usc steam turbine technology for maximum efficiency … · ... maximum power plant efficiency is a...

Copyright © Siemens AG 2008. All rights reserved. 1

POWER-GEN Asia 2008 – Kuala Lumpur, Malaysia October 21-23, 2008

Copyright © Siemens AG 2008. All rights reserved.

USC Steam Turbine technology

for maximum efficiency and

operational flexibility

Dr. Rainer Quinkertz

Andreas Ulma

Edwin Gobrecht

Michael Wechsung

Siemens AG

Abstract

State of the art ultra supercritical (USC) steam parameters provide highest efficiencies (i.e.

lowest fuel costs and emissions like CO2) for coal fired steam power plants. At the same time

these plants also face the requirements of deregulated electricity markets with an increasing

portion of renewable energy. Thus the units have to be capable of flexible load operation and

frequency support. The overload admission has proven to be very effective for flexible

turbine operation at an excellent heat rate. Both main turbine components and controls have

been optimized to minimize thermal stresses, leading to an improved start-up performance.

Despite increased steam parameters and number of load cycles and starts maintenance

intervals have not been shortened. As a result overall life cycle costs are minimized.

Siemens has more than 10 years of experience with USC steam turbines and continues to

optimize associated designs and technologies.

This paper presents the current Siemens steam turbine technology and products for USC

applications and references for operating units.


Introduction

According to common forecasts, the worldwide demand for power will increase significantly

over the next two decades, and the current power plant capacity will need to double by the

year 2030. A considerable share of this huge increase in demand has to be covered by coal-

fired units. To save primary energy resources i.e. to reduce fuel consumption, and to reduce

emissions, maximum power plant efficiency is a crucial parameter for power plants.

Therefore steam parameters will have to be maximized to an economically reasonable extent.

State of the art temperatures and pressures for ultra supercritical (USC) applications are

600°C at 270 bar for the main steam and 610°C at 60 bar for the reheat steam.

Furthermore, the share of renewable capacities will increase further to fulfill global emission

reduction targets. As a result electric grids will face a growing share of volatile capacities.

This again requires additional flexibility of the conventional units and, in addition to

combined cycle units, coal-fired units will also operate in the mid merit market. Thus these

units will experience an increased number of starts and load changes, including rapid load

changes, especially within small electric grids.

Siemens USC steam turbines

50 Hz full speed tandem compound turbo-sets for USC steam power plants (SPP) are

available for gross power outputs from 600 – 1200 MW per unit.

The steam turbine set-up for ultra-supercritical applications depends on the unit rating, the

number of reheats selected, and the site backpressure characteristics. A typical turbo-set

comprises three separate turbine modules operating at different pressure and temperature

levels. These modules are the high pressure turbine (HP), intermediate pressure turbine (IP)

and, depending on the cooling water conditions, one, two or three low pressure turbines (LP).

The generator is directly coupled to the last LP turbine. Figure 1 shows an SST-6000 series

Siemens steam turbine with two LP turbines. A comprehensive description of the specific

turbine features of the whole turbo-set is given in [1].


HP turbine

IP turbine LP turbines

Figure 1: SST-6000 series turbo-set

Design features for optimum turbine efficiency

Siemens steam turbines contribute to overall plant efficiency requirements by providing

highest internal efficiencies. These efficiencies have been proven for decades in various

acceptance tests and continue to be a major evaluation criterion for new developments.

One feature is that clearances between rotating and stationary parts of the turbine are very

small due to minimum relative expansion and thermal deformation during transient operation.

These minimum clearances are achieved by a rigorous symmetrical design, which avoids

local material build-ups (HP barrel type) on the one hand, and ensures uniform thermal

loading on the other hand.

Steam leakage through these clearances along the blade path as well as at turbine pistons and

sealing areas is further minimized by using advanced sealing technologies such as abradable

coatings and brush seals. More details are given in [2].

Within the blade path, 3DV™ reaction blading with minimum secondary losses and variable

reaction across the blade path minimizes flow losses. [3] describes this approach in detail.

References and operating experience

Siemens first supercritical unit was built in 1956 and had a main steam temperature of 650°C.

However power output was limited. Table 1 shows a reference list of large scale USC steam

turbines manufactured by Siemens. Obviously steam parameters have increased slightly over

the last years, but power output capacity has increased considerably compared to the first

large scale USC application in Isogo, Japan. Whereas the Chinese units favor higher


electrical output, European customers consider 800 MW an optimum unit size. This is

basically due to maximum dimension of the air pre-heater for a one-line configuration of the

pre-heater piping. But increasing power output also reduces operational flexibility because

wall thicknesses of components determine start-up times and permissible number and rate of

load cycles. [4]

Plant Country Power output Main steam Reheat

steam

Commercial

operation

Isogo Japan 1 x 600 MW 251 bar / 600°C 610°C 2001

Yuhuan China 4 x 1000 MW 262 bar / 600°C 600°C 2007

Wai Gao Qiao 3 China 2 x 1000 MW 270 bar / 600°C 600°C 2009

Westfalen Germany 2 x 800 MW 275 bar / 600°C 610°C 2011

Eemshafen Netherlands 2 x 800 MW 275 bar / 600°C 610°C 2012

Lünen Germany 1 x 800 MW 270 bar / 600°C 610°C 2012

Mainz Germany 1 x 800 MW 273 bar / 600°C 610°C 2013

Table 1: References of Siemens USC steam turbines

Case Study: First Large Scale USC Operation Experiences at Isogo Power Station,

Japan

The order Isogo was placed at the end of the 90s when Siemens was a sub supplier for the

contract owner FUJI ELECTRIC SYSTEMS. FUJI is a licensee for conventional Siemens

steam turbines and a long-term business partner. The Japanese power plant, which is located

in the Bay of Tokyo, has a very compact design due to the restricted area available and is

equipped with separate HP and IP turbines and a double flow 12.5 m2 LP turbine.

In addition to the parameters given in Table 1, the condenser has once through, sea water

cooling and a pressure of 0.0507 bar (1.500 inch Hg). The boiler is hard coal fired. Figure 2

depicts the inside of the turbine building, and Figure 3 shows the cross section of the turbine.


Figure 2: Turbine building at Isogo

Figure 3: Cross section of Isogo turbine

The Japanese industrial ministry requires that periodical comprehensive inspections are

carried out on the steam turbine. A complete disassembly and detailed inspection of all

turbine components was therefore arranged with the customer, initially after two years of

operation and then every four years. Consequently, the turbine and the valves were subject to

this strict control from March to June, 2008.

This presented a good opportunity for the manufacturer to inspect the highly stressed

components. At the time of inspection the turbine had a base load operation of approximately

48000 equivalent operation hours (EOH).

The design of the HP and IP turbines and of the valves is basically the same as the previous

supercritical design. Design concept changes were consciously avoided to minimize the

operational risk and to achieve good plant availability and to use existing calculations and


manufacturing procedures. However materials for cast and forged parts exposed to USC

operating parameters have been upgraded and coatings have been added.

Figure 4: Isogo HP rotor

In particular 10% chrome steel material was used. This material was developed from the

well-known P91 tube material as a result of COST investigations and is characterized by up

to 30% higher creep rupture strength in comparison with 12% chrome materials used

previously. Figure 4 shows the disassembled HP rotor.

According to material creep laws, components which are hot and under tension are subject to

inelastic changes, particularly in the first operational phase. To validate design calculations, a

validation program was initiated which determined and evaluated changes in component

geometry at exposed areas over time. The creep behavior of shaft and casing components, as

well as of HP main steam valve breechlock nuts of the HP casing and valve casings was

recorded.

The results were consistently positive and showed lower plastic deformation than expected.

This proves the conservative approach of the component design. The maximum creep of the

HP barrel casing, for example, is less than 0.2% in the area of steam admission. This is an

excellent value for 48000 operating hours in comparison with the generally permitted limit of

1% material expansion. This information is extremely valuable for the optimization of new

components and allows a better assessment of the design limits.


Further investigations concern the oxidation and scaling behavior of materials used in real

operation. The knowledge gained here was especially valuable and relevant for the anti-

friction property of seal rings and sliding pads. Guide ways, threads and moving parts in the

steam valves are partly coated. Chrome carbide coatings of valve cones, for example, show

no relevant wear or damage, which verifies the suitability of these coatings also for USC

plants. Chrome carbide coatings have been used for SIEMENS steam valves as standard

protection for approximately 10 years and are, in addition to stelliting, a proven design, see

Figure 5 IP control valve cone.

Figure 5: IP control valve cone

A special feature of the USC design is the choice of blade material. Nimonic 80 A has been

used for the first stages of the HP- and IP- turbines. Experiences made at the beginning of the

90s with large steam turbines were drawn upon as well as experiences of the Siemens gas

turbine design for high temperatures. Nimonic can show an unfavorable friction behavior

with austenitic and martensitic material, which is used, e.g. for seal strips. This has been

considered when designing the stages.

The blades at Isogo were also inspected during the March to June outage. As expected, the

condition of the blades was excellent. Surfaces of airfoils, integrated shrouds and pre-stress

of bladings were without findings. The design of the integrated shrouds as well as the

calculated radial clearance could be confirmed as suitable choices here. See Figures 6 and 7

for first Nimonic stages.


Figure 6: HP Nimonic stage1 Figure 7: IP Nimonic stage1

The shaft seals of a steam turbine shall seal efficiently to reduce steam losses. Furthermore,

the sealing needs to resist high temperature, and static and dynamic forces. The HP turbine at

Isogo, for example, is fully equipped with seal strips made from 13% chrome material, which

is martensitic. The inspection confirms the suitability of this material also for temperatures up

to approx. 580° C (1076° F), which dominate at the HP balancing piston entrance where the

highest load is located, see Figure 8 showing the HP thrust balancing piston.

Figure 8: HP rotor thrust balancing piston

The inspection of the relevant components shows obviously the good suitability of the USC

design concept. The results show that the steam turbine is in a very good condition after

almost 48000 EOH.


Turbine HP stage bypass for frequency control and power increase

Generally there are many different methods for controlling frequency and increasing power

both within the balance of plant and the steam turbine. Whereas on the water steam side e.g.

condensate throttling or preheater bypass address reserve requirements within minutes, valve

throttling on the turbine side provides power increase within seconds.

The HP stage bypass is a design feature applied to the main steam piping, which routes a

second main steam line onto the HP turbine. Stage bypass steam enters the turbine via at least

two inlet belts in the HP blade path section. Figure 9 shows the piping arrangement as well as

details of the turbine design.

bypassstagecontrol valve

main steamvalve

HP turbine

main steambypassstagecontrol valve

main steamvalve

HP turbine

main steam

Main Steam lines

Stage bypass chambers

Figure 9: Stage bypass routing from main steam valve combination to HP turbine and cross

sectional view of HP turbine with HP stage bypass chamber

The HP stage bypass is the most efficient solution for rapid load increase because it creates

two load conditions at which there are minimal throttling losses. The first condition occurs

when the valves of the main steam inlet are wide open and the stage bypass is closed. Usually

this is the 100% load point. The second case with minimum throttling losses is with all valves

wide open (VWO) which represents the maximum load point, usually 105%. Consequently,

bypass governing achieves a better part load performance than throttling of the valves at the

100% load point. Manufacturing this design is also more economical than building a nozzle

governed machine with similar part load characteristic.

Design of the HP stage bypass and stationary performance is described in detail in [5, 6].


Figure 10 illustrates the transient performance of the HP stage bypass. Like throttling of the

main steam valves, the stage bypass is able to provide a most rapid load increase of about 1%

per sec. The maximum load enhancement in both cases is about 50% of the design value i.e.

with full additional firing load of the boiler.

The diagram also shows that a combination of different measures may be required to meet

challenging grid code requirements of islands e.g. UK with 10% load increase within 10 sec.

Figure 10: Dynamic performance of different load increase measures

Advances in start-up time

Beside load changes, start-up performance has become an important attribute of SPP.

Siemens has proven steam turbine cycling capabilities with different approaches. The design

supports quick and uniform heat-up of thick-walled components. Turbine controls provide

three different start-up modes i.e. speeds. The resulting material fatigue of each cycle is

calculated online. This enables the operator to balance power revenue against lifetime

consumption. Details are given in [7, 8].

As a third approach for fast start-up, Siemens improved the overall start-up process of the

total plant. The new concept was first introduced to combined cycle power plants (CCPP).

This enabled a parallel start of gas turbine (GT)/ heat recovery steam generator (HRSG) and

steam turbine and is described in [9]. In principle, it can also be performed for SPP, provided

that boiler and controls have appropriate capabilities.


As a traditional requirement for steam turbine start-up, the steam temperature has to be higher

than the metal temperature. This reduces thermal stresses, especially for hot starts and

restarts.

Different developments have made it more and more difficult or rather time-consuming to

meet this requirement. Firstly, increasing rated steam temperatures from 540°C to 600°C

enlarge the difference between metal temperature and steam temperature after short shut-

down periods (<8 hrs). Then, reduced bypass sizes limit the start-up load, and heating up the

steam physically takes longer at low load because heat transfer is also low. Finally, cost

reduction measures have lead to reduced boiler performances.

Originally, thick walled boiler headers had the main influence on hot start times in steam

power plants because the headers cool down faster than the steam turbine.

With the new boiler design the “hot” turbine became a disadvantage for the overall plant hot

start performance.

For hot start-up after boiler ignition, the plant is kept in bypass operation without producing

power until boiler outlet temperatures meet steam turbine starting conditions (480°C to

500°C). For the customer this is a waste of fuel and the time to synchronization is too long

under deregulated market conditions. Shortening these waiting times increases the steam

turbine dispatch rate.

Thus the focus is on allowing “cold” steam to enter the steam turbine at an earliest possible

point. This enables the turbine to start while the boiler is ramping-up in load without any

additional hold.

As a result, the start-up time is shortened, which leads to an earlier dispatch of the turbine.

Savings are at least the amount of fuel energy which is not dumped into the condenser. The

new start-up method does not reduce turbine efficiency by increasing axial or radial

clearances. Life consumption for the new method is a little higher but still allows more than

6000 hot starts of the steam turbine.


Typical Hot Start-up Curve

0

100

200

300

400

500

600

0 20 40 60 80 100 120 140Time [min]

Tem

pera

ture

[°C

]

0

20

40

60

80

100

120

Load

/ Fl

ow /

Spee

d [%

]

Main Steam Temperature

Main Steam Pressure

Main Steam Flow

Load

Turbine Speed

Pres

sure

[bar

]

150

200

250

300

100

50

"old" turbine

start

"new" turbine

start

boiler waiting for ST to start

Figure 11: Start-up time reduction with new concept

The steam turbine start-up procedure used to be based on static performance curves of the

boiler and did not take into account any possible ramp rates. Differences between boiler start-

up pressure and pressures inside the turbine (i.e. throttling) were not taken into account.

During opening of the turbine valves and picking–up of load, this throttling is reduced more

and more. Hot piping from boiler to turbine will heat-up the “cold” steam before it reaches

the steam turbine. Change from pure static to more dynamic orientation of the start-up

process is the major achievement of the new procedure.

Typically the hot start-up time can be reduced by 15 min if boiler and steam turbine start

simultaneously (see Figure 11).


-30

-20

-10

0

10

20

30

-100 -80 -60 -40 -20 0 20

tem

pera

ture

diff

eren

ces

durin

g co

ol d

own

and

heat

up

g

g

LCF design limit

mid wall temperature minus main steam temperature[K]

heat up

cool down

start-up 2start-up 1

LCF design limit

calculated

Figure 12: Calculated and measured cooling of HP shaft during parallel start-up

Figure 12 shows results of tests carried out at a German SPP which prove the Siemens

approach to allow turbine start-up with steam temperature lower than the turbine shaft

temperature. The boiler at this SPP starts with a ramp-rate of 3% per min. For example, with

main steam 50 K below metal temperature, temperature difference for cool down is only 16

K. This is well below the design for low cycle fatigue which allows 27 K.

Conclusion

Growing worldwide power demand, limited resources for all fossil fuels, CO2 reduction

targets and growing shares of renewable energy set the scene for current and future plants.

Coal will definitely continue to be an important part of the energy mix.

Siemens contributes to these market requirements by offering USC steam turbines for highest

steam parameters i.e. maximum plant efficiency. Besides featuring minimum clearances for

maximum inner turbine efficiency, these turbines are also very flexible to operate. Both short

start-up times and quick load changes are possible. For temporary power increase the stage

bypass is the optimum solution for efficiency and dynamic performance.


Siemens large scale USC steam turbines have been in operation for nearly 10 years now. The

design principles were proven during recent inspections at the Isogo plant in Japan where

blades, casings and rotors exposed to high steam parameters showed far less creep effects,

oxidation and abrasion than expected after 48.000 EOH.

References

[1] Wichtmann A., Deckers M., Ulm W.

Ultra-supercritical steam turbine turbosets – Best efficiency solution for conventional steam

power plants, International Conference on Electrical Engineering, Kunming, China, July

2005.

[2] Neef M., Sürken N., Sulda E., Walkenhorst J.

Design Features and Performance Details of Brush Seals for Turbine Applications, ASME

Turbo Expo, Barcelona, 2006

[3] Deckers M., Pfitzinger E.-W., Ulm W.,

Advanced HP&IP Blading Technologies for the Design of Highly Efficient Steam Turbines,

Thermal Turbine, 2004

[4] Quinkertz R., Then O., Gerber R.

High efficient and most flexible 800 MW Ultra Supercritical Steam Power Plants -

A common approach of RWE Power AG and Siemens AG, CoalGen Europe, Warsaw, 2008

[5] Deidewig F., Wechsung M.

Thermodynamic Aspects of Designing the new Siemens High Pressure Turbine with

Overload Valve for Supercritical Applications, ASME Power, Atlanta, 2006

[6] Wichtmann A., Wechsung M., Rosenkranz J., Wiesenmüller W., Tomschi U.

Flexible Load Operation and Frequency Support for Steam Turbine Power Plants. PowerGen

Europe, Madrid, 2007

[7] Almstedt H., Gobrecht E., Thiemann T., Wallis A., Wechsung M.

Siemens 600 - 1200 MW Steam Turbine Series for Flexible Load Operation. PowerGen

Europe, Madrid, 2007

[8] Quinkertz R., Gobrecht E.

State of the Art Steam Turbine Automation for Optimum Transient Operation Performance,

ASME Power, San Antonio, Texas, 2007

[9] Emberger H., Schmid E., Gobrecht E.

Fast Cycling Capability for New Plants and Upgrade Opportunities, PowerGen Asia,

Singapur, 2005



Permission for use

The content of this paper is copyrighted by Siemens and is licensed to PennWell for

publication and distribution only. Any inquiries regarding permission to use the content of

this paper, in whole or in part, for any purpose must be addressed to Siemens directly.

Disclaimer

These documents contain forward-looking statements and information – that is, statements

related to future, not past, events. These statements may be identified either orally or in

writing by words as “expects”, “anticipates”, “intends”, “plans”, “believes”, “seeks”,

“estimates”, “will” or words of similar meaning. Such statements are based on our current

expectations and certain assumptions, and are, therefore, subject to certain risks and

uncertainties. A variety of factors, many of which are beyond Siemens’ control, affect its

operations, performance, business strategy and results and could cause the actual results,

performance or achievements of Siemens worldwide to be materially different from any

future results, performance or achievements that may be expressed or implied by such

forward-looking statements. For us, particular uncertainties arise, among others, from

changes in general economic and business conditions, changes in currency exchange rates

and interest rates, introduction of competing products or technologies by other companies,

lack of acceptance of new products or services by customers targeted by Siemens worldwide,

changes in business strategy and various other factors. More detailed information about

certain of these factors is contained in Siemens’ filings with the SEC, which are available on

the Siemens website, www.siemens.com and on the SEC’s website, www.sec.gov. Should

one or more of these risks or uncertainties materialize, or should underlying assumptions

prove incorrect, actual results may vary materially from those described in the relevant

forward-looking statement as anticipated, believed, estimated, expected, intended, planned or

projected. Siemens does not intend or assume any obligation to update or revise these

forward-looking statements in light of developments which differ from those anticipated.

Trademarks mentioned in these documents are the property of Siemens AG, its affiliates or

their respective owners.

http://www.siemens.com/

http://www.sec.gov/

usc steam turbine technology for maximum efficiency … · ... maximum power plant efficiency is a...

Documents