Field Assessment of Cavitation Detection Methods in Hydropower Plants
An experimental investigation on Francis turbines suffering from inlet cavitation erosion on the runner blades has permitted to assess several aspects of the actual cavitation erosion detection and prediction methods based on vibrations. Detection tests in three prototypes have consisted in the measurement of vibrations in turbine guide bearing, guide vanes and draft tube during operation. Discussion about RMS levels, auto power spectra and amplitude demodulation results is provided. A dynamic calibration has been carried out using an instrumented hammer. For that, one of the accelerometers has been mounted on the blade and the rest have been kept in their external positions. Frequency response functions between the blade and the remote sensors have been obtained and compared among the machines. The various transmission paths have been investigated.