マイクロフォンによる配管外部からのキャビテーション検知(機械力学,計測,自動制御)

Abstract

Cavitation induced vibration and erosion of pipes are the potential damaging factors in a piping system. To prevent such trouble, it is necessary to develop the detection method of cavitation phenomena. Especially, in power plants, it is preferred to detect them from the outside of piping during operation. In this paper, detection of cavitation phenomena was experimentally investigated using microphones placed outside of piping at upstream and downstream of an orifice. Following results were obtained: (1) According to the progress of cavitation phase, output voltage of the microphone varied, and the amplitude and number of the pulse-shaped signals increased. However, it migh be difficult to distinguish them from surrounding noises in an operating plant. (2) Microphone outputs were confirmed to be radiasion sounds caused by vibration on surface of the piping based on measurements of time difference between accelerometer output and microphone output. (3) The results of 1/3 octave band analysis revealed, noises by cavitation increased in the region of high frequency accoding to the progress of cavitation. In transitional phase, high frequency noises at the downstream from the orifice were larger than upstream. (4) The RMS (root mean square) values ratio of microphone output at upstream and downstream of the orifice varied according to the progress of cavitation, and increased by applying a high band pass filter. Therefore, comparison of RMS values at upstream and downstream of the orifice might make it possible to detect cavitation phenomena in a piping system of operating plants.