The use of creep strength enhanced ferritic alloys, such as Grade 91, in fossil power plants has become popular for high temperature applications. Since Grade 91 has higher stress allowables than Grade 22, a designer can specify thinner component wall thicknesses, resulting in lower through-wall thermal stresses during transient events and lower material and pipe support costs. During the past two decades, Grade 91 has been used successfully in fossil power plants. However, this alloy has had some incidents of premature failures. Case histories discuss such factors as excessively hard material, extremely soft material, overheating failures, and improper mill processing. This compilation also discusses likely root causes and solutions to avoid these potential Grade 91 problems.

This paper presents an evaluation of the failure probability and cost of high energy piping (HEP) failures. Using a conventional definition of risk as the product of failure probability and failure consequence, we propose in this paper a dollar value of consequence in order to develop a quantitative approach to risk-based inspection (RBI) methodology. A 16-year historical database of probability and consequence was evaluated as an RBI methodology for devising a life management strategy for welds in main steam and hot reheat piping systems. This evaluation provides us the raw data necessary for producing a concrete example of this new Richter-scale-like approach. Uncertainty in consequence and probability estimates is also provided in plotting (a) a static consequence vs. likelihood diagram at a specific time for comparing the relative severity of a variety of potential failures, and (b) a dynamic risk vs. time diagram for a specific hardware under continuous monitoring where the effect of life management decisions over a period of time is quantitatively displayed. Significance of this new approach to risk-based inspection strategy for advancing the state-of-the-art of managing aging structures is discussed.

The use of creep strength enhanced ferritic alloys, such as Grade 91, in fossil power plants has become popular for high temperature applications. Since Grade 91 has higher stress allowables than Grade 22, a designer can specify thinner component wall thicknesses, resulting in lower throughwall thermal stresses during transient events and lower material and pipe support costs. During the past two decades, Grade 91 has been used successfully in fossil power plants. However, this alloy has had some incidents of premature failures. Case histories discuss such factors as excessively hard material, extremely soft material, overheating failures, and improper mill processing. This compilation also discusses likely root causes and solutions to avoid these potential Grade 91 problems.

The use of creep strength enhanced ferritic alloys such as Grade 91 in fossil power plants has become popular for high temperature piping applications. Since Grade 91 has higher stress allowables than Grade 22, a designer can specify thinner component wall thicknesses, resulting in lower through-wall thermal stresses during transient events and lower material and piping support costs. During the past two decades, Grade 91 has been used successfully in fossil power plants. However, this alloy has had some incidents of non-optimal weldment microstructure. In this case study, Brinell hardness tests of an ASME A182 Grade F91 (F91) wye block, including upstream and downstream F91 spools, revealed several readings of soft material, as low as 168HB. A study of creep rupture tests of degraded Grade 91 specimens revealed that the lower bound creep rupture curve of the degraded Grade 91 material is above the average creep rupture curve of Grade 22 material for the range of the specific piping operating stresses. Based on the empirical evidence that the average Grade 22 material creep rupture curve is conservative for the creep rupture of degraded Grade 91 material, a life consumption evaluation was performed for the degraded Grade 91 weldments using Grade 22 creep rupture properties. A life fraction analysis was performed considering the redistributed maximum principal stresses, based on simulation of piping displacements obtained from the hot and cold walkdowns. This study also considered the recent history of the specific piping system operating pressures and temperatures. This study also considered dissimilar metal welds, from ASME A182 Grade F91 (F91) to ASME A335 Grade P22 (P22) materials. It was determined that the Grades F91-to-F91 weldments had about 30% life consumption and the remaining lives were at least 7 years. The Grades F91-to-P22 weldments had less than 40% life consumption and the remaining lives were at least 15 years.

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.