GE 1000 MW USC TURBINE

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Recent Advances in Ultra Super Critical

Steam Turbine Technology

M. Boss T. Gadoury S. Feeny M. Montgomery

GE Energy, Steam Turbine Technology

1 River Road, Schenectady, NY 12345

Abstract – With the continuing drive to reduce power plant emissions including green

house gases, coal fired power plants have been moving to higher ultra-supercritical (USC) steam

conditions in addition to advances in technology. GE Energy has designed the next generation USC

steam turbine generator with a rating of 1000 MW to address the need for higher efficiency coal

fired power plants. With inlet steam conditions of 260 bar and 610ºC / 621ºC (3770 psi and 1150F /

1180F), the primary objective for the advanced technology USC 1000 MW steam turbine is high

efficiency. To achieve this higher cycle efficiency, the design utilizes advanced steam turbine

technology and system design and a longer last stage bucket design in addition to ultra

supercritical steam conditions.

Performance enhancing technology is being applied to turbine buckets, nozzles and seals.

In addition to improvements to steam path components, performance gains are achieved by

optimizing stationary components such as valves, inlets, and exhausts using advanced CFD tools.

This USC project illustrates the latest design and technology capabilities of GE Energy and

sets the standard for future 1000 MW USC applications.

1. INTRODUCTION

GE Energy was an early entrant into USC steam turbine technology with the first unit

shipped in 1956 with inlet steam conditions of 310 bar / 621C (4500 psi / 1150F). Since then, GE

has shipped 77GW of steam turbines (125 units) with supercritical steam conditions. GE designed

the world’s most powerful USC steam turbine rated 1050 MW operating at 250 bar / 600C / 610C

(3626 psi / 1112F / 1130F). This 1000 MW USC steam turbine design is a natural evolution of GE’s

USC technology. GE continues to develop and refine USC steam turbine technology.

With the emerging interest in reducing emissions, including green house gases from coal

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fired power generation, GE Energy is striving to increase USC PC generation output and efficiency in

its development of this 1000MW USC PC platform. Every 1% improvement in plant efficiency results

in approximately 2.5% reduction in green house gas emissions. To satisfy this objective, GE Energy

is looking to achieve the following advances in PC generation technologies:

MW rating: 1000MW MGR

HP throttle pressure: 260 bar (3770 psi)

HP throttle temperature: 610C (1130F)

Reheat steam temperature: 621C (1150F)

Condenser pressure: 1.5” Hg (NR Back Pressure: 2.5” Hg)

4 flow, 45 inch Last Stage Blade

Cycle: Single Reheat Regenerative

2. TECHNOLOGY of USC STEAM TURBINE

2.1 Cycle Overview

In the evaluation of steam conditions, the potential cycle efficiency gain from elevating

steam pressures and temperatures must be considered. Starting with the traditional 165 bar /

538°C (2400 psi / 1000°F) single reheat cycle, dramatic improvements in power plant performance

can be achieved by raising inlet steam conditions to levels up to 310 bar (4500 psi) and

temperatures to levels in excess of 600°C (1112°F). Every 28°C (50°F) increase in throttle and reheat

temperature results in approximately 1.5% improvement in heat rate.

The feedwater heater arrangement is designed to obtain the best heat rate for a given set

of USC steam conditions. In general, the selection of higher steam conditions will result in additional

feedwater heaters and a higher final feedwater temperature. The higher final feedwater

temperature will have an impact on the boiler cost. This then requires a system level optimization to

determine the best economical solution for the increase in final feedwater temperature. In many

cases, the selection of a heater above the reheat point (HARP) also is warranted. The use of a

separate de-superheater ahead of the top heater for units with a HARP can result in additional

gains in unit performance.

The selection of the cold reheat pressure is an integral part of any power plant design, but

becomes even more important for plants with advanced steam conditions. Comparing the heat rate

at the thermodynamic optimum, the improvement resulting from the use of a HARP can be about

0.6%. However, economic considerations of the boiler design without a HARP tend to favor a lower

reheater pressure at the expense of a slight decrease in cycle performance. The resulting net heat

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rate gain is usually larger, approaching 0.6-0.7%. Changing the final feedwater temperature, adding

a HARP, and setting the reheater pressure obtain the best relative heat rate impact.

The use of advanced reheat steam conditions strongly affects the inlet temperature to the

low-pressure (LP) turbine section. An increase in hot reheat temperature translates into an almost

equal increase in crossover temperature for a given crossover pressure. However, the maximum

allowable LP inlet temperature is limited by material considerations associated with the rotor,

crossover and hood stationary components. In addition, the selection of hot reheat temperature

(and corresponding effect on LP inlet temperature) impacts the amount of moisture at the L-0

bucket which factors into stress corrosion cracking considerations.

Once the reheat steam conditions are established (pressure and temperature) then the LP

steam conditions can be determined. If the resulting crossover temperature is too high, the energy

ratio between the IP and the LP can be changed to lower this temperature. Increasing the energy on

the IP section will lower the crossover temperature, but it will also impact the cycle efficiency,

increase the number of IP stages, or the loading of the IP stages, increase the height of the final IP

bucket, increase the size of the crossover, or increase the pressure drop through the crossover.

2.2 Steam Turbine Configuration

The appropriate steam turbine configuration for a given USC application is largely a

function of the number of reheats selected, the unit rating, the site back pressure characteristics,

and any special requirements such as district heating extractions. Specific design details will also

determine the number of flows in a turbine section, the number of stages and the last stage bucket

(LSB) length.

In particular, the site ambient conditions and the condensing system will play a huge role in

the selection of the LSB and the number of LP section flows. The 38.1 mmHgA (1.5” HgA) would be

for a direct cooled condenser, or cooling towers in a cold environment. The 88.9 mmHgA (3.5” HgA)

would be for cooling towers in an area with warmer ambient temperatures.

The expected exhaust pressure of the plant at the time of maximum expected power

production should be considered in the design the LP section. At 1.5” HgA, and 1000 MW output, a

45” LSB and a 6-flow LP section would achieve the best heat rate. At 3.5”HgA, and an output of

1000 MW, the 40” LSB, and a 4-flow LP section would be the lowest heat rate choice. In either of

these cases, the high pressure (HP) and intermediate pressure (IP) sections would be essentially the

same.

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The turbine cost increases and plant cost increases would then be compared to the

expected kilowatt outputs to optimize the plant Cost of Electricity. In the case of the 3.5” HgA, the 4-

flow 40” would be higher cost than the 4-flow 33.5”, and the footprint of the 40” LP section would be

larger also. In the case of the 1.5” HgA chart, the 6-flow LP section would require the additional LP

turbine section, and additional condenser, as well as a larger footprint for the 45” LP section.

These considerations resulted the selection of a 4-flow 45” LP section design. The overall

turbine configuration is shown in Figure 1.

Figure 1 - 4-casing, 4-flow LP Configuration

2.3 Evaluation of Ultra Super Critical Technology

The history of steam turbine development is an evolutionary advancement toward greater

power density and efficiency. Improvements in the power density of steam turbines have been

driven largely by the development of improved rotor and bucket alloys as well as improvements in

the design and analysis of the attachment devices for the vanes. This has increased the allowable

stresses and enabling the construction of longer last stage buckets for increased exhaust area per

exhaust flow.

Increases in efficiency have been achieved largely through two kinds of advancements: (1)

improving expansion efficiency by reducing aerodynamic and leakage losses as the steam expands

through the turbine; and (2) improving the thermodynamic efficiency by increasing the temperature

and pressure at which heat is added to the power cycle. The latter improvement is the core of USC

technology.

The design of the Ultra Super Critical steam turbine for the present development will

incorporate the new technologies, which consist of:

© 2007, General Electric Company

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i) Improvement of in the power density of steam turbines such as;

Increased number of stages

Decreased inner ring diameter

Optimized stage reaction levels

Optimized Stage energy levels

ii) Mechanical design elements including:

Advanced sealing

Integral cover bucket (ICB)

Full Arc, hook diaphragm 1st stage

Advanced cooling scheme

iii) Improved HP/IP/LP shell design

iv) Advanced LP design with 45 inch last stage blade

3. HIGH/INTERMEDIATE PRESSURE TURBINE DESIGN

3.1 High Pressure (HP) Section Design

3.1.1 Section Design

Figure 2 shows the HP cross-section. The HP section is designed in a single flow

configuration. This modern design eliminated the partial admission, control stage, and nozzle box.

An overload admission was added for frequency control and capacity margin. Main steam enters

the section through two pipes (top and bottom.) A heater above reheat point extraction is taken

from the lower half. The HP exhaust uses two cold-reheat (CRH) pipes from the lower half

arranged in the pant leg configuration. Elimination of the nozzle box required that two inner shells

be used. All shells are split and bolted at their horizontal joints for ease of maintenance. In this

arrangement, the inlet #1 inner shell is subject to adjacent stage steam conditions on its inner

surfaces and a downstream stage’s steam pressure on its outer surface. The corresponding outer

shell inner surface is subjected to steam conditions at the same downstream steam pressure, albeit

with an enthalpy determined by the flow balance of steam between the packing leakage and the

split location. The exhaust #2 inner shell is arranged in a manner similar to that for the full inner

shell described above. The inner shell split location is judiciously chosen to optimize the horizontal

joint and bolting design of the outer, #1 inner, and #2 inner shells. The outer and # 1 inner shell are

cast 10Cr material. The #2 inner shell is cast CrMoV material. The mono-block rotor material is 12Cr.

The Advancing steam conditions necessitated the addition of inlet cooling for the rotating parts to

allow the rotor design to stay 12Cr materials. Full admission design alleviated the mechanical

challenges associated with partial admission design and the control stage. Thrust is balanced using

the pressure difference across the rotor at the generator end of the HP section. The rotor is

supported by bearings in the front and middle standards. Both standards slide in a manner

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conventional to GE 4 casing, 4-flow turbine construction.

Figure 2 - HP Section Arrangement

3.1.2 High Pressure Steam Path

The HP steam path has 10 stages. The first stage is full-arc admission. The HP staging is

designed in a manner typical for GE’s proven designs using Dense PackTM steampath components.

The buckets of the first four stages are made of nickel-based material due to the high temperature

creep requirements. The remaining buckets are conventional 12Cr materials. All 10 stages will utilize

integral cover buckets (ICB) with advanced tip seals. Figure 3 shows a typical GE ICB design. The

wheel spaces of the first two stages are cooled using external cooling steam. The first five stages of

diaphragms utilize 10Cr materials. The remaining five stages of diaphragms use 12Cr web and ring

material. A combination of brush, variable clearance, and conventional shaft seals are used in the

HP section.

Figure 3 - Typical GE ICB Design



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3.2 Intermediate Pressure (IP) Section

3.2.1 Section Design

Figure 4 shows the intermediate-pressure cross-section. The IP section is designed in a

double flow configuration. Steam enters the section through two pipes in the lower half. Two feed

water heater extractions are taken from the lower half. The IP exhaust uses two cross over

connections from the upper half arranged in the manner conventional to GE 4 casing, 4-flow

turbine construction. Single shell construction is used. The shell is split and bolted at its horizontal

joint to minimize clearances and reduce manufacturing cost. The shell is cast 10Cr material. The

mono-block rotor material is 12Cr. The advancing steam conditions necessitated the addition of

inlet cooling for the rotating parts. The rotor is supported by bearings in the middle standard and

LPA standards. Both standards slide in a manner conventional to GE 4 casing, 4-flow turbine

construction.

Figure 4 - IP Section Arrangement

3.2.1 Steam Path Design

The IP steam path has 8 stages. The IP staging is designed in a manner typical for GE’s

proven designs using Dense PackTM components. The buckets of the first three stages are made of

nickel-based material due to high temperatures. The remaining buckets are conventional 12Cr

materials. All 8 stages will utilize integral cover buckets (ICB) with advanced tip seals. The wheel

spaces of the first two stages are cooled using cooling steam, from HP section. The first two stages

of diaphragms utilize 10Cr materials. The remaining six stages of diaphragms use 12Cr web and

ring material. A combination of variable clearance and conventional shaft seals are used in the IP

section.

3.3 Aerodynamic Design

3.3.1 Advanced Aero Design


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The biggest advance in steam turbines in recent years has been Aerodynamics. GE has

continued to develop advanced aerodynamic vane shapes based on 100 years of experience

coupled with 3D CFD (computational fluid dynamics) and GE’s HP test facility. The addition of CFD in

the last 10-15 years has resulted in great strides in steam turbine aerodynamics. Figure 5 shows the

evolution of aerodynamic shapes over the years.

Stea

m P

ath

Effic

ienc

y %

Free -Vortex• Uniform Radial

Controlled -Vortex• Non -Uniform Radial Flow• Straight Trailing Edge

Advanced -Vortex• Compound Lean • Bowed Nozzle

Partitions

1960’sFree -

VortexDesign

1970’sIm proved

Vane Profiles

1980’sControlVortexDesign

1990’sAV1, adv

RTSS

Late 1990’sAV2, ICBs,

opt. clearance

2000’sDense Pack

2000’sDense Pack ™

w/adv sealing


Flow

Controlled- Vortex• Non Uniform Radial Flow• Straight Trailing Edge• Optimized Reaction

Advanced - Vortex• Compound Lean • Bowed Nozzle

Partitions

-

Stea

m P

ath

Effic

ienc

y %




Partitions

1960’sFree -

VortexDesign

1970’sIm proved

Vane Profiles


1990’sAV1, adv

RTSS


opt. clearance

2000’sDense Pack


w/adv sealing


Flow



Partitions

- New Design

Adv Sealing

Stea

m P

ath

Effic

ienc

y %




Partitions

1960’sFree -

VortexDesign

1970’sIm proved

Vane Profiles


1990’sAV1, adv

RTSS


opt. clearance

2000’sDense Pack


w/adv sealing


Flow



Partitions

-

Stea

m P

ath

Effic

ienc

y %




Partitions

1960’sFree -

VortexDesign

1970’sIm proved

Vane Profiles


1990’sAV1, adv

RTSS


opt. clearance

2000’sDense Pack


w/adv sealing


Flow



Partitions

- New Design

Adv Sealing

Figure 5 - Evolution of Steam Turbine Aerodynamics

GE’s designs using Dense PackTM components further improve efficiency by addressing the

major sources or aerodynamic loss in impulse steam turbines. This is achieved via:

Higher bucket reaction -> Decreased nozzle velocity/turning ->Decreased nozzle profile and

secondary losses

Reduced nozzle and bucket counts -> Decreased friction loss

Decreased root diameter -> Increase bucket active length -> Improved bucket efficiency

In addition, the higher reaction and decreased root diameter result in additional stages (in

order to keep optimum wheel velocity ratio). These additional stages improved section efficiency.

3.3.2 High Pressure Turbine 1st Stage

Older steam turbine designs utilized a control stage to control pressure and load during

transient conditions. The mechanical design requirements of this stage, having to withstand partial

arc stimulus, often result in a very large and in-efficient 1st stage design that can be 5-10% lower


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efficiency than the other HP stages. Although there have been many improvements to control stage

design in recent years as part of GE’s products using Dense PackTM components, the efficiency still

lags other group stages. Therefore, for this 1000 MW USC standard GE chose to implement a full arc

1st stage design, which is much higher reaction and lower aspect ratio to a traditional control stage.

This enables the first stage design to rival the efficiency of other stages. To allow this however,

changes need to be made to allow the turbine to quickly and efficiently respond to load swings.

Therefore, GE will utilize an overload valve that will bypass the 1st stage and allow additional

flow/load response. GE has utilized such a design previously, and has a patent in this area.

3.3.3 High Pressure and Intermediate Pressure Cooling

As throttle temperatures increase, cooling becomes extremely important. For this 1000 MW

USC standard, GE utilizes traditional type IP Cooling, and also HP cooling, which was utilized on GE’s

1st USC machine in 1958. HP cooling takes lower enthalpy steam from the boiler and floods the HP

1st stage wheel and N2 packing areas. This method has minimal performance penalty since lower

enthalpy steam feeds the leakage circuit and the high enthalpy steam stays in the steampath. In

addition, the IP cooling steam is mixed from 2 different sources, allowing better control of

temperature, and minimizing flow and performance loss.

4. LOW PRESSURE TURBINE DESIGN TECHNOLOGY

4.1 Wheel and Diaphragm Configuration Structure

Figure 6 - Wheel and Diaphragm Configuration

As shown in Figure 6, the LP design for the 1000 MW consists of 5 stages with 4 extractions

for the feed water cycle. The last 3 stages (L-0, L-1, and L-2) are designed as a system and the L-0

utilizes the new GE 45” Titanium LSB design. This provides the maximum annulus area for a 4-flow


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configuration. Titanium material is required due to the large load. In addition, advanced curved

axial entry dovetails have been developed to minimize stresses. High strength LP rotor material,

similar to that used on GE’s 40” steel bucket is also used to control stress.

The first 2 LP stages (L-3 and L-4) utilize high reaction stage design for optimal efficiency. In

addition, advance brush seals are utilized to reduce leakage losses.

4.2 Exhaust Hood Design

The LP turbine section is a complex system, which requires a careful optimization, to get

the proper balance of performance, cost, robust operability, and ease of maintenance. The

elements of this LP turbine section are: the outer exhaust hood, the inner casing, the stationary

steam path, and the rotor.

As the steam paths get larger, with the introduction of longer last stage buckets, the

challenges of the exhaust hood design become more critical. The exhaust hood must contain the

vacuum established in the condenser, it must support the rotor bearings, as well as the inner casing,

and it must have a design that allows for the proper diffusion of the steam leaving the last stage.

Three-dimensional (3D) solid models of all of the components of the LP turbine allow for

state-of-the-art analysis techniques, with respect to finite stress calculations, transient heat

transfer calculations, component response to heating and loading, and Computational Fluid

Dynamic (CFD) Analysis. The critical movements of the interfaces of the various components need to

be understood to be able to optimize the clearance calculation throughout the LP turbine. The

required axial and radial clearances can be calculated with the transient loading conditions for

start-up and shut down of the machine. This together with a statistical analysis on the expected

variation from the stack-up, manufacturing tolerances, and calculation uncertainty, yields the

clearances for the machines. Figure 7 shows the exhaust hood outer model, which is a part of this

analysis.

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Figure 7 - LP Exhaust Hood Model

The LP exhaust hood directs the flow from the last stage bucket exit annulus to the condenser. The

high volumetric flows associated with the low exhaust pressure result in high exit Mach numbers

making the recovery of this exit kinetic energy or “Leaving Loss” an important feature of the LP

turbine. To maximize the recovery of the exit kinetic energy, the exhaust hood is designed using an

unstructured CFD mesh. Inlet boundary conditions are set to model the effects of the LSB exit flow

profile.

Results of this model, shown in Figure 8, are used to refine the geometric definition of the

exhaust hood shape such that the flow losses are reduced and the leaving loss is recovered to the

maximum extent possible. In the exhaust, the placement and shapes of the butterfly plates, Herzog

plates, and steam guides were designed for the best performance. These changes reduced the high

velocity regions, minimized separations, and reduced flow turning resulting in reduced inlet and

exhaust pressure losses, contributing to the overall improved performance of the LP turbine.

Figure 8 -LP casing Model and CFD results



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The LP inner casing inlet duct transitions flow from the crossover pipe to the annular inlets

of the steam path. 3D CFD analysis of the inlet region is employed to minimize the losses during this

transition. Results from this analysis provided the inlet boundary condition for the turbine analysis,

including both radial and circumferential variation.

4.3 LP Inlet Design

In addition to the CFD work used to optimize LP Exhaust, similar methods are used for LP

Inlets. Although the velocities are much lower in the inlets, meaning that less performance is lost

than in the exhaust, some improvements can be made. Aside from the basic area rules of crossover

to LP inlet to 1st stage used to avoid acceleration, the LP inlet shape can be designed to reduce

pressure drop. Figure 9 shows a CFD analysis of an LP inlet before and after aerodynamic

optimization. The colors represent areas of different velocity. (red is high, blue is low)

Figure 9 - LP Inlet Optimization

5. ADVANCED SEAL DESIGN TECHNOLOGY

One of the biggest issues with increased reaction is the need for improved seals over the

bucket tips due to the higher pressure drops. To further efficiency improvements, improved seals

are also applied to diaphragms. GE’s wheel and diaphragm construction lends itself well to

application of a variety of advanced seals.

5.1 Elliptical Clearance Packing


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The first, most basic advanced seal used is a standard hi-lo or slant tooth packing with

elliptical packing. Elliptical packing has been shown through experience to be the best solution to

minimize leakage area while preventing most rubs. Clearances tend to be larger at the vertical

position as opposed to the horizontal position due to rotor and stationary part movements. Recent

CFD analyses as well as component testing has shown that rubbed packing shapes have a much

higher leakage than a sharp toothed packing (see Figure 10).

Figure 10 - Rubbed packing CFD

Elliptical clearances are used in conjunction with other advanced seals in most areas.

5.2 Variable Clearance Positive Pressure Packing

In the 1990’s GE introduced its proprietary variable clearance positive pressure packing

(VCPPP). This design provides improved rub resistance, as the packing is “open” while the turbine is

starting up. Unlike conventional packing where the spring pushes the packing ring toward the rotor,

the spring on VCPPP packing pushed the ring away from the rotor by a prescribed amount. Once

the turbine starts to increase in load, pressure builds up and “closes” the ring against the hook. This

is beneficial as the rotor has already gone through its critical speeds and much of the thermal

transient conditions that lead to rubbing have passed. GE applies VCPPP where pressure drops are

large enough to close the rings.

5.3 Brush Seals

Brush seals have been used in GE steam turbines since the 1990’s and have proven to be

very beneficial. Brush seals, unlike traditional packing, are design to contact during normal

operation. The “bristle” material is designed for heavy wear, thus resulting in a minimal clearance

during the life of the turbine. Brush seals are most effective where there are large pressure drops,

however there is some limitation on pressure drop based on backplate stress. Brush seal usage is


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also limited based on rotor dynamics criteria. Since the seals are designed to rub, there is heat

generation in the rotor locally to the rub. This rub causes a thermal bow that can cause rotor

instability. Therefore, brush seals are limited in number, based on the rotor dynamics characteristics

of a given rotor.

6. CONCLUSIONS

GE Energy’s next generation 1000 MW fossil PC steam turbine generator platform provides

increased efficiency and reduction in emissions, which is of paramount importance to the

environment. USC steam conditions enable high efficiency designs that reduce the amount of fuel

required for generation and reduce green house gas emissions.

REFERENCES

1. J. Michael Hill, Sanjay Goel, “Development of the Dense Pack Steam turbine: A New Design

Methodology for Increased Efficiency”, Proceedings of 2000 International Joint Power

Generation Conference, 2000

2. Eichiro Watanabe, Yoshinori Tanaka, “Development of New High Efficiency Steam Turbine”,

Mitsubishi Heavy Industries, Ltd., Technical Review Vol. 40 No.4., Aug. 2003

3. Tom Logan, Un-Hak Nah, James Donohue, “GE and Doosan bring Ultra Super Critical Steam

Turbine Technology to Korea”, Proceedings of 2003 Power Gen Asia, 2003.

4. Daniel Cornell, Klaus Retzlaff, Sean Talley, “DX2 (Dense Pack) Steam Turbines”, GER-4202 GE

Power Systems.

5. Mujezinovic, A., Hofer, D., Barb, K., Kaneko, J, Tanuma, T. and Okuno, K, “Introduction of 40/48

Inch Steel Steam Turbine Low Pressure Section Stages”, Proceeding of the Power-GEN Asia,

(2002), CD-ROM.

6. Hofer, D., Slepski, J., Tanuma, T. Shibagaki, T., Shibukawa, N., and Tashima, T., “Aerodynamic

Design and Development of Steel 48/40 inch Steam Turbine LP End Bucket Series”, Proceedings

of the International Conference on Power Engineering-03 (ICOPE-03) November 9-13, 2003,

Kobe, Japan

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