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Redesigned recycle valvesabate compressor vibration

New recycle valves installed in 1994 on thecompressors in theEast Brae field in the North Seacorrected the noise and vibra-tion problem that damaged theoriginal valves shortly aftercommissioning the platform. The original recycle valves,especially on the second-stagecompressors, showed severedamage.

East BranTo further develop the Braefield, Marathon Oil U.K. Ltd.brought on stream the FastBrae platform (Fig. 1). First oilwas produced in mid-Decem-ber 1993. East Brae primarily serves asa facility for recycling gas, ex-porting natural gas liquids orcondensate to the Forties pipe-line, reinjecting dry gas intothe reservoir, and exportinggas to the SAGE pipeline forprocessing at the St. Fergusgas terminal. The platform was designed tohandle 125,000 b/d from asingle-train separation systemcomprised of four main separa-tion vessels and to process gasat the rate of 670,000 kg/hr(1,460,000 lb/hr) or 626 MMscfd. Gas from the separation trainwas introduced into the first ofthree gas compressor trains in

David E. Laing Marathon Oil U.K. Ltd. Aberdeen, ScotlandHerbert L Miller, John W. McCaskill Control Components Inc. Rancho Santa Margarita, Calif.

the early part of 1994. Each train is driven by a Gen-eral Electric LM5000 gas tur-bine. This engine is used as agas generator to drive a powerturbine manufactured by Euro-pean Gas Turbines Ltd. Thepower turbine develops about33 megawatts (mw) that, in turn,drive the three stages of thecompressor through a gearbox.Suction gas at about 65 barg(940 psig) comes from the sepa-ration train via a glycol waterremoval system.

Gas discharge from the firstcompression stage, at about166 barg (2,400 psig), can be ex-ported to the SAGE pipelineand/or transferred to the nextcompression stage for injectioninto the reservoir. North Brae gas can be im-ported into the second-stagesuction and, along with EastBrae gas, is compressed toabout 290 barg (4,200 psig). Thethird and last compressionstage raises the gas pressureto about 500 barg (7,250 psig)

before entering the manifoldfor injection into the reservoir. Each of the compressortrains can move 339,300 kg/hr(730,000 lb/hr) or 321 MMscfdof gas. Hence, two trains arerequired to handle all of therequired capacity. Each stage of compressionis protected against compres-sion surge by a fast-actingcontrol system provided bythe Compressor ControlsCorp. This system modulatesindividual stage, recycle-con-trol valves. The valves were the multi-stage, drill hole-cage type.Problems with these valvesduring the commissioningstages led to their eventual re-placement in the summer of1994, and the previous prob-lems have ceased

Recycle systemEssentially, a recycle systemfunctions by maintaining theminimum flow required to avoidsurge conditions for the estab-lished compressor speed. Re-gardless of design or applica-tion, centrifugal compressorshave one common characteris-tic. That is, at low flow becauseof high external circuit resis-tance, orderly gas flow throughthe compressor is impossible

During commissioning of the East Brae platform, the compressor recyclevalves caused excessive vibration and noise (Fig. 1).

Reprinted from the June 5, 1995 edition of Oil & Gas JournalCopyright 1995 by PennWell Publishing Company

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because gas velocities are toolow to be converted to the re-quired pressure energy level inthe discharge line. When discharge pressure ex-ceeds impeller discharge pres-sure, backflow occurs until thedischarge pressure is less thanimpeller outlet pressure. At thispoint, forward flow is reestab-lished, and the compressor op-eration is nearing unstable con-ditions, resulting in vibration andpossible damage to the compres-sor.

This phenomenon is calledstall and produces a surgein the system. As a function ofits design, every centrifugal com-pressor has a stall/flow pointat any given operating speed.While the gas flow correspond-ing to this surge point is fixed fora constant-speed compressor, itchanges at each operating speedfor a variable-speed compressor. In any event, good industrypractice dictates that compres-sor gas flow should never dropbelow 5% in excess of the surgepoint for any operating speed. Itis for this purpose that compres-sor recycle valves are used to ap-propriately recycle gas flow fromcompressor discharge to suctionto prevent operation below thiscritical flow.

Recycle valvesCompressor recycle valves per-form two functions: 1. React quickly during start-up and shutdown operations andto emergency situations. 2. Modulate recycle flow dur-ing normal operations to avoidcompressor operation near orbelow the critical surge point. Fig. 2 shows a typical recyclevalve control integration into thecompressor cycle. Fig. 2A indi-cates the relationship of the fourcritical flow points for three typi-cal compressor operatingspeeds. Fig. 2B indicates safetyon response, an operating fea-ture of the control system. If the line is crossed, the con-trol and recycle trip lines will bemoved to the right by X% be-cause their position is assumedto be too close to the surge point.If crossed three times, the com-pressor will trip. The main problems generallyassociated with compressor re-cycle valves include: The potential for high noiselevels. Vibration. Inadequate response timessuch as the time required to fullyopen a closed recycle valve onsystem upset. Control instability (hunting)due to improper actuator andcontrols selection. Inadequate

capacity for all service condi-tions. High noise levels in excess of100 dbA can be experienced withconventional recycle valves. Se-vere vibration is created becauseof the high mass flow at highpressure drop. This can causevalve and trim fatigue and canalso lead to piping failure. Noise levels at East Brae havebeen reduced to below 80 dbAwith accompanying drastic re-duction in damaging vibrationlevels after fitting DRAG trimmedvalves (DRAG is a registeredtrade mark of Control ComponentInc.)

OperationsEach of the three compressorson East Brae normally handles339,500 kg/h (750,000 lb/hr) ofnatural gas at 500 barg (7,250psig), an( 750 C. (1670 F.). Thisvery high discharge pressureoptimizes gas injection back intothe reservoir. Prior to entering each compres-sor train, the nature gas isscrubbed of water vapor. Shortlyafter initial commissioning of theplatform, the drilled cage-type re-cycle valves, especially thoseserving the second stage com-pressors, showed signs of se-vere vibration These problemsinvolved excessive noise and vi-bration that led to valve damage.

Several modifications to theoriginal design were tried toalleviate noise and vibrationproblems. The valve trim wasimproved to try to produce theeffect of multistage velocitycontrol. In the spring of 1994, one valvefailed to the closed position caus-ing its compressor to surge. Atthis point, Marathon Oil decidedto replace the valves with a dif-ferent design.

New valve designInitially, Marathon decided toretrofit the first-stage valves byreplacing the existing drilledcages with a true multistage trimdesign with the existing valvebodies. But this proved im-practical because Marathonfelt that it could not afford totake the first-stage recyclevalves out of service for thetime required to transport themashore and install new multi-

stage trim. Therefore, only two sets of first-stage multistage valve trim wereordered for valve retrofit and onecomplete new first-stage valvewas purchased to initiate the to-tal recycle valve replacementprocess. Because time was short, and anew cast steel body could notbe procured within the requiredaccelerated delivery schedule,the replacement first-stage valvebody was machined out of aforged steel billet with enlargedinternal gas passages (Fig. 3). A forged steel billet uses morematerial than a cast steel valvebody and is therefore heavier buthas the advantage of absorbingmore vibration. Existing face-facedimensions were held, and the

new body was machined withGrayloc fittings to match exist-ing piping requirements. Second and third-stage re-placement valves were procuredagain with bodies fabricated fromforged steel billets, incorporat-ing multistage velocity letdown.First stage (10 in. x 10 in.) andsecond stage (4 in. x 4 in.) gasrecycle valve bodies were de-signed in accordance with ANSI1500 and 2500 requirements re-spectively, but third stage (4 in. x4 in.) valve bodies were in accor-dance with API 10,000. API design rules allow boltedend connections as per Mara-thon specification. Table I shows the essentialoperating parameters of the re-cycle valves. First-stage recycle

valves in each train were char-acterized (Fig. 4) with four, mul-tistage disk groups to producelinearity within the control range(valve coefficient Cv-vs.-valvestroke) up to about 62% of maxi-mum stroke. A velocity-control transitionzone was incorporated from 62to 75% of stroke. From there to100% of stroke, no velocity con-trol was needed. Flow in thisrange is required for settle-outflow, such as the result of a tripand isolation. Discharge flowsback to suction to settle out atsome intermediate pressure. Second and third-stage re-cycle valves were not character-ized; for example, valve coeffi-cient Cv-vs.-stroke was linearfrom 0 to 100% of stroke. For the retrofit of the first-stagevalves, replacement trim had tofit into the existing valve bodiesin all respects without the needfor remachining the existingvalve bodies. This meant thatthe multistage, redesigned trimwas confined to a 6-in. stroke.Subsequently, complete first,second, and third-stage trim wasalso designed with a 6-in, stroke,

allowing all nine replacementvalve actuators to be exact du-plicates. Fig. 5 details the basic recyclevalve design now in use atMarathons East Brae platform.The valve is uniquely suited forcompressor recycle service be-cause it specifically addressesand solves the potential prob-lems in this tough service. Also, this replacement recyclevalve design provides for gasflow modulation. Therefore, inaddition to reacting quickly toprotect the compressors duringstart-up and emergency situa-tions, the valves modulate theflow during normal operations toavoid compressor operation nearor below the surge point

Noise and vibrationThe new recycle valves incorpo-rate a tortuous flow path. Thatis, pressure energy is dissipatedat a controlled velocity headthrough multiple, right-angleturns in a stack of individualdisks. At East Brae, the replacementvalve trim disks are electro-dis-charge machined (Fig. 6). The stackof disks surround the plug through-

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out its stroke in the second andthird-stage valves. In the first-stage valves, these character-ized disks are in only up to 75%of the stroke. Above this, adrilled cage is used because novelocity control is required. Velocity head (pV2/2)through the disk stack is lim-ited to 0.52 MPa (70 psi) to mini-mize noise and vibration. Inaddition, the disk stack incor-porates a pressure equalizingring (PER) on its inside diam-eter to ensure equal pressureacting radially on the valveplug at all times to eliminatevibration that could occur be-

cause of rapid plug radial movement. This propensity for rapid plugradial movement undoubtedlycaused plug guide galling insome valve designs. This tortu-ous path, velocity control designlimits noise levels to below 80 db. These recycle valves are de-signed with an ANSI Class Vplug/seat-ring design and mate-rials to assure tight valve closureat shutoff. In this under-the-plugdesign, gas flow passes throughthe plug/seat-ring area under fullcompressor discharge pressure.The gas then enters the energydissipating disk stack that is de-signed to accommodate changes

in gas density from the expand-ing gas volume. For the same reason, where newforged steel valve bodies wereprovided for Fast Brae, largerbody runs in this area were ma-chined-in to achieve minimumvelocity head.

Valve actuators In an upset situation to pro-tect the compressors from severedamage, the nine recycle valveson the East Brae platform mustrapidly stroke from full closed tofull open. A 2-sec stroking,through the 150-mm (6-in.) fullvalve travel, is achieved withquick exhaust valves on bothsides of the pneumatic, double-acting actuator piston. Fig. 7 applies to all nine identi-cal actuators. The actuator issupplied with 5.5-6.8 barg (80-

100 psig) air that is controlledby a 4-20 ma input signal tothe positioner. Because aspring acts on the actuatorpiston, quick opening is as-sured in the event of eitherpower or air failure. Because all nine actuators in-corporate a 150-mm (6-in.)stroke, the installation has maxi-mum interchangeability shouldthe need arise. All new replacement and ret-rofitted compressor recyclevalves at Marathons East Braeplatform have been installed.Marathons evaluation of theperformance indicates thatnoise and vibration have beensignificantly reduced to back-ground levels and, in its opin-ion, previous problems havebeen eliminated.

David F. Loing is instrument and elec-trical supervisor in the facilities depart-ment of Marathon Oil U.K. Ltd., Aber-deen. He is responsible for the onshoresupport of the maintenance and facili-ties work on the Brae field topsides.Laing has a BS in electronic and elec-trical engineering from Robert Gordons,Aberdeen.

Herbert L. Miller is vice-president oftechnical services of Control Compo-nents Inc. in Rancho Santa Margarita,Calif. He has been involved in the de-sign, manufacture, and application ofcontrol valves for severe service for over20 years. Miller has a BS in mechanicalengineering from Ohio Northern Uni-versity, and an MS in mechanical engineering from Northwestern University.

John W. McCaskill is manager of engineering at Control Components Inc. inRancho Santa Margarita, Calif. He has 26 years of experience in the controlvalve and oil field equipment manufacturing industry. MeCaskill has a BS inmechanical engineering from the University of Houston. He is a member ofASME, Society of Manufacturing Engineers, and the Instrument Society of America.McCaskill is a registered professional engineer and holds five U.S. patents.

Laing Miller

McCaskill

THE AUTHORS