Examination of insertion type electromagnetic inspection for outer side defect on ferromagnetic steel tube by speed effect using only static magnetic field

Abstract

IntroductionWhen an electromagnetic sensor composed of a static excitation coil and a detection coil is moved in a steel tube at high speed, an eddy current is generated in the steel tube. If a defect is present in the steel tube, the defect may be detected by the change in the eddy current. Generally, the heat transfer tubes with ferromagnetism in power plants or oil plants are inspected for defects by moving an internally inserted electromagnetic sensor at a high speed of about 1 m/s [1]. In this research, an inner-type electromagnetic sensor using the static magnetic field that moves inside these steel tubes at high speed and detects on the outer side defect in the steel tubes is proposed. The generated eddy current inside the steel tube by constant velocity movement of the static magnetic field is analyzed by the 3-D nonlinear eddy current FEM analysis using step-by-step method. In addition, verification experiment is also conducted. Inspection Model and Electromagnetic ConditionsFig.1 shows the proposed electromagnetic sensor using only the static magnetic field and an inspection steel tube (STB340SC). This sensor is composed of two static excitation coils in series, a pair of differential detection coils, and a magnetic yoke of permendur material. The direct current of 1 A is passed through two excitation coils in series so that the plus z-direction of the sensor is the S-pole and the minus z-direction is the N-pole. The permendur material is an alloy of iron and cobalt. The two excitation coils and the differential detection coil are 220 turns × 2 and 100 turns × 2, respectively. The outer diameter, length, and thickness of this inspection steel tube are f 19 mm, 460 mm, and 2 mm, respectively. The proposed sensor is inserted in the steel tube at a speed of 1 m/s for minus z-direction. The distance (lift-off: Lo) between the sensor and the steel tube is equal to 0.5 mm. Analysis and Experimental ResultsFig.2 shows the distribution of only one layer in the y-direction of eddy current density inside the steel tube when the sensor is moved in the minus z-direction of the steel tube without defect at 1m/s. This figure denotes that the directions of the eddy currents generated on the upper side and the lower side of the steel tube are opposite. Since the lower part of the sensor in the moving direction is the N-pole, the eddy current is generated inside the steel tube near the lower part of the sensor in the direction of canceling the N-pole magnetic field. On the other hand, since the upper part of this sensor is the S pole, an eddy current is generated inside the steel tube near it in the direction of canceling the S-pole magnetic field. In addition, Fig.2 denotes that both the upper and lower eddy currents in the thickness of the steel tube are concentrated on the inner surface of the steel tube by skin effect.Fig.3 shows the distribution of the flux density inside the steel tube when the sensor is moved in the minus z-direction of the steel tube without defect at 1m/s. This figure denotes the flux density inside the steel tube near the lower side of the sensor is distributed in the plus z-direction, since the eddy current is generated inside the steel tube in the direction of canceling the N-pole magnetic field. On the other hand, the flux density on the inner surface of the steel tube near the upper side of the sensor is offset because the eddy current is generated in the opposite direction to when the N-pole of lower side of the sensor passed. However, the distribution of flux density in the steel tube near the central region of the sensor is almost uniform.Fig.4 shows comparison results of analysis and experiment of the steel tube with an outer slit defect in the circumferential direction. The width for the z-direction and the depth for the x-direction of the defect are 10 mm and 0.5 mm, respectively. The horizontal axis of the figure indicates the moving position of the sensor, and the vertical axis indicates the differential voltage output by the pair of detection coils in the sensor. This figure shows that an electromagnetic sensor using only a static magnetic field is detected the outer side defect on the steel tube. Moreover, this figure denotes that the calculated result is in agreement with measurement. **