Multiple yoke eddy current technique for detection of surface defects on

Abstract

Defects such as corrosion located on the exterior of a component such as a metal pipe, tank, vessel or support structure which is insulated by a coating or cover, are detected by a low frequency eddy current (LOFEC) system. A magnetizing yoke in the shape of an inverted U is placed on the cover insulating a metal component. An excitation coil is wrapped around the magnetizing yoke for carrying an altering current in order to produce an alternating magnetic field by the magnetized yoke. The alternating magnetic field produced by the magnetized yoke induces an eddy current which runs through the metal component between the legs of the yoke. A pair of magnetic flux sensors are used in differential connection beneath one or more excitation coils which can be articulated to conform to varying pipe diameter. In addition, an absolute signal response can be obtained from one of the receiver coils. This arrangement allows for the reduction and minimizing of unwanted and extraneous signal responses caused by variations in the component structure while the yoke is scanned about the cover and a positive separation of loss of metal signals (CUI) from artifact signals. Defects located on the surface of pipes or tanks covered with marine growth can be detected using the DPEC probe with either a signal magnetic flux sensor located beneath the excitation coil or a plurality of magnetic flux sensors differentially connected.