Supercritical Steam Turbines

7/30/2019 Supercritical Steam Turbines

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GE Pow er Generation

Steam Turbines forUltrasupercriticalPower Plants

Klaus M. RetzlaffW. Anthony RueggerGeneral Electric Company

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GER-394

GE Pow er Generation

Steam Turbines forUltrasupercritical

Power Plants

Klaus M. RetzlaffW. Anthony Ruegger

General Electric Company


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W. Anthony (Tony) Ruegger

W. Anthony Ruegger is a former manager from GEs CorporateMarketing component where he provided internal consulting services tovarious GE businesses on m arketing issues. He joined G EPG in 1990 asManager of Steam Turbine Product Planning. Following that position,he was the program manager for the 6FA gas turbine. Presently he is theManager of Steam Turbine Prod uct Development an d Structuring.

A List of Figures appears at the end of thi s paper.

Klaus M. Retzlaff

Klaus M. Retzlaff is a senior steam turbine pro duct d esign en gineer inGEs Power Generation group. He has worked in GEs steam turbinedesign engineering organization for over twenty years. Before joiningGE, Klaus worked in Germany for two German steam turbine suppliers.

Prior to assuming his present position, Klaus was a technical leader invarious mechanical and thermodynamic steam turbine d esign functions.He has co-auth ored several techn ical papers, some o n th e subject ofultrasupercritical steam turbine designs. He has received a U.S. patentfor the design of a single-shaft comb ined cycle steam turb ine.


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STEAM TURBINES FOR ULTRASUPERCRITICALPOWER PLANTS

K. M. Retzlaff and W. A. RueggerGE Power SystemsSchenectady, NY

INTRODUCTIONThe history of steam turbine development

can be described as an evolutionary advance-ment toward greater power density and efficien-cy. Power density is a measure of the amount ofpower that can be efficiently generated from asteam turbine of a given physical size and mass.Impro vements in the po wer density of steam tur-bines have been driven largely by the develop-ment of improved rotor and bucket alloys capa-ble of sustaining higher stresses and enablingthe construction of longer last stage buckets fori n c r e a s e d e x h a u s t a r e a p e r e x h a u s t f l o w .Improvements in efficiency have been broughtabout largely through two kinds of advance-ments. The first type of advancement is improve-ment in mechanical efficiency by reduction ofaerodynamic and leakage losses as the steamexpands through the turbine. The second typeof advancement is improvement in the thermo-dynamic efficiency by increasing the tempera-ture and pressure at which heat is added to thepower cycle. The focus of this paper is predomi-nantly on the latter type of efforts to advance

the state-of-the-ar t in steam turbine tech no logy.

EXPERIENCEE f f o r t s t o i n c r e a s e t h e e f f i c ie n c y o f t h e

Rankine cycle by raising steam pressures andtemperatures are not new. Early steam turbinesp r o d u c e d a t t h e t u r n o f t h e c e n t u r y w e r edesigned for inlet pressures and temperatures ofapproximately 200 psi, 500 F (13.7 bar and 260C), respectively. As time progressed and averageunit size increased, main steam temperatures

and pressures also increased. The 1950s was aperiod of rapid growth in average power plantsize with the average unit shipped by GE increas-ing from 38 MW in 1947 to 156 MW in 1957.During this period , the reh eat cycle became wellestablished commercially and maximum steamconditions were raised from 2400 psi / 1000 F(165 bar / 538 C) up to th ose of the experimen-tal units at the Philo power station with inletconditions of 4500 psi, 1150 F / 1050 F / 1000 F(310 bar, 620 C / 566 C / 538 C). This effortprovided the ba sic knowledge th at led to placing

in service, in 1960, several large capacity cross-compound units with modest, but still for thetime challeng ing, steam con ditions of 3500 psi,1050 F / 1050 F / 1050 F (241 bar, 566 C / 566C / 566 C). At this time a 325 MW 2400 psi,1100 F / 1050 F / 1000 F (165 bar, 593 C / 566C / 538 C) unit was also commissioned.

By 1969, a simpler tand em-compoun d do ublereheat design was placed into service that com-bined 3500 psi, 1000 F (242 bar, 538 C) highpressure and 1025 F/552 C first reheat turbinesections in a sing le opp osed-flow casing . Thesecond reheat flow section at 1050 F/566 C was

designed in a d ouble-flow configuration to pro-vide ad equa te volume flow capability and to con -fine the highest temperature conditions to themiddle of the casing [1 ]. The cross section inFigure 1 illustrates this design, which ha s experi-enced excep t iona l ly good re l i ab i l i t y wh i leexceeding performance expectations.

In addition to units with double reheat, dur-ing the 1960s and 1970s GE placed into servicenumerous supercritical units with single reheatand nominal steam conditions of 3500 psi, 1000

Figure 1. Tandem-Compound Double-Reheat Supercritical Steam TurbineRDC24265-4


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F / 1000 F (241 bar, 538 C / 538 C) as shown inFigure 2. These units ranged in size from 350MW to 1103 MW. In clud ed were un its of ta n-dem-compoun d d esign ra nging in size between350 MW and 884 MW.

The combination of experience with singleand double rehea t un i t s , t oge ther wi th theknowledge gained on the advanced steam condi-tion designs of the 1950s, served as the basis forsevera l Elec t r ica l Power Research Inst i tu te(EPRI) studies conducted during the 1980s ofdoub le-reheat tur bines designed for o perationat the advanced steam conditions of 4500 psi,1100 F / 1100 F / 1100 F (310 bar, 593 C / 593C / 593 C). Such designs have been offered fora number of years and although there appearsto be l i t t le in terest in the United Sta tes foradvanced steam condit ions, other countries,

most notab ly in Asia an d n orth ern Euro pe, havepursued this option. An example of a recentadvanced s team turb ine genera tor recen t lydesigned by G E is a single-reh eat cross-com-pound unit for operation with main steam con-ditions of 3626 psi, 1112 F (250 bar / 600 C)and reheat steam temperature of 1130 F/610 C.This unit is being executed in a four-casingdesign with separate high -pressure and inter me-dia te-pressure section s on t he full speed shaf tand two doub le-flow LP turbines on the h alf-speed shaft.

THERMODYNAMIC CYCLEOPTIMIZATION

Effect of Higher Steam Conditionson Unit Performance

As the first step in the optimization of cyclesteam conditions, the potential cycle efficiencygain from elevating steam pressures and temper-

atures needs to be considered. Starting with thetraditional 2400 psi / 1000 F (165 bar / 538 C)single-reh eat cycle, dr am atic impr ovemen ts inpower plant performance can be achieved byraising inlet steam conditions to levels up to

4500 psi/310 bar and temperatures to levels inexcess of 1112 F/600 C. It has become industrypractice to refer to such steam conditions, andin fact any supercritical conditions where thethro t t le and/or rehea t s t eam tempera turesexceed 1050 F/566 C, as ultrasupercritical.Figure 3a illustrates the relative heat rate ga infor a variety of main steam and reheat steamcond itions for single-rehea t units compa red tothe base 2400 psi, 1000 F / 1000 F (65 bar, 538 C/ 538 C) cycle.

Double Reheat vs. Single Reheat

It has long been understood that improved

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Figure 2. Tandem-Compound Single-Reheat Supercritical Steam TurbineRDC24265-5

Figure 3a. Heat Rate Improvement from SteamCycle with Ultrasupercritical SteamConditions

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plant performance is possible by employing adouble, rather than single, reheat cycle. Theseperformance benefits were recognized by utili-ties in the 1960s and, as a result, many double-reheat machines were built by GE [1]. The ben-efit of using the double reheat cycle is furtherenhanced by the feasibility of using ultrasuper-critical pressures and temperatures. During the

mid-1980s, an extensive development projectunder the auspices of EPRI led to the design oflarge ultrasupercritical 4500 psi, 1100 F / 1100 F/ 1100 F (310 bar, 593 C / 593 C / 593 C) dou-ble reheat units with gross output of 700 MWand below [2,3]. Figure 3b demon strates theperfor man ce gains possible by utilizing a d oublereheat cycle at various steam conditions.

For any particular application, the heat rategain possible with the double reheat cycle willhave to be evaluated against the higher stationcosts at tributable to greater eq uipment com-plexity in the boiler, piping systems and steamturb ine. The result of th is trad e-of f will depend

heavily on local site conditions, fuel costs andenvironmental requirements.

Heater Selection and Final FeedwaterTemperature

In order to maximize the heat rate gain possi-ble with ultrasupercritical steam conditions, the

feedwater heater arrangement also needs to beoptimized. In general, the selection of highersteam cond itions will result in ad ditional feed wa-ter heaters and a economically optimal higherfinal feedwater temperature. In many cases theselect ion of a heater above the reheat point(HARP) will also be warranted. The use of a sep-arate desuperheater ahead of the top h eater forunits with a HARP can result in additional gainsin unit performance.

The use of a HARP a nd the associated h igherfinal feedwater temperature and lower reheaterpressure have a strong influence on the designof the steam turbine and will be discussed inmore detail below.

Oth er cycle param eters such a s reheater pres-sure drop, heater terminal temperature differ-ences, line pressure dro ps and d rain coo ler tem-perature differences have a lesser impact onturbine design, but should also be optimized aspart o f the overa l l power plant cost/perfor-man ce trad e-off activity. Table 1 shows typicalgains for different heater configurations associ-ated with a 4500 psi, 1100 F / 1100 F (310 bar,593 C / 593 C) single reheat cycle and a 1100 F

/ 1100 F / 1100 F (593 C / 593 C / 593 C) dou-ble rehea t cycle. Figure 4 shows a typical single-reheat cycle featuring eight feedwater heatersincluding a HARP.

Reheater Pressure Optimization andUse of a HARP

The selection of the cold reheat pressure is anintegral part of any power plant optimization

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Figure 3b.Heat Rate Improvement from SteamCycle with Ultrasupercritical SteamConditions

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Table 1. Heat Rate Impact of Alternative Feedwater Heater Configurations

Cycle No. of Feedwater Heaters HARP Heat Rate BenefitSingle Reheat 7 No Base Case

8 No +0.2%8 Yes +0.6%9 Yes +0.7%

Double Reheat 8 No Base Case9 No +0.3%9 Yes +0.2%

10 Yes +0.5%


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process, but becomes more important for plantswith advanced steam condit ions. Figure 5ashows the heat rate impact of different f inalfeedwater tem peratures for single-reheat unitswith advanced steam conditions. Comparing theheat rate at the thermodynamic optimum, theimprovement resulting from the use of a HARPamo unts to abo ut 0.5%. H owever, econ omicconsiderations of the boiler design without aHARP will tend to favor a lower reheater pres-sure at the expense of a slight decrease in cycle

performance. Therefore, the resulting net heat

rate gain is usually larger, approaching 0.6 -0.7%.

The use of a HARP results in a lower optimalreheater pressure and a higher optimal f inalfeedwater temperature. Both of these considera-tions significantly impact the design and cost ofthe boiler. As a result, careful plant-level cross-optimization needs to be done, in consideringthe use of a HARP, to ensure an economicallyoptimal cycle selection is made.

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Figure 4. Typical Single Reheat Heater Cycle with Heater Above Reheat PointGT25592

Figure 5a.Effect of Final FeedwaterTemperature and Reheat Pressure onTurbine Net Heat Rate

GT25593Figure 5b.Effect of Final Feedwater

Temperature and Reheat Pressure onTurbine Net Heat Rate

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Reheater Pressure Optimization forDouble Reheat Units

For double reheat units, the above describedoptimizat ion o f various design pa rameters ismo re involved a nd ha s to include a cross-opti-mization process in order to properly select thefirst and second reheat pressures. For doublereheat units without HARP, the best perfor-mance would be achieved with the first reheatpressure of approximately 1450 psi/100 bar.Ho wever, econo mic con sideration s associated

with the boiler and piping systems would typical-ly favor reducing this to a lower level. As with

single reheat units, the use of a H ARP can signif-icantly improve unit heat rate. This relationshipis sho wn in Figure 5b.

An exa mple o f the cross-optim ization of firstand second reheat pressures is shown in Figure6. The typical outcome is that the first reheatpressure is chosen below the thermodynamicoptimum while the second reheat pressure is

generally selected slightly above to reduce theLP in let steam tempera ture. As shown in Tab le1 , t h e d o u b l e r e h e a t c y c l e c a n b e f u r t h e rimproved by using an additional low pressureand/or high pressure heater. A typical doublereheat cycle with ten feedwater heaters, includ-ing a H ARP, is sho wn in Figure 7.

Crossover Pressure Optimization

The use of advanced reheat steam conditionsstrongly affects the inlet temperature to the lowpressure (LP) turbine section. An increase in

hot reheat temperature translates into an almostequal increase in crossover temperature for agiven crossover pressure. However, the maxi-mum allowable LP inlet temperature is limitedby material considerations associated with therotor, crossover and hood stat ionary compo-nents. Of these, the rotor material temperaturelimits are usually reach ed first.

Two basic parameters can be varied to adjustthe LP inlet temperature for a given hot reheattemperature: reheater pressure and crossoverpressure. To lower th e crossover tem pera ture,the reheater pressure has to be increased or thecrossover pressure has to be decreased . Asshown in Figure 5a, there is a direct correlation

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Figure 6. Reheat Pressure Cross Optimizationfor Double Reheat Units

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Figure 7. Double Reheat Cycle with Heater above Reheat PointGT25596


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between reheat pressure and unit performance.Since the use of a HARP is likely to be the eco-nomic choice for most ultrasupercritical cycles,the reheater pressure will be lower to maximizethe h eat rate g ain fro m th e H ARP. This, unfo rtu-nately, will result in increased crossover temper-atures.

This ef fec t can be of fset by lowering thecrossover pressure by an equivalent pressureratio. H owever, this tends to increase the energ yon the reheat section which, in turn, increasesthe number of stages and results in longer bear-ing spans. Also, the crossover volume f low

increases and could pr esent a limitation for verylarge ratings. The correlation between crossoverinlet temperature and second reheat pressure is

shown for double reheat units in Figure 8. Therelationship is similar for single reheat units.

STEAM TURBINE DESIGN &MATERIAL SELECTION

Steam Turbine ConfigurationsThe appropriate steam turbine configuration

for a given ultrasupercritical application is large-ly a function of the number of reheats selected,the unit ra ting, th e site backpressure cha racteris-tics and any special requirements such as districtheating extractions.

Single Reheat Power Generation ApplicationsThe available configurations for single-reheat

applications are shown in Figure 9. For mostapplications, an opposed flow HP/IP section in

a single casing can be util ized. This sect ionwould be combined with either one or two dou-ble-flow LP sections depen ding on the a ctualrating an d design exhaust pressure The use ofthe combined HP/IP section makes possible asmaller overall power island with its resultantsavings in turbine bui ld ing, founda t ion an dmaintenance costs. Supercritical units with thistype of construction have operated successfullyat ra tin gs ab ove 600 MW fo r m an y year s. Tomeet the requirements of specialized applica-tion s an d customer pr eferen ces, sing le-flow HPand IP sections in separate casings are also avail-

able. The H P a nd IP turbine cross-sections ofthese two configura tions are shown in Figures 10an d 11 respectively.

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Figure 9. Single-Reheat Ultrasupercritical Product LineGT25604

Figure 8. Crossover Temperature vs. SecondReheat Pressure

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As unit rating increases, stability requirements

and last IP bucket length make a configurationutilizing a single flow HP section and doubleflow IP section in separate casings the appropri-ate selection. These two high temperature sec-tions can be combined with one, two or threedo uble-f low LP sec t ions depen d ing on thedesign exha ust pressure. Tand em comp oun d

configurations of this type with three LP sec-

tions are capable of the highest unit ratings cur-rently contemplated for ultrasupercritical powerplan ts. The H P and RH cross-section of such aunit is shown in Figure 12.

F o r t h e h i g h e s t u n i t r a t i n g s a n d t h o s einstances where the customer prefers it, cross-compound units are also available. These units

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Figure 11. Separate HP and IP Sections of Ultrasupercritical TurbineGT25606

Figure 12. Separate HP and Double-Flow IP Sections of Ultrasupercritical TurbineGT25607

Figure 10. Combined HP/ IP Section of Ultrasupercritical TurbineGT25605


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include a full speed shaft line having a single-flow HP section an d a do uble-flow IP section, asdescribed, above driving a two pole generator.A second ha lf-speed shaft line con sisting of twodo uble-flow LP sections driving a four po le gen-erator is also included. Steam exhausting fromthe IP section o f th e full-speed shaf t-line is fedto th e inlet of the LP sections in the half-speedshaft line via two crossovers.

Single Reheat District Heating ApplicationsA number of single-reheat ultrasupercritical

projects have been used for district heat ingapplications and this requirement can signifi-cantly affect both the steam cycle parametersand turbine configuration. The optimal turbineconfiguration that meets the functional require-ments of d istrict heating o peration as well as the

h i g h p e r f o r m a n c e a n d e c o n o m i ca l t u r b in eisland arrangement, will depend primarily onthe need for controllability of district heat overthe load range. A study done recently on a 440MW ultrasupercritical district heating applica-t ion concluded that i f part load district heatcontrollability is not a requirement, a compactthree-casing configur ation u sing a n oppo sedflow HP /IP section, such as that shown in Figure10, was the best choice from a systems cost per-spective. With this configu ration , th e d istrictheaters would be fed from uncontrolled extrac-tions in the LP sections and control would be

achieved o n th e water side o f the d istrict heatingsystem [4].

In district h eat ing applicat ions where pa rtload district heat controllability is a require-ment, a four-casing co nfiguratio n such as thatshown in Figure 13 is more a ppropriate. Thisconfiguration, which was developed for another400 MW ultrasupercritical application features afirst casing containing the HP section and thes ing le f low por t ion o f the IP sec t ion in anopposed-flow arra ngemen t. Exhaust from th e

sing le-flow IP section is directed into a separatedo uble-flow asymm etrical IP section in a sepa-rate casing. The two district heating extractionsare taken from the exhausts of this casing andthe district heating pressure is controlled by wayof b utter fly valves in th e crossovers to th e LP sec-tions. In comparison to an alternative construc-tion with totally separate H P an d IP sections, theuse of single-flow IP stag ing fo r the f irst part ofthe reheat expansion enables longer bucketsw i th a s so c i a t e d b e t t e r s t a g e p e r f o r m a n c e .Additional benefits include confining all thehigh tempera ture steam to the center of th e firstsection, better ro tor cooling steam utilizationand overall reduced machine length.

Double Reheat ApplicationsThe a va i l ab le con f igura t ion s for d oub le-

reheat applications are shown in Figure 14. Formany application s, a single-flow HP section in itsown casing can be combined with a second cas-i n g h a v i n g t h e t w o r e h e a t s e c t i o n s i n a nopposed flow arrangement. The high pressureand reheat sections are directly coupled to one,two or three d ouble-flow LP sections depen dingon the application rating and design exhaustpressure.

For units of higher rat ing, a configurat ionwith a sing le-flow H P section an d sing le-flow firstreheat section, located in a common casing andcoupled to a d ouble-flow second reheat section

in a separate casing, is utilized. As with the con-figuration described above, the high tempera-ture sections are directly coupled to one, two orthree d ouble-flow LP sections based o n th e rat-ing and exhaust pressure. Figure 15 shows across-section of th e HP and RH section s of sucha design.

For units of th e high est rating, a cross-com-pound configuration can be used. This configu-ratio n would utilize a full-speed shaft line h avingsections basically the same as the HP and RH

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Figure 13. Ultrasupercritical Steam Turbine Designed for 2-Stage District Heating ApplicationGT25,608o


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sections just described. Rather than being cou-pled to full-speed LP sectio ns, these section swould be directly coupled to a 3600 or 3000RPM gen erato r. A separate h alf-speed LP shafttrain similar to th at used in sing le-reheat appli-cations would be utilized in conjunction withthe full-speed H P/IP shaft train.

Steam Turbine Component/SystemDesign

The design of high temperature steam tur-bines has evolved and is strongly influenced bythe development of improved materials and bythe use of more ef fective cooling steam ar rang e-ments. Both factors are d iscussed for th e variousc r i t ic a l c o m p o n e n t s wh i c h a r e a f f e c t e d b yadvanced steam cond itions.

Rotor MaterialGE has extensive experience with two rotor

alloy steels in high -pressure r oto r a pplications:CrMoV and 12CrMoVCbN. The 12Cr steel isgenerally used when a higher r upture strength isrequired at elevated temperatures, or when ah igher than norma l opera t ing t empera ture(1050 F/566 C) is required.

The first 12Cr rotor was placed into service in1959. This material was developed and patentedby the authors' company in anticipation of amarket need for steam turbines capable of oper-ating at ultrasupercritical steam temperatures.Since 1959, a total of 63 rotors have been builtwith 12Cr forgin gs. These ro tors have successful-ly operated in some of the most challengingapplicat ions in units rated between 500 and1000 MW.

The result of these extensive service experi-

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Figure 15. HP and Reheat Sections of a Double-Reheat Ultrasupercritical TurbineGT25610

Figure 14. Double-Reheat Ultrasupercritical Product LineGT25609


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ences and lo ng-term m ateria l tests has con -firmed that the 12Cr rotor alloy has a rupture

strength at 1100 F/593 C that is equivalent tothe corresponding value for CrMoV material at1050 F/566 C. Therefore, no compromise isrequired for the design of a high temperaturerotor operating at 1100 F/593 C with the 12Crmaterial [5].

Weld Inlay of Rotor Bearing JournalsThe 12Cr ro tor m aterial has very poor journ al

running characteristics due to its high chromecontent. Under abnormal running conditions,the rotor journal surface can gall and parts ofthe surface can be chafed off, resulting in bear-

ing fa i lure. Tradi t iona l ly , th is problem wassolved b y employing shrunk-on low alloy jour na lsleeves. However, the use of shrunk-on sleevesalso req uires the use of shr unk-on couplingsand, depending on the unit configuration, theuse of shr unk-on thr ust runn ers. Altho ugh th esedesigns have been shown to operate reliably,current designs employ a low alloy weld inlay tothe journal and thrust runner surfaces, whichaddresses the galling issue without resorting theuse of shrun k-on compo nen ts. This appro achprovides the additional benefit of allowing theturbine designer to locate the thrust bearing in

a position such that optimum clearance controlin the H P section is achieved.

Rotor CoolingAt the elevated temperatures associated with

ultrasupercritical applications, the first and sec-o n d st a g e o f t h e r e h e a t s e ct i o n s g e n e r a l l yrequire external cooling of the wheel and buck-et dovetail region. This design approach hasbeen successfully employed on many previously

built turbines utilizing conventional materialsand operating at traditional temperatures.

For o pposed flow HP /IP sections, the coolingsteam is extracted from the third or fourth HPstage an d re-ad mitted in to th e mid-span pack-ing. To improve the coo ling ef fectiveness, a por-tion o f the mid -span packing leakage flow canbe bled off prior to mixing. The HP/IP coolingscheme is shown in Figure 16.

For the first stage of a d ouble-flow secon dreheat section, the cooling steam is extractedfrom the first reheat extract ion stage and ispiped into the upstream first stage wheel spacebelow the double flow tub. By judicious use ofbucket dovetail steam balance holes and root

radial spill strips on both sides of the dovetail, itis possible to direct the cooler steam to the sec-ond stage upstream wheel space.

In all cases, the cooling steam effectivenessmust be evaluated at full load and at the loadpoint where the reheat temperature normallystarts to drop off, typically at 40-50% load. Thiseffect is sho wn in Figures 17 and 18.

High Temperature Bucket / Diaphragm Designsand Materials

Buckets for the early HP and reheat stages ofsteam turbines must have goo d h igh-tempera -

ture strength and low thermal expansion to min-imize thermal stresses. For ultrasupercriticalapplications, a 10CrMoVCbN bucket alloy simi-lar to the ro tor fo rging a lloy was developed. Thisalloy possesses a rupture strength nearly 50%higher at 1050 F/566 C than the AISI 422 alloytraditionally used in applications of up to 1050F/566 C. Together with use of a xial ent r y typebucket dovetails, judicious application of rotorcooling schemes, reheat pressure optimization

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Figure 16. Reheat Stage Cooling Configuration for Opposed Flow HP/ IP SectionsGT25611


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an d the use of doub le-flow configura tions forHP control stages at higher ratings, acceptableh i g h t e m p e r a t u r e b u c k e t d e s i g n s c a n b eachieved to cover the rating range of 350 MW to1100 MW.

I n a l l t u r b i n e s e c t i o n s e m p l o y i n g12CrMoVCbN rotors, diaphragms and packingcasings are con struc ted of 12Cr mater ia l tomatch the thermal expansion characteristics ofthe 12Cr ro tor ma terial.

Shells and Nozzle BoxesLow alloy CrMoV materials generally suitable

for stationary components in turbines designedfor conventional steam conditions are not suit-able for the higher temperature regions of ultra-supercr i t ica l s team turbines. High st rengthmar tens i t i c s t a in less s t ee l c a s t ing a l loys(10CrMoVCb) were developed by the authorscompany in the late 1950s for valve bodies andnozzle boxes in applications with 1050 F/566 Cand 1100 F/593 C inlet temperatures. Last year,four large turbine shells were made from thismaterial and work has been completed with avendor to improve its producibility for largecastings.

HP sections of ultrasupercritical steam tur-bines gen erally utilize triple-shell con structionto minimize the thermal and operating stressesthe various pressure containment parts are sub-jected to. The highest pressures and tempera-tures are borne by a nozzle box constructed offorged 12CrMoVCbN steel. The inner shells arecons t ruc ted o f c a s t 10CrMoVCb or C rMoVmaterial depending on the specific tempera-tures associated with the ultrasupercritical appli-

cation. With this type of construction, the outershell is not subjected to elevated temperaturesand can thus be cons t ruc ted o f t r ad i t iona lCrMoV material.

Th e t r a n s i t io n b e t w e e n t h e m a i n st e a mleads and th e outer shell has tradition ally beendesigned as a flanged con nection with therm alsleeves. Today 's u l t rasupercr i t ica l designsemploy a welded connection. The welded con-nection is cooled by the cold reheat steam onthe inner wall to a temperatur e level of 1025 F- 1050 F/550-565 C. To assure sufficient heatt r a n s f e r n e a r t h e w e l d , a s m a l l a m o u n t o fs team is b lown down to the next extrac t ion

point. Figure 19 illustrates the ultrasupercriti-c a l m u l t i -s h e l l H P se c t i o n c o n s t r u c t i o ndescribed abo ve.

IP sections of ultrasupercritical turbines uti-lize do uble shell construction with the h igh tem-perature inner shell being constructed of cast10CrMoVCb material and the outer shell and

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Figure 19. Main Steam Inlet ConstructionGT25600

Figure 17.Typical Boiler Characteristic for USCUnit (Hybrid Pressure 310 bar, 395C/ 593 C/ 593 C Cycle

GT25598Figure 18.Effect of Part Load Operation on

Cooling Effectiveness

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low temperature inner shell constructed of tra-ditional C rMoV material.

Advancements in finite element (FE) calcula-tion capabilities enable designers to assess thelocal stress field in these high temperature com-ponents and, thus, selectively add material onlywhere needed for s t rength purposes . Th isresults in a shell design that satisfies all stress

limitations and is thermally flexible to meet thesho rter start-up times req uired by tod ays cus-tomers. Figure 20 shows an example of a FEmesh for an ul t rasupercr i t ica l HP/IP innershell. Figure 21 shows a typical stress plot for fullload steady state conditions.

BoltingFor shell bolting applications at temperatures

up to 1050 F/566 C, 12Cr alloys and low alloys tee ls have been used . However , the moredemanding ultrasupercritical steam conditionsexceed the capabilities of these materials, thusdictating the requiremen t fo r n ickel-based a lloysin high-temperature regions.

A comparison of candidate bolting materialspossessing higher temperature strength wasrecently made and Inconel 718 was selected asthe material possessing the best combination ofall the bolting req uirements. The use of Incon elbol ts resul ts in smaller bol t d iameters and,therefore, narrower flanges. This, in turn, leadsto lower transient thermal stresses during tur-bine start-ups. This ma terial has been successful-ly used by the authors' company in gas turbine,

aircraft engine and conventional steam turbineapplications for may years.

LP Section DesignThe primary LP section design issue associat-

ed with ultrasupercritical turbines is the elevated

crossover tempera ture th at is frequen tly encoun -tered with th ese power cycles. It has been foundthat conventional NiCrMoV roto r ma terials havea tendency to embrittle at LP bowl temperaturesabove 660 - 710 F/350 - 375 C. In order to avoid

t h i s p h e n o m e n o n , p a s t h i g h t e m p e r a t u r edesigns have used an internal cooling schemethat circulates the exhaust steam of the first LPstage into the upstream wheel space by virtue ofspecial wheel hole scoops and a slightly negativeroot reaction. This design approach, however,results in a per form ance loss.

Studies performed by EPRI and others overthe past several years have demonstrated thatN i C r M o V m a t e r i a l c a n b e m a d e v i r t u a l l yimmune to embrittlement by reducing th e levelsof P, Sn, Mn and Si. Utilization of this super-clean chemistry combined with other enhance-

ments such as raising the nickel content andgashing between the wheels prior to quenching,result in rotor forgings with superior embrittle-ment, fracture toughness and tensile ductilityproperties in comparison to previously available

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Figure 21.Predicted HP/ RHT Inner Shell Stress Distribution at Peak Load (Normalized to MaximumStress)

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Figure 20.Finite Element Model of USC HP/ IPInner Shell

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NiCrMoV materials. This improvement providesadditional freedom to optimize the cycle param-eters, in particular the crossover temperaturefor double reheat units, to achieve higher effi-ciency levels without performance losses associ-ated with previously used cooling schemes.

Advanced Steam Path DesignRecent years have seen the rapid advance-ment of computational fluid dynamics (CFD).Based on this new capability, turbine compo-nents can be better optimized for reduced flowlosses [6]. The performance of steampath com-ponents such as nozzles, buckets and seals havebeen significantly enhanced as a result of apply-ing this new technology and the resultant per-formance gains have been verified both in testturbines and operating units. A segment of anIP section diaphragm utilizing advanced nozzlepar tition designs is shown in Figure 22.

In ad d i t ion to the per forman ce improve-ments attributable to CFD in the steampa th, per-form ance ga ins can also be achieved b y optimiz-ing stationary components such as valves, inletsand exhausts using the same tools. All ultrasu-percritical designs in the futur e will incorpo ratethese CFD-based design enh an cemen ts.

CONCLUSIONIncreased fuel costs, improved techno logy

and an a heightened focus on reducing powerplant emissions have combined to revitalizepower ind ustry in terest in coa l-f ired p owerplants utilizing ultrasupercritical steam condi-tion s. To a chieve an econ om ically optim ized

plant, the cycle conditions under which theseplants operate need to be carefully evaluated,taking into account such param eters as the num-ber of reheats employed, inlet steam conditionsand feedwater heater arrangement. A variety ofsteam turbine configurations for ultrasupercriti-cal applications are available. Each of th ese con-figurations utilizes materials and design featuresappropriate to en sure long turbine life with reli-a b i l i t y l e v e l s c o m p a r a b l e t o c o n v e n t i o n a ldesigns.

Note: This paper was originally presented at

Po wer Gen Europe 96.

REFERENCES1. R .C . Spencer, " Des ign o f Double Rehea t

Turbines for Super-Critical Pressures", pre-sen ted a t the 1980 Amer ican PowerConference, Chicago, Ill.

2. G .P. Wozn ey, M. Akiba, G .L. Touch ton , R.I .Jaffee, S.J. Woodco ck, "Turbin e Research an dDevelopment for Improved C oal-Fired PowerPlants", American Power Conference, April14-16, 1986

3. K.M. Retz la f f an d K. Aizawa , " Turb ineDesigns", First International Conference onI m p r o ve d C o a l -F ir e d P o w e r P l a n t s ,November 19-21, 1986

4. J. Kure-Jensen and K. Retzlaff, A 440 MWExtrac t ion Steam Turbine for AdvancedS t ea m C o n d i t io n s , I n t e r n a t i o n a l J o in tPower Generation Conference, 1994

5. J. Kure-Jensen , A. Morson , P. Schilke, LargeS t e a m Tu r b i n e f o r A d v a n c e d S t e a mCon ditions, EPRI Co nference, March 1993

6. J . I . Cofer IV, "Advances in S team Pa thTechno logy" presented at Po wer Genera tionEurope, April 1995

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1996 G E Compan y

Figure 22.Diaphragm Segment with AdvancedNozzle Partitions

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LIST OF FIGURES

Figure 1. Tand em-Com pound Do uble-Reheat Supercritical Steam Turbine

Figure 2. Tand em-Com pound Do uble-Reheat Supercritical Steam Turbine

Figure 3a. Hea t Rate Improvement from Steam Cycle with Ultrasupercritical Steam Cond itions

Figure 3b. Hea t Rate Improvement from Steam Cycle with U ltrasupercritical Steam Co ndition s

Figure 4. Typical Single Reheat Cycle with Heater Above Reheat Po intFigure 5a. Effect of Final Feedwater Temperature a nd Reheat P ressure on Turbine Net H eat Rate

Figure 5b. Effect of Final Feedwater Temperature a nd Reheat P ressure on Turbine Net H eat Rate

Figure 6. Reheat Pressure Cross Optimization for Double Reheat Units

Figure 7. Double Reheat Cycle with Heater above Reheat Point

Figure 8. Crossover Temperature vs. Second Reheat Pressure

Figure 9. Single-Reheat Ultrasupercritical Product Line

Figure 10. Com bined H P/IP Section of U ltrasupercritical Turbine

Figure 11. Separate H P an d IP Sections of U ltrasupercritical Turbine

Figure 12. Separate H P and Do uble-Flow IP Sections of U ltrasupercritical Turbine

Figure 13. U ltrasupercritical Steam Turbin e Designed fo r 2-Stage District Hea ting Application

Figure 14. Doub le-Reheat U ltrasupercritical Pro duct LineFigure 15. HP and Reheat Sections of a D ouble-Reheat U ltrasupercritical

Figure 16. Reheat Stage Cooling Con figuration for Oppo sed Flow HP/IP Sections

Figure 17. Typical Bo iler Chara cteristic for U SC U nit ( Hybrid Pressure 310 bar, 395 C/593 C/593 C

Cycle

Figure 18. Effect of Part Load Opera tion on C ooling Effectiveness

Figure 19. Main Steam In let Construction

Figure 20. Finite Element Model of USC H P/IP In ner Shell

Figure 21. Pred icted HP /RHT Inn er Shell Stress Distribution a t Peak Load (Nor malized to Maximum

Stress

Figure 22. Diaphra gm Segment with Advanced Nozzle Partitions

LIST OF TABLES

Table 1. Heat Rate Impact of Alternative Feedwater Heater Configurations

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For furt her inform ati on, contact your GE Field SalesRepresentative or w rite t o GE Pow er Generation M arketing

GE Pow er Systems

General Electric Company

Building 2, Room 115BOne River Road

Schenectady, NY 12345

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Supercritical Steam Turbines

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