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Sizing of Inside Diameter Pits in Tubing Utilizing Two-variable Regression Curves

The traditional approach to inside diameter (ID) pit sizing using the eddy current testing technique in heat exchanger tubing involves relating one variable (either signal amplitude or signal phase angle) to pit depth. The phase and the amplitude of an eddy current signal are dependent upon both the depth and the volume of the ID pit. The volume is a function of pit depth and pit morphology (effective diameter). Measurements of signal phase and amplitude can be used to construct a twovariable regression curve relating these parameters to depth. A two-variable curve should attenuate the effect of pit morphology on depth estimates. The principal question is whether a single measurement curve can accurately depth size ID pits of various morphologies. Calibration data sets consisting of machined pits of various diameters and depths were acquired. Two-variable regression curves were used to estimate the depth of machined and in-service pits of various diameters and depths. Traditional one-variable curves were also used to estimate the depth of these indications. The results of these analyses were evaluated to determine sizing accuracy.

ASME, Boiler and Pressure Vessel Code, Section V Nondestructive Examination, Article 8: Eddy Current Examination of Tubular Products, American Society of Mechanical Engineers, New York, New York, 2001. Cecco, V. S., G. Van Drunen and F. L. Sharp, “Eddy Current Manual,” Vol. 1, AECL Report, AECL-7523, Chalk River Laboratories, 1983. EPRI, “Electromagnetic NDE Guide for Balance of Plant Heat Exchangers,” Revision 2, Report TR101772-R2, Project 3232-01, Electric Power Research Institute, December 1992.
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