TRANSMIT-RECEIVE EDDY-CURRENT PROBES FOR CIRCUMFERENTIAL CRACKS IN HEAT-EXCHANGER TUBES

Conventional eddy current bobbin probes are known to be ineffectual in detecting circumferential cracks in tubing Multi-pancake and/or rotat ing pancake probes are required to detect circumferential cracks. It h as recently been demonstrated in CANDU steam generator tubes, with def ormation, ferromagnetic deposits, and copper deposits, that multi-chan nel probes with transmit-receive (T/R) coils are superior to those usi ng surface impedance coils. Unlike rotating probes, the design of a ne w probe denoted as C3 permits fast, single-scan inspection of a full-l ength tube at inspection speeds comparable to conventional bobbin prob es. Since 1992, the probe has been used routinely for steam generator in-service inspection at four CANDU plants. Defective tubes have been plugged and the units returned to service, and they continue to operat e without leaks. This paper describes the basic features of T/R surfac e probes. Two-dimensional voltage diagrams showing computer-derived pr obe response to frequency, lift-off, carbon steel supports, magnetite deposits and copper deposits are presented and compared with correspon ding signals from impedance coils. Theoretical and experimental result s show that T/R probes are able to detect defects in the presence of v ariable lift-off (due to tube-wall deformation) with ten times the sig nal-to-noise ratio as that exhibited by comparable pancake-type impeda nce probes. In addition, T/R probes are less sensitive to magnetite de posits, and possess good phase discrimination to internal defects.