Measurement of ferromagnetic pipe wall thickness by magnetic flux leakage method

Abstract

The paper considers the use of the magnetic flux leakage method for measuring the wall thickness of ferromagnetic
pipes. The method implies longitudinal magnetization of the test pipe section using a short solenoid
and measurement of the spatial longitudinal component of the magnetic field strength in the air gap between
the solenoid and the test pipe using Hall sensors. A numerical model has been developed to analyze the interaction
between the magnetizing field of the short solenoid and the ferromagnetic pipe. The model considers
the nonlinear magnetic properties of the test pipe and its geometrical parameters, including the distance between
the pipe edge and the measurement plane of magnetic field. The accuracy of the model was validated
through physical modeling techniques. A simplified analytical dependence of the longitudinal component of
the magnetic field strength on the pipe wall thickness was obtained. A method was proposed to mitigate the
impact of the distance from the measurement plane to the pipe edge on wall thickness measurement results.
The method entails simultaneous measurement of the magnetic field strength and the distance between the
pipe edge and the measurement plane. The method considers the distance between the pipe edge and the
measurement plane of the magnetic field strength thereby enabling a 10-fold reduction in the error of the wall
thickness measurement induced by the edge effect. The study results can be used for generation, mathematical
modeling, and measurement of the magnetic field, including magnetic inspection of steel drill pipes.