GE Power Generation

Steam Turbines forUltrasupercriticalPower Plants

Klaus M. RetzlaffW. Anthony RueggerGeneral Electric Company

GER-3945A

GER-3945A

GE Power Generation

Steam Turbines forUltrasupercritical

Power Plants

Klaus M. RetzlaffW. Anthony Ruegger

General Electric Company

W. Anthony (Tony) RueggerW. Anthony Ruegger is a former manager from GE’s Corporate

Marketing component where he provided internal consulting services tovarious GE businesses on marketing issues. He joined GEPG 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 Product Development and Structuring.

A List of Figures appears at the end of this paper.

Klaus M. RetzlaffKlaus M. Retzlaff is a senior steam turbine product design engineer in

GE’s Power Generation group. He has worked in GE’s 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 design functions.He has co-authored several technical papers, some on the subject ofultrasupercritical steam turbine designs. He has received a U.S. patentfor the design of a single-shaft combined cycle steam turbine.

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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.Improvements in the power 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 forincreased exhaust area per exhaust flow.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 advancethe state-of-the-art in steam turbine technology.

EXPERIENCEEfforts to increase the efficiency of the

Rankine cycle by raising steam pressures andtemperatures are not new. Early steam turbinesproduced at the turn of the centur y weredesigned 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 reheat cycle became wellestablished commercially and maximum steamconditions were raised from 2400 psi / 1000 F(165 bar / 538 C) up to those 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 basic knowledge that led to placingin service, in 1960, several large capacity cross-compound units with modest, but still for thetime challenging, steam conditions 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 tandem-compound doublereheat 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 single opposed-flow casing. Thesecond reheat flow section at 1050 F/566 C wasdesigned in a double-flow configuration to pro-vide adequate 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 has experi-enced exceptionally good reliability whileexceeding 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 Turbine RDC24265-4

F / 1000 F (241 bar, 538 C / 538 C) as shown inFigure 2. These units ranged in size from 350MW to 1103 MW. Included were units of tan-dem-compound design ranging in size between350 MW and 884 MW.

The combination of experience with singleand double reheat units, together with theknowledge gained on the advanced steam condi-tion designs of the 1950s, served as the basis forseveral Electrical Power Research Institute(EPRI) studies conducted during the 1980s ofdouble-reheat turbines designed for operationat 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 little interest in the United States foradvanced steam conditions, other countries,most notably in Asia and northern Europe, havepursued this option. An example of a recentadvanced steam turbine generator recentlydesigned by GE is a single-reheat 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 interme-diate-pressure sections on the full speed shaftand two double-flow LP turbines on the half-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-reheat cycle, dramatic improvements inpower plant performance can be achieved byraising inlet steam conditions to levels up to4500 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 thethrottle and/or reheat steam temperaturesexceed 1050 F/566 C, as “ultrasupercritical”.Figure 3a illustrates the relative heat rate gainfor a variety of main steam and reheat steamconditions for single-reheat units compared tothe base 2400 psi, 1000 F / 1000 F (65 bar, 538 C/ 538 C) cycle.

Double Reheat vs. Single ReheatIt has long been understood that improved

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Figure 2. Tandem-Compound Single-Reheat Supercritical Steam Turbine RDC24265-5

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 themid-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 demonstrates theperformance gains possible by utilizing a doublereheat 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 attributable to greater equipment com-plexity in the boiler, piping systems and steamturbine. The result of this trade-off 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, thefeedwater heater arrangement also needs to beoptimized. In general, the selection of highersteam conditions will result in additional feedwa-ter heaters and a economically optimal higherfinal feedwater temperature. In many cases theselection of a heater above the reheat point(HARP) will also be warranted. The use of a sep-arate desuperheater ahead of the top heater forunits with a HARP can result in additional gainsin unit performance.

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

Other cycle parameters such as reheater pres-sure drop, heater terminal temperature differ-ences, line pressure drops and drain cooler tem-perature differences have a lesser impact onturbine design, but should also be optimized aspart of the overall power plant cost/perfor-mance trade-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 reheat 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 ConfigurationsCycle No. of Feedwater Heaters HARP Heat Rate Benefit

Single Reheat 7 No Base Case8 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%

process, but becomes more important for plantswith advanced steam conditions. Figure 5ashows the heat rate impact of different finalfeedwater temperatures for single-reheat unitswith advanced steam conditions. Comparing theheat rate at the thermodynamic optimum, theimprovement resulting from the use of a HARPamounts to about 0.5%. However, economicconsiderations 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 heatrate gain is usually larger, approaching 0.6 -0.7%.

The use of a HARP results in a lower optimalreheater pressure and a higher optimal finalfeedwater 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

GT25593 Figure 5b. Effect of Final FeedwaterTemperature and Reheat Pressure onTurbine Net Heat Rate

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Reheater Pressure Optimization forDouble Reheat Units

For double reheat units, the above describedoptimization of various design parameters ismore involved and has 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.However, economic considerations associatedwith the boiler and piping systems would typical-ly favor reducing this to a lower level. As with

single reheat units, the use of a HARP can signif-icantly improve unit heat rate. This relationshipis shown in Figure 5b.

An example of the cross-optimization 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 isgenerally selected slightly above to reduce theLP inlet steam temperature. As shown in Table1, the double reheat cycle can be furtherimproved by using an additional low pressureand/or high pressure heater. A typical doublereheat cycle with ten feedwater heaters, includ-ing a HARP, is shown in Figure 7.

Crossover Pressure OptimizationThe use of advanced reheat steam conditions

strongly affects the inlet temperature to the lowpressure (LP) turbine section. An increase inhot 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 stationary compo-nents. Of these, the rotor material temperaturelimits are usually reached first.

Two basic parameters can be varied to adjustthe LP inlet temperature for a given hot reheattemperature: reheater pressure and crossoverpressure. To lower the crossover temperature,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

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 heat rate gain from the HARP. This, unfortu-nately, will result in increased crossover temper-atures.

This effect can be offset by lowering thecrossover pressure by an equivalent pressureratio. However, this tends to increase the energyon the reheat section which, in turn, increasesthe number of stages and results in longer bear-ing spans. Also, the crossover volume flowincreases and could present 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 rating, the site backpressure characteris-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 ina single casing can be utilized. This sectionwould be combined with either one or two dou-ble-flow LP sections depending on the actualrating and design exhaust pressure The use ofthe combined HP/IP section makes possible asmaller overall power island with its resultantsavings in turbine building, foundation andmaintenance costs. Supercritical units with thistype of construction have operated successfullyat ratings above 600 MW for many years. Tomeet the requirements of specialized applica-tions and customer preferences, single-flow HPand IP sections in separate casings are also avail-able. The HP and IP turbine cross-sections ofthese two configurations are shown in Figures 10and 11 respectively.

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Figure 9. Single-Reheat Ultrasupercritical Product LineGT25604

Figure 8. Crossover Temperature vs. SecondReheat Pressure

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(C)

As unit rating increases, stability requirementsand 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 threedouble-flow LP sections depending on thedesign exhaust pressure. Tandem compound

configurations of this type with three LP sec-tions are capable of the highest unit ratings cur-rently contemplated for ultrasupercritical powerplants. The HP and RH cross-section of such aunit is shown in Figure 12.

For the highest unit ratings and thoseinstances 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

include a full speed shaft line having a single-flow HP section and a double-flow IP section, asdescribed, above driving a two pole generator.A second half-speed shaft line consisting of twodouble-flow LP sections driving a four pole gen-erator is also included. Steam exhausting fromthe IP section of the full-speed shaft-line is fedto the 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 heatingapplications and this requirement can signifi-cantly affect both the steam cycle parametersand turbine configuration. The optimal turbineconfiguration that meets the functional require-ments of district heating operation as well as thehigh performance and economical turbineisland 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-tion concluded that if part load district heatcontrollability is not a requirement, a compactthree-casing configuration using an opposedflow HP/IP section, such as that shown in Figure10, was the best choice from a systems cost per-spective. With this configuration, the districtheaters would be fed from uncontrolled extrac-tions in the LP sections and control would beachieved on the water side of the district heatingsystem [4].

In district heating applications where partload district heat controllability is a require-ment, a four-casing configuration such as thatshown in Figure 13 is more appropriate. Thisconfiguration, which was developed for another400 MW ultrasupercritical application features afirst casing containing the HP section and thesingle flow portion of the IP section in anopposed-flow arrangement. Exhaust from the

single-flow IP section is directed into a separatedouble-flow asymmetrical 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 butterfly valves in the crossovers to the LP sec-tions. In comparison to an alternative construc-tion with totally separate HP and IP sections, theuse of single-flow IP staging for the first part ofthe reheat expansion enables longer bucketswith associated better stage performance.Additional benefits include confining all thehigh temperature steam to the center of the firstsection, better rotor cooling steam utilizationand overall reduced machine length.

Double Reheat ApplicationsThe available configurations for double-

reheat applications are shown in Figure 14. Formany applications, a single-flow HP section in itsown casing can be combined with a second cas-ing having the two reheat sections in anopposed flow arrangement. The high pressureand reheat sections are directly coupled to one,two or three double-flow LP sections dependingon the application rating and design exhaustpressure.

For units of higher rating, a configurationwith a single-flow HP section and single-flow firstreheat section, located in a common casing andcoupled to a double-flow second reheat sectionin a separate casing, is utilized. As with the con-figuration described above, the high tempera-ture sections are directly coupled to one, two orthree double-flow LP sections based on the rat-ing and exhaust pressure. Figure 15 shows across-section of the HP and RH sections of sucha design.

For units of the highest rating, a cross-com-pound configuration can be used. This configu-ration would utilize a full-speed shaft line havingsections 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

sections just described. Rather than being cou-pled to full-speed LP sections, these sectionswould be directly coupled to a 3600 or 3000RPM generator. A separate half-speed LP shafttrain similar to that used in single-reheat appli-cations would be utilized in conjunction withthe full-speed HP/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 effective cooling steam arrange-ments. Both factors are discussed for the variouscritical components which are affected byadvanced steam conditions.

Rotor MaterialGE has extensive experience with two rotor

alloy steels in high-pressure rotor applications:CrMoV and 12CrMoVCbN. The 12Cr steel isgenerally used when a higher rupture strength isrequired at elevated temperatures, or when ahigher than normal operating temperature(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 forgings. These rotors have successful-ly operated in some of the most challengingapplications 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

ences and long-term material tests has con-firmed that the 12Cr rotor alloy has a rupturestrength 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 rotor material has very poor journal

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 failure. Traditionally, this problem wassolved by employing shrunk-on low alloy journalsleeves. However, the use of shrunk-on sleevesalso requires the use of shrunk-on couplingsand, depending on the unit configuration, theuse of shrunk-on thrust runners. Although thesedesigns 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 shrunk-on components. This approachprovides the additional benefit of allowing theturbine designer to locate the thrust bearing ina position such that optimum clearance controlin the HP section is achieved.

Rotor CoolingAt the elevated temperatures associated with

ultrasupercritical applications, the first and sec-ond stage of the reheat sections generallyrequire 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 opposed flow HP/IP sections, the coolingsteam is extracted from the third or fourth HPstage and re-admitted into the mid-span pack-ing. To improve the cooling effectiveness, a por-tion of 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 double-flow secondreheat section, the cooling steam is extractedfrom the first reheat extraction stage and ispiped into the upstream first stage wheel spacebelow the double flow tub. By judicious use ofbucket dovetail steam balance holes and rootradial 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 shown in Figures 17 and 18.

High Temperature Bucket / Diaphragm Designsand Materials

Buckets for the early HP and reheat stages ofsteam turbines must have good high-tempera-ture strength and low thermal expansion to min-imize thermal stresses. For ultrasupercriticalapplications, a 10CrMoVCbN bucket alloy simi-lar to the rotor forging alloy 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 axial entry 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

and the use of double-flow configurations forHP control stages at higher ratings, acceptablehigh temperature bucket designs can beachieved to cover the rating range of 350 MW to1100 MW.

In all turbine sections employing12CrMoVCbN rotors, diaphragms and packingcasings are constructed of 12Cr material tomatch the thermal expansion characteristics ofthe 12Cr rotor material.

Shells and Nozzle Boxes Low 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-supercritical steam turbines. High strengthmartensitic stainless steel casting alloys(10CrMoVCb) were developed by the authors’company 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 generally utilize triple-shell constructionto 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 areconstructed of cast 10CrMoVCb or CrMoVmaterial 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 constructed of traditionalCrMoV material.

The transition between the main steamleads and the outer shell has traditionally beendesigned as a flanged connection with thermalsleeves. Today's ultrasupercritical designsemploy a welded connection. The welded con-nection is cooled by the cold reheat steam onthe inner wall to a temperature level of 1025 F- 1050 F/550-565 C. To assure sufficient heattransfer near the weld, a small amount ofsteam is blown down to the next extractionpoint. Figure 19 illustrates the ultrasupercriti-cal multi -shell HP section constructiondescribed above.

IP sections of ultrasupercritical turbines uti-lize double shell construction with the high 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 CrMoV 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 strength purposes. Thisresults in a shell design that satisfies all stresslimitations and is thermally flexible to meet theshorter start-up times required by today’s cus-tomers. Figure 20 shows an example of a FEmesh for an ultrasupercritical 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 alloysteels have been used. However, the moredemanding ultrasupercritical steam conditionsexceed the capabilities of these materials, thusdictating the requirement for nickel-based alloysin 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 requirements. The use of Inconelbolts results in smaller bolt diameters and,therefore, narrower flanges. This, in turn, leadsto lower transient thermal stresses during tur-bine start-ups. This material 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 temperature that is frequently encoun-tered with these power cycles. It has been foundthat conventional NiCrMoV rotor materials havea tendency to embrittle at LP bowl temperaturesabove 660 - 710 F/350 - 375 C. In order to avoidthis phenomenon, past high temperaturedesigns 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 performance loss.

Studies performed by EPRI and others overthe past several years have demonstrated thatNiCrMoV material can be made virtuallyimmune to embrittlement by reducing the 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 Design Recent 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 nozzlepartition designs is shown in Figure 22.

In addition to the performance improve-ments attributable to CFD in the steampath, per-formance gains can also be achieved by optimiz-ing stationary components such as valves, inletsand exhausts using the same tools. All ultrasu-percritical designs in the future will incorporatethese CFD-based design enhancements.

CONCLUSIONIncreased fuel costs, improved technology

and an a heightened focus on reducing powerplant emissions have combined to revitalizepower industr y interest in coal-fired powerplants utilizing ultrasupercritical steam condi-tions. To achieve an economically optimizedplant, the cycle conditions under which theseplants operate need to be carefully evaluated,taking into account such parameters 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 these con-figurations utilizes materials and design featuresappropriate to ensure long turbine life with reli-ability levels comparable to conventionaldesigns.

Note: This paper was originally presented atPower Gen Europe ‘96.

REFERENCES1. R.C. Spencer, "Design of Double Reheat

Turbines for Super-Critical Pressures", pre-sented at the 1980 American PowerConference, Chicago, Ill.

2. G.P. Wozney, M. Akiba, G.L. Touchton, R.I.Jaffee, S.J. Woodcock, "Turbine Research andDevelopment for Improved Coal-Fired PowerPlants", American Power Conference, April14-16, 1986

3. K.M. Retzlaff and K. Aizawa, "TurbineDesigns", First International Conference onImproved Coal-Fired Power Plants,November 19-21, 1986

4. J. Kure-Jensen and K. Retzlaff, “A 440 MWExtraction Steam Turbine for AdvancedSteam Conditions”, International JointPower Generation Conference, 1994

5. J. Kure-Jensen, A. Morson, P. Schilke, “LargeSteam Turbine for Advanced SteamConditions”, EPRI Conference, March 1993

6. J.I. Cofer IV, "Advances in Steam PathTechnology" presented at Power GenerationEurope, April 1995

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© 1996 GE Company

Figure 22. Diaphragm Segment with AdvancedNozzle Partitions

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LIST OF FIGURES

Figure 1. Tandem-Compound Double-Reheat Supercritical Steam TurbineFigure 2. Tandem-Compound Double-Reheat Supercritical Steam TurbineFigure 3a. Heat Rate Improvement from Steam Cycle with Ultrasupercritical Steam ConditionsFigure 3b. Heat Rate Improvement from Steam Cycle with Ultrasupercritical Steam ConditionsFigure 4. Typical Single Reheat Cycle with Heater Above Reheat PointFigure 5a. Effect of Final Feedwater Temperature and Reheat Pressure on Turbine Net Heat RateFigure 5b. Effect of Final Feedwater Temperature and Reheat Pressure on Turbine Net Heat RateFigure 6. Reheat Pressure Cross Optimization for Double Reheat UnitsFigure 7. Double Reheat Cycle with Heater above Reheat PointFigure 8. Crossover Temperature vs. Second Reheat PressureFigure 9. Single-Reheat Ultrasupercritical Product LineFigure 10. Combined HP/IP Section of Ultrasupercritical TurbineFigure 11. Separate HP and IP Sections of Ultrasupercritical TurbineFigure 12. Separate HP and Double-Flow IP Sections of Ultrasupercritical TurbineFigure 13. Ultrasupercritical Steam Turbine Designed for 2-Stage District Heating ApplicationFigure 14. Double-Reheat Ultrasupercritical Product LineFigure 15. HP and Reheat Sections of a Double-Reheat UltrasupercriticalFigure 16. Reheat Stage Cooling Configuration for Opposed Flow HP/IP SectionsFigure 17. Typical Boiler Characteristic for USC Unit (Hybrid Pressure 310 bar, 395 C/593 C/593 C

CycleFigure 18. Effect of Part Load Operation on Cooling EffectivenessFigure 19. Main Steam Inlet ConstructionFigure 20. Finite Element Model of USC HP/IP Inner ShellFigure 21. Predicted HP/RHT Inner Shell Stress Distribution at Peak Load (Normalized to Maximum

StressFigure 22. Diaphragm Segment with Advanced Nozzle Partitions

LIST OF TABLES

Table 1. Heat Rate Impact of Alternative Feedwater Heater Configurations

GER-3945A

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