the situation in steam turbine construction and current development...

Steam turbine development trends OMMI (Vol. 2, Issue 3) December 2003 www.ommi.co.uk

THE SITUATION IN STEAM TURBINE CONSTRUCTION AND CURRENT DEVELOPMENT TRENDS

Wilfried Ulm, Director, Steam Turbine Engineering, Siemens Westinghouse Power Corporation, 4400, Alafaya Trail, Orlando, FL 32826-2399 (USA) mailto:[email protected]

Dr. Wilfried Ulm studied Power Station Engineering and graduated in 1982. He finalized his PhD on Aerodynamic Calculation of Steam Turbine Finals Stages in 1986. Dr Ulm joined Siemens Power Generation in 1991 and worked as Design Engineer in the blade design department in Muelheim/Germany. Later in 1995 he became the head of the Steam Turbine Basic Development Group. From 1998 through 2001 he worked as Director for Steam Turbine Manufacturing. Since 2001 Dr. Wilfried Ulm is responsible for the Siemens Global Steam Turbine Engineering organization.

1 The market situation

In recent years, investment in the power industry worldwide has been dominated by an unprecedented boom in the U.. After years of restraint in the construction of new power plants, which ultimately led to supply bottlenecks, investment in the U.S. took off like never before. The massive expansion of generating capacity during the almost three-year boom period also blessed steam turbine manufacturers with an order volume twice to three times the long-term average. The ratings of the steam turbines installed were between 100 and 400 MW, depending on the type of power plant.

The boom is now over, and the demand for power in the U.S. has now been satisfied, at least for the time being. The rapid return of the order volume to normal has made the global overcapacities that have been building up for years painfully apparent again and is forcing manufacturers to scale down their production capacities. Cost pressures, shorter delivery times and the pursuit of ever-higher efficiencies are typical manifestations of the intensifying competitive pressure within the industry. And that raises the question as to the outlook for the steam turbine market in the future.

A look across to Europe shows a clear need for action over the long term. The aging power plant fleet is urgently in need of renovation. In the next ten to twenty years, capacities totaling 200,000 MW are going to have to be replaced for age reasons, about 40,000 MW of that total in Germany alone.

In the U.S. the situation is not significantly different. The fact is that there, too, the vast majority of power plants are too old and will have to be replaced with new plants over the long term. Following the surge in the construction of new combined-cycle power plants, there are now signs of a rethink setting in as regards the primary energy source in the U.S. In the throes of wide price fluctuations

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Steam turbine development trends OMMI (Vol. 2, Issue 3) December 2003

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for the noble fuels oil and gas, many of the recently built combined-cycle power plants have already lost their competitive edge. And that brings coal back into play as an economic alternative. The best prospects are to be found in the Asia-Pacific market. This region is on the rebound following the financial crisis of the late nineties, and it is now undergoing strong economic growth. The power generation forecasts show growth rates averaging 3.5% per year through to the year 2020.

Especially China as the country with the largest population and very strong economic expansion is planning large-scale investments in power plants.

Even if combined-cycle power plants continue to be predominant, the generation of power using coal is an option that is likely to become more interesting again in all regions of the world. The price stability and worldwide availability of this fuel speak for themselves. Although domestic hard coal is not competitive in Western Europe, it is still possible to generate power cost-effectively using imported coal.

In Germany, power generated using locally mined lignite is proving competitive on a day-to-day basis on the increasingly liberalized market, while advanced emissions-reducing technology is making lignite more environmentally compatible than ever before.

The latest lignite-fired units constructed in Germany show how it is done. The centerpieces of these plants are modern steam turbines with ratings of up to 1000 MW and more, driven by steam at supercritical conditions. Figure 1 shows by way of example the 900-MW turbine-generator in the Boxberg power plant.

Figure 1: The 900-MW turbine-generator in the Boxberg power plant


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2 The state of the art

The latest generation of steam turbines is no longer comparable with the workhorse machines of the "good old days". Driven by ambitious steam power plant projects in Germany, Denmark, Japan and other countries, steam turbine technology was unobtrusively but consistently revolutionized during the nineties.

The leverage for ever-higher cycle efficiencies is provided by steam parameters with supercritical pressures and temperatures of up to 610°C. Progress made in materials technology and in casting and forging techniques led to the development of steels for rotors, casings and turbine blading capable of operating reliably at these steam conditions. Figure 2 shows the bottom-half of an inner casing made of 10% chromium steel for a steam turbine with a rating of 1000 MW and a reheat steam temperature of 600°C.

Figure 2: Intermediate-pressure turbine inner casing made of 10% chromium for 600 C

Widespread use of finite element methods in mechanical calculations also makes it possible to dimension parts with optimum in-service clearances and at the same time maximum operational reliability. Progress made in the use of computers in flow mechanics paved the way for the development of 3D blading, as illustrated in Figure 3.

This technology was first marketed in the mid-nineties and is already the standard in steam turbine construction. The blades with optimized profile and secondary losses yield internal efficiencies of up to 96%, depending on the rating of the turbines.


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Along with the blading, more and more attention was also paid to the design of other parts exposed to the steam flow.

For instance, 3-D CFD methods are nowadays used to optimize the flow characteristics of parts such as the valves, steam admission and exhaust sections of the turbine.

Figure 4 shows the results of 3-D CFD calculations for the diffuser of an LP turbine. The analyses improve our understanding of the three-dimensional flow characteristics and help to prevent unnecessary losses and thus further improve efficiency.

Figure 4: CFD simulation of an LP turbine exhaust section

Significant progress has also been made in the design of the final blading stages. Thanks to the combined use of advanced flow mechanics, finite element models for strength and vibration calculation, and improved materials, the final-stage moving blades of a full-speed turbine for 50 Hz now reach a length of just on 1,200 mm. Figure 5 shows the rotor of a double-flow low-pressure turbine with an exhaust cross section of 12.5 m². Low-pressure cylinders like this make it possible to build extremely compact turbines and also to achieve unit ratings of more than 1000 MW for a single-shaft plant.

Figure 5: Low-pressure rotor with 1150 mm final-stage blades for 50 Hz

Figure 3: Advanced 3D blading for steam turbines


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One such technological masterpiece in this class is the five-cylinder 1027-MW steam turbine built by Siemens Power Generation for the new lignite-fired unit of RWE's Niederaussem power plant. The plant went into operation late last year. Supercritical steam conditions with temperatures of around 600°C make it possible to achieve a net efficiency of about 43% and thus assure the long-term economic viability of the plant.

European manufacturers and operators are not the only ones to have rediscovered coal as a fuel for power generation. In Asia, too, this goal is being consistently pursued. Japan has taken the lead in this context, with ambitious power plant projects under way. For instance a compact 600-MW coal-fired unit with steam temperatures of 600/610 C went into operation in the Isogo power plant in Yokohama in 2002. China, too, is set to embark on the path to supercritical parameters with ultra-high steam temperatures in the foreseeable future.

Besides paving the way for technological masterpieces, the increased use of steam turbines in combined-cycle power plants has also given rise to new requirements to be met by steam turbine design.

In the U.S. boom phase, large series of steam turbines were built in significant numbers. This makes particular demands on material supplies and logistics, and forces manufacturers to increasingly standardize what used to be a one-off business.

This challenge can only be mastered with the aid of sophisticated building-block systems. Figure 6 shows an example of such a modular product range.

H

E

N

K

M

HE-Serie KN-Serie

HMN-Serie

HH

EE

N

KK

MM

HE-SerieHE-Serie KN-SerieKN-Serie

HMN-SerieHMN-Serie

Figure 6: Steam turbine product range

Combining turbine modules of various types and sizes makes it possible to meet the market's demand for combined-cycle and steam power plant applications throughout the entire rating range from 100 to 1000 MW. At the same time, this meets requirements in terms of standardization and versatility.


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3 Service

The innovative technologies originally developed for new plants are increasingly being used to upgrade existing turbines. Particularly in nuclear power plants, this can extend the service life of the plant while at the same time significantly improving efficiency. Figure 7 shows replacement of the low-pressure turbines at the Emsland nuclear power plant in 2000.

Figure 7: Retrofit of low-pressure turbines in the Emsland nuclear power plant

The revamped blading gives about 32 MW of additional power output with no increase in fuel consumption. As a rule, investing in such conversions pays for itself in just a few years thanks to the improved economic performance of the plant. While in Germany nearly all nuclear power plant turbines have already been upgraded in this way, there is still considerable potential for power plant modernization in the U.S. Encouraged by the extension of licensable service lives under the current U.S. Administration, it is becoming increasingly interesting for nuclear plant operators in the U.S. to upgrade their steam turbines. But replacing the turbine rotor can be economically meaningful for older fossil-fired plants, too. The improvements in efficiency achievable at a comparatively low cost make it possible to keep many plants of an aging fleet operating at a profit.

Figure 8 compares and old and a new design for a combined high-/intermediate-pressure turbine.


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Westinghouse

Siemens

Altes Design

Neues Design

Figure 8: Siemens K turbine upgrade – comparison of Siemens and Westinghouse designs

Elimination of the control blading stage and replacement of the old blades with modern 3D blading make for a considerable improvement in efficiency. Replacing degraded parts at the same time extends the service life. Substituting a state-of-the-art instrumentation and control system for older control technology also improves plant response times and permits much more flexible operations management.

The average potential for efficiency enhancement lies between 3 and 8 percent, depending on the age and condition of the turbines. This potential can be harnessed with a relatively low cost effort and, unlike projects for the construction of new plants, does not involve long drawn-out licensing procedures.

4 Ongoing trends in development Despite more than a hundred years of history and the surge of new developments in the nineties, there are still further potentials slumbering in this supposedly run-of-the-mill steam turbine technology. The physical boundaries on improving the turbine's internal efficiency by optimizing the flow characteristics of the blading and other components have almost been reached. Large-scale conversion to innovative sealing technologies such as brush-type glands or the use of abrasive coatings will contribute only marginally to further enhancing overall efficiency. A further increase in steam temperatures, on the other hand, still holds promise of worthwhile improvements in efficiency, and for this reason this will be one of the main thrusts in the ongoing development of steam turbine technology in the coming years. The efforts being made by


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manufacturers and operators in the power plant initiative Emax in Europe are geared toward supercritical steam parameters with temperatures of up to 700°C.

This goes beyond the application ranges of the ferritic and austenitic steels used up to now and calls for innovative material developments and novel constructions using nickel-based materials. Figure 9 shows by way of example the conceptual design of a high-pressure turbine for such parameters, in which the areas exposed to the highest temperatures are made of suitably temperature-resistant materials. One of the challenges in this context is how to manufacture the large components involved, but also how to make it possible to weld these high-temperature materials together with conventional materials. Intelligent cooling approaches will have to be devised to make it technically possible to cope with these high temperatures in fabrication while at the same time keeping the cost to a reasonable level. Even the introduction of thermal barrier coatings, as used in gas turbine design, may catch on.

The interest of the steam turbine manufacturers will focus not just on raising the inlet parameters but also on further optimizing the "cold end". With blade lengths of about 1200 mm for full-speed 50-Hz applications, the potential for using steel as the blading material is practically exhausted.

An alternative is titanium alloys which, because of their high strength-to-density ratio, make it possible to build even longer blades. This development already got under way some years ago for the 60-Hz market, because here the size limits on the final-stage blading area are particularly bothersome as a result of the higher speeds involved.

Figure 10 shows a 1000-mm titanium blade for a 60-Hz turbine.

Figure 10: 1000-mm titanium blade for 60-Hz low-pressure turbines

Figure 9: Development of an HP turbine for

700°C


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Integral shrouds and/or mid-blade braces are typically used to control vibration in such blading. Titanium blades with blade lengths of up to 1500 mm for full-speed 50-Hz turbines will already be available within the next two to three years.

This paves the way for unit ratings way beyond 1000 MW or for even more compact turbines for lower ratings. For instance, turbines with a single-flow exhaust for ratings of up to about 400 MW will be possible in the future.

5 Summary In a nutshell: the steam turbine has by no means had its day but is going to keep its place in the power engineering environment of the 21st century. Technological progress made in the recent past has made a considerable contribution to improving its competitiveness. As coal comes back into fashion as a fuel for power generation, the steam turbine is set to regain some of its earlier supremacy. And whether the primary energy source is coal, gas or nuclear power, there will be at least one steam turbine turning in every new power plant built, so we need not fear for the prospects of this technology on the market of the future.

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