ger-4201 - structured steam turbines for the combined ... · steam turbine-generator output. in a...

gGE Power Systems

StructuredSteam Turbines for theCombined-Cycle Market

Dave ColegrovePaul MasonKlaus RetzlaffDaniel CornellGE Power SystemsSchenectady, NY

GER-4201

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Cycle Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

IP Admission and Reheat Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2LP Admission Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Structured D-11 Design Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Opposed Flow HP/IP Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Steam Path Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Low-Pressure Section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Application Rules for the Structured D-11 Steam Turbine. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6LSB Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Other Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Heat Balance Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Bypass System Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Advantages of Structured D-11 Steam Turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Delivery Cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Customer Drawing Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Common Spare Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Installation Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Future Structured Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11DX2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11A-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11DX4/GX1 Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Structured Steam Turbines for the Combined-Cycle Market

GE Power Systems ■ GER-4201 ■ (05/01) i

GE Power Systems ■ GER-4201 ■ (05/01) ii

AbstractGE’s variety of robust steam turbine products hasproven to be a valuable choice in today’s highlycompetitive, combined-cycle marketplace. A dis-cussion of the GE steam turbine offering for 2-on-1, “F” technology, gas turbine, combined-cycle plants is the main focus of this paper, withemphasis placed on the structured D-11 product– the customer’s choice for delivery cycle, per-formance, reliability, and availability.

IntroductionTo date, GE has built over 40 steam turbinesused in “F” technology, gas turbine, combined-cycle applications, totaling over 6000 MW insteam turbine-generator output. In a GE SteamAnd Gas (STAG) application, the steam turbineis matched with one or more gas turbines, uti-lizing the exhaust energy from the combustionturbine(s) to produce steam through a heatrecovery steam generator (HRSG). A typical GEconfiguration uses a three-pressure HRSG forthe plant, where steam is supplied from high-pressure (HP), intermediate-pressure (IP), andlow-pressure (LP) drums to the correspondingsection of the steam turbine.

In the past, GE’s design philosophy dictated

standardization of some of the major turbinecomponents, but customization of the steampath for each application. In 1997, in responseto customers’ continual demands for shorterdelivery cycles and higher efficiency, GE recog-nized the need to take a more proactiveapproach to meet the demands of a competitiveand growing marketplace.

To be competitive in this market, GE needed asteam turbine product that was both efficient atbaseload conditions and robust enough to beused in a variety of climates, configurations, andoperating modes. While only a custom-designedunit could operate at peak efficiency in any givensituation, the design and production of such aunit would result in a prohibitively high priceand an excessively long delivery cycle. This wasnot an option for a domestic U.S. market thatwas beginning to add significant capacity for thefirst time in many years. Based on an analysis ofmarket activity, GE focused its standardizationeffort on steam turbines for 207FA and 209FAcombined-cycle plants. GE’s product for theseparticular applications is the D-11 turbine, adesign consisting of a combined, opposed-flow,HP/IP section with single-shell construction, anda two-flow LP section (Figure 1).


GE Power Systems ■ GER-4201 ■ (05/01) 1

Figure 1. GE’s D-11 steam turbine

The results of this design standardization yield-ed five basic D-11 structured configurations,which are listed in Table 1. For the 60-Hertz(Hz) market, three standard LP sections havebeen designed with last-stage bucket (LSB)lengths of 30 in. (76.2 cm), 33.5 in. (85.1 cm),and 40 in. (101.6 cm). For the 50 Hz marketthere are two standard LP sections, based onLSB lengths of 33.5 in. (85.1 cm) and 42 in.(106.7 cm).

Cycle OptimizationThe starting point for designing the structuredD-11 product is the highly efficient and reliable,three-pressure HRSG design, with nominal

1800 psia/1050°F (124 bar/566°C) throttleconditions and 1050°F reheat temperature.Given that the basic bottoming cycle parame-ters were already determined, efforts were cen-tered on determining the optimum IP and LPadmission pressures in terms of overall cycleand steam turbine efficiency.

IP Admission and Reheat Pressure As shown in Figure 2, variation in hot reheat pres-sure does not have a significant effect on steamturbine generator output over the range consid-ered. The reheat pressure will ultimately set theIP admission level since the IP admission is intothe cold reheat line. The hot repeat pressureimpacts the volume flow of the reheat system,and therefore, has a major influence on thedesign of both the HRSG and the steam turbine.Hot reheat pressure for the cycle is set by the flowpassing area of the first IP turbine nozzle. ForGE’s structured D-11 product, the hot reheatpressure for the baseload condition was set at 333psia (23 bar) for the 207FA configuration and366 psia (25.2 bar) for the 209FA configuration.Since these results are very close to the com-bined cycle optimum level, GE’s designs for theHRSG and steam turbine are both cost effectiveand mechanically conservative.



STAG plant 207FA 209FA

Casings 2 2

HP Stages 11 10

IP Stages 7 8

LP Stages (per flow) 5 5

RPM 3600 3000

LSBs 30 in. 33.5 in.

33.5 in. 42 in.

40 in.

Table 1. Structured D-11 configurations

Figure 2: Effect of Hot Reheat Pressure on Steam Turbine Output

-0.4

-0.2

0

0.2

0.4

260 280 300 320 340 360 380 400 420 440

Hot Reheat Pressure ( psia)

Rel

ati

ve

Ste

am T

urb

ine

Ou

tpu

t (%

)

207FA 209FA

Figure 2. Effect of hot reheat on pressure steam turbine output

LP Admission Pressure The second parameter that GE investigated foroptimization was the LP admission pressurelevel, including the place within the steam tur-bine flow path to locate this admission. Theeffect of steam turbine output based on the vari-ation of LP admission preassure is shown inFigure 3. This optimization considered steamturbine output effects, HRSG surface areaeffects and stack exit temperature, volume flowcriteria, and location of admission interfacewith the steam turbine. As a result of the analy-sis of the parameters mentioned above, the low-pressure admission was located in the IPexhaust region of the steam turbine. Becausethe IP exhaust passes directly into the low-pres-sure turbine crossover pipe, the pressure in thecrossover pipe is directly set by the HRSG LPdrum pressure level.

As a result of extensive cycle and steam turbineefficiency optimizations as well as the carefulselection and design of the IP and LP steampaths, GE was able to establish a common LPadmission pressure and effective flow passingarea (AeN). Because of this work on the stan-dardization of the crossover pressure, it was nowpossible to design, for a given class of turbine(207FA or 209FA), a single IP section that was

compatible with a variety of standardized low -pressure sections. The optimized LP Bowl pres-sures were set at 55 psia (3.8 bar) for the 207FAconfiguration and 66 psia (4.5 bar) for the209FA machine.

Steam turbine condensing pressure has a largeinfluence on steam turbine output and variesdepending on the available condensing medi-um. Knowing the optimum required LP admis-sion/LP crossover pressure made it possible forGE to match the fixed IP turbine with a newlydesigned series of standardized low-pressureturbine sections with different last-stage bucketsand annulus areas for different condensingpressures. These LP modules can be inter-changed without impact to the HP/IP turbinedesign.

Structured D-11 Design FeaturesThe optimized 207FA and 209FA thermal cycleshave enabled the development of a standard-ized family of steam turbines. A cross-sectionaldrawing is shown in Figure 4.

Opposed Flow HP/IP Section The structured D-11 steam turbine evolvedfrom the opposed-flow, HP/IP turbine with adouble-flow LP section, a design that has been



Figure 3: Relative Steam Turbine Output vs LP Bowl Pressure

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

35 45 55 65 75 85

LP Bowl Pressure ( psia)

Rel

ativ

e St

eam

Tub

ine

Out

put

(%)

207FA

209FA

Turb

ine

Ou

tpu

t (%

)

Figure 3. Relative steam turbine output vs. LP admission pressure

applied in fossil and combined-cycle applica-tions for many years. Main steam enters the tur-bine at the bottom of the high pressure shell viatwo separate stop and control valves. The flowof HP steam continues to the left in Figure 4 andexits the section via the cold reheat line whereit returns to the HRSG. The reheated, interme-diate pressure steam enters the center of thecasing via the hot reheat piping and flowsthrough the IP section in the direction oppositethat of the HP section. This design results in aneven temperature gradient from the center ofthe casing to the ends, as the highest tempera-ture steam in the system enters at the center ofthe shell and then gradually reduces its temper-ature as it flows outward toward the end pack-ings and bearings.

The combined HP/IP section utilizes singleshell construction that has been proven by suc-cessful operating experience at a maximumoperating pressure of 1950 psia at an operatingtemperature of 1050°F. There are two HP/IPshell designs, one for 207FA, 60 Hz applicationsand one for 209FA, 50 Hz applications. Eachshell design is standard, with the interstagediaphragm grooving and supports alreadydesigned into the shell (Figure 5). Variability inthe steam path design is limited to the high

pressure section, with the HP staging cus-tomized for each application.

Steam Path Design Staging within the HP and IP sections is basedon low reaction design theory, which leads tothe use of wheel-and-diaphragm construction(Figure 6). Rows of rotating blades, or buckets,are machined from blocks of 12Cr steel, utiliz-ing a pinetree dovetail design, as shown inFigure 7. These buckets are assembled tangen-tially on a rotor wheel and locked into place bythe use of several specially designed closurebuckets and by bands or covers, which are fas-



HP Steam PathVariable by Design

HP/IP Shell (fixed)

IP Turbine (fixed)

Crossover (fixed)

LP Turbine (fixed)

IP Admission

Main SteamHot Reheat

Cold Reheat

Figure 4. Cross-section of the structured D-11 turbine

Figure 5. Machining of HP/IP casing

tened or “peened” over several buckets at atime. Stationary blades, or nozzles, are alsomachined from 12Cr steel and are assembled inthe outer ring and inner web portions of thediaphragm (Figure 8). The diaphragm sectionsare then affixed in grooves in the upper andlower halves of the shell.

The HP section was designed to accommodateup to 45% additional throttle mass flow basedon the site-specific requirements for supple-mentary firing. Because of the fixed IP steam

path and the variable range of reheater pres-sure drop, the cold reheat pressure varies with-in a certain range. Hence, this pressure varia-tion requires some customization of HP stagingfor each application. Since two 7FA or 9FA gasturbines provide a predetermined amount ofexhaust energy, and the HRSG surface areas aresomewhat standardized by the constraints dis-cussed earlier, it was possible to optimize HPturbine thermal performance, and to fix thenumber of high pressure stages at 11 for the207FA turbine and 10 for the 209FA turbine.With the fixed staging of the IP section, itbecame possible to closely control the HP/IProtor design in terms of forging size and bear-ing span. Rotor dynamic criteria have beenthoroughly analyzed so that the relatively smallsteam path variations allowed in the high-pres-sure section do not require re-analysis of thedesign for each application.

Low-Pressure Section The low-pressure section designs are based onGE’s established, highly reliable and efficientfamily of last stage buckets (LSBs), shown inFigure 9. These buckets are of the continuously



Figure 6. Assembled HP/IP rotor

Figure 7. Tangential entry “Pinetree” dovetailbucket

Figure 8. Diaphragm section

coupled design, with attachments at both thevane tip and mid-vane to provide a high degreeof rigidity, model suppression, and damping.

Through use of computer modeling of the LPsection, GE found that this section could beoptimized with a 5-stage design. In addition,maximization of the steam turbine outputrequired redesigning the upstream LP stages,utilizing the most advanced, three-dimensionalblade design technology. This redesign effortresulted in an integrated and interchangeableset of low-pressure turbines, specificallydesigned for combined-cycle applications.

In previous designs, provisions for feedwaterheating extractions from the low-pressure tur-bine were included only if required by the spe-cific application of any low-pressure section.Extraction provisions for feedwater heating arenow included on all structured D-11 LP turbinesections.

Application Rules for the Structured D-11 Steam TurbineThe structured D-11 steam turbine is designedfor an 1800 psia inlet pressure at nominal flowconditions. Like most combined-cycle steam tur-bines, normal operation is with valves wide open

in boiler-following mode. Once the guaranteepoint inlet pressure is established, the cor-responding HP turbine flow passing area (oth-erwise known as AeN) becomes fixed, at whichpoint inlet pressure will vary directly with inletflow. Table 2 summarizes the key design parame-ters for the structured D-11 turbine. When sup-plementary firing is applied, the maximum inletpressure for the fired case is allowed to floathigher than the unfired case. This is permissible,given that the additional flow generated by sup-plementary firing causes a greater pressure dropacross the inlet valves and piping, so that thesame pressure will be seen at the high pressurebowl. If the intent is to apply a significant level ofsupplementary firing only during periods ofpeak energy demand, it is necessary to set theunfired inlet pressure at a much lower value.For instance, if up to 20% supplemental firingis anticipated on an intermittent basis, thenthe unfired pressure should be set at 1910 psia/1.2 = 1592 psia. (See Figure 10.)

Note that in Table 2, the inlet AeNs of both theIP turbine and LP turbine are already fixedbecause, unlike the HP turbine, the designs ofboth the IP and LP sections of the steam pathare based on the optimizations mentioned inthe “Cycle Optimization” section of this paper.These inlet AeNs remain fixed, regardless of the



Figure 9. Last stage bucket family

35

40

45

50

55

60

65

70

1000 1100 1200 1300 1400 1500 1600

ENTHALPY (btu/lb)

FL

OW

FU

NC

TIO

N (w

/p)

Figure 10. Flow function vs. enthalpy

amount of supplemental firing. Hence, forgiven mass flows, the pressures at the inlets ofthe IP and LP sections can be established. If thecycle is fired, then the additional flow will resultin higher pressures at these points.

AeN, or the pressure that results from establish-ing the AeN, may be reasonably estimated fromthe equation:

AeN = F/ (w/p) x P ; or

P = F/ AeN x (w/p),

where:

F = Flow in lb/hr

AeN = Flow passing area in sq. in.

(w/p) = Flow function, determined from the graph in Figure 10, once enthalpy is known

P = Initial pressure, in psia

Close attention must be paid to the pressure vs.AeN equation to ensure that the turbine andHRSG are properly matched. Table 2 showsAeNs for the IP and LP inlets, and the nominal

pressures associated with each of these points ifthe thermal cycle is configured around theseparameters.

It is important to note that under all steady stateoperating conditions, both the main steam inletand reheat steam inlet are designed to accom-modate a maximum temperature of 1050°F.

It can be seen from Table 2 that two sets of coldreheat pressure values are given. The firstassumes a total of 6% pressure drop throughthe reheat section of the HRSG including coldand hot reheat piping, while the secondassumes a total of 12% pressure drop. By usingthese pressure drops, the cold reheat valuesmay be predicted knowing that the reheat tur-bine inlet AeN is set at 74.38 in2 (479.87 cm2)for the 60 Hz turbine and 101.78 in2 (656.64cm2) for the 50 Hz turbine. This flow restrictioncontrols the pressure in the reheat section ofthe HRSG and therefore, the pressure at theturbine high-pressure section exhaust.

Similarly, the LP bowl AeN is set at 421 in2

(2716 cm2) for the 60 Hz turbine and 513 in2



ST AG Configuration 2 07FA 207FA 209FA 209FA

M achine Spe e d (RPM ) 3600 3600 3000 3000

Supple me ntary Duct Firing UN FIRED

M AX

FIRED UNFIRED

M AX

FIRED

Machine rating MW 180 265 283 400

Throttle pressure limit psia

Throttle temperature limit F

HP AeN Sq-in 1 1.75 17.27 17.56 25.73

Cold reheat pressure at turbine flange,

based on 12% reheat pressure droppsia 379 509 416 548

Cold reheat pressure at turbine flange,

based on 6% reheat pressure droppsia 355 476 390 513

RHT pressure drop (Min.) %

RHT pressure drop (Nominal) %

RHT pressure drop (Max.) %

Hot reheat pressure psia 333 448 366 482

Hot reheat temperature F

IP/Reheat bowl pressure (Nominal) psia 330 443 362 477

IP/Reheat bowl AeN Sq-in

IP exhaust pressure (Nominal) psia 56 71 69 87

LP admission pressure at valve psia 58 73 71 89

LP bowl AeN Sq-in 421 513

1050 1050

74.38 101.78

6

10

12

6

10

12

1890 1910 1890 1910

1050 1050

Table 2 Thermal Application Data

(3310 cm2) for the 50 Hz turbine. This parame-ter controls the pressure in the turbinecrossover and therefore, the IP turbine exhaust,which is also the LP steam admission point.There is normally a total of about 2-psi pressuredrop across the LP admission strainer, LP but-terfly control valve and LP butterfly stop valve,admission pipe and turbine inlet flange. This isshown in Table 2 as the pressure differencebetween IP nominal exhaust pressure and LPadmission pressure.

LSB Selection When configuring any steam turbine, it is veryimportant to choose the proper annulus areafor the anticipated exhaust flow and condenserpressure. Figures 11a and 11b show potentialchoices of last stage buckets for 60 Hz and 50Hz applications, respectively. Given the designpoint of the turbine and the range of condens-ing pressures, the optimum LSB can be select-ed, and from there, the associated annulus areamay be calculated. Economic factors come intoplay when selecting low-pressure turbine sec-

tions, but the use of Figure 11 together with theLP turbine data shown in Table 3 provides theproper selection for most applications, whereLP exhaust loss is minimized for a particularcondenser pressure.

Other Features Structured D-11 steam turbines have additionalflexibility because of the following thermalcycle variations that were taken into account as

part of the conceptual design process:

1. Two-pressure reheat cycle (no LPadmission). If fuel oil (containingsulfur) is the primary or secondaryfuel, the thermal cycle will not supportthe third level of steam generation inthe HRSG. A structured D-11 turbineapplied to such a cycle should beconfigured without the LP admissionport.

2. Process extraction from HP or IPexhaust piping, as shown schematicallyin Figure 12. The shell connections and



Steam Turbine Output vs. Exhaust Pressure207FA D11 Structured

170

175

180

185

190

195

200

0 0.5 1 1.5 2 2.5 3 3.5 4

Exhaust Pressure ( inch Hg)

Ste

am T

urb

ine

Gen

erat

or

Ou

tpu

t (M

W)

2 F-40.0" LSB

2 F-33.5" LSB

2 F-30.0" LSB

The generator output is approximate, based on typical207FA 1800 psia 1050F / 1050F Combined Cycle Conditions

Figure 11a. Output vs. exhaust pressure – 60 Hz

Steam Turbine Output vs. Exhaust Pressure209FA D11 Structured

255

260

265

270

275

280

285

0 0.5 1 1.5 2 2.5 3 3.5 4

Exhaust Pressure ( inch Hg)

Ste

am T

urb

ine

Gen

erat

or

Ou

tpu

t (M

W)

2 F-42.0" LSB

2 F-33.5" LSB

The generator output is approximate based on typical 209FA 1800 Psia 1050 F / 1050F Combined Cycle Conditions

Figure 11b. Output vs. exhaust pressure – 50 Hz

IP staging are designed to withstandthe additional loads caused by processextraction flows.

3. Feedwater heating deaerationextraction from low-pressure turbinesection. (Generally used for cycleswhere the gas turbine fuel hasrelatively high sulfur content)

4. Application of 1000°F/1000°F cycletemperatures in lieu of the standard1050°F/1050°F, due to economicconsiderations, which allows the use of(less expensive) P22 main steam andhot reheat piping, rather than themore expensive P91 piping.

5. Application of two different GEgenerators at both 50 Hz and 60 Hz toaccommodate the range of output,considering the steam turbine outputdifference between unfired andmaximum supplementary fired cases.

Heat Balance Requirements The information given above will allow a con-ceptual steam turbine design to be successfullyincorporated into the thermodynamic design ofthe plant. It is necessary, however, to pay strictattention to the entire range of operating sce-narios to which the plant will be subjected andto anticipate such occurrences in the design ofthe steam turbine, so that reliability and per-



Note: All pressures are approximate

207FA 207FA 207FA 209FA 209FALP LSB Length inch 40 33.5 30 42 33.5

Back pressure range w/o firing inch Hg 1.0 - 2.3 2.3 - 2.8 2.8 - 3.5 1.0 - 2.5 2.5 - 3.5Back pressure range with firing inch Hg 1.2 - 2.9 2.9 - 3.5 3.5 - 4.5 1.2 - 3.0 3.0 - 4.5

LP bowl pressure w/o firing psia 55 55 55 66 66

LP bowl AeN sq-in 421 421 421 513 513

LP extraction stage for DA - L-4 L-4 L-4 L-4 L-3LP extraction size for DA inch 2x14 2x14 2x14 2x16 2x16LP extraction flow % of LP bowl % 10 10 10 10 10

Table 3. LP turbine data for structured D-11 steam turbines

HP IP

LP

Reheater

LP Admission

CRV

SV/CV

IP-ADM

HRSG

Process Extraction (optional)

DA (optional)

Process Extraction (optional)

GEN

Figure 12. Schematic showing structured D-11 layout with possible extractions

formance targets are met. In addition to theguarantee point heat balance data, GE alsorequires the heat balance data at the maximumand minimum ambient conditions for whichthe plant will be designed. Simply put, cold airis denser than hot air, so that on a cold day thegas turbines will pass a greater mass flow andproduce more power and exhaust energy. Thisin turn drives greater steam production fromthe HRSG, which results in greater flow to theHP turbine, and a corresponding higher throt-tle pressure. On a maximum ambient tempera-ture day, the reverse scenario takes place, butthe decreased steam production will result inpotentially higher steam temperatures. Sincethe plant cannot operate safely at temperaturesabove 1050°F, excess heat must be handled byattemperation, or through features in the over-all plant design. Therefore, at a minimum, thefollowing three heat balances must be available:

1. Cold ambient day steam conditions.

2. Hot ambient day steam conditions.

3. Guarantee point steam conditions.

If these heat balances do not fully describe theoperating envelope with respect to maximumthrottle pressure and temperature, maximumand minimum IP and LP admission flows, andmaximum and minimum process extractionflows, then additional heat balances will berequired. This information is used to ensurethat temperatures and pressures within the tur-bine steam path are accounted for in the designof the HP section, and evaluated against thepre-established design limits of the IP and LPsections.

Bypass System Information Bypass system data is additional informationnecessary to successfully release any steam tur-bine for steam path design. Most modern com-

bined-cycle power plants use the “Cascading”type of bypass system, for which the structuredD-11 steam turbine may be configured as a stan-dard option. Specific bypass system informationrequired is:

1. Bypass configuration (i.e., cascading,or other configuration);

2. HP and LP bypass system capacities,expressed as a percentage of mainsteam flow; and

3. HRSG floor pressure (this parametermust be provided by the HRSGvendor).

This information enables the high pressureexhaust set point to be established, to enablebypass mode thermal modeling of the HP, IP,and LP turbines. This ensures that the low flowforward through the IP and LP turbines, andreverse flow through the HP turbine, do notcause overheating of any stages; a very impor-tant consideration in a machine alreadybrought to 1050°F at the main steam and reheatsteam inlets, and also continuing to rotate atrated speed. The floor pressure information iskey to establishing:

1. Transfer point from reverse flow toforward flow in the HP section;

2. HP turbine exhaust temperatureduring the flow transfer operation;and

3. No excessive windage heating isoccurring in the HP section duringthis low flow, high backpressureoperating regime.

The bypass system flow information is then usedto establish proper sizing for the HP reverseflow valving so that sufficient cooling steam willbe available for all operating situations.



Advantages of Structured D-11 SteamTurbine

Delivery Cycle Design standardization permits the structuredD-11 steam turbine to be offered with 12months ex-factory shipment from release date.Since the design of items which require longlead times will be essentially complete, GE willforecast reserve capacity and volume with expe-rienced suppliers, resulting in shorter deliverycycles for rotor forgings, castings, and exhaustfabrications.

Customer Drawing Availability Critical customer drawings will be availableimmediately after the customer gives GE noticeto proceed. The product is specifically designedso that minor adjustments in the high pressuresteam path to configure the turbine for thethermal cycle conditions of a particular applica-tion do not change the outline dimensions,component weights, sole plate layout or foun-dation loadings. This design consistency allowsarchitect engineers and owners to get an earlystart on the turbine foundation design, over-head crane specification, auxiliary equipmentplacement, and design of piping and electricalsystems.

Common Spare Parts Spare parts inventory can be reduced from thelevels required prior to standardization of theD-11’s design. All possible variants of the struc-tured D-11 steam turbine have common com-ponents throughout. Items such as valve stems,valve discs, journal bearings, thrust bearing,shaft end packing, interstage packing, spillstrips, horizontal joint shell bolting, auxiliarysystem components and various gaskets will becommon to all D-11 turbines.

Installation Time Installation of the structured D-11 turbines hasbeen simplified and will proceed more quicklythan installation of non-structured turbines.When it is shipped from the factory, the HP/IPsection of the turbine will be fully assembledwith diaphragms and rotor installed and prop-erly aligned, and with the horizontal joint shellbolts fully tightened. Delivering the HP/IP tur-bine pre-assembled saves about four weeks offield erection time.

Future Structured ApplicationsThe structuring philosophy that was used tostandardize the D-11 turbine is also beingapplied to other turbines being built by GE.

DX2 The DX2 is GE’s new family of high-efficiencysteam turbines, designed for both 207F and209F applications. These new turbines featureseparate casings for the HP and IP sections,while utilizing the LP sections that were devel-oped in the structured D-11 design program.

A-10 The A-10 design consists of a single HP sectionand a combined IP/LP section and is used pri-marily in 107F and 109F multi-shaft applica-tions. Although this design utilizes separate cas-ings, it is compact, and has the additional fea-ture of not requiring a crossover pipe.

DX4/GX1 Designs GE is currently developing steam turbines forcombined-cycle plants that are designed tooperate with inlet conditions of 2400 psia (165bar) and 1050°F (566°C). Although thisincrease in operating pressure requires use ofmore expensive balance of plant (BOP) com-ponents, the inherent benefit in overall cycleperformance can outweigh the higher initial



capital investment in certain operating environ-ments.

As a result of the structuring process, GE’s deliv-ery cycle for these optimally designed steamturbines will be comparable to that of the struc-tured D-11 line.

ConclusionThe structured D-11 steam turbine is a highlyefficient, highly reliable, cost-effective steamturbine, configured specifically for 207FA or209FA combined-cycles. Within the base design,there is allowance for significant variation onthe basic three-pressure level reheat condens-ing cycle, while maintaining a 12-month ex-fac-tory shipping commitment. The concept of

product structuring has proven to be valuableon the D-11 turbine, and will be equally benefi-cial on future GE steam turbines.

References1. Boss, M., “Steam Turbines for STAG™Combined-Cycle Power Systems,” Paper No.GER-3582D, GE Power Generation TurbineTechnology Reference Library, 1996.

2. Mason, P.B. and Reinker, J.K., “SteamTurbines for Large Power Applications,” PaperNo. GER-3646D, GE Power Generation TurbineTechnology Reference Library, 1996.

3. Gorman, William and Stueber, Henry, “EverDecreasing Cycles,” Power EngineeringInternational, May, 1999.



List of FiguresFigure 1. GE's D-11 steam turbine

Figure 2. Effect of hot reheat pressure on steam turbine output

Figure 3. Relative steam turbine output vs. LP admission pressure

Figure 4. Cross-section of the structured D-11 turbine

Figure 5. Machining of HP/IP casing

Figure 6. Assembled HP/IP rotor

Figure 7. Tangential entry, “pinetree” dovetail bucket

Figure 8. Diaphragm section

Figure 9. Last stage bucket family

Figure 10. Flow function vs. enthalpy

Figure 11a. Output vs. exhaust pressure – 60 Hz

Figure 11b. Output vs. exhaust pressure – 50 Hz

Figure 12. Schematic showing structured D-11 layout with possible extractions

List of TablesTable 1. Structured D-11 configurations

Table 2. Thermal application data

Table 3. LP turbine data for structured D-11 steam turbines



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