Petroleum Gas Compression workbook 3.pdf

POLPetroleum Open Learning

OPITO

THE OIL & GAS ACADEMY

Petroleum GasCompression

Part of thePetroleum Processing Technology Series

3

Contents Page

Training Targets 3.2

Introduction 3.3

Section 1 Basic Theory 3.4 OperatingPrinciplesofaCentrifugalCompressor

Section 2 Design and Construction 3.11 CompressorCasing RotatingAssembly Bearings DiffusersandDiaphragms CompressorSeals

Section 3 Auxiliary Systems 3.32 SealOilSystem LubricationSystem CoolingSystem DriversandCouplings

Section 4 The Operation of Centrifugal Compressors 3.42 PerformanceCharacteristics OperatingProblems,AlarmsandShutdowns TheMainOperationalChecksonacentrifugalCompressor

Check Yourself Answers 3.55

Visual Cues

training targets foryoutoachievebytheendoftheunit

test yourself questionstoseehowmuchyouunderstand

check yourself answerstoletyouseeifyouhavebeenthinkingalongtherightlines

activitiesforyoutoapplyyournewknowledge

summariesforyoutorecaponthemajorstepsinyourprogress

Petroleum Gas Compression - Unit 3 - Centrifugal Compressors(PartofthePetroleumProcessingTechnologySeries)

Petroleum Open Learning

3.

Training Targets

WhenyouhavecompletedUnit3ofthePetroleumGasCompressionseriesyouwillbeableto:

Explainthebasicoperatingprinciplesofacentrifugalcompressor.

Describetheconstructionacentrifugalcompressor.

Explainthefunctionandoperationoftheprincipalcomponentsofacentrifugalcompressor.

Describethelayoutandoperationoftheauxiliarysystemsassociatedwithacentrifugalcompressor.

Explainabasiccentrifugalcompressoralarmandshutdownsystem.

Listthecommonoperatingcheckscarriedoutonacentrifugalcompressor.


3.

In Unit 1 of this Compressor programme, you saw that a centrifugal compressor is a member of the dynamic branch of the continuous flow familyofcompressors.

Dynamiccompressorsincreasethepressureofgasinadifferentwaytothatofthepositivedisplacementmachines.Areciprocatingcompressorreducesthevolumeofatrappedmassofgastoincreaseitspressure.Adynamicmachine,however,usestheprincipleofenergyconservationtoachievepressureincrease.

Therearetwomaintypesofdynamiccompressor,andtheseare:

centrifugal (radial flow) compressors

axial flow compressors

Weareonlygoingtolookatcentrifugalcompressorsinthisunit.However,inUnit4 of the compressor programme, we will look at axial flow machines in a little moredetail.

Centrifugal compressors are large capacity, continuous flow machines with a very smoothoutput.Whenrunattheiroptimumspeedandloading,theyarevibrationfreeandhavefewmovingparts.

Theyarecapableofdeliveringverylargevolumesofgas.Indaysgoneby,theytendedtobeusedforlowerpressureapplicationsthanreciprocatingcompressors.However,modernmachinesarecapableofdeliveringgasatpressuresinexcessof700bar.

TheUnitisdividedintofoursections.

Section 1coversthebasicoperatingtheoryofacentrifugalcompressor.

InSection 2,wewilllookatthedesignandconstructionofatypicalmachine.

InSection 3,wewillconcentrateontheauxiliaryequipment

and

Section 4looksatcentrifugalcompressorperformanceandoperations.

Petroleum Gas Compression - Unit 3 - Centrifugal Compressors

IntroductionPetroleum Open Learning

3.3

Thecompressorconsistsofanimpellermountedonashaftwhichcanberotatedathighspeeds.

Thisassemblyisenclosedinacasingwhichincorporatesadiffuser.Alsobuiltintothecasingarethesuctionanddischargeports,i.e.theinletandoutletofthecompressor.

WewillbelookingatthecomponentsofacompressorinmuchmoredetailinSection.Forthemomentwewillconcentrateonthisrathersimplemachine.

Look again at Figure 1 and follow the flow of gas throughthecompressorfromtheinletorsuctionporttotheoutletordischargeport.

Gasentersthecompressorthroughthesuctionportandisdirectedtotheinletoreyeoftheimpeller.

Astheimpellerrotates,thegasisforcedtorotatewithit,causingthefollowingeffects:

Centrifugal force causes the gas to flow from the eye totheoutsideorrimoftheimpeller.Astherimoftheimpelleristravellingfasterthantheeye,thegasspeedsupasitmovesoutwards.

Operating Principles of a Centrifugal CompressorIntheIntroductionIsaidthatadynamicmachineusestheprincipleofenergyconservationtoincreasethepressureofagas.Thismeansthatonetypeofenergyisconvertedtoanother.Inthiscaseitistheenergyagashasduetoitsvelocitywhichisconvertedintopressureenergy.

Thecompressionofgasinacentrifugalcompressorisatwopartprocess.

Inpartone,gasentersthemachineanditis speededuporaccelerated.Thisincreases theenergyofthegasbygivingitkinetic energyorenergyofmotion.Acertainamount ofpressureenergyisalsoaddedatthis time.

Inparttwo,thegasisrapidlydecelerated. Thisconvertskineticenergyintomore pressureenergy.

LookatFigure 1whichshowsasimplecentrifugalcompressorwithitsmaincomponents.


Section 1 - Basic TheoryPetroleum Open Learning

3.

Figure 2showstheeffectingraphicalform.

The impeller is fitted with blades which act rather likeairplanewings.Thesecreatealiftforcewhichhelpstoforcethegasfromthelowpressureattheeyetothehigherpressureattherim.

Again,thiseffectisshowngraphicallyinFigure 3.

Thesetwoforces,thecentrifugalforceandtheliftforce,acceleratethegasandraisethepressure.Thisis the first part of the process.


3.

Whenthegasleavestheimpellerattherim,itentersthe diffuser. This part of the compressor is a flowchannel. It is designed in such a way that the flowareaisgraduallyincreasedalongitslength.

ThediffusershowninFigureiscalledavolutediffuser.Avoluteisanincreasingspiralshape,asillustratedinFigure 4.Youwillrecognisetheshape if you look at shellfish such as whelks orwinkles.

As the gas flows through the diffuser, the increase in flow area causes the gas to slow down.Thisreductioninvelocitymeansthatthegaslosesitskineticenergy.

Figure 5showsthisreductioninvelocity.

Becausethetotalenergyofthegascannotbereduced,thefallinkineticenergymustbecompensatedforbyariseinsomeothertypeofenergy.Inthiscasethepressureenergyofthegasisincreased.


3.

ThenextFigure, 6,shows,ingraphicalformhowthepressure of the gas increases as it flows through the diffuser.


3.7

So,youhavenowseenhowthetwopartsoftheprocessproduceanoverallpressureincreasefromtheinlettotheoutletofthecompressor.Figure 7showstheoverallprocessfrominlettooutlet,againingraphicalform.


3.

Thecompressorwehavejustbeenlookingatiscalledasinglestagecompressor.Ithasoneimpellerandonediffuser.Itwillhaveacapabilityofincreasing the pressure of gas by a fixed amount.YouwillrememberfromUnitofthecompressorseries,thatthisincreaseisknownasthecompressionratio.

Toremindyourselfofcompressionratio,haveagoatthefollowingsimpleTest Yourself question.

Test Yourself 3.1a)Ifacompressortakesingasatapressureofpsiaanddeliversitatadischargepressureof90psia,whatisitscompressionratio?

b)Ifacompressorhasthesamecompressionratioastheoneinpart(a)andittakesingasat30psia,whatisitsdischargepressure?

You will find the answers inCheck Yourself 3.1onpage3.

Thecompressionratiowhichcanbeachievedlargelydependsonthecentrifugalforcewhichisdevelopedbytheimpeller.Thisinturndependsonthreethings:

speedofrotation

impellershape

impellerdiameter

Thereisapracticallimittosomeofthese.Forinstance,thediameteroftheimpelleranditsspeedofrotationwilldeterminehowfasttherimoftheimpelleristravelling.Thiscannotbeallowedtoexceedthespeedofsoundinthegasitiscompressing.Ifitdoesso,seriousdamagetothemachinemayresultasthesoundbarrierisbroken.

Inordertoachievegreaterpressureincreases,multiple impellers and diffusers can be fitted insideonecasing.Eachimpelleranddiffuserunitiscalledastageandsuchamachineiscalledamulti-stagecompressor.

Inamulti-stagecompressor,thegaspassesthroughanimpeller,thenadiffuser,isthendirectedtotheeyeofthenextimpeller,andsoon.

Eachstageis,ineffect,acompressorinitsownright. In the case of a five stage compressor thereare actually five compressors within one casing.

Now,beforemovingontothenextsection,haveagoatthefollowingTest Yourself question.

Test Yourself 3.2

Ofthefollowing0items,onlybelongtoacentrifugalcompressor.Indicatewithatickintheboxprovidedwhichitemsarecentrifugalcompressorcomponents.

ImpellerDiffuserPistonRodCylinderShaftCrankCasingBladesCrossHeadClearancePocket

You will find the answers inCheck Yourself 3.2onpage3.


3.9

Summary of Section 1

InthisSectiononthebasictheoryofcentrifugalcompressors,yousawthattheprincipleofenergyconservationisusedtoraisethepressureofgas.

You saw that, within the compressor, the gas is first accelerated to give it kinetic energy and some pressureenergy.Itisthensloweddownandthekineticenergyisconvertedintomorepressureenergy.

Welookedatasimplecompressormadeupofthefollowingparts:

impeller

shaft

diffuser

casing

inletport

outletport

Itwaspointedoutthatasingleimpellerhasrestrictionswhichcanlimittheamountofpressureincreaseitcanproduce.Inordertoachievegreaterpressureincreases,multi-stagecompressorsshouldbeused.

InthenextSectionwewilllookinmoredetailatthecomponentsofatypicalmulti-stagecentrifugalcompressor.


3.0

InthisSectionwearegoingtolookatthecomponentpartsofacentrifugalcompressor.Wewillseehowtheyareconstructedandwhattheirfunctionisintheoperationofthemachine.Ihavelistedbelowthecomponentswhichwewillconsider.

Theseare:

casing

rotatingassembly-consistingof

shaft

impeller

balancepiston

bearings

diffusersanddiaphragms

seals

Beforeweproceed,takealookatFigure 8overleaf,andidentifythesecomponents.


Section 2 - Design and ConstructionPetroleum Open Learning

3.


3.

Horizontallysplitcompressorsaremainlyusedinlowerpressureservice.

Ofcourse,acompressorhastobepoweredbysomething.Themotorwhichpowersthecompressoriscalledthemaindriverorsimply,thedriver.InthisSectionwewillnotconcernourselveswiththeconstructionoroperationofthedriver.However,centrifugalcompressorsareorientated,orlinedup,relativetothemaindriver,soitisworthidentifyingtheorientationatthispoint.Theendofthecompressornearesttothedriveriscalledthedrive end orinboard end.Theendofthecompressorfurthestfromthedriveriscalledthenon-drive endoroutboard end.

Letsmoveontothecomponentsnow.

Compressor CasingThecompressorcasingisusedtohousethecomponentpartsofthecompressor.Itmaybeeitherhorizontallysplitorverticallysplit,dependinguponthedesignandapplication.

Horizontally Split CasingsFigure 9showsacompressorwithahorizontallysplitcasing.Thecasingismadeintwohalveswhicharethenboltedtogetheralongahorizontaljoin.

Theinternalassemblyofthecompressormayberemovedonlyafterthetwohalvesofthecompressorcasinghavebeenunboltedandliftedapart.


3.3

Vertically Split CasingsAverticallysplitcasingconsistsofabarrelshapewithendcapsboltedontoeachendofthebarrel.

Theinternalassemblyofthecompressormaybewithdrawnfromthenon-driveendofthecasingaftertheendplatehasbeenremoved.

Verticallysplitcasingcompressorsareoftencalledbarrel compressors.Theyaremainlyusedinhightomediumpressureservice.

Figure 10showsaverticallysplitcompressorcasing.


3.

To minimise vibration, the rotating assembly is finely balanced and not allowed to run near any criticalspeeds.

Everyrotatingassemblyhasanumberofspeeds,whereitwillreachapeakofvibration.Thesespeedsarecalledthecritical speeds.

Criticalspeedscanbecalculatedasthecompressorisdesignedandbuilt.Thenormalrunningspeedofacompressorissettoavoidbeingnearacriticalspeed.Ifanyrotatingequipmentisrunatacriticalspeed,damagecanbecausedtobearingswithinsecondsasthevibrationcausesmetaltometalcontact.

Itisnormalpracticetogothroughcriticalspeedsasquicklyaspossiblewhenacceleratingthecompressortoitsnormalrunningspeed.

Rotating AssemblyYouwillrememberthatthisassemblyconsistsofashaft,impeller(s)andabalancingpiston.Letstakealookateachofthesecomponentsinturn.

The ShaftThecompressor shaftistheheartofthecentrifugalcompressor.Itcarriestheimpellersandbalancepistonandtheyallrotatetogetherwithinthecompressorcasing.

Figure 11indicatesthebasicshapeofthecompressorshaftandtherelativepositions,ontheshaft,ofthevariouscomponents.


3.

The ImpellerTheimpellersprovidethedrivingforceforthegasas it flows through the compressor. They are fixedtothecompressorshaftandrotatewithinthecompressorcasing.

Theimpellersimpartkineticenergytothegasbyincreasingitsvelocitywithinthecompressorcasing.

Impellersareavailableinavarietyofdesignssuchasopen,semi-openorclosed,andthedifferenttypesareshownin Figure 12.


3.

Theopentypeimpellerconsistsofanumberofbladesattachedtoashaft.Itrotateswithinthecasingofthecompressorandthegasisconstrainedbetweentheopenimpellerbladesandthecasingwall.

Thesemi-openimpeller,inadditiontotheblades,hasadiscorhubtowhichthebladesareattached.The hub stiffens the assembly, and helps confinethegastothebladearea.

Theclosedtypeofimpelleristhemostcommoninlargecompressorapplications,andthisistheonewewillconcentrateon.

Ithasbladesofcourse,andalsoahub.Atthefrontoftheimpeller,however,thebladesareattachedtoacoverorshroud.

The shroud also confines the gas to the blade areaandprovidesstiffening.

Closedimpellersaremadeofforgedsteel.Thebladesmaybeweldedtoboththediscandtheshroud.Alternatively,thebladesmaybemachinedfromasoliddiscandthenweldedtotheshroud.

Figure 13showsaclosedimpeller.

YouwillrememberfromSection,thattheimpellercannotberotatedattoogreataspeed-aspeedwhichwouldresultintheoutercircumferenceoftheimpellerexceedingthespeedofsoundinthegasitiscompressing.Thisisduetothepossibilityofdamagebeingcausedasthesoundbarrierisbrokenandturbulenceiscreatedwithinthecasing.

Rememberalsothat,toovercomethisrestriction,amulti-stagecompressormaybeusedtoachieveagivencompressionratio.

Toillustratethisfurther,ifeachstagehasacompression ratio of 1.35 then a five stagecompressorwouldhaveanoverallcompressionratioof.7.

The Balance PistonThelastcomponentoftherotatingassemblyisthebalance piston.

Inacompressorthepressureattheinletisobviouslylessthanthepressureattheoutlet.Thisdifferenceinpressureacrossacompressoractsontheimpellersandshafttocreateathrust force.Theforcetendstopushtherotatingassemblytowardstheinlet,orsuctionend,ofthecompressor.Aforcepushingalongthelineoftheshaftiscalledanaxial force.


3.7

In Figure 14,wecanseehowthethrustforcewhichisgeneratedwillbeimposeduponasingleimpeller.

Ifnothingwasdoneaboutthis,severedamagecouldbedonetothemachineastherotatingassemblytriedtomoveaxiallywithinthecasing.

Onewaytominimiserotatingassemblymovementistoinstallthrust bearingsinthecompressor.Wewillbelookingathowthesebearingsworkshortly.

Onalargecompressor,theamountofthrustforcedevelopedisextremelyhigh.Ifathrustbearingwastheonlydeviceinstalledtopreventthrustforcesfromdamagingthecompressor,thenthebearingwouldprobablybebiggerthanthecompressoritself.

Toreducetheeffectsofthethrustforcesandhencereducethethrustbearingrequirement,abalancing piston is fitted to the shaft. It is designed to reduce thethrustforcestoanacceptablelevel.

Figure 15showsatypicalbalancing(orbalance)piston assembly which is fitted to the high pressure endoftherotatingassembly.


3.

Thepressuredifferentialacrossthedrumproducesanaxialforcewhichopposesthethrustexertedbytheunbalancedforcesactingontheimpellers.

Thebalancepistonisasoliddrumwhichisattachedto,androtateswiththeshaft.Thehighpressuregasatthedischargeofthecompressor,actsontheinboardsideofthedrum.Lowpressuregasfromthesuctionsideofthecompressorisfedtotheoutboardsideofthedrum.Thepressuredifferentialacrossthedrumismaintainedbyhavingsealsbetweenthedrumandthecasing.

Figure 16showshowthelowpressuregasfromthesuctionsideisfedtotheoutboardsideofthedrumviaabalancing line.


3.9

BearingsTherotatingassemblyofacompressorneedstobesupportedwithinthecasingandallowedtorotatefreely.Thismeansthatsomeformofbearingisrequired.Thebearingsusedforthisapplicationareusuallyreferredtoasthemain bearingsorjournalbearings.

Also,asyouhavealreadyseen,a thrust bearingisrequired, but lets look at the journal bearings first.

Lookbacktothedrawingoftheshaft.ItisFigureonPage3..Youwillseethattherearejusttwolocationsforjournalbearings.Theyareattheextremeoutboardandinboardendsoftheshaft.Thebearingscanbeofseveraldifferenttypes,buttheoneIwilldescribeisknownasa tilting pad journal bearing.

Itisadevelopmentofasimplesleeve type journalbearingwhichisshowninFigure 17.


3.0

Thesleevetypejournalbearingconsistsofasimplehousingformingasleevearoundtheshaft.Astheshaft rotates, it causes the film of oil to form awedgewhichholdstheshaftandhousingapart.

Thetilting padtypebearingisadevelopmentofthis.Ithasanumberofpadswhicharelocatedonfixed pivots attached to the stationary housing.

Figure 18showsthearrangementofthepadsinthebearing.

As the shaft rotates, the film of oil again forms awedgebetweenshaftandpad.Inaddition,thetiltingpadsgivethebearingselfaligningproperties.Thesecompensateforanyslightmisalignmentofshaftandbearing.Theyalsotendtodistributeevenlytheloadswhicharecreatedwhentherotatingassemblyisspinning.

Andnow,letustakealookatthrustbearings.


3.

Youhavealreadyseenhowthebalancingdrumhelpstoreducethethrustforcestoacceptablelevels.Anyresidualthrust,however,mustbetakenupbyathrust bearing.

AtypicalthrustbearingisshowninFigure 19andiscalleda pivoting pad thrust bearing.

Thebearinghasacollar which is fitted to and rotates withtheshaft.Locatedinrecessesmachinedintoafixed or non-rotating thrust surface,areanumberofmetal shoesorpads.

Thecollarrotatesagainstthepadswhicharefreetopivot.Anoilwedgeformsbetweenthecollarandthepads,asinthetiltingpadjournalbearing.

Figure9showsathrustbearingwhichiscapableoftakingupthrustinonedirectiononly.Mostthrustbearings,however,cantakeupthrustaxiallyinbothdirections. This requires two fixed or stationary shaft thrustsurfacescontainingtwosetsofpads.

Diffusers and DiaphragmsFromFigurewecanseethatthediaphragmsanddiffusersarenon-rotatingpartsofthecompressor.

FromSection,youwillrememberthatgasleavingthe impeller at the rim enters a flow channel called the diffuser.Inthesimple,singlestagecompressorwhichIusedasanexample,thediffuserwasintheshapeofavolute.Inamultistagecompressor,havingavoluteaftereachstagewouldbeunwieldy.Therefore,in this case, the flow channels are formed by having diaphragms,whichformpartofthecasing,andseparatethestages.Theadjacentwallsofindividualdiaphragmsformadiffuserpassage.


3.

Figure 20demonstratesthelayoutofthediaphragmsanddiffusersinahorizontallysplit.multi-stagecentrifugalcompressor.


3.3

Thediaphragmsalsoguidethegasthroughreturnpassagestotheeyeofthenextimpeller.

AlthoughnotshowninFigure0,thediaphragmsalsocarrylabyrinthsealswhichpreventthebackflow of gas along the shaft from the high to the lowpressuresidesoftheimpellers.Wewillnowhaveacloserlookatseals.

Compressor SealsCompressorsealscanbedividedinto:

internal seals,whicharedesignedtoprevent themovementofgaswithinthecompressor casing

external seals,whicharedesignedtoprevent theescapeofgasfromthecompressorcasing totheatmosphere.

Internal SealsLetusconsider internal seals first.

Labyrinth sealsarethemostcommonformofinternalseal.Theyconsistofaseriesofteeth,across which the gas would have to flow, in order toescapefromahighpressureareaintoalowpressurearea.Figure 21showsalabyrinthsealandhowitworks.


3.

Inalabyrinthsealtheteetharemostcommonlymachinedintosleeveswhichareattachedtothecasing or diaphragm, and are a very close fit withtherevolvingshaft.

Theyoperatebymaintainingaminimumgapbetweentheshaftandtheseal,andcreatingturbulenceinthegasasitpassesacrosseachsealsurface.

Thisturbulenceconvertspressureenergyintoheatenergy,andpressureisthereforelostacrosseachstageofthelabyrinth.

Theteethofthelabyrintharemachinedintosoftalloysofaluminiumorlead.Inordertominimisethegapbetweenshaftandsealtheyareoftenallowedtorubagainsttherotatingelementwhenbeingbeddedin.

Thenumberofteethonthelabyrinthwillvaryfromasfewastwotowellover0.Thenumberusedwilldependupon:

thetypeofgasbeingcompressed

thelevelofsealingrequired

thedifferentialpressureacrosstheseal

Thetypeofsealselecteddependsuponthejobithastodo.However,becauseoftheimperfectsealingnatureofalabyrinthseal,itisneverusedastheexternalshaftsealwhencompressingflammable or dangerous gases.

Typicalusesoflabyrinthsealsinacentrifugalcompressorare:

as interstage seals to prevent the flow of high pressuregasfromthetipoftheimpellertoits eye

tomaintainthedifferentialpressureacrossthe balancepiston

tocontroltheescapeofcompressorgasinto the sealing system of an oil film seal

tocontrolthelossoflubricatingorsealoil alongashaft

External SealsNowletuslookatexternal seals.

When flammable or dangerous gases are beingcompresseditisimportantthatthegasesdonotescapefromthecompressor.Topreventthisfromoccurring,thegapbetweenthecompressorshaftandthecompressorcasing,ateachend,issealed.

Themostcommondevicesusedforexternalsealingareliquid film seals.

Liquid film sealsarecommonlyusedasexternalsealsonhighpressure,heavydutycompressorsastheyprovidecompletesealingcapabilities.Thereare,however,othertypesofsealinuse.Youwillcomeacrosscarbon ring seals, mechanicalcontact seals andincreasingly,dry gas seals.InthisUnit,wewillconcernourselvesonlywiththeliquid film seal and the dry gas seal.

Figure 22 on the next page shows a liquid film sealandthewayinwhichitworks.


3.

Liquid Film SealsThe liquid film seal uses oil as the sealing medium.Oil is pumped between the shaft and two tight fittingsleeveswhichareanchoredtothecompressorcasing.Thetwosleevesformaninboard sealingelementandan outboard sealing element.

The oil flowing across the outboard sealing elementdoesnotcomeintodirectcontactwiththecompressedgasandisreturneddirectlytoasealoil reservoir.

The oil flowing across the inboard sealing elementcomesintocontactwiththecompressedgas.Someofthegasmaydissolveintheoil,andtheoilisthereforeroutedtoade-gassing systembeforebeingreturnedtothesealoilreservoir.

The liquid film seal has no touching parts in itsassembly.Itisthereforenotpronetowear,andisveryreliable.


3.

Dry Gas SealsThedry gas seal isarecentdevelopmentwhichisbecomingincreasinglypopularasanexternalcompressorseal.

Figure 23illustratestheconstructionandoperationofthistypeofseal.

Thesealconsistsof:

arotatingtungstenring,attachedtothe compressorshaft

astaticcarbonring,attachedtothecasing

Thestaticcarbonringispushedtowardstherotatingtungstenringbyasetofcoiledsprings.Whenthecompressorisatrest,thetwofacestouchtoformagastightseal.Eachcontactfaceismachinedtoahigh degree of flatness so that this sealing effectcanbeachieved.


3.7

Aseriesofspiralgroovesarecutintothefaceofthetungstenring,asshowninFigure 24.Whentheshaftbeginstorotate,gasispulledintothesegrooves,andforcedtowardsthecentreofthering.Atinyareaofhighpressuregasiscreatedattheinsideendofeachgroove,calledasealing dam.

Pressureofgasinthesealingdamsforcesthecarbonringawayfromthetungstenring,againstthecoiledsprings.Atthispoint,thetwosurfacesarenolongerincontactand,therefore,nofrictionalheatisbeinggenerated.Thismeansthatnocoolingisrequiredundernormalrunningconditions.

If we were handling a non-flammable gas, some of the gasbeingcompressedwouldbeusedtocreatetheseal.Thesmallamountofleakagethroughthesealcouldthenbevented.

Figure3,however,showsthesealarrangementusedfor flammable or toxic gases.

Alabyrinthsealismountedbetweenthegasbeingcompressedandthedrygasseal.Nitrogenisinjectedintothisspaceatapressureslightlyhigherthanthegasbeingcompressed.Somenitrogenmayleakbackinto the compressor, but no toxic or flammable gas can escapeacrossthedrygassealtoatmosphere.Whenusedinthisway,thenitrogeniscalleda buffer gas.

Drygassealsgiveareliablelevelofsealingwithoutthe ancillary equipment associated with liquid film seals- tanks, pumps, filters, de-gassers, and so on. Apartfromcostsavings,theweightofthisequipmentcouldbeashighassevenoreighttons.Thiswillbeaveryimportantfactoronanoffshoreproductionfacility,andtheuseofdrygassealsinthisenvironmentisexpectedtoincrease.

NowhaveagoatthefollowingTest Yourself question.


3.

Test Yourself 3.3Read through the following statements and fill in the missing words from the list given below:

. Ina.....................splitcasing,thecasingismadeintwohalveswhichare boltedtogether.

. Atarotatingassemblys...............................speeditwillreachapeakofvibration.

3. Inaclosedimpellerthebladesareattachedtoboththe...................andthe.........................

. Thethrustforceactingontherotatingassemblyistakenupbytwoitems.Theyarethe .....................andthe...................................

. The........................guidethegasthroughthereturnpassagestotheeyeofthenext impeller.

. Themostcommoninternalsealisthe............................seal.

LIST OF WORDS

HUB, DIFFUSERS, CRITICAL, SHROUD, THRUSTBEARING, LABYRINTH, BALANCINGPISTON, HORIZONTALLY.

You will find the answers in Check Yourself 3.3 onpage3.


3.9

Theseitemsare:

casing

rotatingassembly(shaft-impeller-balance piston)

bearings

diffusersanddiaphragms

seals

YousawthatthecasingcanbehorizontallysplitorverticallysplitandIillustratedthedifferencebetweenthetwo.

Fromtherewemovedontolookattherotatingassemblywhichconsistsofashaft,oneormoreimpellersandabalancingpiston.Theimpellersaremountedontheshaftandrotatewithit.Yousawthatthereareanumberofbasictypesofimpelleropen,semi-closedandclosed.Thebalancepistonisalsomountedontheshaftanditsfunctionistoreducethethrustforceswhichcanactonacentrifugalcompressor.Ipointedoutthatitdoesthisbyusingapressuredifferenceacrossittocounteractthemainthrust.

Bearingswerethenextthingweconsidered.Welookedatjournalbearingsandthrustbearings.Journalbearingsareusedtosupporttherotorandallowittorotatefreely.Thrustbearingsareusedtopreventmovementoftheshaftinanaxialdirection.Inbothcasesthetiltingorpivotingpadtypebearingwasillustrated.

Nextwelookedatdiaphragmsanddiffusers.Thesearethenon-rotatingpartsofthecompressorwhichreducethevelocityofthegasleavingtheimpeller,thusraisingitspressure.Theyalsoguidethegasfromtheoutletofoneimpellertotheinleteyeofthenext.

Finallyweconsideredseals.Yousawthattheyareeitherinternalorexternalseals.Thelabyrinthtypesarecommonlyusedasinternalseals.

Liquid film seals are most often used for external sealingpurposes,althoughdrygassealsarebecomingincreasinglypopular.Bothtypeswereconsideredindetail.Youshouldbeaware,however,thatotherdesignsmaybeusedasexternalseals.

Summary of Section 2IntheSectionwehavejustworkedthrough,welookedatthecomponentpartsofacentrifugalcompressor.


3.30

BeforemovingontoSection3,wherewewillbelookingatauxiliarysystems,takesometimetotrythefollowingTest Yourself.

Test Yourself 3.4Statewhetherthefollowingcomponentsarepartof:

a) therotatingassembly

b) thesealingsystem

c) thecasingandnon-rotatingassembly

You will find the answers in Check Yourself 3.4onpage3.

. Inboardelement ...................

. Labyrinthteeth ...................

3. Impellerrim ...................

. Diffuser ...................

. Thrustcollar ...................

. Diaphragm ...................

7. De-gasser ...................

. Journalbearing ...................

9. Balancingpiston ...................

0. Inletport ...................


3.3

Inthis,thethirdsectionoftheUnit,wewillbelookingattheauxiliarysystemsassociatedwithcentrifugalcompressors.Theseare:

sealoilsystem

lubricationsystem

coolingsystem

driverandcoupling

Seal Oil SystemLetsstartwiththeseal oil system.

AswesawinthelastSection,highpressure,heavyduty compressors are often fitted with liquid film seals. Thesearedesignedtopreventanygasfromleakingtotheatmospherefromtheshaftends.

LookatFigure 25. This is a simplified view of a compressor which shows the liquid film seals at each endoftheshaft.EachsealwillbethesametypeastheoneillustratedinFigureonPage3..

Figure 25 also shows how the oil flows through the seals. Note how part of the oil in each seal flows outwardspasttheoutboardsealingelement.Thisoildoesnotcomeintocontactwithgasand,therefore,can flow directly back to the seal oil reservoir. The oil which flows inwards past the inboard sealing element becomescontaminatedwithgas.Inorderthatthisoilcanbeusedagainitmustbecleaned.Itisthereforeroutedtoade-gassingsystembeforereturningtothereservoir.

(Wewilltalkaboutthereference lineveryshortly).


Section 3 - Auxiliary SystemsPetroleum Open Learning

3.3


3.33

Thesystemwhichsuppliestheoiltothesealsconsistsofthefollowingitems.

asealoilreservoir

onemainandoneauxiliarysealoilsupply pump

filters

coolers

aheadertank

sealoiltraps(de-gassingsystem)

TakealookatFigure 26whichshowsthesealoilsystem in simplified form.


3.3

Theoilispumpedfromthereservoirviathefiltersandcoolerstotheheader tank.

A pressure reference linetakesgasfromthesealsandfeedsittotheheadertank.Variationsinthisgaspressureatthesealsare,therefore,mirroredinthetank.

However,theoilpressureatthesealmustalwaysbegreaterthanthegaspressurethere,otherwisegaswillescape.Thisisachievedbyalwayspositioningtheheadertankabovethecompressor,and maintaining the seal oil at a fixed level in thetankwithalevel controller.Thesealoilpressure,therefore,willalwaysexceedthegaspressureatthesealbyanamountequaltothestaticheadofoil.

From the header tank the oil flows to the seal.As you have just seen, some of the oil then flowsoutwardsbetweentheshaftandtheoutboardsealing element to the reservoir. The rest flowsacrosstheinboardsealingelement,tothereservoirviathede-gasser

FollowcarefullythelayoutofthesealoilsysteminFigureandensurethatyouarefamiliarwiththecomponentsandthemethodofoperation.

AsyoulookatFigure,rememberthecommentImadeonPageabouttheweight of a liquid filmsealsystem-thatitcouldeasilyreachseveraltons.

Itwillbeclearwhylightersystems,suchasdrygasseals,oftenprovemorepopularinanoffshoreenvironment.

Lubrication SystemWecannowmoveontothelubrication system.

Allrotatingmachinerymusthavealubricationsystemofsomeformoranother.Compressorsarenoexception.Thelubricant,inthiscaseoil,performsthefollowingfunctions:

separatesmovingparts

removesheatgeneratedbyfriction

reducesmetalwear

protectsmetalsurfacesfromcorrosion

The lubrication system normally provides a flow ofoiltothejournalandthrustbearingsofthecompressor.Inaddition,itprovideslubricationforthemaindriver,gearboxandotheraccessories.

Mostlubricatingoils,usuallyabbreviatedtolubeoils, are refined from crude oil. However, to givethemtheirspecialproperties,chemicaladditivesmaybemixedwiththem.

Eachcompressorwillhavealubeoilwhichisspecified for that particular machine. Care must betakentoensurethatnootherlubricantisused,inordertopreventthepossibilityofdamagetothecompressor.

Insomecompressors,thelubricationsystemmaybecombinedwiththesealoilsystemwhichwehavejustbeenlookingat.Here,however,wewill

considerthetwoasseparatesystems.

Atypicallubricationsystemwouldconsistofthefollowingcomponents:

anoilreservoir

amainandauxiliarypump

coolers

filters

ThesystemlayoutisshowninFigure 27,overleaf.Takealookatthisnowandidentifythecomponents.


3.3

Thelubeoilispumpedfromthereservoirthroughcoolerswhichreducethetemperatureoftheoiltoitscorrectoperatingvalue.Fromthereitpassesthroughfilterswhichremoveanydirtparticles.Thecooled, clean oil then flows under pressure to each bearingthroughpipework,groovesandchannels.After performing its lubricating job, the oil flows back tothereservoirundergravity.Thesystempressureismaintainedbyhavingapressure controller inthelinebetween the filters and the compressor.

Notethataheadertankisincorporatedintothesystem.Undernormalrunningconditionsthetankiskepttoppedupwithoilfromthemainfeedline.Asmallamount of oil will overflow back to the reservoir.

Ifthemainlubeoilpumpsfail,thecompressorwillshutdown.Whenthishappens,thebearingsarelubricatedusingagravityfeedfromtheheadertank.Thisensuresthattherewillbenodamagecausedtounlubricatedbearingsduringthetimethatthecompressorisrollingtoahalt.

BeforemovingontothenextpartofSection3,inwhichwewilllookatcoolingsystems,try Test Yourself 3.5.


3.3

Test Yourself 3.5Indicateifthefollowingstatementsaretrueorfalse.Iffalsegivethecorrectanswer.

a) Theexternalsealsofahighpressurecompressorareusuallylabyrinthseals.

b) Inasealoilsystemapressurereferencelinetakesgasfromthesealsandfeedsittoa headertank.

c) Inasealoilsystemtheoilwhichcomesincontactwiththegasisroutedtothereservoirsvia the filters.

d) Lubricatingoilhelpstoremoveheatgeneratedbyfriction.

e) In a typical lube oil system the filters remove dirt particles from the oil.

You will find the answers in Check Yourself 3.5 onpage3.


3.37

Cooling SystemCompressiongeneratesheat.Itisoftennecessaryto cool the compressed gas for safe and efficientoperationoftheplant.Inmanyinstallations,theremaybeanumberofcompressorsworkinginseries.If the hot, compressed gas from the first compressorwerepasseddirectlytothesuctionofthenextmachine,overheatinganddamagecouldoccur.

Insuchasituation,anaftercoolermaybeinstalleddownstreamofeachcompressor.

Figure 28showsatypicalaftercoolerforacentrifugalcompressor,anditspositioninthesystem.


3.3

Largecompressorswillconsistoftwoormoresetsofimpellersmountedonacommonshaft.Inordertoreducethetemperatureincreasewithinsuchamachine, the gas may be discharged after the firstsetofimpellers,cooled,anddirectedtothesuctionofthenextset.

Inthiscase,thegaspassesthroughanintercooleras it flows between the two sets of impellers.

ThisarrangementisshowninFigure 29.


3.39

Drivers and CouplingsIn the final part of this Section, I want to have a brieflookat drivers and couplings.

Centrifugal compressors used in oil and gas field operationsarenormallydrivenby gas turbines orelectric motors.Theseareconnectedtothecompressorbymeansof drive shaftsandcouplings.

Somedrivers,whichrotateathighspeeds,maybecoupleddirectlytothecompressor.However,itisoftennecessarytoincorporatesomekindofspeed increasing gearboxbetweendriverandcompressor.

Couplingsareusedtoconnecttwoshaftstogetherandtransmittherotarymotionofonetotheother.Greatcareistakentoalignthedrivershafttothecompressorshaft.However,duetovariationsinloadingandpossibleunevenheatingofmaterialsandequipment,smallmisalignmentconditionscanoccur.

Becauseofthisaflexible couplingisrequired.You should not misunderstand the word flexible. Acompressorcouplingmaybeover0inchesindiameter, and appear to be far from flexible when handled.

Insomeinstances,itmaybenecessaryforthecompressortorunatavariablespeed,eventhoughthedrivermaybeaconstantspeedmachinesuchasanelectricmotor.Intheseinstances,a variable speed fluid coupling may be fitted between the main driverandthegearboxtopermitthis.

InthisshortpieceondriversandcouplingsIhavenottriedtodescribethecomponentsindetail.However,itisimportantthatyouknowtheterminologyassociatedwiththem.

BeforeIsummarisethisSection,attemptTest Yourself 3.6.

Test Yourself 3.6Inthefollowinglistofcomponents,somebelongtothecompressorauxiliarysystems,somedonot.Ifnot,towhichsystemdotheybelong?

. Impeller

. Lubeoilcooler

3. Coupling

. Driver

. Balancingpiston

. Diaphragm

7. Intercooler

. De-gasser

9. Reservoir

0. Thrustcollar

You will find the answers in Check Yourself3.6 onpage3.


3.0

Summary of Section 3InthisSectiononauxiliaries,wehavelookedatsealoilsystems,lubricatingsystems,coolingsystems,driversandcouplings.

Startingwithsealoilsystems,yousawthattheoilispumpedfromareservoir,throughcoolersandfilters, to a header tank. The level in the tank maintained the correct oil pressure on the seal.Someoftheoil,whichbecomescontaminatedwithgas,iscleanedinadegassingsystembeforebeingusedagain.Therestoftheoilgoesstraightbacktothereservoir.

Turningourattentiontothelubricatingsystem,yousawthatthesystemisverysimilartothesealoil system. Again, coolers and filters are used and the oil is continually circulated round thesystemandbacktoareservoir.Thelubeoilnotonlylubricatesthecompressorbearings,butalsothedriverandgearbox.

A cooling system may be required to reduce the temperature of the gas for safe and efficientoperation.Yousawthatthissystemmaytaketheformofanintercooler,oranaftercooler.

Finally,intheSection,wehadabrieflookatdriversandcouplings.Thecompressorsmaybedrivenbyelectricmotors,orgasorsteamturbines.Theymaybedirectlycoupledorbedrivenviaagearbox.Couplingsconnecttheshaftsofthedriverandcompressor.Thesecouplingsarecalled flexible couplings which are capable of taking up any small misalignments between theshafts.

In the final Section of this Unit on centrifugal compressors, we will take a look at compressoroperations.


3.

In this, the final Section of the centrifugalcompressorunit,wewillbelookingattheoperationofthecompressor.

IhavedividedtheSectionintothefollowingtopics:

compressorperformancecharacteristics

operatingproblems,alarmsandshutdowns

operationalchecks

Letsstartbyconsideringtheperformancecharacteristics.

Performance CharacteristicsTheperformanceofacentrifugalcompressorcanbeshownonasetofoperating curves.Thesearegraphspreparedindividuallyforeachcompressor.They show the range of flows, heads, efficiencies andspeedswithinwhichaparticularcompressoriscapableofoperating.Inotherwords,theyindicatetheperformanceofthecompressorunderdifferentoperatingconditions.

Figure 30isasimplegraphwhichshowsasingleoperating curve for a specific compressor.


Section 4 - The Operation of Centrifugal CompressorsPetroleum Open Learning

3.

Youcanseethat:

theverticalaxisgivestheactualcompressionratio(orhead)asapercentageofthedesigncompressionratio

the horizontal axis gives the inlet flow (capacity)asapercentageofthedesigncapacity

Thesolidlinecurvewhichpassesthroughthedesignpointshowstherelationshipbetweenheadandinletflow when the machine is running at 100% of its design speed.

Thedesign pointforanycompressoristhepointatwhichthemachineis:

running at 100% of its design speed

compressing 100% of the design capacity or inlet flow

producing 100% of the design compression ratio

At this point the compressor is operating at 100% efficiency. As you can see from the operating curve, any changes to the speed, flow or pressure will remove itfromthispoint.

YouwillnoticethreeotherlinesonFigure30,markedas:

optimum efficiency

approximatesurgelimit

stonewall(choke)line

Wewilllookatalloftheseinsomedetaillater.

Infact,theperformanceofacompressorisusuallyexpressedbyafamilyofcurves.Theshapeandpositionofthecurvesdependonanumberofthings.Theseincludethedesign,size,speedandthenumberofimpellers.

AtypicalsetofcurvesisshowninFigure 31,overleaf.


3.3

InFigure3,thecurvesrepresenttherelationshipbetween head and flow for a number of differentspeeds.

The efficiency of the machine at various heads, flowsandspeedsisindicatedinthegraphasaseriesofellipses representing lines of constant efficiency. Youwillalsoseethatoptimum efficiencyatvaryingspeedsisrepresentedbyasinglelinerunningthroughtheellipses.

Themachineiscapableofoperatingatconditionsanywherewithintheenvelopeofthecurves.Thismeansthatthecompressorisabletooperateatconditionsawayfromitsdesignpoint,butatareducedefficiency.

Theboundariesofthisenvelopetotheleftandrightarelabelledapproximate surge limit andstonewall(choke) line.Iwillhavemoretosayabouttheselinesshortly.

Theupperboundaryoftheenvelopeisdeterminedbythemaximumspeedatwhichtheimpelleriscapableofrotating.Thisinturndependsonitssizeandconstruction.

Thelowerboundaryisnotreallyaproblemforthecompressor.Ifthesurgeandstonewalllineswereextendeddown,theywouldmeetatthepointofzeroflow and zero head. So the compressor could operateatmuchlowerspeeds.Thislowerlimitisusuallydeterminedbytheminimumspeedatwhichthedrivercanbeoperated.


3.

LetsnowlookatthetwoboundariesofthecompressorperformancecurvesshowninFigures30and3.

Thesearetheapproximate surge limit and stonewall.Firstofall,surgelimit.

Compressor SurgeThe surge limit is the minimum flow for a given speed at which the compressor can maintainstable operation. At inlet flow rates to the left of this line, the operation of the machinebecomesunstable.

Wecananalyseacompressorcharacteristiccurvetoseehowsurgeinacompressoroccurs.

TakealookatFigure 32 which shows a curve of discharge pressure against inlet flow for aconstantspeed.You should note that I have deliberately exaggerated the shape of thiscurve in order to make the following explanation of surge more easily understood.

ImaginethatthecompressorisrunningatconstantspeedatPointonthecurve.Thismeansthatitwillbedeliveringacertainpressure(P)withacorrespondinginlet flow (F). Everything is normal.

Supposing,however,thatthereisasuddenincreaseindownstream resistance which reduces the inlet flow to Pointonthecurve.

Dontforgetthat,ifthecompressorisrunningataconstant speed, the pressure and flow values mustlieonthecurve.

Atthispointthecompressorwillcontinuetooperateinastablemanner.Eventhoughthereisapressureincreaseinthedownstreampipework,thecompressordischargepressurehasrisentoovercomethis.

NowlookatFigure 33


3.

Supposing,now,thatthecompressorisoperatingatPoint3onthecurvewhichisrightatthepeak.

Now, if there is a reduction in flow due to an increase indownstreamresistance,theoperatingpointmovestoPoint.

ThispointisinaregionwherethecompressoractuallyproduceslesspressurethanatPoint3.Themachinenowcannotproducethepressurenecessarytoovercomethedownstreamresistance.

When this condition occurs, the flow momentarily reverses its direction so there is no forward flow. The operatingpointnowmovestoPoint.

With no flow through the machine, the discharge pressure is reduced. This then allows the gas flow rate tobuildupagaintowardsPoint.

But, at that flow rate, the pressure delivered by the machineislessthanthatrequiredtoovercomethedownstreamresistance.TheoperatingpointthenmovesalongthecurvetowardsPoint3againand,oncebeyondthere,thecycleisrepeated.

ThecyclingIhavejustdescribedcausesoscillationsofthe gas flow in the compressor and pipeline, which is knownassurge.

Inmajorcasesofsurgetherearecompletereversalsofflow which result in a massive shuntingoftherotatingassembly.Eachcyclemayoccurovertwicepersecondandoneverycyclethewholerotatingassembly(whichmayweighinexcessoftwotons)shuntsinsidethecompressorcasingasthethrustforceschangedirectionacrossthecompressor.

Eachshuntcanthrowenormousstressesonthrustbearings,journalbearings,sealsandshaftcouplings.

Theultimateresultofcompressorsurgeistherapidbackwardandforwardmovementoftherotatingassembly,andtheresultingvibrationmay:

destroyinternallabyrinthseals

destroyjournaland/orthrustbearings

seizetherotatingassemblyinitsbearings

stallthemaindriver

shattertheshaftcouplingand/orgearbox

shattertheimpellersand/orthecompressor casing

Theeffectsofcompressorsurgearewellknownandpredictable.Becauseofthis,allhighpressurecompressors are fitted with anti-surge controlloopswhichpreventthesituationfromoccurring.

Theanti-surgecontrolloopsoperatebycontrollingthe flow of gas through the compressor to a pre-setminimum.


3.

Figure3isadrawingofasimpleanti-surgecontrollerwhichre-cyclesgasfromthedischargeofthecompressorbacktothesuctionline.

Youcanseefromthedrawingthattheanti-surge controller isconnectedtoacontrol valve.Thevalveisinapipelinewhichisconnectedbetweenthecompressorsuctionanddischargelinesoutsidethemachine.

The controller senses and compares the rate of gas flow intothe compressor, and the pressure rise across it. If the flow rateatagivenpressureriseislessthanthesettingofthecontroller,the control valve is opened. This allows enough gas to flowfromthedischargetothesuctionofthecompressortokeepthesuction flow rate above the surge value.

Notethattherecycledgasiscooledinaheatexchanger.Thisistopreventarapidincreaseintemperatureashotdischargegasisfedtothesuctionandfurtherheatedasitiscompressedagain.

Inviewofthedamagewhichmayresultfromsurge,mostcentrifugal compressors are fitted with an approaching surgealarmandananti-surge shutdown,inadditiontoananti-surgecontroller.Thesesystemsworkasfollows:

Theanti-surgecontrollershouldnormallypreventsurgefromoccurring.Ifitfailstooperate,thentheapproachingsurgealarmwillbeactivated.

Theapproachingsurgealarmwillalerttheoperatortothefactthattheanti-surgecontrollerhasnotmanagedtorectifythesituation.Ifthesituationisnotcorrectedthentheanti-surgeshutdownisactivated.

Theanti-surgeshutdowndeviceissettoactivatejustbeforethepointatwhichthecompressorwillentersurge.Whenthissafetydeviceoperates,thecompressorwillbestopped.


3.7

Stonewall or ChokeYouwillrememberthatthesecondboundarylineonthecompressorperformancegraphwascalledthestonewall (choke).Figures30and3bothshowthis.

Stonewall is the point at which the flow rate throughthecompressorapproachesthespeedofsoundinthegasitiscompressing.Asthisoccurs,theshockwavesgeneratedresultinachokingeffect,whichpreventsthegasfrombuildingpressure.

Thesymptomsthatthecompressorisapproachingastonewallconditionarearapidlyfallingdischargehead coupled with very little extra flow.

Stonewall limits the maximum flow which themachinecanachieveatagivenspeed.

Beforemovingon,haveagoatTest Yourself 3.7.

Test Yourself 3.7a) List3possibleconsequencesofsurge inacompressor.

b) Whatmeasurementsarecomparedby ananti-surgecontroller?

c) Whatdeterminesthelowerspeedlimit ofacentrifugalcompressor?

You will find the answers in Check Yourself 3.7onpage3.7

Operating Problems, Alarmsand ShutdownsJustlikeanyothermachineryorprocessplant,compressorsareprotectedfrommalfunctionordamagebyinstrumentationsystems.Thesesystemswillgeneratealarmsandcompressorshutdownsifdangeroussituationsshouldarise.Letshavealookatsomeofthesesituationsnow.

Excessive Compressor SpeedIfthecompressorisbeingdrivenbyanelectricmotor, rotational speed is normally fixed and normalrunningspeedcannotbeexceeded.

Ifthecompressorisbeingdrivenbyasteamorgasturbine,normalrunningspeedcouldbeexceededwhichmightresultinseveredamagetothesystem.Inthesecasesthecompressor,andthemaindriver,arenormallyprotectedby:

aspeedgovernoronthemaindriverwhichis settocontrolthecompressoratamaximum of 100% normal running speed

anelectronicspeedsensoronthemain driverwhichwillshutdownthecompressorif its rotational speed exceeds 105% of normal runningspeed


3.

a mechanical trip which flies out from the shaft ofthemaindriverandtripsthefuelsupply, iftherotationalspeedofthecompressor exceeds 110% of normal running speed

Ingress of LiquidsIngressoffreeliquidsintothecompressorcanbeextremelydangerous.Theliquidscouldbeintheformofmistordroplets,orintheformoflargerslugs.

Ifliquidsenteracentrifugalcompressorinmistordropletform,theremaybeerosionoftheimpellers,diaphragmsandcasing.

This erosion may create a loss of efficiency, orproblemsofvibrationastheimpellersbecomeunbalanced.

Inextremecases,theerosionwillresultinbothimpellerandcasingfailure.

Iftheliquidsareintheformofslugswhentheyenterthecompressorthen,asliquidsarebothincompressibleanddenserthangases,theeffectsarelikelytobesuddenanddramatic.

Theresultsarevariedbutmayinclude:

Thecompressormayslowdownorstopand themaindrivermaystall

Thestrainonthegearboxmaycauseittofail orburst

Thecompressorshaftmaybebentbythe suddenandunevenforceswhichoccur.This bendingofthecompressorshaftmayoften resultinacompressorcasingfailure

Thesealsystemmayfailduetoanover pressuringofthesystem

Themostcommonresultisthatconsiderabledamagetothecompressorsshaft,shaftcoupling,impellersandcasingwilloccur.

Inordertopreventliquidsenteringthemachine,allcentrifugal compressors are fitted with suctionknock-out drums.Thesearepressurevesselslocatedinthepipeworkleadingtothesuctionofthecompressor.Theyaredesignedtoseparateanyfreeliquidfromthesuctiongasstream.

Theliquidwhichisseparatedfromthegas,accumulatesinthebottomofthedrum.Ifthisliquidlevelrisesbeyondacertainpoint,thereisadangerofitbeingcarriedoverwiththegas.Topreventthis,alowliquidlevelismaintainedinthevesselbylevelcontrolinstrumentation.Anyfurtherriseinlevel,perhapsthroughalevelcontrolmalfunction,willtriggeranalarmatapre-setvalue.Ifthelevelstillcontinuestorise,theinstrumentswillcauseashutdownofthecompressorplant.

Figure 35,onthenextpage,showsasimplelayoutofcompressorandknockoutdrum.


3.9


3.0

Test Yourself 3.8

Describewiththeaidofasimplesketchananti-surgecontroller.

You will find the answer in Check Yourself 3.8onpage3.7

Lube Oil Pressure and TemperatureIfthepressureonthelubeoilsystemistoolow,thecompressorwillnotbelubricatedproperly.Excessivewearonbearingswouldthenoccur.Topreventthis,pressuresensorsinitiateanalarm,followedbyashutdown,ifthepressurefallsbelowasetvalue.

Similarly,ifthelubeoilgetstoohot,itwillbecometoothinandloseitslubricatingproperties.Temperaturesensorswillinitiatehighlubeoil

VibrationIfthecompressorstartstovibrate,therotatingimpellerscouldrubagainstthecasing.Thiscoulddamagethemachine,andinseverecases,causecatastrophicfailure.

Thevibrationcouldbecausedbysurge,corrosionproductsbeingdepositedinthecompressor,erosionofimpellers,ormisalignmentofthedriverandcompressor.

Toprotectthecompressor,vibrationmonitorsareinstalledatvariouspointsonthemachine.Atpre-determinedvibrationlevels,themonitorswillinitiatealarms,followedbyshutdowns.

SurgeWelookedattheproblemofsurgealittleearlierinthisSection.CheckthatyourememberwhatprotectsthemachineinthecaseofsurgebyattemptingTest Yourself 3.8.

temperaturealarmsandshutdowns.InadditiontothealarmsandshutdownswhichIhave just described, other alarms may be fitted tothecompressor.Thesearecalledinhibit alarms.They are fitted to prevent the compressor beingstartedunderconditionswhichmaybedetrimentaltothemachine.Oncethemachineisrunning,however,theinhibitalarmwillnotcauseashutdown.

Ihaveincludedbelow,asexamples,afewinhibitalarmsituations:

Aninhibitwhichwillnotallowthecompressor lube-oilpumptostartuntilthelube-oil temperatureisatapre-setlevel.

Aninhibitwhichwillnotallowthecompressor tostartuntilthelube-oiltemperatureisata pre-setlevel.

Aninhibitwhichwillnotallowthecompressor tostartuntilthelube-oilreservoirisfull.

Aninhibitwhichwillnotallowthecompressor tostartunlessthesuctionvalveisopen.


3.

The Main Operational Checks on aCentrifugal CompressorWehavelookedathowthesystemiscontrolled,andwhatalarmandshutdownsystemsarecommonlyinstalled.Nowwecanlookathowthecompressorshouldbeoperated.

Thegoldenrulesforoperatingacentrifugalcompressorare:

Before Starting the CompressorCheck that the compressor is purged of all air

Ifthecompressorisnotcompletelypurgedofair,thegas/airmixturemayburnorexplodewhenthecompressorisstarted.

Check that the suction line and compressorcasing is free from liquids

Liquidsareincompressible.Ifwetrytocompressthem,thepressureincreasemaybesohighthatthecompressorisdamaged.

Thehighrotationalspeedsofcentrifugalcompressorsalsomeansthattheydevelophightorquewhenstarting.Tryingtospinacasingfullofliquidmaystallthemaindriverduringthestartsequence.

Check that the valves in the suction anddischarge pipelines and the anti-surge systemare in the correct position, i.e., open or closed.

Wehavetomakesurethatthecompressorhasanuninterruptedsupplyofgastothesuction.Wealsohave to make sure that the gas is able to flow awayfromthecompressortowhereitisintendedtogo.Theanti-surgelineshouldalsobecheckedtoensurethatthecontrolvalveisfullyopenwhenthecompressorisbeingstarted.

Check that dependent systems are operational

Wehavetocheckthatthecompressorisnotgoingtoshutdownbecauseofalackofgas,orbecausethemaindriverhasrunoutoffuel,orforanyotherreasonwhichisnotdirectlyrelatedtothecompressoritself.

Check that the discharge valve is in the correctposition

Somecentrifugalcompressorsarestartedupwiththedischargevalveclosedandallowedtogothroughare-cyclestagebeforetheyareputonline.

Check that all relief valves are operational

Check that the lube-oil system is operatingcorrectly

Weshouldcheckthat:

there is sufficient lube-oil in the reservoir

anylube-oiladdedtothesystemisofthe correcttypeandgrade

mainlubeoilpumpsarerunningandheader tanksarefull

lube-oil flows are within normal limits

auxiliarylubeoilpumpsareavailableforuse

Check that the seal oil system is operatingcorrectly

Weshouldcheckthat:

there is sufficient seal oil in the reservoir

anysealoiladdedtothesystemisofthe correcttypeandgrade

mainsealoilpumpsarerunningandheader tanksarefull

seal oil flows are within normal limits

auxiliarysealoilpumpsareavailableforuse


3.

Check that the lube-oil and seal oil coolingmedium is available

Weshouldcheckthat:

thelube-oilandsealoilcoolersareoperating correctly

thetemperaturesarebeingproperly maintained

Check that no current alarm or shutdown conditions exist

Inadditiontothealarmandshutdowninstrumentation(includinginhibitalarms),centrifugalcompressorsareoften fitted with various by-passes which allow them to startup.Themostcommonbypassesare: An anti-surge shutdown by-pass.Surgeis a function of low flow, and the anti-surge shutdown operates when the flow through the compressorislow.Beforestart-upthereis, of course, no flow. Therefore, the anti-surge shutdownmustbeby-passedduringstart-up, topreventitshuttingdownthecompressor due to the temporary, low flow conditions whichexistatthistime.

A high vibration shutdown by-pass.Asthe compressorstartsupitmaypassthroughtwo ormorecriticalspeedswhichwillcausehigh vibration.Unlessthehighvibrationshutdown isby-passed,thecompressorwillbeshut downasitpassesthroughthesecritical speeds.

When the Compressor is Running Check that the pressures, levels, flows and temperatures are within operational limits. Thesechecksmustbemadeatregular intervals,atleastonceeverytwohours.

Check that the lube-oil and seal oil levels are maintained.Ifitbecomesnecessaryto topupthesystemswithoil,thefollowing pointsshouldbenoted:

Alwaysusethecorrecttypeofoilforeachsystem.

Alwaystopupthesystemwithcleananddustfreeoilofidenticalgrade.

Ihavedescribedafewchecksthatanoperatorofcompressionplantshouldcarryoutaspartofhisroutineduties.Thelistisbynomeansexhaustiveandiscertainlynotmeanttobetakenasanoperatingprocedure.

If you are involved in compressor operationsyou should become completely familiar with theequipment under your control. Your specificoperating procedures should be followed, andsafe working practices adopted at all times.


3.3

Summary of Section 4 In the final Section of this Unit on centrifugal compressors, we have concentrated on the operationofthesystem.

Thesectionwassplitintothreeparts:

In the first part we looked at compressor performancecharacteristics.Yousawthat theperformancecanbeillustratedbyaset ofoperatingcurves.Thesecurvesshowthe range of heads, flows, efficiencies and speedswithinwhichacompressorcan operate.

Westudiedasetoftypicalcurvesandused themtoexplainsurgeandstonewall.You sawhowacompressorisprotectedagainst surgebyananti-surgesystem.

Wethenwentontoconsidersome operatingproblems,alarmsandshutdowns. Welookedatsometypicalproblemsand sawhowacompressorisprotectedagainst thedamagethattheycancause.

Finallywehadalookatsomeofthe operationalcheckswhichshouldbecarried outbeforeacompressorisstartedandwhen themachineisrunning.

NowthatyouhavecompletedSection,youhavecometotheendofUnit3ofthecompressionprogramme.I must emphasise once again that this unit is not meant to take the place of specific manufacturers guidelines or operating instructions.Itisintendedtogiveyouagoodbasicgroundinginthedesign,constructionandoperationofcentrifugalcompressors.

Now,gobacktotheTrainingTargetsandsatisfyyourselfthatyouhavemetthosetargets.


3.

Check Yourself 3.2 Theitemsmarkedwithatickarecentrifugalcompressoritems.Theothersbelongtoareciprocatingcompressor.

Impeller

Diffuser

PistonRod

Cylinder

Shaft

Crank

Casing

Blades

CrossHead

ClearancePocket

Check Yourself 3.1a) Compressionratio dischargepressure = suctionpressure 90psia=3. = psia

b) Dischargepressure = SuctionpressurexCompressionratio = 30psiax3. = 0psia

Check Yourself 3.3. InaHORIZONTALLYsplitcasing, thecasingismadeintwohalves whichareboltedtogether.

. Atarotatingassemblys CRITICALspeeditwillreacha peakofvibration.

3. Inaclosedimpellerthebladesare attachedtoboththeHUBandthe SHROUD

. Thethrustforceactingonthe rotatingassemblyistakenupby twoitems.Theyarethe BALANCINGPISTONandthe THRUSTBEARING

. TheDIFFUSERSguidethegas throughthereturnpassagesto theeyeofthenextimpeller.

. Themostcommoninternalsealis theLABYRINTHseal.

Check Yourself - AnswersPetroleum Open Learning

3.

Check Yourself 3.4

. Inboardelement (b)

. Labyrinthteeth (b)

3. Impellerrim (a)

. Diffuser (c)

. Thrustcollar (a)

. Diaphragm (c)

7. De-gasser (b)

. Journalbearing (c)

9. Balancingpiston (a)

0. Inletport (c)

Check Yourself 3.5 a) FALSE Theexternalsealsareusuallyliquid film seals or other, equally efficient types.

b) TRUEc) FALSE Theoilwhichcomesintocontactwith thegasisroutedviatheDE-GASSERStothe reservoirs.

d) TRUE

e) TRUE

Check Yourself 3.6. Impeller NO-Rotatingassembly

. Lubeoilcooler YES

3. Coupling YES

. Driver YES

. BalancingpistonNO-Rotatingassembly

. Diaphragm NO-Nonrotatingassembly

7. Intercooler YES

. De-gasser YES

9. Reservoir YES

0. Thrustcollar NO-Rotatingassembly


3.

Check Yourself 3.7

a) Anythreeofthefollowing:

- destroyinternallabyrinthseals

- destroybearings

- stallmaindriver

- shattercouplingand/orgearbox

- shatterimpeller

b) Inlet flow rate, and pressure difference between suction anddischarge.

c) Theminimumspeedatwhichthedrivercanbeoperated.

Check Yourself 3.8UsingasketchsimilartothatinFigure 34,yourdescriptionshouldhavebeenasfollows:

An anti-surge controller compares the inlet flow to the compressorwith the pressure rise across the compressor. If the inlet flow rate istoolowforaparticularpressurerisethecontrolleropensavalveinalinebetweencompressorsuctionanddischarge.

This allows gas to flow from the discharge to the suction side of thecompressor to keep the suction flow rate above that at which surgewilloccur.


3.7


3.

Petroleum GasPetroleum Gas Compression workbook 3

Documents

Petroleum Gas Compression workbook 3.pdf