Recently, ultrasonic inspections at a domestic PWR identified localized wall thinning due to Flow Accelerated Corrosion (FAC) in their feedwater piping system. Specifically, in a carbon steel 18 x 12-inch expander located in the auxiliary boiler building that is non-safety related. Based on a fitness-for-service analysis, the projected wall thinning will be below the design minimum wall thickness prior to the next refueling outage. Structural Integrity was contacted in order to help the station reduce identified conservatisms through a more sophisticated analysis.
Using detailed inspection data from the site, we developed a finite element model of the degraded expander for evaluation. The model reflected the actual profile of the remaining expander wall thickness and allowed for structural credit to be taken for all remaining material as opposed to assuming uniform thinning. The finite element model was then further thinned, using corrosion rate data from the plant, such that the future condition of the expander at the next refueling outage could be analyzed. Design loading could then be applied to the model and directional stresses extracted for comparison to the design allowable stress to determine acceptance. If the acceptance criteria were met, future operating cycles of thinning could be applied to predict the maximum life of the expander.
Taking advantage of a higher design allowable stress in more recent ASME Codes (between 1999 and 2000 the ASME changed the safety factor associated with material allowable stress from 4 to 3.5), we prepared a code reconciliation to justify the use of the 14% higher allowable stress. The Code of Construction for the feedwater piping at this PWR is B31.1. Since B31.1 does not provide reconciliation guidance to newer codes, guidance was taken from ASME Section XI, IWA-4200.
By performing a stress evaluation using the developed finite element model and comparing the stress results to the higher design allowable stress, it was determined the expander wall thickness would not violate the design minimum wall thickness by the next refueling outage. In fact, additional operating cycles of thinning were applied to the model and the expander was found to be acceptable for at least 18 more years of future operation if the corrosion rates remain constant.