Detection of Cavitation with Microphone Placed Outside Piping

Abstract

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.

Detection of Cavitation States with Microphone Placed Outside Pipe Components

“…However, the flow distributions were different between the orifice and valves; consequently, the locations where the bubbles reached near the pipe wall were different for each pipe component. In the experiments with the orifice, super cavitation appeared below σ=0.5 [8] , and cavitation bubbles did not develop over 2.0 D downstream from the orifice (cf. Fig.…”

“…We have previously carried out vibration measurements using accelerometers mounted on the outer surface of pipes, and we confirmed that cavitation occurrence could be detected by comparing accelerometer outputs upstream and downstream [7] . Moreover, considering applicability to actual plants, we examined a cavitation detection method using microphones as a noncontact technique [8] [9] and confirmed that microphones could detect a developed cavitation state. However, the cavitation detection characteristic with microphones was inferior to that with the accelerometer, and it was difficult to judge a weak cavitation state.…”

“…The vertical axis shows the ratio of accelerometer or microphone output RMS (root mean square) at 75 mm downstream to that at 90 mm upstream from the orifice (from now on, RMS ratio: RMS downstream / RMS upstream ). Microphone output ratios were also obtained from the waveform treated with the 8 kHz high band pass filter (HPF), because it was advantageous to use high frequency bands over 8 kHz [8] from the cavitation noises. Accelerometer output RMS ratio expressed well the locations of the bubble collapses as shown in Fig.…”

Evaluation of Incipient Cavitation Erosion on Pipe Walls Downstream from an Orifice

“…However, these methods are for detecting cavitation in a pump, and studies on cavitation detection at valves and orifices in an operating plant have not been reported. Therefore, we have investigated cavitation detection methods in operating power plants, and we developed the method which compares accelerometer or microphone outputs upstream and downstream from a pipe throttle (5)(6) . However, this method can detect only the stage of developed cavitation under low cavitation numbers.…”

“…Comparison of the average value of the maximum forces, F 1st , in Fig. 10 with erosion pit occurrence in Table 1 showed that there was a minimum force to make erosion pits, and it was about 2 N. Figure 13 compares results of the erosion experiments with cavitation detection characteristics using accelerometers (5) and non-directional microphones (6) …”