Research On The Applications Of Wavelet Analysis In Cavitation Fault Diagnosis For Centrifugal Pumps

Cavitation has always been a problem seriously concerned by irrigation works, navigation shipbuilding, aero-space, nuclear industry, fluid machinery and so on. Cavitation has also been a long-existed difficult problem for centrifugal pumps. Although the research on cavitation has a history of nearly one hundred years, its diagnosis methods still mainly rely on traditional ones. With the development of modern diagnosis method and technology, the research on cavitation diagnosis for centrifugal pumps has made considerable progress. However, the present diagnosis methods are mostly limited to the cavitation in serious state, and always fail in the diagnosis of the early-stage cavitation, and do nothing of determining the degree of cavitation.In this paper, the characteristics of cavitation vibration signal from the centrifugal pump is analyzed by fast Fourier transform(FFT), wavelet decomposition and wavelet package decomposition. And the difference between the vibration signal under normal working state and in cavitation state is found out: under normal working state, the vibration in the high frequency domain is very weak, while it is notably strengthened when the cavitation is initially developed in the pump. Then, through analyzing the distribution characteristics of vibration signal in the high frequency and low frequency domain, the diagnosis of the early-stage cavitation for centrifugal pumps can be realized.Secondly, since wavelet package decomposition has the ability of decomposing the vibration signal into various frequency domains, it is used to decompose the cavitation vibration signals of the centrifugal pump working in various cavitated state. From the results of decomposition, it is found out that the vibration frequency of the cavitation is relatively high in the early-stage of cavitation, and gradually develops to the low frequency domain with the development of cavitation. In this way, it is convenient to determine the degree of cavitation for centrifugal pumps by analyzing the level of vibration frequency and its distribution characteristics.Lastly, based on the fact that in centrifugal pumps the volume of bubbles produced by cavitation is small in the early-stage cavitation and gradually becomes larger with the development of cavitation, it can be concluded that in centrifugal pump the vibration frequency caused by small bubbles is higher, while that induced by larger bubbles is lower, for the cavitation vibration is mainly caused by the rupture of bubbles. It is instructive for the further investigation of the cavitation mechanism, and it is also very useful for improving the new-developed diagnosis method for cavitation.