An Update on Improved Flange Design Methods

This paper details further progress made in the PVRC project “Development of Improved Flange Design Method for the ASME VIII, Div.2 Rewrite Project” presented during the panel session on flange design at the 2006 PVP conference in Vancouver. The major areas of flange design improvement indicated by that project are examined and the suggested solutions for implementing the improved methods into the Code are discussed. Further analysis on aspects such as gasket creep and the use of leakage-based design has been conducted. Shortcomings in the proposed ASME flange design method (ASME BFJ) and current CEN flange design methods (EN-1591) are highlighted and methods for resolution of these issues are suggested.

Effects of Weld Strength Mismatch on Estimation Procedures for J and CTOD Fracture Parameters Using SE(B) Specimens

This work presents an exploratory development of J and CTOD estimation procedures for welded fracture specimens under bending based upon plastic eta factors and plastic rotation factors. The techniques considered include: i) estimating J and CTOD from plastic work and ii) estimating CTOD from the plastic rotational factor. The primary objective is to gain additional understanding on the effect of weld strength mismatch on estimation techniques to determine J and CTOD fracture parameters for a wide range of a/W-ratios and mismatch levels. Very detailed non-linear finite element analyses for plane-strain models of SE(B) fracture specimens with center cracked, square groove welds provide the evolution of load with increased load-line displacement and crack mouth opening displacement which are required for the estimation procedure. The results show that levels of weld strength mismatch within the range ±20% mismatch do not affect significantly J and CTOD estimation expressions applicable to homogeneous materials, particularly for deeply cracked fracture specimens. The present analyses, when taken together with previous studies, provide a fairly extensive body of results which serve to determine parameters J and CTOD for different materials using bend specimens with varying geometries and mismatch levels.

Shakedown Limit Load Determination for a Kinematically Hardening 90-Degree Pipe Bend Subjected to Constant Internal Pressure and Cyclic Bending

A simplified technique for determining the shakedown limit load of a structure employing an elastic-perfectly-plastic material behavior was previously developed and successfully applied to a long radius 90-degree pipe bend. The pipe bend is subjected to constant internal pressure and cyclic bending. The cyclic bending includes three different loading patterns namely; in-plane closing, in-plane opening, and out-of-plane bending moment loadings. The simplified technique utilizes the finite element method and employs small displacement formulation to determine the shakedown limit load without performing lengthy time consuming full cyclic loading finite element simulations or conventional iterative elastic techniques. In the present paper, the simplified technique is further modified to handle structures employing elastic-plastic material behavior following the kinematic hardening rule. The shakedown limit load is determined through the calculation of residual stresses developed within the pipe bend structure accounting for the back stresses, determined from the kinematic hardening shift tensor, responsible for the translation of the yield surface. The outcomes of the simplified technique showed very good correlation with the results of full elastic-plastic cyclic loading finite element simulations. The shakedown limit moments output by the simplified technique are used to generate shakedown diagrams of the pipe bend for a spectrum of constant internal pressure magnitudes. The generated shakedown diagrams are compared with the ones previously generated employing an elastic-perfectly-plastic material behavior. These indicated conservative shakedown limit moments compared to the ones employing the kinematic hardening rule.

Scoping Analyses of Parameterized Cool-Down Transients Associated With Reactor Shutdown

The current regulations, as set forth by the United States Nuclear Regulatory Commission (NRC), to insure that light-water nuclear reactor pressure vessels (RPVs) maintain their structural integrity when subjected to planned startup (heat-up) and shutdown (cool-down) transients are specified in Appendix G to 10 CFR Part 50, which incorporates by reference Appendix G to Section XI of the ASME Code. The technical basis for these regulations contains many aspects that are now broadly recognized by the technical community as being unnecessarily conservative. During the past decade, the NRC conducted the interdisciplinary Pressurized Thermal Shock (PTS) Re-evaluation Project that established a technical basis to support a risk-informed revision to current PTS regulations (10CFR Part 50.61). Once the results of the PTS reevaluation are incorporated into a revision of the 10 CFR 50 guidance on PTS, it is anticipated that the regulatory requirements for the fracture toughness of the RPV required to withstand a PTS event (accidental loading) will in some cases be less restrictive than the current requirements of Appendix G to 10 CFR Part 50, which apply to normal operating conditions. This logical inconsistency occurs because the new PTS guidelines will be based on realistic models and inputs whereas existing Appendix G requirements contain known and substantial conservatisms. Consequently, a goal of current NRC research is to derive a technical basis for a risk-informed revision to the current requirements of Appendix G to 10 CFR Part 50 in a manner that is consistent with that used to develop the risk-informed revision to the PTS regulations. Scoping probabilistic fracture mechanics (PFM) analyses have been performed for several hundred parameterized cool-down transients to (1) obtain insights regarding the interaction of operating temperature and pressure parameters on the conditional probability of crack initiation and vessel failure and (2) determine the limits on the permissible combinations of operating temperature and pressure within which the reactor may be brought into or out of an operational condition that remains below the acceptance criteria adopted for PTS of 1 × 10−6 failed RPVs per reactor operating year. This paper discusses the modeling assumptions, results, and implications of these scoping analyses.

Finite-Element Simulations and Probabilistic Fracture Assessments of the Response of Alternate Rifling Geometries

A 3-D finite-element model was used to simulate the severe and localized thermal/pressure transients and the resulting stresses experienced by a rifled ceramic-barrel with a steel outer-liner; the focus of the simulations was on the influence of non-traditional rifling geometries on the thermoelastic- and pressure-stresses generated during a single firing event. In order to minimize computational requirements, a twisted segment of the barrel length based on rotational symmetry was used. Using this simplification, the model utilized uniform heating and pressure across the ID surface via a time-dependent convective coefficient and pressure generated by the propellant gasses. Results indicated that the unique rifling geometries had only a limited influence on the maximum circumferential (hoop) stresses and temperatures when compared with more traditional rifling configurations because of the compressive thermal stresses developed at the heated (and rifled) surface.

Serviceability Characterization of Composite Storage Tanks

This paper outlines an analytical technique enabling serviceability characterization of a storage tank made of a Polymer Matrix Composite (PMC) with regards to a specified profile of long-term operation of the tank. The technique combines force-temperature exposure (conceivably changing over a tank’s service life) and fatigue properties of a composite utilized within the tank structure. Along with a serviceability assessment, the technique is capable of providing a well-grounded specification of design knock-downs and safety factors relevant to the conventional structural design procedure.

Utilization of Modern Test Methods and Analyses for Pressure Vessel and Piping Gasket Design

Modern, state-of-the-art equipment was recently introduced to the marketplace which allows gasket material manufacturers, fabricators, and end users to test gasket materials and analyze gasket behavior more accurately and precisely than ever before. Previous methods lacked the sophistication of this equipment, or conversely were so complex that the methods were not user friendly. Therefore, this research focuses on providing data from new equipment using newly adopted standards and applying this information to an operational bolted gasketed joint. In addition, the paper describes the test methods, summarizes and analyzes the test results, and discusses incorporating the data into Finite Element Analysis (FEA) modeling of a gasketed joint. The intent of the work is to provide practical information about the behavior of compressed non-asbestos type gaskets under varying conditions, examining this ubiquitous material and how it provides a seal, while elucidating the problems and pitfalls of pressure vessel sealing.

Gross Plastic Deformation of a Hemispherical Head With Cylindrical Nozzle: A Comparative Study

The plastic load of a hemispherical head with a cylindrical nozzle subject to an internal pressure and/or moment is established using two new plastic criteria; the plastic work curvature (PWC) criterion and the ratio of plastic to total work curvature (RPWC) criterion. The calculated plastic loads are compared to ASME Design by Analysis stress categorization and limit analysis allowable loads. The stress categorization approach is seen to be dependant on the appropriate choice of stress classification lines and the classification of primary, secondary and peak stresses. The limit load approach is simpler to apply but does not consider material strain hardening and/or large deformation behaviour and may often lead to conservative design loads. In the plastic analyses, strain hardening material models and large deformation analyses are considered. The calculated allowable pressure-moment interaction surfaces are assessed and compared for the different methods.

Finite Element Analysis of a Compressor Housing Used in High Pressure Refrigeration System

In the design of compressors running with high pressure refrigerants, safety aspects must be a mandatory concern. Moreover, when dealing with high pressure levels, compressor components have their original design adapted to withstand such a high pressures, particularly acoustical mufflers, external housing, and compression mechanism. Regarding the external housing, the design approach goes beyond acoustical and aesthetics features as mostly observed in current refrigerating compressors. In order to safety enclose the compression mechanism the application of a proper design methodology is mandatory to safeguard the structural integrity of both the compressor external housing and the whole refrigerating system. Looking for acceptable, cost effective safety factors, a simultaneous design approach including advanced structural mechanics techniques, experimentation, safety Codes revision, and Computer Aided Engineering (CAE) tools application is mandatory. The aim of this work is to present a new development approach, concerning structural design of a compressor housing used in high pressure refrigeration system. Numerical and experimental results will be compared among each other aiming to evaluate some ASME Codes criteria and design procedures.

Flange Joint Bolt Spacing Requirements

The bolted flange joint assembly is a complex system. System stresses are dependent on elastic and nonlinear interaction between the bolting, flange and gasket. The ASME Code design rules provides a method for sizing the flange and bolts to be structurally adequate for the specified pressure design conditions and are based on an axi-symmetric analysis of a flange. The ASME rules do not address the circumferential variation in gasket and flange stress due to the discrete bolt loads. Proper bolt spacing is important to maintain leak tightness between bolts and to avoid distorting the flange. This paper provides an analytical solution for the gasket and flange stress variation between bolts. The analytical solution is validated with 3-D Finite Element solutions of standard flange designs.