Proposal of electromagnetic inspection method of outer side defect on steel tube with steel support plate using optimal differential search coils

Steel tubes are used in the heat exchanger in the petrochemical plant etc. Since these steel tubes are stored as a bundle by the support steel plate (baffle), it is necessary to inspect the existence of defects from the inside of each tube. In these steel tubes, the defect is generated between the steel tube and the baffle at the outer side of steel tube. In this paper, problems in applying the alternating flux leakage testing method for the inspection of the outer side defect on steel tube with the baffle are examined using 3-D nonlinear FEM. Moreover, the differential search coil that detects the leakage flux from the outer side defect on steel tube is proposed. The optimal dimension of the differential search coil is determined using the Rosenbrock´s method.