Measurement of Accumulated Height of Exfoliated Oxide Scales in Austenitic Boiler Tubes Using Pulsed Eddy Current Testing

Abstract

Austenitic boiler tubes are key components in modern coal-fired power plants but often suffer from blockage or even rupture caused by the accumulation of steam-side oxide scales. Existing nondestructive testing methods are somewhat deficient to quantify the accumulated height of exfoliated oxide scales. Pulsed eddy current (PEC) technique has unique merits such as deep penetration depth and diverse signal features, offering a great possibility to address this issue. This work serves as an exploratory study for applying PEC technique to measure the scales accumulation. Finite element modeling and experimental work are conducted to validate the feasibility and effectiveness of PEC measurement under different circumstances. The PEC signal peak shows a linear relationship with the accumulated height and thereby is used as the signal feature for the quantitative measurement of accumulated height. A flexible sensor consisting of a replaceable flat cable with connectors at the ends is designed, following by the analyses of sensor offset and liftoff effects. To distinguish the unevenness of accumulation, the sensor-coil array scheme is proposed and the output array signals successfully characterize the profile of inner accumulation. Finally, a referencing strategy is explained to eliminate the interference due to magnetism transformation of the tube wall.