Evaluation, repair restoration of generator magnetic retaining rings

摘要

There has been much attention in the Power Generation Industry over the last ten to 15 years on corrosion mechanisms and failure prevention of non-magnetic retaining rings. Even though there have been just a few failures, the catastrophic damage, which has occurred, justifies this attention and concern. The 18Mn5Cr retaining ring composition has had extensive laboratory testing and been shown to be highly susceptible to stress corrosion cracking in the presence of moisture or other corrodants. Original Equipment Manufacturers (OEMs) have recommended replacement of damaged 18Mn5Cr retaining rings with 18Mn18Cr rings. Generator magnetic retaining rings have received much less attention, even though rotors with these types of rings exist in significant numbers, and some have failed. Magnetic rings operating in a hydrogen environment have shown a susceptibility to hydrogen embrittlement and subsequent cracking. Older rings, manufactured before certain steel making advances, are at greater risk of failure. The same is trure for rings with higher tested hardness levels, with correspondingly higher tensile strength and lower fatigue strength. Industry concern for cracking of magnetic retaining rings should be as high as concerns with non-magnetic rings. Not all damaged retaining rings need to be replaced. Often, with a proper engineering evaluation, they can be repaired and restored to full service. This paper outlines the problem areas commonly found on generator magnetic retaining rings. Case histories of "problem rings," along with successful engineering evaluations and repairs, are presented. This paper should be useful to those power generation equipment owners and users who are faced with "11'roblem" generator retaining rings.