The Effects of Remanent Magnetization on Magnetic Flux Leakage Signals

Abstract

The Magnetic Flux Leakage (MFL)Technique is the most commonly used technique to inspect large diameter transmission pipelines [1–5]. A typical MFL inspection system uses permanent magnets to apply an axially oriented magnetic field to the ferromagnetic pipe material. The magnetic field is perturbed by a metal-loss region (usually caused by corrosion) to produce flux leakage outside the pipe, which can be measured by field sensors. The magnetization system in an MFL inspection system should ideally produce a magnetic field that is

• strong enough to cause a measurable amount of magnetic flux to leak from the pipe material at metal-loss regions,

• uniform from inside to the outside surface of the wall thickness so that the measured signal is more linearly related to metal-loss depth, and

• consistent in magnitude along the length of a pipe so that flux leakage measurements can be compared at different locations during an inspection run.