Magnetic methods for wall thickness measurement and flaw detection in ferromagnetic tubing and plate

The need has arisen for continuous wall thickness measurement, often through insulation of ferromagnetic piping such as refinery tubing, used drill pipe, and used coiled tubing. An earlier method((1, 2)) which employed an encircling coil to measure the magnetic flux therein determined only the average wall of the tube. Move recently, a method employing solid-state sensors that will measure the wall thickness of tubing immediately below it has been introduced, and will detect localised losses from eccentricity, erosion and corrosion. The method is very useful wherever long runs of tubing are to be inspected and where ultrasound does not couple well to the material, for example refinery tubing through insulation used drill pipe with slip cuts and outer surface damage, and used coiled cubing with varying degrees of rust and contamination. The sensing system, in flat form, is applicable to the measurement of flat steel strip, and steel floors in oil storage tanks. Along with Magnetic Flux Leakage (MFL), which can be detected with the same sensors, a powerful technique for inspection has emerged.