PrimarySuperheater

SecondarySuperheater

Main FWpump

L.P.Heaters

Polishingequip.HTR drain pump

HP Heaters

204

203202-1

202-2

200

201

207240

220

221 230

231

241

205

210

250

Capillarysystem

Boiler

FlashTank

Dea

storage

cond pump

Turbine

Condenser

CCI Severe ServiceApplications in Fossil Power Plants

2

3

This handbook was published thanks to the creation and direction of Curtis Sterud, CCI Valve Doctor.

With over 40 years of experience in the valve industry, Curtis is one of the industry’s most respected

engineers and severe service valve experts.

4

I. SEVERE SERVICE APPLICATIONS OVERVIEW–SCHEMATICS AND

DIAGRAMS

a)FossilOnceThruUnits

II. DRUM BOILER

III. CONDENSATE SYSTEM

a) CondensatePumpRecirculationValve

b) DeaereatorLevelControlValve

IV. FEEDWATER SYSTEM

a) BoilerFeedwaterPumpRecirculationValve

b) BoilerFeedwaterRegulatorValve(B&W100)

V. MAIN STEAM SYSTEM

a) Start-UpSystemValves(B&W)

i. 501

ii. 502

iii. 518

iv. 519

b) SuperheaterAttemperatorSprayValve

c) ReheaterAttemperatorSprayValve

d) TurbineBypassValve(B&W510—Thisisasmallbypass)

e) DeaereatorSteamPeggingValve

f) SootBlowerValve

VI. HEATER DRAIN SYSTEM

a) HighPressureHeaterDrainValves,EmergencyHeaterDrain

b) LowPressureDrainValves

Table of Contents

5

VALVE PURPOSE No. 205 LPSuperheaterStop205C LPThrottleControl 207 SecondaryBypass 210 TurbineBypass 218 SSHOutlerSteamAttemporator 219 ReheatSHSteamAttemperator 220 HPHTRSteamLevel 221 FlashTanktoHTR 230 DEASteamPegging 231 FlashTankLevel 240 FlashTankPressure 241 FlashTankLevel 401 HPThrottleControl AheadoftheSSHTR

A. Fossil Once Through Units

Thesediagramsshowtheprocessesand

componentsoftypicalpowerplants,

inwhichCCIsevereservicevalves

arecommonlyemployed.Therestof

thepaperlogicallybreaksdownand

expandstheprocessesandapplications,

explaininginfurtherdetail.

6

ECONOMISER

PRIMARY

SUPERHEATER

MAI

N S

TREA

M

10 T

EHEA

T

FWN

STACK

GENERATOR

CONDENSER

HEATER

DRUM VALVEAIRHEATER

5788

542

578

501 VALVE

BOILER FEEDER

RECIRC VALVES

SECONDARY

SUPERHEATER

REHEAT

SUPERHEATER

HIGH PRESSURE

HEATERS

FEEDWATER

CONTROL VALVE

BN100

502 VALVE

PUMP ATTEMP

SPRAY

CONDENSATE

MOTOR DRIVEN PUMP

(STARTUP)

MAIN FEEDWATER

TURBINE DRIVEN PUMP

DEAERATOR LEVEL

CONTROL VALVE

EXTRATION STREAM

LOW PRESSURE

HEATERS

MAINSTREAM

ATTEMP SPRAY

FEEDER CONTROL

VALVE

STEAMDRUM

500VALVE

A FeedWaterRegulator

B FeedPumpRecirculation

C D.A.LevelControl

D BoosterPumpRecirculation

E MainSteamAttemperation

F ReheatSteamAttemperation

G HeaterDrains

H AuxiliarySteam

J Sampling—VariousLocations

II. Drum Boiler

SevereServiceApplicationsinaTypicalDrumBoiler

Drumboilersvaryinsize,fromsmallboilersusedtogenerate

steamindustrialheatinguptothelargepublicutilityboilers

whichproduceenoughsteamtogenerateupto900MWof

power.

n Combined-cycleplantsoftenrequireturbinebypassvalves

of50–100%capacity.

n Largeutilitydrumboilershavemoresevereservice

applicationswhichrequireDRAG®valvetechnology.

Theseboilersalsorequireturbinebypassvalves.

Shownhereisatypicalschematicanddiagramofareaswhich

requireaDRAG®valve.Theseapplicationsseeeitherthe

potentialforcavitation,flashing,oracombinationofhigh-

pressuredropandlowflowrate.

7III. Condensate System

Thisparticularapplicationintheplantiswherethecondensateistakenfrom

thecondenserhotwell,circulatedthroughthelowpressureheaters,andtothe

deaereator.

Thecondenseractsasaheatexchangerthatservesthepurposeofcreating

avacuumwhichincreasestheefficiencyoftheturbineandforrecoveryof

qualityfeedwater(condensate).

Shownbelowisaschematicofatypicalcondensatesystem:

a)CondensatePumpRecirculationValve:

Thecondensatepumpmusthaveaminimumamountofflowthroughitat

alltimestopreventitfromoverheatingandtoprotectitfromcavitation.

Therefore,arecirculationvalveandlinerunsfromthepumpoutletlineback

tothecondenser.Whentheboilerloadislowtheflowofcondensaterequired

islessthanthepumpminimumflowrequirement.Therecirculationvalve

isusedtoallowtheadditionalflowrequiredthroughthepump.Thepump

outletpressurevariesfrom300psito600psiwithfluidtemperaturefrom

100°to150°F.

Therecirculationlinedumpsintothecondenserwhichisatvacuum.Usageof

aconventionalvalveinthiscircumstancecanleadtohighlevelsofcavitation

(theoutletpressureatthevalveishigherthancondenservacuumbecauseof

pipefriction,elevationandspargerbackpressure).

Thisvalvemusthavepositiveshutoff.Toassurepropershutoff,thevalvemust

haveasoftseat.

200

0 25 50 75 100

400

600

800

TURBINE

CONDENSER

HOT WELL

CONDENSATE PUMPRECIRC VALVE

CONDENSATEPUMP

DEA LOAD LEVEL CONTROLVALVE OPERATING RANGE

LOW PRESSUREHEATER DEAERATOR LEVEL

CONTROL VALVE

DEAERATORREA PRESSURE

PUMP PRESSURE

100

90

80

70

60

50

40

30

20

10

10 20 30 40 50 60 70 80 90 1000

% STROKE

% C

v

% Cv V5 % STROKE

ExampleofCharacterizedTrim

8 Condensate System

b)DeaereatorLevelControlValve:

Thepurposeofthisvalveistomaintainalevelinthedeaereator,anopen

styleoffeedwaterheater.Itcontrolstheamountofcondensateflowintothe

deaereatingvessel.Theserviceconditionsofthisvalvevarydirectlywiththe

plantload.Duringstart-up,thepumpingloadissmall,thevalveinletpressure

ishighandtheoutletpressureislow,becausethedeaereatorpressureisnot

builtupyet.Inthiscase,thereisaneedforcavitationpreventionandtheflow

capacityrequiredisverylow.

Astheplantloadincreases,theneedforhighflowsandthecondensate

pumpcan’tmaintainthesamepressureheadatthesehigherflows.Theresult

islowerinletpressuretothevalve.Concurrently,thelinepressuretothe

deaereatorisbuilding,puttingbackpressureonthevalve.Thesehigherflows

withlowerpressuredropscreateaneedforhighercapacityofthevalvebut

lessresistanceinthetrim.

Therequirementsofthisvalveare:

n Highrangeability

n Cavitationprotectionatlowflows

n Lowresistanceatincreasingflows

n Tightshut-offisnotessentialbecausethisvalveisopenatalltimesthe

plantisupandrunning.

Thefeedwatersystemiswherefeedwateristakenfromthedeaereatorbythe

boilerfeedpumpsandsentthroughthehighpressureheaters,theeconomizer,

andfinallyintotheboiler.Thefluidisbroughttofulloutletpressureofthe

boileranditstemperatureraisedbyheatrecoveryforefficiencyofthesystem.

a) BoilerFeedpumpRecirculationValve:

b) BoilerFeedwaterRegulator:

Angle Globe

9IV. Feedwater System

DependingontheA/E,utilityandboilermanufacturer,thefeedwaterflow

willbecontrolledeitherbyavariablespeedfeedpumporahighcapacity

controlvalve.

Theboilerpumpsmaybemotordriven,whicharegenerallyconstant

speedandthereforeconstantoutletpressure,orsteamdrivenwithvariable

output.(Afluidcouplingonaconstantspeedmotordrivenpumpcanbe

utilizedtogetvariableoutput.)

Inanycase,acontrolvalveforfeedwaterregulationtotheboilertakesthe

fluidfromthepumpoutletandregulatestheoutletflowratetotheboiler

demand.

TheserviceofthisvalveissimilartotheDEAlevelcontrolvalve,exceptat

asignificantlyhigherpressure.

ThefluidistakenfromtheDEAintotheboilerfeedpumpandthepressure

israisedtoboileroperatingpressure(mostcasesareover3,000psi).

Thisistheinletpressuretothefeedwaterregulator.Atstart-upandlow

loads,thepumpingloadissmallandthepumpoutletpressureishigh

andthedrumpressureisnotbuiltupyet.Inthiscase,thereisaneedfor

cavitationpreventionandtheflowcapacityisverylow.Astheplantload

increases,drumpressureincreasesandflowrateincreases.Thepump

cannotmaintainthesamepressureheadatthesehigherflows.Theresultis

lowerinletpressuretothevalveandhighbackpressureonthevalve.These

higherflowswithlowerpressuredropscreateaneedforhighcapacityof

thevalvewithlessresistanceinthetrim.Manyplantsutilizeastart-up

valveandamainvalveforthisservice.Thestart-upvalvewouldhavetrim

tocopewiththelowflowandcavitationcondition,andthemainvalve

takeovertheflowincreasedanddifferentialpressuredecreased.CCIcan

provideacustomizedvalvefeaturingDRAG®technologythatcanbebuilt

withcharacterizedtrimtocoverthefullrangeofoperationconditionsin

onevalve.

Requirementsofthisvalve:

n Highrangeability

n Cavitationprotectionatlowflows

n Lowresistanceatmaximumflow

n ThisvalveshouldhaveatleastClassIVshutoffInacharacterizedstackalldiscsarenotthesame,butratherarechosentoprovideprecisevariableflowversuspressuredropoverthefullrangeofthevalve.

10 Typical Feedwater Systems

3500

0 10 20 30 40 50 60 70 80 90 100

3000

2000

1000

BOILER FEEDPUMP

DEAERATORBOILER FEEDPUMPRECIRC VALVE

HIGH PRESSHEATER

STARTUPFEEDWATERREGULATOR

MAIN FEEDWATERREGULATOR

DRUM

DRUM PRESSURE

PUMP PRESSURE

START UPFWR MAIN FWR

Typical2ValveFeedwaterSystem

TypicalFeedwaterSystemwithDRAG®

11V. Main Steam System

ApplicationsofValvesinaTypicalFossilFieldDrumBoilerPowerPlant

Themainsteamsystemcoverstheportionoftheplantthattakesthesteamfromtheboiler,sendsitthroughthesuperheaters,

andintothehigh-pressureturbine.Thesteamexitingthehigh-pressureturbineissentthroughareheater,thenfedintothe

low-pressureturbine.Finally,afterallpotentialenergyisextractedfromthesteam,itisdumpedintothecondensertostart

thewholeprocessoveragain.

Largegeneratingunitsweredesignedgenerallyforbase-loadedoperation.However,withincreasedemphasisonplannedcycling

operationoffossil-firedboilers,therearenewdemandsonthecontrolofboilersduringstart-upandlowloadoperation.

Conventionaldrumboilerscanbeoperatedwithwidevariationinload,includingcompleteshutdownandre-start,without

sacrificingheatrateorcycliclife.Modesofoperationincludevariabledrumpressure,constantthrottlepressureanddualpressure.

With“variabledrumpressure,”theturbinethrottlevalvesarenearlywideopenandthethrottlepressurevarieswithdrum

pressure.Thisoperationisrelativelyslowinresponsetoloaddemand.

With“constantdrumpressure,”theturbinethrottlevalvesarenearlywideopenandthethrottlepressurevarieswithdrum

pressure.Thisoperationisrelativelyslowinresponsetoloaddemand.

SevereServicevalves:

n BW100—FeedwaterRegulator

n BW501—SecondarySuperheater

Stop&BypassValve

n BW502—PrimarySuperheater

BypassValve

n BW518—MainSteam

Attemperator

n BW519—ReheatOutletSteam

Attemperator

n BW510—TurbineBypass

12 Typical Start-up After Overnight

Shut-down

With“constantdrumpressure,”theturbineloadischangedbymodulatingtheturbinethrottlevalves.Thelowloadefficiency

isachievedbysequencedturbinecontrolvalvesandpartialARCthrottlingattheexpenseofalargechangeinimpulse

chambertemperature.

“Dualpressure”operationinvolveswidevariablethrottlepressure,withthepressurecontrolledbywiderangesuperheater

divisionvalves.Thedrumpressureiscontrolledatahighpressureabove2000psi.Forthistypeofcontrol,thereislittle

changeinturbinemetaltemperatures,orindrumsaturationtemperatureovertheloadrange.Loadresponsewillbeatleast

comparabletothatfor“constantthrottlepressure”operation.

The“dualpressure”modeofoperationisasystemincorporatedinsomeB&Wdrumboilers.B&WincorporatedCCIDRAG®

valvesinfivelocationsofthissystem.

Thesuperheaterdivisionvalves(BW500andBW501)areusedbelowabout70%loadtomaintaindrumpressure,yetprovide

reducedthrottlepressuretotheturbine.TheBW502valvepermitsfiringproportionaltosteamflowduringanunloaded

transient,andlimitedover-firingatlowload.The502fromthedrumwiththesteamexitingthesuperheaterandreheaterto

holdtemperatureattheturbinewithouttheconcernforwaterintotheturbine,whichmightresultfromwaterattemperation.

TheBW100feedwaterregulatorisahighrangeabilityvalveforthisservice.

The500and501valveshavebeenclosedto“bottle”uptheboilerovernight.Theboilerpressurewillhavedecayed

somewhat,soinitialfiringwillbetobringdrumpressureandtemperatureup.The502valveisusedtobypassthesteamto

thecondenser.Whensteamtemperatureisestablished,the501valveisopenedtoadmitsteamthroughthesuperheaterand

initiallythroughthe510valvetothecondenser.Thisifforwarmingflow.The510valveisclosed,andturbinethrottlevalves

areopenedto70%.Theturbineisrolled(turbinethrottlepressurizedatabout200to300psi.The501valveopenstoincrease

theturbinethrottlepressurewhichisturbineload.Withthelowflowsinvolved,thesteamattemperationatsuperheateroutlet

(518valve)andreheateroutlet(519valve)controlstheturbinetemperaturewithoutthedangerofwaterintotheturbineat

lessthan15%load.Asloadgoesabove70%,theBW500valveisopenedto100%openandtheturbinethrottlevalvescontrol

loadoftheturbinefromtheretofullload.

The502valvestartswithlowtemperaturewateratdrumpressure(~2000psiand300°F).Theflowrapidlychangesin

temperatureasthelegofwaterisdisplacedby2000psisaturatedsteamwhichisapproximately650°F;thisisasignificant

thermaltransient.Thevalveshouldbeovertheplug,gasketseal,withlineardiskstackforthisservice.

13

The501valvesstartswithhighinletpressure(approximately2400psi)andverylowoutletpressure(0to100psi).Thevalve

mustcontrolflowtoloadtheturbineandthencontrolflowasturbineload(pressure)israised.Thisrequiresacharacterized

diskstacksimilartotheBW100feedwaterregulatorvalve.Thisallowsforthesystemtohaveaninherentlinearcharacteristics,

i.e.valvestrokelinearwithloadincrease.

The518and519valveflowconditionsareaboutthesame,i.e.~2000psisaturatedsteam~650°Finletand0to300psioutlet

pressure.Thetrimcanbelinear,andunderplugflow.

Thereareothersevereserviceapplicationswhicharecommontobothdrumandonce-throughunits.Theseareattemperator

sprayvalves,sootblowercontrolvalves(forplantswhichusesteamforsootblowing),andauxiliarysteamvalvesforsteam

frommainsteamtoboilerfeedpumpturbine.

Typical Start-up After Overnight

Shut-down (continued)

14 Sootblower Header Control Valve

Aregulating(modulating)valveisrequiredtocontrolthepressureinthe

sootblowerheader.Thevalvemusthavehighrangeabilityduetothehigh

levelofflowvariationduringthesootblowercycle.Asthesootblowers

openandclose,theheadercontrolvalvemustrespondquicklytoavoid

pressuresurgeswhichwouldpopthesafetiesontheline.

AClassVshutoffisrequiredbecauseanyleakagethroughtheheader

controlvalvewouldincreaseheaderpressureandpopthesafetieswhenthe

sootblowersareclosed.

Anotherconsiderationisthermaltransients.Avalveclosedforaperiod

oftimewillcoolsomewhat.Whenopened,thetrimheatsupmuchfaster

thanthebody.

Itiscriticaltocontrolthenozzlevelocitytomaximizesootblowingeffect

andsimultaneouslycontrollingtemperaturetominimizetheerosiveeffects

ofwetsteam.

ThisCCIvalveforsootblowerheadercontrolhasbeendesignedtomeet

theaboverequirements.Thediskstackischaracterizedwith14-turnand

eight-turnexpandingdiskswithPressureEqualizingRing(PER)grooves

forminimumfluidvelocitiesandhighrangeability.Theflowisoverthe

plugtoprotecttheseatfromtrashdamage.Theplugisunbalancedwith

highactuatorloadforgoodshutoff.

MAIN STEAM LINE

SOOTBLOWERHEADER

CONTROL VALVES PRESSURE CONTROLLERAIR SUPPLY

RELIEFVALVE VENT

PRESSURESENSINGELEMENT

SOOTBLOWERSHUTOFFVALVES

SOOTBLOWERHEADER

FILTER REGULATORAIR

MAIN STEAM LINE

SOOTBLOWERHEADER

CONTROL VALVES PRESSURE CONTROLLERAIR SUPPLY

RELIEFVALVE VENT

PRESSURESENSINGELEMENT

SOOTBLOWERSHUTOFFVALVES

SOOTBLOWERHEADER

FILTER REGULATORAIR

Multi-StageDisk(ShowingRightAngleFluidTrim)

%ValveStroke(Cv=18.6SootblowerFullValveCapacity)

15Sootblower Header Control Valve

SootblowerControlValveSectional

Plug/SeatInterface

StaggeringofAlternateDisks

FlowStreamlines

?Box

Bonnet

Body

DiskStackSeatRing

Stem/Plug

16 Attemperator Spray Control Valve

Attemperatorspraycontrolvalvescontroltheamountofwaterrequiredto

controlthesteamtemperatureexitingthesuperheaters.(Primary,secondary

andreheat).

Thewatersourcefortheattemperationaftertheprimaryorsecondary

superheaterisfromthemainfeedpumpgenerallyaftertheeconomizer

sectionoftheboiler.Thepressuredropacrossthevalveislowand

conventionalvalveshaveusuallybeenused.However,thereisaverywide

rangeabilityrequirement.Singledrop-typevalvesthrottlingattheseattendto

wireopenandleak.

Thewatersourceforthereheatattemperationmaybethesameasforthe

superheatoritmaybefromsomeintermediatestageofthepump.Ineither

case,thepressuredropacrossthevalveissignificantlyhighandvelocity

controltrimisrequired.

Thesamevalveforbothareas.Thediskstackischaracterizedwith14turnand

8turnexpandingdiskswith(PER)forminimumfluidvelocitywithreheat

sprayandwiderangeabilityinbothapplications.Theflowisovertheplugto

protecttheseatsurfacesfromtrashdamage.Theplugisunbalancedwithahigh

actuatorloadforshutoff.

Sprayvalvesinglobeandangleconfiguration

17Heater Drain System

GlobeDRAG®HeaterDrainValvewithFlowDistributerIntegralwiththeSeatRing

Therearetwosetsofheatersystemsinanormalpowerplant.Thelow-

pressureheatersheatthecondensatecomingfromthecondensatepumpso

itisnearsaturationwhenitgetstothedeaereator.Theotherset,calledhigh-

pressureheater,heatsthefeedwatercomingfromtheboilerfeedwaterpump

sothatitisnearsaturationwhenitenterstheboiler.Bothsystemsworkin

similarmanners,withtheexceptionoftheheatingfluid.Inthelow-pressure

heaters,exhauststeamfromtheLPturbineisused,whilethehigh-pressure

heatersuseextractionteamfromthereheatsection.Seetypicalschematic,

below.

Thelevelofcondensateintheheatersmustbecontrolledforthehighest

levelofsystemefficiency,sothedrainsystemisfairlyelaborate.Thereare

emergencyheaterdrainvalveswhichbypassthefluidtothecondenser.Each

heaterisatalowerpressurethantheprecedingheater.Thefluidinthefirst

heaterissaturatedwaterasthefluidflowsthroughthedrainvalvetothenext

heater,thefluidflashes,theflashedsteampassesoverthetubescontaining

thecondensate,andtheheatofthesteamisabsorbedbythetubeswarming

thecondensate.Atthesametimethesteamtempisreducedtosaturated

water.Thissaturatedwaterisletdowntothenextheaterandthesameprocess

occurs.

18

Theproblemisthatthecondensateinthebottomoftheheaterisatsaturation.Whenthecondensateisdrainedandloses

justasmallamountofpressure,itflashes,anderosiondamagetothecontrolvalveandassociatedpipingiscommon.The

importantthinginchoosingcontrolvalvesinthisapplicationistouseacharacterizeddiskstacktorangelowflowawayfrom

theseat,withsufficientturnstokeepvelocityaslowaspossible.Anglevalves,orglobevalveswithoversizedendsandaflow

distributorintegralwiththeseatringtolimitvelocitywillalsocombaterosion.Inaddition,thebodymaterialshouldbe

A217C5,orA182F5.

Heater Drain System (continued)

19B&W Once-through Start-up System Using Bailey 201 & 207 Valves

Allboilersrequireaminimumfluidflowthroughthefurnacewalltubesat

evenaminimumfiringratetoprotectthefurnacetubesfromoverheating.

Protectionisprovidedbycirculationofaminimumamountoffeedwaterand

theuseofastart-upbypasssystem.

20 Start-up Bypass System in Once-through Critical Pressure Units

Manyvariationsexistwhencomparingthestart-upsystemsprovidedbyeachoftheboilermanufacturers.Functionally,they

allhaveacommonpurposeaselaboratedbelow.NoticeinFigures3,4and5,thephysicaldifferencesillustratedforeachof

theonce-throughboilermanufacturers.CombustionEngineering,Inc.,(Figure5onPage44)providescirculationpumps

torecirculatefluidthroughthefurnacepumpsandconvectionwallsand,inthisway,protectthetubesfromoverheating.

(Figure7onPage44)Babcock&WilcoxCompanyandFosterWheelerCorporation(Figure3onPage42)requireaminimum

pumpingratebeestablishedtoprovidethissameprotection.

Becauseoftheseindividualdifferences,controlsystemsvaryoneachoftheseunitsinsofarastheactualcoordinationofthe

valvesineachstart-upsystem.However,againanalyzingthejobthathastobeperformed,thefunctionalobjectivesofall

systemsarethesame,namely:

1) Asnotedabove,provideprotectiontokeepfurnacetubesfromoverheatingbymaintainingaminimumflowoffluid

throughthefurnace.Caremustbetakentokeepthepressureofthefluidinthefurnacecircuitatapressurewellabove

saturation,thuspreventinganyflashingfromoccurringinthefurnacecircuit.

2) Allsystemsprovidesomemeansofcirculatingwaterthroughthesystemforbothacoldandhotwatercleanupthrough

theuseofapolishingsystem.

3) Allsystemsprovideforanorderlysequencetostart-upandinitiallyloadtheunitasfollows:

a) Byrejectingflowtotheflashtankorseparatorduringstart-up,provisionsaremadeforhotcleanupoperationand

initialbuildupofenthalpy.

b) Toassistinbuildinguptheheatintheboiler,inaminimumtime,boththewaterandsteamintheflashtankareput

backintoheatrecoveryinthedeaereatorand/orfeedwaterheatersduringstart-upandlow-loadoperation.

c) Whentheenthalpylevelintheflashtankorseparatorreachessomeminimumdesiredlevel,steamcanbeadmitted

tothesuperheaterandmainsteamlinesforwarmingpurposes.

d) Bybypassingsteamtothecondenserthroughaturbinebypassvalveand/ortheturbineaboveseatdrainswhile

regulatingtheheatinput,bettermatchingofsteamtemperaturetoturbinemetaltemperatureisachievedpriorto

rollingtheturbine.Thisimprovementisrelativetothatobtainedwithadrumtypeboilerwheretemperatureis

obtainedasafunctionofthefiringrequirementsforthepressurerequired.

Theturbinebypassvalves’functiontoprovideinitialsteamtoheatsteamlinesandrolltheturbine.Throughthese

valves,itispossibletoestablishsteamflowthroughthesuperheatertoturbinerollandsynchronizetheturbine

withminimumupsets.(B&W=210,FW=U,CE=SD)

e) Followingturbinesynchronization,turbineloadisincreasedusingflashtankorseparatorsteamavailable.Whenthe

availableflashtankorseparatorsteamisdepleted,additionalloadisobtainedbyopeningthein-linestopvalves,

thus,admittingfurnaceoutletfluiddirectlyintothesuperheater.InthecaseofaCombustionEngineeringorRiley

boiler,pumpingratemustbeincreasedatthesametime.Oncethestopvalvesarefullyopened,theflashtankor

separatoraretakenoutofservice.

21

Controlsystemsmustbeproperlyprogrammedtorecognizethefollowingfacetsduringthisperiod:

1) Furnacecircuitpressuremustalwaysbemaintained.

2) Throttlepressureisincreasingduringthisperiod;thus,theamountofstoredfluidandheatmustbeincreased.Loadand

steamtemperaturesarelikewiseincreasing,whichalsodemandsadditionalheatandfluidstorage.

3) Saturatedsteamfromtheflashtankorseparatortothesuperheaterisbeingreplacedwithsteamdirectlyfromthe

evaporatingsectionoftheboiler.Thismeanstheenthalpyleavingthefurnacesectionmustbemaintainedatanenthalpy

levelapproximatelythatofsteamleavingtheflashtank.Byproperlyprogrammingtheopeningoftheinlinestopvalves

(B&W=200,FW=Y,CE=BT)andchangesinpumpingandfiringrate,outletsteamtemperaturescanbemaintained

duringthistransfertostraightthroughoperation.

Thusduringstart-upandlowloadoperationpriortotheturbineloadexceedingtheminimumfeedwaterflow,thecontrol

systemmustutilizethebypasssystemvalvesasanextensionofpressurecontrolandfeedwaterflowcontrol.Duringthis

period,theheatinputmustbeproperlycontrolledtoprovidetherequiredsteamconditionsattheturbine,recognizing

thatsomeheatisbeinglostthroughthebypasssystemuntilitistakenoutofservice.

B&WUnitsforBaseLoadedOperation

B&Wonce-throughunitswereoriginallydesignedwithBaileyvalvesinthestart-upsystem.(Figure7onPage44)

BaileyvalveswasownedbyB&W.B&WacquiredCCIin1971toutilizetheadvantagesoftheDRAG®valveinthesevere

serviceapplicationsinthesteamgenerationunits.Atthesametime,themanufacturingoftheBaileyvalvewasmovedtoCCI

inCalifornia.

So,CCIsupportsthestart-upsystemvalvesofoldornewB&Wunits,whetherBaileyorCCIDRAG®valvesareused.

B&W Units for Base Loaded Operations

22

Note: Flow rate in lbs/hr is estimated by multiple of 7000 x MWE of the unit. Example: 500 MWE unit would have 3500000#/hr flow at 100% loadThe 202 valve generally was 2 or more valves is the total flow: if there are two valves then each valve is sized for the flow

ThePreviousdiagramsshowtypicalB&WunitswithBaileyvalvesinanolderplant,andCCIDRAG®valvesinaplantafter1974.

Thisdesignrequiresastart-upflowof25%to30%ofratedcapacity.

Forstart-up,theseunitscirculatethefluidthrutheboiler,202&207valves,theflashtankandtothecondenser,untilthefluid

temperatureisproperforsteamatabout600psiintheflashtank.Thensteamflowisallowedthroughthe205valvetothe

secondarysuperheaterand210valveforwarming.Then,whenthetemperatureisOK,the210isclosedandtheturbineisloaded

tominimumload.Thiscouldbe800psithrottlepressureandabouta25%load.Pressureupstreamoftheprimarysuperheater

wouldbeabout3500psiforsupercriticalunitsandabout2800psiforsub-criticalunits.Theturbinethrottlevalvesaresetat

thisminimumloadsetting.Thenthe201valveisopenedasthe202and207areclosedtopressurizethedownstreamsecondary

superheater.

Duringthestart-up,primarysuperheaterpressurecontrolismaintainedbythe202valve.Whenthepressuredropacrossthe200

(201)valveisabout300psi,the200valveisopened100%.Notethatwhensecondarysuperheaterpressureishigherthanthe

flashtank,the205valvecloses(itsacheckvalve).Loadisnowincreasedto100%byopeningtheturbinethrottlevalves.Atthis

time,thecontrolsaresettoopenthe202valveiftheprimarysuperheaterpressureexceedsacertainlimit.

Theloadcanbevariedbyturbinethrottlevalves.However,theloadmustbechangedveryslowlybecausewhenthethrottle

changestochangeload,thereisachangeinsteamtemperatureacrosstheturbine.Turbinesarenottobethermallycycled.

Thereforethistypeofunitisbaseloaded,thatis,operatedatconstantload.

B&Wintroducedcyclingoftheironcethroughunitsbyincorporatinga“401”valve.Thisvalvereplacesthe200&201valves.

B&W Once-Through Unit (Older Style Unit)

23History of B&W Configurations Schematics

OriginalSchematicoftheStartupofaBase-LoadedB&WOnce-ThroughUnitOlderunitwithBaileyE40&D10valvesinthestart-upsystem

SchematicofabaseloadedB&Wonce-throughunitusingDRAG®valvesinthestart-upsystem

After1974,CCIDRAG®valves

replacedtheBaileyValvesinthe

StartupSystem

24 Detailed Description of a Typical Start-up Sequence of a Base-Loaded B&W Unit

Coldwatercleanupmode,nofiringinthismode.

1. Approximately25%offullloadflowisestablished.

2. The200,201,and202valvesareclosed.

3. The207issettomaintain600PSIattheinlettotheprimarysuperheater.

4. The241(flashtanklevelcontrolvalve)isopen,dumpingallflowtothecondenserandtothecondensatepolishing

system.Theflashtankwillbefloodedduringthismodetoallowthe241topasstotalstart-upflow.

5. The242valveiskeptcloseduntiltheflashtanklevelstartstofallbelowtheflashtankcenterline,itwillthenopenuntil

theflashtanklevelhitsapredeterminedhighlevelsetpoint.

6. Circulationismaintainedinthismanneruntilthecationconductivityenteringtheeconomizerandatthe207valveinlet

isbelow1µΩ.

25Detailed Description of a Typical Start-up Sequence of a Base-Loaded B&W Unit (continued)

InitialFiringMode

1. Firingisinitiatedintheboiler.

2. Allflashtankdrainflowwillbetransferredfromthecondensertothedeaereator.The241valveisheldclosedand230

(deaereatorwaterpeggingcontrolvalve)isheldopenuntilthedeaereatorispeggedatitsfull-loadoperatingpressure(approx.

140PSI).Afterthedeaereatorispeggedthe230valvewilllimitflowtothedeaereatortomaintainitspressureatsetpoint.

3. The241willcontrolflash-tanklevelaboutitsnormallevelafterthedeaereatorispegged.

4. Whenthefluidtemperatureattheprimarysuperheaterinletexceeds300F,theprimarysuperheateroutletpressuresetpoint

willberampedautomaticallyfrom600to3650PSIattheprimarysuperheateroutlet.

5. Asatemperatureleavingtheprimarysuperheaterincreasesthe207operatestomaintainaprogrammedprimary

superheateroutlettemperature.

6. Atthetemperatureof300F,the207valveopenstoaminimumposition.Asthetemperatureleavingtheprimary

superheaterincreases,the207operatestomaintainaprogrammedprimarysuperheateroutlettemperature.

7. The220(H.P.heatersteamcontrolvalve)andthe240(flashtankoverpressurecontrolvalve)willopentolimittheflash

tankpressureatitssetpointof500PSI.

8. Duringthisperiod,thesecondarysuperheaterwillbeboilingouttoremoveallwater.

9. Theflash-tankpressureincreasesasfiringiscontinued.

26 Detailed Description of a Typical Start-up Sequence of a Base-Loaded B&W Unit (continued)

InitialTurbineRollMode

1. Ataflashtankpressureof300PSI,205(lowpressuresuperheaternonreturnvalve),willopen.

2. The210(turbinebypassvalve),isopenedtopassapproximately2%offullloadthroughthesuperheatertowarmthe

mainsteamlines.Theflashtankpressurewillcontinuetoincreaseasfiringiscontinuedtoitssetpointof500PSI.

3. Attheflashtankpressureof500psi,theturbinecanberolled,approximately2%offullloadflowisrequiredtorollthe

turbine.The210valveshouldbekeptopentopassanadditional2%flowtothecondenser.

4. Thefiringrateshouldbeadjusteduntilthegastemperatureisapprox.50ºFabovethedesiredtemperaturetotheturbine.

5. Aftertheturbinesteamrequirementshavebeenmet,the220(flashtanksteamtoH.P.heater)valvewillbeopenedto

limitflashtankpressureto500psi.

6. The241valveisstillmaintainingflashtanklevel,the230valveismaintainingdeareaterpressure

(approximately140PSI).

7. Whenthecapacityofthe220valveisexhausted,thesteamenteringtheturbineshouldbeincreasedto1000PSI.This

willincreasetheflashtanksetpointto1000PSI.

8. The220and240valvesareautomaticallysettoholdtheflashtankatitssetpointof1000PSI

9. Ataflashtankpressureof1000PSI,theturbinecanbesynchronizedandloaded.Theunitloadisrampedto

approximately7%load.

10. The210valveisclosedaftertheturbineissynchronized.

27Detailed Description of a Typical Start-up Sequence of a Base-Loaded B&W Unit (continued)

TransfertoOnce-ThroughOperation

1. Whentheloadontheturbinereachesapprox.7%,the201(pressurereducingvalve)willbegintoopen.Thiswillallow

steamtoflowdirectlytothesecondarysuperheater,ratherthantotheflashtank.

2. Pressurizationofthesecondarysuperheateroccursasthe201isopened.(Theturbinethrottlevalvesaresettomaintain

approximately7%to25%loadasthesecondarysuperheaterispressurized.Whenthesecondarysuperheaterpressure

exceedstheflashtankpressure,the205valvewillclose.

3. The201valvewillcontinuetobeopenedatapredeterminedratetoallowtheturbineloadtoincreasetoapprox.25%offullload.

4. Asthe201opens,the207willclosetocontroltheprimarysuperheateroutletpressureatitssetpoint.

5. Theflashtankdrainflowtothedeaereatorwilldecreaseasthe201valveisopened.Thedeaereatorpressurewilldecayas

theflashtankdrainsincrease.Whenthedeaereatorpressuredecreasesbelow25PSI,the231valvewillopentoholdthe

deaereatorpressureat25PSIwithflashtanksteam.

6. Theflowtothesecondarysuperheateristhroughthe201valveuntilitscapacityisexhausted,whichistypicallyaround25%

offullload.The200(high-pressurestopvalve)willthenbeopenedtoachievefullpressurizationofthesecondarysuperheater

7. Asthe200and201valvesareopened,the202valvewillclosetoholdprimarysuperheateroutletpressureat3650PSI.

8. Astheflowtotheflashtankdecreases,theheatersanddeaereatorwillbepressurizedbysteamfromturbineextractionpoints.

9. Astheloadontheturbinereaches25%the202and207valveswillcloseandtheiropeningsetpointwillbe4250PSI.

Thesevalveswillnowactasreliefvalvesduringaunittrip.

10. The260valve(flashtankwarminglinenon-returnvalve;notshown,bypassesthe231valve)willbeopenedtopassasmall

amountofsteamfromthedeaereatorbacktotheflashtank.Thisisrequiredinordertokeeptheflashtankwarmincase

the202or207valvesopenforoverpressurerelief.

11. Duringthistimethe241valveoperatestomaintainflashtanklevel.

28 B&W Units for Sliding Pressure Operation

Figure1-ConstantPressureSystem

Figure2-SlidingPressureSystem

Inaturbinegenerator,theelectricalpoweroutputisdependantonthepressure

enteringtheturbine.First,theboilerisfiredandbroughtuptoaconstant

dischargepressure.Theturbineisequippedwithseveralvalves,knownas

turbinethrottlevalves,whichregulatetheturbineinletpressure.Asload

decreases,thevalvesmaymoveclosedtoreduceturbineinletpressure.All

valvesmaymoveclosedequalamountsinunison(fullarcadmission)orthey

mayclosesequentially(partiallyarcadmission).Thisisknownaconstant

pressureoperation.

Constantpressurehastwoadverseeffectswhenlargeloadchangesoccur.First,

theturbinewillexperiencetemperaturefluctuations,whichwillcreatefatigue

andreduceitslife.Second,thenetthermalefficiencyorheatrateoftheturbine

dropsatlowerloads.

Slidingpressureoperationisdesignedtoeliminatetheseproblems.

Figure1showstheconstantpressuresystem.Withthissystemtheturbine

throttlevalvescontroltheinletpressuretotheturbineproportionaltoplant

load.

Figure2showsaslidingpressuresystem.Here,acontrolvalve(401)isinstalled

upstreamofthesecondarysuperheater.Althoughtheturbinethrottlevalvesare

stillinthesystem,theyareheldwideopenandplantload(turbinepressure)

isvariedbytheslidingpressurecontrolvalves.Thetemperaturechange

duetothrottlingatthe401valvesisadjustedatthesecondarysuperheater.

Temperatureattheturbineisconstantatallloads.

Afterstart-upandtransfertotheonce-throughoperation,theloadwillberaised

to100%.

29

Withconstantpressuresystems,thetransferisatapproximately25%load.Pressureattheturbinethrottlevalvesisbroughtupto

approximately3500psiandloadonturbineraisedviatheturbinethrottlevalves.

Withslidingpressurecontrolthereareoptionsof70%or100%slidingpressurecontrol.

With70%slidingpressurecontrol,alarger201valveisinstalled.Theturbinethrottlevalvesaresetat70%loadandthe

turbinethrottlepressureiscontrolledbythe201valveuptothatload.The200valveisopenedandthentheloadisraisedon

constantpressurecontrolbytheturbinethrottlevalvesto100%.

With100%slidingpressurecontrol,the201and200valvesarereplacedbyvalveswhichcombinethefunctionsofthe201and

200valves.Thesevalvesarecalled401valves.Systemswiththisdesignoperatebysettingtheturbinethrottlevalveswideopen

andcontrollingtheturbinethrottlepressurewiththese401valvesthroughouttheloadrange.

B&W Units for Sliding Pressure Operation (continued)

30 Sliding Pressure Control

Theslidingpressureunithasafewmodificationsdesignedtosatisfythreebasicrequirementsofoperation.Theserequirementsare:

1. Capabilitytobereliablystartedupandshutdowntomakethemavailablefortwo-shiftoperations.

2. Extendedunitloadturndownwhileoperatingintheonce-throughmode.Tomaximizethecapabilitytoreductunitloadduring

off-peakdemandperiodswithoutplacingtheboilerbypasssysteminservice;thusmaintainingreasonableheatratesatreduced

loads.

3. Capabilityofvariablepressureoperationstheonce-throughmodetooptimizeoperationoftheunitforloadcycling.Toextend

therangeofoperationoftheunitintheoncethroughmodeitisnecessarytoreducetheboilerminimumfeedwaterflow

requirementsforfurnaceprotection.

Modifications

1. Inconstantpressureunits,thefirstpassofthefurnaceconsistsoffourparallelrisercircuits(sidewallsandfrontandrearwalls).

Inslidingpressureunitsthisconsistsoftwopassesinseries;pass1isthroughsidewallrisersandpass2isthroughfrontand

rearwallrisers.Inadditionabypassvalvearoundpass1isinstalled(263valve),tolimittheflowthroughthesidewallsto

125%oforiginaldesign.Thisbypassistolimittheadditionalpressuredropcreatedbythedualpassarrangementofthelower

furnace.Asecondbypassvalve(264valve)aroundpass2isaddedtofurtherreducethispressuredrop;thisallowsaminimum

feedwaterflowrateofapproximately10%offullloadflow.

2. Forcapabilityoftheboiler/turbinetemperaturematchingduringstart-upandforaccuratemainstreamandhotreheat

controlwhileoperatingonthebypasssystem,steamattemperatorsareaddedforthesecondarysuperheaterandreheater.The

secondsuperheatersteamattemperatorrequirestheadditionofvalves218and205C.Thelatterisusedtomaintainenough

differentialpressurebetweentheflashtankandthethrottletosupporttheattemperationfunction.Theflashtanksteamis

usedforsuperheatsteamattemperation,since

thisfunctionisrequiredonlywhileonthe

bypasssystem.Thereheatsteamattemperation

requiresa219valvewhichtakessteamfrom

downstreamofthe401valve,sincethisreheat

steamattemperationisneededwhileon

thebypasssystemandalsoduringonce-

throughoperationatlowloadgenerator.

3. The202valveiseliminatedfromthe

cycleandthe207valveisusedforall

functionsformerlydonebythe202

and207valves.

4.Thekeyvalve(s)forslidingpressure

isthe401valve.Thisvalveisthe

turbinepressurecontrolvalve.

Thevalvemustcontrolload

fromaslowas10%at2500PSI

pressuredropto100%atapprox.

50PSIpressuredropwitha

linearinstalledstrokevs.load

characteristic.

31

PRIMARY SUPERHEATER

207VALVE

(CLOSED)

205 VALVE(CLOSED)

SECONDARY SUPERHEATER

TURBINE

CONDENSERFLASHTANK

1P

2P

100

90

80

70

60

50

40

30

20

10

10 20 30 40 50 60 70 80 90 1000

% STROKE

% C

v

% Cv V5 % STROKE

4000

010 20 30 40 50 60 70 80 90 100

3500

2000

3000

1000

PESSUREPSI

% LOAD

2P

1P

EXISTING

UPDATE

401 Valve

DiskStackCharacterization

PRIMARY SUPERHEATER

207VALVE

(CLOSED)

205 VALVE(CLOSED)

SECONDARY SUPERHEATER

TURBINE

CONDENSERFLASHTANK

1P

2P

100

90

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30

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10

10 20 30 40 50 60 70 80 90 1000

% STROKE

% C

v

% Cv V5 % STROKE

4000

010 20 30 40 50 60 70 80 90 100

3500

2000

3000

1000

PESSUREPSI

% LOAD

2P

1P

EXISTING

UPDATE

PRIMARY SUPERHEATER

207VALVE

(CLOSED)

205 VALVE(CLOSED)

SECONDARY SUPERHEATER

TURBINE

CONDENSERFLASHTANK

1P

2P

100

90

80

70

60

50

40

30

20

10

10 20 30 40 50 60 70 80 90 1000

% STROKE

% C

v

% Cv V5 % STROKE

4000

010 20 30 40 50 60 70 80 90 100

3500

2000

3000

1000

PESSUREPSI

% LOAD

2P

1P

EXISTING

UPDATE

n Byreplacingthe201and200valveswithequivilantcapacity401valves,thestartupsequencewillbesimplifiedandtheunitcanbeoperatedonslidingpressurefromtheexistingminimumload(25%)upto100%load

n Sothestartupsequencechangesasfollows.

n Theflashtanksteamisadmittedthroughthe205valvetothesecondarysuperheater.Thesecondarysuperheaterbringsthesteamtotheturbinetoapproximatley1000deg.F.Theturbineis‘loaded’toabout7%andslowlyincreasedtoabout25%loadontheflashtanksteam.Thepressureattheturbineatthisloadwillbeapproximatly900psi(flashtankpressureabout1000psi).Theflowisstillthroughthe205valve.

The401valveswillopenandthe207valvewillstartclosingallowingsteamfromtheprimarysuperheaterdogodirectlytothesecondarysuperheater.

The207and401arecoordinatedtotransfertheloadofsteamtotheSSHsuchthatthereisaminimaleffectoftheprimarysuperheaterpressure.

n Theturbinethrottlevalvesaresetatapproximatly80%open.

n WhenSSHpressexceedsFlashTankpress,205willclose

n The401valvescontinuetoopenuntilallthesteamflowatthetransferload(approx.25%)isthruthe401valves.Whathappensisthatthe401valvestakethepressuredroptothepressureattheturbineatanygivenload.Thetemperaturedropacrossthe401’swillbecorrectedbythesecondarysuperheatertomaintainaconstant1000degreesattheturbineatallloads(turbineinletpressure).Thisis‘SLIDINGPRESSUREWITHCONSTANTTEMPERATURE’.

n Asthe401sopen,207willclosetocontrolPSHoutletpressatitssetpoint

n At25%fullload,theSSHpressurewillbeapprox.1000psi.

n Loadisraisedbyopeningthe401valves.

n Athigherloads(>80%),throttlingthe401valvesforloadchangesissomewhat‘soft’.The401valvesthereforecouldgo100%openandtheturbinethrottlevalvescontrolloadfrom80%to100%load.

n Startupissignificantlysmootherbecausethetransferfrom‘Flashtankoperation’to‘once-thru’operationisdonebyopeningthe401valvesandclosingthe202/207valves.The201and200valvesareeliminated

n Theunitcanoperatewiththeturbinevalveswideopenandtheturbinepressure(load)iscontrolledbythe401valves.

n The401valvescanvarytheloadasrequiredfrom25%to100%loadwithconstanttemperatureattheturbine.

32 Once-Through Systems by Foster Wheeler

Once-throughsteamgeneratorsbyFosterWheelerincorporatestartupbypasssystemstomaintainaminimumcoolingflow

throughthefurnacecircuitswhenstartingup.Otherprovisionsarealsobuiltintothesystemtosatisfyturbinethrottlesteam

requirementsandtogivemaximumheatrecoveryduringstarting.Therearetwodesigns;oneusinganexternalflashtank,

andoneusingintegralseparators.

1. EXTERNALFLASHTANKSYSTEM

Abypasssystemutilizinganexternalflashtanksystemforrolling,synchronizingandinitialloadingoftheturbineisshown

first.Atsomeload,usuallythatwhichmatches1000psithrottlepressure,thesteamflowtotheturbineisswitchedfromthe

externalflashtankcircuittothemainlineflowpath.Inthissystem,shownintheschematic(Figure2onPage45),throttle

steamtotheturbineisinitiallyfurnishedthroughvalveNwithdivisionvalveVclosed.Whenthrottlepressureistobe

rampedfromthe1000psileveltofullpressureforloadingto25%offullload,divisionvalveVisslowlyopenedandvalveN

isclosed.ValvePclosestogeneratethepressureramp.

SystemDrawbacks

Whentheexternalflashtanksystemisusedwithacyclingunit,itisdifficulttoprovideoptimumsteamconditionstothe

turbine(temperatureandpressure)duringloadingandrampingtoobtainminimumstartingtimewithoutdegradationof

turbinecyclinglife.Thischaracteristicisespeciallytruewhenthepressurerampachievesfullpressureat25%load.

Ingeneral,theflashtanksystem,toachieveproperfluidenthalpyattheboilerdivisionvalve,startstherampatahigherthan

optimumloadand,asaresult,turbinecontrolvalvescloseslightlyduringrampingtoproperlyfollowtherampprogram

(pressureversusload).Inadditiontothiseffect,throttlesteamtemperaturetotheturbinemaydecrease(dip)orexhibitreversing

trendsduringrampingasaresultofchangingfromtheexternalflashtanklooptothemainflowpath.Thecumulativeeffect

oftheforegoingforahotstartistocauseadecreaseinturbinefirststageshelltemperature.Thefirststageoutletinnershell

temperatureismeasuredandusedasanindicationofadjacentshaftsurfacetemperature.Forrepeatedhotstarting,fatigue

damagecausingsurfacecracksontheshaftmustbeavoided.Tokeepcyclinglifeexpenditurefortheturbineatachosenlevel

whenstartinginthismanner,eitherthetimeforhotstartingmustbegreatlyincreasedorthenumberofhotstartsatminimum

timemustbelimited.Forcyclingservice,thisrestrictiononoperationisunacceptable.

33Application Schematic of Foster Wheeler in in Flash Tank Systems

Valves

‘W’ Pressurereduction.Thisvalveisusuallysizedforapproximately25%boilerflowat300psidifferentialpressurewithanequalpercentagecharacterizeddiskstack.

‘P’ Superheaterbypassvalve.Thisvalvedischargestotheflashtank.Duringstartupthevalveisusedinserieswith“W”valvetocontrolboilerpressureastemperatureisraised.Whentheunitisonlinethe“P”valveisclosedandfunctionsasapressurereliefvalvefortheboiler.Thevalvemusthavegoodshutoff.Thepressurizedseatvalve,likethe207valveintheB&Woncethroughsystem(Figure4A),shouldbeusedhere.ThediskstackcanbeMS200016turntypewithlinearcharacteristics.

RecirculationValvesforCondensate,BoostersandMainFeedwaterPumps

ThesevalvesarethesameasontheB&Wsystem.

‘D’ Flashtanklevel.ThisvalveisthesameastheB&W241valve.

‘E’ Flashtanklevel(alongwith“D”valve).ThisvalveisthesameastheB&W221valve.

‘C’ FlashtanktoH.P.heater.ThisvalveisthesameastheB&W220valve.

‘B’ Flashtanktodeaereator.ThisvalveisthesameastheB&W231valve.

‘A’ Flashtanktooverpressurecontrolvalve.ThisvalveisthesameastheB&W240valve.

ThisisFosterWheeler’sversionofastart-upsystem

utilizingaflashtank.Themaindifferenceisthatflowto

theflashtankinstart-upisthroughtwovalvesinseries;

thisisthe“W”valveandthe“P”valve.

34 Schematic of Foster Wheeler Cycling Super Critical Unit with Integral Separator

Applications for CCI valves in integral separator system

RecirculationValvesforCondensate,BoostersandMainFeedwaterPumps

ThesevalvesarethesameasontheB&Wsystem.

‘D’ Flashtanklevel.ThisvalveisthesameastheB&W241valve.

‘E’ Flashtanklevel(alongwith“D”valve).ThisvalveisthesameastheB&W221valve.

‘C’ FlashtanktoH.P.heater.ThisvalveisthesameastheB&W220valve.

‘B’ Flashtanktodeaereator.ThisvalveisthesameastheB&W231valve.

‘A’ Flashtanktooverpressurecontrolvalve.Thisvalveisthe

sameastheB&W240valve.

Valves

‘W’ Pressurereduction.Thisvalveisusuallysizedforapproximately25%boilerflowat300psidifferentialpressurewithanequalpercentagecharacterizeddiskstack.

‘P’ Superheaterbypassvalve.Thisvalvedischargestotheflashtank.Duringstartupthevalveisusedinserieswith“W”valvetocontrolboilerpressureastemperatureisraised.Whentheunitisonlinethe“P”valveisclosedandfunctionsasapressurereliefvalvefortheboiler.Thevalvemusthavegoodshutoff.Thepressurizedseatvalve,likethe207valveintheB&Woncethroughsystem(Figure4A),shouldbeusedhere.ThediskstackcanbeMS200016turntypewithlinearcharacteristics.

35Once-Through Systems by Foster Wheeler

2.FOSTERWHEELERINTEGRALSEPARATORSYSTEM

Toovercomethedisadvantagesoftheexternalflashtanksystem,theIntegralSeparatorStartUpSystemwasdeveloped,which

incorporatedmainlineseparatorsinthehigh-pressurecircuitry.Theschematiconpage37(seealsoFigure5onPage44)

showsthissystem.

Toavoidthick-walledpressurevesselsthatwouldlimitstartingandloadingrates,multipleseparatorsareemployedatthe

primarysuperheaterinlet.Forthecasewherethefurnacecircuitsoperateatfullpressure,apressurereducingstation(W

valves)isinstalledupstreamoftheseparatorstoprovidevariablepressureoperationofthesuperheaters.

Steamandwateratlowerpressuredownstreamofthepressure-reducingstationenterstheseparators.Separatedsteamflowsto

theprimaryandfinishingsuperheaterandthentotheturbine.Aspraystationbetweentheprimaryandfinishingsuperheater

controlsfinalsteamtemperatureduringstart-up.Drainflowleavingtheseparatorsiscollectedinadrainmanifoldandrouted

throughbreakdownvalvesPtoheat-recoverysubloopsand/orthecondenser.

Byaddingcontrolstoadjustthefiringratetoholdseparatorpressuretosetpoint,thisstart-upoperationbecomessimilarto

thatusedfordrum-typeboilers.Theminimumstart-upflowrequiredisrecycledthroughtheheatrecoverysubloopsbackto

thesteamgeneratorandcanbeconsideredsimilartotherecirculateddowncomerflowofthedrumunit.

Bymaintainingcirculatingflowalwaysinthemainlineflowpathforbothstartupandon-lineoperation,asimplicityof

operationresults.Thereisnomatchingofbypassflowenthalpytosaturatedsteamenthalpybeforestartingthepressureramp,

andasaresult,thesysteminherentlyavoidsathrottletemperaturedecreaseorerratictemperatureswingsduringpressure

ramping.Inaddition,thepressurerampcanmatchtheturbinecharacteristicwithouttheneedtoadjustturbinecontrolvalves

toamoreclosedposition.

ByvirtueoftheseinherentcharacteristicsoftheIntegralSeparatorSystem,theturbinefirst-stageshelltemperatureduring

rampingismaintainedatsteadyorincreasingvalues.Forhotstarting,thismodeofoperationpermitsmorerapidstarts

whilemaintainingfullturbinecycliclife.Forapplicationtocycling(twoshift)units,thesystempermitsrapidwarmandhot

startingforthe7500cumulativecyclesrequired.

ThesimplicityandrepeatabilityofoperationoftheIntegralSeparatorSystemmakesitamoreacceptablesystemforthe

operators.Repeatabilityofconditionsforstartingtherampiseasilyachieved.

TheIntegralSeparatorSystemachievesfullthrottlepressureat25%to70%loaddependingonthecapacityofthepressure

reducingvalves(Wvalves).

Fortheturbine,slidingpressureoperationtothe70%loadplateaugivesamoreuniformthermalgradientfortheturbine

whilechangingload.

Ata70%turbinevalveopening,turbineandsteamgeneratorhotstarttimesmatchatapproximately90to100minutesfrom

lightofftofullload.Thebasisforthecalculationsisthatturbinelifeexpenditureisnottoexceed.01%percycle,orpermissible

cyclesare10,000.Thecomparabletimefor25%turbinevalveopeningis140to150minutes.Furtherincreasesincapacity

beyond70%yieldonlymarginalreductionsinhotstarttimes.

36 Schematic of Combustion Engineering Super-Critical Unit, Base-Loaded Design

Thecombustionengineeringcombined-circulationsupercriticalboilerincorporatesastart-upsystemwhichhasmanyof

thesamefeaturesandbenefitsastheB&Wdesign.ThemainuniquefeaturewhichdifferentiatestheCEunitfromtheB&W

unitistheintegralrecirculationsystemintheboiler,whichseparateswaterwallprotectionfromflowrequirements.Integral

recirculationallowsforlowerminimumflowofapproximately10%offullboilerloadflowwhichnotonlyminimizesheat

rejectionduringstart-upbutallowsthetransferfromthebypasssystemtoonce-throughoperationtotakeplacewithouta

suddendropinsteamtemperature.FortheCEunit,thetransferfromrecirculationtoonce-throughoperationoccurswithout

operatorintervention.Increasingwaterwallpressuredrop,whichisduetoincreasedflow,causesareversalofpressureacross

thecheckvalveintherecirculationline.Oncethecheckvalveisclosed,theoperatorhastheoptionofleavingtherecirculation

pumpsinserviceorshuttingthemdown.Theconceptofseparatingwaterwallprotectionfromplantcyclerequirementsisalso

usedinCE’sslidingpressureunits.(Figure3onPage42,Figure6onPage44)

37

100

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0 10 20 30 40 50 60 70 80 90 100

3000

2000

1000

FEEDWATER REGULATOROPERATING RANGE

% LOAD

PESSUREPSI

WATER WALL

BT VALVE OPERATING RANGE

TURBINE THROTTLEBT

1

3 4

5

BT 2 3

4

WATER WALL

RECIRCSYSTEM

BE

BT VALVE

SA

PSH SSH

TURBIN

COND

13 PR

13

BT Valve

DiskStackCharacterization

38 Description of CE Once-through Unit and a Modification for Sliding Pressure to 70% Load

BE Boilerextractionvalve.Startupflowandwaterwallpressurecontrolvalve.

BTB Boilerthrottlebypass.Controlwaterwallpressureattransferfromstart-upsystemtooncethroughoperation.AlsopressurecontrolwhenopeningBTvalves.

BT Boilerthrottlevalves.

SA Steamadmission.Passessteamfromtheseparatortanktotheturbinewhileonthestart-upsystem.Itisacheckvalvewhenononcethroughoperation.

SP Spillovervalve.Controlsseparatorpressurewhenonthestart-upsystem.

WD Waterdrainvalve.Controlslevelintheseparator.

IS Superheaterspraycontrolvalve.

IR Reheaterspraycontrolvalve

SD Steamdrainvalve.Bypassturbinewhenwarminglinesanddepressatshutdown.

FWB Feedwatercontrolvalve.Controlsfeed-waterflowfrom5%to25%unitflow.

SUBJECT:Startup valve modification to allow frequent turbine load changes without affecting the turbine life.

BACKGROUND:The unit shown is a Combustion Engineering–designed once-through critical pressure 565 MW system. The start-up system incorporates Sulzer valves. The schematic is shown below:

TheCEunitincorporatesa“CombinedCirculation”system.

Inadrumunit,circulationofwaterwithintheunitprovidesacoolingflowin

thefurnacetubestopreventoverheating.Inaonce-throughsteamgenerator,

aturbinebypasssystemisusedduringstart-upandlowloadoperationto

handletheminimumflowrequiredforfurnacewallcooling.Thisflowis

usually30%ofmaxflow.

Forlargesupercriticalunits,30%turbinebypassisatechnicalandeconomic

handicap.Thereforethe“CombinedCirculation”designutilizesfurnacewall

recirculationratherthanaturbinebypasssystem;arecirculationlinetakes

thefluidfromthefurnacewalloutletanddischargesitintotheinletofthe

furnacewallsystem.Acirculatingpumpattheinletofthefurnacewallsystem

maintainstherequiredminimumfurnacewallvelocities,atlowerloads

automatically,byrecirculationsuperimposedontheonce-throughflow.

Theunitthrough-load(flowtotheturbine),asmaintainedbytheboiler

feedpump,increasesindirectproportiontounitload.Therecirculatedflow,

asmaintainedbythecirculatingpump,supplementsthethrough-flowover

thelowerloadrangeinamannerwhichprotectsthefurnacewallsbyraising

thetotalflowthroughthewallstoasafelevel,regardlessoffeedwaterflow.

Atapproximately10%load,thepressuredropthroughthefurnacewall

systemequalstheheadproducedbythecirculationpumpandthestop-check

intherecirculationlineautomaticallycloses.Therecirculationthenceases

toproducerecirculationoffurnacewallflowbutcontinuestocontributeits

positiveheadtothetotalunitthrough-flow,inthismanneractingasabooster

totheboilerfeedpump.

39The Start-up for the CE Unit, and the Start-up Valves Interface is Shown for a Base-Loaded Plant

Thestart-upshownisforabaseloadedunit.TheSulzervalvesandactuatorsaredesignedtobeoperatedwithintheconditions

shown.

Forinstance,thefirstBTvalveisshowntostarttoopenat18%load.Thepressuredropacrossthisvalveat18%loadis1700psi.

TheBTactuatorsaresizedforthatpressuredrop.Thevalveshouldnot(andprobablycannot)beopenedatlessthan18%load

becauseofthegreaterpressuredrop.Thevalve,pluspackingfriction,istheactuatorload.Thus,actuatorloadisproportionalto

systempressuredrop.

Forlowloadoperationatlessthan30%,theunitmayoperatewiththeturbinethrottlevalvesat30%andtheBTandBTBvalvesin

controlbetween10%and30%load.Theunitcouldalsooperateatlessthan10%bytransferfromonce-throughtotheBEvalve

andseparatorstart-upsystem.

40

Toreduceloaddowntothelowloadisareverseofthestart-up.Theturbine

throttlevalvesareusedtochangetheloaddownto30%,theBTvalvesare

broughtdowntocontrol,andthentheBTBvalvesareaswell.Theloadreduction

viatheturbinethrottlevalvesmustbedoneslowenoughsoastominimize

thermalstressesontheturbine.Thereisafluidtemperaturereductionwith

throttling.OnceontheBTvalves,thepressurethrottlingisacrosstheBTvalves

andthetemperaturetotheturbineremainsconstantbecauseofthesuperheaters

aftertheBTs.ChangingloadwhileontheBTvalvesis“sliding”pressure.This

meansthatvaryingtheloadinthe30%to100%loadrangeontheturbine

throttlevalvesisslow,butonceontheBTs,theloadcanvaryrelativelyfastasfar

astheturbineisconcerned.

TheSulzerBE,BTB,andBTvalveareextremelyheavy-dutyvalves.Theywere

designedforthestart-upandshutdownoftheunitasdiscussedsofar.However,

extendedtimeatlowloadusingthesevalvesandactuatorswasnotinthe

originalscope.

Sothequestionis,“Whatshouldbedonetoaddressextendedlowloadorsliding

pressureoperationoftheunit?”

Ifslidingpressure(extendedservicetime)isdone,theBTvalvetrimshouldbe

changedfrom‘linear’to‘equal-percentage’flowcharacteristic.Alsotheactuator

sizeshouldbeincreasedtoenablefullpressurerangeoperationontheBTvalves.

Theequal-percentagecharacteristicisrequiredsothereisasmoothchange

inflowaseachsuccessiveBTvalveisopenedorclosed.Especiallyimportant

iswhenthefirstBTisopened.AtthattimetheBTBiscontrollingwaterwall

pressure.TheBTBtrimhasabout10timeslessplugareathantheBTplugarea.

WhentheBTopens,theBTBvalvemustclosetomaintainwaterwallpressureas

theflowisincreasedthroughtheBTvalve.Withthesignificantdifferenceinplug

size,theequalpercentagetrimintheBTwouldallowsmoothincreaseofflow

whileminimizingwaterwallpressureswing.Theequal-percentagetrimrequires

alargerseatringboreintheBTvalveinordertomaintainthesamemaximum

capacity(Cv).TheBTvalves’combinedmaximumcapacitymustbeatleastthe

sameasbefore.Theresistancesbetweenthepumpandturbine(called“parasitic

power”)mustnotincrease.Theforgoingisaddressingextendedoperationonthe

BTandBTBvalvesdowntoabout10%loadononce-throughoperation.

Theunitcouldtransfertothestart-upsystemforlowloaddownto

approximately7%.However,transferfromonce-throughtothestart-upsystem

introducesproblemsoffeedwatercontrolandfeedwaterchemistry.Thefrequent

cyclingoftheunitforslidingpressureorforlowloadoperationisbestdoneon

once-throughoperation.

The Start-up for the CE Unit, and the Start-up Valves Interface is Shown for a Base-Loaded Plant (continued)

41

WithBTvalvesmodifiedwithequal-percentagetrim,increasedseatringsizeforcapacity,andlargeractuators,theunitcanbe

operatedon“slidingpressure”toahigherloadthan30%.Shownbelowisa70%systemshowingthestart-upandonce-through

rangesoftheSulzervalves:

ThissystemcouldbeconfiguredwithonlytheBTvalves.HowevertheBTBcapacityadditionat100%loadisofbenefitfor

minimizingparasiticpower.

The Start-up for the CE Unit, and the Start-up Valves Interface is Shown for a Base-Loaded Plant (continued)

42

Control Valve Function

Primary Superheaterbypass

Secondary Superheaterbypass

PressureReducing

H1 PressSuperheaterstopvalve

Turbinebypass

Manufacture

B & W Base Lord 202 207 201 200 210

Cycling N/A 207 401 N/A 210

Foster Wheeler

Base Lord N/A P W Y U

Cycling N/A P W (large) Y U

Comb. Eng.

Base Lord BE N/A BTB BT SD (U)

Cycling BE N/A BT by CCI N/A SD/U

Control Valve Function Superheater

stopvalve

High pressure heater steam

Deaerater Pegging

Flash tank over pressure

Flash tank level controlManufacture

B & W 205 220 231 240 241

Foster Wheeler N C B A/F D

Comb. Eng. SA ? dea peg SP WD

B&W, FW and CE, Nomenclature of Start-up Valves Listed According to Function

Control Valve Function

Primary Superheaterbypass

Secondary Superheaterbypass

PressureReducing

H1 PressSuperheaterstopvalve

Turbinebypass

Manufacture

B & W Base Lord 202 207 201 200 210

Cycling N/A 207 401 N/A 210

Foster Wheeler

Base Lord N/A P W Y U

Cycling N/A P W (large) Y U

Comb. Eng.

Base Lord BE N/A BTB BT SD (U)

Cycling BE N/A BT by CCI N/A SD/U

Control Valve Function Superheater

stopvalve

High pressure heater steam

Deaerater Pegging

Flash tank over pressure

Flash tank level controlManufacture

B & W 205 220 231 240 241

Foster Wheeler N C B A/F D

Comb. Eng. SA ? dea peg SP WD

Theoperationofthebypasssystemcanbebrokendownintotwobasicareasofcontrol:thelowloadandpressureportionof

thepumpingandfiringcontrols,andtheflashtanksub-loopcontrols.

Thepumpingandfiringratecontrolsincludethecontroloftheboilerfeedpump,thefiringrateandcontrolofcriticalcontrol

valves.

Thesecriticalvalvesforthethreemajoronce-throughboilermanufacturersareshownbelow:

Theflashtanksubloopcontrolsincludethefollowingvalvesinthethreeboilerdesigns.

43Figures 1 Through 3 Base-Loaded Configurations

Figure1

Babcox & Wilcox

OnceThruUnit

Figure2

Foster Wheeler

OnceThruUnit

Figure3

Combustion Engineering

OnceThruUnit

44 Figures 4 Base-Loaded Configuration

Figure4

Riley Stroker

OnceThruUnit

45Figures 5 Through 7 Sliding Pressure Configurations

Figure7

Combustion Engineering

Once-ThroughUnit

Figure5

Babcox & Wilcox

Once-ThroughUnit

Figure6

Foster Wheeler

Once-ThroughUnit

46

TypicalLayoutforHP

andLPBypassValves

Power Plants Which Utilize Turbine Bypass Systems

47CCI DRAG® Control Valves with Downstream Water Injection for Turbine Bypass Applications

48 Heat Recovery Steam Generator

49B&W Benson Boiler

50 Stein Boiler (Benson Boiler)

51

52

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