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

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1 T URBINE B YPASS S YSTEMS VELOCITY CONTROL TECHNOLOGY

Transcript of 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

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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.

HeadquartersNihonKOSOCo.,LTD.1-16-7,Nihombashi,Chuo-KuTokyo,Japan,103-0027Tel:81.3.5202.4100Fax:81.3.5202.1511www.koso.co.jp/en/

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