Transmission of High Frequency Vibrations in Rotating Systems. Application to Cavitation Detection in Hydraulic Turbines

Abstract

Abstract:One of the main causes of damage in hydraulic turbines is cavitation. While not all cavitation appearing in a turbine is of a destructive type, erosive cavitation can severely affect the structure, thus increasing maintenance costs and reducing the remaining useful life of the machine. Of all types of cavitation, the maximum erosion occurs when clouds of bubbles collapse on the runner surface (cloud cavitation). When this occurs it is associated with a substantial increase in noise, and vibrations that are propagated everywhere throughout the machine. The generation of these cavitation clouds may occur naturally or it may be the response to a periodic pressure fluctuation, like the rotor/stator interaction in a hydraulic turbine. Erosive bubble cavitation generates high-frequency vibrations that are modulated by the shedding frequency. Therefore, the methods for the detection of erosive cavitation in hydraulic turbines are based on the measurement and demodulation of high-frequency vibrations. In this paper, the feasibility of detecting erosive cavitation in hydraulic turbines is investigated experimentally in a rotating disk system, which represents a simplified hydraulic turbine structure. The test rig used consists of a rotating disk submerged in a tank of water and confined with nearby axial and radial rigid surfaces. The excitation patterns produced by cloud cavitation are reproduced with a PZT (piezoelectric patch) located on the disk. These patterns include pseudo-random excitations of different frequency bands modulated by one low carrier frequency, which model the erosive cavitation characteristics. Different types of sensors have been placed in the stationary and in the rotating parts (accelerometers, acoustic emission (AE), and a microphone) in order to detect the excitation pattern. The results obtained for all the sensors tested have been compared in detail for the different excitation patterns applied to the disk. With this information, the best location and type of sensor to detect the different excitations have been identified. This study permits improving the actual technique of detecting erosive cavitation in hydraulic turbines and, therefore, to avoid operation under these circumstances.

Detection of Hydraulic Phenomena in Francis Turbines with Different Sensors

“…In the accelerometer located in the shaft (ASH), the RSI frequency viewed from the rotating frame appears for one operating point (P/P rated = 0.8), which is the point that presents the highest amplitude in the demodulation obtained with the AGV. This could mean that at this operating condition the cavitation presents more erosive characteristics, as was discussed previously in [34,35].…”

“…In that way, after applying the demodulation technique, it is expected to find the periodic hydraulic phenomena. Those frequencies are normally the frequency of the vortex rope or hydraulic resonances when those phenomena are taking place in the turbine or the RSI frequencies when cavitation appears, especially for the inlet cavitation [35].…”

“…To detect cavitation, the demodulation technique has been used in the past [34,35]. The theory says that inlet cavitation is modulated with the RSI, hence if the RSI frequencies appear once demodulating high-frequency signals, this means that cavitation is taking place.…”

“…RMS values were also calculated from the same 60 s signal but applying different bandpass frequency filters to detect the different phenomena. Finally, the demodulation analysis was performed by applying an FFT to the absolute value of the Hilbert transform of 20 s of high-frequency signals filtered in different high-frequency ranges [34,35]. To know which the best high-frequency ranges were to detect every phenomenon, several analyses at different ranges of 2 kHz width were carried out.…”

Anti-Cavitation Design of the Symmetric Leading-Edge Shape of Mixed-Flow Pump Impeller Blades

“…The collapsing bubble may release shock waves and form re-entrant jets. These waves and jets cause noise, pressure pulsation, and vibration [3,4]. If cavitation happens near the surfaces of materials, the shock waves, re-entrant jets, and following rigid particles [5] cause material damage, like spotted erosion [6].…”