Assessment of wall thinning in insulated ferromagnetic pipes using the time-to-peak of differential pulsed eddy-current testing signals

Abstract

Pulsed eddy current testing (PECT) is a powerful candidate for the detection of wall thinning of insulated ferromagnetic pipes in petrochemical and power generation plants. The main purpose of this study is to find an efficient and easy-to-use signal feature for the assessment of wall thinning. Analytical modeling for a PECT probe over the insulated piping system is performed and its result is verified by experimental test. Two time-related features, the peak value and the time-to-peak, are found in the differential signal obtained by subtracting the test signal from the reference signal. The time-to-peak is superior to the peak value due to its linear variation with wall thickness. Influences of various conditions in practical testing on the PECT signal are investigated. Results show that the time-to-peak is independent of the insulation thickness and the probe lift-off. Robustness of time-to-peak to probe configuration is also validated by employing three probes of different dimensions and structures. To determine the linear range of time-to-peak with amount of wall thinning, differential signals based on different reference thicknesses are examined. Results show that the time-to-peak only keeps linear for the relative wall thinning less than 60%, but still can be effectively used for calibration purpose in periodical in-service inspection of insulated pipeline.