Options for management of retaining ring inspection

Abstract

The high-stress, high-energy nature of generator retaining rings makes it essential that defects are detected early to avoid propagation and catastrophic failure. This paper explores how combinations of automated ultrasonic and eddy current methods can be tailored to give the most complete retaining ring inspections possible, reducing the time and inconvenience involved with the inspection. As part of a management programme to ensure the integrity of the rings, ultrasonic inspection is periodically carried out, often with the rings in situ on the rotor. Conventional pulse-echo inspection using low frequency shear waves and Time-of-Flight Diffraction (TOFD) inspection have been the primary search techniques for cracking. More recently, ID creeping waves, which have the ability to detect and resolve shallower defects than those found with conventional shear waves, have been introduced to the inspection. This paper compares and contrasts the use of shear waves and ID creeping waves for the detection of stress corrosion. Array eddy current can be used to complement the ultrasonic inspection, providing recordable data on surface breaking and sub-surface defects not achievable with conventional dye penetrant methods.