New material reduces wall thickness 54 percent
Schkopau power station in Germany is using a new material, P91/T91 (pipes/tubes), for the first time in an entire high-pressure piping system. The material`s history dates back to the early 1970s when U.S. research on fast breeder nuclear reactors created a need for better 9-percent to 12-percent chromium steels. This led eventually to Oak Ridge National Laboratory`s developing a ferritic CrMo steel with a 100,000-hour creep rupture strength of 14.5 kpsi at 1,100 F. The steel`s long-time rupture strength combined with its toughness and machinability also made it an excellent candidate for use in conventional power stations at higher live steam temperatures.
In the early 1980s, the American Society for Testing and Materials included into its product standards a modified version of the Oak Ridge steel (9 percent Cr /1 percent Mo), setting the stage for the perfection of the material`s chemical composition. Perfecting the material meant the new steel could be categorized as a martensitic material, achieving its full potential in the air-hardened condition. After development work on fast breeder technology waned in the United States, German companies Mannesmann Research Institute, Mannesmannröhren-Werke and Mannesmann Anlagenbau became involved in internationally backed projects whose aim was to qualify the steel through wide-ranging material tests. As a result of this research, P91/T91 (designated X 10 CrMoVNb 91 in Germany) was cleared for use in German power stations at temperatures up to 1,148 F. Reliable extrapolations based on long-time testing under real operating conditions revealed a 13-kpsi, 100,000-hour creep rupture strength at 1,112 F, a figure which is 53-percent higher than X 20 steel and 155-percent higher than P22 steel. The material also has improved welding properties compared to X 20.
Reduced wall thickness
The material`s improved long-time rupture strength properties means manufacturers can substantially reduce wall thickness compared to other steels when using P91/T91 (see Figure). The smaller wall thickness results in lighter power plant components and support structures. At the same time, the reduced wall thickness gives greater elasticity to high-performance components such as steam headers and fittings and interconnecting pipe systems. Bulk material reduction in a recently completed power station saved approximately $1.73 million per 800-MW unit compared to X 20 steel.
The main application for P91/T91 is in new power stations, primarily in high-pressure piping systems. For about 30 years, the chief steam parameters in hard-coal power stations were constant at approximately 2,900 psi and 995 F on the live steam side and 725 or 870 psi and 995 F in the hot reheat pipe.
Efficiency for these power stations was 36 percent to 37 percent at best. However, over the same period, turbine performance has increased from 150 MW in the early 1960s to about 700 MW today. Consequently, a major means for further improving efficiency lies in increasing steam pressure and temperature in the live steam and reheat systems, increasing their difference with respect to the turbine. With steam parameters for recently completed stations at about 1,013 F/3,770 psi and 1,050 F/800 psi, P91/T91 can effect savings through efficiency increases by allowing even higher temperatures.
The final breakthrough for the new material, and an example of increased efficiency through higher temperatures, came with the Schkopau project. This new, 900-MW ultra-modern plant became the first ever in Europe to use the material for its entire high-pressure piping system. As a result of using P91/T91, the plant has live steam parameters of 1,022 F/4,134 psi with hot reheat steam at 1,040 F/1,030 psi and has an efficiency above 40 percent.
Induction-bent P91/T91 pipe at the Schkopau power station. Photo courtesy of Mannesmannrohr GmbH, Mülheim, Germany.
Seamless Mannesmann pipe made from P91/T91. Photo courtesy of Mannesmannrohr GmbH, Mülheim, Germany.