otc-7742-new separator internals cut revamping costs

8/2/2019 OTC-7742-New Separator Internals Cut Revamping Costs

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OTC 7742New separator internals cut revamping costsF. Koene, Burgess ManningProf. Dr. Ir. J. de Graauw, Technical University of DelftDr. R.A. Swanborn, CDS Engineering BVCopyright 1995, Offshore Technology ConferenceThis paper was presented at the 27th Annual OTC In Houston, Texas, U,S.A., 1-4 May 1995,This paper was selected for presenlatlon by the OTC Program Commit tee fol lowing review of Information contained In an abstract submitted by the.author(s), Contents of the papar,as p r e s ~ n t e d , have not been reviewed by the Offshore Technology Conference and are subject to correction by the aUlhor(s), The malerlal, as presented. does not necessarily reflectany position ~ the OffshoreTechnology Conferenceor itsofficers. PermIssion to copy Is restrIcted to an abstractof notmore than 300words. Illustrations may notbe copied, The abstractshould contain conspicuous acknowledgment of where and by whom the paper Is presented,

ABSTRACTThis article describes the deboltle-neckingoperation of the production separators on the ShellLeman AK platform in the southern North Sea.The original, compactly built separators becameoverloaded through decreasing well pressures.Gradually they were forming a serious restriction inthe production flowsheet.Two alternatives were considered to overcome thisproblem: to extend the platform and to install newseparators or to retrofit the old separator vesselswith state-of-the-art internals.The second alternative was chosen because ofconsiderably lower costs (totalling only 10% of firstalternative) and considerablyshorter downtime.This separator upgrade project was a jointcooperation between the operator of the field, theircentral research and engineering facilities, and theabove mentioned companies and institutions.BACKGROUNDThe debottle-necking operation formed part of alarger project in which gas production of an ageinggas field was boosted through, among others, theinstallation of new compressors.The process flowsheet of the a.m. platform isdepicted in fig. 1.The function of the platform is to remove liquidsfrom the produced gas and to secure transport ofthe gas onshore through additional compression.References, nomenclature and figures at end ofpaper

441

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The produced well fluids first pass through theslugcatcher that removes the bulk of the liquid.Downstream of the slugcatcher two parallelseparators remove the finer liquids, so the gas is'fit' to enter the centrifugal compressors.In recent times well pressures were at a level of6,9 bar, with typical flowrates of 200 MMSCFT/Dper vessel. It was doubtful whether under suchconditions the second stage separators functionedeffectively, although the original design stated thatthe separators could handle 600 MMSCFT/D pervessel.After the planned installation of the compressors,flowrates were expected of 175 MMSCFT/D pervessel at pressures of 2,9 bar.The vendor of the original separators re-rated max.flowrates per vessel at these pressures at 72MMSCFT/D, and it became clear that theseparators would not sufficiently guard thecompressors against incoming free liquids.Therefore, it was considered necessary beforeinstalling the new compressors to improve theperformance of this separation stage.ORIGINAL SEPARATOR DESIGNThe original separators (fig. 2) consisted ofhorizontal vessels of 1520 mm diameter, 3630 mmlength with 11 horizontally mounted 8" cyclonetUbes in the top half of the vessel.Although this arrangement functioned well underthe conditions the separators were originallydesigned for, a clear deterioration of performancewas observed with increasing flowrates. In theend, no more liquid was separated at all (fig. 9).


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442

Koene, De Graauw, Swanborn

OPTIONS TO IMPROVE SEPARATION PERFORMANCEThe two main alternatives to improve theperformance of the second separation stage were:1. to replace the current separators with biggerones able to handle the increased flowrates.2. to remove the internals from the currentseparators and to install high throughputretrofit internals.The generally accepted advantage of new vesselswas that it would be certain that performance willimprove, disadvantage the lack of space on theplatform, requiring a platform extension.Advantages of the retrofit option are mainly themuch lower costs and the much shorter downtime.As disadvantage it was considered that a retrofitoperation under these conditions had not oftenbeen carried out before and that a range of novelengineering techniques had to be applied toguarantee success. In the following the feasibilityand consequences of each of these alternativeswill be detailed.NEW SEPARATORSIf new separators were to be designed for thisoperation, current design and engineeringprocedures prescribe vertical vessels withdiameter of approx. 2000 mm and length betweentangents approx. 2900 mm. Weight would amountto approx. 11.500 kgs (per vessel). Costs, inc!.internals approx. GBP 320.000,-- (per vessel). Asthe existing horizontal separators are difficult toremove, a considerable amount of platformreconstruction / extension work would benecessary. Estimated total costs to be in the orderof magnitude of GBP 2,0 Million. Completion time(project start 1993) was projected for mid 1995.DEBOTTLE-NECKING EXISTING SEPARATORSThe succes of the above depends on fulfilling thefollowing requirements:1. selection of a gas-liquid separation internal thatcan be fitted inside the existing vessels andthat can process the current flowrates.2. ensuring a proper flow distribution to and fromthe internal, taken into account that theremaining area inside the vessels will berestricted.

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3. ensuring that the internal can be brought inthrough the manway / outlet nozzle and can bemounted inside the vessels within a shortperiod of time (3 days) after removal of thecurrent internals.

1. Selection of a suitable internalUnder the conditions specified, the operatingprinciple of the separation internal will rely oninertial forces. That is, centrifugal forces areevoked and because of the difference in densitybetween the gasphase and the contaminants,separation is achieved. The current internalsconsisted of 11 8" cyclone tubes that functionalong this principle. Using inertial forces as theprincipal force of separation, normally particlesdown to 5 micron can be separated, which is inline with the purpose of the separators. The use ofdifferent separation principles (e.g. diffusion) onlyleads to larger separators, which was in this caseunwanted.Inertial separator intemalsTwo main types of inertial separator internals exist:vane type- and cyclone type separators.In vane type separators, the gas flows through abundle of parallel, curved, blades. The droplets inthe gas are flung onto these blades (see fig. 3a)and subsequently drained. Various designs existon the market, but all have one property incommon, that is, that relatively large area isrequired for these internals, and, moreover, thatthey are sensitive to maldistribution andsubsequent local overloading in restrictedsurroundings.For this particular application approx. 3,5 m2 ofintake area of a sophisticated upflow vane typeinternal would be required.The available area !n the vessel between thetangents amounts to only 5,5 m2 See fig 3b for apossible mounting configuration. Because of thealmost impossible task (lit 1) of achieving an evendistribution across the vane area, this option wasrejected.Cyclone type intemalsIn cyclones, the gas/particulate phase mixture isbrought into a spinning motion. The heavierparticulates are flung out and collected on thecyclone wall (see fig. 4a). There are variousoptions to apply this principle of operation inpractice.

---- - - ----


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OTC 7742 New separator internals cut revamping costs 3

The operating properties of cyclone typeseparation internals are determined by twoparameters: the cyclone type the number of parallel cyclones that arearranged into 'cyclone bundles'.Cyclone typesI. The conventional cyclone is the so-called'Reverse-flow' c y c l o n ~ (fig. 4a). Normally the gasenters through one or more tangential inlets thatinduce a strong rotational motion. On the waydown, the particulate phase is slung against thecyclone wall and discharged into a hopper. Thegas reverses direction and flows out through thecentrally located gas outflow pipe. Principleadvantages of this cyclone are its robustness andits fairly tolerant design and lay-out rules. Disadvantages are its pressure drop and bulkiness.II. The 'Axial' cyclone operates on a slightlydifferent principle. The gas (fig. 4b) enters thecyclone tube through a spinning device ('swirldevice'), which is in this case responsible forgenerating the swirling flow.Particulates are again collected on the wall, anddischarged through longitudonal slits in the wall.The cleaned gas flows out of the cyclone withoutreversing direction. Principle advantages of thiscyclone are the possibility to built compactmulticyclone bundles that have very low pressuredrops. Because of an unsolved problem in thisdesign, flow rates above maximum throughputslead to strongly decreasing separation efficiencies.This is caused by the fact that a certainpercentage of the gas throughput (approx. 10%) isrequired as a discharge 'force' to drive out theseparated liquids collected on the cyclone wall.This discharge gas is heaVily loaded with liquidsand has to join the clean gas flow after a relativelycrude (secondary) separation step: Normally abaffle that forces this gas to flow around a bend isused to this purpose (fig. 4b).At higher flow rates, this step is insufficient toclean the purge gas, and liquids are reentrained inquickly increasing quantities.III. The 'Recycle-axial cyclone' (fig. 4c) overcomesthis problem by recycling the purge gas throughthe cyclone, and thereby using the cyclone itselfas a secondary separation step (lit. 4).Advantages are a strongly increased throughput,

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leading to very compact assemblies. Disadvantageis that the mechanical design of the cyclone iscomplex. Because of this, and the fact that for thedesign of the actual cyclone sophisticatedengineering tools must be used, hardly any robustand practice-proven recycle-axial cycloneseparators are commercially available.Single cyclone VS. multicyclone internalDesigning a cyclone separator means carrying outa trade-off between separation efficiency, pressuredrop and cyclone size. High pressure drops aremostly accompanied by high erosion rates insidethe cyclone (lit 2), because of high internalvelocities. The smaller the cyclone, the higher thepressure drop, but also the better small particlesare separated. This is approximately a quadraticrelation. Therefore it pays off to feed into amultiple of smaller cyclones: this way highseparation efficiencies can be combined with smallseparator sizes and low separator pressure drops(low erosion). Unfortunately a more complicatedconstruction is another consequence. Theadvantages are made clear in fig. 5, in whichseparator efficiency is depicted as a function ofload factor A for a variety of cyclonetype- and otherseparators (lit. 2).Cyclonetype internal selectionIt had become clear that if any, only a multicycloneconsisting of recycle-axial cyclones would fulfillboth the size and separation efficiencyreqUirements that are posed on the retrofitinternals for the overloaded separators. In a jointresearch project carried out at the University ofDelft (Holland), sponsored mainly by Shell IntI.Petro Mij. and the Dutch Ministry of EconomicAffairs, a compact recycle-axial cylone wasdeveloped (lit. 3). It had been operationally provenin on- and offshore gas cleaning applications andwas commercially available at the time of thisproject (The A-X Axiflow cyclone of CDSEngineering licensed to Burgess Manning). Withthe practical experience gained with this cyclone, itwas feasible to design a reliable cyclone internalwith a surface area of 2,9 m2 , containing 600 2"cyclones and still offering a tolerance to 25% localoverloading.2. Ensuring an even flow distributionWith the availability of the described multi-axialcyclone type internal, the first requirement for a

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4 Koene, De Graauw, Swanborn OTC 7742

succesful separator upgrading had been fulfilled.With respect to the gasflow distributionrequirement through the internal, it still needed tobe assured that the variations in the distributionremain within the 25% overload limit tolerated bythe internal. It was expected, in case of separatorupgrading, that this could reliably be achievedthrough the installation of gasflow distributionbaffles of which size and location were to bedetermined by computational fluid dynamics (CFD)techniques.3. Ensuring feasibility of installation.It was decided, in case of separator upgrading,that a mock-up of the vessel in the state after theremoval of the current internal would be necessaryto ensure that the new internal could be locatedand fastened where foreseen.NmvSEPARATORSRUPGRADINGFCURRENTONES?The costs of two new sets of retrofit internals (incl.installation) amounted to approx. GBP 275.000.It was expected that the removal of the currentinternals and the installation of the newmulticyclones could take place in mid 1994. Thesefacts compared very favorably with those linkedto the installation of new, bigger separators (seetable 1). Given that the risks, as previouslydescribed, for installing retrofit internals were wellquantified and considered acceptable, it wasdecided to go ahead with the upgrading of thecurrent separators.

Total Costs (GBP) On StreamNew Separators 2,700,000 1995Retrofit Internals 275,000 1994Table 1:Comparison of key parametersENSURINGA PROPERFLOWDISTRIBUTIONA Computational Fluid Dynamic (CFD) package(Fluent) was used to determine the measure ofmaldistribution of the gasflow through the internal.With such software tools, gas flow patterns can bepredicted in various geometries.With the cyclone internal mounted in the vesselwithout any further precautions a flow pattern waspredicted as schematically depicted in fig 6a. Itbecame clear that the upstream cyclones wouldhardly be flowed through at all, or even in the

wrong direction, whilst the downstream cycloneswould handle the bulk of the flow.These cyclones would handle approx. 35% morethan the average design flowrate, which is morethan the tolerated overload.This flow pattern is caused by the fact that thestatic pressure below the upstream cyclones is low(high local gas velocities)and high above thesecyclones (low local velocities). This effect is evenworsened by the tailoring of the gas flow whenflowing into the narrow space between internal andvessel.Various gas flow patterns were simulated withdifferent flow baffle configurations. The optimumsimulated flow pattern obtained is the result of thebaffle configuration in fig. 6b. The upstream baffleis to compensate the tailoring effect, the down-stream baffles are to improve flow distributionthrough the cyclones by influencing the staticpressure distribution below the cyclones. Theresulting flow pattern is depicted in fig. 5b: localoverloading is reduced to less than 15Y0,which thecyclones should easily be capable of handling.ENSURINGTHE FEASIBILITYTO INSTALLINTERNALThe three aspects which would complicate theactual mechanical installation of the internal most,are:1.

2.

3.

all internal parts had to pass through a 24manway, or through the vessel outletno welding should take place within the vessel,although 3000 kgs. of internal would have tobe fitted in,the vessel would still contain the remnants ofvarious parts of the previous internals, whichcould prove to be unovercomable obstacleswhen encountered unexpectedly during theinstallation

Therefore, the multicyclone internal consisting of600 cyclones, was split into 60 modular units of 10cyclones, weighing each 35 kgs. (fig. 7 shows amodular unit of 10 Axiflow Cyclones). These couldbe handled by one person, and could passthrough the manway. Because no welding wasallowed, two clamped-rings were used.These rings were mounted with flexible packingsinside the vessel, and functioned at the same timeas a seal between internal up- and downstramcompartment and as mechanical mounting points444


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OTC 7742 New separator internals cut revampin~ costs 5

for the supporting structure for the cyclonemodules. In order to make sure that everythingcould be fitted between the stumbs and remnantsthat were left over of the previous internal, a mock-up was built. In this mock-up all obstacles werereconstructed. In this way, it could be assured thatthe internal could actually be fitted inside theexisting vessels through the available openings.This mock-up was later used for training purposes:installation personnel did dry runs, and installationvideos were prepared.Fig. 8 shows a picture of the partially mountedinternal in the mock-up.RESULTS AND CONCLUSIONSIn the summer shut down of 1994, the old internalswere removed, and the existing welded baffles andsupports in the vessel were ground off as far aspossible. The new internals could be installed asplanned without any major difficulties. Thisoperation took place within two weeks.The separators came onstream in Iate 1994, butbecause of low flowrates, the load on theseparators was minimal. Under these conditionsthe slugcatcher (fig 1) separates out most of theentrained liquids.In early 1995 the first serious capacity tests werecarried out to simulate the conditions to beexpected when the new compressors have beeninstalled. This was achieved by blocking off theliquid outlet of the slugcatcher and by diverting allflow through only one of the two upgraded secondstage separators. Under these conditions, the loadfactor on the internal amounted to approx.X = 0,56, 10% higher than design conditions.Although it is not exactly clear how much liquid ispresent in the feed, the tests have convincinglyconfirmed that the second stage separators nowseparate out considerably larger amounts of liquidthan before (fig. 9).This separator upgrade project shows howrecently available process and mechanicalengineering techniques can be applied to stretchthe performance and lifetime of existing equipmentin a way that was previously unthinkable.

SYMBOLS

=vc3max x (L%/(~L-~G))O5j

in which:A = Ioadfactor (m/s)vGma~= max. gas velocity (m/s)PG = gasdensity (kg/m)PL = liquid density (kg/ms)

LITERATURE1.

2.

3.

4.

Verlaan, The effect of gas density on theperformance of vane-type demisters, Report,Technical University DeIft, 1991.Oranje, Cyclone separators score high incomparative tests, Oil & Gas Journal, 1, 1990Swanbom, New gas-liquid separators for theoff shore industry, PhD Thesis, TechnicalUniversity DeIft, 1988.Jackson, Mechanical equipment for removingdust and grit from gases, The British CoalUtilisation Research Association, 1963.

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6 Koene, De Graauw, Swanborn OTC 7742

slug- 2nd Stage Compressorscatcher Separators

Fig. 1: Simplified flowsheet

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Fig, 3b: Retrofitted with upflow vanes.5 -

Fig. 4b: Axial Cyclone Fig. 4c: Recycle-AxialCyclone


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OTC 7742 New separator internals cut revamping costs 7

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otc-7742-new separator internals cut revamping costs

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