The pulsed eddy current (PEC) system has been developed for the detection of the wall thinning in the pipelines of nuclear power plants (NPPs) without removing the insulation. The PEC signal response from the stainless steel sample which the thickness varies from 1 mm to 5 mm was investigated at the position of 6 mm above the sample surface. A LabVIEW based data acquisition program was developed to continuously display the thickness variation of the tested sample. The signal analysis technique such as power spectrum density (PSD) of Fourier transform is applied to obtain an optimum parameter to describe the wall thinning of pipeline steel. This technique can be used as a potential tool to detect the local wall thinning of a pipe without the removal of the insulation.

Detection of corrosion and measurement of the thickness of stainless steel in industrial structures, such as pipelines, has been an important issue to avoid unwanted leakages. A recently proposed transient eddy-current oscillations (TECO) method has been used in this paper to measure thickness in non-magnetic stainless steel. The TECO method works based on the electromagnetic induction principle, as the conventional eddy-current testing method. An inductor (excitation coil) connected in parallel with a capacitor to generate damped oscillations is used to induce eddy currents into the test material beneath the coil. A Hall sensor, which is fixed at the bottom side and at the axial center of the excitation coil, detects the resultant magnetic field due to the induced eddy currents. Experiments were performed on the stainless steel specimens, having different thicknesses to test the potential of the TECO method. The results were analyzed using the fast Fourier transform. This paper proposes a lift-off point of intersection in the frequency domain that can be used to eliminate the false indications due to the lift-off variations during the thickness measurement of the test material. The results reveal that the proposed method can be applied to detect the thickness variations in metal structures covered with non-conductive paints or insulation coatings.

A differential probe which is used in the pulsed eddy current (PEC) system has been fabricated for the detection of wall thinning of insulated pipelines in a nuclear power plant (NPP). The differential PEC probe consists of two hall sensors in a differential arrangement. The tested sample is a stainless steel of thickness variation from 1 to 5 mm, the flat side of the sample is laminated by a plastic insulation having a uniform thickness to simulate the pipelines in NPP. The PEC response to varying metal thickness was measured at various thicknesses of insulations on the tested sample. The time-domain feature such as peak value of the detected pulse is used to interpret the thickness of the test sample. The signal analysis technique, such as power spectrum density, is applied to obtain an optimum parameter to describe the wall thinning of pipeline steel. This technique can be used as a potential tool to detect the corrosion or the wall thinning of the pipelines without removing the insulation.