Array coil probe design optimization

To overcome the limitations of conventional bobbin style eddy-current probes, it has become common practice to perform some portion of a steam generator tube inspection program with array coil probes. Typically, eight surface-riding pancake coils are utilized in a probe head, either as eight independent coils (8 x 1) or in a parallel combination of four serially connected coils (4 x 4). The advantages of the array coils are improved sensitivity to intergranular attack (IGA), reduced interference from tube denting, and improved sensitivity to circumferential cracking. The current status of commercially available probes is such that several weaknesses in array coils have limited the widespread application of this technology. Task S 301-6 was conceived to address each of the weaknesses in order to produce a more reliable and cost-effective probe. The emphasis was on reliability, since the cost associated with downtime for a probe change far exceeds the price of a probe. The array coil probe optimization has progressed to the completion of the prototype stage. A total of seven probes were fabricated in the second stage to inspect two different ranges of tube inner diameters (0.58 to 0.70 in. and 0.66 to 0.78 in.). The probes have a range of 0.12-in. variation on the diameter, and each has eight spring-loaded pancake coils of approximately 1/8-in. diameter. Two size ranges were built in order to validate the design criterion of maximizing the common parts in probes over a wide range of sizes. Several design improvement, have been successfully demonstrated