Return-to-Service Evaluations for a Scotch Marine Boiler Furnace With Non-Compliant Corrugation Geometry

Abstract During a routine visual inspection of a set of six Scotch marine boiler furnace tubes, Hanford WTP discovered numerous shallow discontinuities on the inward peaks of the fire side of the corrugated furnace tubes. This discovery led to the initiation of three parallel efforts: (1) to determine the root cause of the cracking, (2) to quantify the depth of cracking, and (3) to perform an engineering evaluation under NBIC Part 2 in support of an action plan to return the boilers to service. Several panel sections were removed from one of the boiler furnace tubes to support the laboratory analysis. The root cause evaluation determined that the cracks most likely formed from poor control of thermo-mechanical processing parameters during corrugated tube formation. The material was found to be otherwise in compliance with the material condition expected for SA-516, Grade 70 plate material. Furthermore, the discontinuities were found to have blunt tips, and there was no evidence that they were growing under service conditions. Non-destructive testing of the boiler furnace tubes was performed using an eddy current array to perform general screening and an individual eddy current probe for precise depth sizing of the most severe flaws. Ultrasonic measurements were also performed to quantify wall thickness both in the flaw regions and in the regions between corrugation peaks. One of the furnace tubes examined was found to have several regions thinner than the minimum wall thickness calculated using the ASME BPVC Section I PFT-18 equation. Detailed geometric measurements of the panel sections revealed an additional challenge to return-to-service. The general corrugation profile was not compliant with any of the shapes defined in PFT-18. Engineering evaluations were therefore performed to demonstrate that the as-found shape has a safety factor exceeding Code rule requirements. These calculations further demonstrated that the as-found geometry and wall thickness pattern have sufficient margin to remove the deepest flaws without reducing the safety factor below that expected by BPVC Section I. This paper focuses on the engineering evaluations performed to support return-to-service of the subject boilers.