The Characteristics of Rotating Instabilities in Low Pressure Steam Turbines at Low Volume Flow Operation
For flexible operation steam turbines may operate occasionally at low load. Operation away from the original design regime looks set to be an increasing trend mainly due to the presence of intermittently available renewable energy sources in the grid. This paper sets out an approach for considering low flow effects on turbine designs. At low load operating conditions rotating instabilities (RIS) can occur in the rear stages of LP steam turbines. The instabilities are comparable in many ways to rotating stall in compressors. Ideally the turbine blade natural frequencies should be designed to avoid the frequencies generated by the RIS system. The characteristics of RIS systems were experimentally investigated to understand the dependency with both flow coefficient and exhaust configuration. Correlations have been developed to characterize the dynamic pressure amplitudes and the fractional speed of the RIS moving around the wheel. The presented correlation based method is shown calibrated for a specific blade design. Two different test rigs provide the basis for the work presented. A low pressure model steam turbine provided detailed information for key blade/exhaust combinations. A simplified small scale air turbine was used to provide additional input for the behavior with alternative exhaust back wall position. Observations of the characteristic RIS behavior from model turbine tests are set in context with observed changes in the flow field.