Pulsed eddy current probe optimization for steel pipe wall thickness measurement

Abstract

Inspection of bulk wall loss, due to far side surface corrosion in ferromagnetic steel pipe, is a common requirement across multiple industries, including chemical processing, and oil and gas. In the nuclear industry, there is a requirement for inspection of tile holes and balance of plant inspections, where insulated pipe is present. Inspection is typically done using ultrasonic testing, which necessitates a coupling agent and removal of any insulation. Pulsed eddy current (PEC) technique does not require direct contact and has a larger spot size, thereby facilitating more rapid inspection without insulation removal. To further develop the potential of PEC to inspect under these conditions, a reflection type PEC probe was examined for inspection of thickness of steel plate. PEC data was taken for different plate thicknesses so that sensitivity to wall loss due to corrosion could be assessed. The PEC signal was analyzed by fitting the tail of the transient decay with an exponential curve, with curve fitting parameters correlated to wall thickness variation. An analytical model was developed and partially validated in order to examine a wider range of factors, such as probe dimensions, number of turns, and magnetic permeability and conductivity of the steel. The modelled data was also used to perform a sensitivity analysis on probe dimensions in order to realise the largest range of wall thickness with highest spatial resolution. A finite element method model of the reflection probe over steel plate was used to provide insights into the observation from the analytical model that outer pickup coil radius was the most significant optimization parameter.