With global warming being one of mankind’s greatest challenges together with, an increasing demand for electricity world-wide, and studies showing that fossil resources like coal and gas will remain a major source for electricity for the next couple of decades, research into the development of highest efficiency fossil power plants has become a top priority. Calculations for coal fired power plants have shown that by increasing the live steam parameters to 700°C and 350bar CO2 emissions can be reduced by as much as 8% compared to the current state-of-the-art. This is equivalent to a reduction of 24% compared to the current steam power plant fleet within the European Union. To achieve the desired operating hours at this temperature the application of nickel (Ni) based alloys for the main steam turbine components such as rotors, inner casings and valves is necessary. The use of Nickel base alloys for selected gas turbine components is common practice. But with steam turbine rotors being solid, 1000mm in diameter and casings with wall thicknesses >100mm the gas turbine application range and experience for nickel base alloys are well exceeded. This paper discusses a basic product design concept in order to identify the core challenges in developing Ni based steam turbine components. These include casting, forging, non-destructive testing and welding. The material property requirements for such components (steam-oxidation resistance, creep and fatigue resistance) are also identified. Based on these challenges and requirements a number of research projects have been carried out in Europe which have selected Alloy617 as being most suitable for forged components and Alloy625 for cast components. Further projects are currently being initiated. The last major step in steam turbine development for high temperatures was to switch from low alloyed chromium (Cr) steels to high alloyed Cr steels. The identified challenges in using Nickel base alloys for large steam turbines are compared to this last material switch to characterize the level of complexity and difficulty of the development of the 700°C steam turbine technology.

Recent developments like the significant introduction of renewable energy sources to the electricity networks worldwide have led to more frequent and extended operation of fossil power plants in part load conditions. As a result the typical load spectrum of large steam turbines used for electricity generation has changed over the last years and will continue to do so. A number of papers has already been published on how to optimize the water-steam cycle and the steam turbine from a thermodynamical and aero-dynamical point of view for this new load regime in order to improve the average efficiency. But the changed load regime also poses a challenge for the mechanical design and structural integrity assessment of steam turbines. Reason for this is that the rated conditions are not necessarily the most challenging boundary conditions and therefore not necessarily a suitable, conservative envelope for all other load cases for mechanical design. Pressures decrease, but steam temperatures in part loads can increase and heat transfer coefficients and the influence of radiation on the component temperatures change. With an increasing demand for and a wider range of part load operation it for this reason becomes more important than ever to consider these load cases in the mechanical design. This paper uses a large, double-flow intermediate pressure steam turbine as an example to investigate the impact of extended part load operation on the design. Both an analytical model and finite element calculations are used to compare from a structural integrity point of view a low part-load load case and the rated load case and to evaluate the significance of heat radiation.

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