내삽형 자기카메라를 이용한 습분분리재열기의 비파괴검사 및 평가

Abstract

This study analyzes the nondestructive testing and evaluation of moisture separator and reheater (MSR) in the secondary system. The MSR is a heat transfer tube in a heat exchanger, which increases the quality of steam used in a high-pressure turbine to reuse it in a low-pressure turbine. As the MSR tube is made of ferromagnetic material (SS439), it shows high strength under the conditions of high temperature and pressure. Nondestructive testing applying existing eddy current inspection technology cannot be carried out on ferromagnetic tubes due to electromagnetic characteristics such as high magnetic permeability and low conductivity. Moreover, the shape of a fin installed at the external wall of this tube to increase heat transfer area leads to leakage flux or distortion of eddy currents, thereby reducing fault detection performance. This paper proposes a method of signal processing and analysis, which detects faults in the ferromagnetic MSR fin tubes electromagnetically to remove the distribution of eddy currents distorted by external fins and extract valid fault information effectively. Magnetic flux leakage testing (MFLT), eddy current testing (ECT) and combined ECT were performed and evaluated by using a Bobbin-type magnetic camera to examine the MSR tube. The test results indicate that stripes are repeatedly shown by the external fin in MFLT and that the S/N ratio increases due to the minimized number of stripes in ECT. However, signal strength is higher in MFLT than that in ECT. The results obtained by conducting combined ECT applying both MFLT and ECT verify that the number of stripes generated by the external fin is lower than that in MFLT and that the S/N ratio increases due to stronger fault signals. For this reason, combined ECT was applied in this study. In addition, the fault detection performance of this method was validated by using a SS439 specimen in which artificial faults such as flat bottom holes, slits, tapered wear, and grooves were added.