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Pag 81

KeyX heat release, expressed in wattsY flame length (including any lift-off), expressed in metres

1 fuel gas (!8-mm stac")# $lgerian gas well% catalytic reformer & recycle gas ('1!-mm stac") catalytic reformer & reactor effluent gas ('1!-mm stac") dehydrogenation unit (%!-mm stac")' hydrogen (8-mm stac") hydrogen ('#-mm stac")

*+ 1 his figure was con.erted from /igure 80*+ # ultiple points indicate separate o2ser.ations or different assumptions ofheat content0

/igure & /lame length .ersus heat release & 3ndustrial si4es and releases (53units)

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Pag 8#

KeyX heat release, expressed in 6ritish thermal units per hourY flame length (including any lift-off), expressed in feet1 fuel gas (#!-inch stac")# $lgerian gas well% catalytic reformer & recycle gas (#-inch stac") catalytic reformer & reactor effluent gas (#-inch stac") dehydrogenation unit (1#-inch stac")' hydrogen (%1-inch stac") hydrogen (%!-inch stac")*+ ultiple points indicate separate o2ser.ations or different assumptions ofheat content0

/igure 8 & /lame length .ersus heat release & 3ndustrial si4es and releases (75units)

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Pag 8%

Key

X ( ) 9 u u : ;Y or y < x < = = ; ;u : is the lateral wind speedu 9 is the 9et exit .elocity

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3nsert shows the flame geometry in still air and lateral wind0

/igure > & $pproximate flame distortion due to lateral wind on 9et .elocity fromflare stac"

Pag >1

a) 5elf-supporting 2) ?uy-supported c)@erric"-supported

Key1 derric"/igure 1# & /lare structures

'00%0' $uxiliary flaring eAuipment'00%0'01 3f used, the purpose of a liAuid seal in a flare system includes thefollowingB to pre.ent any flash2ac" originating from the flare tip from propagating 2ac"through the flare systemC to maintain a positi.e system pressure to ensure no air lea"age into the flaresystem and permit use of aflare gas reco.ery systemC

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to pro.ide a method of flare staging 2etween an enclosed flare and a full si4eemergency flareC to pre.ent an ingress of air into the flare system during sudden temperaturechanges or condensation offlare gas, such as can occur following a ma9or release of flare gas or following asteaming operation0

Pag0 >%

$73+* & Purge-reduction seals are not flame arrestorsC that is, they will not stopa flash2ac"0 heyare designed as energy-conser.ation de.ices to reduce purge-gas flowsreAuired to mitigate airinfiltration into the stac"0 3n the e.ent of loss of purge-gas flow (and assuming thatthere is no otherwaste-gas flow) the oxygen le.el 2elow the .elocity type seal will almostimmediately 2egin toincrease0 3n the case of the 2uoyancy seal there is a time delay 2etween the

moment the purge gasflow stops and the time the oxygen le.el 2elow the seal 2egins to increase06uoyancy and .elocity seals are typically not applica2le to multi-point, stagedflares0 3n staged systems, thepiping downstream of the staging de.ice may 2e purged with a non-condensa2le inert gas following eachclosure of the staging de.ice0

/igure 1% & Purge-reduction seal & 6uoyancy seal

'00%0'0% he following methods of controlling steam (or compressed air, water andso on) for smo"elessflare control are common (many other strategies are possi2le)Ba) manual operationBanual control usually in.ol.es remote operation of a steam .al.e 2y operatingpersonnel assigned to a

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unit from which the flare is readily .isi2le0 his method is satisfactory ifshort-term smo"ing can 2etolerated when a sudden increase in flaring occurs0 Dith a manualarrangement, close super.ision isreAuired to ensure that the steam flow is reduced following the correction of anupset0 +perating costs

can 2e excessi.e if monitoring is not timely02) tele.ision monitoring with manual controlBhe philosophy is the same as with manual operation except that a tele.isionmonitoring system is addedso that control room operators can monitor and control the steam flow moreeffecti.ely0

Pag 11

/igure 1' & @etermination of drag coefficient

whereE is the .iscosity of the gas, expressed in megapascal-seconds (centipoise)C

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F. is the density of the gas, expressed in "ilograms per cu2ic metre (pounds percu2ic foot)CFl is the density of the liAuid, expressed in "ilograms per cu2ic metre (pounds percu2ic foot)C@ is the particle diameter, expressed in metres (feet)0

*+ Gefer to the section on particle dynamics in the hemical ngineersHIand2oo" J'0

P 118

he second step in si4ing a "noc"out drum is to consider the effect any liAuid

contained in the drum can ha.e on reducing the .olume a.aila2le for .apourLliAuiddisengagement0 his liAuid may result from

a) condensate that separates during a .apour release, or

2) liAuid streams that accompany a .apour release0

he .olume occupied 2y the liAuid should 2e 2ased on a release that lasts #!min to %! min0

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$ single contingency results in the flow of #,# "gLs (#!! !!! l2Lh) of afluid with a liAuid density of >',' "gLm% (%1 l2Lft%) and a .apour density of #,>"gLm% (!,18 l2Lft%), 2oth at flowing conditions0 he gauge pressure is 1%,8 "Pa (# psi), and the temperature is 1> M (%!! M/)0 he .iscosity of the .apour is !,!1 mPaNs (!,!1 cP)0 he fluid eAuili2rium results in %,> "gLs (%1 !!! l2Lh) of liAuid and #1,% "gLs (1'>

!!! l2Lh) of .apour03n addition, 1,8> m% (!! 75 gal) of storage for miscellaneous drainings fromthe units is desired0 heschematic in /igure 1 applies0 he droplet si4e selected as allowa2le is %!! Em(!,!1# in) in diameter0

P 11>

Key1 .apour and liAuid safety relief .al.e releases

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# le.el instrument to indicate when slop and drain .olume has 2een consumed% minimum .apour space for dropout .elocity liAuid hold-up from safety relief .al.es and other emergency releases slop and drain liAuid' to flare pumpout

/igure 1 & /lare "noc"out drum

he .apour rate, G . , in actual cu2ic metres per second (cu2ic feet per second), isdetermined as followsB

P 1#!

he dropout .elocity, u c , is calculated as followsB

$ hori4ontal .essel with an inside diameter, @ 3 , and a cylindrical length,

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$ total of %,> "gLs (%1 !!! l2Lh) of condensed liAuids with a density of >',' "gLm%(%1 l2Lft%) accumulated for %! min occupies a cross-sectional segment Jsee0%0#010# a) as followsB

he cross-sectional area remaining for the .apour flow is expressed as followsB

P1#1he .ertical depths of the liAuid and .apour spaces are determined usingstandard geometry and the total drum diameter, h t , is calculated using Auation(')B

whereh

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where is the liAuid dropout time, expressed in secondsCh . is the .ertical drop a.aila2le for liAuid dropout, expressed in centimetres (inches)Cu c

is the dropout .elocity, expressed in metres per second (feet per second)0he .elocity of * .apour passes, 2ased on one .apour pass, is determined fromAuations (>) and (!) for a.olume flow rate of ,% m%Ls (#'! ft%Ls)B

where $ . is the cross-sectional area, expressed in sAuare metres (sAuare feet)C* is the num2er of .apour passesCu . is the .apour .elocity, expressed in metres per second (feet per second)0P 1##

he drum length reAuired, < min , is determined as followsBmin . < u * Q N N (1)

< min shall 2e less than or eAual to the a2o.e assumed cylindrical drum length,

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3t can 2e concluded from a2le 1 that all of the drum si4es a2o.e fulfill the design reAuirementsC the most suita2le drum si4e should 2e selected according to the design pressure,material reAuirements and corrosion allowance as well as layout, transportation andother considerationsC the choice of two-pass flow, as shown in /igure 1, is optional0

a2le 1' & +ptimi4ing the si4e of a hori4ontal "noc"out drum (75 units)

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3t can 2e concluded from a2le 1' that all of the drum si4es a2o.e fulfil the design reAuirementsC the most suita2le drum si4e should 2e selected according to the design pressure,material reAuirements and corrosion allowance as well as layout, transportation andother considerationsC and the choice of two-pass flow, as shown in /igure 1, is optional03f a .ertical .essel is considered, the .apour .elocity is eAual to the dropout.elocity, which is !,1 mLs (#,% ftLs)0 he .olume flow rate is ,% m%Ls (#'!ft%Ls)0 he reAuired cross-sectional area, $ cs , of the drum, in sAuare metres(sAuare feet) is determined as followsB3n 53 unitsB

he drum diameter, @, is determined as followsB

P1#

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0%0#010% @uring flaring situations, the liAuid le.el in all flare "noc"out drums should2e monitored0 Iigh-le.el alarms should 2e installed to alert the operators ofa2normal "noc"out conditions0 herefore, these alarms should 2e set at arelati.ely low le.el so there is the reAuired hold-up time 2etween the alarm pointand the high liAuid le.el for normal facility shutdown0 Gedundant le.el transmittersmay 2e considered if high alarm relia2ility is needed0 inimum le.els in "noc"out

drums should 2e maintained to ensure sufficient free .olume is a.aila2le in thee.ent of a flaring situation0

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specified .acuum0 3t is important that the purchaser states this performancereAuirement on the data sheet03f possi2le, a minimum height of % m (1! ft) should 2e considered0 $ tan"-2lan"eting regulator, pressure regulator andLor pressure switchLtransmitter thatdumps extra purge gas into the system in the e.ent of .acuum can also 2econsidered in addition to or in lieu of the water seal0

he flare header pressure at which gas 2egins passing through the seal can.ary depending upon the purpose of the water seal (i0e0, pre.ent airinfiltration, act as a flame arrester, act as a staging de.ice or pro.ide 2ac"pressure to a flare gas reco.ery system)0 he gas pressure at the start of flowthrough the liAuid seal can .ary from ! mm (# in I#+) to % !! mm (1#! in I#+)or more0 ypical seal depths are eAui.alent to a gauge pressure of 1%,8 "Pa to %,"Pa (# psi to psi) for staged flares or ',> "Pa to 1%,8 "Pa (1 psi to # psi) where aflare-gas reco.ery system is used0 /or general applications, a seal depth of 1! mm(' in) is common0 3n normal operation, a gas flow that exceeds the lower-stagecapacity or capacity of the flare gas reco.ery unit causes waste gas to start flowingthrough the seal0

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P1#

$ properly designed and operated liAuid seal should allow gas to pass through theseal with minimal surging in gas flow andLor upstream gas pressure0 he design ofthe liAuid seal internals and the si4ing of the .essel can ha.e a significant impacton the a2ility of the seal to meet this performance o29ecti.e0 /or example, acommon design for the end of a dip-leg pipe uses RTS notches cut into the end ofthe pipe wall0 he design is less effecti.e than the proprietary designs de.eloped 2y.endors of liAuid seals0 hese proprietary designs that use alternati.e designguidelines can offer economic or operational ad.antages0 he purchaser shoulddetermine the applica2ility of such designs to the purchaserHs system and situation0$dditional discussion of liAuid seals and examples of alternate seal head designscan 2e found in Geference J110

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0%0#0#0%

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P1#'

$73+* & +nly open the seal-drum-access manway after the complete flaresystem has 2een shut down, 2linded and purged of all hydrocar2ons, gases

and .apours0 hec" oxygen le.el and follow plant safety procedures 2eforeentry (confined space)0 3nspection approximately once e.ery two years or less isrecommended if liAuid-seal pressure drop increases or pulsations occur0

D$G*3*? & Possi2le gas lea"s from the liAuid-seal drum can 2e caused 2y thefollowing0a) excessi.e 2ac" pressure from fouling, free4ing or plugging of the flareeAuipment (arrestor, 2urner or flare stac"), which can displace water from theo.erflow andLor drain u-trap thus allowing gas to escape0 Pressures downstreamof seal should 2e monitored to ensure pressures are not exceeded02) loss of liAuid in water seal and trap, which shall always 2e filled with liAuid tocontain the flare gas0

0%0#0#0 he area for the gas flow a2o.e the liAuid le.el should 2e at least threetimes the inlet pipe cross-sectional area to pre.ent surges of gas flow to the flare0$s descri2ed in 0%0#0#01, the flare-header pressure at which gas 2egins passingthrough the drum can .ary0 he su2mergence depth is set 2y this pressure with dueconsideration for the specific gra.ity of the seal fluid Jsee Auations (') and ()0

his 2ac" pressure should not exceed the maximum 2ac" pressure allowa2le in the.ent header (see 0%010%)0 his 2ac" pressure sets the maximum distance, h JseeAuations (') and (), that the inlet pipe is su2merged0

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whereh is the maximum distance the inlet pipe is su2merged, expressed in metres (feet)Cp is the maximum header exit a2solute pressure, expressed in "ilopascals (poundsper sAuare inch)CF is the sealing liAuid density, expressed in "ilograms per cu2ic metre (pounds percu2ic foot)0

he area for the gas a2o.e the liAuid surface should eAual at least that of a circlewith a diameter, @, that is eAual to #d, where d is the diameter of the inlet gas pipe0his can 2e deri.ed assuming a .ertical .essel that has an internal area eAual to(U@ #)L and an inlet pipe with an area eAual to (Ud #)L0 he annular areais (UL)(@ # V d #)0 5ince the suggested ratio is 1B%, thenB

he height of the .apour space in a .ertical seal drum should 2e approximately !,to 1,! times the diameter, @, to pro.ide disengaging space for entrained seal liAuid0

$ minimum dimension of 1 m (% ft) is suggested0

3n some situations, special considerations can affect the si4e of a seal drum0 +nesuch occurrence is a large flow of hot .apour into the .ent header0 he .acuumcreated when this .apour cools can pull sufficient liAuid into the header to 2rea"the seal, thus allowing air to 2e drawn into the flare system0 o pre.entthis occurrence, the inlet line should 2e constructed to form a .acuum leg0 he total.ertical height of the inlet leg at the seal drum is determined 2y the maximum.acuum expected0 he .olume of liAuid in the inlet line at the maximum .acuumshould 2e o2tained from the seal drum0 his reAuirement can necessitate anincrease in the si4e of the drum0

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P 1#

0%0#0% 5i4ing a Auench drum he si4ing criteria for Auench drums depend soclosely on the design of the drum internals, the liAuid loading, the amount ofcondensation and other features specific to the particular installation that nogenerally meaningful si4ing rules can 2e esta2lished0 $ common criterion is toreduce the temperature of the stream so that the exit liAuid and .apour do notexceed the range of '' M to >% M (1! M/ to #!! M/) and, typically, to assume thatno more than ! W to ! W of the liAuid fed is .apori4ed0 5cheimans articles J11',J11 co.er the si4ing criteria for one type of internals freAuently used in thisser.ice0

0%0#0 @esign details$ con.enient way to state the detailed design reAuirements for a seal drum and a"noc"out drum is to use data sheets from $P3 5td %0@esign details that can 2e applica2le to "noc"out drums and seal drums include thefollowing0a) $nti-swirl or anti-.ortex 2affles should 2e used on the liAuid outlet lines02) 3nternally extended liAuid outlet no44les should 2e used so that sediment settlesout in the drums, not in low spots in the lines0c) $ntifree4e, siphon-type drains should 2e used for normal manual drains if afree4ing pro2lem exists0d) Pro.isions should 2e made for water leg or 2oot and water remo.al if three-phase separation is expected0e) Iandholes J@* 1!! to @* #!! (*P5 to *P5 8) no44les should 2e present onthe 2ottom of the drum to permit thorough cleaning0 hese no44les should ha.e @*! (*P5 1-1L#) or @* ! (*P5 #) .al.es in the 2lind flange to permit completedraining of the .essels 2efore opening0f) $llowance should 2e made for 2linding, .enting, purging (steaming) andpreparing the .essel for entry where manways are pro.ided0

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g) Pro.isions should 2e made for heating the contents of the .essel if coldweather, auto-refrigeration, .iscous or congealing liAuids can introduce pro2lems0 3finternal coils are needed, consideration should 2e gi.en to coil drainage0 he coilsshould ha.e a generous corrosion allowance and adeAuate support to pre.entmechanical failure0 6ecause one side of the .essel shell cannot 2e inspected,heating 9ac"ets that use the .essel shell as one wall should 2e a.oided0

h) ost "noc"out drums and seal drums are operating at relati.ely lowpressures0 o ensure sound construction, a minimum design gauge pressure of% "Pa (! psi) is suggested for "noc"out drums in su2sonic flare or other low-pressure applications0 $ .essel with a design gauge pressure of % "Pa (!psi) should not rupture if a deflagration occurs0 5toichiometric hydrocar2on-airmixtures can produce pea" explosion pressures on the order of se.en to eighttimes the a2solute operating pressure0 ost su2sonic-flare seal drums operate inthe range of gauge pressure from ! "Pa to % "Pa (! psi to psi)0i) 3n designing .essel no44les, attachments, supports and internals, oneshould consider shoc" loadings that result from thermal effects, slugs of liAuid orgas expansion09) ry coc"s for liAuid-le.el detection can 2e desira2le in addition to or instead of

le.el gauges0") /acilities to pro.ide for continuous remo.al or intermittent manuals"imming of hydrocar2ons that can accumulate should 2e considered0 onstants"imming 2y means of continuous addition of seal liAuid and o.erflow to drain can2e used0 Pro.isions for periodically raising the le.el of the seal liAuid to force lighterfluid out through a s"immer connection are permissi2le0 he designer is cautionedto re.iew the proposed system to ensure that lighter material cannot 2uild up to thepoint at which a false (non-design) sealing effect is esta2lished0

P1#8

l) 3nstrumentation components should 2e the simplest and most ruggeda.aila2le and should 2e easily maintained (externally mounted and .al.ed)0 heuse of seals instead of .al.es and of .al.es instead of traps is preferred, primarily2ecause of the nature of the materials handled and the conditions underwhich it is necessary that these components operate0 +n-off .al.es with large flowareas are freAuently preferred to small-passage throttling .al.es0m) Pro.isions for esta2lishing and maintaining an adeAuate seal le.el arerecommended0n) 3f corrosion can occur at the seal fluidL.apour interface, an adeAuate corrosionallowance should 2e used0

5uch corrosion can occur e.en in hydrocar2on systems that use water as the sealfluid or in areas where water can collect at low points in the system03n addition to these common details, some details are specific to the .arious typesof eAuipment0 Knoc"out drums can 2e of the hori4ontal or .ertical typeC and theyshould 2e pro.ided with a pump or draining facilities and instrumentation to remo.ethe accumulated liAuids to a tan", sewer or other location0 he actual type ofdisposal used depends on the characteristics and ha4ards associated with theliAuids remo.ed0 he design of liAuid-remo.al facilities for a "noc"out drum

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depends on the si4e of the .essel and the extent or pro2a2ility of liAuid occurring inthe system03n the simplest system, the .essel might ha.e only a manually operated drain .al.eand a liAuid-le.el sight glass for reference0 $ liAuid-remo.al pump is freAuentlyused on "noc"out drums0 Knoc"out-drum transfer pumps are si4ed for aminimum net positi.e suction-head reAuirement0 heir specification should

also consider the maximum liAuid temperature that can 2e encountered0

$ high-le.el alarm, a manual starting switch and an automatic shutoff switch to thepump motor are generally pro.ided0 ore ela2orate arrangements can alsoha.e high- and low-le.el alarms and le.el controls that operate a motori4eddrain .al.e or a liAuid-remo.al pump0 Dhere a drain .al.e is used, the on-off type ismore commonC howe.er, a throttling type may 2e employed0 he high-liAuid le.el inthe drum is limited so that the cross-sectional area of .apour passage is notreduced0 he low-liAuid limit is esta2lished to pre.ent .apour from entering theliAuid remo.al system0

he seal drum should 2e located 2etween the stac" and the other header drums

and as close to the flare stac" as is practical0 $ .ariation of a seal drum is oftenincorporated into the 2ase of the flare stac" where the flare line enters the stac"0he configuration of a seal drum may consist of a .essel partially filled witha sealing liAuid (e0g0 water)0

he pro2lem of surging in seal drums can 2e minimi4ed 2y the use of slots or Tnotches on the end of the dip pipe so that increasing flow area is pro.ided asthe gas flow increases, utili4ing a principle similar to that in.ol.ed in thedesign of a 2u22le cap0 +ccasionally, it can 2e desira2le to increase the si4e of theinlet line inside the drum to reduce gas .elocity and allow enough circumference forthe slots0 he desired sealing le.el may also 2e maintained 2y means of anautomatic controller operating on the liAuid supply line0

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collection 4one to "eep the system operati.e in situations where low am2ienttemperatures can 2e a factor0