suppliers in the world market is Z = z/(djR ~) presented for the benefit of both groups of readers. Subscripts f = flame i = i'th component of flare gas mixture 2. THE NEED FOR FLARING j = flare tip discharge conditions L = lean limit The flaring of gases in the petroleum industry occurs = ambient air in three ways. First, there is a requirement for safe I. INTRODUCTION disposal of flammable gases in connection with produc- ing oil fields, in those cases where no provision exists Flaring is the combustion process which has been the for collecting and processing the gas. There was a time traditional method for the safe disposal of large when almost all gas released in this way was flared, quantities of unwanted flammable gases and vapours in but the great value now placed on natural gas has made the oil industry. With the advent of air quality gas recovery economical in many fields. Nevertheless, standards, flaring has also taken on an added if the gas occurs in quantities which are too small for importance as a method of industrial environmental economical processing or if it is so sour that processing control, since most gases which could previously be it would be very expensive, it may still be flared. Such vented to the atmosphere must now be burned in a flaring is usually called production flaring. flare. Flaring also takes place in petrochemical plants, oil Flaring has only recently attracted the attention of refineries and gas processing plants where the flare combustion scientists. It is now recognized that corn- system is one of the s0-called "offsite" facilities. Pro- bustion in the common elevated flare occurs in a cess flaring and emergency flaring can occur in this turbulent diffusion flame in a cross-wind. Such a flame setting. In process flaring, the gas which leaks past the presents a number of fascinating and challenging safety valves protecting the various process units is phenomena for study, including: the effect of cross-wind brought to the flare and burned. This gas feeds the small on its shape an d length, its radiation field, the formation flames which burn almost continuously on refinery flare and dispersion of smoke and gaseous pollutants, the stacks. Process flaring at much greater rates can occur action of steam in suppressing the formation of smoke, when process units are evacuated during a shutdown the completeness of combustion of any toxic gases or when off-specification products are produced during 129
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Prog.EnergyCombust.Sci.,Vol. 2, pp.129-141,1976.PergamonPress.Printed in GreatBritain FLARI NGI NTHEENERGYI NDUSTRY T . A . B R Z U ST O WSK ITh erma l EngineeringGroup,Dep a rtmentofMech a nica l Engineering,Univ ersityof Waterloo, Waterl oo,Ontario,CanadaN 2 L 3 GIAbstraet--Flaring is thecombustion processused for the safedisposal of largequantities of flammable gasesand vapoursin thepetroleum industry. This paperisacriticalreviewof thetechnology of flaring and of the state of knowledge on whichdesign information canbe based.Discussion includes the length and shapeof theflameonanelevatedflare,itsradiationfield,aswellasnoiseandairpollution from flares. NOTATIONSreleasedtotheflare,andvariouskindsofnoi se-- CL =leanlimitconcentrationofflaregasinair,volumeparticularly thenoiseofsteam j etsandlow-frequency fractionvibrations ofthe flare system. d=diameterAthoroughunderst andi ngofallthesephenomena F=fraction of heat releaseradiated by the flameis now required forthe design of modernflare systems, which are generally larger and, at the same time, subject F r-~ gd~U~\ P ~ - P f / ( Pl tdensityFrudenumber foraflame of constanttomuchstricterregulationst hantheirforerunners. L=flame length measured from the nozzle along the flameThe design guidelines which have evolved over the years axisfromexperiencewithsmallerflaresmustbeputona M=- pu 2 momentum fluxfirmbasis,andreliablemethodsforextrapolationin =molecular weightbot hsize and performance mustbe developed. ~h=mass flow p=densityThepurposeof thispaperistoassistthisdevelop- R= - M J M ~ ment intwoways.Itiswrittenbothtoacquaint s=curvilinear coordinate along flameaxisfromtheflarecombust i on specialists withtheparticularcombust i on tipproblemsencounteredinflaring,andtointroduce S=- s/ ( d j R~)engineers whospecify anddesignflaresystemstothe u=velocity x=horizontal coordinate downwind from the flaretipresearchresultswhichbearonthesecombust i on X=mole fractionproblems. A critical descriptionof anumber of generic X=- x / ( d~R )andproprietarydesignsofflareswhichareavailable z=verticalcoordinate upward from the flare tipfromtheprincipalsuppliersintheworldmarketis Z=z / ( d j R~)presentedforthebenefit ofbothgroupsof readers. Subscripts f=flame i=i' th component of flare gas mixture2.THENEED FORFLARING j=flaretip dischargeconditions L=lean limitThe flaring of gases in the petroleum industry occurs =ambient airinthreeways.First,thereisarequirementforsafe I.INTRODUCTIONdisposal of f lammable gases in connection with produc- ingoilfields, inthosecaseswherenoprovisionexists Flaring is the combust i on processwhich has been thefor collecting and processing the gas.Therewasatime traditionalmethodforthesafedisposaloflargewhenalmostallgasreleasedinthiswaywasflared, quantities of unwant ed f lammable gases and vapours inbut the great value now placed on nat uralgas has made theoilindustry.Withtheadventofairqualitygasrecoveryeconomicalinmanyfields.Nevertheless, standards,flaringhasalsotakenonanaddedif thegasoccursinquantitieswhicharetoosmallfor importanceasamethodofindustrialenvi ronment al economical processing or if it is so sour thatprocessing control,sincemostgaseswhichcouldpreviouslybeit wouldbe very expensive, itmaystillbeflared.Such ventedtotheatmospheremust nowbeburnedinaflaring is usually called product i on flaring. flare.Flaring alsotakesplaceinpetrochemicalplants,oil Flaringhasonlyrecentlyattractedtheat t ent i onofrefineriesandgasprocessingplantswheretheflare combust i onscientists.Itisnowrecognizedthatcorn-systemisoneof thes0-called"offsite"facilities.Pro- bust i oninthec ommonelevatedflareoccursinacessflaringandemergencyflaringcanoccurinthis turbulent diffusion flame in across-wind.Suchaflamesetting. I n processflaring, the gaswhich leakspastthe presentsanumberof fascinatingandchallengingsafetyvalvesprotectingthevariousprocessunitsis phenomena for study, including:the effect ofcross-windbrought to the flare and burned. This gas feeds the small on its shape an d length, its radi at i on field, the f ormationflames which burn almost continuously on refinery flare anddispersionof smokeandgaseouspollutants,thestacks.Processflaring atmuch greaterratescanoccur act i on of steam in suppressing the f ormationof smoke,when processunitsareevacuatedduri ngashutdown thecompletenessofcombust i onofanytoxicgasesor when off-specification productsare produced duri ng 129 130T.A.BRZUSTOWSKI tFxG. 1.Principalelementsof a' steam-ring' smokelessflare tip:(1)high-temperaturealloytip,(2)refractorylining,Steam (3)pilotlight,(4)nozzleliporperforated' flame-holder' ,I nl et(5)centre-steamnozzle,(6)steam-ringheader,(7)steam-Fl~.2a.Principalelementsofthe Flaregas FS Antipollutant ringnozzle,flaretip. start-up.Emergency flaring occurs whenlargevolumesorderof magnitude.Ontheotherhand,processflaring ofvolatileliquidsorf lammablegaseshavetoberatesareusuallynomorethanafewpercentof the disposedof safely in an emergency suchas afire, poweremergencyflaring rates. failure,coolingwaterloss,compressorfailure,over-Agoodintroductiontothesystemsavailablefor pressureinaprocessvessel, etc.processandemergencyflaringisgiven byJones. Underemergency conditions, the flaring ratethrough asingle refinery flare mightbeof theorderof 105 kg/ h,3.THETECHNOLOGYOFFLARING withaheatreleaserateof theorderof1000 MWfor afew minutes.ThelargestflareinstallationscanreachThemostc ommontypesoftipsforelevatedflares flaringrateswhichareevenhigher,perhapsbyanare shown in Figs.1-5andare describedin TableI. The Air ~ AirSteam and ai r mixture / N'ji FIG. 2b.Detail of theCoandanozzle which induces air intothe conicalmixing section. Flaring in the energy industry131 i FXG. 3.PrincipalelementsoftheIndairSmokelessFlare:"1[II (1) conical nozzle inside contoured burner tip, (2) adjustable annular nozzleforh.p.gas,(3)h.p.gasnozzleadjustment mechanism,(4)annularh.p.gasduct,(5)l.p.gasduct,FIG. 5.Principal elementsoftheZink Series SA Field Flare (6) pilot lightassembly,tip:(1)ejectortubescarryingsteamandinducedairinto the flame, (2) ejectorentrance nozzle,(3) steam-ring header firstfigureandtablerefertothreegenericdesignsofandnozzles,(4) centre-steam nozzle,(5)acousticalshroud. flares:the"utility"flare,the"centre-steam"flareand the"steam-ring"flare.Figures2,3,4and5andseveralofthefiguresarebasedondetaileddrawings Table1-Dreferstofourproprietaryflaredesigns.Thepresentedinthesalesliteratureof someofthemanu- tablescont ai nself-explanatorycommentsabout thefacturers;therestaretheauthor' srendition basedon constructionandtheparticularadvantagesanddis-thefarlessdetaileddat areleasedbytheother advantagesofalltheflares.Flaretipsof thesedesignsmanufacturers. are generally available forflare stacksvarying betweenFigure6andTable. 2refertogroundflares.Other 0.4and1.2mindiameter,but someflarestacksofgroundflares,inparticularthe"Multij et"flare 1.8 mhavealsobeenbuilt.Itshouldbenotedthatdeveloped by the ( now) ExxonResearchandEngineer- ingCompany, aredescribedindetailbyJones. 1A somewhatsimilardesignisalsoofferedbyAlphons CustodisK.G.ofDiisseldorf.Typicaldesignheat releaseratesforgroundflaresofthetypeshownin Fig.6lieintherangefromlessthan1 MWtomore than100MW, withaflareheightoftheorderof 10-20 m. FIG. 4.Principal elementsoftheSmoke-BanModelSVLFIG. 6.Principal elements of atypical ground flare: (1) ex- flare tip: (1) shroud, (2) steam-ring header,(3) hollow spokesterior frame, (2) refractory lining, (3) flare gas header,(4) flare carrying gas from the central duct totheradialslit nozzles,gas burners, (5) pilot lightswith an independent gassupply, (4) pilot light.(6) louvred enclosure. J PEC S Vol . 2,No. 3 - - B 1 3 2 T . A . BRZUSTOWSKITABLE 1.Tipsfor elevatedflaresditionsmayproducebot hj etnoiseand low-frequency noise (chugging) A."Utility" flare tipssteamringandnozzlesaresusceptibleto damagebyflameintheabsenceof steam ConstructiontipasinFig.1butwithoutcentresteamflow nozzle5, steamring6orsteamnozzles7Availability- - f r omseveralsuppliers - - may havewindscreens surroundingtipof e.g.NationalAiroilNRC flareSamiasmokeless flare Advantages--c heap,quiet,requires no utilities except forZinkseries STF-S ignitionsystem andgasforpilots - -mi ni mumofpartsexposedtothe flameD.Proprietaryandspecial purposeflaretips Disadvantages - -no provision for smoke suppression, can be usedforflaringH2,CH4,H2SandCOFLAREGASSERIES FSANTIPOLLUTANT FLARE TIP butevensmallproport i onsof heavyorConstructiontipasinFig.2a.Principalfeatureisa unsaturatedhydrocarbonscausesmokingperf oratedconicalmixingsectioninto Availabilityf rommostsupplierswhichairisentrainedbysteam-driven e. g. --Flaregas series FNCoanda j etejectors (detailinFig.2b) --Nat i onal AiroilNCGAdvantagesconfinedmixingof hydrocarbonwith --Sami ast andardflaresteam and induced air leads to very efficient --Zi nkseriesSTF-Usteamuse - - st eamwall j etsintheCoandaejectorsare muchquieterthansonicsteamj etsfrom B."Centresteam" flaretipsnozzles intypicalsteam-ring designs Disadvantages - -expensive construction Constructiontip as in Fig.1 butwithoutsteamring6or--openi ngsinmixingsectionmaycause steamnozzles 7problems with flame stability atlow flaring - - may havewindscreens surroundingtipof ratesinastrong wind flare Advantages--c heapestprovisionforsmokesuppressionINDAIRSMOKELESSFLARE ( GKNBirwelco) forinstallationsinwhichonlysmall Construction- - t i p as inFig.3.Principalfeatureisatulip- quantitiesof heavysaturatesandany unsaturatesareflaredshapednozzle with low-pressure gasflow- - h y dr oc ar b ons andsteamarewellmixedingt hroughitandhigh-pressuregas atlow flaring rateseven inastrongwindflowingoverit.Theh.p.gasremainsina - - st eamnozzlenot susceptibletoflamewallj etandfollowsthecurvatureof the nozzlewall( Coandaeffect) whileentrain- damageintheabsence ofsteamflow ingair.The1.p. gasburnswithinh.p.gas Disadvantages - - st eam is not usedtoentrainairintoflame,flame as a result a greater proport i on ofsteamto Advantages--speci al purposeflareforproduci ng fields, hydrocarbonisrequiredforsmokesup- pression t haninothersmokeless flaresoff-shore rigs, etc.,whereh.p.gasisavail- -steam noise is possible at peaksteam flows,able depending on location anddesign ofsteam--ent rai nment of airintoh.p.wallj etis nozzlerelativelysilentandefficient,smokeis suppressed, flameluminosity islow Availability- - f r omseveralsupplierswall j etofh.p.gascoolsnozzlewall,high e. g. --Flaregas seriesFCtemperaturematerialsareunnecessary --Nat i onal AiroilNCS- - maybe operated vertically orhorizontally, - - Zi nk(notadesignatedmodel)withsignificant liquid carry-over Disadvantagescannotbeusedunless h.p.gasisavailable --devel opment toreplaceh.p.gasbysteam C."Steamring" flaretipsseemspossible,butitmaycompromise combustionperf ormance since the1.p. gas Construction- - t i p as in Fig.1, some designs do not includej etwillnotbe surroundedbyaflame the centresteamnozzle5 Advantages--c heapest provisionforsmokesuppressionSMOKE-BANMODEL SVL forinstallationsinwhichlargeflowsofheavysaturatesandunsaturatesmaybeConstruction- - t i pasinFig.4.Principalfeatureisa flaredconverging-diverging conicalshroud.The - - steamisusedtoentrainairintotheflame,flared gas flows from the stackintoacentre lowering the proport i on ofsteam to hydro-bodyfrom whichitemergesthroughslots carbon forsuppression ofsmokeinradialspokes.Steamnozzlesmount ed - - wh en fitted with a centre steamnozzle, canonasteamringundertheshroudinduce be usedover widerangeofflaringratesanairflowoverthespokesinsidetile - - t h i s is the smokeless flare most widely usedshroud inrefineriesandpetrochemicalplants,Advantageslarge area for mixing the hydrocarbon with provendesignsavailablefromseveralairandsteam vendors- - st eamusedeffectivelytoinduceairflow Di s adv ant ages -steam ringmaybenoisy (jet noise) athighinside shroud smokeless flaring rates- - sh r ou dprovidessomeacousticalshielding - - st eamringaloneisnotveryeffectiveforfor thesteamnozzles lowflaringrates,particularlyinastrong--l argecapacityfor smokeless flaring wind,excessivesteamuseatthesecon-Di sadvant ages--expensi ve construction Flaring inthe energy industry133 TABLE 1.Continuedof water sealswhichdivert gastotheelevated flare when the ZTOF is operating at capacity.The system ZINK SERIESSASMOKELESSFIELDFLAREoffersunobtrusiveburningatcapacitylevels sufficientforroutineplant-wideflaring,withthe Construction--t i pasinFig.5.Principalfeatureisthe location of the steamringunderashroudstand-by capacity of an elevated flare for emergency near the bottom of the flare tip. The steamflaring.The one drawbackof thissystemisitshigh cost. nozzlesaredirectedintoejectortubes whichcarrytheinducedairflowthrough the tip up to its exit plane, where the steam- airflowmixeswiththehydrocarbon.ATABLE 3.Suppliers of flare equipment centre-steamnozzle is providedbelow the steamringFlaregasEngineeringLimited,BentinckHouse,Bentinck Road,WestDrayton,Middlesex, EnglandUB7 7SJ Advantages--large area for mixing the hydrocarbon withGKNBirwelcoLimited,MucklowHouse,MucklowHill, airandsteam --steamusedeffectivelyin air ejectorsHalesowen, WestMidlands,EnglandB628DG --shroudprovides some acoustical shieldingNationalAiroil BurnerCompany,1284 EastSedgleyAve., for the steamnozzlesPhiladelphia,Pa.19134, USA --largemaximumcapacityfor'smokelessSamia,S.p.A., 20145Milano-Via Guerrazzi,27, Italy flaringSmoke-Ban Manufacturing, Inc., 711 E. Curtis St., Pasadena, --centre-steamnozzlegivesgoodper-Texas,77502, USA formance atlow flaring ratesJohnZinkCompany,P.O.Box7388, Tulsa,Oklahoma, Disadvantages -- expensive construction74105, USA TABLE 2.Groundlevel flaresTable3isalisting oftheprincipalsuppliersof flare equipment.Anumberof othercompaniesalsobuild Construction- a s inFig.6.Theprincipalfeatureisaflares, buttheygenerallyusetheflaretipsandcontrol refractory-linedelevatedductstanding insidealouvredenclosure.Anumberofsystemsprovidedbytheprincipalsuppliers. small burnersismountedonheadersandFlares are also used in other types ofchemical process providedwithpilotburnersusinganin-plants,e.g.heavywaterplantsusingtheH2Scycle. dependentgas supplyTheyarealsof oundinsteelmillswhereoff gasfrom Advantages-- cheap and unobtrusive, small ground flaresthe basic oxygen furnace andother gases mighthaveto may be built among process units and closebeflared.I nwhatfollows,however,theemphasiswill to property lines because the flames are notbeonflaring inthepetroleumindustry. visible,combustionissmokelessand radiantheating is minimum --manywellseparatedsmallflamesare inherentlycleanerthanasinglelarge4.CURRENTPROBLEMS INFLARING diffusionflame --good design of theindividualburnerscanThreekindsofproblemsareassociatedwithflaring:leadtosmokelessburningoverawideturn-economi cproblems,environmentalproblemsand downratio --turn-downratiocanbeincreasedbysafetyproblems. staging the gas flow to headersOneeconomi c probl em is simply the lossofvaluable --utilitiessuchassteamoraircanbeusedmaterials.Itis generallythou~ghtthatprocessflaring to burndifficultmaterials cleanlyleadstoalossof anaverageof about 0.15-0.5~oofDisadvantages--small capacity, compared to elevated flares,feedstockinrefineriesandpetrochemicalplants.I na Groundflaresoflargecapacitycoverarefineryof200,000bpd(barrelsperday)capacityand large landarea --requireextremereliabilityofflame-outwithcrudeoilat$11perbarrel,thiscouldmeana protectionwhenburningtoxicgasesordailylossofasmuchas$11,000. gasesheavier thanairsince dispersionbyTheenvironmentalproblemsassociatedwithflaring windatthetopof astackis notavailableareprincipallythoseof airpollutionandnoise.Air in the event of flame-out --i nmany designsthe burnersare setupforpollutionisprimarilyinthef ormof visiblesmoke. cleancombustionof specificgases,there-However, unburnedhydrocarbons, SO2,andNOxare foretheflarecanbeusedforroutinepossiblepollutantswhoseappearanceisfarlesscon- flaring associated with given process units,spicuous.I nthecaseof H2Sflares,thedispersionof butnot for plant-wide emergency flaringthistoxicgasastheresultof anyincompletenessofAvailability-- from mostsupplierscombust i on may be a problem. The potentially far more e.g.--Flaregas GroundFlaresseriousprobl emof f lame-outonanH2Sflareisvery --NationalAiroil NPAC --ZinkZTOF (see note)unlikely to arise because the pilot light systems usedon flareshavebeendevelopedtoaveryhighlevelofN ore- - The ZinkZTOFisagroundflare of large capacityreliability.For example,ithasbeenshownthatpilot intheformofatallcylindricalstackoflargelights on ahydrogenflare haveprovidedignitionupto diameterstandingongrade.Withfourstagedwindspeedsof134km/ h. 2 Oncetheflame wasignited, burnersmountedatthe base, the ZTOFisinmany ways similar to an induced-draft furnace.The ZTOFwindsupto176 km/ handsimulatedrainof 114 mm/ h is integrated with an elevated flare througha systemfailedtoextinguishit. 134T.A. BRZUSTOWSKI To suppress smoke formation in processflaring,it isprecisebutactuallyinvolveunprovenformulaefor customary toinject steamintotheflared gas.Variousbasic quantities suchasflame length, flamedeflection waysof doing this weredescribed in the Tables above,in awind, etc.Someof theseformulae arereasonable Theamountof steamperunitmassofhydrocarbonguessesbasedonexperience,butsomeothersare depends both on the type of hydrocarbon (least for lightquestionableAsummarydesignguideforflaresys- saturatedhydrocarbons,increasingwithmolecularterns,contained intheAmericanPetroleumInstitute weight and degreeofunsaturation, most for aromatics)GuideRP521, 8 acceptsmanyoftheserecommenda- andonthedesignoftheflaretips.Inpractice,thesetions uncritically Theaccuracy of thisdocument itself ratiosrangefromsome0.2kgsteamper1 kghydro-is questioned by Heitner.9 carbontoover1 kgsteamperkghydrocarbon.This processofsteamadditionistheprincipalsourceof5.1.Ch oi ceo f T i p D i a m e t e rnoise in processflaring. Steam injection may produce noise in two ways.TheThe choice of the flaretip diameter depends on many first is jet noise associated with the high-velocity steamfactors,includingthe performance of the pilot lights and jets.(Someoftheflaretipsdescribedinthepreviousthe flame-holding characteristics of the tip selected,the sectionhavebeendesignedtominimizethiseffect)composition of thegastobe flared,theavailability of The second is a more indirect effect of the steam injectorpurge gastokeepair from entering theflarewhen the processontheacousticsoftheentireflaresystem,flowof flaregasisveryloworintermittent,1'11 and includingthe stack, the liquid-seal drum in its base, andtheturndownratiorequiredoftheflare.Ofthese theflareheaders.Itismanifestinasub-audibleconsiderations, theturndown ratioappearsnowtobe vibrationatlowflaringratesandhighsteamflows,the most important factor.Modern flaretipsandpilot which becomes an environmental noise problem whenlights appearcapableofstabilizing flamesatgasdis- it rattles windows in aneighbouring community Thischarge velocitiesover100 m/s. Gas seals, such as Zink' s appearstobe j ustoneof severallow-frequency noise"Molecular Seal"andNationalAiroil's "Fluidic Seal" phenomena associated with flares,havereducedtherequiredflowsofpurgegastovery Combustionnoiseisnotgenerallyaprobleminlow values. 12 processflaring.However,atemergencyflaringcon-It is generally easier topredict the maximum flaring ditions whenthegasdischargesatsome100m/stherateatemergency conditionsunderwhichparticular flame could be noisy. Itshould benoted, though,thatprocessvesselshavetobedepressurized,thanitisto in suchcircumstances themostseriousconcern isthepredict the minimum rate of process flaring likely to be safetyproblemcausedbythermalradiationfromtheencountered. Nevertheless,the flare should be designed flame.Theremayalsobemassive smoking, sinceitisin suchawaythatthedischargevelocityof theflare entirelyimpracticaltomaintainastandbysteamgasisaslargeaspossibleunderallconditions. The capacityofsome105 kg/hwhichcanbeinstantlyreason for this requirement is aerodynamic. diverted tothe flareduring an emergency.Theparameterwhichgovernsthebehaviourofthe The principal problem in emergency flaring is one offlaregasjetinthecross-windintheregionnearthe safety.PersonnelandsensitivestructuresmustbeflaretipisR-~M j / M ~ =p j uZ / p ~ u2 ~ , theratioofthe protectedfrom the intense radiation emitted by alongmomentum flux in the jettothemomentum flux of the bent-overflame(flamelengthsoftheorderof100mwind. At sufficiently high values of R,the jetpenetrates havebeenreported,byeyewitnesses,butnotrelatively farintothecross-wind beforeitbends over. documented, in anumber of emergencies atrefineriesAt the same time, the entrainment of air into the jet near and petrochemical plants. Grumer et a l . 3 have reportedthetipisveryrapid.Atlowvaluesof R,ontheother aflame90 mlongonaflare0.75 mindiameterdis-hand, the jet is shearedoff atthedischargeorificeand charging hydrogen at 720 m/s). One way of doing this isissuckedintothewakeofthetipwhereentrainment to"sterilize"anareasurroundingtheflare.Atthisof air is very slowand a lazy smoky flame is produced. point, the safety problem leads to an economic problem.This effectof Risshown inFigs.7and8reproduced The sterilizedspace can be very expensive,particularlyfrom Reference13. 2forafixednozzle,flaregas,if it lies within a plant which is surrounded by built-upSinceRvariesasmj areas and which is being expanded to increase capacity,and wind, a turndown ratio of 1000 between the design emergencyflaringrateandtheminimumexpected 5.CURRENT STATE OFKNOWLEDGEprocess flaring rate through the same tip means that the flamewillbehavesomewhatasinFig.8ratherthan ThisdescriptionofthecurrentstateofknowledgeFig. 7 much of the time.Smoke suppression under such on flaring iswritten withreferencetotheneedsof theconditions isaproblemwithatleastthesteam-ring "offsite"designer whomustselectthediameterof theflaretip(see Tablel-C).Ontheotherhand,Ralso flaretip,andthendecideontheheightofthestack,variesas1/ d4atconstantvhj andgasandwindcon- and on the extent of the restricted area around it.ditions.Thismeansthatiftwotipswereusedin Severalarticlesonthedesignof flaresystems4"5'6'7conjunction, one of themhaving dj fourtimes smaller appeared in the technical literature in the1960s. All ofthan the other, a turndown in flaring rate of 1000 could them suffer to some degreefrom the same shortcoming,beaccomplishedatadecreaseofRofonlyabouta namely, presenting calculations which appear to be veryfactor of four, provided that only the small tip was used Flaring inthe energy industry135 FIG. 7.Theturbulentdiffusionflame in a cross-wind: a detachedflame athighratioRof jetmomentum fluxto cross-windmomentumflux. Exposure for the flame failstoshowthemodelflare whichislocated attherightendofthexintervalmarkedunderthephotographs.Flamecross-sectionsarelong-time averageshapesobtainedbyimmersingpaintedscreensintheflameforseveralminutes.Temperature increasesfromthegraycolouroutsidethecontour,throughtheblackoutline,intothewhiteoxidized region within.(FromReference13.) atlowflaringrates.Itiseasytoseethatifbothtipswouldrequirethelargeflare.Bothflarescould,ofhadthesamedesigndischargevelocity,thesmalltipcourse,be mount edononetowerorerectedasasingle couldhandleupto1/16of designemergencyflaringstack,as hasoftenbeendonewithso-called"sour-gas" rate.Athigherflaringrates,theexcessflowcouldbeflaresalongsideregularflares. directedtothelargerflarebyasystemof suitable waterseals.Suchsystemsexistandarenotexpensive.5.2.Len#th and Shape ofthe Flame If twotipswereusedinthisway,theratiooftheir diameterswouldbechosensuchthatall ofthenormalTheheightof theflarestackandtheextentofany processflaringcouldbehandledbythesmallflarerestrictedareaarounditdependsonthelengthand andonlyashutdownoremergencydepressurizationshapeof theflameandtheradiationf romitunder d j =5m muj = 2 , ~ mls. . ~T=| Se T=I / I 000sec dj3~2 8 4 3FIG. 8.The turbulentdiffusionflame in a cross-wind: an attached flame at alow ratio Rof jetmomentum fluxto cross-windmomentumflux. (FromReference 13.) 136T.A.BRZUSTOWSK! emergency flaringconditions.At themoment , thereisTABLE 4.Calculation of flame shapeandlength 16 nocompletelyreliable met hodofpredictingthelength andshapeofafull-scaleturbulentdiffusionflameinaThemethodisbasedonthecold-flowcorrelationsof cross-wind.AnalyticalmodelsbyEscudier 14andcncentratininwind' blwnhydrcarbnj etslS Brzustowski 15basedonbent -overcircularj etswith1.Requiredinformation: 54~, pj,uj, dj,CL, uo~, p~ , / ~top-hatprofileshaveyieldedsomeinsightintothe problem, but muchmoreneedstobedonebeforea2.Calculatedimensionlesslean-limitconcentrationof fuel reliable predictive procedureemergesf romthiswork.CL =Cd u/ u~ ) ( ~ l / I 9 1)For themoment , theonlydesignprocedurewhich 3.Calculatethedimensionlessco-ordinateSLofthecon- takesintoaccountthedeflectionoftheflamebywindcentrationCL on the axisof the jet andfrom it the down- inevenapproximatelyrealisticfashionisthemodelwindco-ordinateXL. Thisco-ordinateisidentifiedwith proposedbyBrzustowski 16andelaboratedbythe location of the flame tip Brzustowskiand Sommer.17The calculation procedure(i)if C'L 0. 5: SL=2. 51( CL)-'62~ combinesthemeasuredcold-flowcorrelationsforif SL >2.35 ,~L =SL- 1.65 hydrocarbonj etsincross-flow iswiththesuggestionif SL ~< 2.35 SL ~- 1.04 , ~ +2.05 .~o.2s t.oIn this case XL asafunctionof SL isshowninFig.9 4.Calculate the dimensionless rise ZL of the flame tip above the flare tipZL =2.05 .~o.2s 5.CalculatetheratioRof the jetmomentumfluxtothe cross-windmomentumflux U22 R=( p j j ) / ( p ~ u ~ )046.Calculatethedimensionalco-ordinatesof theflametip relative ro the flare tip zL=Z L d j R aleXL=X L d ; R1/2 i [ j ^N ot es : (i)This procedureshouldnot be usedfor uj / u~ >llO (ii)For a mixture of flammable gases,the leanlimit canoften be approximatedby 0.011/ CL=~(XI/CL,~) i components This equation is only an approximation.CL = forinertcomponents.Steamisnotstrictlyan inertandtheuseof thissimplemixingrulefor hydrocarbon-steammixturesmaynotbeac- curate. o.12oiiLI 0 0 ' 5 I ' 0 ~ L 1'5~=2 . 0 2 5afactorof two. 2I nthecaseof propane, theflame FIG. 9.TherelationbetweenSL andXL for SL
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