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PROGRAMA DE
INGENIERIA ELECTRICA
SISTEMAS DE GENERACIISTEMAS DE GENERACINSTEAM TURBINESSTEAM TURBINES
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STEAMTURBINEThesteam turbinegenerator is theprimarypowerconversion
component of the power plant. The function of the steam
turbinegeneratoristoconvertthethermalenergyofthesteam
from the steam generator to electrical energy. Two separate
components are provided: the steam turbine to convert the
thermal energy to rotating mechanical energy, and the
generator to convert the mechanical energy to electrical
energy. Typically, the turbine is directly coupled to the
generator.
Theprincipalmediumtolargesteamturbine manufacturers
in theUnited States areGeneral Electric and Westinghouse.
The major European manufacturers who supply turbine
generators to the United States are ASEA Brown Boveri,
Siemens, MAN, and GEC Alsthom. The major Japanese
manufacturers are Hitachi, Mitsubishi, Toshiba, and Fuji
Electric.
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OPERATINGPRINCIPLESTheoperationofthesteamturbinegeneratorinvolvesthe
expansion of steam through numerous stages in theturbine, causing the turbine rotor to turn the generator
rotor.
The thermal energy of the steam is converted to
mechanical energy by expanding the steam through the
turbine.Theexpansionof thesteamoccurs in two types
ofstages:impulse andreaction.Theimpulsestagecanbe
compared toawaterwheelonwhicha streamofwaterstrikes the paddles, causing the wheel to turn. The
reactionstagecanbecomparedtoarotatingsprinkler in
thatthejetofwaterfromthesprinklercausesthearmsto
rotate.
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Impulseturbine.Thisstagedesign isoftencomparedwithawater wheel because nozzles direct the steam that flows
through highvelocityjets. These steamjets, which containkinetic energy, flow against themoving turbine blades orbuckets.Thisenergy isconverted intomechanicalenergyby
rotatingtheshaft.Inapureimpulseturbine,whenthesteam
passes through the stationary blades, it incurs a pressuredrop. There is no pressure drop in the steam as it passes
throughtherotatingblades.Therefore,inanimpulseturbine,
allthechangeofpressureenergyintokineticenergyoccursin
thestationaryblades,whilethechangeofkineticenergyinto
mechanical energy takes place in themoving blades of the
turbine.
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Impulse Stages. An impulse stage
consists of a stationary nozzle with
rotatingbucketsorbladesFig.The steam expands through the
nozzle, increasing in velocity as a
result of the decrease in pressure.
The steam then strikes the rotatingbuckets and performs work on the
rotating buckets, which in turn
decreases the steam velocity. The
impulse stages can be grouped
togetherinvelocitycompoundstages
orpressurecompoundstages.
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Reaction turbine. This design uses the reaction forceresulting from the steam accelerating through the nozzles.
Thenozzlesareactually createdby theblades,as shown inFig.Each stageof the turbineconsistsofa stationary setof
bladesandarowofrotatingbladesonashaft.Sincethereisa
continuousdropofpressurethroughouteachstage,steamis
admittedaroundtheentirecircumferenceofthebladesand,therefore, the stationary blades extend around the entire
circumference. Steam passes through a set of stationary
blades thatdirect the steamagainst the rotatingblades.As
the steam passes through these rotating blades, there is a
pressure drop from the entrance side to the exit side that
increasesthevelocityofthesteamandproducesrotationby
thereactionofthesteamontheblades.
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The velocity compound stage involves a stationary nozzlefollowed by several rotating and stationary buckets. The
nozzlehasa largepressuredropwitha resulting increase invelocity.The first setof rotatingbucketspartiallydecreases
thevelocityasaresultoftheworkperformedonthebuckets.
The velocity compound stage can consist of the stationarynozzlesandmany rotatingandstationarybuckets;however,there usually are only two rotating bucket rows and one
stationarybucketrow.Thevelocitycompoundstage typicallyisusedasthefirststageofaturbinebecauseof itsabilityto
withstand highpressure reductions and the resultant
efficiency in quickly reducing pressure and minimizing the
requirements for highpressure casings. The velocity
compoundstageisalsocalledaCurtisstage.
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The Curtis stage (see Figure).After steam passes through the
nozzles,itpassesthroughthefirstsetofmovingblades. In the first
set of moving blades, work is
extractedfromthesteamcausing
thevelocitytodrop.Afterpassingthrough the moving blades, the
steam then passes through the
nonmoving blades. The only
purpose the nonmoving blades
serve is to redirect steam from
the first set of moving blades to
thesecondsetofmovingblades.
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TheCurtisstageOnan impulse turbine,nonmovingbladesdonothaveany
effecton thepressureor the velocity of the steam passingthroughthem.Afterleavingthenonmovingbladesthesteam
passesthroughanothersetofmovingblades.Thissetupofa
nozzle followed by a set of moving blades, nonmoving
blades, and moving blades makes up a single Curtis stage.After steam exits the nozzle there are no further pressure
drops.However,acrossbothsetsofmovingbladesthereisa
velocitydrop.ThiscausestheCurtisstagetobeclassifiedas
velocitycompoundedblading.
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TheRateaustage.The remaining stagesof theHP
turbine are a series of Rateaustages. It consists of a nozzle
diaphragmfollowedbyarowof
movingblades.Assteampasses
through the nozzle, velocity isincreased and pressure is
decreased. After leaving the
nozzle, steam then enters the
moving blades where onceagainworkisextractedfromthe
steam.
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TheRateaustage.As work is extracted from the steam, its velocity will once
againdecreaseeventhoughitspressurewillnotbeeffected.consideredpressurecompounded.
Even though there is a velocity increase and a velocity
decrease ineachRateaustage, theoverallvelocity from the
inlet of the first Rateau stage to the exhaust of the finalRateaustage isnotchanged. Incontrast,there isapressure
drop in each Rateau stage, resulting in an overall pressure
dropfromtheinletofthefirstRateaustagetotheexhaustof
thefinalRateaustage.ThisoverallpressuredropcausestheRateaustagingtobetobeconsideredpressurecompounded.
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Low pressure (LP) turbine:The LP turbine (see Figure6) is located next to the HP turbine. The LP turbine is a
pressure compounded, either single or dual axial flow,condensing reaction turbine.
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The major difference betweenthe HP turbine and the LPturbine is the type of blading
used. Because the steamentering the HP turbine is at ahigh pressure it is more efficientto use impulse blading. The
steam entering the LP turbineis at a significantly lowerpressure than the steamentering the HP turbine. In
order to efficiently extract workout of this lower pressuresteam, reaction blading is usedon the LP turbine
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Reaction blading works on the same concept as a jet engine.Similarly, each moving reaction blade, is designed to act as anozzle. As the steam passes through a reaction blade it
causes the reaction blade to be propelled forward, resulting inrotation of the LP turbine rotor. Both the moving blades andthe non-moving blades of a reaction turbine are designed toact like nozzles. As steam passes through the non-moving
blades, no work is extracted. Pressure will decrease andvelocity will increase as steam passes through these non-moving blades. In the moving blades work is extracted. Eventhough the moving blades are designed to act like nozzles,
velocity and pressure will decrease due to work beingextracted from the steam
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Impulse Versus Reaction Comparison. Three significant
differences related to the nature of the expansion process
are thenumberof stages, thebucketdesign,and the stage
sealingrequirements.
Peak efficiency is obtained in an impulse stage with more
workperstage than ina reactionstage,assuming thesame
bucketdiameter.Relative toan impulse turbine, this resultsina reaction turbine requiringeither40%morestages,40%
greaterstagediameters,orsomecombinationofthetwoto
obtain the same peak efficiency. This contrast is more
prevalent in the high and intermediatepressure turbines.The contrast is less in the lowpressure turbineswhere the
long bucket lengths significantly increase velocity of the
bucketfromtheroottothetipandrequirebothimpulseand
reactiondesignfeaturesintheblades.
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The pressure drop in an impulse turbine occurs across thestationarynozzle,whereasthereactionturbine haspressure
dropacrossthestationarynozzleandtherotatingbucket.
Pressuredrop across a rotating bucket causes thrust in the
rotor.Tominimizethethrustloadtotherotor,thehigh and
intermediatepressuresectionsofreactionturbineshavethe
rotatingbladesmounteddirectlyon the rotor, resulting inasmall overall diameter and the need for a large number of
stages.Impulseturbinesdonothavethisthrustconcern,and
the buckets are mounted on disk extension of the rotor
(wheels), resulting in largeroveralldiameters, smaller rotordiameters,andfewerstagesthanreactionturbines,asnoted
previously.
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Turbineshave internalsealingsystemsbetweentherotating
bucketsandthestationarycasingandbetweenthestationary
nozzlesandtherotor.
However, itshouldbenotedthattrue impulsestageshaving
0%reactionandreactionstagesthatalwayshaveatleast50%
reactiondonotexistinpracticalturbinedesign.
Theamountofreactioninabladevariestoaccommodatethenatural variationof reactionwith thebladeheight. Impulse
stages typically have 3% to 5% reaction at the base of a
rotatingbladeinordertoavoidzeroornegativereactionthat
results inefficiency lossandmay lead to flow separation intherotatingbladeorbucket.For longreactionstageblades,
thereactionpercentageatthemeandiametermaybeaslow
as40%.Thus, impulseand reaction turbines in the classical
definitiondonotexistinpracticalpowerplantapplications.
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SteamExpansionThisexpansionprocesscanbeexaminedbyplottingitonaMollier
chart{hsdiagram).Theexpansionofthesteaminasteamturbine
foratypicalthermalpowerplantisshowninFigAsshowninFig.8
4,thesteamenterstheturbineat2,414.7psia(16.65MPa),1,000
F(537.8 C),and1,460.15Btu/lb(3,396.31J/g).Thesteamexpands
throughthehighpressure turbineandexhauststhe turbinetothe
cold reheat lines at 550 psia (3.79 MPa), 640 F (337.8 C), and
1,318.54 BtuAb (3,066.92 J/g). The steam then flows to the
reheaterofthesteamgeneratorwhereitisreheatedandreturned
tothe inletofthe intermediatepressureturbineat500psia(3.45
MPa),1,000 F (537.8 C),and1,520.74BtuAb (3,537.24 J/g).The
steamthenexpandsthroughtheintermediatepressureturbineand
the lowpressure turbine, exhausting to the condenser at a
pressureof1.5in.HgA(5.08kPa)withanexpansionlineendpoint
enthalpyof1,010.00 (2,349.26J/g).
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SteamExpansionThe energy extracted from each pound of steam can be
determined for each turbine section by subtracting the
exhaustenthalpyconditions (hout.act) from the inletenthalpy
conditions (hin.act). Dividing this quantity by the amount of
energy thatwouldhavebeenextracted if theprocesswere
isentropicorideal(hin.act hout.ideal)providestheefficiencyofthe turbine section. This is expressed by the following
equation.
For the turbine section shown in Fig., the highpressuresection of the turbinehas an efficiencyof 78.65%, and the
combined intermediate and lowpressure turbine sections
haveanefficiencyof90.20%.
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SteamExpansionIdeally,theefficiencywouldbe:
Theturbineworkperunitmasspassingthroughtheturbineis
simplythedifferencebetweentheentranceenthalpyandthe
lowerexitenthalpy:wt =hin hout
Thepowerdeliveredbytheturbinetoanexternalload,suchasanelectricalgenerator,isgivenbythefollowing:
TurbinePower=mswt =ms(hin hout)[Btu/hr|kW]
wherems[lbm/hr|kg/s]isthemassflowofsteamthough
thepowerplant.
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TURBINETYPESSteam turbines are divided into many types with various
designations. The designations may indicate the various
combinationsofturbinetypesthatmakeupaturbineaswell
as the turbine size. Figure shows various representative
turbinetypes.The
commonlyusedturbinetypesaredescribedin
thefollowingsections.
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TURBINETYPESExhaustConditionsTwo designations exist based on the turbine exhaust
conditions: condensing and noncondensing. The condensing
turbineexhauststoacondenserwherethesteamiscondensed
at subatmospheric pressure (vacuum). The low pressure
turbinesofatypicalpowerplantcyclearecondensingturbines
inthattheyexhausttoasteamsurfacecondenserortoadirect
condensingaircooledcondenser.Thecondensingturbineshave
large exhaust areas since the steam is expanded to low
pressures,extractingasmuchoftheusefulenergyasreasonablypossiblepriortobeingexhausted.Thelowpressuresresult ina
large volume of steam, requiring a large exhaust area to
minimizeenergylossintheexhaustingprocess.
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TURBINETYPESTurbines are classified in two ways: (1) steam supply and
exhaustconditionsand(2)casingorshaftarrangement.
Steam supply and exhaust conditions. When classifyingsteamturbinesbytheirsteamsupplyandexhaustconditions,
they are categorized as condensing, noncondensing or
backpressure, reheatcondensing, and extraction andinduction.
1. Condensing turbine. This type of steam turbine exhauststeamatlessthanatmosphericpressuretoacondenser,they
can be has one or more extraction points. This extractionpointwithdrawssteamthat isusedtoheatfeedwater inthe
feedwaterheaters,oritcanbeusedforsomeplantprocess.
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TURBINETYPES2.Noncondensingorbackpressureturbine.Thistypeofturbine
is used primarily in process plants, where the exhaust steam
pressureiscontrolledbyaregulatingstationthatmaintainstheprocesssteamat the requiredpressure.These turbinesshown
canbedesigned for initial steam conditionsofup to1450psi
and930F,anditcanproduceoutputsbetween2and28MW.
3. Reheatcondensing
turbine. These turbines are used
primarily in electricityproducing power plants. In these units,
themainsteamexhaustsfromthehighpressuresectionofthe
turbineand isreturnedtotheboiler,where it isreheatedwith
theassociatedincreaseinsteamtemperature.Thesteamisnowat a lower pressure but often at the same superheat
temperatureastheinitialsteamconditions,anditisreturnedto
the intermediate and/or lowpressure sectionsof the turbine
forfurtherexpansion.
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TURBINETYPES4. Extraction and induction turbine. This type of turbine is
alsofoundprimarilyinprocessplants.Onextractionturbines,
steam is taken from the turbineatvariousextractionpoints
and is used as process steam. In induction turbines, low
pressuresteamisintroducedintotheunitatanintermediate
stagetoproduceadditionalpower.Thisextractionsteamcanbeusedforfeedwaterheatingorforsomeprocess.
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STEAMTURBINE Types
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Lasdosfuncionesdelosprensesysellosdelaturbinason:1. Prevenir o reducir las fugas de vapor entre las componentesrotatoriasyestacionariasdelaturbina,silapresindelvaporesmayor
quelaatmosfrica.
2. Prevenir o reducir la entrada de aire entre las componentesrotatoriasyestacionariasdelaturbina,silapresindelvaporesmenorque laatmosfrica.Lasltimasetapasde lasturbinasdebajapresin
normalmentetienenunapresindevaco.
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Una prdida de energa se asocia con las fugas devapor o entrada de aire. As, el diseo de los prenses y sellos seha optimizado para reducir las fugas. Las turbinas de vapormodernas utilizan sellos de vapor de laberinto para restringir lasfugas de vapor y de aire. Sin embargo, los prensaestopas enanillo de carbono se utiliza todava en algunas turbinas de msantiguas.
Las fugas de vapor ode aire ocurren
cuando el eje de laturbina se extiendems all de lasparedes de la misma
hacia la atmsfera.
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