内挿コイルを用いたパルス渦電流試験による強磁性伝熱管の探傷

抄録

Normal ECTs (eddy current tests) using inner probe coils are widely used in the in-service inspection of non-ferromagnetic tubes in various heat exchangers. However, in 70% of such heat exchangers, ferromagnetic tubes are used. If normal ECT was applied to ferromagnetic materials, only degraded signals disturbed by magnetic noise would be detected. In such cases, magnetic saturation methods are successfully used in the production line of steel pipes and rods. However, these methods can not be employed in bore side inspection using inner probes because of the overheating of the excitation windings caused when generating sufficient DC saturation current. Because pulsed eddy current testing employs a short but powerful impulsive excitation current, not only is the average power consumption of excitation windings suppressed low and the probe temperature does not go high enough to overheat, pulsed eddy current test can also be done using the eddy current generated by pulsed excitation simultaneously. A Ferromagnetic tube has been tested with 20A peak current of pulsed excitation. Magnetic noises were suppressed by pulsed excitation and artificial flaws having 50% wall depth were detected on the tube’s outer surface. Accordingly, if a neodymium magnet is used together with pulsed ECT probe, the flaw detection ability is enhanced so that 30% depth flaws on the outer surface can be detected clearly.