predicting steam turbine performance

8/12/2019 Predicting Steam Turbine Performance

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PREDICTING STE M TURBINE PERFORM NCEJames T. Harr iz , EITWater land , Viar Assoc ia t es , Inc .Wilmington, Delaware

BSTR CTTracking the performance of ext rac t ion, backpressure and condensing steam turbines is a cruc ia lpart of minimising energy and maintenance costs forlarge process indust r ies . A thorough unders tandingof key equipment performance character is t icspromotes economical in-house power generation.Proper Scheduling of of maintenance reducesdowntime. Rankine Efficiency as a function ofsteam flow can be accurately predicted from severalsources of avai lab le data . Sample analysis ofpredicted performance data (Willans Line, flowversus ext rac t ion enthalpy, flow versus exhausttemperature and flow versus used energy) and t e s tdata are presented. Techniques for derivingeff ic iency curves from each source are described.These techniques can be applied d i r ec t ly to anysteam turbine r e l i abi l i ty s tudy e f fo r t .

INTRODUCTIONs the cost of energy resources continues to r i se ,

so does the incentive to produce mechanical ande lec t r ica l power while meeting process steam loadrequirements. Cogeneration or iginal ly arose from aneed for re l iab le power, before the pub l ic ut i tysystem could support large indust r ia l loads. Whilethere is s t i l l a need for the generat ing capacityof private indus t ry , th is need is being met withincreasingly expensive fuel . With the introductionof PURP regula t ions , addit ional incentive has beenplaced on the pr ivate sec tor to produce and se l le lec t r ic i ty . Even without going into the e lec t r icbusiness , private industry will keep operatingcos ts down by producing prime shaf t power witheff ic ient topping turbines to reduce purchases ofpub 1ic power.The key to economical power production (e lec t r ica lor mechanical) is to employ e f f ic ien t turbines , andmaintain them at an acceptable performancestandard. Since equipment tends to degrade withtime, procedures must be established to monitorperformance. Over a period of t ime, the degree ofdegradation can be observed and main tenance can bescheduled on a convenient or as-necessary basis ,and the resul ts of maintenance can be verif ied .There are several approaches to predic t ing andver i fy ing the performance of steam turb ines , justas there are several dif ferent types of tu rb ines .However, a l l machines are governed by the samef i r s t and second laws of thermodynamics, and can bet reated in fa i r ly standard form.

N LYSISFive separate sources of information may beavai lab le to the engineer to begin turbineperformance ana lys i s .1. Equipment data sheets2. Wi11ans l ine or ext rac t ion curves3. Exhaust or ext rac t ion steam Enthalpy curves4 . Exhaust or ext rac t ion Used Energy curves5. Actual performance t e s t dataOf these five sources , the f i r s t four come from theturbine manufacturer. While a l l of these are notprovided with al l equipment, some (and sometimessevera l ) usual ly are . Scrounge around the archivesa b i t and see what turns up.The object ive is to predic t as-new expansioneff ic iency for a machine under the conditions i t isac tua l ly subject to, for comparison to on-si te t e s tdata . The fol lowing examples demonstrate theconvers ion of raw data in to usefu l information.1. EQUIPMENT D T SHEET - - For equipment rated 200BHP or less , the only manufacturer ' s data of tenavai lab le is a Data Sheet . (Figure 1)

Figure 1 .This catagory of equipment is generally designedfor on/off operation to support pumping loads(cooling water, condensate , and product t r ans fe r ) ,or draf t fans and a i r blowers for boi lers orprocess.

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Proceedings from the Seventh National Industrial Energy Technology Conference, Houston, TX, May 12-15, 1985


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In add i t ion to const ruc t ion d e t a i l s , t he Da ta Sheet This ef f i c iency value represents expected ~ s n w inc ludes the fo l lowing des ign informat ion: p er fo rm an ce u nd er design condi t ions . I f any

machine ever s ee s d e si gn condi t ions , i t i s , rare , 0 Steam Condit ions t h r o t t l e pressure , temp- but within a m o d er at e r a ng e of t h r o t t l e and exhaust

era tu re , and exhaust pressure . (600 psig , pressures i t wil l cont inue to funct ion wel l . 720FTT, 35 psig)

0 Speed. (3560 RPM) 2. WILLANS LINE OR EXTRACTION CURVES Larger 0 Brake-Horsepower. (121 BHP) turb in es a re of ten designed to run part ly loaded.0 Steam (or Water) r a t e . (31. 3 LB/HP-HR) Turbogenera to r s can be se t up to cont rol an

e x ha us t o r ex t r ac t ion header pressure , for e ~ a m p l e , In some circumstances , th i s in fo rmat ion i s given and vary e l e c t r i c genera t ion ( a t cons tan t speed) to for s e v e r a l p o s s ib l e o p e r a t in g speeds . meet a s team demand. Compressor dr ives are jusualyvar i ab Ie load machines. Another var i ab lei load

From th is data , design e f f i c i e nc y can b e ca lcula ted arrangement is the common-shaft coup ling ! of aas fol lows: blower to a motor and turbine dr iver , wh ich ,allowsload sharing between the two.a . Ideal (100% ef f i c iency) expansion for a t u rb ine

would occur' a t constant entropy ( i s en t rop ic ) . A Wil lans l i n e d e f in e s net BHP or KW over a ~ a r i e t y Use steam tables , c u rv e s o r a s team proper t ies o f t h r o t t l e flow r a t e s . Again, i t is keyed to acomputer program to d ete rm ine in le t s team spec i f i c t h r o t t l e t empera tu re /p ressure a nd e :xhaus ten thalpy and entropy from temperature and pressure pro f i l e (F igure 4) . From it w,e canpressure (1361.75 Btu/ lb &1.594 Btu / lbOR) . predic t ef f i c iency and exhaus t cond i t ions over theb . Find the enthalpy value turbines range o f opera t ion. icorresponding to in le t " i ;. ,T. ; ' I ; . ' . .- ' . "Jen t ropy and exhaust pressure . ! : 6 J S ~ G ; 7 ; ' L O ~ F : . - 1_c? tG - i' 1 1(1125.90 Btu/ lb) . .: ; 7 4 0 0 HFj 10000; RPM I I :

c . The di f fe rence (f).h 235.86) , 0 0 0 0 0 IE ) PECTr , ; ,O P E J ~ F O : t ' A t ' J # - ~ E !I I jrep resen t s the theo re t i ca l i ". ~ ~ , I ? ~ INtlNITE i"-'Z.L,: r - 0 N T ' ~ raximum energy (Btu ' s ) tha t could be removed from a ' ' '! !jpound of s te am u nd er these .J:L" &TJ .[:Ll61. 75 ~ ~ - ~ - Q _ ~ ~ : : _ ~ ~ . : ? : : - ; . _ : . - = - ~ _ _ _ ----j-S\"C>" ~ U ; L B ~ 1. 594 .'.. 0. , . : fOOO : 2000 'lOO 4Q (X ): . j . I I HOR5E..P,::)vvER; :

FIGURE 4 . EFF :0.79 a . As in l a , b, c , and d above, ca lcu la te the: idea l

EFF lb.bJJ - steam r a t e . The r esu l t s are shown in Fi g ur e 5 . ~ S : G 5C : EFF 2:.58 b . From the Wil lans l i ne , ca lcu la te steam raFe for

25J EFF 30.83 a range of flow/power pa i r s . W Flow or Flow Figure 2. BHP (KW x Generator Eff )

e . The r a t io of idea l to manufac tu rer ' s ra ted water where genera to r ef f i c iency can generaqy. be r a t e y i e lds rated e f f i c i e nc y (Figure 3) . concidered 93 to 97%. Space d oes n ot peJtmlt a

thorough discuss ion of var iable g e n ~ r a t o r ~ T E ~ ASR Hc;ual tl>1 RATED l o s se s , and a f ixed value wil l not be far of f . LB/HP-HR BTtJ/LB f


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W1PRESSOR DRIVE TURBINE ~ R E D I C T E D PERFORMANCE CURVECOMPRESSOR DRIVE TURBINE80 T I - - - - = ~ ~ = ~ = ~ = = . t - - - - ~ I TURBINE STEAl RATE - PWID5 H ~ 1 I l ~ I E N T PRESSURE IS 14.7 PSIA

HUT PRESSURE PSI6 li15.0IUT T E J l ~ i l A T U R E -f 720.0INLET E.'f"HIlLPY BTll/LBI3liO.9O

i ~ L f T EMROPY IlTU/LBR 1.591

EXtA!ST PRESS EFF 15.4S _

liSl EFf 23.n1:0 PSIS SOl EFF 30.91

35l EFf 44.15Figure 5.

W1PRESSOR DRIVE TURBINEilrldicted Efficil!llCy tro.Manuficiurer's IHLUlNS LINE13liO.9O BTU/L811 THROTTLE ENTIIlPV15.4SO L ~ / H P HR TlEORETICll. SIDlIl RATE

DELTA H WTl.ET RSR RAN


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COMPRESSOR DRIvE TURBINEOredlctea Efficiency from Manufacturers EXHAUST TEMP vs FLCIj curveINLET C()/DITICJ/S:PRESSURE (PSIS) 615.00 615.00 615.00 615.00TEllPERIlTURE - (Deg Fl 720.00 720.00 720.00 720.00ENT;R.PY - IBTU/Uill 1360.91 1360.91 1360.91 1360.91ENTROPY - IBTU/UiI Rl 1.591 1.591 1.591 1.591'L(ft; - U0E3 LB/HR 160.000 120.000 100.000 80.000

EXIAJST I E X T R l C T I ~ l ~ D I T I E : PRESS IJlSlGl 110.00 110.00 110.00 110.00

T E ~ P (Deg Fl 425.00 430.00 440.00 455.00ACTUAL ENTlRPY 1237.28 1239.97 1245.33 1253.27lDEIll ENTHALPY 1196.21 1196.21 1196.21 1196.21ACTUAL STEAM RIlTE ILB/HPHR 20.59 21.04 22.02 23.65lllEAi. STEAM RATE tLB/HPHR 15.45 15.45 15.45 15.45

tET HP m2 3 5702.2 4541.5 3383.3RANKINE EFFICIEtf:Y 75.06 73.43 70.18 5 5" Figure 9.

PREDICTED PERFORM NCE CURVECOMPRESSOR DRIVE TURBINE III

III

40 60 8'0 100 i 1'20 140 160 I l la20THROTTLE FLOW - 1000's LB/HR

D EXHAUST TEMP DATA + WILLANS LINE DATAFigure 10.

4. EXHAUST OR EXTRACTION USED ENERGY CURVES - - Dataof th is var ie ty is usually provided forintermediate or t a i l sect ions of s ingle- andmult ip le-ex t rac t ion turbines. This is j u s t anotherway of representing an extract ion enthalpy curve,except that an in le t condition (exhaust enthalpyfrom the upstream sect ion) must f i r s t be determinedusing another method: one of the three previoustechniques wi 11 provide that value. From there,the process i s ident ical to the previous example.Figure 11 represents the data from Column 3 ofFigure 6, and produces the same eff ic iencyversus flow curve, Figure 7.S. PERFORMANCE TEST DATA Once al l themanufacturer 's data has been reviewed and analyzed,i t is time to leave the off ice and go out in to theharsh environment of real equipment. Turbinesnever look l ike the sales brochures show; they getcovered with insulat ion (and grime) , and are hookedup to some piece of cr i t ica l equipment that nobodywants you to ( t e s t ) mess with.

t - - - ~- - - - ~ + - - - ,-- I

...... lI61S -7200 FTT-liO#G7400 HP 6000

Figure 11.I

However, you rarely need to disturb the tJrbinet e s t i t . What you do need is some r e l i ~ l e t eequipment. Recently cal ibrated p r e s s ~ r e antemperature gauges are the best source of data . Iyour equipment has thermowells, use them. I I f nothen a long thermocouple can be i n s e r t e ~ betweepipe and insulat ion in the th ro t t le andJ exhaul ines to get fa i r ly accurate reading. . (Thtemperatures wi 11 be a bi t low, but about ' the samamount both up and downstream.) pressure! readingshould be taken as close to the equiPment apossible , to avoid penal iz ing the turbiner for piphydraulics induced pressure losses. IAlso required is some indicat ion of steam flothrough the turbine, corresponding to the measuresteam condit ions. A recent ly cal ibrated steameter wil l provide the bes t data. Flow-bdi fference should be the l a s t r e so r t . To thextent possible , take data for a var iety of turbinflow ra tes (un t i l an operator gets rtired ohumoring you) . Then convert the t e s t I dataef f ic iency, for comparison to the as-new :predic teef f ic iency. Iia . From in le t temperature and pressure , Ide termin

in le t enthalpy , as in L a . above, for leach t econdi t ion observed.b. S i m i l a ~ l y determine actual exhaust enthJl py c. Calculate actual Ah a t known flow ra tes jId. From in le t temperature and pressure , 4nd outlepressure, calculate ideal ah and ideal stea

r a t e as in l . b , c, and d above. Ie. Calculate ac tua l steam ra te , i f th ro t t le flow

known. :f . Calculate and plot turbine eff ic iency versu

flow against manufacturer 's p r e d i c t i o n s ~ IFigures 12 and 13 suuunarise f ie ld t e s t data

calculat ion of ac tua l performance parameters, ancomparison of re su l t s to manufacturer 's ; predictee f f ic ienc ies . This machine appears [ to havsuffered minor performance loss , but I may nwarrent immediate repai r . Continued lnonitorinover time wil l warn of fur ther p e r f o r m a ~ c e decayor comfirm continuing sat isfactory p e r f o r m ~ n c e

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: J ~ P R E S S O R "URBil> ~ R : V E =:: D TEST DA Q AAALYSIS;',-F C C ~ D I T : O N S : P(1 630.00 630.0(1689.00 689.00 689.00

1341.78 1341.78 1341.781.572 1. 572 l.S72

105.000 119.000 140.000

81.00 81.00 81.00383.00 378.00 372.00

1219.15 1216.45 1213.181159.59 1159.59 1159.59

20.75 20.31 19.7913.97 13.97 13.97

5059. (, 58bO.4 7074.167.3! &8.79 70.58

The procedure for t racking steam turbineperformance is qui te s t ra igh t forwarded. Fi r s t ,est imate turbine eff ic iency, independent of steamcondit ions, from any of several sources ofmanufacturer 's data. Then, t e s t the equipment inoperation and convert raw da ta in to terms ofeff ic iency. Plot both resul ts on a common axis andident i fy performance trends. I f the equipmentseems to be doing poorly, check to see i f there issomething simple to correct , l ike par t ly open handvalves, speed cont rol by th ro t t l ing , or operatingspeed far of f spec. I f these do not seem to becontributing to the problem, then closer examination may be ju s t i f ied .

., RE.D iC TED PER ,,- -j M A t ~ C E C. UR' rCOMPRESSOR DRIVE TURBINE8 0 ~ ...-.--.- -------.-- ----- - ....----- ----- -- - -------- ------......., -olr_ ._. :r r_. _ - __ ....._w

' 60-jI / 'II -!"Test Data40) jc.> ,H 1!201

Z iH -l: Io - t - - - , - - - r - - r - - , - - - , - _ . - - r ~ r - . . . , - - - r - - - r - - - - r - ~ - - r - - r - - - i o 20 40 60 80 100 120 140 160 180THROTTLE FLOW - 1000's LB/HR

Figure 13.The methods descr ibed are s t r a igh t forward, andreadi ly adapt ib le to computer-assisted ca lcula t ionand da ta base record keeping to simplify trendanalyses; most of the f igures presented here weregenerated by computational programs.Testing a t regular in te rva l s wi l l provide achronological record of turbine performance, andchanges can be t racked to indicate when waterwashing or repa i r is required. Time ly main tenancecan improve product ion ra tes by avoidingunscheduled down t ime, and head off more cos t lyrepai rs .The long range benef i t s are l ess t rouble frombe t te r maintained equipment for less money,support ing improved production. Sounds pre t tygood, doesn t i t? Sta r t searching those archives

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predicting steam turbine performance

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