Risk-Based Inspection Applied to Main Steam and Hot Reheat Piping Systems

Abstract

Many utilities select critical welds in their main steam (MS) and hot reheat (HRH) piping systems by considering some combination of design-based stresses, terminal point locations, and fitting weldments. The conventional methodology results in frequent inspections of many low risk areas and the neglect of some high risk areas. This paper discusses the use of a risk-based inspection (RBI) strategy to select the most critical inspection locations, determine appropriate reexamination intervals, and recommend the most important corrective actions for the piping systems. The high energy piping life consumption (HEPLC) strategy applies cost effective RBI principles to enhance inspection programs for MS and HRH piping systems. Using a top-down methodology, this strategy is customized to each piping system, considering applicable effects, such as expected damage mechanisms, previous inspection history, operating history, measured weldment wall thicknesses, observed support anomalies, and actual piping thermal displacements. This information can be used to provide more realistic estimates of actual time-dependent multiaxial stresses. Finally, the life consumption estimates are based on realistic weldment performance factors. Risk is defined as the product of probability and consequence. The HEPLC strategy considers a more quantitative probability assessment methodology as compared to most RBI approaches. Piping stress and life consumption evaluations, considering existing field conditions and inspection results, are enhanced to reduce the uncertainty in the quantitative probability of failure value for each particular location and to determine a more accurate estimate for future inspection intervals. Based on the results of many HEPLC projects, the author has determined that most of the risk (regarding failure of the pressure boundary) in MS and HRH piping systems is associated with a few high priority areas that should be examined at appropriate intervals. The author has performed many studies using RBI principles for MS and HRH piping systems over the past 15 years. This life management strategy for MS and HRH critical welds is a rational approach to determine critical weldment locations for examinations and to determine appropriate reexamination intervals as a risk-based evaluation technique. Both consequence of failure (COF) and likelihood of failure (LOF) are considered in this methodology. This paper also provides a few examples of the application of this methodology to MS and HRH piping systems.

A Quantitative Approach to a Risk-Based Inspection Methodology of Main Steam and Hot Reheat Piping Systems

“…As illustrated in Cohn's 2007 paper [2], it was qualitatively indicated that 1) the girth weld failures are high likelihood with medium consequence, 2) the clamshell weld failures are medium likelihood with high consequence, and 3) the seam weld failures are very low to low likelihood with very high consequence.…”

“…Cohn wrote a paper in 2007 [2] illustrating the risk analysis evaluation of a HRH piping system. This piping system has a design pressure of 575 psig (3.96 MPa) and typically operates at 430 psig (2.96 MPa).…”

A Risk-Uncertainty Formula Accounting for Uncertainties of Failure Probability and Consequence in a Nuclear Powerplant

“…During the same period (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) [51][52][53][54][55][56][57][58][59][60][61], and risk-informed decisionmaking [62] were also published by industry and academia for the 3-prong (ASME-NRC-Utilities) effort of securing a "sound" foundation for a risk-based engineering practice. Ayyub, Prassinos, and Etherton [62] summarized this practice in a "risk matrix" as shown in Fig.…”