Magnetic Flux Leakage Scanning Velocities for Tank Floor Inspection

Abstract

The non-destructive testing technique known as magnetic flux leakage (MFL) is used for the inspection of storage tank floors (mild steel plate). This paper focuses on the finite-element analysis of the velocity effects on the induced flux and its distribution in the plate thicknesses of 6 through to 20 mm thick 1020 grade steel. When there is a velocity difference between the plate and the excitation system, eddy currents are induced opposing the applied magnetic field distorting its profile. This has an adverse effect on MFL inspection by altering signal responses thus limiting achievable inspection velocities. An investigation of velocities ranging from 0.5 to 3 m/s is undertaken on both defect-free plates and plates containing spherical 40% through wall thickness defects simulated for both near/top and far/bottom surfaces. From the results, it is indicated that the flux density distribution through the plate decreases as a function of depth. Once a plate depth of 16 mm is reached, it is shown that the flux density is significantly reduced toward the underside of the plate. These combined effects influence defect detectability on larger plate thicknesses for the specific yoke design used in this paper.