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DESCRIPTION
CCI valve Handbook
Citation preview
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
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
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
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
3500
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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Throughout the world, customers rely on CCI companies to solve their severe service control valve problems. CCI has provided custom solutions for these and other industry applications for more than 80 years.
DRAG® is a registered trademark of CCI.
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