Coherent Phase Wide Band Demodulation Technique for Turbomachinery Cavitation Detection and Monitoring

Abstract : This paper discusses a Coherent Phase Wide Band Demodulation (CPWBD) technique for turbopump cavitation detection and monitoring. The principle of cavitation detection is based on the unique nonlinear modulation phenomenon associated with pump cavitation in which the periodic shaft rotational motion (and/or its harmonic motion) amplitude modulates the wideband noise generated from the collapse of cavitation bubbles. However, these periodicities associated with shaft rotation are hidden during the physical wide band modulation (WBM) process and cannot be identified within the conventional power spectral density (PSD) of the monitored dynamic signal. Such periodicity, hidden well within wide band noise, thus provides a unique signature and serves as the basis for effective cavitation detection. Existing techniques for cavitation detection utilize envelope analysis or full wave rectification (FWR) spectral analysis to demodulate dynamic measurement signals over an isolated band of high frequency wide band noise in order to recover hidden cavitation-generated periodicities. However, the conventional FWR technique is subject to a practical limitation in that any discrete components present in the high frequency regions of interest, due to other vibration sources (mechanical structural, etc.), will erroneously generate discrete peaks in the resulting demodulated signal appearing as genuine recovered hidden periodicities. The CPWBD technique provides an effective alternative to avoid such discrete interference. The method is based on an inherent signal property associated with the WBM signal, where a unique coherent phase relationship exists among all interacting spectral components associated with the cavitation process. The CPWBD technique can thus identify hidden periodicity by searching for such a coherent phase relationship.