VIBRATION CAVITATION DETECTION USING ONBOARD MEASUREMENTS

Experimental tests have been carried out in a Francis turbine suffering from inlet edge cavitation erosion on the runner blades. The tests have comprised an onboard accelerometer mounted on the shaft in radial direction just above the turbine guide bearing. Simultaneously, vibrations in fixed parts have also been monitored such as turbine guide bearing pedestal and guide vane. Besides, an acoustic emission sensor has been located next to the fixed accelerometer in the bearing pedestal. The machine has been operated at various output loads and the corresponding signals have been recorded and analyzed following traditional cavitation detection techniques such as amplitude demodulation of high frequency bands. This permits to investigate the erosion process from a location which is directly connected to the blades. The advantage of such a situation is that the influence of the fluid film bearing and of the rotating frame of reference are avoided compared with measurements from a fixed position. As a consequence, the validity of external measurements to detect cavitation dynamic behavior can be checked. In the paper, the suitability of the telemetry system used for the tests is analyzed and then the results are presented and discussed. NOMENCLATURE Rotating speed N Number of runner blades Zb Number of guide vanes Zv Fundamental frequency ff= N/60 [Hz] Blade passing frequency fb= Zb⋅ff [Hz] Guide vane passing frequency fv=Zv⋅ff [Hz] GVO Guide Vane Opening [%]