Flow accelerated corrosion (FAC) is a major pipe wall thinning phenomena in power plants. The management of pipe wall thinning has been carried out to pipe elements such as elbow, orifice, etc. of the piping system in power plants. The combination of analytical codes and measurement of pipe wall thinning is usually applied to thinning estimation methods. In piping system, several pipe elements which are connected in series may generate swirl flow. Therefore the arrangement of pipe elements is considered to affect thinning phenomena seriously. So the behavior of swirl flow is one of the major research targets to improve the accuracy of simulation codes. In Mihama Unit 3 pipe degradation accident in 2004, it was pointed out that the swirl flow caused by the piping layout might influence thinning rate behind orifice.
In the article Part1, we focus on the conditions and parameters (combination, distance, etc. of pipe elements) that affect the generation of swirl flow and analyze the effect of such parameters in mass transfer coefficient behind pipe elements. The generation of swirl flow is confirmed analytically in this study.
In the article part2, we develop new defined effective friction velocity considering the ratio between the change of average flow velocity and the change of turbulent velocity in the straight pipe. The effect of swirl flow on geometry factor is also evaluated by the new defined effective friction velocity. We also focus on the flow field of main-stream in the swirl flow. The result of analysis shows the generation of twin vortex under some combinations of pipe elements and suggests that twin vortex may affect the distribution of geometry factor. We compare the analytical results and estimate the cause of Mihama Unit 3 pipe degradation accident. The results of this study will be reflected to the evaluation of fluid-dynamic factor of FAC and improve the accuracy of simulation codes in near future.
