Cavitation-induced vibration and erosion of pipes is a potentially damaging factor in piping systems. To prevent it, a detection method for cavitation phenomena should be developed. In power plants, especially, it is desirable to detect them from outside pipes during operation. Detection of cavitation phenomena was experimentally investigated in this paper using accelerometers mounted on the outer surface of a pipe upstream and downstream from an orifice. The following results were obtained. (1) With the progression of cavitation, output voltage of the accelerometer varied, and the amplitude and number of the pulse-shaped signals increased. However, it would likely be difficult to distinguish them from noises in an operating plant. (2) It was difficult to recognize the characteristic frequency of cavitation, because the power spectrum density was broad up to the accelerometer limit of 45 kHz. (3) The flow directional distribution of RMS (root mean square) values of accelerometer output voltage varied greatly with the progression of cavitation. Therefore, from comparison of RMS values obtained upstream and downstream from the orifice it seems possible to detect cavitation phenomena in the piping systems of operating plants.

Cavitation induced vibration and consequent erosion of pipes are potential damaging factors in the piping systems of power plants. In order to prevent them, we previously developed a cavitation detection method using accelerometers or microphones placed outside pipes during operation which compares RMS (root mean square) values of sensor outputs upstream and downstream from an orifice (i.e. pipe throttle). However, this method can detect only the stage of developed cavitation. Therefore, in the present study, in order to confirm the effectiveness of the cavitation detection method, we evaluated incipient cavitation erosion on pipe walls using impulsive force detectors and aluminum erosion specimens and compared the cavitation detectable conditions with the erosion occurrence conditions. Results obtained by the impulsive force detectors were insufficient due to weak forces, but the cavitation number σ at the incipient erosion was obtained from observation of erosion pits on aluminum specimens. The cavitation detection method could detect cavitation in the region of 9 0. ≤ σ and erosion pits occurred in the region of 8 . 0 ≤ σ . As the results, we confirmed that the cavitation detection method could be effective to monitor and prevent cavitation erosion on pipe walls.

Cavitation-induced vibration and erosion of pipes are potential damaging factors in piping systems. To prevent damage, it is necessary to develop the detection method for cavitation phenomena. In power plants, it is especially desirable to detect their occurrence from outside the piping during operation. In this paper, detection of cavitation phenomena was experimentally investigated using microphones placed outside the piping at positions upstream and downstream from an orifice. The following results were obtained: (1) According to the development of cavitation state, the microphone output varied, and the amplitude and number of the pulse-shaped signals increased. However, it might be difficult to distinguish them from background noises in an operating plant. (2) Microphone output was confirmed to be radiated sounds caused by vibration on the surface of the piping based on measurements of the time difference between accelerometer output and microphone output. (3) The results of the 1/3 octave band analysis revealed that noises due to cavitation increased in the high frequency region according to development of the cavitation state. In the developed cavitation state, high frequency noises downstream from the orifice were larger than those upstream. (4) The RMS (root mean square) ratio of the microphone output upstream and downstream from the orifice varied according to the development of the cavitation state, and increased by applying a high band pass filter. Therefore, from comparison of RMS values of the microphone output upstream and downstream from the orifice, it is possible to detect cavitation phenomena in piping systems of an operating plant.

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