Generator Retaining Rings: In Situ Inspection and Life Assessment

Stress corrosion cracking (SCC) in retaining rings of generators has been a major issue since large generator manufactures started to advise generator owners to replace and inspect retaining rings made from certain materials in the mid 1990’s. KEMA has been inspecting generator retaining rings with automated systems from the seventies on. Since 1995 new developments led to the possibility of also testing rings in situ, eliminating the need of removing the rotor from the stator and thus saving considerable down-time. Over the years, step by step improvements to the system and its application to generator retaining ring inspections have been implemented, one of which is the use of smaller probes and manipulators. Another improvement is the implementation of eddy current testing (ECT) to increase the detection capability for cracking at the outside surface in parallel to the automated ultrasonic testing (AUT). Experience obtained with this method contradicts some of the common ideas about reliability of other Non-destructive testing (NDT) methods, in particular the application of Dye Penetrant inspection (DPI) which appears to be less reliable. By performing fracture mechanics calculations to evaluate detected and sized flaw indications, predictions of remaining life of the rings can be made (Fitness-for-Purpose analysis). This paper presents results of applying the in situ retaining ring inspection system in the last 14 years and gives an overview of cases — some of those from recent inspections — where defect indications were detected, sized and evaluated. In some cases this resulted in recommendations to replace the retaining rings.