Proposal of Thickness Measurement Method of Steel Plate With High Liftoff Using Pulsed Magnetic Field

As for the steel tube for liquefied natural gas in a petrochemical plant, the steel tube with a large diameter is covered with the thermal insulation material and the sheet metal. Generally, the thickness of its thermal insulation material is about 80 mm. On the inspection of the thickness or the corrosion in this steel tube, the sheet metal and the thermal insulation material are removed from the steel tube. However, much cost and time are needed for this inspection. In this paper, the pulse electromagnetic inspection method for detecting the defect in the steel tube from the upper side of the insulation material and the sheet metal is proposed. The defect is inspected by measuring the slope of the output voltage from the maximum output voltage value obtained immediately after cutting off the pulse wave magnetic field. In addition, the proposed inspection method is investigated by the 3-D nonlinear finite-element method taking into account hysteresis magnetizing properties and eddy current in the steel tube.