Surge Control in Pumping StationsWord文件下载.docx

Surge Control in Pumping StationsWord文件下载.docx

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Pumps&

Systems,March2007

Thisprimerpresentsbasicsurgecontrolprinciplesandthefunctionsofvariousvalvesassociatedwithpumpingstations.

Waterpipelinesanddistributionsystemsaresubjectedtosurgesalmostdaily,whichovertimecancausedamagetoequipmentandthepipelineitself.SurgesarecausedbysuddenchangesinfluidvelocityandcanbeasminorasafewPSItofivetimesthestaticpressure.Thecausesandeffectsofthesesurgesinpumpingsystemswillbediscussed,alongwithequipmentthatisdesignedtopreventanddissipatesurges.Referencewillbemadetotypicalinstallationsandexamplessothatanunderstandingoftheapplicableconstraintscanbegained.

Figure1illustratesatypicalwaterpumping/distributionsystemwheretwoparallelpumpsdrawwaterfromawetwell,thenpumpthewaterthroughcheckandbutterflyvalvesintoapumpheaderanddistributionsystem.Asurgetankandreliefvalveareshownaspossibleequipmentonthepumpheadertorelieveandpreventsurges.Eachofthesewillbediscussedingreaterdetail.

CausesandEffects

Surgesarecausedbysuddenchangesinflowvelocitythatresultfromcommoncausessuchasrapidvalveclosure,pumpstartsandstops,andimproperfillingpractices.Pipelinesoftenseetheirfirstsurgeduringfillingwhentheairbeingexpelledfromapipelinerapidlyescapesthroughamanualventorathrottledvalvefollowedbythewater.

Beingmanytimesdenserthanair,waterfollowstheairtotheoutletatahighvelocity,butitsvelocityisrestrictedbytheoutlet,therebycausingasurge.Itisimperativethatthefillingflowratebecarefullycontrolledandtheairventedthroughproperlysizedautomaticairvalves.Similarly,linevalvesmustbeclosedandopenedslowlytopreventrapidchangesinflowrate.

Theoperationofpumpsandsuddenstoppageofpumpsduetopowerfailuresprobablyhavethemostfrequentimpactonthesystemandthegreatestpotentialtocausesignificantsurges.Ifthepumpingsystemisnotcontrolledorprotected,contaminationanddamagetoequipmentandthepipelineitselfcanbeserious.

Theeffectsofsurgescanbeasminoraslooseningofpipejointstoassevereasdamagetopumps,valves,andconcretestructures.Damagedpipejointsandvacuumconditionscancausecontaminationtothesystemfromgroundwaterandbackflowsituations.Uncontrolledsurgescanbecatastrophicaswell.Linebreakscancausefloodingandlineshiftingcancausedamagetosupportsandevenconcretepiersandvaults.Lossescanbeinthemillionsofdollars,soitisessentialthatsurgesbeunderstoodandcontrolledwiththeproperequipment.

SurgeBackground

Someofthebasicequationsofsurgetheorywillbepresented,soanunderstandingofsurgecontrolequipmentcanbegained.First,thesurgepressure（H）resultingfromaninstantaneousflowstoppageisdirectlyproportionaltothechangeinvelocityandcanbecalculatedasfollows:

H=av/g

where:

H=surgepressure,ftwatercolumn

a=speedofpressurewave,ft/s

v=changeinflowvelocity,ft/s

g=gravity,32.2ft/s2

Thespeedofthepressurewave（a）varieswiththefluid,pipesize,andpipematerial.Foramediumsizedsteelline,ithasavalueofabout3500-ft/s.ForPVCpipes,thespeedwillbefarless.Fora12-insteellinewithwaterflowingat6-ft/s,themagnitudeofasurgefromaninstantaneousflowstoppageis:

H=（3500ft/s）（6ft/s）/（32ft/s2）

H=656ftwatercolumn

Thissurgepressureof656-ft（285-psi）isinadditiontothestaticlinepressure;

therefore,theresultantpressurewilllikelyexceedthepressureratingofthesystem.Further,thishighpressurewillbemaintainedforseveralsecondsasthewavereflectsfromoneendofthepipingsystemtotheotherend,causingoverpressurizationofpipesealsandfittings.Thenafterareflection,thepressurewavemaycauseanegativepressureandvacuumpocketsforseveralseconds,allowingcontaminatedgroundwatertobedrawnintothesystemthroughsealsorconnections.

Evenhighervelocitiesthanthepumpingvelocityareattainableinlongpipingsystems.Ifthepumpsaresuddenlystoppedduetoapowerfailure,thekineticenergyofthewatercombinedwiththelowinertiaofthepumpmaycauseaseparationinthewatercolumnatthepumporatahighpointinthepipeline.Whenthecolumnsofwaterreturnviathestaticheadoftheline,thereversevelocitycanexceedthenormalvelocity.Theresultantsurgepressurecanbeevenhigherthanthe656-ftcalculatedabove.

Transientanalysiscomputerprogramsarenormallyemployedtopredictcolumnseparationandtheactualreturnvelocitiesandsurges.Transientprogramscanalsomodelmethodsemployedtocontrolcolumnseparation,suchastheuseofasurgetank,vacuumbreaker,orairvalve.Thesesolutionswillbediscussedingreaterdetail.

Thusfar,thechangesinvelocityhavebeendescribedas"

sudden."

Howsuddenmustchangesinvelocitybetocausesurges?

Ifthevelocitychangeismadewithinthetimeperiod,thepressurewavewilltravelthelengthofthepipelineandreturn,thechangeinvelocitycanbeconsideredinstantaneous,andtheequationforsurgepressure（S）givenearlierapplies.Thistimeperiod,oftencalledthecriticalperiod,canbecalculatedbytheequation:

t=2L/a

t=criticalperiod,sec

L=lengthofthepipe,ft

a=speedofthepressurewave,ft/s

Fortheearlierexampleofthe12-inline,thecriticalperiodwouldbeasfollowsfora4-milongsteelpipeline:

t=2（21,120ft）/（3500ft/sec）

t=12sec

Tocausesurges,apumpdoesnotneedtostopquicklynordoesthevalveneedtocloseinstantaneously（orevensuddenly）.Anormalflowstoppageof5or10secondsmaycausethemaximumsurgeinlongpumpingsystems.Itfollowsthatsurgecontrolstrategiesshouldbeemployedonalllongpipelines.

Pumps

ReferringagaintoFigure1,akeytocontrollingsurgesinpumpingsystemsistocontroltherateofincreaseanddecreaseoftheflowvelocityintothesystem.Pumpsshouldbesizedfortheexpectedflowrequirements.Multiplepumpscanbeusedtomatchvaryingdemandsforwater.Oversizedpumpscancreatehavocincertainpumpingsystems.

Specialpumpmotorcontrolsystemsareavailabletoslowlyrampupandrampdownthepumpsbycontrollingtheelectricaldriveofthepump.Thesesystemscontrolsupplyandcanpreventsurgesduringnormalpumpoperation.However,afterapowerfailurethemotorcontrolsbecomeinoperativeandthepumpwilltripinstantlyandcauseasuddenstoppageofflow.

Somepumpstationdesignsemploymultiplepumpssothatwhenoneofthepumpsisstartedorstopped,thestoppedpumphasaminorimpactontheoverallpipelinevelocity.However,thesestationsarelikewisefacedwiththesevereimpactofapowerfailure.Almostallpumpingsystemsneedadditionalsurgeequipmenttopreventsurgesafterapowerfailure.

VerticalPumpsandWellServiceAirValves

Verticalpumps,asshowninFigure2,liftwaterfromatankorwetwellintoapipeline.Whenthepumpisoff,thesuctionwaterlevelisbelowthepumpdischargepipe.Thepumpcolumnrefillswithairaftereachpumpstoppage.

Airvalvesplayanimportantrollinautomaticallyventingthepumpcolumnairandcontrollingsurgesinpumpcolumns.Iftheverticalturbinepumpisstartedwithoutanairvalve,theairinthepumpcolumnwouldbepressurizedandforcedthroughthecheckvalveintothepipeline,causingairrelatedproblems.Airvalvesforpumpdischargeservice,calledwellserviceairvalves,aresimilartoair/vacuumvalvesbutareequippedwitheitherathrottlingdeviceoranti-slamdevice,andaredesignedtoexhaustaironpumpstart-upandadmitairuponpumpshutdown.

AsshowninFigure3,thewellserviceairvalveisanormally-open,float-operatedvalvewhichrelievestheairinthepumpcolumnrapidly.Whenwaterentersthevalve,thefloatautomaticallyrisesandclosestopreventdischargeofthewater.

Throttlingdevicesareprovidedontheoutletof3-inandsmallervalvestocontroltherateofairrelease,especiallywithslowopeningpumpcontrolvalves.Thethrottlingdeviceisadjustedwiththeexternalscrewtoslowtheriseofthewaterinthepumpcolumn.However,afterpumpshutdown,asecondportonthetopofthethrottlingdeviceprovidesfullflowintothepumpcolumntorelievethevacuum.Thedualportthrottlingdeviceisimportantbecauseitprovidesfullvacuumflowandpreventscontaminatedwaterfrombeingdrawnintothepipeline,whichcanhappenifthedevicehasacommonexhaustandvacuumconnection.

Whenapoweroperatedpumpcontrolvalveisusedwithaverticalpump,anairreleasevalveequippedwithavacuumbreakercanbeused,asshowninFigure4.Inthiscase,thepumpisstartedandtheopeningofthecontrolvalvedelayedafewsecondssothattheairreleasevalvecanexpeltheairslowlythroughitssmallorifice.

Duringtheprocess,thepumpcolumnwillbecomepressurizedtothepumpshutoffheadandforcetheairoutathighpressure.Themomentarilytrappedairwillactasacushiontocontroltheriseofthewaterinthepumpcolumn.Thevalveorificeissizedtocontroltheriseofthewatertoasafevelocity,typically2-ft/s.

CheckValves

Anotherkeyelementinpumpingsystemdesignistheproperselectionandoperationofthepumpdischargecheckvalve.Everypumpstationdesignerhasbeenfacedwithcheckvalveslam,whichiscausedbythesuddenstoppageofreverseflowthroughaclosingcheckvalve.

Topreventslam,thecheckvalvemusteithercloseveryqu