Transmit-Receive Eddy Current Probes for Circumferential Cracks in Heat Exchanger Tubes

摘要

Conventional eddy current bobbin probes are known to be ineffectual in detecting circumferential cracks in tubing. Multi-pancake and/or rotating pancake probes are required to detect circumferential cracks. It has recently been demonstrated in CANDU steam generator tubes, with deformation, ferromagnetic deposits, and copper deposits, that multi-channel probes with transmit-receive (T/R) coils are superior to those using surface impedance coils. Unlike rotating probes, the design of a new probe denoted as C3 permits fast, single-scan inspection of a full-length tube at inspection speeds comparable to conventional bobbin probes. Since 1992, the probe has been used routinely for steam generator in-service inspection at pur CANDU plants. Defective tubes have been plugged and the units returned to service, and they continue to operate without leaks. This paper describes the basic features of T/R surface probes. Two-dimensional voltage diagrams showing computer-derived probe response to frequency, lift-off, carbon steel supports, magnetite deposits and copper deposits are presented and compared with corresponding signals from impedance coils. Theoretical and experimental results show that T/R probes are able to detect defects in the presence of variable lift-off (due to tube-wall deformation) with ten times the signal-to-noise ratio as that exhibited by comparable pancake-type impedance probes. In addition, T/R probes are less sensitive to mag-netite deposits, and possess good phase discrimination to internal defects.