7 Steam Turbines

7/29/2019 7 Steam Turbines

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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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STEAMTURBINE Types


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STEAMTURBINE

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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STEAMTURBINE

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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7 Steam Turbines

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