Examination of Thinning Inspection Method for Ferromagnetic Steel Tubes Using Velocity Effect of Static Magnetic Field

The high-speed movement of an electromagnetic sensor comprising static excitation and detection coils in a steel tube generates an eddy current in the tube. The presence of a defect in the steel tube may be detected via changes in the eddy current. In general, the ferromagnetic heat transfer tubes in oil and chemical plants are inspected for defects by moving an internally inserted electromagnetic sensor at a high speed of 1 m/s. Although previous studies have focused on the detection of localized thinning in the steel tube, there are no reports on a wide range of thinning in the steel tube. Therefore, this study investigated a thickness measurement method for a wide range of thinning in the steel tube using only a static magnetic field. Consequently, the possibility of measuring the thickness in the steel tube based on the flux density inside the detection coil of the proposed static magnetic inspection sensor moving at 1 m/s was demonstrated. The flux density in the steel tube was evaluated by the 3-D nonlinear finite-element method (FEM) considering the velocity effect.