234-1. the Steam Turbine p

8/3/2019 234-1. the Steam Turbine p

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APPROVAL ISSUE Course 234 - Turbine' and Auxiliaries - Module ODeModule 2341

THE STEAM TURBINENOTES & REFERENCES

OBJECTIVES:After completing this module you will b e . able to :1.1 a) State th e effect of turbine load on th e tu rb in e s te am p re ssur e ~ Pages 3-4

proffie.b) E xplain the effect of turbine load on the perform ance of: ~ Pages 4-8

i) Th e l as t s ta ge (s );ii) T he rem ain in g sta ges.

1.2 a) L ist fo ur m ajor adverse consequences of m oisture in turbine ~Page 9steam.b) E xplain tw o reasons w hy m oisture in turbine steam reduces tur- ~ Pages 9-10

bine effic iency .c) D escribe three types o f erosion, and for each give one exam ple ~ Pages 11-12

of a typical com ponent that m ay be dam aged by itd) E xplain how w et steam prom otes: ~ Pages 12-13

i) General co rros ion :ii) S tress c orro sio n crac kin g o r c orro sio n fa tig ue;iii) Erosion-corrosion.

e) E xplain how the presence o f m oisture in turbine steam increases < = > Page 13tu rb in e o v er s p eed po te nti al .

1.3 Desc rib e fiv e o pera tio na l c au ses o f ex cessiv e m oistu re c on ten t in < = > Page 14tu rb in e s te am .1.4 a) D es crib e tw o a dv erse c on se qu en ces /o pe ra tin g c on ce rn s c au se d ::> Page 15by excess ive axial thrust

b) D esc rib e th ree c au ses o f ex cessiv e ax ia l thru st. ::> Pages 15-16c) L ist tw o in dic atio ns o f ex cessiv e axial thrust ~ Pages 16-17

1.5 E xplain how the turbine pow er level affects the operation of the H P ~ Pages 17-20and L P turbine gland seals.

1.6 S tate the m ajor concern w ith all turbine seals. ::> Page 20

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Course 234 - Turbine and Auxiliaries - Module One APPROVAL ISSUE& R E F ER EN C ES

Pages 2~21 < = >Pages 21-23 < = >Pages 23-25 < = >

Other operational prob-lems, such as thermalstresses, axial differen-tial expansion and tur-bine vibration, are dis-cussed in other modules.

Page 2

1.7 D escribe the adverse consequences/operating concerns caused byea ch o f the follow in g u psets:a) Rubbing in a turbine seal (3);b) E xcessive gland sealing steam pressure (1);c) Insuffic ient g land sealing steam pressure (3).

* * *

INSTRUCTIONAL TEXT

INTRODUCTIONT he previous turbine courses gave you the basic inform ation about a typicalsteam tu rb in e u sed inCANDU stations. Y ou learned there about the m ajorturbine com ponents and the princip le of turbine stage operation . Y ou alsolearned how m oisture in steam and axial thrust problem s are dealt w ith dur-ing norm al operation . And, you also located the m ajor bearings and sealsan d learn ed how they o perate.T his m od ule c overs th e fo llowing topics with som e em phasis placed on ab-n orma l o pe ra tin g c on ditio ns a nd p ote ntia l o pe ra tio na l p ro blems:- T urbine operation at partial loads;- M oisture in turbine steam ;- E xcessive axial thrust;- T urbine gland seal operational problem s".W hat you will learn from this m odule will help you understand better thesignificance of som e of the operational lim itations im posed on the turbinea nd p ossib le c on se qu en ces o f o pera tin g u nd er a bn orm a l c on ditio ns.

TURBINE OPERATION AT PARTIAL LOADSThis sec tio n d escrib es the effec ts o f tu rbine loa d o n:- T he turbine steam pressure profile;- D ifferent perform ance of the last stage(s) as com pared w ith the rem ain-in g sta ges o f the tu rbin e.


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APPROVAL ISSUE Course 234 - Turbine and Auxiliaries - Module OneTurbine steam pressure profileA t partia l loads, steam flow to the turb ine is thro ttled by the governorv alv es" . A s th e ste am flow ra te is re du ce d, s o is th e tu rb in e in le t p re ss ure .Con sequen tl y, th e in te rs ta g e p re ss ur es a re d ec re as ed a s we ll . It turns outth at, e xc ep t fo r v ery sma ll l oa d s, a ll interstage pressures are p re tty we llproportional to turbine load, as shown in Fig . 1.1. For exam ple, at7 0% o f fu ll p ower, in te rsta ge p re ssu re s a re a bo ut 7 0% o f th eir v alu e a t fu llpower. ,The turbine outlet pressure (approximately equa l to condenserp ressu re) v aries m u ch less w ith tu rb in e lo ad a nd , fo r sim p lic ity . is as-sumed here to stay constant o ver th e w ho le lo ad ra ng e. H ow th e tu rb in ein le t p re ss ure c ha ng es w ith th e tu rb in e lo ad d ep en ds, stric tly sp ea kin g, o nh ow th e g ov ern or v alv es o pe ra te ( in u niso n o r se qu en tia lly ) to control tur-bine load . For sim plicity , Fig . 1 .1 covers only the m ore comm on m ode(u se d in n ea rly all CANDU sta tio ns) o f th e g ov ern or v alv es o pera tin g inu niso n. F or th is m o de o f v alv e o pera tio n, th e tu rb in e in let p ressu re is a p-p rox ima t ely p ropo rti ona l to tu rbine load .

) Effect of turbine load on .... pr.. au... atv.-lou.loc.tton.

Turbine In'-l pr'eAUnt

ob) llIrbln t.. m pnuure profile IIIvariou. IGIId.

Fromgovernorv.......

LocallonAg. 1_1. llIrbln ... eam prM8ure atwriou. lo.t .

NO T E S & R E F ER EN C ES~ Obj. 1)

... In some stations, theyare called control valvesor main control volves.

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COW'Se234 - Turbine and Auxiliaries Module One APPROVAL ISSUE& RE FE RENC e S

Obj. 1b) < = >

Page 4

The p ro po rtio na l rela tio nsh ip b etw ee n tu rb in e lo ad a nd in tersta ge p ressu resis w id ely u tilized in co ntro llin g th e statu s o f v ario us iso latin g. d rain o r v en tvalves. For proper operation , these valves are requ ired to open or close atcertain tu rb in e lo ad s. In stea d o f m easu rin g th e lo ad . tu rb in e steam p ressu re(u su ally. in som e ex tractio n steam lin es) is u sed to co ntro l th e v alv e o pera-tion.The other two reasons why you should know the effect of tu rbine load onthe steam pressure profile along the turb ine are: .1 . C han ges in the pressu re p rofile affect the p ressu re and tem peratu re o fthe available extractio n steam an d hen ce the feedheatin g system per-

formance . This is co vered in m ore d etail in m odu le 234-6.2. The perform ance of the turbine stages can be affected when their

in le t a nd ou tle t p re ss ure s c ha ng e. This is ex pla in ed in th e fo llowin gtext,

Turbine stage performance - introductionLet u s first recall the p rin cip le o f tu rb in e stag e o peratio n. B riefly, th e fix edb la des a cc elera te steam a nd d irec t steam jets to th e m ov in g b lad es w hich aredriv en by the steam . D urin g these p ro cesses. steam heat en erg y is con vert-ed ftrst into steam kinetic energy and then in to m echanical w ork by the ro-to r. U nfo rtun ately, this co nversio n is no t p erfect b ecau se so me heat tu rn sin to u nd esira ble fo rm s o f en erg y (m a in ly fric tio na l h ea t), g en era lly referredto a s lo sses.T he larg er the lo sses, the less efficien t th e stag e, an d thu s th e w ho le tu rb in e.T o com pen sate fo r d ecreased efficien cy , eith er m o re fu el m u st b e "b urn ed "in the reactor or the MW output m ust be reduced - both options being verycostly . particu larly in the long run. This is why we strive to m ain tain thetu rb in e effic ien cy a t a h ig h lev el.The effect of turbine load on the performance of variousstagesT here are m any facto rs. ego d ep osits o n the blad es o r m oisture in the steam ,tha t can red uce the efficiency of a tu rb in e stage. Am on g these fac to rs, tu r-b ine load is very im portan t because it can cause deterio ration of the flowp attern in so me turb ine stag es. .Y ou m ay wonder what this term now pattern m eans. In essence, it refersto the d irection and velocity of the steam flow w ith respect to the turb ineblades.For em cien t operation , the steam now pattern must match theb lade shape and velocity . If this condition is no t m et, efficiency


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APPROVAL ISSUE Course 234 - Turbine and Auxiliaries ~Module Onedecreases. For exam ple, this happens when the steam collides w ith theb la de s ra th er th an flow s smo oth ly o ver th eir su rfa ce.S ince the blade shape and velocity stay constant" , there is on ly one flowpattern that m atches them in the b est p ossib le w ay. W hen this hap pen s, thestag e o perates at th e m ax im um efficien cy . Any deviation from this flowpattern must cause the stage efficiency to decrease.The flow pattern in a turb ine stage depends on the steam velocity, andhence. its k in etic en erg y. R ecall that this energ y o rigin ates fro m heat T hele ss h ea t th at is a va ila ble per k ilo gram o f steam . th e slower th e steam flows.W hen the availab le heat is greatly reduced , the steam moves too slowlyto be able to drive the moving blades. W orse than that, the steam re-tards the blade m otion . C hurn ing of the steam by the b lades produces fric-tiona l heat, com monly referred to as windage losses. Note that theselo sses can be su bsta ntia l d ue to the high velocity" of the m oving blades.S in ce the steam retard s the b lad e m o tio n, the stage must be driven, eitherb y the o ther stag es o f the tu rb in e o r b y th e g en erato r (d urin g m o to rin g).H ow is all o f this asso ciated w ith tu rbin e lo ad ? It tu rns o ut that tu rb ine lo adaff~cts the am ou nt o f heat that is a vailab le p er k ilo gram o f th e steam . R eca llth at w ith d ecreasin g tu rb in e lo ad , the tu rb in e in let p ressu re d ro ps, w hile th eex hau st p ressu re stay s ap pro xim ately co nstan t T herefo re, the lower theturbine load, the less heat Is avaUable from each kilogram of steam .H ow ev er, the effect is n ot d istrib uted u nifo rm ly b etw een all th e stag es. In-stead . it occurs on ly in the last few stages. The reason for this is ex-p la in ed b elow .F irst o f all. the am o un t o f h eat av aila ble to a tu rb ine stag e depen ds on its in -let and outlet pressures. N ote that the am ount of this heat decreases w henthe in let pressure d ro ps an d/o r the o utlet pressu re rises. B ut w hat is the ef-fect o n this h eat w hen b oth these p ressu res in crease o r b oth d ecrease as tu r-b ine load changes? The answer to this question is g iven in Fig. 1.2 on then ex t p ag e w here a sim plified M o llier d ia gram is u sed to show the am ount ofh ea t a va ila ble (A hav ) to v ario us tu rb in e stag es o peratin g at d ifferen t steampressure.

NO T E S & R E F ER EN C ES

Tbe blade velocitychanges only during tur-bine runup and rundownwbicb are ignored here.

In the last stage, witb thlongest blades, this ve-locity can reach 500 mfsat the blade tip!

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Course 234 - Turbine and Auxiliaries - Module One APPROVAL ISSUE& REFERENCES

We neglect here other fac-to rs, eg o moisture insteam, that can be affectedby turbine load. However,they are of secondary im-portance in comparisonwith the pressure ratio.

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SPECfF lCENTHALPYkJJkg

S PE C IF IC E N TR O PY

Fig. 1.2. Relation between Inlet and outlet pressuresand heat available In various turbine stages:c a s e P lnl [M PaJ PO ll (UPal AP (UPa] Pinl IPgut A . h a Y [kJIkg]

20 . 50.1

20 . 50.1

ABC222

410.2

130130120

Note that in all three cases, Allav is nearly constant (about 120-130 kJlkg).the pressure ratio (Pinl/pouVis the same, and yet the pressure drop variessubstantially- from 2 MPa (incase A) to 0.1 MPa (in case C). The conclu-sion we can draw is that the available heat remains reasonably constant aslong as the pressure ratio does not change. Note that this statement in-volves the pressure ratio, and not the pressure drop. across the stage.ITyou now refer back to Fig. 1.1. you will recall that the interstage pres-sures are proportional to turbine load. Therefore, the pressure ratio in anyone but the last stage stays constant when turbine load varies. Only inthe last stage, the pressure ratio changes because the outlet pressure (ap-proximately equal to condenser pressure) stays nearly constant. whereas theinlet pressure drops with decreasing load.This has a profound effect on the performance of various stages. Since allbut the last stages operate at a constant pressure ratio. their availa-ble heat (per kilogram of steam) does not change. Hence. the flow patternin these stages does not deteriora~e. Needless to say. their efficiency re-mains approximately constant .


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Course 234 - Turbine and Auxiliaries Module One APPROVAL ISSUE& REFERENCES

Pages 27-28 ~

Page 8

It is w orth em phasizing that a p oor flow pattern in the last few stages createsm ore operational concerns than just reduced turbine effic iency. First.large w indage losses produce so m uch heat that a specia l cooling system isrequired to prevent overheating of the LP turbine exhaust. Second , theflow becom es so turbu lent that it can induce excessive b lade vibration .T hese im portant op erational problem s are discu ssed in detail in o ther m od-ules of this course; from the above descrip tion of the last stage operation ,you can understand w hy they can occur.No te that poor condenser vacuum aggravates these p roblem s be-cause it reduces further the already low pressure ratio in the last stagers),th ere by c on trib utin g to d ete rio ra tio n o f th e flow p atte rn .

SUMMARY OF THE KEY CONCEPTS E xcept for v ery light loads, tu rb ine interstage pressures chang e propor-

tio na lly w ith th e tu rb in e lo ad . T he same a pp lies to th e tu rb in e in le t p re s-sure in the turbines w here the g overnor valves o perate in u nison. T heexhau st p re ssu re is a ssum ed to sta y c on sta nt S tage perform ance depends on the steam flow pattern . The latter isg reatly a ffec ted b y the amo un t o f a va ila ble hea t The am ount of heat availab le to a turbine stage depends on the pressure

ra tio a cro ss th e stage. W hen turbine load is reduced, the p ressure ratio in anyone bu t the laststage stay s reasonably co nstan t T hus, the flow pattern rem ains un -

ch an ged , a nd the sta ges o pera te as effic ien tly as a t fu ll p ow er. In the last sta ge, the p ressu re ra tio d ec reases w ith red uc ed tu rb in e lo ad .

As le ss h ea t is a va ila ble , th e s te am v elo city d ec re as es . C o nse qu en tly ,th e sta ge o utp ut d ro ps .

A t a certa in load, the last stage starts d raw ing po wer from the other stag-es . The flo w p attern in th is sta ge is so d eterio ra ted that th e stage isch ok ed w ith the flo w. T herefo re, the in let p ressu re c ea ses to d ec reasep ro po rtio na lly w ith th e lo ad . This redu ces the pressu re ratio in the sec-o nd last stag e w ho se p erfo rm a nce b eg in s to deteriorate.

T hese effects p ro pagate gradually u pstream thro ugh the turbin e as itsload is fu rthe r r educed.You can work now on assignm ent questions 1-5.


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APPROVAL ISSUE Course 234 - Turbine and Auxiliaries - Module OneMOISTURE IN TURBINE STEAMThe fo llow ing to pic s a re d is cu ss ed i n this section:- T he adverse consequen ces of w et turbine steam ;- Abn orm a l o pera tin g c on ditio ns th at c au se ex cessiv e wetn ess o f tu rb in esteam.A DV ER SE CO NS EQ UE NC ESAs you have a lre ady le arn ed in th e p revio us tu rb in e cou rse s, th e p re senc e o fmo is tu re in tu rb in e ste am has two s eri ou s adv ers e con sequen ce s:1. R educed turbine emclency whic h in crea se s th e fu el c osts a s mo refu el m u st b e "b urn ed " to maintain th e output:2. Erosion of turbine components which inc reases the ma in tenance

c osts a nd , in th e worst c ase, c an c au se fa ilu re.However, mois ture in tu rb in e ste am a ls o c re ate s o th er p robl ems such a s:3. Accelerated corrosion of turbine components;4. Increased turbine overspeed potentfru.Each o f th es e con sequen ce s is discussed below.Loss of turbine efficiencyT here are tw o major re aso ns why mo istu re in tu rb in e ste am d ec rea ses tu r-bine efficiency:1. Water droplets disturb the steam Dow, resulting in increasedfriction losses and hence, reduced efficiency.

Wa ter d ro ple ts d istu rb th e ste am flow b ec au se th ey move slowe r a nd ind if fe ren t d ir ec tions than the s team .T he rea so n why w ater a nd steam mov e indifferen t d i rec tions is tha twhen ev er th e d irec tio n o f flow is c ha ng ed , water droplets in thes te am a re centrifuged (F ig . 1 .4 o n th e n ex t p ag e) b ec au se th eir d en si-tv is much h is he r than th at o f th e s te am .

NO T E S & REFERENCES

< = > Obi. 1.2 a)

~ Obi. 1.2 b)

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Course 234 - Turbine and Auxiliaries - Module One APPROVAL ISSUE& REFERENCES

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a) Flow through the fixed blades b) Flow downstream of the fIxed blades

CAS ING

. . . . . . . . . . . . . . ~ . I ~ _ , . . . . . .... .. ~ ..... .. ~-- - - . _ - . .... _ _ -~ - ~- .... . . .... _ . _ - - - - - - - -~- ~.---- _ . - . .F IX E D B LA D E AX ED BLADE S MO VIN G BLADE S

Fig. 1.4. Steam and water trajectories in a turbine stage:...... - - .. - Steam Water

T he reason w hy w ater m oves slow er than steam through th e turbine isthat w ater, as opposed to steam , does not fill the chan nels b etw een theblades. T hus, w ater is not accelerated directly by th e tu rb in e nozz le s.Rather, water droplets are driven by the steam much the sam e wayas s now fla ke s are blow n by a strong wind.The nature of this process is such that the d riv in g fo rc e, th at th e steamexerts on a w ater droplet, w ould decrease to zero if the steam and w aterv elo citie s e qu aliz ed . Th ere fo re , when s te am a cc el era te s in a nozzle, w a-ter d rop lets alw ay s fa ll b ehin d. T he v elo city differen ce in crea ses w iththe size o f the d ro plet beca use it is more difficult for the steam to m oveit T hu s, the larg er the w ater dro plet is, the slo wer it m ov es.

2. Water droplets collide with moving blades, thereby retardingtheir motion.T his is a direct consequence of the fact that w ater droplets move slowerthan th e s te am as descri bed above. As a resu lt, som e d rop lets that en term oving blades m ove slow er than the blades them selves. W hen them ov in g b lad es hit the d rop lets fro m the b ack . these co llisio ns p ro du cefo rc es whic h reta rd th e b la de mo tio n. As the d riv in g to rq ue d ev elo pedby the steam is reduced due to this retarding effect, the turbineMW output is decreased a s w ell.

T hese tw o effects com bined reduce turbine stage effic iency by about I% pereach 1% o f the a vera ge m oistu re c on tent o f steam .


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APPRO.VAL ISSUE Course 234 - Turbine and Auxiliaries - Module ODeErosionWet steam ca uses three d ifferen t ty pes o f ero sion :1. Im pact erosion - caused by collisions of droplets with the sur-

face of a component In steam turbines, the leading edge of mov-ing blades close to their tip is the m ost typical site of this type o f ero -sio n. R eca ll tha t this reg ion o f the b lad es ex perien ces in tensiv eb om bardm ent b y w ater d ro plets as exp la ined ab ov e.

2. Wire drawing erosion - caused by the cutdng acdon of waterwhich is forced to leak through a joint due to a large pressuredifference across the jo int. Note that this erosion is a self-accelerating process - once a sm all groove has been cut, the leakage ratein creases. cu ttin g the jo in t ev en faster. T his type o f erosio n is qu itecomm on in steam piping w here it can quicldy dam age a leaking flangejoint.In steam turbines, this erosion can happen, for instance, in the hori-zontal jo in t between two halves of a diaphragm (Fig . 1 .5). D ur-in g. n orm al o peratio n, steam leak ag e throu gh suc h a j oin t is very u nlik e-ly . H ow ever, som e im proper operating practices can establish a leakpath through the jo int. For exam ple, this can happen during turbinestartup or pow er m anoeuvres if th e diaphragm is subjected to abnormal-ly la rg e therm al d eform atio ns d ue to an excessive rate o f heatin g o r co ol-ing . Because steam flows m ainly through the blades, the outer part ofthe d iap hragm fo llo ws steam tem peratu re c han ges faster tha n the in nerpart does. T he resultant deform ation of the diaphragm is show n, gross-ly exaggera ted , in F ig. 1 .5 .

a) Normal shape b ) D e fo rm atio n d ue toexcessive heat ing 0) De format io n d ue toexcessi ve cool ing .

HalfJoint

Fig. 1.5. Thermal deformations of a diaphragm.

NO TE S & RE FE RENCE S:::> Obi. 1.2 c)

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Course 234 - Turbine and Auxiliaries - Module One APPROVAL ISSUE& RE FE RENCE S

Obi. 1.2 d) ~

Recall that stress corro-sion cracking refers to acombination of a highconstant tensile stresswith a corrosive envi-ronment. Likewise, cor-rosion fatigue is a c yc lic'stress combined with acorrosive environment.

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3. Washing erosion - caused by a cutting action of water movingvery quickly along a surface. This erosion is promoted when a wa-ter film is created on a surface and th e fllm is dragged quickly by thesteam. For example, this situation can occur in steam pipelines atthe HP turbine outlet where steam is very wet which promotes for-mation of a water film. particularly in pipeline elbows. Inside the tur-bine. this type of erosion can damage the concave surface of fixedblades onto which water is centrifuged (Fig. 1.4 a) and quicklydragged by the steam whose velocity is very high.

Excessive erosion of turbine components Increases maintenancecosts and promotes equipment fallure by weakening components andconcentrating the local stress in the eroded areas. The latter is caused bysurface imperfections produced by erosion. Advanced erosion of theturbine blades also reduces the turbine emciency by increasing theblade surface roughness and spoiling the blade shape.The rate of any erosion increases dramatically with increasing moisture con-tent of the steam. Therefore improper operating practices or malfunction ofthe equipment used for moisture removal from the turbine steam can resultin an unacceptable rate of erosion. The most likely causes of excessive wet-ness of steam are identified later on in this module.CorrosionWet steam also promotes various corrosion mechanisms. The presence ofwater (which always contains dissolved oxygen and other impurities. egochlorides) in a steam turbine sets up good conditions for general elec-trochemical corrosion because it provides an electrolyte. without whichelectrochemical corrosion is impossible.Much more dangerous, however, are other types of corrosion which at-tack locally and can form deep cracks. Among these. stress corrosioncracking and corrosion fatigue'" are known to have caused many fail-ures of turbine components such as blades, discs and rotors. Wet turbinesteam promotes both these types of corrosion because water can depositimpurities in highly stressed places such as blade roots. This is particu-larlybad during cold startups due to large condensation of steam. When theturbine becomes hot later, the condensate evaporates, leaving con-centrated impurities on the surface inhard-to-reach places where flow-ing steam cannot provide a washing effect. Unfortunately. these hard-to-reach places are often heavily stressed.A combination of corrosion and erosion known as erosion/corrosion cancause damage faster than either of them acting independently. This is due toerosion wearing away the protective layer (eg. magnetite on carbonsteel) and exposing more surface to the corrosive environment. The cor-rosion products that are formed are usually less erosion ..resistant


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APPROVAL ISSUE Course 134 - Turbine and Auxiliaries - Module ODethan the parent m etal, thereby prom oting further erosion. In o th er words .erosion helps corrosion in wearing away the metal and corrosion recipro-cates in a sim ilar way.An understanding of the various corrosion m echanism s enables you to real-ize how ~me incorrect operating practfces can promote corrosiondamage. Poor chem istry control of boiler water and steam . and prolongedoperation w ith excessively w et steam are some. to nam e a few.Increased overspeed potentialFinally, wet steam increases turbine overspeed potential when theturbine generator is suddenly disconnected from the grid. As you know. anautom atic response to this upset is to cut off steam flow to th e turbine in or-de r to rem ove the driving torque. This action causes pressure inside the tur-bine to drop quickly because the turbine is directly connected to the co n-denser. As a result, a pordon of the moisture inside the turbineflashes to steam which continues to drive the turbine, thereby in-creasing its overspeed. T he wetter th e turbine steam is. the m ore water canflash to steam an d hence the ov erspeed p oten tial is increased.

SUMMARY OF THE KEY CONCEPTS Moisture in the turbine s tearn reduces turbine efficiency , erodes an d cor-rod es turbine comp onen ts, and increases the overspeed potential. Turbine efficiency is reduced by moisture in th e steam due to i ts retard -ing effect on the m oving blades and the 'disturbance of the steam flow by

wa te r d roplets . Three types of e ro sio n - impact, washing and wire drawing - can dam-a ge v ario us tu rb in e c ompo nen ts. W ater in steam supports various corrosion m echanisms because it acts

as an elec tro lyte. dep osits im purities on the surfa ce o f heavily stressedcom ponents, and erodes away th e p ro tective l ayer .

Wet steam increases the turbine ov erspeed potential as w ater flashes tosteam upon a rapid pressure drop inside the turbine caused by cuttingoff the steam supply.

NOTES & REFERENCES

~ Db}. 1.2 e)

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Course 234 - Turbine and Auxiliaries Module One APPROVAL ISSUE& R E F ER EN C ESObj. 1.3 < = >

A total loss of reheat basbeen estimated to acceler-ate the erosion rate abouttenfold!

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Operational causes of excessive wetness of turbine steamD uring norm al operation the average m oisture content of steam in any tur-b in e sta ge is kept w ithin an acceptable range, ie. below about 10%. Someoperating conditions m ay, however, resu lt in excessive wetness ofsteam in some or all turbine stages. L isted below are a few ex am ples:1. Abnorm ally high moisture conten t of steam at the HP turbine

inlet.This can be caused by a few reasons. For one thing , the boiler m aysupply steam m uch wetter than expected. T he next m odule covers thisoperational prob lem in detail. P oor operation of the m ain steam pipe-lines betw een the boiler and the HP turbine can be another reason. Itcan be caused by m alfunctioning steam traps and /or drain v alv es, o r ex-cessive steam condensation in the p ipelines. M ore inform ation on thistopic is giv en in m odule 234-3 .

2. L oss of reheating.Partial or total valv ing out of the reheaters causes steam w etness in th eLP tu rb in e to in crease. Wh ile this a gg rav ates all the a dv erse c on se-quences of m oistu re in turb in e steam , erosio n is affected most", This isthe m ain reason w hy full lo ad o peration w ithout reheating can be tolerat-ed only over a very short period of t ime a nd sh ou ld be avo id ed a lto g eth -er to preserve the tu rbine life. M odu le 234-4 covers this topic in m oredetail.

3. M alfunctioning m oistu re separator(s) and/or reheater(s),S uch a m alfunction can be caused , for exam ple. b y m echanical dam ageto the chev ron plates in a m oisture sep arato r. reheater o r m oistu re separ-ator drains level con trol pro blem s or reheater tu be fouling . N o m atterwhat the actual cause of the m alfunction is, it results in an i nc re as edwetness of the steam in the who le L P tu rb in e.

4. Significan t reduction in extraenon steam Dow.A large loss of feedheaters (eg. one bank of L P feedheaters valved out)red uces the total flow of ex traction steam . S ince ex traction steam con-trib utes to m oistu re rem ov al from th e tu rb in e. this resu lts in somewhatin crea sed w etn ess o f tu rb in e steam.

5 . C logged turbine drains.Th is r es ul ts in in crea sed m o istu re c on ten t o f steam in the d ow nstreamstages. In addition to the ad verse consequen ces described earlier in thismod ule, c lo gg ed tu rb in e d ra in s c an resu lt in accum ulation of w ater at thecasing bo tto m. S in ce th e accu mulated w ater im pedes heat flo w fromsteam to the c asin g. th e la tter g ets som ew ha t c oo ler, a nd therefo re shrin ksaccordingly. The resultan t hum ping deform ation of the w ho le casing canb e la rg e e no ug h to c au se ru bb in g a nd in crea sed tu rb in e v ib ra tio n.


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APPROVAL ISSUE Course 234 - Turbine and Auxiliaries - Module OneSUMMARY OF THE KEY CONCEPTS

Excessive wetness of turbine steam can be caused by a loss or m alfunc-tio n o f a reheater, p oo r p erfo rm an ce o f m oisture sep arato r(s) o r p lu ggedtu rb in e d ra in s. A sig nific an t red uctio n in the extrac tio n steam flow o rabnonnally high w etness of the HP tu rb in e in let ste am c an be o ther re a-sons.

You can now work on assignm ent questions 6-10.

AXIAL THRUSTT he previous turbine courses described how axial thrust on the turbine gen-erator is created and w hat m ethods are used to carry or m inim ize it duringnormal operation. This course discusses an abnonnally high thrust: youw ill learn how it can dam age the turbine, w hat m ay cause it and how to rec-ognize th at i t is ta k ing p la ce.Operating concerns caused by excessive axial thrustY ou w ill recall that the axial thru st is carried b y a thrust b earing w hich flx esth e ro tor in the axial d irectionw ith respect to the casing and hence preventsru bb in g o f tu rb in e in tern als. An ex cessiv e thru st c au ses therefore tw o m a-jo r a dv ers e c on se qu en ce s/o pe ra tin g c on ce rn s:1 . It accelerates wear of the thrust bearing an d may lead to its d amage .

Ift he th ru st is s ev era l time s n orma l, th e bearing m ay get dam aged im -mediately.

2. Ifdam age to the thrust bearing is substantial, the rotor can m oveenough in the axial d irec tio n to c au se axial rubbing that can dam agetu rb in e in terna Is (sea ls, b la des, discs an d d ia phragms).

Operational causes of excessive axial thrustExc essiv e a xia l th ru st c an be c au sed b y:1. A xial rubbing.

This can be d ue to ex cessiv e d iffe ren ce b etw een th e a xia l th erm a l ex pa n-sions of the casing and rotor * , d am a ge d o r lo ose ne d tu rb in e g en era to rin tern als, o r in co rrect a ssem bly o f the m achin e. T he ax ial fo rc es ac tin gbetw een the ru bb in g p arts ex ert a n ex tra lo ad o n the thru st bearin g.

2. W ater induction to the turblne".Slugs of w ater can block blade passages, resulting in an abnorm al pres-sure profile in the turbine. T his can generate a load on the thrust bearingm any tim es in excess of its n orma l v alu e.

NO TE S & RE FE RENCE S

< = > Pages 28-29

< = > Obj. 1.4 a)

< = > Obj. 1.4 b)

This problem is discussedin module 23411.

More information on wa-ter induction is given inmodule 234-13.

Page 15


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Course 234 - Turbine and Auxiliaries - Module One APPROVAL ISSUEE S & R EF ER EN CE S

You may remember thatthis is done on purposeto determine the direc-tion and magnitude ofthe axial thrust producedduring normal operation.

Db}. 1.4 c) < = >

isplacements are in theof a few tenths ofllimeter, at the most.

Page 16

3 . Change in the steam flow distribution wough the turbine due to:a) Change in the ex traction steam Dow distribution .

This can be ca used b y rem ov in g m a ny feed hea ters fio m serv ic e.dam age to ex traction steam p iping. or m alfun ction o f a check valv ein the p ip in g. N ote tha t th e ex tra ctio n steam lin es are n ot p erfectlysymm etrical w ith respect to the tw o halves of a dou ble flo w tu rb ine.T his m ak es the steam flo w thro ug h the tu rb in e a symmetrica l b ec au sethe resistances of each half of the turbine to the steam flo w are n otthe sam e". C onsequen tly . a change in th e e xtra ctio n s te am d ema ndis no t d istributed evenly an d can actually enhance the asy mm etry o fth e m a in ste am flow a nd p res su re d is trib utio n in t he tu rb in e . As are su lt. a n e xtra th ru st c an be generated.

b) Excessive deposits on turbine b lades, their advanced ero -sion or som e other dam age.Even in d ouble flow tu rb ines. any o f th e above can produce an ex trat hr us t b e cau se th e b lading in the tw o halves o f the turbine m ay be af-f ec ted asymmet ri cal ly .

Indications of excessive axial thrustH ow can we know that the ax ial thrust is excessive? T wo m ost reliable in-d ic atio ns o f th is p ote ntia lly d an ge ro us s itu atio n a re :1. Abnorm al shaft axial posit ion.

As y ou k now. th e tu rb in e g en era to r ro to r is fixed in th e a xia l d irec tio n a tthe thrust b earin g. T his bearing . as everything else. is n ot infin itelyrig id . Therefore, it deflects under the thrust. The larger the thrust,th e l arg er th e d efle cti on . As a resu lt. th e sha ft a xial p ositio n c ha ng es a c-c ord in gly , a lb eit v ery little", H ence. m on ito ring of the shaft axia l posi-tion allow s us to check if the thru st a ctin g u po n the b ea rin g is excessive.

2 . Thrust bearing m etal tem perature abnorm ally high.Mo re fric tio na l h ea t is produced in the bearing w hen it is loaded w ith in-c reased a xia l thru st T herefo re. an ex cessiv e a xia l th ru st c an raise thrustb ea rin g me ta l tempe ra tu re a bn orma lly high. T hough the bearing oil o ut-let tem pe ra tu re c an a lso be a ffe cte d. o p era ti on al e xp eri en ce s hows thatth e bear ing me ta l t empe ra tu re is mo re s en sitiv e to th e ax ia l th ru s tH ow ever, a high bearing m etal tem peratu re indication m ay also becaused by a few other reasons, such as:a) Inadequate o il supp ly to th e bearing (eg . o il too hot or in insufficient

q ua ntity to p ro vid e a deq ua te c oo lin g a nd lu bric atio n);b ) A d va nc ed d ete rio ra tio n o f th e b ea rin g itse lf, re su ltin g in increased

f ri cti on and impai red l ub ri ca ti on ;


17/32

APPROVAL ISSUE Course 234 - Turbine and Auxiliaries - Module Onec ) F au lty tem p era tu re in strum en ta tio n.T herefore som e inv estigation (eg. extra checks in the lubricating oil sy s-tem ) m ay be necessary to pinpoint the actual cause of this ind ication . Hit is , h owever , concurrent with an abnormal shaft axial position,an excessive axial thrust is almost certainly the cause of trouble.

B oth these param eters are con tin uously m on ito red. T yp ically, the m onitor-in g is perform ed by m ore than one system , because these param eters are soessen tia l to tu rb in e safety . Ifa lim it has been reached or exceeded , the tur-bine m ay have to be unloaded (to low er the pressure d istribu tion in the tur-bine, thereby reducing the thrust) or tripped, or further loading during astartup m ay have to be postponed ; W hich action should be taken is clarifiedin th e a pp ro pria te o pe ra tin g m a nu al. Details in th is m a tte r will b e cov er ed inth e sta tio n sp ec ific p hase o f the a uth oriza tio n tra in in g.

SUMMARY OF THE KEY CONCEPTS E xcessive axial thrust results in accelerated w ear of the thrust bearing .It can also severely dam age the turb ine - first, b y w rec kin g its thru stbearing, and then by axial rubbing of turbine internals such as seals.

Possible causes of excessive ax ial thrust in clude axial rub bing (eg. d ueto ex cessiv e a xia l d ifferen tial ex pa nsio n), w ater in du ctio n to the tu rb in e,and a change in the steam flow distribution through the turbine (eg. dueto tak ing m any feedheaters ou t of service).

An e xc es siv e a xia l thrust is in dic ated b y a n a bn orm al sha ft a xia l p ositio na nd hig h thru st b ea rin g m eta l tem p era tu re.You can now work on assignment questions 11-14.

TURBINE GLAND SEAL OPERATIONIn the previou s tu rbine co urses, yo u learned abou t the fu nctio ns and prin ci-ples of operation of various turbine seals. The description given there waslimited to norm al turbine operation at full load. In this m odule, seal opera-tion at partial loads and m ajo r up sets in seal operation will be d iscu ssed .

Operation of an LP turbine gland seal at partial loadsLet us first recall the principle of operation of a typical gland seal of anLP turbine. The major task of this sea l is to p rev en t air ingress in to the L Pturbine which operates under high vacuum . This is achieved by supply ofsealing steam at low pressure (a few kPa(g to the seal at port A as shownin Fig. 1.6 on the next page.


< = > Pages 30-31

< = > Obj. 1.5

Page 17


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Course 234 - Turbine and Auxiliaries ~Module One APPROVAL ISSUE& RE FE RENCE S

Page 18

Gland exhauststeanv'airPB < Palm

LP IUrbineexhauetPr Palm AJ .r a - AlmosphereIun" Palm. . _ _ - - - . . . . - . ~

PressureI:PB,,,,,

Location

Fig. US. Normal operation of LP turbin. gJ.nd ... 1:- SIeam: -- - Air; A and B .. Externa lconnectiona to !he gland sealing steam syetem.

To prevent egress of this steam in to the turb ine hall. port B of the seal isconnected to a g land exhaust condenser w hich operates slightly below at-m o sp heric p ressu re. T he resu ltan t steam an d air flow paths and a sim plified .pressure distribu tion are shown in Fig . 1.6. N ote that turbine load doesnot affect operation of this seal because high vacuum is maintainedat the LP turbine exhaust at all power levels.Operation of an HP turbine gland seal at partial loadsThis is more c omp lic ate d b ec au se :a) The HP turbine exhaust pressure varies, depending on the turbineo pera tin g c on ditio n a s o utlin ed b elow a nd ;b) W hile LP tu rb in e g la nd se als a re v ery sim ila r in all s ta tio n s, th is u n fo r-tu nately d oes n ot ho ld tru e for HP tu rb in e g la nd sea ls. Many varlous

d esig ns of these seals are used in C ANDU statio ns, as their tu rb in esh av e b ee n bu ilt b y d iffe re nt ma nufa ctu re rs.

A t high and m edium loads, the H P turb ine exhaust pressure is well aboveatm o sp heric so th at air ingress in to the turbine is of no concern . Instead ,h igh p re ss ur e steam egress in to the turb ine hall m ust be prevented. Butat very light loads. during turbine startups, as well as those rundowns andshu tdo wns w hen co nden ser vacuum is still m ain tain ed , the HP tu rb in e e x-h au st p ressu re d ro ps b elow atm o sp heric an d th e seal m u st then p rev en t airingress.


19/32

APPROVAL ISSUE Course 234 - Turbine and Auxiliaries - Module OneT hese p rob lem s hav e been so lved b y turb ine manutacturers in a few d iffer-en t ways. The seal shown in Fig . 1.7. is fairly sim ple as it has on ly twoex tern al c on nec tio ns. Wh en th e tu rb in e o perates at h ig h a nd m ed ium lo ad s,th e se al is self-sealin g (ie. -it d o es n ot n eed an y sea lin g steam ), a nd its leak -off (at connection A) is typ ically u tilized as sealing steam in LP tu rb in eg lan d sea ls. W h en ev er tu rb in e lo ad d ro ps b elow a certain lev el, this s ea l re -q uires sealing steam to p rev ent air in gress.' T he p ressu re d istrib utio n an dsteam /a ir flow p ath s in th e seal then b ecom e sim ilar to those in the LP tur-b ine g lands (com pare Fig. 1.6 and 1.7 b) .

) O.-atlon At High And Medium Turbine Lad.

b) ap..tion at V.y Ught Load., Durin. St.tup. E~.

:i,,:::::IIIIIIII:I- - - t - - - - - - - - - - - - - - -II

Flg.i.7. NonnaJ oper.tlon of. HP turbine gI.nd ... 1:- - S team; - - - - Air; A and B E xternalc on nect io n 1 0 th e g la nd s team s ea lin g s ys tem .

N O T E S & R E F ER EN C ES

Page 19


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Course 234 - Turbine and Auxiliaries Module One APPROVAL ISSUENO T E S & R EFER EN C ES .

The HP turbine glandsshown in Fig. 4.6 at theend of module 2344 arean example of suchseals.

Obj. 1.6 ~

Obj. 1.7 a) < = >

Page 20

Other designs o f the H P turbine gland seal m ay have m ore external connec-tions. their leakoff m ay be utilized in another w ay (eg . for feedheating) andsom e of them m ay require sealing steam all the tim e (ie. they are never self-sea lin g)". S in ce a ny attem p t to co ver all th ese v aria tio ns WOu ldbe impracti-cal and confusing , this m odule describes only one type of an HP turbinegland seal. As it m ay not be exactly the type used in your station , the sta-tion specific tra in ing w ill supplem ent the required inform ation . For no w,just rem em ber that the inform ation on the HP turb ine glan d seal that is pre-se nted in this module is very general and m ay no t be fu lly ap plic ab le to y ou rstation . N evertheless. you have learned that, at least in some stations,the HP turbine gland seals are greatly affected by the turbine oper-ating condition - they m ay be self-sealing at high and m edium loads, butreq uire se alin g ste am when the HP tu rb in e ex ha ust p ressu re d ro ps b elo w a t-mospheric.Rubbing in turbine sealsAll tu rb in e s ea ls , b oth e xte rn al a nd in te rn al, are d es ig ned w ith th e c le ara nc esbetw een the rotating and the stationary parts as smal l as possib le. W hilethis m in im izes steam leak ag e, a nd hen ce k eep s the tu rb in e effic ien cy hig h, italso p romo te s ru bb in g.Rubbing 18the major operational problem with all turbine seals be-ca use their sm all c learan ces ca n be clo sed u p relativ ely ea sily . T his u su allyhap pen s d ue to hig h v ib ra tio n o r ex cessiv e therm al o r m ec han ic al d efo rm a-tio ns o f the tu rb in e c asin g an dlo r ro to r.S ea l ru bb in g is p artic ula rly lik ely d urin g initial startup s of the turb ine. T hisis because som e m anu facturers m ak e the radial c learances w ithin turbinese als slig htly to o tig ht in order to avoid undue leakage losses caused by ex-cessive clearances. T hus. during a few initia l startups. som e m ild rubbingis expected to increase clearances to their proper values. N eedless to say.th es e s ta rtu ps mu st be c arrie d o ut e xtremel y c are fu lly .A ll m od em tu rb in e sea ls a re d esig ned to a ccommo date light rubbing with-out any damage to the turbine: For exam ple, the stationary parts ofgland seals and diaphragm seals are flexib ly supported and they are m ade ofsoft m aterials (eg . lead bronze) to prevent/m inim ize dam age to the shaftS im ilarly . the tips of free-standing (ie. shroudless) m oving b lades arethin ned so that rubbing can wear them out without bending or breaking theblades them selves. W hile these m easures are effective against light rub-bing. they cannot protect the turbine in case of more intensive rubbing.Listed below . in the order of severity . are the adverse consequences!operating concerns caused by such rubbing:1. Damage to the rubbing seal(s).

D am ag e to tu rb in e seals resu lts in in creased c leara nces b etw een the "ru eda nd mo vin g c omp on en ts. T herefo re, tu rb in e effic ien cy ispermanentlyred uced d ue to in creased steam lea ka ge thro ug h the d am ag ed sea lfs).


21/32

APPROVAL ISSUE CoW'Se 234 - Turbine and Auxiliaries Module OneThis a dverse c onsequ enc e ap plies to b oth intern al an d ex tern al seals.B ut severe dam age to an external seal has add it iona l consequences.which are the sam e as th ose c au sed b y in su ffic ien t sea lin g steam p res-su re (d esc rib ed in the fo llowin g sec tio n).

2. High turbine rotor vibration due to:a) D irect effect of the forces generated by the rubbing and;b) Thermal b en ding of the ro to r.

Here is how it happens. W hen rubbing occurs in a turbine seal,only a sm all arc of the shaft surface is, at any given tim e, incontactwith the statio nary p art of the seal. The re fo re , fric tio na l he at a t th esite of rubbing does not heat the shaft evenly . Instead , it produces ahot spot on the shaft surface. T he resultant therm al expansion ofthe slu itt causes it to bow . T his increases it s u nb ala nc e a nd he nc e,vibration.

H igh rotor vibration can force a turbine trip a nd re su lt inodamage* toth e machine .3. Severe damage to other turbine internals.

For exam ple, deep grooves can be cut on the shaft surface by the sta-tio na ry p arts o f the ru bb in g seals. Or, som e blades can get bent or evenbro ken o ff if their seals are in vo lv ed in the ru bb in g. A lso , lo calizedheating of the shaft m ay produce thermal stresses so la rg e tha t th e sh aftcan bow perm anently . In any case, dam age w ould be accom panied byv ery h ig h v ib ra tio n.

In the extrem e case, a long outage m ay be n ec essary fo r co stly repa irs to theturbine.Abnormal pressure of gland sealing steamT ro ub le-free o peratio n o f an y tu rb in e g la nd seal req uires p ro per ad ju stm en tof its sealing steam pressure. When this pressure increases aboveits normal value, the sealing steam flow through the seal increases andpressure at port B rises as more steam flows out of the seal and into thegland exhaust condenser. E ventually , pressure at port B can rise above at-mospheric, causing hot steam* to blowout of the seal. This is shownin Fig . 1 .8 on the next page.


Turbine generator damagedue to high vibration is described in module 23414.

~ Obj. 1.7 b)

At temperature of about140150'C.

Page 21


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Course 234 - Turbine and Auxiliaries ~Module One APPROVAL ISSUE& REFERENCES

Page 22

In the drawing below, part a) illustrates an HP turbine gland seal whoseleakoff is utilized as sealing steam in LP turbine gland seals. Part b) of thisfigure shows an LP or HP turbine gland seal operating against turbine ex-haust pressure below atmospheric.

IIIIIII,IIipeI- - ~ t ~ ~ ~ - ~ ~ - - ~ - - - - - ~ ~ - - ~ ~ ~ l ~ - - - ~ - + - - ~: I ~

I I II I ~I I I: I:I r ~

P"-Uni

L.ocalion

b) Turbtne exh .,.. ........ HIow etmoephtricQIendIiIIh.uItttnmPB > Palm

PrHlUII'fI,,,,,,,,I,,---t"--------- -IIi,

I,,,- - - - - - - - r - - - - - - ~ - -,, ,,:,:,L.ocallon

Fig. 1.8. OperllUon of .1urblne gillnd Hal wHh excessivepre&lure 01 n. eeallng .1eam: A and B ...Externalc on ne ctio ns to th e glan d s te am s ea lin g s ys tem.


23/32

APPROYAL ISSUE Course 234 - Turbine and Auxiliaries Module OneIn either case, the steam blow ing out of the seal can overheat the adja ..cent bearing (if leak ag e is larg e en ou gh). Contamination of oil in thebearing with water c an a lso resu lt b ec au se the lea kin g steam c an p en etratethe bearing seal and condense inside. B oth these problem s are covered inmo re d eta il in other m odules of this course. N ote that the axial d istance be-tween turbine gland seals and bearings is very sm all (to reduce the tota llength of the turbine generator). T herefore, it is not d ifficu lt for leakingg la nd s te am to re ac h th e n ea re st b ea rin g.O f course, any steam leak represents a safety hazard and causes increasedconsum ption of m akeup water. In this case. however, the safety hazard isn ea rly n on ex isten t b eca use d urin g tu rb in e o pera tio n it is v ery u nlik ely tha tsom eone w ill be close-to the leaking seal. L ikew ise. the cost of increasedm akeup is sm all in com parison w ith the cost of a bearing repair whichw ou ld req uire a tu rb in e shu td ow n.When the sealing steam pressure at the inlet to a gland seal is toolow, air can leak into the turbine through the malfunctiOning seal.This problem can occur in any L P turb ine gland seal at any pow er level or inan H P turb ine g land seal whenever the H P turbine exhaust pressu re is be-l ow a tmospher ic .The actual steam and air flo w p aths in sid e the m alfu nc tio nin g sea l d ep en don its design w hich. in th e case of the H P turb ine gland seal, varies fromone s ta ti on to ano ther. A long w ith this. the type of the gland sealing steamsu pp ly fa ilu re (ie. to ta l o r partial lo ss to all the sea ls o r o nly to o ne sea l) a lsoaffects the flow paths. To cover all these cases would be im practical andconfusing . T herefore for sim plicity . Fig . 1 .9 on the next page illustrateso nly o ne case: an lP turbine gland seal m alfunctioning du e to a total loss o fg la nd sea lin g steam to all th e sea ls .

NO T E S & R E F ER ENC ES

< = > Obj. 1.7 c)

Page 23


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Course 234 - Turbine and Auxiliaries _Module One APPROVAL ISSUE& R E F ER EN C ES

Page 24

Alrlstnm mixturelrom other gland sealsvia common headers

Rg. 1.'. Oper.Uon of LP turbine g.. nd... 1with lnautllclenlpreuur. or 10 . of I...... In g . " 0 1 :'- - S leam; __ - A ir; A an d B - Ex tema lconnections to Ih . g la nd ,team ,ealing system.

Note in Fig . 1 .9 that the sucking action of vacuum inside the turb ine canlo wer p ressu res a t ports A and B so m uch that the m alfunctioning seal cana ctu ally d raw a n a ir/ste am mixture from other seals via comm on' headers inth e g lan d sea lin g steam sy stem tha t co nn ec t all th e glands together,Air In leakage that results from a loss of gland sealing steam pressure toone or m ore glands causes the follow ing adverse consequencesloperating concerns:1. Increased air concentration in the condenser, resulting in:

a) Reduced condenservacuum with all its adverse consequences asoutlined inmodul e 2 34 -5 ;Ifman y seals are leak in g a nd n o c orrectiv e a ctio n is ta k en , conden serp ressu re m ay q uic kly (w ithin sev era l m in utes) rise en ou gh to ca useautomatic turbine unloading or even trip, b oth rep resen tin g ac os tl y lo ss o f p ro du ctio n.

b) Increased concentration of dissolved oxygen in the conden-sate.H the gland m alfunction does not force a turbine trip and operationis continued, accelerated corrosion in the condenser and thecondensate system is our m ajor concern . T he boiler feedw ater and


25/32

APPROVAL ISSUE Course 234 - Turbine and Auxiliaries - Module Onesteam system s, as w ell as the boiler and the turbine, are also affectedif the h yd ra zin e in jec tio n ra te is n ot in crea se d p ro perly .T o m inim ize co rro sion d am ag e, the allow ab le d ura tio n o f o pera tio nis lim ited a nd p ro per action s m ust be tak en w hen certain lim its o nthe oxygen content are reached. In the extreme case, the unitmust be shut down. Deta ils o n tho se limits an d a ctio ns a re speci-fied in th e a pp rop riate op erating m an ual an d left fo r the sta tio n sp e-c if ic t ra in ing .

2. Quenching of hot parts of the malfunctioning sea1(s) by cool In-leaking air. T he q uen chin g can pro duc e therm al stresses larg e en ou ghto ca use cracking of the seal segments andlor increased turbinevibration. Also, abnormal axial djfferenUaI expansion c an o cc ur" .

3 . A total loss of sealing steam to a turbine gland can cause condenser vac-uum to suck in lube oil from the adjacent bearing . T he resultant con-tamination of boUer feedwater can ca use foaming and asphalt.like deposits In the boilers.

SUMMARY OF THE KEY CONCEPTS Operation of LP turbine gland seals is n ot a ffec ted b y tu rb in e lo ad . Inmany s ta tio ns , th e HP turbine g lan d seals are self-sealin g w hen theH P tu rb in e ex ha ust p re ssu re is su ffic ien tly a bo ve a tm o sp heric b ut th ey

require sealing steam w hen this condition is not m el In o ther s ta tions ,these seals req uire se alin g ste am a ll th e t ime. R ubbing is the m ajor operational concern w ith all turb ine seals because

o f the tig ht clea ra nc es used . While light ru bb in g can b e a ccommod ated , m ore inten siv e ru bbin g c andam age seals and other turbine internals as well as increase turbine

v ib ratio ns. A fo rc ed o utag e an d c ostly rep airs m ay result, E xcessive gland sealing steam pressure results in steam egress that cano verh ea t th e b ea rin g a dja ce nt to th e m a lfu nc tio nin g se al. C on tam in atio n

of th e o il in the b ea rin g w ith co nd en sin g steam is also p ossib le. Insufficient pressure of gland sealing steam causes air ingress in to the

tu rb in e, resu ltin g in re du ced c on de nse r v ac uum a nd in cre ased d isso lv edo xy ge n c on te nt in t he condensa te . In th e ex trem e c ase , tu rb in e u nlo ad -ing and trip on low condenser vacuum can occur, or the increased dis-solved oxygen content in the condensate m ay force a unit shutdow n.S eco nd , q uen chin g o f th e malfunct ioning-seal by in-leaking air can dam-a ge the sea l, in crease tu rb ine vibratio n an d p ro du ce ab no rm al axial diffe-.rentia! expansion . T hird , bearing oil m ay get sucked into the turbine '


Axial differential expan-sion is explained inmodule 234-11.

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Course 234 - Turbine and Auxiliaries Module"One APPROVAL ISSUES "& R E FE R EN C E S

P ages31-32 ~

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a nd co ntam in ate b oiler feedw ater w hich m ay lead to fo am in g a nda sp ha lt- lik e d ep osi ts in th e b oile rs .You can now work on assignment questions 1519.


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APPROVAL ISSUE Course 234 - Turbine and Auxiliaries Module ODeASSIGNMENT

1. Except for operation at very light loads. all turbine i n te rs tage pressu reschange with turbine load as follows: _

2. a) For efficient operation , the flow pattern must match _

b) - The flow pattern in a m rbine stage changes w ith the am ount ofhea t av ai la b le per kg of steam because _

c) T he am ount of the available heat stays approxim ately constant aslong as the steam pressure (drop ' ra tio) across the stage does notchange.

3 . W hen turbine load is reduced, th e amou nt o f th e h ea t a va ila ble to th eturbine, per kilogram of steam , (decreases' increases). T he effect (isd is trib ute d e ve nly b etwee n all th e s tag es I o ccu rs o nly in the first fewstages' occurs only in the last few stages).

4. a) At light m rbine loads, th e flow p attern in th e (first 'last) sta ge iss ig ni fic an tly d ete rio ra te d. The s te am in the stag e m ov es to o (fa st'slow ly) to be able to d riv e th e mo vin g b la des. Large _losses occur when the steam is churned by the blades. T his m aye ve ntu ally le ad to the stage being driven by the other stages of thetu rb in e o r the g en erato r. (F alse' tru e)b) T he flow deterioration discussed in point a) above propagates(d ow nstream ' u pstream) the m rb in e w hen its lo ad is sufficien tlyredu ced . T his hap pen s a s follows:


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Course 234 - Turbine and Auxiliaries Module One APPROVAL ISSUE& RE FE RENCE S

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c) T he effect o f turbine load on the perform ance of the rem ain ingturb ine s tages i s neglig ib le because ~ _

5 . P rolonged operation at very lig ht o r n o lo ad . co nd itio ns p rom otes thef ol low ing oper ati on a l p rob lems :a)b)

6. a) M oisture in turbine steam disturbs th e s te am flow b ec au se

b ) W ater droplets and steam m ove in (d ifferent I th e s ame ) d ire cti on sbecause ___

c) W ater droplets m ove (faster I s lowe r) th an ste am b ec au se

7 . a) Im pact erosion is caused by """""

E xam ple o f a typ ical com ponent that m ay be d am aged by im pacterosion:

b ) W ashing erosion is caused by _

E xam ple o f a typ ical com ponent that m ay be d am aged by w ashingerosion:


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APPROVAL ISSUE Course 234 - Turbine and Auxiliaries - Module Onec) W ire draw ing erosion is caused by _

E xam ple o f a typ ical co mp on en t that m ay b e d am aged b y w iredraw ing erosion : _

8 . W et steam promotes:a) G eneral elec trochem ical corrosion because _

b) S tress corrosion crack ing and corrosion fatigue because _

c) E rosion-corrosion because _

9. W et steam increases turb ine overspeed potentia l because _

10. E xcessiv e w etn ess of tu rb in e steam can be caused by the follow ing o p-e ra ti ng condit ions :a)b)c)d)

, e)

NOTES & REFERENCES

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Course 234 - Turbine and Auxiliaries - Module One APPROVAL ISSUE& R E F ER EN C ES

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11. An excessive axial th ru st c an ha ve th e fo llowin g a dv erse c on se qu en ces:.a)

b)

12. Indouble flow turbines used in C ANDU stations, the steam flow andp ressu re d istrib utio ns in b oth ha lv es o f th e tu rb in e (a re I a re n ot) id en -tical.13. a) An excessive axial th ru st c an be cau sed b y:

i)il)ill)

b) A c ha ng e in th e s te am flow d is trib utio n th ro ug h th e tu rb in e c an bec au se d b y:i)il)

c) W hen the steam flow distribution through the turbine is changed,an increased axial thru st o n the roto r can result bec ause

14. a) An ex ce ssiv e a xia l thru st is in dic ated b y:i)ii)

b) An excessive axial thrust resu lts in an abn orm al shaft ax ial p osi-tionbecause __

c) H igh thrust bearing m etal tem perature can be caused not only byan excessive axial thrust, but also by _


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APPROVAL ISSUE Course 234 - Turbine and Auxiliaries Module One15. a) HP turbine gland seals are always self-sealing as long as som esteam is admitted to the tu rb in e. (F alse / tru e)

b) Operation of LP turb ine gland seals (does / does not) depend ontu rb in e lo ad .

16. The major operational concern with all turb ine seals is _Its occurrence is promoted by the u se of sm all _in tu rb in e sea ls.

17. a) Rubbing in the tu rbin e ca n ha ve the fo llo wing ad verse c onse-quences:i)

ii)

iii)

b) Rubbing in a turbine seal can increase tu rb in e ro to r v ib ra tio n d ue to :i)

ii)

18. W hen the gland sealing steam pressure is to o h ig h, (air in gress / steameg ress) o ccu rs, p ossib ly resu ltin g in:a)b)

19. a) Insufficient g land sealing steam pressure causes air i ng re ss i ntoth e conden se r, re su lti ng in:i)

ti)


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Course 234 - Turbine and Auxiliaries - Module One APPROVAL ISSUE& REFERENCES b) In a dd itio n, q uen chin g o f the ho t sea l b y the leak in g air can

cause:i)

ii)

iii)

c) A lso , a to tal loss of sealing steam to a tu rbin e gland causes thef ol low ing concer n:

Before you m ove on to the next m odule, check the objectives to m akesu re that y ou ha ve n ot o verlo ok ed a ny im po rta nt issu e.

Prepared by: J. Ju ng , ENIDRevised by: 1. Juag, ENID

Revision date: April, 1994

234-1. the Steam Turbine p

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Transcript of 234-1. the Steam Turbine p