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 GEs 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

GEs 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 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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GER-3945A

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

GT25590

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.