Abstract
In this study, a novel method is proposed to detect flaws directly under tube support plate (TSP) of heat exchanger tube using a cylinder-type magnetic camera (CMC). The CMC measures and images magnetic flux density changes occurring around a defect by applying a time-varying magnetic field to a metallic object through magnetic sensors which are arrayed in a matrix. If there is a flaw directly under the TSP, the magnetic flux density is concentrated in the TSP due to its higher permeability. Accordingly, flaws directly under the TSP are difficult to be detected. In order to solve this problem, the principle that the magnetic flux density distribution around the TSP is uniform in the circumferential direction is used. When the magnetic flux density signals obtained at the position where the TSP is placed are set as reference signals and the signals obtained by rotating the sensor probe in the circumferential direction are compared with the reference signals, it is possible to detect the flaw signals from which the TSP signals are removed. If the flaw is formed in the circumferential direction, the flaw can be detected by moving the sensor probe in the axial direction and comparing the signals with the reference signals. The proposed method to detect flaws directly under the TSP was verified using an artificially flawed heat exchanger tube (HXT) test specimen made of titanium alloy.
