Understanding the Necessity for Multimodal Inspections of 18 mn-5cr Generator Rotor Retaining Rings

Generator rotor retaining rings were initially manufactured from magnetic steel alloys. Nonmagnetic rings were later introduced to replace lesser efficient magnetic rings. Commonly referred to as 18/5 rings, an 18% manganese, 5% chromium alloy quickly became the material of choice. Although more efficient than its predecessors, later discoveries showed this alloy to be susceptible to stress corrosion cracking (SCC) when exposed to moisture. Generator rotor retaining rings, by virtue of their task, are generally considered to be the most highly stressed component in a turbine generator, and although rare, SCC failures caused by moisture have been documented. Such ring failures can be structurally and financially catastrophic, not to mention the potential for loss of life. As of a 1994 worldwide study, a total of 39 retaining ring failures were known to have occurred; of these, nine involved 18/5 rings. And of the nine, two were from domestic designs and failed due to inter-granular stress corrosion cracking (IGSCC). These are the two main failures at the heart of 18/5 ring assessments. Nondestructive examinations (NDE) are routinely performed to help mitigate such risks, but the inspections offer little value if ring stresses, inspection capabilities, and failure mechanisms are not properly understood and considered. This paper will discuss why multiple volumetric techniques should be used, how to select the best surface technique and the advantages of conducting a probabilistic condition assessment as part of an effective ring maintenance strategy.

References
1. Nottingham, Ammirato, MacDonald, Zayicek and Elmo, TR 104209 - Evaluation of Nonmagnetic Generator Retaining Rings, EPRI, 1994. 2. Nottingham, Comprehensive Turbine and Generator Assessment, Structural Integrity Associates, Inc., 2010
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