Condition monitoring assessment of risk informed thinning rate of Darlington feeders

The reduced thickness near the outlet feeder Grayloc is caused by the combination of the grinding for weld-prep during the construction and Flow Accelerated Corrosion (FAC) through the feeder service life. The commonly used thinning rate, based on the difference of one inspection data and initial nominal thickness, has been proven to be overly conservative at the area adjacent to the Grayloc weld. In addition, there is the lack of repeat measurements to establish a reliable repeat thinning rate. A risk informed thinning rate was developed at OPG (Ontario Power Generation) based on the average value of five different thinning rates. Using this method, the predicted thickness has shown excellent agreement with the measured data from last 5 planned outages. The reliable prediction of feeder local thinning at Darlington prevents premature feeder replacement while ensuring the feeder structure integrity. A description of the risk informed rate and condition monitoring assessment results are presented in this paper. (author)