OTC 3909AGUIDE TO FLARE SYSTEM DESIGN, WITH PART-ICULAR REFERENCE TO OFFSHORE PLATFORMS

by Robert McMurray, Brian J. Oswald, andRodney E. Witheridge; Kaldair Limited

@Copyright 1980Offshore Tecllil-ology Conference

ThIs paper was presented at the 12th Annual OTC In Houston, Tex., May 5·8,1980. The material Is subject to correction by the author. Permission to copy Is restricted 10 an abstract of not more than 300 words.

ABSTRACT

If an offshore flare system is designed withoutbeing integrated from the that of theoverall platform, the resulting system will be lesseffective than need be in terms of safety, cost,energy efficiency and operability.

This paper presents a guide to the mostimportant. factors to be considered in designingoffshore flare systems, including safety, processrequirements and pollution, noise and radiation fromflares. The methods used to calculate flareradiation are reviewed critically. Case studiesare cited to illustrate the cost and materialbenefits which can be achieved if the flare systemis considered from the outset as an integral partof the production process equipment.

INTRODUCTION

In land based oil fields flaring has not beenan important part of the process since the flarecould, until recently, be sited well away from theplant and its environmental impact minimised.Engineers were often tempted to write the words'To the Flare' on an arrow pointing off the edgeof a process flow sheet and then forget them! Quiteoften this represented the bulk of the initialdesign information for the flare system and thedetailed design was not considered until near the endof the project. The flare system then had to meet thediverse and often conflicting requirements of a multi-plicity of gas sources, making design optimisationdifficult if not impossible. _

For offshore fields, particularly in deep waterlocations, the flare is an important part of theprocess because economics often prohibit locatingit sufficiently far away to have no impact on theplatform. __

It thus needs to be considered in detail at theinitial design stage since to some extent the flaresystem can have a significant influence on the processdesign and oil production rate. Early consideration

References and illustrations at end of paper

makes available a greater number of options and leadsto the most efficient and cost effective system.

A previous paperl described how the developmentof new high technology flares has made on-platformflaring. viable, and by eliminating the need for aseparate flare platform has resulted in enormous costsavings for deep water locations such as the NorthSea. The flare is much closer to the operator andpreviously used design calculation methods involvingmany implicit assumptions are no longer satisfactory.Under-design will lead to potentially unsafeoperation whereas over-design carries enormous costpenalties. This paper presents a guide to the mostimportant factors in designing offshore flare systemsand looks critically into one of the most importantaspects - accurate prediction of flare radiation.Case studies are presented which highlight the factthat offshore flare design is not a simple matter andthat fairly detailed analysis is required to producethe most economic system.

FUNDAMENTAL CONSIDERATIONS

At the start of an offshore project the flareengineer is required to produce a system conceptwhich will fully meet the requirements of safety andease of operation as defined by the Codes andStandards produced by the oil operator (and oftenadditionally by the major contractor). The systemmust also meet any existing government legislationon safety, design and environmental pollution.He should also be aware of any impending legislationwhich may come into force during the development ofthe project and reflect this in the system offered.In addition to the skills of the process engineer,the flare system designer also requires a thoroughunderstanding of the techniques for predictingpollution, dispersion, radiation and noise from flares.

These topics are discussed briefly below.

PROCESS AND SAFETY REQUIREMENTS

There is increasing economic pressure toconserve associated gas produced offshore, either byre-injection into the formation, or by transportation

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to shore as gas or liquid. This has tended toreduce continuous flaring requirements, but has ledto the use of increasingly complex, high pressuregas systems, involving moving or rotating machinery.In the case of mechanical failure or other emergency,eg fire, large volumes of gaseous hydrocarbon mustbe disposed of quickly and safely. The use offlares has long been recognised as one of thesafest methods available to the industry.

To dispose of this gas safely, the flare chosenmust offer total reliability of ignition on suddenventing (often under very high wind conditions),and assured combustion under all foreseeableclimatic conditions. Some recent developments inflare ignition and combustion reliability have beendescribed previously.l

To design a safe flare system, at optimal cost,the flare engineer must have a detailed appreciationof the limits imposed by the various sources of gas,and he must be fully aware of the options opento him which could result in cost savings. This isone instance where the flare system design must beconsidered as part o£ the process design as earlyin the project as possible.

The gas sources associated with oil productionoperations can cover a wide pressure range - fromnear-atmospheric for surge tanks and drains, to3000 psig or more for gas injection blowdowns.

Limitations will be set by vessel design,relief valve types.?lld settings, process blowdown andcontrol considerations. The disposal system mustsatisfy many combinations of operating conditionswithout overpressuring individual sources. The'worst' case in terms of. relief discharge may notalways be obvious.

It has already been suggested that usingbalanced relief valves to maximise allowable flareheader backpressure, and segregating high and lowpressure gas sources into seRarate headers, cansave piping and valving costs. 2 ,3 A material costsaving of 34 per cent was quoted in the first ofthe two references given. A recent paper alsopoints out the savings to be made in flare supportstructures by holding and aontrolling backpressureon flare blowdown systems. Holding a high pressureon the flare system also allows smaller knockoutvessels to be used, as superficial velocities canbe kept constant at lower flow·areas. As well assaving equipment weight, valuable platform spaceand structure are released.

grouping gas according to pressuretolerance, the designer can reduce system costsand can also utilise the energy available in thehigh pressure gas where. it is most useful, inpromoting efficient combustion at the flare.High pressure, high efflciency flares have alreadybeen shown to offer significant savings in supportstructure costs. l Case studies 2 and 3, at theend of this paper, give comparisons of performance,size and weight for high pressure and low pressuresystems.

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POLLUTION FROM FLARES

On land based installations there is growingpressure to eliminate smoke and reduce airbornepollution in the flare plume, pressure whichincreasingly carries legislative weight. Althoughthe same pressures are not so much in evidenceoffshore at present, it is to be expected thatlegislation will be applied in the future.

The most obvious source of pollution fromflares is smoke, resulting from insufficientaeration of the hydrocarbon flame. Smoke isproduced by cracking and polymerisation reactionswhich take place in regions of the flame whereoxygen is lacking, and high temperatures exist(eg the flame core). The chemical intermediatesof such reactions and the carbon particlesthemselves are strong emitters of radiation in theinfra-red band and contribute significantly to theradiation emitted by the flare. Smoke can beeliminated by improved mixing of atmospheric air(oxygen) into the flame, since the oxidationreactions are kinetically much faster than thoseinvolving formation of the smoke precursors.Reduction in the concentration of carbon particlesand precursors automatically reduces the radiationemitted. Conventional techniques of steam orair injection into the flame are not suitable foroffshore use, as such services are not usuallyavailable in sufficient capacity. Installation ofsuch services imposes a large cost and weightpenalty as well as providing an increasedmaintenance load. Happily the pressure energy ofthe gas itself can be used to inspire air andprovide smokeless, low radiative combustion.A range of Indair and Mardair flares, based on theCoanda effect, which can utilise this energy forsmokeless combustion have been describedSegregation of high and low pressure gas sourcesto optimise energy utilisation can bring otheradvantages in cost and system flexibility. Theseare discussed in other sections of this paper.

No method of predicting dispersion ofpollutants and ground level concentrations fromflares has been published. Our own methods havebeen developed from equations used to. predictdispersions from chimneys but are not, as yet,backed by the extensive field measurements whichwe normally require to substantiate our computations.

Tower mQunted on-platform flares rarelypresent any dispersion problems since wind blowsany pollutants away from the platform. More careis needed for remote or boom mounted flares whereit is essential to ensure that there can be nopossibility of the hot, oxygen deficient gas plumebeing blown over manned areas under adverseconditions. An operator caught in the plume couldpossibly collapse and die, whereas he might surviverelatively high radiation exposure with superficialinjuries by escaping to shelter.

Equally important is consideration of bothH2S and S02 dispersion from sour gases. Thethreshold limiting value (TLV) must not be exceeded

under any circumstances and the design should,for all but exceptional cases, give acceptable airquality standards of such pollutants back on theplatform to provide a reasonable working environmentfor personnel.

Finally, consideration must be given to thepossibility of gas flowing to the flare without beingignited. Current nare technology has, to all··intents and purposes, eliminated the risk of flameblow-off, but it is still within the power of therig operator to blow-down gases without lightingthem.

RADIATION FROM FLARES

In the absence of flame temperatures, F providesa more reliable guide as to how much radiation isemitted. However, other literature5 ,lZ,13,14 hassuggested that F is a property of the fuel and thatfor instance F = 0.17 for.methane. A quickexperiment with a bunsen burner will convince youthat this is only half the story. Measurementstaken with the air hole closed (yellow flame) thenopen (blue name) showed that radiation fell by afactor of 0.78! Recent experiments on small flames15have shown that F is not simply a fuel property.Large scale field trials confirm this (see Figure 1).The most important influences on F are exit velocityand nare type (mainly how efficiently is air mixedinto the name).

The formula quoted is

The method given inAPI-RP-521 is notmathematically correct but a slightly modified formgives good results. The modified method6 ,although based on a sound premise, appears tooverestimate flame rises observed in practice.

A knowledge of the radiation field produced byflares is essential to the designer of flare systems.Current literature on this subject is very limitedand the 'standard' treatment of this subject iscontained in API-RP-5213 • The method is attributedto Hajek and Ludwig5 who in turn refer back to aconference on flare stacks and seal pots inJanuary 1958.

NOISE FROM FLARES

For land-based systems the above uncertaintiesmay not matter since a safety factor can be appliedto the final design without serious cost penalty.For offshore applications however, personnel cannot'run away' if th"e system is underdesigned and thecost penalties of overdesign can be enormous.In particular it is clear that the long thin flametypical of most nares cannot be adequatelyrepresented by a single point source except in limitedcircumstances. Practically no data have been putforward to support the model represented byequation 1. One paper8 highlights the inherentinaccuracy of it by stating that a higher effectiveF needs to be used to explain some experimentalresults. In light of the dearth of publishedinformation we have, over the last ten years,collected a substantial amount of radiation datafrom controlled field trials involving flowratesup to ZOO MMscfd of gas. A typical sample of rawtest data is shown in Figure 2. Careful analysisof field test results has confirmed the inadequacy ofequation 1 .in certain areas of the radiation field,particularly at shallow view angles to the flameand at locations relatively close to the flare.Kent's modellZ ; which considers a series of pointsources located along the flame length (by integra-tion), overestimates radiation in areas. Oneof the few other published modelsl considers theflame as a diffuse radiator located at the'flame centre' (a point source modified by a viewfactor sin Q such that the flare radiates mostperpendicular to the flame axis and zero along theflame axis). This model underestimates radiationin those areas. We have thus developed our ownflare model7 from analysis of measured data. Thismodel consists of spherical and diffuse radiatorslocated along the flame axis and takes into accountthe real shape of the wind-blown flame and itsorientation with respect to the receiver. The modelis simple enough to fit onto a programmablecalculator but when used on a computer with plottingfacilities provides a powerful analytical tool.With the computer, the main incentive for usingequation 1 - its simplicity - disappears andaccurate predictions of radiation can be made withoutsignificant increase in effort or time expended.

Very little has been published on the noiseradiated by flares. United Kingdom codes of practicesuggest a limit for continuous exposure over an8 hour period of 90 dBA for personnell7 , and allowthis to be adjusted to 88 dBA for a lZ hour shift.This figure assumes broad band noise, with nonarrow band or impulsive characteristics. Noise from

• (Z)

•• (1)

eX f.T 4fFQ

This apparently simple formula is in realityfar more complex, since d is the distance from the'centre of radiance' to the observation point andinvolves a mathematical description of the length,shape and position of the flame under the influenceof crosswinds'6 Flame Length has been the subjectof some debate and it should be.realised that itis dependant upon flare type. For example,Indair type flares give flame lengths? around onehalf of that predicted by API-RP-521 and Mardairtype flares give even shorter flame lengths.Recent work8 ,9,10,11 on small scale flames hassuggested that cold flow correlations of jetdistortion in crosswinds based on momentumconsideration may be applicable to flares. The APIguide uses a velocity-based which hasbeen suggested should be modified. Whilst themomentum-based method gives good predictions nearthe flame tip, it underestimates the rise over thelatter part of the flame.

There is also some doubt as to the location ofthe 'centre of radiance'. Hajek and Ludwig5 givean example calculation in which it is implicit thatit is taken at the top of the flare stack. The APIguide, however, takes it as being located at one-halfof the total downwind distance that the flame isdistorted by (which can be grossly different fromlocating it at one half of the curvilinear flamelength). Perhaps the most serious area of uncertaintyis the selection of F factor. This often seems to beconfused with emissivity £. These two quantitiesare fundamentally different and are related by

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the flare itself could then be limited to around85 dBA, which allows a non-flare source of equalintensity to be present at the same time. This isapplied to the continuous flaring rate only, with theemergency rate being assumed to be of short duration.

More recently a new concept of equivalent noiselevel (Leq) has been introduced and futurelegislation may adopt this criterion rather thanprevious blanket levels. An Leq of 90 dBA relatesnoise exposure to the accumulated noise energyreceived over a period of time. ‘bus - ‘eq ‘f90 dBA could be composed of 9~dBA continuously for8 hours, or 93 dBA for k hours or 94 dBA for 2_hoursfollowed by 85 dBA for 10 hours etc. With theavailability of Le dose meters, a more rationaJ.approach to noise ?imitation may be adopted.

Noise from flares may be split into two discretesources - combustion noise and jet noise.Combustion noise dominates the low frequencies(63, 125, 250 and 500Hz octave bands) whereas jetnoise dominates the high frequencies (lk, 2k, Xkand 8k Hz octave bands). In general.there is nosignificant interaction between these two sourcesand they may be added logarithmically to obtainthe overall noise for any flare. Figure 5 shows atypical spectrum for a sm~l, near sonic pipefl,areboth burning and cold-venting (modern pipeflares incorporate flame stabilisation devices SOthat the so-called blow-off limit of 0.2 Mach nolonger applies - the limiting factor is now usuallythe available pressure).

The estimation of jet noise is difficult sinceit depends on burner type. No general rule cs-nthus be given to estimate it. However, it is worthnoting that jet noise is highly directional incharacter and if it dominates the spectrum thetotal flare noise will be directional.in charactertoo (typically falling by ~10 dBA from perpendicularto axial positions to the jet). High frequency (jet)noise can also be attenuated very effectively(up to 15 dBA) by correctly designed and positionedacoustic shielding.

For most pipe flares__operatingnear 0.2 @ch thecontribution from jet noise is minimal.and combustionnoise dominates the whole spectrum. For pipe flaresof higher Mach numbers, jet noise must also beconsidered. Unlike jet noise, combustion noise isnot very directional in character. The followingempirical formula, derived from field trials, maybe used to estimate combustion noise

@kwL = 50+810g10Q . . . . . ...(3)

The concept o? an A weighted sound power level(dBAs~) is useful in correlating field data andproviding a single value.for flare noise but shoedbe used with care since in estimating sound ~ressurelevels at distances over -200 feet no allowancefor atmospheric attenuation of high frequency noisecau be made. In calculating sound pressure levelsthe flare tip itself should be used as the‘noise source! rather than the flame centre.

FLARE SYSTEM DESIGN

The design of a flare system is an iterativeprocess since feedback on the maximum boom length

or tower height which can be accommodated on theplatform may affect feasible production rates andhence flaring rates. Safety, economic andstructural considerations all influence the design,right back to the point where individual gas sourcesenter the flare headers.

If tackled at an early stage in the projectseveral flare options should be available and theproject team will be able to evaluate which is bestsuited to the particular application. If leftuntil process and platform details are firmed upthe flare system will, at best, be unnecessarilycostly. At worst, operating throughput could berestricted to meet safety requirements.

The following sections give suggested stepsin carrying out a typical system design. Casestudies are included at the end of this paper toillustrate the advantages of the variousrecommendations.

DEFINE FLARX LOADS

It is essential that an estimate of flare ratesis made available as early as possible in thedevelopment of the project. The composition,source and duration of each individual flare rateshould be defined and concurrent rates identified.These can be divided into four main grouPs: -

(a) Continuous - a high probability of flaringat this rate for long periods throughout theoperational life of the platform.

(b) Semi-continuous or short term - forexample at start-up or until sufficient wellsare operating to satisfy Gas Export systems or forshort periods during the life of the platformassociated with planned maintenance.

(C) ELowdown - the controlled letdown oflocked-in hydrocarbon.

(d) Emergency - major, and usually uncontrolled,upset conditions. Normally a rare occurrence andwould often lead to automatic shutdown of the pl=t.involved.

ASSIGN ENVIRONMENTAL LIMITS

Having broken down the flare rates into thesegroups the engineer should then set broad radiationand noise limits for each group based on duration offlaring, likelihood of occurrence etc.

The platform layout should then be studied and‘sensitive areas’ identified. Typical examplesare areas where radiation must be kept to anacceptable level to meet safety requirements (eg life-boat access-ways, helipads, overhead working areas),or operational.requirements (eg areas such aspipe-racks or drilling derrick where personnelwill be required to work a full shift with littleor no protection from the flare). There are alsomany other areas where it is essential that noiseand glare must be kept at a low level (eg sleepingand accommodation area). Once all such areas havebeen identified specific limits can be set for eacharea for each different type of flaring load. Again,this analysis should be done as early as possible.

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The imposition of ‘blanket limits f for noise and IDENTIFY OTHER PROBLEM AREASradiation, without regard for platform layout canlead to costly overdesign. For example, we have The foregoing analysis is likely to be based onfound that substantial savings can be made by the assumption that the gas is clean and dry.installing purpose-designed local protection for Whilst every responsible operator takes allsensitive or critical areas. precautions to ensure that liquids will not be

carried over into the flare system, practice showsSELECT lfETEOROLOGICALCONDITIONS that under extreme conditions this does occur.

If significant quantities of liquid reach the flareIn order to calculate radiation levels, end to any detailed consideration of flame length, shape,

a lesser extent noise levels, it is necessary to radiation etc which may have been carried out ondefine a wind speed and direction as a basis for hydrocarbon gas alone will be nullified. Such adesign. In the very early days of North Sea Platform condition is normally a grossly aberrant situationdesign we encountered many examples of flare system which will probably lead to rapid shutdown.design based on the wind speed which had been used Consequently, design need not be specificallyfor structural design. This structural wind speed tailored to cater for it as long as the probablewas often based on the ‘once in a hundred year effects are evaluated to ensure that adequateperiod’ wind, of very high speed but short duration. protection and escape routes are available forThis approach is obviously essential in structural. personnel should it occur even for a short time.design but as a basis for flare design it csn lead In this context, high pressure flares, like theto a grossly over-designed and uneconomic system. Indair normally have high-shear discharge portsOur design objective is to protect personnel and as such give better atomisation and combustionand equipment from excessive radiation levels or of associated liquids than low-velocity pipe flares.more precisely from excessive temperature. Forcontinuous exposure personnel should be able to Situations may arise in production emergencies,work without suffering from heat exhaustion (and be eg blowout, where it is essentisl to extinguish theable to touch metal surfaces etc) whilst for emergency platform flares quic~y to prevent vapour-cloudconditions personnel should be able to take cover ignition. The technology to achieve emergency flarebefore exposure is sufficient to cause radiation snuffing has been de eloped and is now in use onburns. Use of very high wind speeds for flare North Sea platforms~z.design is illogical since production might thenbe curtailed and personnel need to take shelter from DEFINE TILECONTROLLING CASEthe buffeting (and chilling) effects of the wind.Case Study 1 (appended) shows that for exposed The foregoing analysis will have identifiedequipment wind cooling effects rise faster than the information needed to finalise the design.increasing radiation so that the worst situation Ideally, every permutation of flare rate andarises under calm conditions! Such arguments selected meteorological conditions should be “studied.are directly applicable to exposed locations such In practice the experienced designer quicklyas the crown block. For sheltered areas, where recognises which cases are trivial and which areconvective cooling is less, more care nraybe needed. likely to be significarrt. However, it is importantOnce again detailed consideration of the platform to realise that the maximum flowrate seldomlayout becomes important. controls the flare system design in offshore

locations, as is illustrated in Case Study 2.We thus strongly recommend that wind speed and High semi-continuous rates such as occur on blowdown

direction are based on a sensible consideration of the plant can be controlled to give a moreof wind rosettes for the area and a range of economic system4. Often the maximum continuousmoderate wind speeds examined to determine the worst flaring rate is found to control boom length orcase. tower height, while relief and process safety

considerations dictate the process layout.A similarly realistic approach must be taken

with regard to the effects of solar radiation when CASE STUDY 1being added to flare radiation. It is not unusualto find instances of systems designed on the basis Any body subject to flare radiation will heatof maximum flare rate with worst wind and solar up to an equilibrium temperature when the incomingradiation taken at mid-day maximum. Solar radiation is balanced by heat lost by re-radiationradiation varies throughout the day and is not simply and convection. Wind cooling is an importantadditive to flare radiation. (eg consider the flare contribution and even at surface temperatures as highsited on the North side of a North Sea platform). as 350 degrees centigrade, convective losses canFurthermore, the platform’s geographical location exceed re-radiative losses. This case study considersmust be considered to establish whether high solar an elevated flare burning at a fixed flowrate inradiation levels are likely to coincide with various winds. From computer plots similar tohigh wind speeds. Simple addition of flare and Figure 3 the maximum radiation at a given elevationsolar radiation thus allows for a multi-jeopardy can be found, and using the appropriate wind speedoccurrence on the flare system whereas the remainder to estimate convective losses an equilibrium surfaceof the plant may be based on only double jeopardy. temperature corresponding to that radiation can beWe are not advocating that such occurrences should established. Figure b shows a plot of temperaturenot be evaluated but the effect on production should against wind speed with the perhaps unexpectedbe balanced against the likelihood of them all conclusion that calm conditions are more severe thanoccurring at the same time and the increased cost windy conditions. This result is directly applicableon the flare structure to meet such coincidence. to exposed locations such as the crown block but needs

FE.1

more care in applying it to areas which could be upstream system design, operating philosophy,sheltered from the full effects of convective end flare type. The effects of carryover have beencooling. Similar conclusions are applicable to boom discussed earlier in this paper, in the sectionmounted flares with the wind blowing directly entitled ‘Identify other Problem Areas’.towards the platform. . .

Finally in this Case Study, in line withCASE STUDY 2 Case Study 1, there are instances where calm

conditions represent the controlling design case.For ersonnel exposed to radiation, a figure of

?440 Btu h- ft-2 has been the recommended maximum CASE STUDY 3for continuous exposure, with 1500 Btu h-ift-2for short duration emergency exposure. A figure of It is required to dispose of 100 MMscfd of5000 Btu h-~ft-2 is recommended for unattended but gas having a molecular weight of 24 from an offshoreexposed plant and steel surfaces. As a general platform. In this case study, the gas is consideredguide, these levels are realistic, but must be to be coming from sources which are capable ofconsidered against the specific requirements tolerating an imposed back pressure of about 70 psigof the system, design, and locations. (ie near atmospheric pressure gas sources have

already been segregated). The study sets out toRegardless of the design levels chosen, these compare the sizes and weights of two flare systems

values are usually taken as inclusive of solar which would handle 100 Mllscfdgas, one at a headerradiation. In North Sea offshore locations, high pressure of about 65 psig (HP) the other at aboutwinds are seldom accompanied by significant levels 2 psig (LP).of solar radiation. In the present case studya solar contribution of 18o Btu h-ift-2 was taken The results of the study are shown in Table 2for calm conditions, with zero solar contribution and are briefly discussed below. To simplifywhen the wind speed was 100 fk see-l._ This gives matters various assumptions have been mademaximum flare radiation limits on the platform of 440, concerning equipment. These assumptions are260, 1500 and 1320 Btu h-ift-2 for continuous and explained at the end of the case study.relief conditions with and without wind. The studyconcerns a typical three-stage separation process Direct comparison of the weights and volumesgiving high pressure (HP), intermediate (1P) and of the two systems shows that a saving of arrsundlow pressure (LP) gas streams and considers two 50 per cent weight or 75 per cent volume is possibleflare schemes - the first considers all gas piped in the header systems and knockout drum when usingto a common header and burnt via a conventional a high pressure system. When the high pressureflare; the second segregates the LP gas and burns system is coupled with a high efficiency flare,this via a separate conventional flare and utilises further savings arepossible in flare supportthe pressure energy in the combined HP and 1P structure, because of reduced flare radiation.gas streams using ar.tidair high pressure flare As with Case Study 2, the actual size of structure(which also handles the relief flowrate). From is influenced by the radiation limits setcomputer plots similar to figure 3, boom lengths/ (ie depending on whether flaring is for emergency-tower heights required to meet given levels are relief or continuous duty), and by the volume ofextracted and shown in table 1. The tower height/boom low pressure or atmospheric gases which are notlength required by the system is the largest of compatible with the HP system and which must beany set of four numbers (limiting case). Two burned separately. In our experience segregation ofimportant conclusions can be reached .- continuous such streams results in cost savings and increasedflaring controls the flare design with relief operational flexibility, whether a high pressureflaring a secondary case; tower .heights are shorter flare or pipe flare is used as the primary flare.than boom lengths.

Table 2 shows the structural savings possibleContinuous, rather than relief flaring where a flare system can be converted entirely

controls the required distance between the flare from LP flaring to HP flaring. Case Study 2and the platform because of a combination of two discussed a mixed HP/LP system, and showed thatfactors. Firstly, the allowable radiation levels considerable structural savings were still possible.governing relief and continuous flaring aredifferent. Secondly, the ratio of the relief The following account of the assumptions madeflow to the maximum continuous flow is much less than in Study 3 will show that, had alternativethat commonly encountered in, for example, refinery assumptions been made, the overall conclusionssystems. would remain. Firstly, a header velocity limit of

200 ft see-l has been used for the HP and LP case.With its lower weight, the tower should offer Increasing the allowable velocity would reduce the

the most cost effective solution to flare support. size of each header, but would not significantlyHowever, other considerations should be taken into alter the size ratio. One calculation methodaccount before a final decision is reached, eg thetrade-off between deck area required by a tower

GuggeSt.Sthat 12 inch and 24 inch headers could beused for the HP and LP”systems, but at velocities of

and the effect of the overturning moment imposed around 300 ft see-l. 19by a boom. In some cases, the use of high pressure,reduced radiation flares allows a lighter vertical Standard wall piping has been assumed for HPstructure to be used, and this can be cantilevered and LP headers, in line with common North Seaoff the platform side without taking up deck spacel. practice. In the case of the HP system, theA second factor which must be evaluated in deciding knockout drum is designed to 75 psig, operating atbetween tower and boom mounted flares is the risk 65 psig. The LP drum is designed to the commonof carryover. This risk is entirely a function of lower limit of 50 psig, although operating at 2 psig.

Some weight could be saved by cutting LP vessel

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design pressure and HP or LP header schedules, at 5. Hajek, J.D. and Ludwig, E.E., ‘How to Designthe expense of corrosion allowance and mechanical Safe Flare Stacks’, Petro./Chem. Eng., Pt. 1,strength, but volumes would remain unchanged. Vol. XxXII, No. 6, (1960) c.31-8 and Pt. 2,

Vol XXXII, No. 7, (1960), C 44-51.A header length of 500 ft has been used in

each case. No allowance has been made for the fact 6. Heitner, I., 1A Critical Look at API RP-521’,that a shorter header would be possible if a high Hydrocarbon Processing, (Nov. 1970) 209-212efficiency flare were used to reduce radiation. (~, No. 11).

A droplet removal size of 400-microns has been 7. McMurray, R., ‘Predicting Radiation from Flares’taken in the knockout drum, using API-RP-521 Ksldair Technical Report TR262, December 1978.design3. This is totally adequate where highpressure Indair type flares are concerned, but a 8. Brzustowski, T.A. and Sommer, E.C., Jr.,smaller droplet size might be required for a low ‘Predicting Radiant Heating from Flares’ ,pressure pipe flare if continuous liquid carryover API Preprint, No. 64-73.were present, and a larger LP knockout vessel wouldbe needed. Surge loads have been ignored. 9. Brzustowski, T.A., Gollahalli, S.R. and

Sullivan, H.F., ‘The Turbulent HydrogenThe comparison is based on tower-mounted Diffusion Flame in a Cross-Wind’, Combustion

flares, but relative support structure savings Science and Technology, (1975) Q, 29-33.apply equally with boom-mounted flares.

10. Brzustowski, T.A., Gollahalli, S.R. andACKNOWLEDGMENT Sullivan, H.F., ‘Characteristics ~f a

Turbulent Propane Flame in a Cross-Wind’,We wish to thank staff of the BP Research

Centre, Sunbury, for providing field data frompaper presented at 25th Can. Chem. Eng.Conference, Montreal, November 1975.

flare trials. Permission to publish this paperhas been given by the British Petroleum Company 11. Brzustowski, T.A., ‘Flaring in the EnergyLimited. Industry’, Progress Eng. Combust. Sci.,

(1976) q, 129-141.NOMENCLATURE

12. Kent, G.R., ‘Practical Design of Flare Stacks’,d = distance (ft) Hydrocarbon Processing and Petroleum Refiner,

(Aug 1974), 121-125 (~, No. 8).F . Fraction of heat radiated (Dimensionless)

13. Tan, S.H., ‘Flare System Design Simplified’,Q = Heat Release from combustion (Btu h-l) Hydrocarbon Processing, (Jan. 1967)

172-176 (& No. 1).K . Incident Radiation (Btu h-ift-2)

14. Reed, R.,~ = Flame Emissivity

‘Design and Gpe$ation of Flare(Dimensionless) Systems’, Chem. Eng. progress, (June 1968) 64.—

Tf . Flame Temperature (“R) 15. Brzustowski, T.A., Gollahalli, S.R., Gupta, M.P.

-1 Kaptein, M. and Sullivan, H.F., ‘RadiantMl/ = Molecular Weight lb.lb mole Heating from Flares’, ASME Paper 75-HT-4 (1975).

REFERENCES 16. Owen, L.E. and Rothrock, W.C., ‘CantileverFlare Boom Design for Offshore Platforms’,

1. Wilkins, J., Witheridge, R.E., Desty, D.H., Paper_OTC 2482 presented at 1976 Conference.Mason, J.T.M., Newby, N. ‘The Design,Development and Performance of Indair and 17. Anonymous, ‘Offshore Installations - GuidanceMardair Flares!, OTC Paper No. 2822, on Design and Constructiont, Part 2, Section V.Presented OTC.MaY 1977. Department of Energy (United Kingdom)

Publication, Her Majesty’s Stationery Office,2. Anonymous: ‘Recommended Practice for the 1977.

Design and Installation of Pressure-RelievingSystems in Refineries!, API RP-520, Part 1 18. Anonymous, ‘Emergency Flare Snuf”fingSystems’,- Design, &nertcan Petroleum Institute, Kaldair publication MP610\80.Refining Department (1976) 16.

19. Tan, S.H., ‘Simplified Flare System Sizing’,3. Anonymous: ‘Guide for Pressure Relief and Hydrocarbon Processing (Oct. 1967) 149-154

Depressuring Systems’, APIRP-521, Americ~ (~, No. 10):Petroleum Institute,”Div. of Refining (1969).

4. Paruit,B., Kimmel, W., ‘Control Blowdown tothe Flare’, Hydrocarbon Processing (Ott.1979)117-121. (~, No. 10).

553

!l’able1Comp~ison of Righ PressureIndairand

ConventionalPipe Flare Systems

.

Radiation(fromflare) Eoom Length/TowerHt. (ft)Limit Btu h-ift-2 to meet RadiationLimit

Flare System

m

HP Indair + W Pipe @ C xWc xf: x

Pipe Flare @c xWc x;; x

Notes:-

1.

2.

3.

4.

5.

==t=-345267180

x 203

x

415363

?3

45° E!oom

28026o115165

310345133230

Tower

22022596132

2?58285

la

HP Indair is burning45.5 MNscfd,25.8 MWgas, mixed from HP and IPseparatorstreams.

LP pipe is burning3.97 NNscfd .50.3MW gas from third stage separator.

Pipe flare (for comparison)is burningtotal HP + 1P + LP mixed gas.

Relief is 83.5 MMscfd,25.O MW gas.

Desigm Eeom/Towerfor a given configurationis the largestof thefiguresappearing.

C . continuous R = relief

@ = calm conditions “ W . 100 ft.see-l wind

Table 2Case Study 3 - Comparisonof Righ and Low

PressureFlare SystemsI 1~—--”- “I

Knockout 15.5 ftdia/ 2 I k30I&urn 17 ft LG

Total 16 1100

Relief Duty 115 ft I ’35 -

ContinuousDuty 255 ft I 95 -

LP System

2 psig at 100 NMscfd

Size

32 inch

8.5 ft dia2.5.5ft LC

180 ft

400 ft

ieight Volume[tons) (fts)

....-..——...

28 2800

5 1450.

33 4250

60 -

160 -~I

Note:- Comparativedata only - no allowancemade for secondarylow pressureand atmosphericgas sources.

‘ JJJJ_—Lo 10 20 30 40FLOW RATE MM scfd

FIG 1. VARIATION OFF FACTORWITH THROUGHPUTOF MARDAIR FLARE BURNING 17 MW GAS

NOTE THAT FQ INCREASES WITH FLOWRATE

FIG 3. COMPUTER DRAWN RADIATION ISOPLETHSIN A VERTICAL SLICE OFTHE RADIATION FIELD

CONTAININGTHE WIND BLOWN FLAME. 0.4 MACHPIPEFLARE BURNING 45.5 MMscfd OF 25”8 MW GAS

IN A 30 mph CROSSWIND

—

FIG2. VARIATION OF MEASURED RADIATION LEVELOVER A 10 MINUTE PERIOD SHOWING THAT

FLARE RADIATION ISA TIME AVERAGED PHENOMENONAND MEAN VALUES ARE OFTEN * 100/0

’0000 INDAIR ON 155f t TOWER900 - A PIPEFLARE ON 185ft TOWER

45.5 MMscfd OF 258 MW GASpoo -

:700 -

:600 -

500 -’

400

# 60 -3~ 50 -

g 40 -

l!430 ; ~ I I I I 1

20 30 50 100Wl;DSP#D (mph)

FIG 4. MAXIMUM INCIDENT RADIATION AT A GIVENELEVATION, AND ASSOCIATED EXPECTED METAL

TEMFIERATURESAT VARIOUS WINDSPEEDS

. .OCTAVEBAND CENTRE FREQUENCY (Hz)

FIG 5. NOISE FROM 4 INCH PIPEFLARE BURNING8.3MM scfd OF 17 MW GAS