Improved Steam Turbine Design for Optimum Efficiency and Reduced Cost of Ownership

The cost of ownership of a power plant is partly governed by the efficiency of the turbine island. The turbine stands for the production revenues when transforming the energy in fuel into electric power and district heating. One gauge of the quality of the individual processes is the component efficiency - the current work
addresses the turbine part of the power plant. The turbine efficiency is dependent on both process parameters and the blading aerodynamics. The former is typically the steam data (e.g. temperature, pressure and mass flow) that influences the volumetric flow through the turbine. The blading aerodynamics is the local flow process in each stage in the turbine. The efficiency of a stage (i.e. a stator and a rotor) is limited by losses due to dissipation in boundary layers,
losses due to secondary flows, leakage mixing and lost work, etc. Most previous development efforts in the industrial steam turbine segment have been towards reduced first cost and not necessarily the efficiency. Most industrial steam turbines utilize prismatic (or un-twisted) blades for shorter stages. A constant section rotor blade typically is milled in a two-axis machine whilst more advanced
shapes require five-axis flank milling. The costs associated with the latter have today leveled with the simpler manufacturing methods. This technology step has been introduced in larger size utility type of turbines with very high attainable efficiency levels. An industrial size steam turbine cannot reach the same level of efficiency and lags several points behind as for a utility type unit, because of the lower volumetric flow and the cylinder pressure ratio. The focus in the present work has been to reduce the losses and hence increase efficiency in an industrial size steam turbine stage. Profile losses and secondary losses being the two main targets to improve performance, the work is focused on both the loss mechanisms. The work has been carried out by state-of-the-art turbine design tools and comprises: one-dimensional tool, two-dimensional blade-to-blade flow analysis and full three-dimensional high-fidelity CFD. The datum stage is a
tall stage in an assumed Siemens SST- turbine. The work, however, is generic for low-reaction steam turbines. The work shows that the stage performance can be increased. The significant improvement obtained from
numerical prediction makes a strong case for the proposed design modifications to be considered.