No abstract

Industrial steam turbines cover a wide capacity range from a few MW up to about 150 MW and usually they are individually designed for their application for mechanical and generator drive according to the specific customer requirements. For both applications, the extraction of steam at various pressure and temperature levels is often required to supply external steam consumers. The resulting compact and non-rotational symmetric design of the turbine casing with significant temperature differences between the various regions of the casing has to meet both the requirement of safe operation with sufficient clearances at any operational mode and at the same time of high inner turbine efficiency by minimum clearances. The paper presents a semi-analytical procedure for the estimation of thermal casing distortions and resulting radial clearances, which allows a rapid prediction and covers a multiplicity of design variants. The method is based on an analytical calculation model and the casing is considered as a sequence of cylindrical sections with different geometry and thermal loading. After solving a system of equations the thermal bending line of the casing is found. However, the pure analytical approach doesn’t incorporate nozzles or cut outs of the casing that are necessary for steam extraction. Their influence on casing distortion is considered by systematic finite element parameter studies. In combination with the dynamic rotor deflection the radial clearances can be predicted. The authors report about the application of the procedure for the example of a turbine test run, where temperature and displacement measurements were conducted. The boundary conditions for the calculation, i.e. the temperature distribution in every section, were derived from the measurements. The predicted casing distortion is in good accordance with the results of a complete 3D FE analysis and the measurements.