Comparison of cavitation erosion rate with liquid impingement erosion rate

Both cavitation erosion and liquid impingement erosion are phenomena that can cause pipe wall thinning in power plants. The Code for Power Generation Facilities, Rules on Pipe Wall Thinning Management, was published by the JSME (Japan Society of Mechanical Engineers) in 2005. The code says that cavitation erosion shall be prevented either in the design stage or by daily inspection. On the other hand, liquid impingement erosion can occur in any location where a working fluid attacks a pipe wall at high flow velocities. Therefore, it is very important to evaluate the amount of erosion by liquid impingement for pipe steels quantitatively from the viewpoint of aging management. In this study, we carried out both cavitation erosion and liquid impingement erosion tests, and clarified the relation between the two erosion rates. As a result, we found that the erosion rate by cavitation increases in proportion with the 5.2th to 6.8th power of the flow velocity and that by liquid impingement with the 6.0th to 7.4th power. Moreover, a good correlation was obtained between erosion rates by cavitation and by liquid impingement. We also discussed the erosion mechanism with SEM photography, and proposed an erosion model. INTRODUCTION Both cavitation erosion and liquid impingement erosion are phenomena that can cause pipe wall thinning in power plants. The Code for Power Generation Facilities, Rules on Pipe Wall Thinning Management [1], was published by the JSME (Japan Society of Mechanical Engineers) in 2005. The code says that cavitation erosion shall be prevented either in the design stage by a daily inspection. On the other hand, liquid impingement erosion can occur at locations where a working fluid attacks a pipe wall at high flow velocity. The liquid impingement damage occurs downstream of elbows and Ttubes, and just behind orifices and valves. The most effective parameter for liquid impingement erosion is the flow velocity. The impact load increases in proportion with the first to second power of the velocity and the erosion rate with the 4th to 5th power. Therefore, it is possible to prevent the occurrence of liquid impingement erosion by operating power plants at low flow velocities, but it is not realistic from the viewpoint of power generation efficiency. Many studies of liquid impingement erosion have been carried out for steam turbines [2], but very few for pipe wall thinning. It is very important to evaluate the amount of erosion by liquid impingement for pipe wall thinning quantitatively from the viewpoint of aging management. Heymann [3] indicated that the processes of liquid impingement erosion and cavitation erosion are very similar. In our laboratory, about one thousand test data of cavitation erosion have been accumulated [4], which will be available for comparison with liquid impingement erosion data. In this study, we carried out erosion tests of pipe steels by Figure 2: Test apparatus Table 1: Chemical compositions Material C Si Mn V P S Cu Ni Mo Cr Al Fe Ti Al 0.06 0.01 0.01 Bal 0.12 0.01 S15C 0.16 0.21 0.45 0.01 0.016 0.01 0.02 0.05 Bal SUS304 0.06 0.2 1.67 0.034 0.027 8.0 18.73 Bal STPA24 0.12 0.35 0.47 0.002 0.012 0.004 0.02 0.02 0.93 2.12 0.002 Bal 0.001 (mass%) Figure 1: Shape of test specimen Φ12