Feeder Grayloc Hub Local Allowable Thickness - An Application of FFSG Appendix E Level 2 Evaluation

Abstract

Flow assisted corrosion (FAC) caused the highest rates of wall loss at outlet feeder pipes in the regions close to the Grayloc end fittings. In the past, almost all stress analyses on reduced thickness, above or below pressure based value, were conducted based on ASME III Code compliance. However, above assessments were feeders specific in nature and time consuming. For a large scale planned outage, more than several hundreds outlet feeder baseline and repeat inspections may be conducted. There is a reasonable risk that a number of feeders may be found with a wall thickness at or below the pressure based thickness. It is possible that unanticipated results from the inspection could extend the outage duration due to the need to perform unplanned stress analyses to demonstrate the fitness for service of the feeder pipes. To be effective to prevent outage delay it is necessary to conduct a generic feeder structural integrity assessment prior to the start of the outage. FFSG Appendix E (Reference 1) provides acceptance criteria and evaluation procedure for assessing local thinning near the Grayloc region for thickness below pressure based value. The structural evaluation procedure in Appendix E is not feeder specific in such a preventive assessment could provide predetermined acceptable thickness limits to be compared with inspection data. The acceptable thickness limits shall result in significant reduction of feeder disposition time.