Multi-frequency eddy current inspection and signal analyzing unit system

In the steam condensers for thermal and nuclear power stations, the number of cooling tubes per unit is 15,000 ∼ 24,000 in thermal power stations, and nearly 44,000 in nuclear power stations. The brass tubes and titanium tubes of 25.4 ∼ 38.1 mm diameter, 9 ∼ 23 m length and 0.5 ∼ 1.2 mm wall thickness are used. One of the secular deterioration due to the operation of turbines is the decrease of the wall thickness of condenser tubes on the outside and inside surfaces, accordingly, the regular inspection of condenser tubes is carried out. As the method of detecting defects and flaws on the outside and inside surfaces of many cooling tubes one by one, electromagnetic induction eddy current flaw detection test has been widely employed as the nondestructive test. In this paper, the problems in conventional eddy current flaw detection test, the outline of the damage due to the decrease of wall thickness of condenser tubes, multi-frequency phase analysis type eddy current flaw detection method, the method of inspecting the depth of defects and the volume of defects, the circumstance of using eddy current flaw detection method in power stations, and the signal analysis and processing computer system used at plant sites are reported. (Kako, I.)