A Case Study on Generation of Acceptance Criteria in Feedwater Supply Tube Bends

Inspection of components utilizing advanced examination techniques is becoming standard practice in the power industry. One of the critical aspects during this type of inspection is to have on hand criteria for acceptance or rejection of flaws found during these inspections. The examinations typically include the use of advanced ultrasonic techniques such as Linear Phased Array (LPA), Focused Annular Phased Array (APA), and Time of Flight Diffraction. These techniques can detect and size flaws that are often “Fit for Service” for at least one additional outage cycle. However, schedule limitations during an outage often do not permit for extensive analysis time. Repair decisions need to be made immediately and having criteria available for these decisions are critical. The following is a case study for the generation of acceptance / rejection criteria for power boiler feedwater supply tube bends. The determination of “allowable flaw sizes” is needed to determine which of these supply tube bends need to be removed during the outage. The acceptance criteria in this work is based on the criteria of API 579-1 / ASME FFS-1 “Fitness for Service Standard” [1] to evaluate the rate of growth of the flaws found. Discussion is also made as to the sensitivity of the examination methods used to detect the flaws expected and how this relates to the acceptance criteria generated. NOMENCLATURE APA – Annular Phased Array BR – Bend Radius of Tube C, n – Crack Growth Rate constants da – Change in flaw depth dN – Change in Number of cycles K – Change in Stress Intensity Factor FAD – Failure Assessment Diagram FFS – Fitness for Service KIc – Critical Stress Intensity Factor KI – Stress Intensity Factor Kr– Stress Intensity Factor Ratio (KI / KIc) LPA – Linear Phased Array Ultrasonics Lr – Load Ratio based on Primary Stress NDE – Non-Destructive Evaluation P Pressure R – Radius of Tube 1max – Maximum Principal Stress in Bend t – Thickness of Tube INTRODUCTION Disposition of components identified to contain defects during a maintenance outage is often of critical importance in getting units back on-line. Replacement components are typically not readily available and could involve significant delays in outage schedules. The following is a case study on the use of API 579-1 / ASME FFS-1 Fitness for Service (FFS) [1] for the generation of acceptance criteria for feedwater supply tube bends. A typical configuration of this type of bend is used here for the case study. The bends considered are: Tubes – 4 1⁄2 inch diameter x 0.375 inch min wall thickness Bend Radius – approximately 4 x tube diameter (18 inches) Operating Pressure – 2080 psi at sat. temperature of 641°F Material – SA-210 Grade C Carbon Steel [3],[4] Yield Strength is 40 ksi at room temperature Yield Strength is 29.6 ksi at 641°F Tensile Strength is 70 ksi at room temperature Cracking in these bends is often a result of either fatigue or corrosion fatigue. The corrosion is often caused by the water