Steam Turbine Uprates

g GER-4199 GE Power Systems

J. F. Lesiuk GE Power Systems Atlanta, GA

Steam Turbine Uprates

Contents

Summary ...... 1 Background ...... 1 Dense Pack Section Replacement ...... 2 High Efficiency Steam Path ...... 3 Sustained Heat Rate and Solid Particle Erosion (SPE) Resistance...... 7 Conclusions ...... 9 Acknowledgement ...... 10 References...... 11 List of Figures ...... 11

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GE Power Systems GER-4199 (10/00) ii Steam Turbine Uprates

Summary features achieve additional improvement in efficiency. The thermodynamic performance of a steam turbine is primarily determined by the steam As a natural progression from the ADSP experi- path components. Because the efficiency of the ence, GE introduced the “Dense Pack” redesign entire power plant cycle is largely dependent on approach to achieve additional section efficien- the efficiency of the energy conversion in the cy improvement. turbine, it is important to minimize aerodynamic and steam leakage losses in the steam Background path. During the mid-1980s and early 1990s the prin- Nozzle and bucket aerodynamic-profile losses, ciple reason for making steam turbine uprate secondary-flow losses, and leakage losses decisions was the aging of power plants, (see account for roughly 80-to-90 percent of the total Figure 1), and the attendant poorer reliability of stage losses. In order to ensure high-efficiency older equipment, as shown in Figure 2. turbine designs without sacrificing turbine reliability, it is necessary to use both highly-efficient 35 nozzle and bucket designs to minimize profile 30 and secondary losses, and advanced clearance 25 controls to minimize leakage flows. 20

GE’s development effort has been underway for 15 more than a decade in order to reduce these 10

losses. The objective of this long-term program Capacity of Percent 5 is to develop specific design features for both new turbines and retrofits of existing units that 0 1970 1975 1980 1985 1990 1995 2000 maximize overall turbine efficiency and main- Year tain a high degree of reliability and cost effec- Figure 1. Utility dependence upon units greater tiveness. The development program has includ- than 30 years old ed activities conducted for all of the steam turbine product lines in cooperation with other company components such as Aircraft Engines, Utilities evaluated their aging fleet and con- Gas Turbine, and Corporate Research & cluded it was more economical to extend the Development. The result of this development life of the equipment than to retire the unit. effort was the introduction of the Advanced This resulted in turbine life extension evalua- Design Steam Path, ADSP, in 1995. tions to identify equipment that needed repair or replacement in order to operate beyond the Since 1995, GE has installed over 40 first- and generally-accepted design life of 40 years. second-generation ADSP retrofits on a variety of units. Field test data from these units has indi- Talk of deregulation of the power industry dur- cated steam path efficiency improvements rang- ing the mid-1990s created considerable uncer- ing from 1.5 percent to 3.0 percent. Along with tainty among the utilities. Decisions regarding this, GE has been developing new NPI features, the sale of generating and/or transmission and such as enhanced advanced-vortex designs, distribution assets were analyzed. One clear integral cover buckets, and brush seals. These conclusion from the deregulation discussions

GE Power Systems GER-4199 (10/00) 1 Steam Turbine Uprates

electricity with additional capacity would be the victor in the new, unregulated market.

8 Today, in addition to addressing the aging fleet

7 issue, utilities are looking for the competitive Rate (%) Rate

5 edge that additional capacity and better per-

4 formance will provide. Other factors that enter 3 into the economic model are the desire for sus- 2

1 tained performance with minimal degradation Forced Outage Outage Forced 0 over period of at least ten years and the desire 0 51015 20 25 30 35 40 45 50

Age in Years to extend time between major overhauls to at least ten years. The balance of the paper dis- Figure 2. Typical aging reliability trend cusses the technologies and product solutions developed to address these utility needs. was that the utilities were going to be in an extremely competitive environment in the Dense Pack Section Replacement future. The Dense Pack turbine section performance, The initial reaction to deregulation within the as shown in Figure 3, is the latest evolution of GE utility industry was to take a “wait and see” posi- steam design that began in 1903 with a 5000 kW tion. As decisions regarding the retention or turbine. The design limits, operational experi- selling of assets were made, new drivers in the ence, and rugged dependability synonymous cost model developed. Evaluations for replace- with GE turbine design is not changed with a ment power during peak periods soared and it Dense Pack. became clear that additional capacity and improved efficiency were the principle objec- The design goal of a Dense Pack retrofit is to tives for the future. The low cost producer of put the most efficient steam path into an exist-

98 97

90 HP Steam Path Efficiency - % HP Steam Path 89

88 1960’s 1970’s 1980’s 1990’s Late 1990’s 2000’s 2000’s Free-vortex Improved Step RTSS, AV1, RTSS AV2, ICBs Dense Pack Dense Pack design Vane profiles SPE, CV stgs adv opt. clearance w/adv Sealing

Figure 3. Efficiency evolution of a single HP stage

GE Power Systems GER-4199 (10/00) 2 Steam Turbine Uprates ing high-pressure outer shell. The high efficien- installed onto the existing steam turbine rotor cy steam path will produce a lower heat rate and and inner shell. This limited the designers’ increased output for the same steam flow. In flexibility because the number of turbine stages designing the most efficient steam path, two and rotor diameters was fixed. interesting by-products also resulted. The With more than 40 ADSP packages currently in design parameters utilized to increase efficien- operation, the ADSP program provided a good cy, such as bucket and nozzle solidity and experience base for the Dense Pack. ADSP reduced rotor diameters, also had the benefit of incorporates some of the features to be includ- reducing solid particle erosion. This led to a ed in Dense Pack such as steam flow manage- steam path that not only is more efficient but ment, optimized packing clearances, and also has a greater, sustained efficiency. Coupled advanced shaft sealing. Pending deregulation with the supply of a solid rotor (elimination of and decreasing reserve margin have driven the the rotor bore), the sustained efficiency results utility marketplace to seek ever more aggressive in a turbine section that should not require methods of lowering bus bar cost and increas- internal repair or inspection for ten or more ing capacity. Armed with the experience gained years. These complementary, interacting bene- from the ADSP program, improved turbine fits produce additional megawatts at a lower life- design and modeling tools, shared technology cycle cost. from GE’s aircraft engine and gas turbine prod- High Efficiency Steam Path uct lines, and customer desire for steam path In the early 1990s, GE produced an Advanced enhancements, the Dense Pack team was Vortex bucket and diaphragm retrofit known as formed. Advanced Design Steam Path (ADSP), Figure 4. The Dense Pack team began by reviewing the Although ADSP remains a cost-effective effi- turbine to determine where to focus design ciency retrofit, the potential efficiency benefit efforts to improve efficiency. Figure 5 is a Pareto of ADSP is limited because the retrofit is chart of unrecoverable turbine losses by turbine

Advanced aero buckets All stages, with advanced bucket tip sealing Advanced aero partitions All diaphragms Additional Diffusion coated 1st stage nozzle with SPE Profile Partition Diamond Tuff™ HVOF coating (stage 1 buckets, 1st reheat diaphragm,1 st reheat buckets)

Figure 4. Advanced design steam path

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3.0 (48.9%) Typical 700 MW unit

2.5 HP section irreversibility

2.0

1.5

1.0

0.5 irreversibility, % inlet availability

0.0 valves IP inlet HP inlet LP inlet IP turbine IP exhaust IP cross over LP turbine HP turbineHP LP exhaust LP HP exhaust HP feed pumps feed reheat cycle piping losses Heat rejection condensor losses condensor feedwater heaters feedwater Figure 5. Efficiency loss by system section. Unrecoverable losses are essentially more controlled, and more efficient incre- inefficient use of the steam energy. A review of ments. Reducing the stage root diameter results the Pareto chart shows that except for the heat in increased bucket and nozzle partition losses of the condenser, the HP and IP turbine heights. Secondary losses decrease as the parti- sections have the greatest amount of irre- tion height increases. In addition, reducing the versibility. rotor diameter results in a reduction in shaft sealing leakage area, further improving effi- The efficiency upgrade focused on HP and IP ciency. The smaller steam path diameter also sections and attacked the tip leakage and steam results in lower steam velocity and reduces the path secondary and profile losses. effect of any solid particle carryover. The Dense Pack team drew on cross-functional The steam path design incorporates reduced GE engineering resources from steam and gas solidity concepts or less buckets and nozzles per turbine design, Aircraft Engines, and Corporate row. The reduction in steam velocity coupled Research and Development (CRD). Their with fewer partitions results in lower profile loss- objective was to design the most efficient steam es per stage. Typical efficiency gains vs. sources path that could be installed into an existing of irreversible losses for a conventional and outer shell that included incorporating addi- Dense Pack steam path are shown on Figure 6. tional staging into the turbine section. Hence, The first step in the steam path redesign is to the name “Dense Pack,” for additional staging establish the mechanical constraints. The min- in the same axial spacing. imum rotor diameters are determined through Given the latitude of supplying a new steam tur- an analysis of the torsional stress requirements bine rotor and inner shell, the team capitalized and a rotor dynamics investigation including a on adding staging and reducing the rotor stage rigorous review of rotor stability. Once the min- diameter. Both of these concepts result in an imum rotor packing diameters are determined, inherently more efficient steam path. By adding the minimum steam path diameter can be stages, the energy may be extracted in smaller, established from the known radial height

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baseline dense pack Specific Irreversibility, BTU/lbm valves exhaust C.S. End iling Edge iling Edge inlet /bridge inlet Tip Leakage C.S. Windage Nozzle Profile Root Leakage Windage Loss Bucket Profile turnaround duct CS. Notch Block C.S. Tip Leakage Packing Leakage Notch Block Loss Nozzle Secondary Bucket Secondary C.S. Root Leakage C.S. Nozzle Profile C.S. Bucket Profile leakage and mixing Nozzle Trailing Edge Bucket Tra C.S. Packing Leakage C.S. Nozzle Secondary C.S. Bucket Secondary C.S. Nozzle Trailing Edge C.S. Bucket Tra Figure 6. Typical loss mechanism comparison requirements for the various bucket attach- stage number and bucket reaction levels for ments available to the turbine designer. This each steam path. Figure 7 shows a typical opti- allows the steam path optimization to proceed. mization contour plot of stage count vs. stage GE has always utilized some reaction levels in root reaction. Additional parameters are com- the design of high-pressure steam turbine buck- pared in order to arrive at the optimum value ets. Building on ADSP experience, the Dense for the key parameters that define the Dense Pack increases the bucket reaction to approxi- Pack steam path. mately 20-25 percent. Modern design and mod- Figure 8 shows the results of the Dense Pack opti- eling tools allow the computational iterations mization approach as applied to the redesign of necessary to individually optimize both the a single-flow, high-pressure section. The base-

Increasing Eff’y 25

15 Root Reaction

5 567891011

Number of Stages Figure 7. Optimization contour plot

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Baseline DoubleDouble flow inletinlet

6 stagesstages

Dense Pack Single flow inlet

Adv seals More stages Longer buckets Integral covers

Single flow control stage Smaller rotor and wheel diameters

Figure 8. Dense Pack redesign of a single-flow high-pressure section line design employed a 6-stage section with a zle diaphragm, depending on unit size and double-flow first stage. By lowering the steam number of control-stage admissions. path diameters and increasing the stage reac- Figure 9 is a photograph of an 11-stage, single- tion, a more efficient 14-stage steam path flow high-pressure rotor that was redesigned results. The major components affected by this from a baseline configuration of 7 stages includ- redesign include replacement of the bucketed ing a double-flow first stage. rotor, diaphragms, inner shell, and end packing heads. In addition, the original double-flow, Once the high efficiency steam path is funda- first-stage nozzle box is replaced with either a mentally designed for the number of stages, has single-flow nozzle box, a nozzle plate, or a noz- solidity and has sufficient rotor root diameter,

Figure 9. Dense Pack redesign of a single-flow high-pressure rotor

GE Power Systems GER-4199 (10/00) 6 Steam Turbine Uprates then design engineering adds sealing features Sustained Heat Rate and Solid Particle such as optimized packing clearances, integral Erosion (SPE) Resistance covered buckets, and advanced tip sealing (See Another GE design objective with the Dense Figure 10). Bimetallic seal rings complete the Pack redesign was to supply a steam path that offering and maximize steam efficiency. would be resistant to SPE degradation. In the

IntegralIntegral Covered Covered Buckets Buckets IntegralIntegral Covered Covered Buckets Buckets w/Adv w/Adv Sealin Sealingg

DenseDense Pack Pack Diaphragms Diaphragms

Figure 10. Dense Pack features

The Dense Pack design approach has been ver- 1980s GE performed extensive computer mod- ified through a rigorous Six Sigma Design of eling and analysis of solid particle trajectories in Experiment process in GE’s Steam Turbine Test the steam path. The comprehensive develop- Vehicle (STTV) located in Lynn, Massachusetts. ment effort resulted in the following: This state-of-the-art test facility was completed A fundamental understanding of the in 1999 and the first test was completed in April erosion mechanism in the steam of that year. The STTV is a fully-instrumented, turbine steam path. multi-stage test turbine using high-pressure Development of plasma spray and steam. The initial test in April of 1999 was to diffusion coatings that significantly establish a baseline representing the installed enhanced the SPE resistance of the fleet designs. This was followed by a series of steam path. tests using the Dense Pack design approach applied to the same steam conditions. Figure11 A combination of design changes shows the test facility and Figure 12 shows the identified by the trajectory analysis test rotors for the baseline and one of the Dense and SPE-resistant coatings that led to Pack tests. minimizing particle carryover damage.

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Figure 11. Steam Turbine Test Vehicle

Figure 12. Baseline and Dense Pack test rotors

These design changes included increased axial particle erosion. Clearly, the model and the GE clearance between the stationary nozzles and analysis were on target. The high-efficiency rotating buckets for the reheat section, steam path of the Dense Pack will virtually elim- increased nozzle scale factor for all stages, and inate solid particle erosion. Figure 13 depicts the redesigned nozzle profiles that allow smoother solid particle erosion of a conventional steam and less damaging passage of solid particles path nozzle partition. The severity of the ero- through the steam path. sion varies from zero, shown in blue, to the The study resulted in the redesign of the high- highest shown in red. pressure nozzle partitions to distribute the solid Figure 14 shows the benefits of the reduced noz- particle impact over a broader area of the parti- zle count, lower solidity, and the redesigned tion. Customer experience indicated a 75% nozzle partition. These features (lower nozzle reduction in the degradation attributed to solid and bucket solidity, redesigned partitions,

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Figure 13. Solid particle erosion on conventional steam path

Figure 14. Solid particle erosion on Dense Pack steam path

reduced steam velocities, and solid coatings) Conclusions yield a steam path that is more resistant to solid The Dense Pack section replacement is the lat- particle carryover. This will allow units to be est option in GE’s long history of steam path operating for longer periods between major efficiency improvements. Incorporating tech- overhauls because the rate of performance nologies from gas turbine, Aircraft Engines, degradation will be significantly reduced, if not and Corporate Research and Development, the virtually eliminated.

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Dense Pack provides an alternative to the proven solid particle erosion protection features, Advanced Design Steam Path design. The Dense address a utility concern of sustained perform- Pack alternative supplies the user with a ance. The resultant damage to the steam path redesigned steam path including a new bucketed from particle carryover is virtually eliminated, rotor, diaphragms, and inner shell. The increase enabling utilities to extend the time between in output and the reduction in heat rate address major overhauls and reduce life-cycle costs. the two major competitive issues facing the utility industry today. Acknowledgement The inherent features of Dense Pack, including a The author wishes to acknowledge the contribu- lower solidity design steam path, and fewer noz- tions of the Dense Pack team led by James zles and buckets per row, combined with GE’s Maughan.

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References 1. J.I. Cofer, IV, J.K. Reinker, W.J. Sumner, “Advances in Steam Path Technology,” GE Power Systems Paper, GER-3713. 2. R.S. Couchman, K.E. Robbins, P. Schofield, “GE Steam Turbine Design Philosophy and Technology Programs,” GE Power Systems Paper, GER-3705. 3. M.F. O’Conner, “Upgrade Opportunities for Steam Turbines,” GE Power Systems Paper, GER- 3696. List of Figures Figure 1. Utility dependence upon units greater than 30 years old Figure 2. Typical aging reliability trent Figure 3. Efficiency evolution of a single HP stage Figure 4. Advanced design steam path Figure 5. Efficiency loss by system Figure 6. Typical loss mechanism comparison Figure 7. Optimization contour plot Figure 8. Dense Pack redesign of a single-flow high-pressure section Figure 9. Dense Pack redesign of a single-flow high-pressure rotor Figure 10. Dense Pack features Figure 11. Steam Turbine Test Vehicle Figure 12. Baseline and Dense Pack test rotors Figure 13. Solid particle erosion on conventional steam path Figure 14. Solid particle erosion on Dense Pack steam path

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