Design modification of a feedwater heater impingement baffle to mitigate shell wall thinning by flow acceleration corrosion

The feedwater heaters of many power plants can experience severe wall thinning damage, which accelerates as the plant ages. Several nuclear power plants in Korea have undergone this wall thinning damage around the impingement baffle inside feedwater heaters installed downstream of the turbine extraction stream line. At the baffle point, the extracted steam from the turbine exists as a two-phase fluid at high temperature, high pressure, and high velocity. Since it flows in the reverse direction after impinging the impingement baffle, the shell walls of feedwater heaters may be affected by flow acceleration corrosion. This paper describes a comparison of a numerical analysis using the FLUENT code and downscaled experimental data in an effort to determine the root causes of shell wall thinning of feedwater heaters. The numerical analysis and experimental data were also confirmed by actual wall thicknesses as measured by flow visualization. To compare the degree of shell wall thinning, a test of the mitigation rate on squared-type impingement baffle plates compared with the mitigation rate of other types of impingement baffle plates (squared, curved, mitigating, and multi-hole type) was done inside the shell of a feedwater heater. This paper describes the experimental procedures and the numerical analysis of these comparison data. These experiments were done using conditions similar to those found in a real feedwater heater. Flow visualization was done in order to verify the experimental results and the numerical analysis. From the comparison of the results of the local velocity profiles and the wall-thinning measurements, this study shows that mitigating type and multi-hole type impingement baffle plates are more effective than other baffle plates in preventing decreases in pressure concentration and pressure value.