Evaluation of superimposed flaws using the remote field eddy current technique

摘要

The Remote Field Eddy Current (RFEC) technique is currently recognised as the method of choice for the examination of ferromagnetic tubes such as those found in boilers and heat-exchangers. A basic RFEC probe consists of an exciter and detector coil in a send-receive configuration. Metal loss alters the signal received at the detector coil, and flaws can be characterized by their signatures on the Strip Chart and Voltage Plane Polar Plot (VPPP) displays. An advantage of RFEC over conventional eddy current is that a simple, computerised phasor analysis can yield both the depth and circumferential extent of flaws. Each flaw has its own unique phasor, so the effects of overlapping flaws can be calculated by mathematically combining the two phasors. A procedure for obtaining the correct depth of metal loss when two flaws overlap is illustrated using data from a reboiler examination.