principles turbine gen aux.pdf

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NUCLEAR TRAINING CENTRE

COURSE 134

FOR ONTARIO HYDRO USE ONLY

This course was originally developed for

the use of Ontario Hydro employees.

Reproduced on the CANTEACH web

site with permission


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September 1976

NUCLEAR TRAINING COURSECOURSE 134

1 - Level3 - Equipment & System Pr inc ip les4 - TURBINE, GENERATOR & AUXILIARIES

Index

Objec t ives

Turbine Theory

Turbine Opera t iona l Performance

Turbine Opera t iona l Problems

Turbine Sta r t -up

Facto rs Limit ing Star tup and Rates o f Loading

Rel iab i l i ty and Test ing Requirements

Maintenance


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Turbine , Generator & Auxi l i a r i e s - Course 134OBJECTIVES

At the end of t h i s course you wi l l be able to :Courses 434, 334 and 2341. Meet the o bje ctiv es f or the Courses 434, 334 and 234.

134.00-1 Turbine Theory1. Sta te a working de f in i t i on o f:

(a ) entropy(b) enthalpy(c ) percent mois ture(d ) qua l i ty .2. Sketch and l abe l a Moll ie r Diagram showing:

(a) the sa tu ra t ion l ine(b) cons tant pre ssure l ines(c) cons tant temperature l ines(d) cons tant pe rcen t moisture l i ne s(e) cons tant degree of superheat l i nes .3. On a ske tch o f a Moll ie r Diagram, p lo t the condi t ionl ine fo r the steam sys tem in your p l an t showing:

(a) ou t l e t of steam genera tor(b) i n l e t to HP turb ine(c ) ou t l e t o f HP turb ine(d) i n l e t to moisture separa to r(e) ou t l e t o f mois tu re separa to r(f) i n l e t to rehea te r(g) ou t l e t of r ehea te r(h) i n l e t to LP tu rb ines(i) ou t l e t of LP tu rb ines .(Personnel not a t a genera t ing s ta tio n w i l l usePicker ing NGS as it i s t yp ica l of l a rge un i t s . )

4. Expla in what i s meant by Rankine cyc le and Carnot cycle .5. Calcula te Carnot Cycle Eff ic iency and expla in it'ss ign i f i cance .6. Expla in the advantages of superheated steam and whysuperheated cannot be produced in our nuc lear steamgenera tors .September 1976

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7. Exp la in u sin g an enthalpy-entropy diagram the extrac t ionof usefu l energy from the steam passing through a tu rb ines tage including:(a) i n i t i a l t emperature , pressure and enthalpy(b) useful energy ext rac ted(c) loss of entropy(d) f r i c t iona l reheat(e) exhaust pressure(f) ac tua l exhau st e nth al py(g) i sentropic exhaust entha lpy .

8. Define and expla in the s ig ni fic an ce o f:(a) s tage eff ic iency(b) expansion eff ic iency(c) diagram eff ic iency(d) f ixed b lad e leak ag e fac to r(e) moving blade leakage fac to r(f) dryness fac tor .

9. Sta te and expla in the fac tors a f fec t ing s tagee f f i c i ency inc lud ing:(a ) expansion eff ic iency(b) diagram eff ic iency(c ) f ixed blade leakage fac tor(d) moving blade leakage fac to r(e) steam moisture percentage .

10. Expla in the s igni f icance of carryover from a turb ines tage and the s igni f icance of carryover from the f i na lturb ine s tage (exhaust l o s s ) .

11. Draw a typ ica l condi t ion l ine fo r a mult i -s tageturb ine and ind ica te and expla in :(a) i n i t i a l pressu re , temperature and enthalpy(b) s tage pressures(c) pressure drop across th ro t t l e valve(d) i sen t rop ic enthalpy drop fo r each s tage(e) ac tua l enthalpy drop fo r each s tage(f) exhaust pressure(g) exhaust loss .

12. Explain th e fo llowin g:(a) Curt i ss Stage(b) Rateau Stage(c) Reaction Stage(d) Impulse Stage .


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13. Expla in the fac to rs i nf lu en cin g th e choice o f turb ineblading inc luding:(a) maximum diagram e f f i c i ency(b) enthalpy drop per s tage(c) veloci ty r a t io(d) steam pressure drop across the stage(e) ax ia l t h rus t(f) moisture e f f e c t s .

14. Expla in what i s meant by "nozzle governing" and" th ro t t l e governing and the advantages and disadvantagesof each.15. Explain how each of the fol lowing a f fec t s tu rb inee f f i c i ency :

(a) superheat ing(b) moisture(c ) moisture separa to r(d) feedhea't ing(e) pressure drop in piping and va lves .

134.00-2 Turbine Opera t ional Performance1 . Define:

(a ) Sta t ion Heat Rate(b) Turbine H eat Rate(c ) Derat ing .2. Expla in why s ta t ion hea t ra te and tu rb ine heat ra teare n ot eq ua l.3. Expla in the e f f e c t s of each o f the fol lowing on tu rb inehea t r a t e :

(a ) condenser vacuum(b ) moisture in steam passing through a turb ine(c) pressure drop through i n l e t valves(d) bo i l e r pressure(e) f i na l feedwater temperature(f) blade t ip leakage(g) a i r in leakage to condenser(h ) fau l ty gland sea ls o r gland sea l steam opera t ion(i) fau l ty a i r ex t rac t ion system operat ion .

4. Given a design hea t balance , compute a Design TurbineHeat Rate fo r your s t a t i on .5. Expla in which p lan t components, opera t ing parametersand flow ra te s have a major e f fec t on heat r a t e . - 3


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6. Develop a sys temat ic approach to improving a degradedhea t r a t e .7. Discuss the fac to rs which could cause d era tin g of atu rb ine-genera to r un i t .8. Li s t the major fac to rs which c ou ld ca use a decrease incondenser vacuum and expla in how you would d i f f e ren t i a t ebetween them .9. Li s t the major fac to rs which could decrease thee ff ic ie nc y o f th e f ee dh ea tin g system and how you wouldd i f fe ren t i a t e between them .

134.00-3 Turbine Operat ional Problems1. Discuss the fac to rs a f fec t ing the sever i ty of thefol lowing opera t iona l problems, the poss ib leconsequences and the des ign and opera t iona l considerat i ons which minimize t he i r frequency or e f f e c t :

(a ) overspeed(b) motoring(c) low condenser vacuum(d) water induc t ion(e) condenser tube l eak(f) blade fa i lu re(g) expan si on bel lows fa i lu re(h) bear ing fa i lu re or de te r iora t ion(i) low cycle fa t igue cracking .2. Explain the advantages of using FRF as a hydraul ic f lu idfo r turb ine con t ro l .3. Explain the precaut ions wh ich must be exerc ised withFRF and an e lec t r ica l -hydraul ic con t ro l system.

134.00-4 Turbine Sta r tup1. Describe the s e q u e n c ~ of events on a un i t s ta r tupinc luding:

(a ) genera tor sea l o i l(b) turb ine l ub r i ca t ing o i l system(c ) jacking o i l pump(d) turn ing gear(e) pos i t ion of governor steam valves , in te rceptvalves and steam re l ease valves(f) posi t ion of speeder gear(g ) posi t ion of emergency s top valve


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1 . (Continued)(h) t empera ture in deae ra t o r( i) condensate ex t rac t ion pumps( j) bo i l e r feed pumps(k ) a i r ex t rac t ion system(1) gland sea l system(m) condenser cool ing water system(n ) s t a t o r cool ing system(0) hydrogen coo l ing system(p ) bo i l e r s top valve pos i t ion(q ) condenser vacuum(r) lube o i l temperature(s) runup to ope ra t ing speed( t) synchroniz ing(u) load ing of gene ra to r .

2. Explain th e reason fo r each o f th e fol lowing in th es t a r tup sequence:(a) gland sea l ing system(b) a i r ex t rac t ion system(c) condenser c i r cu l a t i ng water system(d ) main lube o i l system(e) con t ro l o i l system(f) s ea l o i l system(g) genera tor cool ing systems(h) tu rn in g g ea r.

134.00-5 Fac tors Affec t ing Sta r tup and Rates of Loading1 . Explain th e reasons fo r each o f the fo l lowing:

(a) COLD, WARM and HOT s t a r tup procedures(b) b lock load on synchroniz ing(c) l imi t a t ion on r a t e s o f loading(d) HOLD and TRIP tu rb ine s uperv is o ry pa rame te rs .2. Discuss th e fac to rs which l im i t th e r a t e a t which al a rge steam t u rb ine may be s t a r t ed up and loadedinc luding:

(a ) steam pressure(b) dra in ing steam piping and tu rb ine(c ) condenser vacuum(d) the rmal s t r e s s e s in cas ing and ro t o r(e) d i f f e r en t i a l expansion between cas ing and ro t o r(f) lube o i l temperature(g) gene ra to r ro t o r temperature(h) s h af t e c ce n tr ic it y( i) v ib ra t ion( j) c r i t i c a l speeds .-


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134.00-6 Rel iab i l i ty and Tes t ing Reguirements1 . Expla in the hazards of an unterminated tu rb ine overspeed.2. Discuss the two fac to rs which determine con t ro l valveunava i lab i l i ty : valve unava i lab i l i ty and t r ippingchannel unava i lab i l i ty .3. Discuss the e f f e c t of t es t ing frequency on t r ippingc i r cu i t unava i l ab i l i t y .

134.00-7 Maintenance1 . Out l ine a program of prepara t ions pr io r to shut t ing

down a t ur bi ne g en era to r u nit p rio r co overhaul .2. Discuss items which should be examined dur ing ove rh au linc luding:

(a) blading(b) glands(c) diaphragms and nozzles(d) al ignment(e) t h rus t bear ing(f) r ad i a l bear ings(g) casing(h) ro to r( i) casing drains( j) evidence of presence of water(k) c learances between f ixed and moving blades(1) shroud c learances(m) turb ine f lange faces .3. Outlin e the bas ic fac to rs to be considered in turb inemaintenance.4. Outlin e the fac to rs which determine when a majorturb ine overhaul i s scheduled.

R.O. Schuelke


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T u rb in e, G e ne ra to r & A u x i l i a r i e s - Course 134TURBINE THEORY

The s u b j e c t of t u r b i n e th eo ry len ds i t s e l f t o a r a p i dd i g r e s s i o n i n t o a maze o f e s o t e r i c s c h o l a r s h i p which i s o fuse only t o t h e design e n g i n e e r s . On t h e o t h e r hand ast u r b i n e u n i t s become l a r g e r and push f u r t h e r toward t h el i m i t o f e x i s t i n g knowledge, the need f o r o p e ra ti n g ands u p e r v i s o r y p e r s o n n e l t o unders tand t h e reasons f o r t h el i m i t a t i o n s p l a c e d on t h e u n i t becomes a p a r t o f d a i l ye x i s t e n c e . It seems u n l i k e l y we can ever r e t u r n t o t h ehalcyon days of judging t u r b i n e u n i t performance by t h e"rumble of t h e engine and t h e smoke from t h e e x h a u s t " . Thepurpose of t h i s l e s s o n i s t o d i s c u s s t h e b a s i c theory o ft u r b i n e and steam c y c l e o p e r a t i o n from t h e s t a n d p o i n t ofunderstanding why t u rb in e s a r c c o ns t r u ct e d i n a c e r t a i nmanner. It i s hoped t h a t t h i s approach w i l l g i v e t h e r e a d e ran a p p r e c i a t i o n f o r t h e design f e a t u r e s of a t y p i c a l l a r g en u c l e a r t u r b i n e u n i t without t h e need t o r e s o r t t o a d e t a i l e dmathemat ical t r e a t m e n t . Those who d e s i r e a more r i g o r o u st r e a t m e n t a r e r e f e r r e d t o t h e l a r g e number o f e x i s t i n g textbookson power p l a n t theory and a p p l i e d thermodynamics.THERMODYNAMICS

The second law of thermodynamics t e l l s us t h a t it i simpossible t o c o n s t r u c t a system o p e r a t i n g i n a c y c l e whichcan c o n v e r t a l l t h e h e a t energy i n p u t from a h e a t sourcet o u s e a b l e work. It f u r t h e r d e f i n e s the maximum e f f i c i e n c yo f any c y c l e can by d e r i v e d from the e q u a t i o n :n = (1 .1)

n =

where: T, i s t h e a b s o l u t e temperature a t whichh e a ~ i s s u p p l i e d

T2 i s t h e a b s o l u t e temperature a t whichh e a t i s r e j e c t e dA CANDU n u cl ea r g e ne ra ti ng s t a t i o n s u p p l i e s h e a t i n thesteam g e n e r a t o r s a t approximately 250C and r e j e c t s h e a t t o t h ec i r c u l a t i n g water i n the condenser a t approximately 33C.Using e q u a t i o n 1 . 1 between t h e s e two temperatures we g e t :

523 0 K - 306K523 K217 K= 523K

= 42%- 1 -

September 1976


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In a system o p e r a t i n g between 250C and 33C a maximum o f only42% o f t h e h e a t s u p p l i e d can be converted t o work. It i sobvious t h a t t h i s maximum e f f i c i e n c y can be i n c r e a s e d byi n c r e a s i n g t h e temperature a t which h e a t i s s u p p l i e d o r bylowering the temperature a t which h e a t i s r e j e c t e d . In aCANDU n u c l e a r power p l a n t , however, t h e s e temperaturescannot be v a r i e d s u b s t a n t i a l l y i n a d i r e c t i o n which w i l l improvee f f i c i e n c y . The upper l i m i t i s imposed by m a t e r i a l l i m i t sw i t h i n t h e f u e l e lem en ts, w hile t h e lower l i m i t i s im posed bythe a v a i l a b l e temperature o f condenser c o o li n g w a te r from t h el a k e o r r i v e r and a b s o l u t e ly l im i te d by t h e f r e e z i n g temperatureo f water a t OC.

It i s w e l l t o remember t h a t t h i s 42% r e p r e s e n t s an upperl i m i t on the e f f i c i e n c y o f a CANDU g e n e r a t i n g s t a t i o n . As longas a CANDU system i s used t o conver t h e a t energy t o e l e c t r i c a lenergy the c y c l e cannot be more e f f i c i e n t than 42%.1 T

Condenser

Boiler4 CompressorI--+.

3 2

(a l

3

THE CARNOT CYCLEFigure 1 . 1

(b)s

Figure l . l ( a ) shows a system which has an i d e a l e f f i c i e n c yas d e s c r i b e d by e q u a t i o n 1 . 1 . T his system i s d e s c r i b e d asa Carnot Cycle. Heat i s added i n the b o i l e r a t 250C, worki s e x t r a c t e d i n the t u r b i n e , h e a t i s r e je c t e d i n t h e condenser a t 33C u n t i l about 80% o f t h e steam i s condensed andthen t h e wet stearn i s compressed t o s a t u r a t e d water a t t h ep r e s s u r e i n the b o i l e r . While t h i s c y c l e would have at h e o r e t i c a l e f f i c i e n c y equal t o t h e maximum of 42% it hass e v e r a l p r a c t i c a l draw backs:(al it i s d i f f i c u l t t o s t o p the condensing process s h o r t o fcomplete condensat ion t o w a t e r ,(bl t h e compressor must handle a low q u a l i t y wet stearn whichtends t o s e p a r a t e i n t o i t s component phases f o r c i n g thecompressor t o d e a l with a non-homoqen",ous mixture .- 2 -


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(c) the volume of f l u id handled by th e compressor i s high andth e compressor must be c omparab le in s i ze and cos t to th etu rb ine ,(d ) because th e compressor consumes a l a rge percentage of the

tu rb ine outpu t power, t h i s cyc le i s very sens i t ive toi r r eve r s ib i l i t i e s . While t h i s cyc le i s idea l ly 42%e f f i c i en t , i f the compressor and tu rb ine are only 80%e f f i c i en t , the cyc le e f f i c i ency drops to about 28%. I fthe compressor and tu rb ine e f f i c i ency drop to about 50%,the cyc le becomes a ne t consumer o f energy.Most of the prac t i c a l problems of the Carnot Cycle can beavoided by al lowing the steam to completely condense and thencompressing the l i qu id to bo i l e r pre ssure with a smal l feed pump.The re su l t ing cyc le , shown in Figure 1 .2 , i s known as aRankine Cycle .

Boiler

4

1

(a )

3

T

Turbine

Condenser

THE RANKINE CYCLEFigure 1 .2

4

(b) 5

It i s ev iden t without ca lcu la t ion t ha t the e f f i c i ency o ft h i s cyc le wi l l be l e s s than t ha t of the Carnot Cycleopera t ing between the same t empera tu res , because a l l thehea t suppl ied i s no t t rans fe r red a t the upper t empera tu re .Some hea t i s added whi le the temperature o f the l iqu id i sincreas ing from T4 to T s By comparing the work outpu t perkilogram o f steam (the shaded area of the T-s diagram) , iti s apparen t t h a t the s team consumpt ion i s l e ss in theRankine Cycle . In add i t ion s ince the power requi rements o fthe pump i s a smal l percentage of the tu rb ine ou tpu t, thee f f e c t o f i r r eve r s ib i l i t i e s i s s ign i f i c an t ly l e ss than withthe Carnot Cycle. While the Rankine Cycle has a lower i d ea le f f i c i ency than the Carnot Cycle, the prac t i ca l l y a t ta inablee f f i c i ency i s no t much d i f f e r en t and the p lan t i s ce r ta in lysmal le r and l e s s cos t ly .

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In th e Rankine Cycle shown in Figure 1 .2 , the s teamexhaust ing from th e t u rb ine has a mois tu re con ten t o f 28%.This i s much too high fo r any economic tu rb ine . The waterdrop le t s which a re c a r r ie d in th e wet steam cannot move asr ap id ly as the steam and as th e water passes through th e movingb l ade s , th e back of th e b la de s c o n ti nu a ll y s t r i ke the slower movingmoving d rop l e t s . This exer t s a r e ta rd ing e f f e c t on th e movingblades which decreases e f f i c i ency . On the o rde r of 1% o ftu rb in e s tag e ef f ic iency i s lo s t fo r each 1% average mois tu rein th e s t age . In ad di t io n th e eros ion e f f e c t of the waterdrop le t s fo r mois tu re pe rcen t s much above 13-14% would shor tenthe blade l i f e to an economically una t t rac t ive po in t .

The e f f e c t t h i s has on th e design ef f ic iency o f at u rb ine un i t can be seen on th e Mo ll ie r diagram in Figure 1 .3 .

,,,,

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I f t h e e xh au st mo is tu re must be l i m i t e d t o around 10%, asit i s on most l a r g e t u r b i n e u n i t s , then t h e t u r b i n e mustexhaus t a t p o i n t 2 (10% m o i s t u r e , 33C) r a t h e r t h a n a tp o i n t 1. That i s , we cannot des ign t h i s t u r b i n e t o bei s e n t r o p i c because we c an no t h an dle t h e i n c r e a s e d m o i s t u r eo f a complete ly r e v e r s i b l e expension .

To d e c r e a s e t h e e xh au st mo is tu re t o a c c e p t i b l e v a l u e swe must a c c e p t a c o n s i d e r a b l e i n c r e a s e i n entropy whichi m p l i e s a l o s s o f a v a i l a b l e energy and a d e c r e a s e i n e f f i c i e n c y .I n t h i s case t h e t u r b i n e e f f i c i e n c y must be l i m i t e d t o onlyabout 50% o f t h e i d e a l e f f i c i e n c y because we cannot copewith t h e mois ture c o n t e n t a h i g h e r e f f i c i e n c y would imply.While t h e t u r b i n e u n i t shown i n F i g u r e 1 . 2 has an i d e a le f f i c i e n c y o f 35%, t h e problem o f ex hau st moistu re alo nel i m i t s t h e p r a c t i c a l e f f i c i e n c y t o about 17%.MOISTURE SEPARATION

To improve t h e e f f i c i e n c y o f t h e c y c l e above t h a t p o s s i b l ew i t h a s i n g l e t u r b i n e , it i s common t o remove t h e steam from t h et u r b i n e a t 10% m o i s t u r e , s e p a r a t e t h e water from t h e s team, andthen u t i l i z e t h e steam i n a second t u r b i n e . While t h e e x a c tp r e s s u r e t o remove t h e steam f o r m o i s t u r e s e p a r a t i o n dependson a number o f f a c t o r s , p la n t e ff i c ie n c y i s g e n e r a l l y opt imizeda t a p r e s s u r e i n t h e 500-700 KPa (g) r a n g e . F i g u r e 1 . 4 showst h e e f f e c t o f such m o i s t u r e s e p a r a t i o n .

The dashed l i n e (1245) shows t h e i d e a l i s e n t r o p i c p r o c e s s .While t h i s p r o c e s s still r e s u l t s i n a m o i s t u r e c o n t e n t abovet h e maximum a c c e p t i b l e , t h e r e a l t u r b i n e p r o c e s s (1346) i smuch c l o s e r t o t h e i d e a l than was p o s s i b l e i n t h e s i n g l et u r b i n e . The i s e n t r o p i c e f f i c i e n c y o f t h i s process i ss l i g h t l y over 35%, a smal l improvement over t h e i s e n t r o p i ce f f i c i e n c y w i t h o u t mois ture s e p a r a t i o n . However, t h er e a l i s t i c a l l y a l l o w a b l e p r o c e s s i s almost 25% e f f i c i e n twhich i s a c o n s i d e r a b l e improvement over t h e 17% f o r t h ep ro c e ss w i t h o u t mois ture s e p a r a t i o n .

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MOISTURE SEPARATOR~ i g u r e 1 . 4

REHEATINGReheat ing i s o f t e n used t o f u r t h e r improve t h e c y c l ee f f i c i e n c y . Figure 1 . 5 shows a t y p i c a l n u c l e a r t u r b i n e systemw i t h r e h e a t i n g and m o i s t u r e s e p a r a t i o n .A p e r c e n t a g e o f t h e steam produced i n t h e b o i l e r i sl e a d t o a r e h e a t e r where it i s used t o s up e r h e a t t h e steameXhaust ing from the m o i s t u r e s e p a r a t o r .

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The dashed l i ne (12456) in Figure 1 .6 shows the i d ea li s en t r op i c process which has an ef f i c iency o f 38% which i sstill no t s i gn i f i c an t l y above the 35% a t t a i nab l e by a s ing letu rb ine wi th o ut mo is tu re sepa ra t ion o r r ehea t ing . However,the r e a l i s t i c a l l y a l lowable process (13457) i s a lmost 30%e f f i c i en t . It should be noted how much more c lose ly thea l lowable cond i t ion process fo l lows the i d e a l process inFigure 1 .6 than occurred in Figure 1 .3 .

In add i t ion it should be noted t h a t th e average moisturecon ten t in th e low pressure t u rb ine wi th moisture sepa ra t ionalone i s abou t 5% when th e e xh au st mo is tu re i s held to 10%.However, wi th r ehea t ing th e average moisture con ten t i s nomore than 1% w ith the same 10% l im i t on exhaus t mois tu re . Notonly does t h i s decrease e ro s ion in the low pressure t u rb i ne ,bu t th e decrease in e f f i c i ency due to water d rop l e t impingementon th e moving blades i s subs t an t i a l l y r educed .SUPERHEATING

Almost withou t e xcep ti on, c onven ti on a ll y fue l l ed powerp lan t s supe rhea t steam before sending it to th e high pressuret u rb ine . Not only does t h i s give the high p re ss ur e t ur bin eth e same bene f i t s t h a t r ehea t ing gives to the low pressuretu rb ine bu t in add i t ion the r a i s i ng of steam tempera ture aboves a tu r a t i on t empera tu re in cre as es th e average tempera ture a t whichhea t i s ex t rac t ed from the hea t source and thus in cre as es th eCarnot e f f i c i ency . Unfor tuna te ly , we are unable to add anyapprec iable amount o f supe rhea t to the 4000 KPa(g) sa tu ra t eds te am p ro du ce d in our steanl gene ra to r s . The same meta l lu rg i ca ll im i t a t i on s which r e s t r i c t steam gene ra to r t empera tu re to250C, r e s t r i c t th e tempera ture in a hypo th e ti ca l s u pe rh e at erto about 250C; t h a t i s , no supe rhea t .

While a CANDU r eac to r could produce superhea ted steam a t apressure lower than 4000 KPa(g) t h i s i s una t t r a c t i v e no t onlyfrom th e s tandpo in t o f a lower s a tu r a t i on t empera tu re in thebo i l e r and , t h e r e fo r e , a lower Carno t e f f i c i ency but a lso fromth e lower steam dens i ty which would r equ i r e l a r ge r pip ing andcomponents fo r the same power outpu t .PRESSURE DROPS IN PIPING AND VALVES

The pressure drops which occur as steam passes down themain steam piping and through valves can be considered at h r o t t l i ng process . Thro t t l i ng i s a cons tan t en tha lpy process ;t h a t i s , hea t con ten t o f th e ste am does no t change, eventhough th e p re ss ure and tempera ture dec rease . A t h ro t t l i ngprocess can be shown as a hor i zon ta l l i ne ( cons t an t entha lpy)on a Mol l i e r diagram. These pressure drops have to be heldto a minimum because the en t ropy o f the steam inc reases and,t h e r e fo r e , th e ava i l ab i l i t y o f energy dec reases .- 8 -


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INLET VALVE PRESSURE DROPFigure 1. 7

Figure 1 .7 shows th e p ressu re drop across th e i n l e t valves toth e high pressure t u rb ine . I f th e exhaust pressure does no tchange, then th e p re ssu re drop r e su l t s in l e ss ava i l ab leenergy. In fh i s case the 25% pre ssu re drop (12) r e su l t s inonly 87% (13 /13) as much energy ava i lab le in the highpressure tu rb ine . Typica l ly th e p ressu re drop between thesteam genera tors and i n l e t to the high pressure i s held tono more than 5%. The e f f e c t of a pre ssu re drop across themois ture s ep ar ato r, r eh ea te r and va lves between th e HP andLP tu rb ines i s s imi l a r and t h i s pre ssu re drop i s l ikewiseheld to a maximum of about 5%.FEEDHEATING

The t heo re t i c a l asp ec ts o f regenera t ive feedheat ing i sfu l ly discussed in the 225 Heat and Thermodynamics courseand does not r equ i re a complete red iscuss ion in t h i s l e s son .- 9 -


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The advantages o f ex t rac t ing steam from th e high and lowpressure tu rb ine fo r use in hea t ing feedwater i s f a i r l y obvious .With turb ine exhaus t wetness l imi ted to 10%, o nly about 10%o f th e l a t en t hea t o f vapor iza t ion can be u t i l i zed by pass ingthe steam through th e remaining s tages o f th e tu rb ine .However, if the steam i s ex t rac ted from th e t u rb ine and usedto hea t feedwater a l l of th e l a t en t hea t of vapor iza t ion canbe u t i l i zed . Of course , we are in a sense robbing Pete r( turbine output) to pay Paul (heat ing feedwater) so there i sa po in t o f diminishing r e tu rns bu t th e i n i t i a l e f f e c t i squ i t e pronounced in favor o f inc reas ing cycle e f f i c i ency . Inaddi t ion th e ex t rac t ion of steam from the low pressure turb inehelps to reduce the vas t volumes of steam which the l a t t e r s tageso f th e low p re ss ur e tu rb in e must handle . The e f f e c t of ahigher f i n a l feedwater temperature can be seen on th e T-sdiagram in Figure 1.8.

T

65 F - - - - - - - - - - - - - ~ 4

s

EFFECT OF FEEDHEATINGFigure 1. 8

Feedheat ing has r a i sed th e feedwater temperature from T6to Tl so th e bo i l e r must only inc rease the temperature fromTl to Tz before steam product ion beg ins . This r a i s e s th eaverage temperature a t which th e bo i l e r adds hea t energyand the re fo re i nc rea se s the Carnot e f f i c i ency . Feedwatert yp ica l ly en te r s a nuc lear steam genera ted heated to near175C. This r e su l t s in the steam genera to r adding hea tenergy a t an average temperature 35C ho t t e r than withoutfeedhea t ing .- 10 -


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It i s worth not ing t h a t in p lan t s such as Bruce N.G.S.where th e prehea te r i s loca ted ex te rna l to the bo i l e r , it i sth e t empera tu re of feedwater e nte rin g th e p re he ate r whiche f f e c t s e f f i c i ency . Thermodynamically th e prehea te r i snot a fe ed he ate r b ut ra the r an extens ion o f th e steam gene ra to r .

40000 300FINAL FEEDWATER TEMPERATURE. of

..-r .z/ ' ~ ~ ~~ STAGES OF- ............

A V FEEDWATER HEATING 2'--.../ ' V ---. ..... ""-/ / .............

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Figure 1 . 9Figure 1 .9 shows th e t yp ica l e f f e c t of feedheat ing on cyclee f f i c i ency . Since the temperature dif ference between th eex t rac t ion steam en te r ing a feedhea te r and the feedwater

leaving a feedhea te r i s t yp ica l ly SOC or l e s s , th e f i n a lf eedwa te r t empe ra tu re c lose ly approximates th e t empera tu reof the highes t temperature ex t rac t ion steam.Examination of th e curves in Figure 1 .9 rev ea ls th efo l lowing:

(al as the number of feedhea te rs inc reases , th e optimumtemperature and, the re fo re , p ressu re o f th e ex t rac t ionsteam to th e l a s t feedhea te r inc reases ,(bl the re i s little advantage to be gained in going beyonds ix to e igh t s tages of feedhea t ing , and(cl s ince the curves are r e l a t i ve ly f l a t on top th e ex t rac t ionsteam pressures can vary subs tan t i a l ly from th e optimumwithout much e f f e c t on e f f i c i ency .TURBINE STAGE EFFICIENCY

Thus fa r we have discussed the e f fec t s of var ious cyc lecomponents on id e a l e f f ic ie n cy whether i s en t rop ic or t ha tr e a l i s t i c a l l y imposed by exhaust mois ture . Tur bin es c anno talways be designed to work as we would l ike them to work andwe must accept e f f ic ienc ies lower than theore t ica l ly poss ib le .

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h

1

Frict ional Reheat*2

s

STAGE EFFICIENCYFigure 1.10

Figure 1.10 shows the condi t ion l i ne fo r a t u rb ine s tageopera t ing on wet steam between pressure PI and P 2 The dashedl i ne (12) represents the i dea l i s en t rop ic path between thesetwo pressures and hI - h 2 represents the i dea l work done bya kilogram o f steam pass ing through the s t age . In a r e a lturb ine we would find t h i s much work was not done and theac tua l path through the s tage (13) would r e su l t in l e s s hea tenergy being ex t rac t ed from the steam. The r a t i o

i s known as s tage ef f ic iency and fo r a wel l designed s tage i st yp ica l ly between 75% and 90% depending on the type o f s t age ,th e en tha lpy drop across the s tage and the moisture content ofthe steam.There a re a number of reasons why s tage ef f ic iency i snot 100%, bu t a s i gn i f i c an t source of ine f f i c iency i s f r i c t i onbetween the steam and blading. This f r i c t i on adds hea t energyback in to the s team and r e su l t s in a leaving en tha lpy higherthan i d ea l . Because of the s ign i f i cance of t h i s f r i c t i onhea t ing , the amount o f i s en t rop ic enthalpy drop not u t i l i z ed

in a s tage i s known as f r i c t i ona l rehea t even though f r i c t i oni s not the only cause of i n e f f i c i enc i e s . Although f r i c t i ona l- 12 -


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r ehea t r e su l t s in a g rea t e r enthalpy of the steam a t theou t l e t of the s tage than one would t heo re t i c a l l y expec t ,there i s also an increase in entropy which r ep resen t s a lossin ava i l ab i l i t y of energy.Stage e f f i c i ency i s a product o f f ive fac to r s as describedbelow:

Stage ~ E x p a n S i o n ~ ~ D i a g r a m ~ ( ~ ~ ~ ~ ~ ~ ( M ~ ~ ~ ~ ~ ~ ~ D r Y n e S JEf fi ci en cy = ,E ff ic ie n cy Eff i c i enc Leakage Leakage Fac torFac tor Fac torwhere: Expansion Eff ic iency = Steam Kine t ic Energy ProducedSteam Enthalpy Supplied

Diagram Eff ic iency = Work Done On RotorSteam Kine t ic Energy ProducedDryness fac to r accounts fo r the decrease in e f f i c i encydue to moi$ture impingement on the moving blades .

Of prac t i ca l s ig nif ic an ce in tu rb ine design i s thee f f i c i ency of the convers ion o f steam k ine t i c energy to work.I f t h i s diagram e f f i c i ency o f the turb ine i s not 100%, thensome steam kine t ic energy i s l o s t as steam l eaves the movingb la de s w ith some ve loc i ty . This loss of kine t ic energy i sknown as ca r ryove r . I f the subsequent s tag es a re wel ldes igned, th is c ar ry ov er can be pa r t i a l ly or fu l ly recovered;however, the c a r r yove r from the l a s t s tage represen t s anunrecoverable loss o f energy. After the steam leaves thel a s t s tage , t h i s k ine t i c energy i s converted to hea t andappears on the Moll ie r diagram as an unexpected in cre ase inexhaust entha lpy. This l eaving loss or exhaust loss as iti s ca l l ed must be minimized by insur ing the ve loc i ty of thesteam leav in g th e l a s t stage i s as small as poss ib le . For th i sreason, the annular area of the l a s t row of blading i s madeas l a rge as economical ly poss ib le .THE TURBINE CONDITION LINE

F igure 1 .11 shows a t yp i ca l tu rb ine condi t ion l ine fo ra seven s tage sa tu ra ted steam tu rb ine .You wi l l note the pre ssure drop across the i n l e t valvesand steam s t r a i n e r . At the normal opera t ing load o f thet u rb ine , the des igner at tempts to ach ieve an equal enthalpydrop in each s tage so the work produced in each s tage i sapproximately equa l . The abrup t increase in enthalpy a t theturb ine exhaust i s the appearance o f the exhaust loss ashea t energy. The turb ine e f f i c i ency would be expressed by

h 2 - h gh 2 - h lO '

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Pg

Pe

Ps

7

9 l~ ~ ~ - - - Exhaust Lossf ~ - -

5 thStage

7 6 thStageI

III

4 thStage

3rdStage

EnthalpyKJ/kg

EntropyKJ/kg - oK

F ig ure 1 .11- 14 -


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As a r ep resen ta t ive va lue the e f f i c i ency of thePicker ing N.G.S. high pressure tu rb ine i s approximately 75%while the e f f i c i ency o f the low pressure tu rb ine i s approxi-mately 85%. I f exhaust losses were e l imina ted complete lyin the LP tu rb ine , the ef f ic iency would be near ly 89%. Theg r ea te r e ff ic ie n cy o f the lo w p re ss ure tu rb in e i s a combinat iono f the e f f e c t s o f rehea t ing and lower average mois tu re .Figure 1.12 shows th e c on ditio n l i ne fo r a t yp ica l l a rget u rb ine un i t with r ehea te r s and mois ture separa to r s . Points 1through 6 show the ex t rac t ion po in ts fo r feedhea t ing .,.;>r,n ,

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TURBINE STAGE TYPESThere are two bas ic types o f tu rb ine s tage s : the r eac t ions tage and the impulse s tage . The fundamental di f fe rence betweenthe two types o f s tag in g i s the pa r t o f the s tage in which hea tenergy i s conver ted to steam k ine t i c energy_ In the impulses tage , t h i s conversion t akes place only in the f ixed blades ;

in the reac t ion s tage , t h i s conversion takes place in both thef ixed and moving blades .THE IMPULSE STAGE

As the steam passes through the f ixed b la de n oz zle s o f animpulse s tage , th e e nth alp y o r hea t energy o f the steam i sreduced and the ve loc i ty i s grea t ly increased . This highve loc i ty steam i s then di rec ted by the f ix ed b la de s in to themoving blades . The s te am changes d i rec t ion in the movingblades and impar ts an impulse (force x t ime) to the movingblades . Figure 1.13 shows th e pre ssure , ve loc i ty and enthalpychange across two impulse s t ages .

h

P

MOVING

v I - - - - - ; - - - - - ; - - - ~ - - - - - . . . : . -

RATEAU STAGESF igu re 1.13

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This type o f impulse s tag e (fixed nozz le , moving blade)i s known as a Rateau Stage . The pressure and e n th a lp y d ec re as eacro ss the nozz le as hea t energy i s conver ted to steam k ine t i cenergy (ve loc i ty ) . Across th e moving b lades , th e steam veloci tydecreases as k ine t i c energy i s t r an sf er re d to the moving b lades .You wi l l note the absence o f a pressure drop across the movingblade . The tu rb ine shown in F igu re 1.13 would be re fe r red toas a two s tage Rateau t u rb ine .

Figure 1.14 shows another type o f impulse s tage arrange-ment known as a Cur t i s s Wheel.

h

FIXED MOVING FIXED MOVING

p

v

TWO STAGE CURTISS WHEELFigure 1.14

The second s e t of fixed b lades are no t nozzles and onlyserve to r ed i r e c t the steam in to the second s e t of movingb lades . Because the second f ixed blades only r ed i r e c t thesteam t he re i s no change in steam ve loc i ty across theseb lades . The tu rb ine shown in Figure 1.14 i s a two s tagetu rb ine . These two s tages are col lec t ive ly ca l l ed a Cur t i s s

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Wheel.s t age ,

You wi l l note t h a t in th e Cur t i s s Wheel, as in the Rateauthe re i s no pressure drop across th e moving blades .

THE REACTION STAGEThe f ixed blade nozzles in a r ea ct io n tu rb in e conver thea t energy to steam k ine t i c energy in the same manner as th e

Rateau s tage . The high ve loc i ty steam impar ts an impulse toth e moving blades .The reac t ion s tage d i f f e r s from th e Rateau s tage in t h a tthe moving blades a re shaped l i k e a nozzle so t h a t hea t energyi s conver ted to k ine t i c energy in the moving as wel l as the

f ixed b l ades . This conversion fo rces the moving blades awayfrom the expandi ng s te am in a reac t ion e f f e c t s imi l a r to arocke t rea ct in g to the e sc ap in g e xh au st gasses .

h

p

FIXED FIXED MOVING

v t - - - - - ~ - - - ~ - - - - ' . - - 2 : : : : : : : : : . . -

REACTION STAGESFigure 1.15

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Figure 1.15 shows the pres su re , enthalpy and ve loc i tychange across a two s tage r eac t ion tu rb ine . Heat energy i sconverted to k ine t i c energy in both th e f ixed and moving blades .The moving blades move in response to both an impulse and ar eac t ion e f f e c t . You wi l l note t ha t a pressure drop occursacross the moving blades .The d i s t i nc t ion between im pU lse and react ion s tages i smore c l e a r cu t in theory than in p rac t i c e . Turbine s tagesare c l a s s i f i ed by t h e i r degree o f r eac t ion o r th e r a t i o o fthe enthalpy drop across the moving blades to the enthalpydrop across the en t i r e s tage . The degree of r eac t ion may varyfrom 0% to 100%; zero r eac t ion being p ure im pu lse. It i s no ta t a l l uncommon fo r impulse s tages to have a smal l amount o f

react ion to improve t h e i r e f f i c i ency . General ly if th e degreeo f r eac t ion i s no more than 5-10%, th e stage i s ca l l ed animpulse s tage , otherwise it i s ca l l ed a r ea ctio n s ta ge .CHOICE OF TURBINE STAGE

The dec is ion o f which type o f s tage to use in a turb inei s never c l e a r cu t . Each type o f s tage has i t s pa r t i cu l a radvantages and disadvantages and an a pp lic at io n in which iti s the supe r io r cho ice .AXIAL THRUST

React ion tu rb ines have a pressure drop across the movingblades . Because o f t h i s , the fo rce on th e high pre ssure s ideo f the b lade wheel i s g rea t e r than the counte rac t ing fo rceon the low pressure s ide . This force d i f fe rence means the rei s a tendency of th e wheel to move in th e d i rec t ion ofdecreas ing pressure . In a s ing le f low, high pressure r eac t iont u rb ine , the cumulat ive fo rce can be very l a rge and the t h rus tbear ing necessary to handle t h i s force would be extremelyl a rge and cos t ly . Although th e re are methods ( fo r example adummy pis ton) o f compensating fo r t h i s t h rus t in a s ing le flowhigh pressure r ea ctio n t ur bin e , th e l e a s t compl ex method o fhandling ax i a l t h r u s t in a s ing le flow t u rb ine i s to useimpulse s tag ing . Since the impulse s tage has no pressuredrop across the moving blades , it produces no ax ia l t h rus t .

In a low pressure tu rb ine , the pressure drop across themoving blades of a r ea cti on tu rb in e i s much l e s s . For at yp ica l 50% r eac t ion nuc lea r steam tu rb ine , th e pre ssure dropacross the moving blades of th e HP turb ine would be 200 KPaper s tage while the pressure drop across the moving blades ofth e LP turb ine would be 25 KPa per s tage . It i s p oss ib le toeconomical ly cons t ruc t a t h rus t bear ing which wi l l handle thet h rus t of a s ing le flow low pressure re ac tio n tu rb in e. Ther e s u l t i s t ha t while most s ing le flow HP t u rb ines have impulseblading, many s ing le flow LP turb ines have r eac t ion blading.

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In l a rg e tu rb in e un i t s with l a rge diameter low pressure bladewheels , even th e smal l p res su re drops across th e moving blades o fa low pressure reac t ion t u rb ine produce a l a rge ax i a l t h r u s t .In such un i t s th e low p ressu re tu rb ines a re t yp i ca l l y doubleflow to compensate fo r t h i s t h ru s t .EFFICIENCY AND ENTHALPY DROP

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Figure 1.16 Figure 1.17Figure 1.16 shows how th e diagram e f f i c i ency fo r th e t h reemajor types of tu rb ine s tages changes as the enthalpy dropper s tage va r i e s . I f th e e nth alp y drop pe r s tage i s kep t smal l

th e re ac tio n tu rb in e i s a t t r a c t i ve due to i t s h ighe r maximume f f i c i e ncy . T yp ic al ly , th e number o f s tages in a reac t iontu rb ine i s high to keep th e e nth alp y drop low. In someins tances it i s d i f f i c u l t to keep the en tha lpy drop acrossa s ta ge w ith in the proper range fo r a r ea ct io n tu rb in e. Inthese cases , th e Rateau s tage and occas iona l ly even thecu r t i s s Wheel i s used to keep th e e f f i c i ency up. The useo f an impulse s tage in th e f i r s t s tage o f a nozzle governedhigh pressure tu rb ine i s w idesp read . In add i t ion , under ce r t a incondi t ions o f re he at in g, th e en tha lpy drop ac ross the f i r s ts tage of the low p re ss ur e tu rb in e may be qui t e l a rge andrequ i re an impulse s t age .

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VELOCITY RATIOVeloci ty ra t io i s the r a t i o o f blade t angen t i a l speed tosteam ve loc i ty and each s tage type has a d i f f e r en t ve loc i tyr a t i o a t which it runs most e f f i c i en t l y . Figure 1.17 showsthe re l a t ionsh ip between ve loc i ty ra t io and diagram e f f i c i ency .As blade wheels become l a rge r to accomodate the high volumesof steam in modern t u rb ine un i t s , the b lade ta n ge n tia l v elo c it yinc reases and the ve loc i ty ra t io inc reases . As a r e su l t ther eac t ion turb ine become more a t t r ac t i ve as the ve loc i ty ra t ioinc reases . Large turb ines and pa r t i cu l a r ly l a rge low pressuretu rb ines are commonly r ea ct io n t ur b in e s.

MOISTURE EFFECTSReact ion turb ines are more sens i t ive to the e f f ec t s ofwater drople ts decreas ing ef f ic iency by impact with the movingblades . Typical ly a 1/2 - 3/4% reduc t ion in s tage e f f i c i ency

fo r each 1% moisture i s encountered in an impUlse s tage . Thise f f e c t i s on the order o f 1 - 1-1/4% fo r each 1% mois tu re in ar ea ctio n s ta ge . In those t u rb ines which encounter wet steamcondi t ions such as the high pressure turb ine in a nuc lear un i t ,th i s fac t has an inf luence on turb ine design. One a l t e rna t i v ei s to make th e HP turb ine an impulse tu rb ine ; if, however, th eHP turb ine i s a r ea ct io n t ur bi ne the need to keep the moisturecontent low can be r ead i ly apprec ia ted .BLADE LEAKAGE

Since th e react ion t u rb ine produces a pressure drop acrossthe moving b lad es , th ere i s a tendency in th e r eac t ion t u rb inefo r th e steam to crawl over the end o f th e moving blades . Thise f f e c t can be qu i te pronounced in a high p re ss ur e t ur bi ne wherethe pressure drop pe r s tage can be f a i r l y high. This type o fleakage i s l e ss of a problem in the impulse s tage which makesi t s use in HP t u rb ines a t t r ac t i ve in minimizing th e movingblade leakage fac to r . In HP react ion tu rb ine s , a higher bladet ip leakage must be expected and usua l ly an increased numbero f s tages i s requi red to keep th e pressure drop pe r s tagereasonably low.TYPES OF GOVERNOR VALVES

There are two bas i c types of governor valves in widespreaduse: nozz le governor valves and t h ro t t l e governor va lves . Notonly i s the type o f governor valve i nd ica t ive o f the se rv icethe un i t was designed to see , but in addi t ion i s a determinerof the co ns tru ct io n of the high pressure tu rb ine .THROTTLE GOVERNORS

In t h r o t t l e governor v alves , th e steam flow to th e t u rb inepasses through a governor valve which con t ro l s steam flow tothe turb ine by t h ro t t l i ng th e steam and thereby con t ro l l ing the

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stearn pressure a t the i n l e t to th e high p re ss ur e tu rb in e . Whethert he re i s a s ing le governor valve o r seve ra l valves in pa r a l l e l ,the governor valves t h ro t t l e the steam flow equa l ly . At 25%t u rb ine f u l l power a l l the governor valves are pass ing 25% o ft h e i r design f low. At 50% o f f u l l power a l l are pass ing 50%o f t h e i r design flow and so on. The advantage o f t h ro t t l egoverning i s the s impl ic i ty o f con tro l and cons t ruc t ion . Thisi s pa r t i cu l a r l y t rue o f the stearn i n l e t to the f i r s t s tagenozzles s ince a l l o f the nozzles a re used a t a l l t imes with thegovernor valves regu la t ing steam flow through th e f i r s t s t age .

Pg

h

s

EFFECT OF THROTTLE GOVERNINGFigure 1.18

Figure 1.18 shows th e cond i t ion l i nes fo r a t h ro t t l egoverned t u rb ine a t various power l eve l s . In sp ec tio n o f t h i sdiagram read i ly shows the disadvantages o f t h ro t t l e governing.At power l eve l s below 100%, the re i s a l a rge pressure dropacross the t h ro t t l e governor valves . This r e su l t s in a l a rgein crea se in entropy and a corresponding decrease in a va ila ble- 22 -


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energy. s i n c e each kilogram o f steam does l e s s work a t lowpower, t h e low power steam consumption p e r k il o w a t t - h o u r i smuch g r e a t e r than a t high power. This i s c l e a r l y i n e f f i c i e n t .The r e s u l t i s t h a t t h r o t t l e governing i s seldom used on t u r b i n e st h a t a r e used f o r v a r i a b l e load s e r v i c e . A t h r o t t l e governedt u r b i n e i s designed t o o p e r a t e a t a n e a rl y c o n s t an t high powerl e v e l .NOZZLE GOVERNING

Nozzle governor valves a r e arranged as shown i n Figure 1.19and a r e opened s e q u e n t i a l l y e i t h e r s i n g l y o r i n p a i r s .

Stearn Chest With Bar L i f tNozzle Governing F i r s t StageNozzles Bypass ToSecond Stage

NOZZLE GOVERNINGFigure 1.19

As t h e nozzle block moves up the valves open s e q u e n t i a l l yand as a r e s u l t only one valve i s t h r o t t l i n g s team flow a t anyone t ime. This r e s u l t s i n a n e a rl y c o n s t a n t i n l e t p r e s s u r et o t h e f i r s t s t a g e and e l i m i n a t e s much of t h e adverse p r e s s u r edrop a s s o c i a t e d with t h r o t t l e governing a t low power l e v e l s .To p re ve nt t h e e f f e c t of t h e one valve which i s t h r o t t l i n gfrom e f f e c t i n g the p r e s s u r e a t the i n l e t o f t h e f i r s t s t a g eit i s necessary t o r e s o r t t o each valve a d m i t t i n g steam t o ad i f f e r e n t a r c on the f i r s t s t a g e nozzle r i n g as i s shown i nFigure 1 . 1 9 . This complicates t h e s t r u c t u r e o f the i n l e t t othe f i r s t s t a g e nozzles and makes t h e use o f a s i n g l e flowt u r b i n e v i r t u a l l y manditory w ith n oz zle governing.

To a p p r e c i a t e one o f t h e o t h e r design f e a t u r e s o f anozzle governed t u r b i n e , it i s necessary t o understand t h a tt h e flow o f steam through a t u rb in e s ta g e i s roughly p r o p o r t i o n a l t o t h e p r e s s u r e drop a c r o s s t h e s t a g e . This means- 23 -


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as the flow through a s tage i nc rea se s , the pressure drop acrossthe s tage must i nc rea se .

'".."lUl'"..p.

Turbine In le t Pressure- - - - - - - - - - - - - - - -

Steam Flow

VARIATION OF STAGEPRESSURE WITH FLOW

Figure 1.20

h Second StageIn le t Pressure

25%50%75%00%

S

EFFECT OF NOZZLEGOVERNING

F igure 1 . 21Since the ou t l e t pressure o f the turb ine i s re la tiv e lycons tan t , the i n l e t p re ssu re to each s ta ge in cre as es as the

power l eve l inc rea se s . However, the i n l e t pressure to thef i r s t s tage i s a t steam genera to r pressure and does no t change.Figure 1.20 shows the i n l e t pressure to the f i r s t and seconds tages o f a nozzle governed t u rb ine . At low power l eve l thepressure drop across the f i r s t s tage o f th e t u rb ine i s veryl a rge and the f i r s t s tage does a l a rge percen tage o f th et o t a l work o f the t u rb ine . For t h i s reason th e f i r s t s tagemust be e f f i c i en t with a l a rge enthalpy drop and i s usua l lyan impulse s t age . As th e power l eve l i n c reases th e pressuredrop across the f i r s t s tage decreases un t i l it may be so smal lt h a t the f i r s t s tage becomes unable to pass enough s team todevelop th e requ i red turb ine power. To increase the maximumpower of the tu rb ine when ef f ic iency i s o f a secondary importance ,the f i r s t s tage may be bypassed by some o f th e governor valvesto se nd ste am d i r ec t l y to the second s t age .

Fig ure 1 .2 1 shows th e c on dit io n l i n e s fo r a nozzle governedtu rb ine a t var ious power l eve l s . Even though th e nozzle governedt u rb ine has a higher steam consumpt ion (kg /kw-h r) a t low powerthan a t high power, it i s more e ff ic ie n t a t low power l eve l s thana t h ro t t l e governed t u rb ine .- 24 -


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Nozzle~ G O v e r n i n g

/ Throttle/ Governing

Load in KW

COMPARISON OF NOZZLEAND THROTTLING GOVERNING

Figure 1. 22

134.00-1

Figure 1 . 22 i s acomparison of steam ra te sfo r nozzle and t h ro t t l egoverning . The nozzlegoverned t u rb ine consumesl e ss steam pe r k i lowat t -hour fo r a l l power l eve l sup to th e maximum. Thenozzle governed tu rb inei s more e f f i c i en t as avar iab le load tu rb ine ;however, a base loadt u rb ine which seldom runsbelow f u l l power canobta in good ef f ic iencywith a much s implergover ning sys tem.

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1 3 4 . 0 0 - 1

ASSIGNMENT

1 . Expla in t h e e f f e c t s t h a t each o f t h e f o l l o w i n g have on t h ee f f i c i e n c y o f a t u r b i n e steam c y c l e :(a) e x c e s si v e m o is tu re i n t h e t u r b i n e .(b ) p r e s s u r e drop a c r o s s t h e i n l e t v a l v e s .(c) m o i s t u r e s e p a r a t o r .(d ) l i v e steam r e h e a t e r .(e) s u p e r h e a t i n g .( f ) r e g e n e r a t i v e f e e d h e a t in g .

2. Draw and e x p l a i n a c o n d i t i o n l i n e f o r a t y p i c a l l a r g eCANDU t u r b i n e u n i t having one HP t u r b i n e and t h r e e LPt u r b i n e s .

3. What a r e t h e problems i n v o l v e d i n b u i l d in g a g e n e r a t i n gs t a t i o n u t il i z i n g a Carnot c y c l e .4. Expla in why we u t i l i z e f e e d h e a t i n g i n o u r g e n e r a t i n gs t a t i o n s .5.

lPTurbine

LPTurbine

LPTurbine

The diagram above i s o f a f o u r c a s i n g t u r b i n e u n i t useda t a c o n v e n t i o n a l l y f i r e d , s u p e r h e a t e d steam g e n e r a t i n gs t a t i o n . Discuss p o s s i b l e r e a s o n s f o r :(a) s i n g l e flow high p r e s s u r e t u r b i n e .(b ) s i n g l e flow i n t er m e d i at e p r e s s u r e t u r b i n e .

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(c) double flow low pressure t u rb ine .(d ) cu r t i s s Wheel as f i r s t two s tages o f HP tu rb ine .(e) React ion s tages in LP tu rb ine .

The diagram above i s o f a four cas ing t u rb ine un i t useda t a nuclear fue l , sa tu ra ted steam genera t ing s t a t i on .Discuss poss ib l e reasons fo r :(a) double flow high pressure tu rb ine .(b ) double flow low pressure t u rb ine .(c) reac t ion s tages in LP tu rb ine .(d ) r ea ct io n s ta ge s in HP tu rb ine .

R.O. Schuelke

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Turbine, Generator & Auxi l i a r ies - Course 134TURBINE OPERATIONAL PERFORMANCE

The purpose o f any steam power p l an t i s to conver t hea tenergy r e leased by th e fue l in to genera tor e l e c t r i c a l ou tpu t .Under th e bes t circumstances t h i s process i s r a the r i n e f f i c i en t .In a t yp i ca l CANDU genera t ing s t a t i on something on the o rdero f 70% o f the hea t generated in the r e ac to r i s l o s t throughthe product ion of waste hea t and through i n t e rna l e l e c t r i c a lpower requirements . The ab i l i t y to produce higher t empera turesuperheated s team, a llows c onv en ti on al foss i l fue l steam p lan t sto be somewha t more e f f i c i en t bu t even in these p lan t s onlyabout 40% o f th e hea t energy i s conver ted to e l e c t r i c a l outputfrom th e g en era to r.Because the conversion of hea t energy to e l e c t r i c a lenergy i s a t bes t r a the r i n e f f i c i en t and because the conversionp ro ce ss in vo lv es a grea t po t en t i a l fo r a degrading o f event h i s e f f i c i ency , steam p l an t opera tors have long been concernedwith asse ss ing the amount of hea t energy requi red to produce

a ki lowa t t -hour o f e l e c t r i c a l output . The amount o f hea tenergy which must be produced in fue l burnup to produce aki lowat t -hour of e l e c t r i c a l outpu t i s known as the s t a t i onhea t ra te and the opera to r who saw an upward t rend in t h i ss t a t i on hea t ra te was concerned not only because o f wastedfue l do l l a rs but also because it ind ica ted something unpleasan twas happening to h is p lan t .

Theore t i ca l ly the computat ion o f a s t a t i on hea t ra te fo ra nuc lear genera t ing s t a t i on i s s impler than fo r a convent iona lp l an t . The reason i s t h a t no convent iona l p lan t opera to r knowsas much informat ion about h is hea t source as a nuc lear opera to rdoes. For reac to r con t ro l and r egu la t ion the prec i se powerl eve l in the reac to r must be known a t a l l t imes . For a reac to ropera t ing a t a cons tant power l eve l the number of k i lo jou leso f hea t ene rgy p roduced in an hour i s simply

KJ 6--h = (Thermal MW power) (3.6 x 10 KJ/MW-hr.) 2.1r .The s t a t i on hea t r a t e , SR, can, the re fo re be eas i lvcomputed by the fol lowing formula:

SR = KJKW-hr output 2.2I f a nucl ea r g ene ra ti ng p l an t produces 1743.5 MW o f thermalpower in the reac to r and 543 ~ v of e l e c t r i c a l ou tpu t , then thes t a t i on hea t ra te can eas i ly be computed as :

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SR =

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(Thermal MW power) (3 .6 x 10 6 KJ/MW-hr)Klv ou tpu t= (1743.5 MW) (3.6 x 10 6 KJ/MW-hr)(543,000 KW)= 11559.1 KJ/KW-hr

It does no t r equ i r e any pa r t i c u l a r i n s igh t to r e a l i z et h a t if t h i s va lue i n c r e a se s to 11 ,609 .1 KJ/KW-hr, o r by about.4%, th en th e p l an t i s ope ra t ing l e s s e f f i c i e n t l y , fue l i s be ingwas ted , and p l an t components a re b ein g tax ed unnece s sa r i l y .

The problem comes in th e f an t a s t i c a l l y i nexpens ive c o s to f nuc l e a r f ue l . The 1976 cos t o f CANDU nuc lea r f ue l i s onlyabout $ .07 to $ .12 per 1 ,000 ,000 KJ. This should be comparedto $ .60 to $ .90 fo r coa l and $1.80 to $2.20 fo r o i l . Althought h ese va lues wi l l undoubted ly change r ap i d l y , th e r e l a t i v es t and ing shou ld remain n ea rly c on sta nt . Thus over a yea r , th ei n e f f i c i e ncy desc r ibed above would cos t th e o i l f i r ed s t a t i onabout $450,000 , th e coa l f i r ed s t a t i o n abou t $ 17 0,00 0 an dth e n n c ~ e a r s t a t i on anont . ~ 2 ~ , ~ ~ ~ . It s n o u ~ a cOllie a s nosu rp r i s e t h a t you can g e t a l o t more exc i t ed about a .4%inc rease in s t a t i on hea t r a t e a t a conven t i ona l gene ra t i ngs t a t i on than a t a nuc l e a r gene ra t ing s t a t i o n .

The obvious economic conc lus ion o f t h i s i s t h a t basedon fue l co s t s you cannot j u s t i f y th e expense o f va s t sums o fmoney t r ack ing down i n e f f i c i e n c i e s in a nuc lea r gene ra t i ngs t a t i o n . On th e o the r hand t h e r e i s ano the r im p lic at io n to ani n c r e a s i ng hea t r a t e . I f the s t a t i on hea t r a t e i s i n c r e a s i ngt hen someth ing i s no t working as it shou ld . As th e e f f i c i e ncyo f th e p l a n t decreases due to a g radua l de t e r i o r a t i on o fcomponent c ap ab i l i t i e s , th e p robab i l i t y o f a fo rced outagei n c r e a s e s . In th e even t th e p l a n t must be shutdown fo r r equ i r edmain tenance , th e cos t o f a l t e rna t e energy sources to r ep laceth e megawat t hour s o f lo s t e le c t r ic a l ou tpu t can be s t agge r i ng .At th e t ime o f t h i s wri t ing th e e st im a te d co s t fo r a l t e r n a t eenergy fo r a Picke r ing s i ze n u cl ea r g e ne ra ti ng s t a t i on i s onth e o rde r o f $5,000 pe r hour . This f igure depends on th e cos td i f f e r e n t i a l between conven t i ona l and nuc l e a r f ue l bu t it isdoub t f u l it w i l l decrease ove r th e f ore se ea ble fu t u r e .

The t yp i ca l nuc l e a r s t a t i o n i s thus faced wi th a delemma,th e horns o f which a re a very marg ina l r a t e o f re tu rn ondo l l a r s inves ted fo r improvement in p la n t e f f ic ie n cy and a t r u l yt remendous co s t fo r an unplanned p l an t outage . It should bereasonab ly obvious t h a t th e t r a deo f f in do l l a r s between t h esetw o extremes i s a major cha l l enge .

The p r a c t i c e o f assess ing steam p l a n t ope r a t i ona l performanceas a method o f de te rmin ing th e cond i t ion o f the un i t i s fundamentalto le ng th in g th e t ime be tw een ma jo r o ve rh au ls w h ile a vo id in g afo rced outage . However t h e r e a re seve ra l f a c t o r s which compl ica teth e assessment o f th e cond i t ion o f an opera t i ng t u r b ine un i twhich dese rve some a t t e n t i o n .


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Accuracy an d Adequ acy o f I ns tr umenta tio nOnce the ope ra to r has de tec ted a t ren d o f in crea s in g s t a t i ohea t r a t e he i s faced w ith th e dual problems o f f i r s t de t e r mining p rec i s e ly where th e problem l i e s and second decid ing

whether th e problem j u s t i f i e s th e expendi tu re o f down t ime andmaintenance do l la r s to co r re c t it. The ab i l i t y to t r a ce ade t e r i o r a t i ng hea t r a t e to i t s ul t ima te cause requ i re s th eab i l i t y to determine accu ra te ly th e a ctu al cond i t ions a tvar ious loca t ions . This i s of t en d i f f i c u l t and occas iona l lyimpo ss ib le u si ng ex i s t i ng i n s t rumen t a t i on . For example, toassess the con dit ion o f th e t u rb ine un i t it i s des i rab leto determine th e hea t energy de l ive red to th e un i t pe rk il ow a tt -h o ur o f e l e c t r i c a l ou tpu t . The Turbine Heat Rate(THR) can be determined by th e formula:

MI (h - h f ) + M2 (h 2 - hI)THR ,,; s w 2 3KWwhere: MI = s team flow through th e s top valve (KJ/hr)h = s team en tha lpy a t th e s top valve (KJ/Kg)h S = en tha lpy o f f i n a l feedwate r (KJ/Kg)M ~ w = s team flow through r ehea t e r (Kg/hr)h 2 = s team en tha lpy from r ehea t e r (KJ/Kg)hI = s team en tha lpy to reh ea te r (KJ/Kg)= t o t a l e l e c t r i c a l outpu t in KW

In a na ly zin g t h i s t h eo r et ic a ll y r at he r s imple formula itbecomes obvious t h a t th e a cc ur ate d ete rm in atio n o f t u rb inehea t r a t e us ing t h i s formula requ i re s th e s team flow a ttwo poin t s as wel l as four s team e nt ha lp ie s t hr ee o f whichrequ i re s team qua l i t y dete rmina t ions . Before applying t h i sformula these quan t i t i e s must be known with s u f f i c i e n t accuracyto y ie ld an accura te t u rb ine hea t r a t e . I f th e s team flowscan only be determined wi th in 5%, then th e p robab i l i t y o fco r r ec t l y de tec t ing changing un i t e f f i c i ency i s r a t he rdoub t fu l . While n e ce ss ar y p ar ame te rs can of t en be es t imatedo r der ived from othe r parameters , t h i s i s a t be s t of t end i f f i c u l t and the i n s t a l l a t i on o f permanent o r por t ab lea c cu ra te i ns tr ument at io n may wel l be th e only way to assessun i t performance p rope r l y .R e p ro d uc ib il ity o f I n i t i a l Condi t ions

Although a va r i e ty o f c on clu sio ns can be drawn from anin cr ea sin g n et s t a t i on hea t r a t e , one has to be ca r e fu l not tobe chas ing a wi l l -o ' - t h e -w i sp . A l a rge number o f fac to rs cane f f e c t th e n e t s t a t i on hea t r a t e which have little o r noth ingto do with a de t e r i o r a t i on o f the tu rb ine o r s team/feedwatersystem performance . Whenever a hea t r a t e i s conducted it i sabsolute ly e s sen t i a l t h a t uniform initial cond i t ions beu t i l i z e d on which to base compar isons . Normal va r i a t i on s incondenser vacuum, makeup water f low, s team pre ssure , s teamgene ra to r le ve l, a d ju ste r ro d motio n, xenon inven to rv ,

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genera tor hydrogen pur i ty and hea t t ranspor t temperature can r e su l tin hours of searching fo r nonex i s t en t problems.The method o f turb ine un i t ana lys i s which can y ie ld th emost product ive r e su l t s i s based on i n i t i a l condi t ions which arereasonably easy to reproduce. Ful l power with the steam andfeedwater system in a "normal l ineup" wi l l probably r e su l t inthe fewest induced e r ro r s . In a dd it io n th ese c on ditio ns should be

h eld c on sta nt fo r some t ime i n t e rva l - say f ive o r ten minutes- p rio r to t ak ing th e hea t ra t e da ta .The fol lowing i s a pa r t i a l list o f condi t ions which shouldbe met p r i o r to conducting a hea t ra te dete rmina t ion .(1 ) genera tor producing approximatl ly 100% of r a ted grossoutput(2 ) steam flow from steam genera tors equa l(3 ) feed flow to steam genera tors equal(4 ) xenon a t 100% ~ q u i l i b r i u m(5) steam genera tor l eve l s cons tant(6) condenser vacuum a t some re fe rence l eve l(7) reac to r r e ac t iv i t y a t equ il ib r ium condi ti on(8 ) genera tor hydrogen puri ty a t some re fe rence l eve l(9) hea t t ranspor t temperature cons tant

(10) steam p re ss ur e c on st an t(11) no steam genera tor blowdown in progress(12) no steam flow to bulk steam p lan t or through r e j e c tsystemThe precise i n i t i a l condi t ions and acceptable range of parameterEshould be spec i f i ed fo r a ne t s t a t i on hea t r a t e . In add i t ion , ifthese condi t ions cannot be met the qua l i ty of the hea t ra te w il l bedowngraded.

Frequency of Heat Rate Determinat ionA deta i led determinat ion of hea t ra te s on the var ious comp-onents of a generat ing s ta t ion i s the most accura te method ofmeasuring turb ine un i t performance and a ss es sin g th e properopera t ion of the turb ine and i t s aux i l i a r i e s . However, t h i s can beexpensive in both t ime and manpower. For th i s reason it i s


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genera l ly considered good prac t i ce to ca rry ou t such t e s t sonly once per year and during pre - and post -overhaul t e s t s .On the o ther hand, when hea t ra te s are computed in f requen t lyit i s l ik e ly th at comparabi l i ty of r e su l t s su f f e r s . Whatprobably represents the bes t compromise i s f requent ca l cu la l ions of ne t s t a t i on hea t ra te using equat ion 3 .1 toi nsu re rep roduc ib i l i ty o f r e su l t s and to de tec t any majorchanges , and to conduct a de ta i l ed hea t r a te a t annuali n t e rva l s . The ne t s t a t i on hea t ra te conducted a t th e t imeo f the de ta i l ed ana lys i s cou ld then be used to check reproduc i b i l i t y .Turbine Heat Rate

Once a change has occurred in s t a t i on hea t r a t e , itbecomes necessary to make a dete rmina t ion o f the impl i ca t ionof the change. General ly t h i s impl ica t ion can f a l l i n to th efo l lowing four ca tegor ies :

(a) i n s ign i f i c an t(b) co r rec t th e problem withou t shu t t ing down thetu rb ine(c) co r rec t th e problem inc luding shu t t ing down theturb ine(d) cor rec t th e problem dur ing the next scheduledturb ine outage .

The c l a s s i f i ca t i on in to one of these categor ies can log ica l lyoccur on ly by knowing what i s caus ing th e change in hea t r a t e .Since s t a t i on hea t ra te i s ef fec ted by a wide var i e ty o fth ings from th e reac to r to the g en era to r, th e problem becomesone o f loca l iza t ion of cause .Turbine hea t ra te i s def ined as the number o f Kilojoulespe r hour del ivered to the turb ine un i t pe r Kilowat t ofgenera tor e l e c t r i c a l outpu t . As such it i s sen s i t i v e onlyto changes occur ing in the s teamj feedwater system and i sindependant o f problems associa ted with the r e ac to r , hea tt ranspor t system and steam gene ra to rs .

THR =Turbine hea t

MI (h sra te i s computed from equat ion 2 .3 .- h fw ) + M2 (h 2 - hI)

KW

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where: = steam flow through th e s top va lve (Kg/hr)= steam entha lpy a t th e s top valve (KJ/Kg)= enthalpy o f f i na l feedwater (KJ/Kg)= steam flow through r ehea te r (Kg/hr)= steam enthalpy from r ehea t e r (KJ/Kg)= steam enthalpy to reh ea te r (KJ/Kg)= t o t a l e l e c t r i c a l output in KWRefer r ing to th e diagram 2 .1 you wi l l not ice t h a t TurbineHeat Rate would be computed as :

THR = (108 0) (3600) (2793. 39 - 72 6 2 3 ) + (8 8 352) (3600) (2793. 39 - 11790699=

=

8.0645 X 10 9 + .5373 X 10 97906998.6018 x 10 9790699

= 10878.7 KJ/KW-hrIn l i eu of equat ion 2.3 tu rb ine hea t ra te can be computed fromthe fol lowing:

2.4KW

where: M3hM ~hM ~ wh rhdKW

= stearn flow from stearn genera tor (Kg/hr)= stearn entha lpy from steam genera tor (KJ/Kg)= feedwater flow (Kg/hr)= entha lpy of f i na l feedwater (KJ/kg)= r eh ea te r d ra in flow (Kg/hr)= enthalpy of r ehea t e r dra ins (KJ/Kg)= t o t a l e l ec r i ca l output in KW.

The choice ' o f equat ion 2.3 o r 2.4 wi l l depend l a rge ly on theaccuracy to which the parameters used in the equa t ions canbe determined. In e i t he r case , the accu ra te determinat ion o fflow r a t e s i s probably th e most l im i tin g f ac to r. From anopera t iona l s t andpo in t equat ion 2.4 i s probably the mostconvenient and can be fu r the r s impl i f i ed by making someassumptions .

2.5HR =

Since l i qu id flow can genera l ly be determined moreaccurate ly than vapor flow it i s of ten eas ie r to approximateM3 as th e sum of M4 and Ms. In add i t ion s ince the entha lpyo f sa tu ra ted steam a t steam gene ra to r pres su re , it can of tenbe t rea ted as a cons tan t Equat ion 2.4 , t he re fo re , becomes:

M4(hs - h fw ) + Ms(h s - h rhd )

where: h sKl']

= steam enthalpy of sa tu ra ted steam a t th edesign steam genera tor p re ssure . (KJ/Kg).-


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Since th e major i ty o f th e hea t energy l os s (roughly 90%)occurs in th e s tearn /feedwater system, a change in tu rb ine hea tra te i s r e f l e c t ed v i r tua l ly one to one in s t a t i on hea t r a t e .This means if tu rb in e h ea t r a t e inc reases by;;5% we wouldexpect an inc rease o f about .5% in s ta t ion hea t r a t e .

Once th e cause fo r inc reas ing s t a t i on hea t r a t e has beent racked to th e s team/feedwater system, th e problem i s "simply"one o f t racking down the pa r t i cu l a r offend ing component.Condenser BackpressureI f the condenser back pressure inc reases , the tu rb in eoutpu t wi l l decrease and tuvb ine hea t ra te w il l inc rease .

Figure 2.2 shows t h i s e f f e c t . The r e su l t has such a g rea timpact on tu rb in e h ea t r a t e , t h a t two otherwise comparabletu rb in e h ea t r a t e determinat ions with d i f fe r ing condenservacuums wi l l y ie ld widely d i f f e r en t r e su l t s .At a cons tan t power l eve l an i nc rea se in condenser backpre ssure can be caused by only four th ings :(a) inc rease in the average t empera tu re of the condensercool ing water in th e tubes

(b) f looding of th e condenser tube surfaces due to highhotwel l l eve l(c) a i r leakage in to the s he l l of the condenser"(d ) a decrease in th e ove r a l l hea t t ra n s fe r c o ef fi cie n to f th e tubes .The cool ing water temperature wi l l vary considerablybetween summer and winte r . Under normal cond i t ions , however,the t empera tu re r i se across the condenser i s reasonably cons t a n t . This can be seen from th e formula e xp re ss in g th e hea t

r e jec ted to th e condenser c oo lin g wa te r:Q = m Cp liT 2 .5where: Q = th e h ea t re je cte d to th e ccw (KJ/sec)m = ccw f low (Kg/sec)Cp = th e co ns tan t pressure spec i f i c hea t capac i tyof water (KJ/KgOC)liT = th e temperature r i se in ccw across th e con-denser (OC)


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2 4 6 8 10 12 14 16CWo INLET TEMPERATURE IN C

6.51.9

G < O ' " , ~ 1.8Ii'.


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I f th e r a t e a t which hea t i s re j ec t ed and th e cew flow r a t eremain cons tan t , th e ccw ~ T wi l l remain cons t an t . At a cons t a n t gene ra to r power, th e r a t e a t which hea t i s r e j ec t ed toth e ccw i s reasonab ly cons tan t fo r smal l changes in condense rvacuum. This impl ies t h a t if vacuum i s decreas ing and condense r ~ T i s r emain ing cons t an t , then th e cause i s poss ib lyan i n c rease in th e t empera tu re o f wate r e nte rin g th e ccwsystem from th e l ake . This can be eas i l y checked by moni tor ingccw i n l e t t empera tu re .

A change in l ake wa te r tempe ra tu re usua l ly occurs due toseasonal o r d iu rna l f luc tua t ions . Occas ional ly , however , ahigh wind has produced a su f f i c i e n t cu r r en t to r e t u rn th e wate ra t th e condense r ou t l e t back in to th e ccw in take with ar e su l t i ng ra pid in cr ea se in ccw t empera tu re .

Other fac to rs which i n c rease th e average t empera tu re o fth e condenser cool ing wate r in the t ubes a re u su a lly a ss o ci ate dwith a decrease in ccw flow from such causes as cav i t a t i on o ra i r bind ing o f the ccw pumps, marine growth in th e condensert ubes , sand in th e condense r t ubes , b lo ck age of th e screenso r t ubes with deb r i s , and a i r bind ing o f the t ubes o r wate rboxes.

Returning to equat ion 2.5 you wi l l not i ce t h a t if th e ccwf low r a t e decreases whi le a n ea rly c on sta nt ra t e o f h ea tr e j ec t i on i s mainta ined then th e ccw ~ T wi l l i nc rea se .~oQ + t= m Cp ~ T

The impl ica t ion i s t h a t if vacuum i s decreas ing and ccw ~ T i sincreas ing then t he re i s a decrease in ccw f low. A dec reasein ccw flow can be pa r t i cu l a r l y t roublesome if it r e su l t s ina pa r t i a l blockage o f some tubes . This wi l l in cre ase th eccw flow th rough th e remaining c l e a r tubes wi th pos s ib l e tubecav i t a t i on and f a i l u re r e su l t i ng .A ir in leakage to th e condenser wi l l lower th e condenservacuum ( increase t he b a ck p re ss ur e) . However, s ince th e increasein backpressure i s a t t r i bu t ab l e to a i r pressure as opposed tosteam pre ssure , th e t empera tu re of th e condensing steam(condensate tempera ture) wi l l not r i s e app rec i ab ly as thebackpressure i nc rea se s .A good measure o f th e t i gh t ne s s o f th e condense r andas soc i a t ed subatmospher ic sys tems i s th e disso lved O2 l e ve l inth e condensa te leav ing the ho tw ell . I f t h i s l eve l i s r i s i ngthen rega rd le s s of what i s happening to condense r vacuum a i r i s

ge t t i ng i n to th e sys tem. This add i t i ona l a i r ingress can occure i t h e r from increased l eaks o r decreased a i r ex t rac t ioncapab i l i t y . In e i t h e r case th e cause of the problem must be10 -


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found and cor rec t ed no t only to e l imina te th e i n e f f i c i enc i e scaused by increased backpressure bu t because th e long terme f f e c t s of g en era l and loca l i zed corros ion on the condensate ,feedwater and steam genera tor wi l l eventua l ly produce s ign i -f i c an t problems.I f th e ove ra l l hea t t r ans f e r coe f f i c i en t o f the tubesdecreases due to cor ros ion , sca lin g o r fou l ing of the tubesu r faces , th e e f f e c t i s qu i t e s im ila r to a i r ing res s . Condenser Cooling Water ~ T remains cons tan t and backpressureinc reases . However, s in ce the in crea se in backpressure i sa t t r i bu t ab l e to steam pressure as opposed to a i r pres su re , th econdensate temperature wi l l r i se corresponding to th e sa tu ra t iontemperature fo r the condenser backpressure . In add i t ion , thecondensate dissolved O2 wi l l not change if th e cause of th evacuum decrease i s not a i r .The below t ab l e summarizes the type of parameter changesfo r var ious condi t ions a f fec t ing vacuum:

CAUSES OF POOR PERFORMANCE OF CONDENSER

SATURATIONTEMPERATURE

CCW CORRESPOND- CONDEN-BACK- ING TO BACK- SATE

INLET OUTLET toT PRESSURE PRESSURE TEMP.SITUATION C C C KPa(a) C C

NormalOperation 15 .0 25.0 10.0 4.50 31. 0 31.0Decrease inccw Flow 15.0 30.0 15.0 5.17 33.5 33.5Increase inccw I n l e tTemp. 20.0 30.0 10.0 5 .94 36.0 36.0Air Leak-age 15.0 25.0 10.0 6.27 37.0 31. 5Tube Sur-face Foul-ing 15.0 25.0 10.0 1 5.17 3.3 .5 33.5

Feedwater Heat ing SystemThe feedwater hea t ing system can be the source o f as ign i f i c an t inc rease in turb ine hea t r a t e . Turbine hea t r a t ei s pa r t i cu l a r ly s en si t iv e to f i n a l feedwater temperature and

a 3C change in f i n a l feedwater temperature can a f f e c tturb ine hea t ra te by as much as .2%. General ly th e performance

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o f th e feedheat ing system can be m onitored by watching:(a) the f i na l temperature o f feedwater leaving eachfeedhea te r , and(b ) the t e rmina l t empera tu re di f fe rence between ex t rac t ion

steam and feedwater le av in g th e hea t e r .I f these parameters remain c lose to those o f the design hea tbalance then t he re cannot be too much wrong with th e sys tem.

When one feedhea te r does not r a i s e the feedwater a t th eou t l e t to the design value , then th e nex t feedhea te r wi l lrequ i re more ex t rac t ion steam if t h i s t empera tu re lo s s i s tobe rega ined . Because t h i s a d di ti on a l e x tr ac ti on steam i st aken from a poin t nea re r the steam genera tor , energy whichi s normally u t i l i z ed in th e tu rb in e i s bled o ff as ex t rac t ions team. For a cons tan t load outpu t under these condi t ions ,add i t iona l steam to th e high pressure t u rb ine i s requ i redand t u rb ine hea t ra te i nc rea se s .

The fo l lowing problems a re l i ke ly to encompass them ajo ri ty o f feedheat ing system de f i c i enc i e s :(al Long term contaminat ion o f f ee dh ea tin g su r faces .This can occur due to o i l ingress from thetu rb ine o r bu ildup o f c o rr os io n p ro duc ts .(b ) Extrac t ion steam valves no t fu l ly open.(c) In su f f i c i en t ven t ing of th e fe ed he ate r s he l l . Thiscan be caused by fu l ly o r par t i a l l y shu t valves inth e ven t l i ne s .(d ) Increased l eve l in th e fe ed he ate r she l l . This f loodsou t some o f the tubes and reduces the hea t t r an s fe rarea .(e ) Tube blockage due to fo re ign mate r i a l in th e feed-l i ne s .( f l Changes in e xtra ct io n steam p ressu re o r qua l i ty dueto problems in th e tu rb ines . I f th e en tha lpy o f theex t r ac t i on steam decreases then th e flow o f ex t r ac t -

ion steam wi l l i nc rea se .By ca r e fu l l y analyz ing th e feedwater 6T and 6p, th e t e rmina lt empera tu re d i f fe rence , th e fe ed he ate r she l l pre ssure , andth e she l l dra in temperature and comparing these parameterswith design values , the cause o f th e d ef ic ie nc y in feedhea te rperformance can be l o ca l i z ed .

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TUrb i r l ' e In te rna l Ef:fi:ciencyThe p rim ary c au se s o f a reduc t ion in i n t e rna l e f f i c i encya re :(a) chemical depos i t ion on tu rb ine blades ,(b) i nc rea se in blade t i p c learances due to e ros ion o rp hy sic al c on ta ct between f ixed and moving pa r t s ,(c) changes in blade su r face .Because o f wet steam cond i t ions in th e high pre ssuretu rb ine , low steam g en er ato r p re ss u re (and, t h e re fo re ,t empera tu re) , and th e s h i f t to vo l a t i l e steam gene ra to r

chemis t ry , chemical depos i t ion on t u rb ine blad ing i s no tf requent ly a cause of lo ss o f tu rb in e e f f i c i ency on nuc learsteam t u rb ine s . There have been cases of chemical corros ionof blad ing due to s team genera tor car ryover in p lan t s us ingso l i d steam genera tor chemistry and, t h e r e fo r e , th e e f f e c t so f p oss ib le chemical a t tack cannot be complete ly ignored .

Tip rubbing can b e m in im ize d by ca r e fu l adherence torun-up and load ing procedures and avoidance o f cond i t ionsl i ke ly to produce excess ive d i f f e r en t i a l ax i a l and r ad i a lexpansion between th e cas ing and ro t o r . Contro l o f excess iveblade eros ion due to wet steam o r s tand ing water cond i t ionsi s l a rge ly a des ign problem r e l a t ed to adequate mois tu reremoval from each s tage . However, e r ro r s in des ign suchas improper s iz ing o f s tage dra ins and inadequate ab i l i t y toremove mois tu re from moving blades can cause s ign i f i c an tdecreases in e ff ic ie nc y due to e ros ion .with th e wet steam cond i t ions which ex i s t in a nuc lea rs team t u rb ine , th e b lades wi l l gradua l ly be eroded due to

mois tu re impengement. This damage i s usua l ly most severe inth e high pressure t u rb ine and l a t t e r s t ages of th e low pre ssuretu rb ine and usua l ly f i r s t a f f ec t s the t r a i l i ng edge of th e f r on ts ide of f ixed blades and th e l ead ing edge o f the back s ideo f moving blades . The r e su l t o f t h i s eros ion i s t h a t th epro f i l e and su r face o f the blade wi l l change w ith tim e. I fth e wear becomes ex tens ive , th e b lades may change to th eex t en t t ha t s tage e f f i c i ency i s reduced. It i s very d i f f i c u l tto de t ec t such blade wear withou t shu t t ing down th e tu rb in eand examining it i n t e rna l ly . However, ca r e fu l observa t ion o fth e pre ssure drops across a s tage o r group o f s tages maymake th e change apparen t .

It i s more l i ke ly in p rac t i c e t h a t if th e blade wear i ssuch t ha t t h e r e i s a no ti ce ab le in c re as e in steam consumption ,it wi l l show up in th e form o f excess ive v ib ra t ion due to theou t -o f -ba lance of the blade wheels .

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Steam Genera to r - Water ChemistryRemoval o f impur i t i e s in th e steam genera to rs can havean e f f e c t on s t a t i on hea t ra te because hot water l o s t throughblowdown must be rep laced through cold makeup water . I fthe amount o f blowdown i s s ign i f i c an t , t he re may be a no t ice ab le e f f e c t on s t a t i on hea t r a t e , however , th is e ff e c t i s

fa r ou t weighed by th e consequences o f running with ou t -o f spec i f i c a t i on steam genera to r chemis t ry .The long term e f f e c t on hea t r a t e through tube fou l ing ,t u rb ine blade depos i t s o r d era tin g fa r exceeds the advantagegained by minimizing blowdown. Each ou t -o f - spec i f i c a t i oncond i t ion i s s i gn i f i c an t and the ca use s ho uld be rap id lyc orre cted to av oid b oth th e sho r t and long term e f f e c t s .

Gland S te am Consump tionProblems in th e gland s ea l system are usua l ly not

s u f f i c i e n t to cause a no tic ea b le i nc re as e in t u rb ine hea tr a t e . Since only about.08% o f the steam flow from the steamgenera to rs i s used to se a l th e tu rb in e glands , the e f f e c t onhea t ra te i s minimal. However, steam flow to th e glands i sa good i nd i ca to r o f the bas i c cond i t ion o f the gland and fo rt h i s reason can be valuab le in diagnos i s o f gland problems.Dete rio ra tio n o f tu rb in e la by rin th glands usua l ly occursfrom thermal bending o f the sha f t o r r ad i a l rubbing in th eglands dur ing s t a r t up . These problems can cause s i gn i f i c an tvib ra t ions on s t a r tup bu t tend to become s e l f l imi t ing as th eun i t speed i s increased above th e c r i t i c a l speed and the un i twarms up. This i s pa r t i cu l a r ly t rue o f r ad i a l rubbing andas a r e s u l t gland de te r io ra t ion can occur withou t th e ope ra to rbeing fu l ly aware o f th e problem.The problems can be l a rge ly e l imina ted by:(a) co r r ec t ope ra to r i n t e rp re t a t i on o f v ib ra t ion ons t a r t up ,(b ) co r r ec t s team to ro to r and steam to cas ing d i f f e r en t i a l t empera tu res ,(c) avoidance o f low bear ing o i l temperature ,(d) avoidance o f prolonged low speed ope ra t ion , and(e) proper a l ignment o f r o to r and cas ing during overhau l .Providing th e t u rb ine un i t i s reasonably f ree o f a i rl e aks , th e l eve l o f dissolved oxygen in th e condensate , th el eng th o f t ime to draw a vacuum, and th e length o f t ime to

lose vacuum when th e a i r ex t rac t ion system i s shutdown a regood measures o f th e cond i t ion o f the gland.14 -


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Derat ingDerat ing o f a g e n e r a t i n g s t a t i o n i s t h e process o fr e s t r i c t i n g g e n e r a t o r o u t p u t below f u l l power because of someabnormali ty i n t h e system. Because t h e g e n e r at i n g s t a t i o n i s

forced t o run a t l e s s than i t s design c a p a b i l i t y , d e r a t i n gcan have a s i g n i f ic a n t e f fe c t on s t a t i o n h e a t r a t e .The u l t i m a t e d e r a t i n g occurs when t h e h e a t source systemi s no longer capable o f producing s a f e l y t h e number o f k i l o -j o u l e s per hour r e q ui r e d t o produce t h e g e n e r at o r design o u t p u t .I n t h e case o f a r e a c t o r t h e r e i s an a b s o l u t e upper l i m i t t opower o u t p u t . Although t h e g e n e r a t i n g s t a t i o n i s designedt o allow some i n c r e a s e i n s t a t i o n h e a t r a t e b e f o r e reachingt h i s upper l i m i t , a d e c r e a s i n g s t a t i o n e f f i c i e n c y w i l le v e n t u a l l y reach the p o i n t where the r e a c t o r p l a n t reachesi t s l i m i t b e f o r e t h e g e n e r a to r g e t s t o 100% o f i t s des igno u t p u t .When t h i s occurs t h e r e a r e only two p o s s i b l e a l t e r n a t i v e s :(a) i n c r e a s e t h e design c a p a b i l i t y of t h e r e a c t o r , andthereby allow the r e a c t o r t o produce more powerby lowering the s a f e t y margin. T h i s , of c o u r s e ,would r eq u ir e c o n s u lt at io n with t h e r e a c t o r d e s i g n e rand with t h e AECB t o o b t a i n consensus t h a t t h e r e a c t o rp l a n t was overdesigned i n t h e f i r s t p l a c e .(b ) f i n d and c o r r e c t t h e cause o f t h e i n c r e a s i n g s t a t i o nh e a t r a t e so t h a t t h e r e a c t o r can once more produce

design g e n e r a to r o u t p u t w i t h in design r e a c t o rs p e c i f i c a t i o n s .Dera t ings o f a more temporary n a t u r e may occur when t h eg e n e r a t i n g s t a t i o n cannot be s a f e l y o p e r a t e d a t 100% o f design

g e n e r a t or o u tp u t . While the problems i n t h e c o n v e n t i o n a l endo f a n u c l e a r s t a t i o n which may r e s u l t i n d e r a ti n g a re almoste n d l e s s , t h e fol lowing have been o c c a s i o n a l sources o f d e r a t i n g s .Condenser C i r c u l a t i n g Water ~ T

To l i m i t a l g a e growth i n the v i c i n i t y o f t h e o u t f l o w ,environmental a u t h o r i t i e s have imposed a l i m i t a t i o n o f 10Con t h e temperature d i f f e r e n t i a l of t h e CCW a c r o s s t h e condenser .I n a b i l i t y t o meet t h i s l i m i t a t f u l l power n e c e s s i t a t e s ad e r a t i n g u n t i l t h e temperature r i s e i s w it h i n t h e l i m i t . I nt h e case o f some o l d e r s t a t i o n s , t h e l i m i t was imposed a f t e rt h e s t a t i o n was b u i l t and has r e su lt e d i n what amounts t o p e r -manent d e r a t i n g .

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Genera tor Hydrogen Pres su reLoss o f hydrogen from th e g en er ato r r e su lt s in a decreasein e ffe ct iv

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