Limit Load Analysis (LLA) is a powerful tool for design at temperatures below the creep range, and there is desire to extend the method to the elevated temperature (creep) regime. However, there is no direct relationship between LLA and elevated temperature allowable stresses and failure modes, such that the basic LLA methods or results must be manipulated in some way to be generally meaningful for elevated temperature design. The most direct way to judge simplified methods is against a rigorous solution; this requires an inelastic material model consistent with allowable stresses. Such a creep model was described in Part 1 of this work (the Omega model), and subsequently applied in a rational way to the problem of primary load carrying capacity and design using a simple sample problem. In this paper, extension is made to the case of a typical steam header, and existing simplified (LLA) methods are compared against further detailed transient inelastic finite element analysis. Recommendations are then made for the application of LLA to evolving ASME high temperature design rules. Paper published with permission.

Section I of the ASME Boiler and Pressure Vessel Code currently has a Code Case that provides rules for the construction of unstayed flat heads with integral flanges (butt welded hubs) from forged material. This Code Case is currently being considered for adoption into the Section I rules; therefore, the existing requirements of the Code Case are under review. In particular, feedback on the Code Case indicated that the required minimum hub length presents manufacturing challenges in certain situations, so the necessity of this hub is being reviewed. As this type of flat head is commonly used in high temperature operation, evaluation of the expected creep behavior is critical, especially in the proposed weld area at the head junction to the attached shell (typically a header). Example geometries were developed using the current Code Case requirements and their suitability for high temperature operation was initially evaluated using elastic analysis. Detailed inelastic analysis was then performed to further quantify the expected time-dependent high temperature behavior. Additionally, a series of failures of end plates designed to the TRD code was reported to have occurred in Europe in the 1990s. In order to assess the potential relevance of these failures to heads designed per the proposed Section I rules, additional examples were developed using the design rules of the TRD code rules. The predicted creep behavior and damage for these examples were then compared to the Code Case designed examples.

Chaboche unified viscoplasticity model and uncoupled plasticity and creep models (nonunified) are evaluated for their capability in simulating low-cycle fatigue, creep and creep-fatigue responses of Grade 91 steel. The primary objective of this study is to develop a constitutive model incorporating various advanced modeling features for design-by-analysis of elevated temperature power plant components. For validation of the model a broad set of experimental responses of Grade 91 in the temperature range 20–600°C are collected from literature. Performance of the models is demonstrated against simulating these experimental responses. It is demonstrated that the unified Chaboche model simulation capability can be improved through implementing strain range dependence, cyclic hardening through kinematic hardening rule and static recovery modeling features.

Creep is progressive deformation of material over an extended period when exposed to elevated temperature and stresses below the yield strength. Poor Creep ductility and cracking can be a problem above 900 °F (482°C) in the HAZ of low alloy (Cr-Mo) steel. High stress areas, including supports, hangers and fittings are more vulnerable to cracking. Creep cracking has occurred in longitudinal pipe welds with excessive peaking or welds with poor quality. Numerous incidents of cracking in low alloy (Cr-Mo) steel have been reported in the power industry and in refineries with major concern in longitudinal seam welds as well as highly stressed welds in reactors-heaters interconnecting piping. This paper presents the results of an assessment performed on reactors-heaters interconnecting piping in a catalytic reformer unit with a maximum operating temperature of about 950 °F (510 °C) at 250 psig (1.7 MPa) (> 40 years in-service). Comprehensive inspection including visual, dye penetrant testing, thickness measurements and peaking measurements have been performed. Phased Array Ultrasonic Testing (PAUT) was utilized to detect crack-like defects and flaws. Detailed pipe stress analysis and finite element analyses (FEA) were also performed.

This article discusses how ASME Boiler and Pressure Vessel Code (BPVC) has continued to evolve in the past 100 years to meet the needs of the engineers using the most advanced technology. The first edition of what has now become the ASME BPVC was published in 1914. The Code has continued to expand and adapt over the years to meet the needs of new technologies, many unimagined 100 years ago. The Code continues to meet new challenges and to extend its influence in the cause of safety around the globe. The volunteers who meet four times a year to maintain and extend the Code are completely dedicated to translating sometimes painfully gained experience into rules that strive to protect people. It's why competitors come together and share critical knowledge with one another and the public, and why volunteers dedicate time that almost universally extends well beyond the traditional 40-hour work week.