Modeling Emergency Shutdowns of Centrifugal Compressors

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Transcript of Modeling Emergency Shutdowns of Centrifugal Compressors

Copyright2009, Pipeline Simulation Interest Group andSolar Turbines Incorporated ThispaperwaspreparedforpresentationatthePSIGAnnualMeetingheldinGalveston, Texas, May 12 - May 15, 2009. ThispaperwasselectedforpresentationbythePSIGBoardofDirectorsfollowingreviewof informationcontainedinanabstractsubmittedbytheauthor(s).Thematerial,aspresented, does not necessarily reflect any position of the Pipeline Simulation Interest Group, its officers, or members. Papers presented at PSIG meetings are subject to publication review by Editorial Committees of the Pipeline Simulation Interest Group. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of Solar Turbines Incorporated is prohibited. Permission to reproduce in print is restricted to an abstract ofnotmorethan300words;illustrationsmaynotbecopied.Theabstractmustcontain conspicuous acknowledgment of where and by whom the paper was presented. ABSTRACT Emergencyshutdownsincompressorstationscausefast transientsintheoperatingconditions.Thepaperand presentation will address the physics of compressor surge , as wellasthephysicsthathavetobemodeledtodescribethe systembehaviorduringthesefasttransients.Sample calculations are presented. Surge avoidance generally falls into two classes: Avoidance of surgeduringnormal(slow)processchangesandsurge avoidanceduringfasttransients,which,farexampleoccur during emergency shutdowns. Inthispaper,datafromacompressorthatsurgedduringan emergencyshutdownarepresented.Thedataareanalyzedto determine the effects of surge and the rate of deceleration. The issueoftherateofdeceleration,inparticularfordifferent drivers, is discussed.Amodeltosimulateshutdowneventsisdevelopedand possiblesimplificationsareevaluated.Thecompression systemisanalyzed,thusverifyingthemodelandthe simplifications.Theimpactofpipinggeometry,valvesizing, and instrumentationon the results are also covered.

NOMENCLATURE AFlow area cvFlow coefficient CCompressible valve coefficient Fp Piping geometry factor hHead HcoolerGas cooler heat transfer (W) JInertia kIsentropic exponent kConstant Kv Valve coefficient LPipe length NSpeed (1/s) pPressure QVolumetric flow SGSpecific gravity TTemperature TDTurndown tTime VVolume YCoefficient ZCompressibility factor ,,Constants Density Subscripts availAvailable comprCompressor opOperating point surgeAt surge stdAt standard conditions ssSteady state vValve 1Compressor inlet 2Compressor discharge INTRODUCTION Acentrifugalcompressor,operatingwithinacompressor station (Figure 1), will exhibit a stability limit that prevents it from operating at conditions that require a flow lowerthan the flowatsaidstabilitylimit(Figure2).Thestabilitylimitis usually referred to as surge limit, because once the compressor operatingpointcrossesthesurgelimit,theflowthroughthe compressorwillreverse.Thiscancausedamagestothe compressor.Itisimportanttonotethatsurgeisadynamic systembehavior,thatfollowsfromtheinteractionofa centrifugalcompressor(expressedbyitshead-flow characteristic)withthepipes,valves,coolersetc.ofthe compressorstationupstreamanddownstreamofthe compressor. PSIG 0913 Modeling Emergency Shutdowns of Centrifugal Compressors. Rainer Kurz, Solar Turbines Incorporated, Robert C. White, Solar Turbines Incorporated 2RAINER KURZROBERT C. WHITEPSIG 0913 The possible operating points of a centrifugal gas compressor arelimitedbymaximumandminimumoperatingspeed, maximumavailablepower,chokeflow,andstability(surge) limit(Figure2).Surge,whichistheflowreversalwithinthe compressor,accompaniedbyhighfluctuatingloadonthe compressorbearings,hastobeavoidedtoprotectthe compressor.Theusualmethodforsurgeavoidance(anti-surge-control) consists of a recycle loop that can be activated byafastactingvalve(anti-surgevalve)whenthecontrol system detects that the compressor approaches its surge limit. Typicalcontrolsystemsusesuctionanddischargepressure andtemperature,togetherwiththeinletflowintothe compressorasinputtocalculatetherelativedistance (turndown) of the present operating point to the predicted or measuredsurgelineofthecompressor(Figure2).Turndown is defined by: const Hopsurge opsQQ QTD==(1) Ifthesurgemarginreachesapresetvalue(often10%),the anti-surgevalvestartstoopen,therebyreducingthepressure ratioofthecompressorandincreasingtheflowthroughthe compressor.Thesituationiscomplicatedbythefactthatthe surge valve also has to be capable of precisely controlling low. Additionally, some manufacturers place limits on how far into choke (or overload) they allow their compressors to operate. Properly designed centrifugal compressor systems can provide an extremely large operating range, that is further enhanced by theappropriateuseofrecyclesystems.Aproperlydesigned recycle system will allow to keep the compressor online down tozeroflowintothesystem,withoutupsettingtheprocess duringthetransitionfromoperatingwithaclosedrecycle valvethroughafullyopenvalve.Asummaryonapplication guidelinesforsuregcontrolsystemsisgivenbyBrunand Nored in [1]. For normal operation, as part ofprocess control,the recycle valve must be sized to allow fine tuned control of the recycle flow.Thisrequiresavalvethatisproperlymatchedtothe compressormap,inordertoallowbumplesstransferfrom recycle to normal operation and vice versa. Duringemergencyshutdowns,thesituationisdifferentfrom regular, controlled recycle. Emergency shutdowns are initiated toprotectthecompressorstation,andrequiretheimmediate shutdownofthecompressoranditsdriver.Ingasturbine drivencompressors,thisisinitiatedbycuttingoffthefuel flow into the gas turbine. In electric motor driven stations, the electric power to the motor is turned off. Parameterssuchasgasvolumecapacitanceintherecycle path,compressorpowertraininertia(Figure3)andtheflow capacityofrecycleandantisurgevalves,thedynamic behaviorofthesevalvesandtheiractuators,aswellascheck valvedynamiccharacteristics,arecrucialindeterminingthe dynamic behavior of the system. When an emergency shutdown is initiated,the recycle valve, oradedicatedantisurgevalvehastobeopenedasfastas possibletoavoidthecompressorformsurgingwhileitspins down under the train inertia1.Conceptually, the problem is as follows:Withthedriverpowercutoff,theinertiaofthe compressortrainworksagainstthedeceleratingforceofthe gastobecompressedAtthesametime,thedischargecheck valvecloses..Withthecompressorrapidlyslowingdown,it loosesitscapabilitytogeneratehead,andthustoovercome thedischargepressureimposedbythesystem.Thedischarge pressurehastobeloweredfastbyflowinggasthroughthe recycleline.Therateofpressurereductionisdeterminedby theflowthroughtherecyclevalveandthevolumeofgas trappedbetweenthecompressordischargenozzle,thecheck valveandtherecyclevalve.Thespeedatwhichthepressure canberelievedofthepressurenotonlydependsonthe reactiontimeofthevalve,butalsoonthetimeconstants imposedbythepipingsystem.Thetransientbehaviorofthe piping system depends largely on the volumes of gas enclosed bythevariouscomponentsofthepipingsystem,whichmay include, besides the piping itself, various scrubbers, knockout drums,andcoolers.Thesystemboundariesforthisstudyare thefirstdownstreamcheckvalve,whiletheupstream boundary may be either a check valve or an infinite plenum (at constantpressure)Figure4.Thevalvearrangementcanvary, forexamplewithasinglevalveinacooledloop,adedicated hot bypass valve (for fast transients) in parallel with a cooled recycle valve, or two valves in parallel, with one of them used forslowprocesscontrol,andbothforfasttransients(Figure 4).Thearrangementsdescribedarejustafewamongmany other feasible arrangements (White and Kurz [2]) SURGE PHENOMENONFigure 2 shows the head-versus-flow characteristic of a typical centrifugal compressor, including the areas of unstable operation. At flows lower than the stability point, the compressor initially shows a reduced capability to generate head with reduced flow, until it experiences reverse flow, that is, the gas now flows from the discharge to the suction side (Figure 5). It must be noted here that the stability point (surge line) of the system does not automatically coincide with the onset of stall in the compressor, because surge is a system behavior. Thus, the stability limit in question is determined by the interaction of the compressor and the piping system. Many manufacturers will place the surge line on the map at the onset of stall (especially for high pressure compressors) to avoid unduly high vibration levels caused by stall.

1 Some installations maintain fuel flow to the turbine for 1 to 2 seconds while the recycle valve opens. However, this can generate a safety hazard.PSIG 0913Modeling Emergency Shutdowns of Centrifugal Compressors. 3

Once flow reversal occurs, the amount of flow depends on the pressure ratio across the compressor, since in this situation the compressorsactsmoreorlesslikeanorifice.Theflow reversalmeansthatthepressuredownstreamofthe compressorisgraduallyreduced.Thespeedofpressure reductiondependslargelyonthesizeofthevolume downstreamofthecompressor.Oncethepressureisreduced sufficiently,thecompressorwillrecoverandflowgasagain from the suction to the discharge side. Unless action is taken, theeventsrepeatagain.Ongoingsurgecandamagethrust bearings (due to the massive change of thrust loads), seals, and eventuallyoverheatthecompressor.Detailsoftheenergy transfer from the compressor into the gas are described in [3].

MODELLINGTHEPIPING-SURGE CONTROL INTERACTIONDesignofthepipingandvalves,togetherwiththeselection andtheplacementofinstrumentswillsignificantlyaffectthe performanceofananti-surgecontrolsystem.Thisisamajor issueduringtheplanningstagebecausethecorrectionof designflawscanbeverycostlyoncetheequipmentisin operation.Typicalconfigurationsforrecyclesystemsare outlined in Figure 4. In its simplest form, the system includes aflow-measuringelementinthecompressorsuction, instrumentstomeasurepressuresandtemperaturesatsuction and discharge, the compressor, an aftercooler and