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# OAR Box 1851 Prepped by Keeia Richards



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## EPA-600/2-76-079 A-90-1 9 Marc h 197 6 ll-A-1 2

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Wate rseal s ar e use d t o preven taflam e fron tan d ai rfro m enterin g th e flar e ga s collectio n system . Th e weigh to f th e wate rsea l cause s i tt o b e locate d a to rnea rgrad e an d therefor e th e sea l canno tb e use d t o ore - ven tai rfro m enterin g th e stack . Knockou tdrum s ar e locate d a to rnea rth e bas e A elevate d ''•*r* eat o sep arat e liqui d fro m gase s bein g burned . I f th e lar b - Viqui d dr - let s ar e no t removed , the y coul d bur n al l th e wa y t o th e ground . Designe d to rgases , flar e line s ca n contai n liquid s fro m liqui d expansio nrfUefs , liqui d carrie d ove rfro m ga s reliefs , an d condense d vapors . Th e knockou tdru m i s use d t o remov e thes e liquid s befor e th e gase s ar e flared . Wate r»e»l s an d knockou tdrum s ar e tovm d o nmos tflar e system s fo rsafet y veasons . }. 2 Groun d Flare s A groun d flar e consist s o f aburne ran d auxilaries , suc h a» . aseal , pilo t jurne ran d igniter . Tw o type s ar e found . On e consist s o f conventiona l burner s dischargin g horizontall y wit h n o enclosures . Thi s flar e mus tb e installe d i n alarg e ope n are afo rsaf e o p • atio n an

d , e protect : i . I f th e ignitio n syste m fail s thi 6 i s no ta s capabl e i ndispersin g th e ga** s a s a n elevate d flare . Fo rthes e reason s thi s typ e of groun d flar e ha s foun d onl y limite d applications . Groun d flare s ma y als o consis to f multipl e burner s enclose d withi nare fractor y shel l a s i n th e recentl y develope d "lo wlevel " flare s (Figs . J - 1 \ an d 3-14) . Th e essentia l purpos e o f alo wleve l flar e i s complet e con cealmen to f

th e flar e flam e a s wel l a s smokeles s burnin g a talo wnois e level . Th e Rare d gase s ar e connecte d by amanifol d t o aserie s o f burne r head s wuic h discharg e th e ga s int o arefractor y enclosure . Mixin g o f th e ga s an d ai ri s accomplishe d b y aserie s o f multi-je tnozzles . Combustio n ai ri s provide d b

y tl • natura l drafto f th e enclosure . Smokeles s burnin g i s obtaine d wit h littl e o rn o stea m becaus e o f th e turbulenc e an d tempera tur e o f th e burnin g zc. clu e t o th e natura l draftan d th e enclosure . Th e siz e of th e enclosur e depend s upo nth e capacit y o f th e flar e bu

tca nbe com e quit e large . A n en>:lope d groun d flar e wit h acapacit y o f 25,00 0 lb/h rha s a n enclosur e 1 O D fee tnig h an d 2 0 fee ti ndiamete r(Ref. 91 . Thi s nam e capacit y coul d b e h.-.ndle cb y a n8-inc h diamete relevate d flare . Th e initia l cost s of

a n enclose d groun d flar e usuall y limit s thei rcapacit y t o |us taportio n o f

aplant s emergenc y dum p rates . However , th e groun d flar e ca n b e designe d t o handl < .nos tHar e occurrence s an d th e remainin g larg e release s ca n b e diverte d t o a n elevate d flar e Figur e 3-1 S i s a schemati cshowin g ho wsuc h asyste m migh twork . Thi s typ e of inte grate d Rar e syste m i s no wbecomin g commo nespeciall y i npopulate d areas .

\

. N ror e e d Dr af

ti-'l are s Th e tcrce d draf

tflar e use s ai rprovide d b y ablow- rt o suppl y primar y ai ran' ! turbulenc e necessar y t o provid e smokeles s .nirnin g o f relie i ^ase s withou tth e us e o f steam . Figur e }-1 6 show s tw o commo ndesign s

2 2P. 4
LMSC-HRE C T RD39019 0

o f force d draftflares. Thi s typ e o f flare combine s smokeles s burnin gwit h lo woperatin g coo tan d reliabilit y becaus e onl y pilo tga s an d electricit y ar e required . Th e flame i s als o suffe rand , becaus e o f th e force d draft , i s les s affecte d b y th e wind . However , thi s flare als o ha s ahig h initia l cost . In e cos tca nru n tw o t o thre e time s th e cos to'aconventiona l flare, mainl y sinc e tw o stack s ar e necessar y t o kee p th e ai ran d ga s separate d unti l the y ar e mixe d an d ignite d a tth e tip . A blowe rflar e shoul d hav e a nautomati cai rturndow n devic e t o e reven texces s ai rfro m quenchin g th e fl*m < an d creatin g smok e i f th e fla : f

;;a s rat e it

reduced . Variabl e spee d blower s o rbaffle s couple d t o flows«*nsin g device s hav e bee n use d o nthes e flares t o exten d thei rturn dow n ratio . Becaus e o f cost s an d turndow nrati o limitations , thi s flare ha s b"». nuse d mostl y i nspecia l applications . I tha s bee n use d mainl y t o provid e smoke)/-* • burnin g wher e stea m i s no tavailable . I tha s als o bee n use d i n tankag e transfe ran d ventin « an d i n co n functio n wit h asmokin g elevate d flare t o provid e smokeles s burnin g fo rday-to-da y flaring . !. 4 Comparativ e Cost s o f Flar e System s Th e capita l an d operatin g cost s toragive nflMrr syste m depen d o n man y factor s suc h a s th e availabilit y o f steam , th e siz e o f

th e flare, th e com positio n o f

th e wast* * ga s an d th e frequenc y o f flaring. Eac h installatio n i s aspecia l problem , th e economic s o f whic h mus tb e solve d fo rth e spe - cifi ccase . VanderMnd e (l< . (, '4) estimate d th e relativ e cos to f equipmen tuse d i nth e smokeles s flare systems . Equipmen trost s includ e aguye d stack , ignitio n piping , pilo tpiping , th* * burne rrin g an d accessaries . A s show nI nTabl e 1- 1 h e loun d tha tth e relativ e cos to f •rr»^*le* « (\*rr- system s wa s no t
• tac kdiamete rdependent . Onth e othe rhand , retail* * cn* ttl th e equip * men tfo raforce d ai rsyste m i s diamete rdependent , becaus e astar k Tabl e j. j RELATIV E COST S O rrl.AHK SVmKM* Typ e o l Flar» - Equipmen tCi»»t! »

12-in . Diim . £4*in (Ham , Stunkin g Standar d I i p Smokeles s Stea m Ti p ('•as Ti p Wate rTi p Force d Draf

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t *<J I .2 0P- 5 , l.MSC-HRE C T RD3901«> 0
SECTIO N V RECOMMENDE D DESIG N METHO D Genera l desig n criteri aapplicabl e t o flar e selectio nan d sizin g wer e discusse d i n detai l i n Sectio n I V i n orde rt o establis h th e state-of - • he-ar tfoundatio n o f existin g desig ncalculations . Th e purpos e o f thi s sectio n i s t o summariz e th e sizin g method * tha tar e considere d representativ e o f

method s bein g use d a tpresent . Alon g wit h Sectio n IV , Refs.2,4 , 5 , 12 , 1 3 an d 1 4 ar e recommende d fo rdetaile d explana tio n o f th e sitin g calculations . A recen trevie warticl e b y Horto n e tal . (Ref . 17 ) provide s adetaile d qualitativ e descriptio no f auxiliar y flare components . A revie wo f Vanderlind e (Ref . 9 ) i s recommende d alon gwit h Sectio n III o f thi s repor tfo radiscussio no f th e flar e stac k component s includin gmethod s o f stea m injectio navailabl e fo rsmoke - les s flaring. Th ? basi s o f

th e followin g discussio n i s a nintegrate d disposa l syste m include s l>ot

h * * elevate d an d a nenclose d groun d (lo wlevel ) fl»re. I n th e e K. !<--

d flare , eithe rai ro rstea m injectio n i s recommende d a s th e preferre d contro l metho d fo rsmok e an d particulates . Wate rinjec tio n i s ales s desirabl e mean s o f contro l (Ref. b). I n th e integrate d s . stem , th e lo wleve l flar e i s use d fo rhandlin g routin e discharge s t o reduc e noise , smoke , luminosit- . an d therma l emissions . Th e elevate d flare, wit h ai ro rstea m fo rparticulat e contro l i s provide d fo rhandlin g full-scal e emergenc y upsets . A doubl e seal " o r"diversio n seal , - >mmo n t o bot h flare s i s use d t o diver tth e wast e ga s strea m fro m th e elevate d flar e towar d th e lo wleve l flar e (Fig . iM . Onl vdurin g maio rupset s d o gase s pas s throug h ti e uppe rleve l u l th e liqui d »«a l an d bur n a tth e elevate d flare . Toxic , noxiou s o rodorou s gase s o rgase s whic h viel d hazardou s * . o m - ••ustio n t>roductj > shoul d b e collecte d i n segregate d pipin g s \stem s an d prcfera' Kdispose d of : vsom e mean s othe rtha nflaring . I f flarin go f sue" , stream s i s required , thes e shoul d b e discharge d directl y int o th e elevate d flar e burne rrathe rtha nint o th e diversio nseal . Dependin g or . strea m volume , thes e stream s ca n ofte nb e treate d accordin g t o usua l method s to rsegregatio n an d disposa l <Ref.«. 4 an d IM . y i r . levate d Flar e <\*te: : 5.1. 1 Require d I'Jesig n Informatio n Th e tollo'Ain g informatio ni s recommende d (Bets . 2 , 4,? , 12 , 1 ) an d 14) . Muc h o f thi s inforrnatio n ca n normall \*» e obtaine d tro m relie f valv e sizin g calculation* .

!v:>< - <> : Materia l t o b e Flare d

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LMSC-HRE C T R039019 0 ) . Percen tUnsaturatio n 4 . Lowe r(Net ) Heatin g Value , U Btu/sc f 5 . Soecifi cHea tRatio , k= C/ C Pv 6 . Mas s Flo wRat e a tMaximu m Discharge , W lb/h r 7 . Averag e Vapo rTemperature , T ° F 8 . Flowin g Pressure , p psi g 9 . Percen tToxic , Odorou s o rNoxiou s Gase s 5.1. 2 Flar e Purne rDiamete r Desig n parameter s applicabl e t o th e calculatio no f th e flar e burne r diamete rinclud e mas s flo wrat e an d discharg e conditions , th e typ e o f flame holde ruse d b y th e manufacturer , an d densit y an d hea tcapacit y ratio s whic h «e.rv e t o defin e th e soni cvelocit y i n th e flui d stream . Principa l desig rc-msideration s Includ e pressur e dro p an d flam e stahilit \considerations . Eithe rma y serv e t o defin e th e diameter . Pressur e dro p rathe rtha nflo wvelocit y i s usuall y th e controllin gfactor . Th e flare burne ri s usuall y limite d t o apressur e dro p o f 2 psi g (6 0 in . H,Ol . T o facilitat e th e sizin g calculations , th e maximu m discharg e rat e i s converte d t o th e equivalen tvolum e o f ai ri nscfh . Th e followin g orific e equatio n i s use d t o calculat e th e burne rdiameter . V ^ = 165 6 (K)(A ) ^I p (5.1 )

wner e V volum e equivalen tflowrate , scfh
r . orilic * factor , dimensionles s A - are aof

flare burne rtip , ft * A-. ' allowabl e pressur e dro p a ttip , in . "-'>& T-. e >>ri:ii e tarto rKi s normall y abou t0.9 . Th e orific e facto ran d th e effectiv e interna l ti p are aar e somewha tvariabl e dependin g o nth e flare desig n an d manufacturer . Maximu m Discharg e Velocity : Th e soni cvelo i it y i s determine d b y assumin g perfec tga s behavio r(Refs . 2 , 12 , l), 14 an d 83 ) accordin gt o th e tollowm g equatio n

^ g KR T M (5.2 )wh e r e P. 7 LMSC-HRE C T RD39019 0

c K R M - . th e acousti cvelocity , o rvelocit y o f soun d i nth e fluid, ft/sec

- dimensiona l constant , 32.1 7 Ib-ft/lbf-se c

= rati o o f

specifi cheats , C / C v p ' r

- ga s constant , 154 6 ft-lbf/°R-lb-mol e

= averag e molecula rweigh t Th e maximu m discharg e velocit y fo rflam e stabilit y depend s somewha t o n th e desig n o f

th e flam e holde ran d th e heigh to f th e flar e pilot 3 abov e th e flare tip . A maximu m vapo rvelocit y o f Mac h 0. 2 i s frequentl y recommende d (Refs.2 , 12 , 1 3 an d 14) . Discussion s wit h flar e vendor s indicat e tha thighe rdesig nvelocitie " ma y b e acceptabl e i nsom e cases , bu tthi s figur e seem s t o b e asaf e desig n basis . I f thi s velocit y i s exceede d i n Eq . (5.1) , th e pressur e dro p o f th e ti p shoul d b e reduce d accordingly . Pressur e losse s i n th e flare seal , riser , entr y an d drum s ma y als o requir e areductio n i n vapo rvelocit y dependin g upo nth e availabl e flowing pressur e o f

th e ven tstream . Fo rconvenience , th e pressur e losse s fro m thes e component s i s commonl y expresse d i n term s o f cor - respondin g los s i n velocit y hea d (Refs . 8 3 an d 84) . Fo rai runde rturbulen t flowconditions , th e equivalen tlengt h o f pip e (i npip e diameters ) i s approximatel y equa l t o th e velocit y hea d los s multiplie d b y n . facto ro f abou t55 . Pressur e losse s fro m th e variou s flar e component s ar e sum marize d i n Tabl e 5-1 . Wate rSea l an d l lam e Arrestors : Wate rseal s an d flam e arrestor s ar e discusse d i n detai l i n Sectio n4.4. 6 dealin g wit h flar e safety . Fo rpurpos e o f th e presen tdiscussion , selectio nrathe rtha nsizin g criteri aar e significan ti n determinin g th e pressur e drop . I ti s th e resul to f thi s stud y tha tflame arrestor s o f conventiona l desig nar e unacceptabl e becaus e o f thei rtendenc y t o fil l wit h solid s an d becom e plugged . Eithe rwate rseal s o rflame arrestor s ar e require d whe nflammable mixture s ar e bein g flared , bu tawate rsea l i s normall y include d i nal l flar e f.ystem s a s aprecautionar y measure . Disentrainmen to r"Knockout " Drum : A disentrainmen tsyste m i s require d wheneve rth e dewr->in to f th e flared strea m i s highe rtha nambien ttempera ture . A variet y o f

design s hav e bee nemploye d fo rliqui d disentrainrrent . Thes e ma y b e eithe rvertica l o rhorizonta l an d ma y b e combine d wit h th e wate rseal . I n som e cases , aslope d pipin g arrangemen twit h adrai na t th e lowe ren d ha s bee n foun d adequat e fo rremova l o f smal l amount s o f

liquid . Typically , liqui d remova l requirement s ar e difficul tt o determin e an d a r e p arat e dru m i s installe d betwee nth e flar e stac kan d header , locate d externa l t o th e flare stack .P. 8



### LMSC-HRE C T RD39019 0

Tabl e 5. 1 PRESSUR ELOSSE S I NELEVATE D FLAR ESYSTE M Componen t
P'lar e Ti p Fluidi cSea l Flar e Rise r Molecula rSea l Knockou tDru m

* 

### Wate rSea l Equivalen tPip e Configuratio n Orific e Orific e Equivalen tLengt h o f

Straigh tPip e 2 t o 3 Clos e Retur n180 ° Bend s 1 Clos e Retur n 180 ° Ben d 1 Clos e Retur n180 ° Ben d an d Dept h o f Diptub e Immersio n Velocity ^ Hea d Los s Equatio n 5. 1 Equatio n 5. 1

— 

### 3.0-4. 5 1. 5
1. 5

*» » 

### If th e temperatur e of

th e ven t

ga s i s significantl y highe r tha nambient , amateria l an denerg y balanc e o nth e wate r sea l i s require dt o determin e th e fina l temperatur e an dth e mol e fractio nof wate ri nth e flared stream
. Unit s inequiva'en t

pip e diameters

. Se e Rcfs.8 ^ an d8 4 fo r details

.

7 4P. 9

l.M.sC-HRE C T RD39019 0
Knockou l dru m desig n an d sizin g criteri aar e give ni nSectio n4.4.8 . Referenc e 6 3 i s recommende d fo raqualitativ e discussio no f th e knockou t drum , a s i taffect s operatin g safety . Dru m location , liqui d holdin g capacity , pum p capacity , othe requipmen trequirement s an d adiscuseto n o f sizin g method s ar e give n i n No . 1 0 o f Ref . 2 . Fo rpurpos e o f th e '^•sur e dro p calculation , th e dru m i s approximate d b y aclos e retur nl 0 u-degre e ben d (Tabl e 5-1) . 5.1. 3 Utilit y Requirement s Stea m an d Ai rRequirements : Eithe rstea m o rai ri s require d fo rflaring mos tgaseou s hydrocarbon s othe r« n component s suc h a s methanol , carbo n monoxide , hydroge n sulfid e an d i> . cfnane o rnatura l ga s whic h bur nsmokelessly , Discussion s wit h flare manufacturin g firm s indica ttha tstea m requirement s commonl y cite d (Refs.2 , 1 2 an d 13 ) ma y no tb e suitabl e fo rdesign . Th e followin gempirica l guidelin e i s suggeste d fo ruse :

wher e W_/ W = a- 10/ M S tH e w (5.3 )

W / W W S t*H c M w a steam-to-hydrocarbo nmas s rati o averag e molecula rweigh to f flared gase s empiricall y determine d paramete rdependen t o n th e typ e o f

materia l bein g flared

e - Fo rparaffin s heavie rtha npropane , a ~ 0.50 . Fo rolefins , a'• 0.60 . Othe r variable s includ e nozzl e desig nan d poin to f injection . Becaus e o f thes e variables , stea m utilizatio npredicte d b y Eq.(5.i ) ma y var y b y +25 r .> . R quire d stea m pressur e i s a tleas t1 0 psi g a tth e poin to f discharge . Ai rrequirement s fo r-onokeles s rwrnin g ar e somewha thigher . Fo rparaffin* , th e recommende d valu e fo rQ i s 0.55 , increasin g t o 0.6 9 fo rolefins . Fo r force d draf

tflares , blowe rrequirement s ar e abou t0. 8 h p fo reac h 100 0 lb/h r o f ga s flared. A s mentione d previousl y (Sectio n4 . i . i) , us e o f wate rt o contro l particulate s i s no tr'- . on : mende d becaus e o f flame quenching , limite d turndown , win d effect s am i othe rproblems . Wate rrequirement s ar e als o fairl y high ; 1. 0 t o 1. 2 l b wate rar e require d fo reac h poiui d o f hydrocarbo nga> . I lit; h Pr e -

^ s u r e H y d ro ta-rhon Streams : Hydrocarbo n- tream s havin g pressure ^ greate rtha n <iT>ou

t? . jjs i ma y b e burne d smokelessl y b y mean s o f specia l tlar e ti p design s whic h increas e air-fue l mixing . Thi s i s arathe rspecialize d applicatio n an d desig n criteri aar e no tgenerall y availabl e excep ta s the y ma y appl y t o specifi cflar e ti p designs .

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