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Turbine bypass sysTems
V E L O C I T Y C O N T R O L T E C H N O L O G Y
Applications:
• HPtocoldreheat
• HRH(HotReheat)tocondenser,alsoknownas:
- IP/LPbypasstocondenser
- LPbypasstocondenser
• HPtocondenser
Purpose:Turbinebypasssystemsincreasetheflexibilityinoperationofsteampowerplants.Theyassistinfasterstart-upsandshutdownswithoutincurringsignificantdamagetocriticalandexpensivecomponentsinthesteamcircuitduetothermaltransients.Insomeboilerdesigns,turbinebypasssystemsarealsousedforsafetyfunction.
Majorhardwarecomponentsofturbinebypasssystemsare:
• Steampressure-reducingvalve
• Desuperheater
• Spraywatercontrolvalve
• Spraywaterisolationvalve
• Dumptube/sparger(onlyforbypasstocondenser)
• Actuator
Performanceoftheturbinebypasssystemhasastronginfluenceonplantheatrateandcapacity,effectiveforcedoutagerate(EFOR)andlong-termhealthofcriticalcomponentssuchasboilertubes,headersandsteamturbines.Therefore,correctsizingandselectionofallcomponentsinturbinebypasssystemsisesentialforsmoothoperationofasteamplant.
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Frontcover:Custom engineered turbine bypass system for a 600 MW supercritical unit. One of two HP bypass systems and four LP bypass systems supplied.
HP bypassstation
Figure 1. Locations of turbine bypass systems.
Figure 2. Typical layout of an LP bypass system for a 500 MW supercritical unit
Koso’s530D/540Dbypasssystemprovidesacost-effectivesolutioninthisseveredutyapplication.Itmeetsapplicablecodesinthepowerindustryandisengineeredtakingawealthofindustryexperienceintoaccount.The530D/540Ddesignmeetsthecriticalfunctionalrequirementsofturbinebypasssystemswhichare:
• High reliability–necessarytoachievehighplantavailability
• Low vibration and noise–forpersonnelandequipmentsafety
• Fine control–forsmoothnessofstart-upsandshutdowns,aswellasforlong-termlifeofcriticalhigh-pressure,high-temperaturecomponents
• Tight shutoff–necessarytoavoidpenaltyinheatrateand/orreductioninplantoutput;classVorMSSSP-61shut-offisavailableuponrequest
• Excellent, reliable desuperheating performance–forlong-termprotectionofthedownstreamequipment
• Ease of maintenance–noweldedseatorcage
Turbinebypasssystemsaregenerallysizedforaspecificpercentbypass,whichdependsontheend-users’intentanddesireforfunctionality.Commonpracticesforbypasscapacityare30–35%,60–70%and100%ofthedesignflow.Eachofthesereflectsdifferingintentofhowtheplantwillbeoperatedand/orthefunctionalitydesiredinoperation.
Koso’s530D/540Dbypasssystemsareconfiguredwithpneumaticactuators;electro-hydraulicactuationisavailableuponrequest.Electricactuatorsaregenerallynotusedforthisapplicationunlessslowerresponseispermittedbythesteamsystemdesign.
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Table 1. Typical range of sizes and capacities of HP bypass to Cold Reheat systems for fossil plants
Unit size (MW) x % bypassBypass flow
# of lines
Inlet/outlet
Capacity (Cv) required
1000 MW x 30% (supercritical) 1160 MT/hr 2 16” / 24” 1894800 MW x 30% (supercritical) 930 MT/hr 2 14” / 20” 1517800 MW x 60% (supercritical) 1856 MT/hr 4 14” / 20” 1517600 MW x 30% (supercritical) 695 MT/hr 2 14” / 18” 1136600 MW x 60% (sub-critical) 1390 MT/hr 2 16” / 24” 2273500 MW x 60% (sub-critical) 1160 MT/hr 2 16”/ 24” 1894250 MW x 60% (sub-critical) 580 MT/hr 2 12”/ 16” 947350 MW x 100% (sub-critical) 1350 MT/hr 1 24”/ 36” 4419
Unit size (MW) x % bypassBypass flow
# of lines
Inlet/outlet
Capacity (Cv) required
1000 MW x 30% (supercritical) 1000 MT/hr 1 14” / 20” 461800 MW x 30% (supercritical) 800 MT/hr 1 12” / 18” 367800 MW x 60% (supercritical) 1600 MT/hr 2 12” / 18” 367600 MW x 30% (supercritical) 600 MT/hr 1 10” / 16” 278600 MW x 60% (sub-critical) 1200 MT/hr 1 14” / 22” 845500 MW x 60% (sub-critical) 1000 MT/hr 1 14”/ 20” 740250 MW x 60% (sub-critical) 500 MT/hr 1 12”/ 18” 369350 MW x 100% (sub-critical) 1150 MT/hr 1 14”/ 22” 820
Table 2. Typical range of sizes and capacities of HRH bypass to condenser systems for fossil plants
Table 3. Typical range of sizes and capacities of HP bypass to condenser systems (combined cycle plants)
Steam turbine MW x % bypassBypass flow
# of valves
Inlet/outlet
Capacity (Cv) required
150 MW x 100% 500 MT/hr 1 14”/ 24” 56390 MW x 100% 300 MT/hr 1 12”/ 18” 33860 MW x 100% 200 MT/hr 1 10”/ 14” 221
Table 4. Typical range of sizes and capacities of HRH bypass to condenser systems (combined cycle plants)
Unit size (MW) x % bypassBypass flow
# of valves
Inlet/outlet
Capacity (Cv) required
150 MW x 100% 580 MT/hr 2 12”/ 16” 110090 MW x 100% 350 MT/hr 2 8”/ 12” 66060 MW x 100% 230 MT/hr 2 8”/ 10” 440
Notes:
(1)Thetablesaboveonlyforthepurposesofillustrationoftypicalconfigurations,sizes,flows,Cv’setc.Therewillbe
differenceswithspecificinstallations.
(2)ThebypasssystemsreferencedinthetablesaboverefertothesteamPRVandthedownstreamdesuperheater
combined.Theinlet/outletsizesstatedarethetypicalsteamPRVinletandDSHoutletsizesrespectively.Thesecan
befittedwithreducerstomatchthepipesizesforeaseofinstallation.
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Steam pressure reducing valve (PRV):Thesteampressure-reducingvalveinturbinebypasssystemsistheprimarymechanismofcontrollingtheupstreampressure.Itisavailableindifferentcon-figurations–intheKososystem(a)in-lineglobebody(530D)oranglebody(540D),and(b)flow-to-openorflow-to-close.Thisresultsinfourcombinations.Thefinalchoiceshouldbemadebasedontheplantlayoutandusers’preferences.Anyofthecombinations,whencorrectlydesigned,canmeetthecriticalfunctionalrequire-mentsforturbinebypassservice.
Angle-body,withflow-to-openconfigura-tion,generallyresultsinthemostcompactandlowerweightpackage.Thisconfigu-rationisadvantageousinseveralotherrespectsincluding:
• Supportingrequirementsarelessdemanding
• Pre-warmingrequirementsaresimpler
• Valveandupstreampipecondensatedrainagerequirementsaresimpler
• Specialtreatmenttoeliminatenoiseattheoutletpipeislesslikely
Typicalflowconditionsatfullflowformodernturbinebypasssystemsare:
• HPbypassinlet-180barApressureforsubcriticalUnitsand260barAforsupercriticalUnits;temperatureof550°C
•IP/HRHbypassinlet-Pressureof40barAandtemperatureof560°C
Forbypasstocondenser,pressureattheoutletisdeterminedbycapacityofthedumptubeorthedevicedischargingintothecondenser;typically,itrangesfrom4-15barAatfullflowconditionThetriminKososteamPRV’sisspeciallydesignedtokeepnoiseandvibrationwithinacceptablelimits.Theoutletcagedissipatesthelargehighenergyjet,whichwouldotherwiseformfromtheseatringregionintotheoutletpiping.Thisoutletcageispartofthequickchangetrimandcanbeeasilyinspectedduringregularmaintenance.Thisisamajoradvantageoverdesignswheresimilarbafflesareweldedinthebodyoutletregion.Screwedinseatorcageshouldbeavoidedinthisapplication.
Table 5. Typical materials of construction
Design temperature
Up to 540 °C (1005 °F)Above 540 °C (1005 °F) and
up to 600 °C (1132 °F)
Body A182 F22/A 217 WC9 A182 F91/ A217 C12A
Bonnet A182 F22/A 217 WC9 A182 F91/ A217 C12A
Inlet cage 10CrMo910/A182 F22 X20CrMoV121
Plug 10CrMo910/A182 F22 X20CrMoV121
Stem Inconel 718 Inconel 718
Seat 10CrMo910/A182 F22 X20CrMoV121
Outlet cage 10CrMo910/A182 F22 10CrMo910/A182 F22
Alternatematerialsareavailabletometspecificdesignrequirements.
(Lookingdownfromthetop)PhotographofabrokenbaffleplateinanHPbypasssteamvalve.Whenthistypeoffailureoccurs,itrequirescuttingandreplacingthewholevalve.
Figure 3. Cross-section of a steam pressure reducing valve with a VECTOR™ A trim.
Spraywater control valve:Thefunctionofthespraywatervalveistoregulatethecorrectamountofspraywaterflowintothedesuperheater.Thesearegenerallysmallvalves,typically2to4inchinsizeandareavailableinangle-bodyorin-lineglobebodyconfigurations.Directionofflow-to-closeispreferredbecausethisisaliquidservice.
Criticalfunctionalrequirementsforthespraywatercontrolvalveare:
• Highrangeability
• Quickresponse
• Goodcontrollability
• Tightshutoff
Correctsizingofspraywatervalvesiscriticaltoproperoperationofturbinebypasssystems.Excessiveover-capacityinspraywatervalvesresultsinpoorcontrolatlowflowrates.
Equalpercentageormodifiedequalpercentagecharacteristicsisrecommendedtoachievegoodcontrollability.Highthrustforseatingisrecommendedtoachieverepeatabletightshutoffinservice.
VECTOR™ velocity control trim: TheHPbypasssprayapplicationrequiresavelocitycontroltrimlikethetypeshowninFigure4.Fluidkineticenergyalongtheflowpath,withinacceptablelimits,whicheliminatespotentialproblems(cavitation,vibration,noise,prematureerosionetc).
LPbypasssprayandspraywaterisolationusuallyarenotsevereservices.
Spraywater isolation valves: Thesearerecommendedasprotectionforthedesuperheater.Theyareintendedtopreventcoldspraywaterfromdrippingonto,orcomingintodirectcontact,withhotmetalinthedesuperheater,incasethatthespraywatercontrolvalvedevelopsaleak.
5Figure 4. Velocity control valve eliminates cavitation.
Koso’s VECTOR trim delivers reliable control, long life and freedom from cavitation, erosion, vibrations and noise problems.
Example of a spraywater valve.
Actuation:Pneumaticactuatorsarecommoninmodernturbinebypasssystems.Actuatorsaresizedtoprovidefinecontrolandthehighthrustthatisrequiredtoensuretightshutoff.
Specialpneumaticcontrolcircuit,whichislocaltothecontrolvalve,controlsactionoftheactuatoraccordingtotheDCSsignals;thisincludesfastopen/closeactionandtripmodes.
KOSOalsoofferselectrohydraulic(EH)actuatorsforthisservicewhererequested.
Actuatortypeisoneofthekeydescriptorsofturbinebypasssystems.Choiceofactuationsystemistypicallymadebytheend-userbasedontheirpriorexperienceandontheplantdesign.Untilthemid-1980’s,mostoftheturbinebypassvalvesfeatured“unbalancedtrim”designs,whichrequireveryhighactuatorthrust.Asaresult,EHactuatorsweretheonlypracticalsolution.TherewasnochoiceevenwhenusersexperiencedproblemswiththeirEHactuators,thetypicalproblemsbeing:highmaintenancerequirement,potentialforfires,unreliability,limitedtoleranceforextremeenvironment(dust,humidity,heat,etc.).
Double-actingpneumaticactuatorshavebeenaroundforaverylongtime.Theyaresimpleinconstruction–thatmeanspotentialforhighreliability.Also,thedevicesthatcontroltheseactuatorshadbeenwell-knownandreadilyavailable,intheindustry.Theevolutionofmodernturbinebypassdesignswith“balancedtrim”designsresultedinthrustrequirementsthatwerewithinthecapabilityofthepneumaticactuators.Withthatdevelopment,double-acting-pneumaticactuatorsfoundeasyacceptanceintheindustry.ItprovidedaviableoptionforenduserswhodonotpreferEHsystems.
Today,withtheaddedbenefitsofeasiermaintenanceandeconomy,pneumaticactuatorshavebeenacceptedasastandardforturbinebypasssystemsinmanypowerplantdesigns,althoughelectro-hydraulicactuatorscontinuetobeusedinsomecases.AcomparisonofactuatoroptionsisshowninTable6.
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Customengineeredturbinebypasssystemforasupercriticalplantwithalow-volume,double-actingpistontypepneumaticactuator
Table 6. Comparison of actuator options (typical performance)
AttributePneumatic
(Double-acting, piston)Electro-
hydraulicElectric
(fast)
Stroke time< 2 seconds
(< 1 second possible)< 1 seconds < 5 seconds
Positioning accuracy < 2% < 0.5% < 2%
Step change response < 1% overshoot No overshoot No overshoot
Reliability Very highModerate to
highHigh
Maintenance requirement Low High Moderate
Maintenance cost Low High Moderate
Pneumaticpistonactuator Electro-hydraulicactuator
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Desuperheater:Desuperheatingforturbinebypassapplicationsischallengingbecausetheamountofspraywaterishuge.TypicalrequirementforLPturbinebypassserviceis30–35%oftheincomingsteamflow.Evenfora30%bypasssystemina600MWstation,thismeansaspraywaterflowrateofabout100t/h,ortheequivalentoffivefire-hydrants,sprayinginthepipedownstreamoftheLPbypasssteamvalve.Withintheenvelopeofthepipethedesuperheaterdesignhastoensurethat:
• thecoldspraywaterdoesnotimpingeonthehotpipewall–thisisimportantforavoidingexcessivethermalstressesandtheresultingpotentialforcrackingofpipes
•allthespraywaterhastobeevaporatedwithintheshortdistancedownstreamthatisavailable
ForHPbypasstocoldreheatsystems,thesituationissimilar,eventhoughthespraywaterisabouthalfthatfortheLPbypasssystems.
Theperformancegoalsdescribedaboverequirefineatomizationandproperdispersionofthespraywater.Largedropsarenotbeneficialforthesystem.Theytendtoimpingeonthesidewallsand/orfalloutofthesteamflowduetotheirhighinertia.Evenwhentheydon’tdropoutofthesteamflow,theyrequirealongtimetoevaporate.Generationofsmalldropsdependsontheinherentnozzleinjectioncharacteristics(orprimaryatomization)aswellastheenergyofthesteamflowintowhichthesprayisinjected(secondaryatomization).
AtomizationofliquidinasteamisgovernedprimarilybyWeberNumber(We),whichisdefinedinFigure5.Thisrelationshipshowstheimportanceoftherelativekineticenergy(1/2ρU2)ofthesteaminachievingfineatomization.Itisthekeyprincipleusedinthedesignofspraywaternozzlesaswellasforthespraynozzle-steamsystemasawhole.
Dropletsizerule:Thedropletdiameterneedstobelessthan250μmunderalloperatingconditionsforoptimumdesuperheaterperformance.
Twootherimportantconsiderationsindesuperheaterdesignisspraypenetrationandcoverage.Spraypenetrationdependsprimarilyonthemomentumratiooftheinjectedspraywaterandsteam,initialsizeoftheinjectionjetanddownstreamdistance.SeeFigure6.
Spraypenetrationrule:Thespraypenetrationiscontrolledwithin15and85%ofthepipediameterinawell-designeddesupereheater.
Steam
�ow
Drop�attens
Formscup
Thickrim
Rim breaksintoligaments
Fragmentsof bubblein center
Final breakup intodrops of various sizesHalf-
bubbleBubblebursts
Figure 6. Schematic of penetration of spraywater in a cross-flow of steam. h=spraypenetration,qL=momentumofspraywaterjet,qG=momentumofsteam,d=jetdiameter
Figure 5. Break-up of a water droplet by interaction with steam when Weber number (We) is greater than 14. We=ρU2d/σ where ρ =steamdensity,U=relativevelocityofsteam,d=dropletdiameter,σ=surfacetensionofwater.
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Coverageiscontrolledbythenumberofspraynozzlesused,theirinherentcharacteristics,aswellastheirplacementinthesteamflow.Thisisimportantforthoroughmixingofspraywaterinsteam,whichisessentialforefficientevaporation.
Theselecteddesuperheatermustmeetalloperatingconditionsforasystem–notjustthefull-loadorsizingcondition.Thisrequiresrecognitionofthedifferingcharacteristicsofeachsystem.Secondaryatomizationoflargedropsfromspraynozzlesrequiresthatthesteamflowhassufficientenergy.FromtheWerelationshipdescribedearlier,250μmdropletsizecorrespondstoabout2kPa(0.3psi)ofsteamkineticenergy.Thislimitstheperformancecapabilityofthedesuperheateratlowflowrates.ThiseffectismoresevereforHPbypasstocoldreheat(CRH)thanforturbinebypasstocondenser.
Asprayringtypedesuperheaterisbest-suitedforsteambypasstocondenserservice.Itoffersbothsimplicityandeconomy,whilemeetingallperformancerequirements.Kineticenergyofthesteamattheoutletofthesteamvalveissufficientlyhighatalloperatingconditionsinbypass-to-condensersystems;asaresult,alltheinjectedspraywaterisbrokenupintofinedrops.Spraywaterinjectionfromalargenumberofjetsisespeciallybeneficialinachievingpropercoverageacrossthesteampipes,whichtendtobelargeinlowpressureturbinebypassapplications.(SeeFigure7.)
Multi-nozzleringdesuperheatershavediscretespraywaternozzlesdistributedaroundthesteampipecircumference.Thisdesigniswell-suitedespeciallyforHPbypasstoCRH.Spraywaterisinjectedthroughthevariable-area,spring-loadedspraynozzlesinthisdesign.Thesespeciallydesignedspraynozzlesensurethatapre-determinedΔP,whichissufficientforatomization,isavailableforspraywaterinjection-thisensuresthatthespraywaterdoesnotdribbleatlowspraywaterflowrequirements.(SeeFigure8.)
Figure 9. Cross-section of a variable-area, spring-loaded spray nozzle
Figure 8. Multi-Nozzle Ring Desuperheater schematic
Figure 7. Spray-ring desuperheater
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Dump tube:Thefunctionofthedumptube(see
Figure10)istodumpthebypasssteamsafelyintothecondenser.Propersizing,selectionanddesignofthedumptubeisnecessarytoensurethatthepotentialforexcessivenoiseandvibration
iseliminated.
Typicalmaximumpressureatfullflowconditionusedforsizingdumptubesrangesfrom4to15barA.Selectionofthispressureisanimportantpartofturbinebypasssizingandhasamajorimpactontheoverallcost.Itaffectsthesizeofthevalveoutlet,outletpipesize,spraywatervalvesize,desuperheater
design,sizeofthedumptube,etc.
Dumptubesshouldbesizedforthehighestpressurepractical.Thismeanssmallersizepipebetweenthesteamvalveandthecondenser,lessdemandonsupportstructuresetc.,allofwhichleadstolowercost.Carehastobetakensothattheselectionofdumptubedesignpressuredoesnotcompromisethe
spraywatersystem.
Dumptubedesignmustalsoconsiderthepotentialforerosionduetotwo-phaseflowfromthebypasssystems.Anoptimaldesignavoidsthefailurerisksassociatedwithanunder-sizeddumptubesystemaswellastheunnecessaryadditionalcostofanover-
sizedsystem.
Noisegeneratedbythedischargefromdumptubesisamajorconsiderationinthedesignofturbinebypasssystems.Itcanbecontrolledwithinacceptablelimitswithgooddesigns.Thisisgenerallynotaconcernwithwater-cooledcondensers.However,dumptubesforair-cooledcondensersrequirespecialattention.Koso
haslow-noisetechnologiestomeetsuchrequirements.
Control algorithm:Goodcontrolofturbinebypass
systemsisessentialbothforsmoothoperationof
powerstationsandtoavoidprematurefailureof
high-pressure,hightemperaturecomponentsinthe
system.Signalgenerationforthesprayflowflow
controlvalveisacriticallinkfromthestand-pointof
controlofturbinebypasssystems.Atypicalcontrol
algorithmisshowninFigure11.
Forturbinebypasssteamvalves,theplant
controlsystemprovidesthesignaltomaintainthe
respectiveupstreamsystempressure.Thesignal
forthespraywatervalveintheHPbypasssystem
isbasedonafeedbackcontrollooptomaintain
thedownstreamtemperatureset-point.Afeed-
forwardcontrolalgorithmforspraywaterflowis
recommendedforsteambypasstocondenser.Koso
technicalexpertsareavailabletoassistintheproper
set-upofcontrolsonsite.
Figure 11. Typical control logic for turbine bypass to condenser
Figure 10. Dump tube installation schematic
stage pressureReheater
outlet pressure
Max set pt
Min set pt
PT
f{x}
<
>
T
F(x)
LP bypasssteam PRV
kf
AT
AT
A
A T A
F(x)
LP bypassspraywaterblock valve
LP bypassspraywater
control valve
C%
F(x)
Spraywater
LP bypasssteam PRV
Reheateroutlet
temperature
LP bypass steamenthalpy setpoint
PT TT PT
TT PT PT
kf
A T
LP bypassspraywater
control valve
0% 100%
Dump tubepressure
A
A
A
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AcustomVECTOR™velocitycontrolHPturbinebypasssteampressure-reducingvalveswithdesuperheaterforapowerstationinEasternEuropefeaturingtwoinletsandfast-strokingelectricactuatorsasrequestedbythecustomer.
Customization of turbine bypass systems: Customizationismoreofarulethananexceptioninthepowerindustry.Customizationmaybedrivenbythesystemoperationorbyspecialperformancerequirements.Commoninstancesrequiringcustomizationsarepre-definedpipinglayout,noiserequirements,systemoperation,etc.Specialattentionmayberequiredfortransitionsbetweenthesteamvalveandthedesuperheater,andfromthedesuperheatertotheoutletpipe,toensurethatexcessivenoisewillnotbeaproblem.Similarmaterialsofconstructionarepreferredatthepipejointtoavoidweldingofdissimilarmaterialsinthefield.
Acollaborativeeffortbetweentheplantdesignersandturbinebypasssystemprovidersisessentialinpracticallyallsituations.Itresultsincost-effectivesolutionsthatmeetalltherequirementsandachieveoptimumperformance.Mostimportantly,itreducestheriskduringthecommissioningandforthelong-termoperation.
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Related technical literature from KOSO (available upon request):1.GuidelinesforSelectionandSizingofsteampressure-reducingvalvesinturbinebypasssystems2.Desuperheatingforturbinebypasssystems3.ActuationforTurbineBypassSystems-AReviewofRequirements,OptionsandRecommendations4.InstallationGuidelinesforTurbineBypassSystems5.TurbineBypassSystems-FAQ’s(FrequentlyAskedQuestions)6.TurbineBypassSystems-CommonProblems,TheirRootCausesandSolutions
1©2010NihonKosoCo.Ltd.
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