Flow accelerated corrosion (FAC) rate downstream from an orifice was measured in a high-temperature water test loop to evaluate the effects of flow field on FAC. Orifice flow was also measured using laser Doppler velocimetry (LDV) and simulated by steady RANS simulation and large eddy simulation (LES). The LDV measurements indicated the flow structure did not depend on the flow velocity in the range of Re = 2.3×10 4 to 1.2×10 5 . Flow fields predicted by RANS and LES agreed well with LDV data. Measured FAC rate was higher downstream than upstream from the orifice and the maximum appeared at 2D (D: pipe diameter) downstream. The shape of the profile of the root mean square (RMS) wall shear stress predicted by LES had relatively good agreement with the shape of the profile of FAC rate. This result indicates that the effects of flow field on FAC can be evaluated using the calculated wall shear stress.

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.

Flow accelerated corrosion (FAC) thinning rate downstream from an orifice was measured under different velocity conditions in a high-temperature water test loop to understand the effects of flow velocity on FAC thinning rate. The FAC tendency differed downstream and upstream from the orifice. The metal loss increased linearly with time downstream from the orifice, though metal loss rate gradually decreased with time upstream. FAC rate increased as flow velocity increased, particularly from 1D to 3D. The maximum FAC thinning rate increased in proportional to the 0.51th power of the mean cross-sectional velocity in this experiment. The root mean square (RMS) of wall shear stress predicted by large eddy simulation (LES) had a clear relationship with FAC thinning rate. This result indicated that FAC thinning rate can be described as a function of the wall shear stress. Additionally, the mass transfer coefficient estimated from the RMS of wall shear stress had an almost linear correlation with FAC thinning rate.

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