Rationalised product lines aim to anticipate future markets

The merger of Siemens and Westinghouse has quickly led to a highly beneficial combination of their most advanced steam turbine development technologies in a way which has resulted in an remarkable rationalisation of steam turbine modules across the wide range of possible combinations from medium sized combined cycle units to 1000 MWe supercritical units. The use of titanium blades in the LP turbine modules has also greatly reduced the overall dimensions of large utility turbines, which in turn gives the benefits of smaller turbine house buildings and lower site area requirements.

Another driver has been the ever increasing demand for higher temperatures, coming from the requirement for greater efficiency.

Combined cycle market needs

In the combined cycle sector, an important further factor has been accommodating the requirements of the US market. During the last five years, the power plant market has enjoyed a remarkable boom in the USA, sadly now over. A large number of combined cycle plants were ordered during the boom period and for many of these projects it was not just a question of optimising heat rate and initial capital costs. There was also a need to provide highly flexible transient operation modes, short delivery times and long life, with an overriding need for high reliability and availability. To achieve these different targets in an optimum way, main components, such as gas turbines, steam turbines and generators, were developed as parts of an integral system (see Figure 1) not as individual stand-alone products.

HE product line

In the lower power output range, in which single flow exhausts are sufficient, an HP cylinder is coupled together with a combined IP/LP turbine design. This flat floor mounted design is well established as the HE product line (see Figure 2).

However, to meet the anticipated efficiency demands of future markets, it was necessary to develop an advanced E turbine module, shown in Figure 3.

This design has been developed for 50Hz and 60Hz applications and is capable of handling reheat temperatures of up to 600°C in order to support future combined cycle plant efficiencies beyond 60%.

The turbine is optimised with regard to transient operation modes in order to support cycling and fast start-ups. Subsequently, and to maintain a stable level of reliability, the increased temperatures have led to a double shell casing design and other features, for example cooled piston and welded rotor in straight-flow design. All these features are well known in principal from the former E turbine designs developed by Siemens and Westinghouse.

In addition, the HE product line equipped with this advanced E turbine is also suitable for steam power plant applications.

This advanced design combined IP/LP turbine is offered in anticipation of future combined cycle applications. The first version with 12.5m2/50Hz LP was released by Siemens for bids in July 2003. Further versions with different exhaust areas (50Hz and 60 Hz market) are forthcoming.

KN product line

In cases where, due to the volumetric flow, a switch from single flow exhaust to double flow arrangements is necessary (eg, 2x1 applications), another two-cylinder design, known as the KN, can be employed.

Specific to this compact cost-effective steam turbine design is a combined HP/IP cylinder coupled with a double flow LP turbine (see Figure 4).

The basic design of the K turbine was developed several years ago and since then has been continuously improved with regard to efficiency, increased steam temperatures (565°C/565°C) and transient operational performance. The KN has a double shell design with horizontally split inner and outer casings (see Figure 5).

As already mentioned in the context of the advanced E turbines, it will be necessary in the future to achieve plant efficiencies even higher level than currently projected. Therefore it has been necessary to improve the K module by raising inlet and reheat temperatures to 600°C. This has been done and the turbines are now commercially available.

Steam power plants

Advanced combined cycle power plants have not been the sole focus of steam turbine development in the recent years.

Building on the success of the KN product line in the combined cycle market, as described above, the design is now being introduced to the steam power plant market, notably for subcritical plants in the 50 and 60 Hz sectors, with a range of outputs from 200MW to 700MW (see Figure 6). Three modules have been developed especially for applications of up to 177bar/566°C/566°C (see Figure 7).

This advanced design is now commercially available, having been released for bids this year.

In Europe and Japan in particular, there has been a demand for modern coal and lignite-fired steam power plants as large base load units. Even though the efficiencies of combined cycle plants cannot be matched by those of steam power plants and the initial capital costs of steam power plants are much higher than those of combined cycle plants, coal prices are generally much lower and more stable than gas prices. Therefore coal and lignite are likely to continue to play a key role in the power generation industry. To achieve good economics a major goal has been to raise plant efficiency by increasing steam parameters to 300bar/600°C/620°C.

Under the auspices of international material development programmes such as COST 501 and COST 522 the high-chromium steels needed have been were developed and based on these materials, Siemens has taken the proven HMN series one step further, see Figure 8.

At the heart of this HMN series is the barrel type H module, which has no horizontal joint. With its symmetrical outer casing and the non-existent horizontal joint, it is the ideal design to accept the highest possible pressures and temperatures. The deflections due to thermal stresses are minimal and therefore clearances can be optimised, resulting in even better efficiency. This proven design can now accommodate the steam parameters mentioned above and makes use of 10% chromium steel (see Figure 9).

On the basis of the features of the HMN design, Siemens is engaged in ongoing future development projects such as the 550 MWe Referenzkraftwerk Nordrhein-Westfalen. This regional government backed project aimed to develop the technology to design and build "the most modern coal fired power plant in Europe" for operation in 2007, but no commitment has yet been made to build a demonstration plant. The steam turbines for this will still be based on the use of 10-12 per cent chromium steel and no nickel based alloys, but taken to the highest possible temperature. Plant net efficiency is intended to be in the range from 45 to 47%.

State financed up to 40% within the framework of the Future Energies North-Rhine Westphalia initiative, the project was due to run from 1 October 2002 to 30 September 2003. Project partners of this joint research project are manufacturers, operators, and scientific institutes situated in the coal-rich state of North Rhine Westphalia with VGB PowerTech as co-ordinator.

Siemens Power Generation is providing the technical concept study for the turbine hall with optimised steam turbine, entire plant concept, I&C technology as well as planning and construction for the entire plant.

Figure 10 shows the design of the IP turbine. This design is also well-suited for ultra-supercritical steam conditions in that a double shell design is used. In this design, cool IP exhaust steam is passed between the inner and outer cylinder, thereby cooling the inner cylinder. This allows the horizontal joint bolting to be designed in such a way as to accommodate the steam conditions seen in the IP inlet. Moreover, if required, cooling steam from the HP exhaust can be directed into the IP cylinder for the purpose of cooling the middle section of the IP rotor. Two principal designs are available depending on power output: single flow; and double flow.

The table below shows the supercritical plants to which Siemens delivered steam turbines in the 1990s.

Figure 11 shows the longitudinal section of the Niederaußem steam turbine.

Future development needs

The EU-funded AD700 initiative (formerly called Thermie) is investigating a 700°C coal-fired steam power plant, with the specified target of achieving plant efficiency levels of about 50%.

High chromium steels are no longer suitable for these steam conditions. As a consequence, nickel-based or cobalt-based alloys are necessary because they are the only alloys capable of achieving the required creep and yield strengths in such high temperatures.

Even if it was decided that well known alloys such as A617, deriving from combustion turbine technology, could be used, it would nevertheless still be necessary to investigate long-term mechanical behaviour of large parts, eg forged rotors or casings. Furthermore, for the blade alloys such as Waspalloy or Nimonic 80A would be needed. A feasibility study has been done for a 700°C H turbine (see Figure 12).