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Annex C (informative)

Sample calculations for sizing a subsonic flare stackC.1 General

This annex presents examples of the to metho!s use! to size subsonic flare stacksbase! on the effects of ra!iation. The first metho! covere! is the simpleapproach presente! in Clause "# the secon! is the more specific approachusing $rzustoski%s an! Sommer%s metho! &'. The height an! location of the flarestack shoul! be consi!ere!* base! on gas !ispersion if the flame is extinguishe!(see ".+).

C., -xample 1 Sizing a flare stack using the simple approachC.,.1 $asic !ata

/n this example* the material floing is h0!rocarbon vapours. The mass flo

rate* m* is 2 +"3 kg4h (133 333 lb4h). The average relative molecular mass ofthe vapours* 5* is "*1. The floing temperature* T* is ,, 6 (7"3 89). Thecompressibilit0 factor* :* is 1*3. The heat of combustion is 23 333 k;4kg (,1 233$tu4lb).

The absolute pressure ithin the flare tip hile flaring* p ,* is 131*+ k<a (1*7 psi). The !esign in! velocit0 (u =) is +,*, km4h (>*' m4s) &,3 mph (,'*+ ft4s).

C.,., Calculation of flare !iameter The 5ach number is !etermine! from -uation(,7) or (,>) from 7.+.1.+.+?

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C.,.+ Calculation of flame length The heat liberate!* @* is calculate! as follos (see igures 7 an! >)?/n S/ units?

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rom igures 7 an! >* flame length* B* is 23 m (173 ft). See igure C.1.C.,. Simple calculation of flame !istortion cause! b0 in! velocit0 The vapourvolume flo rate* vap * is !etermine! as follos?

The flame !istortion cause! b0 in! velocit0 (see igure ') can be represente! b0-uation (C.1)?

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C.,.2 Calculation of reuire! flare stack heightor the basis of the calculations use! in C.,.2* see "..,.+. See igure C.1 for!imensional references.

The !esign basis for these calculations is as follos. The fraction of heat ra!iate!* * is 3*+. The heat liberate! (see C.,.+)* @* is "*+

132 kD (,*12 13' $tu4h).Assume it is necessar0 for the flare stack !esign to limit the maximumalloable ra!iation* 6* at 2*7 m (123 ft) from the flare stack to "*+ kD4m, (, 333$tu4hEft,).

/n -uation (,) in "..,.+.+* the value of F shoul! be assume! to be 1*3. The!istance from the flame centre to the gra!elevel boun!ar0 (that is* the obHectbeing consi!ere!)* I* is then calculate! accor!ing to -uation (,)?

The ph0sical arrangement shon in igure C.1 is the basis of the remainingcalculations in C.,.2.

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6e01 in! !irection

igure C.1 Iimensional references for sizing a flare stack

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C.+ -xample , Sizing a flare using $rzustoski%s an! Sommer%s approachC.+.1 $asic !ata/n this example* base! on $rzustoski%s an! Sommer%s metho! &'* thematerial floing is h0!rocarbon vapours. The flo rate* m * is 1," kg4s (1 333333 lb4h). The relative molecular mass of the flare gas* 5 H * is "*1* an! therelative molecular mass of air* 5 = * is ,'. The normal average in! spee!*u = * is +,*, km4h (>*' m4s) &,3 mph (,'*+ ft4s). The velocit0 of the flare gas atthe flare tip* u H * is measure! in m4s (ft4s). The insi!e !iameter of the flare tip* ! / *is measure! in metres (feet). The absolute pressure ithin the flare tip hileflaring* p H * is 13> k<a (12*7 psi). The average relative humi!it0* 9 J * is 23K. The heat of combustion is 23 333 k;4kg (,1 233 $tu4lb). The compressibilit0factor* :* is 1*3. The loerexplosivelimit concentration of the flare gas in air* C B *measure! as a volume fraction* is 3*3,1 (see C.+.".1). The absolute temperature ofthe flare gas* T H * is ,, 6 (7"3 89)* an! temperature of the air* T = * is ,>' 6 (2,389).

The fraction b0 hich the flame ra!iation is re!uce! hen transmitte! through theatmosphere is in!icate! b0 F . The fraction of heat ra!iate! is in!icate! b0 . Theheat release* @* is measure! in kD ($tu4h)* an! the alloable ra!iation intensit0* 6*is measure! in kD4m, ($tu4hEft,).

C.+., Calculation of flare !iameter The 5ach number is !etermine! as follos (see7.+.1.+.+)?/n S/ units?

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6e0L B C * the loerexplosivelimit concentration parameter for the flare gas* see-uation (C.7)

M x c* horizontal !istance from the stack to flame centre* expresse! in metres

a (! HE9) is the parameter for Het thrust an! in! thrust* see -uation (C.>).

igure C., lame centre for flares an! ignite! vents Jorizontal !istance* x c (S/units)

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6e0L B C * the loerexplosivelimit concentration parameter for the flare gas* see-uation (C.7)

M x c* horizontal !istance from the stack to flame centre* expresse! in feeta (! HE9) is the parameter for Het thrust an! in! thrust* see -uation (C.>).

igure C.+ lame centre for flares an! ignite! vents Jorizontal !istance* x c(NSC units)

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6e0L B C * the loerexplosivelimit concentration parameter for the flare gas* see

-uation (C.7) M 0 c* vertical !istance from the stack to flame centre* expresse! in metresa (! HE9) is the parameter for Het thrust an! in! thrust* see -uation (C.>).

igure C. lame centre for flares an! ignite! vents Oertical !istance* 0 c (S/units)

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6e0

L B C * the loerexplosivelimit concentration parameter for the flare gas* see-uation (C.7)

M 0 c* vertical !istance from the stack to flame centre* expresse! in feeta (! HE9) is the parameter for Het thrust an! in! thrust* see -uation (C.>).igure C.2 lame centre for flares an! ignite! vents Oertical !istance* 0 c (NSCunits)

C.+. Calculation of the !istance from the flame centre to the obHect or point beingconsi!ere!

The !esign basis for this calculation is as follos? The fraction of heat ra!iate!* * is3*+. The heat liberate! (see C.,.+)* @* is "*+ 13" kD (,*12 1313 $tu4h). Sa0

the flare stack !esign must limit the maximum alloable ra!iation (see"..,.+)* 6* is '*2 kD4m, (+ 333 $tu4hEft,)./n -uation (,)* the value of F shoul! be assume! to be 1*3 (see C.+.".+ an!C.+.".). The !istance from the flame centre to the obHect or point beingconsi!ere! (that is* the !istance to the limit of the ra!iant heat intensit0*such as gra!e level* an euipment platform* or a plant boun!ar0)* I* is thencalculate! as follos?

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Thus* at a maximum alloable 6 of '*2 kD4m, (+ 333 $tu4hEft,)* I P 1," m (1+ ft)from the flame centre.

Similarl0* if the maximum alloable 6 is "*+ kD4m, (, 333 $tu4hEft,)* I P 12 m

(237 ft) from the flame centre.

C.+.2 Ietermination of flare stack height The limiting height of the flare stack !epen!s on the !esign criteria selecte! an!the facilities near the flare. At gra!e level* !irectl0 un!er the flame centre* ith 6 upto '*2 kD4m, (+ 333 $tu4hEft,)* the minimum flare stack height* h* is !etermine! asfollos?

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C.+." -xplanator0 notes

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C.+.".1 Boer explosive limits for pure components ma0 be obtaine! fromAGA L63131 &7 or from Q<A JA:31 &+7. The loer explosive limits ofmixtures ma0 be calculate! using Be Chatelier%s 9ule as follos?

hereC B1* C B,*R* C Bn is the loer explosive concentration (i.e. loer flammablelimit) of the component 1* ,*..n in air#0 1* 0 ,*R*0n is the mole fraction (or volume fraction) of the component 1* ,*..n inthe mixture.C.+."., The graphs in igures C.,* C.+* C. an! C.2 are base! on to in!epen!entvariables* C B an! a mo!ifie! form of (! H E9). The variable (! H E9) as mo!ifie!(from that propose! in the $rzustoski an! Sommer article &') to inclu!e gasan! air temperatures an! relative molecular masses instea! of !ensities. The i!ealgas la as assume!. Some a!Hustments ere ma!e in the graph curves over the CB range from 3*2 to 1*2 to smooth out !iscontinuities. Qo significant !ifference*compare! ith han! calculate! results* is intro!uce! ith the !ata smoothing. Seethe original article for the !etails of the han! calculation proce!ure.

C.+.".+ $rzustoski an! Sommer recommen! the use of the fraction of heatintensit0 transmitte!* F * to correct the ra!iation impact. The folloing is uote!from the original article &')?

/n the case of flares* atmospheric absorption attenuates 6 b0 about 13 K to ,3 Kover !istances of 123 m (233 ft). The empirical -uations (C.13) an! (C.11) areobtaine! b0 crossplotting absorptivities calculate! from the Jottel charts. /t isstrictl0 applicable onl0 hen a luminous* h0!rocarbon flame is ra!iating at 1 ,,7 8C

(, ,3 8)* the !r0 bulb ambient temperature is ,7 8C (>3 8)* the relative humi!it0is more than 13 K* an! the !istance from the flame is beteen +3 m an! 123 m(133 ft an! 233 ft)# hoever* the euation can be use! to estimate the or!er ofmagnitu!e of F un!er a i!er range of con!itions.

here F is the fraction of 6 transmitte! through the atmosphere#9 J is the relative humi!it0* expresse! as a percentage#I is the !istance from the flame to the illuminate! area* expresse! in metres(feet).-uations (C.13) an! (C.11) shoul! prove a!euate for most flare gases* except J,an! J,S hich burn ith little or no luminous ra!iation. /f the anticipate! !esign

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con!itions are ver0 !ifferent from those un!er hich -uations (C.1) an! (C.,)ere !erive!* the !esigner shoul! revert to the Jottel charts.C.+.". Dhere steam inHection is use! at a rate of about 3*+ kg (3*7 lb) of steam perkilogram (poun!) offlare gas* then the fraction of heat ra!iate!* * is !ecrease! b0 ,3 K.

Annex I (informative)

T0pical !etails an! sketchesigure I.1 shos a t0pical horizontal flare seal !rum.igure I., shos a uench !rum.igure I.+ shos a t0pical flare installation.Iimensions in centimetres (inches)

6e01 to flare, flare hea!er+ Tr0 cocks for checking for h0!rocarbons vent2 to seer" ater suppl07 submerge! eir el!e! on en! of flare line> ater level' baffle

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13 !raina The seer seal shoul! be !esigne! for a minimum of 172 K of the !rum%smaximum operating pressure.

igure I.1 T0pical horizontal flare seal !rumIimensions in centimetres (inches)

6e0 1 cooling ater, h0!rocarbon+ vent to atmosphere or flare hea!er !rain2 vent

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" ater an! con!ense! h0!rocarbon out to seera The seer seal shoul! be !esigne! for a minimum of 172 K of the !rum%smaximum operating pressure# see 7.+.,.,. for !etails.

igure I., @uench !rum

6e01 oil0 ater seer (to sour ater s0stem if largeuantities of J,S are flare! continuousl0), knockout !rum+ steam!riven pump an! electricall0!riven spare

molecular seal2 purge gas" flomeasuring element7 flare stack> igniter line' flamefront generator13 from knockout !rum11 to flame stack1, ater

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1+ steam1 from relief or vent hea!er s0stem12 vent1" to oil recover0 facilities or slop17 panelmounte!1> ratio

1' steam to nozzle manifol! for smokeless burning,3 poer suppl0 for spark ignition,1 air suppl0,, fuel gas to pilots an! ignition,+ steam for smokeless burning, slope toar!s !ruma /nsert shos alternative sealing metho! (ater seal).

igure I.+ T0pical flare installation

igure I.+ represents an operable s0stem arrangement an! its components. Thearrangement of the s0stem varies ith the performance reuire!. Correspon!ingl0*the selection of t0pes an! uantities of components* as ell as their applications*shoul! match the nee!s of the particular plant an! its specifications.

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