Defects are usually developed on the outside of heat exchanger tubes due to large friction between the support plate and the tube. In this paper, in order to improve the accuracy of the MFL (magnetic flux leakage) testing for heat exchanger tubes, the influence of support plate on MFL testing is investigated. Firstly, the influence of support plate on the magnetization status is theoretically analyzed by magnetic circuit analysis. Then, numerical simulations are conducted to investigate the magnetization intensity of the tube at different locations. Finally, MFL experiments for heat exchanger tubes are performed, in which a differential induction coil array is proposed to eliminate the background signal caused by support plate. The testing results show that the MFL signal amplitude is increasing with the defect moving from the center of the support plate to the edge, which should be taken into consideration in the evaluation of heat exchanger tubes.
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