Wrought Tee Intersections Operating in the Creep Range, Part I: Service Experience and Root Causes

Abstract Over the last decade hundreds of events in wrought tee intersections designed to ASME B16.9 and stamped as a fitting to A- or SA-234 have been documented by industry. These components are frequently installed in main steam and hot reheat systems, operate in the creep range, are specified to be Grade 22, Grade 91 or Grade 92, and have experienced extensive damage, cracks or leaks in ∼40,000 to 150,000 hours of operation. Part I of this two-part paper will review several failure case studies to confirm the damage mechanism and identify the contributing root cause factors in the design, operation, fabrication, and metallurgy. Thorough and detailed metallographic observations will be provided for several case studies, and where relevant, the results of destructive mechanical testing. Several key vulnerabilities will be highlighted through the presented case studies to help inform a comprehensive integrated life management strategy that must screen, identify, and assess susceptible tees, and ultimately repair or plan for eventual replacement. The broader activities to address root causes, improve the serviceability of new or replacement tees operating in the time-dependent range to improve the specified requirements and/or use of the codes by tee purchasers and manufacturers of the relevant ASME codes are addressed in Part II.