Addressing the Accuracy Concerns of Measuring MOV Stem Nut Wear From Diagnostic Testing

Excessive stem nut thread wear represents a potential common-cause failure mode that could impact all rising-stem valves. The consequence of unexpected failure of stem nuts emphasizes the importance of improving condition monitoring and maintenance practice activities by identifying, quantifying and minimizing stem nut thread wear. In the nuclear industry, motor-operated valve (MOV) diagnostics estimate stem nut thread wear on safety-related valves using the stem-to-stem nut transition time (zero plateau). But the stem-to-stem nut transition time could also be affected by other variables that would lead to an inaccurate calculation of wear. Using stem-to-stem nut transition time to estimate wear, coupled with generally erring on the conservative side, usually indicates wear that is more severe than actual. This method, combined with all of the unknown variables, results in nuclear plants using valuable outage resources and dose to pull good stem nuts unnecessarily. This white paper identifies some of the variables that can be mistakenly construed as wear and offers a method that more accurately and efficiently validates this wear. Paper published with permission.