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IN THIS VOLUME


Design and Analysis

2007;():3-9. doi:10.1115/PVP2007-26089.

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.

Topics: Flanges
Commentary by Dr. Valentin Fuster
2007;():11-15. doi:10.1115/PVP2007-26146.

Flanges in pressure vessels are, in most cases, submitted to non-concentric loading conditions producing bending stresses in the bolts that have to be taken into account for design purposes. The VDI 2230 Guideline [5] provides an excellent tool for the design of bolted joints, especially those in which the bolts are eccentrically loaded, as is commonly the case in pressure vessels. When cyclic loading conditions that can lead to fatigue failure are prevalent, special attention should be paid to the fatigue criteria used in the design. This paper will analyze the general principles of the design of bolted joints, giving particular attention to the use of the VDI 2230 Guideline. The calculation of the stiffness of the joints using this guideline will be introduced and a comparison with the more commonly used approaches will be made. Finally an example of the calculations involved in the design of a bolted flange in a pressure vessel will be shown and a comparison of the different design and fatigue criteria made.

Commentary by Dr. Valentin Fuster
2007;():17-24. doi:10.1115/PVP2007-26197.

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.

Commentary by Dr. Valentin Fuster
2007;():25-32. doi:10.1115/PVP2007-26621.

Bolted joints with gaskets such as non-circular flange connections have been widely used in mechanical structures, nuclear and chemical industry, and so on. They are usually used under internal pressure as well as other loadings such as thermal, impact loadings and so on. In designing the non-circular flange connections with gaskets, it is important to evaluate the sealing performance of the non-circular flange connections with gaskets subjected to internal pressure. An important issue is how to evaluate the sealing performance in the box-shape bolted flange connections by using the contact gasket stress distributions at the interfaces, how to reduce a number of bolt and nuts, that is, how to enlarge the bolt pitch, and how to determine the initial clamping bolt force (preload) by using the new gasket constants. In this paper, the stresses of box-shape flange connection with gaskets subjected to an internal pressure are analyzed using the finite element method (FEM), taking account a hysteresis in the stress-strain curves of the gasket. The contact gasket stress distributions when the internal pressure is applied to the connection are analyzed. The leakage tests were conducted using an actual box-shape flange connection with a gasket Using the contact gasket stress distributions at the interfaces under an internal pressure (Helium gas was used) and the amount of the leakage measured in the experiment, the sealing performances are evaluated experimentally and theoretically by changing the bolt pitch in the connections. Discussion is made on the effect of the bolt pitch on the sealing performance in the above connections.

Commentary by Dr. Valentin Fuster
2007;():33-43. doi:10.1115/PVP2007-26643.

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.

Commentary by Dr. Valentin Fuster
2007;():45-50. doi:10.1115/PVP2007-26644.

This paper details recent testing that was performed as an extension of earlier work on nut factor and high temperature breakout performance of selected anti-seize products. Comparison is made between results obtained using bolt diameters from 3/4 inch to 2 inch, two different anti-seize products (Molybdenum and Nickel) and two different bolt materials (ASTM A193-B7 & ASTM A193-B8M). In addition, common equations used for the determination of achieved bolt load from a given torque are examined and compared from a practical perspective in light of the nut factor test results. The test methods that were used are designed to closely mimic actual bolt assembly in a process plant environment. The paper, therefore, presents useful information that will enable more accurate assembly of bolted flanged joints on pressure vessels and piping in any process plant environment.

Topics: Manufacturing
Commentary by Dr. Valentin Fuster
2007;():51-57. doi:10.1115/PVP2007-26649.

In order to minimize the likelihood of leakage from flanged piping joints, it is a good practice to maximize the initial bolt assembly stress. Present bolting guidelines (ASME PCC-1 [1]) outline the use of a percent of bolt yield across all flange sizes and classes to set the assembly stress level. These guidelines do indicate that aspects such as component strength and gasket stress should be considered, however the most common application of the approach is to use a standard percentage of bolt yield across all flange sizes and classes. This approach does allow for adjustment for differences in material yield strengths (carbon steel versus stainless steel) and raised face (RF) versus ring type joint (RTJ) flange configurations. It does not, however, adjust for the difference in strength between standard pipe flange sizes nor the actual gasket stress achieved across all flange sizes and classes. Since there is no assessment of flange strength, such an approach may cause failure of joint components. In addition, because the standard percentage of bolt yield technique does not look at gasket stress, it is prone to gasket leakage due to low stress or gasket destruction due to over-compression for some joints. In addition, some joints may require bolt loads well in excess of the standard value to develop an acceptable gasket stress level in order to prevent leakage. This paper is a continuation of the paper presented during PVP 2006 in Vancouver (Brown [2]), which examined the variables that must be considered and drew some preliminary conclusions regarding the use of flange stress limits in determining the maximum allowable bolt load for a given flange size. Subsequent to writing that paper, further investigation found that the code calculated flange stresses are a poor indicator of the maximum acceptable bolt load. The most practical measure of this load is obtained by using elastic-plastic finite element analysis (FEA) to determine the point of gross plastic deformation of the flange. This paper details the maximum bolt load limit results of elastic-plastic FEA on most sizes of standard ASME weld neck flange sizes. The practical application of this method is in the development of standard bolt assembly stress (or torque) tables for standard pipe flanges using a given gasket type. In addition, a new code equation and additional limits are developed, by comparison to the elastic-plastic FEA results, which allow the determination of the maximum assembly bolt load for non-standard weld-neck flanges and standard weld-neck flanges with different bores, materials or gaskets than used in the elastic-plastic FEA presented in this paper.

Commentary by Dr. Valentin Fuster
2007;():59-65. doi:10.1115/PVP2007-26841.

It is common to rate a piping system to its weakest component to maximize flexibility for future operations. In many situations, the bolted flange joint is the lowest rated component. Rating a system for its full flange rating reduces the flange’s capacity to carry external bending moments. In the past, moments on flanged joints have been evaluated by using the concept of equivalent pressure, first presented in the Kellogg Design of Piping Systems. Operating moments are converted to an equivalent pressure. This equivalent pressure is added to the design pressure and compared against a limit. According to conventional practices, the design pressure plus the equivalent pressure must not exceed the rating pressure. Consequently, designing up to the flange rating pressure presents an issue, since no margin is left for the effects of external moments on flange joints. Depending on the circumstances, many designers have compensated by permitting the combined design pressure and equivalent pressure to be as high as twice the flange rating. In this paper, the authors demonstrate a robust methodology to define an appropriate limit for operating moments on bolted flange joints. Using the calculation methodologies of EN-1591-1, the authors calculate the maximum external moment that various classes of standard ASME B16.5 flanges (for Group 1.1 materials) can tolerate over a range of temperatures and present a representative sample. Conclusions are drawn about appropriate limits for moments on flanges and are compared to results using the equivalent pressure method.

Commentary by Dr. Valentin Fuster
2007;():69-78. doi:10.1115/PVP2007-26010.

Rectangular metal expansion joints are commonly used in flue gas ductwork, turbine exhaust systems, and heat exchanger applications. The Standards of the Expansion Joint Manufacturers Association (EJMA) are widely referenced for the design of rectangular metal expansion joints. The 8th edition, 2005 addenda, contains equations for rectangular metal bellows pressure stresses, movement stresses, beam lateral deflection, fatigue life, and spring rate. This paper evaluates the accuracy of the EJMA equations based on failure reports and FEA studies. Both linear elastic and non-linear limit analyses are used for the evaluations. Improved methods are proposed for the determination of pressure capacity, beam deflection, moment of inertia, and corner rigidity. New lateral deflection limits and acceptance criteria are provided herein. This paper also includes design considerations for the pressure stops used to support bellows rails.

Commentary by Dr. Valentin Fuster
2007;():79-87. doi:10.1115/PVP2007-26013.

This paper describes the analytical and empirical analyses conducted in the catastrophic failure of a flexible hose utilized in a petro-chemical environment. Specifically, the issues associated with the instability of the metal u-shaped bellows, from which the hose derives its overall flexibility and name, are reviewed and discussed in detail. In an effort to provide a comprehensive examination of the flexible hose’s use in the petro-chemical industry, a discussion of the applied mechanics associated with column buckling of the bellows (also known as “squirm”) is presented. In addition, the fabrication details that also proved detrimental to the structural adequacy of the subject flexible hose are highlighted. Results from an elastic-plastic finite element analysis of the u-shaped bellows are described and compared against previously published theoretical works on the instability of shells of revolution and most specifically, toroids. The applied loads in the finite element analyses include both internal pressure and transverse displacements (i.e., lateral offset). Furthermore, the guidance provided by the rules of the Expansion Joint Manufacturers Association Standards (EJMA) with regard to squirm are also reviewed and discussed. Finally, the results of the theoretical, empirical, and analytical investigations into the squirm phenomenon are utilized to identify some very practical solutions and recommendations to avoid the possibility of catastrophic failure of u-shaped bellows from column type instability.

Commentary by Dr. Valentin Fuster
2007;():89-94. doi:10.1115/PVP2007-26361.

The aim of this paper is to examine the utility of nonlinear analysis for structural response to thermal and mechanical loads, for a structure containing alternating regions of high and low stiffness as a result of inclusion of bellow sections. Utility is measured by comparison of results of linear and nonlinear analyses. The specific example used is that of an exhaust manifold for a large diesel engine. The paper discusses modeling of geometric and material nonlinearity, and makes recommendations in regard to which nonlinear effects are thought to be significant, based on the linear/nonlinear comparisons. The paper also contains general comments on the finite element modeling of structures containing bellows.

Commentary by Dr. Valentin Fuster
2007;():95-103. doi:10.1115/PVP2007-26769.

This paper is the third in a series that describes the materials, fabrication, installation, and applied mechanics considerations surrounding the catastrophic failure of a bellows component within a metallic flexible hose. The subject flexible hose was utilized in a compressor piping system attachment juncture to a petro-chemical piping system designed in accordance with the ASME B31.3 Process Piping Code. Specifically, the ultimate failure mode issues related to the instability of the metal u-shaped bellows, from which the hose derives its overall flexibility and name, are reviewed and discussed in detail. In an effort to provide a comprehensive examination of the use of the flexible hose in the petrochemical industry, a discussion of the materials, fabrication methods, installation, and, applied mechanics associated with column buckling of the bellows (also known as “squirm”) are presented. A metallurgical failure analysis is presented to identify and document the mode of failure and metallurgical condition of the wire braid and bellows components of the hose. In addition, material examination results, including the discovery of inherent flaws from the fabrication process, are presented and the significance of the findings is presented. The selection process for this particular type of flexible hose (and bellows component) for eventual installation in a vibratory service environment is reviewed in light of the published recommendations provided by the rules of the Expansion Joint Manufacturers Association Standards (EJMA) with regard to squirm are also reviewed and discussed. Finally, a summary of the elastic-plastic finite element analysis of the u-shaped bellows is briefly described and compared against previously published theoretical works on the instability of shells of revolution and most specifically, toroids. The results of the theoretical, empirical, and analytical forensic investigations into the squirm phenomenon are utilized to identify some very practical recommendations in an effort to minimize the probability of catastrophic failures of u-shaped bellows from column type instability.

Commentary by Dr. Valentin Fuster
2007;():107-113. doi:10.1115/PVP2007-26102.

Piping thermal expansion stress limits for ASME Section III Class 2/3 piping are directly based on the original thermal expansion stress rules given in B31.1-1955. Cyclic thermal expansion stresses are limited by Equation 10 to SA where SA is a function of the expected number of full temperature cycles. The Eq. 10 limit may be exceeded, if the pressure-plus-weight stresses are below their limit. This requirement is expressed as Eq. 11. The basis for Eqs. 10 and 11 is not well understood in the industry, and has caused confusion. One typical comment is — Why have an Eq. 10 limit if it can be exceeded? The history and development of the thermal expansion stress limits are presented. The thermal expansion stress limits from B31.1 are based on relaxation considerations and prevention of yield or creep strain even though the failure mode of concern is fatigue cycling. Hence, the thermal expansion stress limit is an implied and approximate limit on fatigue. Recommendations for changes to the Section III Class 2/3 rules are provided. A direct fatigue based stress limit is proposed.

Commentary by Dr. Valentin Fuster
2007;():115-123. doi:10.1115/PVP2007-26228.

This paper quantifies effects of the bend angle and the length of the attached straight pipe on plastic limit loads of the 90° pipe bend, based on small strain FE limit analyses using elastic-perfectly plastic materials with the small geometry change option. It is found that the effect of the length of the attached straight pipe on plastic limit loads can be significant, and the limit loads tend to decrease with decrease of the length of the attached straight pipe. Regarding the effect of the bend angle, it is found the plastic load smoothly changes from the limit load of the straight pipe when the bend angle approaches zero to the plastic load of the 90° pipe bend when the bend angle approaches 90 degree.

Topics: Stress , Pipes , Pipe bends
Commentary by Dr. Valentin Fuster
2007;():125-131. doi:10.1115/PVP2007-26425.

Leak-before-break (LBB) evaluations involve the use of deterministic fracture mechanics to establish the margin between critical and leakage flaw sizes in order to assure that leaks can be detected by the plant leak detection system before a through-wall flaw reaches critical flaw size. When the material is semi-ductile, the fracture mechanics evaluations involve the use of elastic-plastic fracture mechanics (EPFM) consisting of the J-integral and tearing modulus (J/T) analyses. An important input into the J/T analyses is the Ramberg-Osgood (R-O) material stress strain parameters which describe the stress-strain curve of the material of interest. These are also key inputs in the determination of leakage associated with through-wall flaws. If the stress-strain curve of the material of interest is available, the R-O parameters can be determined from power law curve fit. However, in most cases, archival material of existing plant piping is not readily available to determine the actual stress-strain curve. In the absence of the actual stress-strain curve, several approximate methods for determining the R-O parameters from basic mechanical properties have been proposed in the literature. These approximate methods however produce different sets of R-O parameters. In this paper, the effect of using different sets of R-O parameters from three R-O formulations on LBB analyses results is investigated. EPFM analyses are performed to determine the critical through-wall flaw lengths with the various sets of the R-O parameters for various materials and various pipe sizes. The same sets of parameters are then used to determine the leakage associated with through-wall flaws. The results of the evaluation indicate that different sets of R-O parameters can yield different critical flaw sizes as well as leakage flaw sizes, thus resulting in different margins in LBB evaluations. Considering the margins involved in LBB evaluations (factor of two on critical flaw size and factor of 10 on leakage), it is believed that these differences are small enough that any of the three correlations presented in this paper for determination of the R-O parameters can be adequately used in LBB evaluations employing EPFM analyses.

Commentary by Dr. Valentin Fuster
2007;():133-139. doi:10.1115/PVP2007-26580.

Inelastic finite element analysis offers an alternate procedure to evaluate damaged piping components for their fitness-for-service purposes. Redistribution of stress resultants beyond yield taken into account in a typical inelastic analysis becomes significant as a damaged piping component may not satisfy code stress criteria based on elastic analysis. A complete inelastic analysis to estimate the limit load of the component may be a numerically intensive and cumbersome process. This paper involves a two step analytical process — unloading a component to satisfy code stress categories after a prior plastic distribution of stress resultants is setup in the component. Linearization of stresses in the thinned section has shown reduction in the general membrane and membrane plus bending stress intensities when analyzed using this simplified method. To illustrate the method, an analysis is performed on both thick and thin pipe with local thinning.

Topics: Stress , Pipes
Commentary by Dr. Valentin Fuster
2007;():141-151. doi:10.1115/PVP2007-26701.

The feeder pipes of a typical CANDU reactor supply each of 380 fuel channels with coolant and are individually connected to the large bore primary piping. In some designs the longer pipes are interlinked with spacer rods, which mitigate vibrations and prevent contact between adjacent feeder pipes. Under severe loading conditions, the spacer rods will yield, which prevents overstressing the pipes as the strength of the coupling gets reduced. Spacer yielding also provides increased damping for dynamic loads. It has recently become practicable to model the entire interlinked system in a single model. This paper presents the modelling approach for seismic analysis and for fuel channel creep (a radiation induced slow unidirectional anchor motion). Results with and without spacer bars are compared to illustrate the effect of spacers. The importance of representing spacer yielding in the model is discussed.

Topics: Modeling , Pipes
Commentary by Dr. Valentin Fuster
2007;():155-160. doi:10.1115/PVP2007-26093.

Structural pressure vessel internals such as flow inlet/outlet baskets, bed supports etc. often use anisotropic screens with wires (or beams) oriented in mutually orthogonal directions. Discrete modeling of the screen details result in excessive model size and prohibitive computing costs. It is preferable to model the structural response of these screens using an anisotropic plate. Commercial FEA codes offer linear elastic models with anisotropic inputs. Elastic coefficients for use in the commercial code ABAQUS™ were developed using anisotropic plate theory and were validated by comparing results from rigorous, discrete beam FEA models with isotropic material properties to anisotropic shell element models for static load, vibration and buckling analyses with various edge boundary conditions. Some of the limitations of the application of this particular method are discussed. Further, a procedure is outlined to extract discrete elastic screen stresses from the anisotropic results.

Commentary by Dr. Valentin Fuster
2007;():161-165. doi:10.1115/PVP2007-26233.

In Code Case 2286-1, λc , the slenderness factor for column buckling, plays a major role in determining the allowable buckling stress, Fca . For λc > 0.15, consideration of column buckling is required but does not include end forces due to external pressure. A reduction factor, RF, based on λc is calculated to determine the allowable buckling stress. Conversely, for λc ≤ 0.15, consideration of column buckling is not required but axial force due to external pressure is included. In this paper the authors provide a method of determining an acceptable value of λc for multi-diameter and multi-thickness pressure vessels.

Commentary by Dr. Valentin Fuster
2007;():167-173. doi:10.1115/PVP2007-26486.

The structural safety of the Ethylene Oxide (EO) reactor of an ethylene glycol plant project should be verified by computational analysis. The heat transfer analysis has been performed using the finite volume method program, Fluent and the stress analysis has been performed using the finite element program, ANSYS. The applied loads are dead weight, internal pressure, nozzle load, earthquake load, wind load and thermal load. The analysis results for the main load conditions are presented. In addition, abnormal operating conditions such as runaway, pre-ignition and post-ignition are analyzed. The structural integrity of an EO reactor is investigated in accordance with the ASME Boiler and Pressure Vessel Code, Section VIII, Div. 2. It is concluded that the design of the EO reactor complies with the design criteria for all design load conditions.

Commentary by Dr. Valentin Fuster
2007;():175-183. doi:10.1115/PVP2007-26647.

This paper presents the results of the analytical evaluation supporting the technical justification of increasing the amount of temperbead welding, currently limited to an area of 100 in2 , that can be performed on low alloy steel (LAS) nuclear power plant components. The need to expand the application area limitations is increasing for ambient temperature Gas Tungsten Arc Weld (GTAW) temperbead weld overlay repairs on LAS components. As nuclear power plants age and as inspection techniques continue to improve increasing the area limit becomes increasingly important since more indications are being identified. Existing limitations of temperbead welding area of 100 in2 imposed in the ASME Code and in Code Cases 606 and 638 for ambient temperature temper bead welding are arbitrary and overly conservative. This paper presents the analyses supporting: 1) a weld overlay repair greater than 100 in2 on a Reactor Pressure Vessel (RPV) nozzle and 2) a weld cavity repair on an RPV of 500 in2 vertical shell weld. Based on the results of these cases, conclusions regarding temperbead welding in excess of the current 100 in2 limit are made.

Topics: Welding , Alloy steel
Commentary by Dr. Valentin Fuster
2007;():185-194. doi:10.1115/PVP2007-26722.

Equations describing the hoop stresses in a pipe due to water hammer have been presented in the literature in a series of papers, and this paper discusses the complete derivation of the pertinent equations. The derivation considers the pipe wall response to a water hammer induced shock wave moving along the inner wall of the pipe. Factors such as fluid properties, pipe wall materials, pipe dimensions, and damping are considered. These factors are combined to present a single, albeit rather complicated, equation to describe the pipe wall vibrations and hoop stresses as a function of time. This equation is also compared to another theoretical prediction for hoop stresses, which is also derived herein. Specifically, the two theories predict different maximum stresses, and the differences between these predictions are graphically displayed.

Topics: Shock waves , Stress
Commentary by Dr. Valentin Fuster
2007;():195-201. doi:10.1115/PVP2007-26740.

Real loading-unloading behavior of an A5083 aluminum alloy was recorded experimentally. This real behavior was incorporated in the variable material properties (VMP) numerical algorithm to predict residual stresses resulted from autofrettage and shrink fit in two kinds of autofrettage compound tubes. Using an analytical method, residual stresses induced by bore material removal via machining were calculated. Using developed VMP code and isotropic hardening rule in both autofrettage compound tubes, residual stresses resulted from shrink fit and autofrettage processes and via post machining, were estimated. Machining process simulation was performed for 10 mm bore material removal. ANSYS commercial FEM software was used for validating the results of VMP code. The results of VMP method and ANSYS were in good agreement with each other. Finally with the help of recorded real unloading behavior of A5083 aluminum alloy, also aforementioned simulations were performed on various tubes. The material removal simulation was modeled for 5 mm to 15 mm material removal from the bore.

Commentary by Dr. Valentin Fuster
2007;():203-209. doi:10.1115/PVP2007-26780.

Considering reinforcement pad and the cylindrical shell as an integral model and a contact model, stress analysis for opening-reinforcement structures of a cylindrical shell is performed by elastic and elastoplastic FEM. By comparison of two sub-models and two material constitutive relations (elastic and elastoplastic), the stress distribution of cylindrical shell intersections by the contact model is similar to that by the integral model, but there are some differences of the stress at contact surfaces of the shell and the reinforcement pad between by the contact model and by the integral model. In general, the stress analysis of the integral model for pad reinforcement can approximately represent that of the contact model. Finite element analyses for different nozzle diameters and different oblique angles in nozzle and cylinder shell intersections are carried out. The stress distribution and the maximum stress are affected by oblique angle. But the difference of the maximum stress intensity among different diameters is small.

Commentary by Dr. Valentin Fuster
2007;():213-221. doi:10.1115/PVP2007-26060.

In an effort to address inelastic creep behavior for very high temperature (VHT) applications, a unified state variable material model was used in a time dependent finite element analysis to generate isochronous curves. The resulting isochronous curves were then used in an efficient time-independent plastic analysis to predict the creep behavior of components. This simplified inelastic time-independent (SITI) method can significantly reduce the geometric and load uncertainties, and the over-conservatism in predicting inelastic strain levels. SITI is an effective and computationally efficient approach for predicting inelastic strains of components operating at high and very high temperatures such as the case in the Next Generation Nuclear Plant. This work compares the SITI inelastic strains to those obtained using fully inelastic time-dependent elastic-plastic-creep analysis, and illustrates the effectiveness of the approach in obtaining creep strain predictions without elaborate full inelastic time-dependent simulation.

Topics: Creep
Commentary by Dr. Valentin Fuster
2007;():223-230. doi:10.1115/PVP2007-26095.

This paper evaluates the use of isochronous stress-strain material data for the creep analysis of metals in high temperature applications. Performing an inelastic analysis using isochronous stress-strain data is a simplified approach for computing time dependent behavior using an implicit time embedded method. This method has been widely used as an effective means to evaluate the creep behavior of complex components without performing a detailed time dependent creep analysis. In order to examine the effectiveness and limitations of this method, isochronous stress-strain material data was numerically constructed from a time dependent creep law at various temperatures, sustained stress levels, and time durations. Component stresses and strains are compared from results obtained by running both the isochronous time embedded inelastic and time dependent creep analyses for some example problems. The effectiveness and limitations of this method under different loading conditions, such as primary and secondary stresses, are demonstrated and explained. It is recognized that this method was not intended to apply to thermal stress problem, and the thermal problem was studied to understand constraint effects.

Topics: Creep
Commentary by Dr. Valentin Fuster
2007;():231-247. doi:10.1115/PVP2007-26672.

The Alloy 617 draft Code case does not permit the use of simplified methods to assess ratcheting when temperatures exceed 649°C. This restriction was placed due to an apparent difficulty in distinguishing between creep and plastic deformation at various strain rates for this material. A numerical evaluation of the B-1 and B-2 Tests for a tube under constant pressure and cyclic thermal gradients (linear and nonlinear) was made. Analysis results indicate that simplified methods currently in Appendix T of ASME-NH are applicable for Alloy 617 in excess of 649°C.

Topics: Alloys
Commentary by Dr. Valentin Fuster
2007;():251-260. doi:10.1115/PVP2007-26025.

Metallurgical examinations and life predictions were performed for six water-cooled tubing bends. All of the tubes had axially oriented cracks on the inside surface of the tube bends near the neutral axis position and one of the tubes had failed. The cracking was intermittent or semi-continuous indicating multiple crack initiation sites. The cracks were transgranular, oxide-filled, and branched. One sample was cold fractured and examination of the fracture surface in the SEM revealed beachmarks and thumbnail shaped cracks. The tube shape is nominally oval and the ovality varied from 5% to 8-1/2%. Increased hoop stresses are expected on the inside surface at the neutral axis of a bend due to ovality. The failure mechanism is corrosion fatigue. Life predictions were performed as an aid in inspections and fitness for service assessments. Because the cracking was near the neutral axis, the straight tube geometry was used for the crack growth calculations. A parametric approach was used with three initial crack depths and stress amplification values of 1, 1.2, and 1.5 to account for stress increases due to ovality. The failure criterion was the reference stress equal to the minimum flow stress of carbon steel. For a stress amplification of 1.5 times the pressure induced stress, the calculated lives were 2,420 cycles for an initial normalized crack depth of 10% and 130 cycles for an initial normalized crack depth of 30%.

Commentary by Dr. Valentin Fuster
2007;():261-264. doi:10.1115/PVP2007-26053.

The paper derives an expression for the effective opening area of a uniform stress process zone at fracture initiation. The expression relates this area to the area that is appropriate to the idealized case of a semi-infinite crack in an infinite solid, and is a two term expansion in terms of the ratio of the square root of this area to a characteristic structural dimension.

Commentary by Dr. Valentin Fuster
2007;():265-271. doi:10.1115/PVP2007-26133.

Effect of WPS in ferritic alloys 15Kh2MFA (CrMoV) and 18MND5 increases with prestress level more rapidly than Chell’s model predicts; this observation had not been understood until now. In this work, experimental data are presented concerning behaviour of steels under simple Load – Unload – Cool – Fracture history. Chell’s model uses as input quantity true fracture toughness and predicts, for simple temperature and loading histories, apparent fracture toughness. A new proposed model based on local approach corrects value of true fracture toughness as an input quantity for Chell’s formulas. Idea of the true fracture toughness increase consists in deactivation of fracture initiation particles during preload. But another interpretation is also possible: Retarding influence of dislocation substructure formed during preload (phase L) on the cleavage microcrack growth at low temperature during loading to fracture (phase F) may cause similar effect. Universal behaviour found in both steels indicates that the second interpretation is more realistic one.

Topics: Stress
Commentary by Dr. Valentin Fuster
2007;():273-280. doi:10.1115/PVP2007-26138.

A series of finite element computations have been conducted to establish the limit pressures for a range of piping branch junctions with d/D ≤ 0.5. The results have been compared with those obtained using existing assessment procedures. In general, the latter have been found to be conservative. An approximate method for calculating the limit pressure of a branch junction is proposed. This method supposes that the branch junction may be treated as a plain cylinder containing an axial through-wall crack. The crack length is dependent upon the type of branch junction. The limit pressure can be established using a modified form of a standard solution limit load solution for a cylinder containing an axial through-wall crack. This approximate method is shown to be valid for a wide range of junction geometries.

Commentary by Dr. Valentin Fuster
2007;():281-287. doi:10.1115/PVP2007-26144.

We consider a steady state thermal stress problem of a long hollow cylinder under thermal striping in this paper. The outer surface of the cylinder was adiabatically insulated, and the inner surface was heated axisymmetrically by a fluid with sinusoidal temperature fluctuations, whose temperature amplitude (ΔT) and angular velocity (ω) were constant. The heat transfer coefficient h was also assumed to be constant. The stress intensity factor (SIF) due to the thermal stress for a given cylinder configuration varies not only with these three parameters ΔT, ω and h, but also with time. It is in fact possible to calculate the transient SIF for a specific combination of cylinder configuration and the three parameters numerically. However, for a given cylinder configuration, we think it is of practical importance to know the maximum SIF for all possible combinations of ΔT, ω and h. This maximum SIF evaluation is time consuming. Thus in this paper, we present this maximum transient SIF for four type surface cracks inside a hollow cylinder for all possible combinations of ΔT, ω and h. Thin to thick-walled cylinders in the range of mean radius to wall thickness parameter rm /W = 10.5 ∼ 1 were considered. Crack configurations considered were 360 deg continuous circumferential, radial, semi-elliptical in circumferential and radial direction. Normalized crack depth for all cases was in the range of a/W = 0.1 ∼ 0.5. In case of semi-elliptical crack, the normalized crack length a/c was all in the range of 0.063 ∼ 1.

Commentary by Dr. Valentin Fuster
2007;():289-299. doi:10.1115/PVP2007-26235.

One of the many design challenges in offshore industry is the fatigue life estimation of risers due to the loading that is generated by vortex induced vibration (VIV). In deep waters, where the long risers are subjected to sever VIV-induced stresses and may encounter multi-modal vibration, the VIV-induced stresses could be the most significant contributor to the overall damage of the structure. The variable amplitude nature of the stress-time history often creates significant errors in the estimated fatigue life of the structure. The irregularities in the loading scenario could also create a considerable degree of plasticity at the crack tip, thus leading to. variability in material response. The uncertainties in the estimated fatigue damage under such a variable amplitude loading has resulted in the use of large safety factors by industry for establishing the fatigue life of the risers.

Commentary by Dr. Valentin Fuster
2007;():301-317. doi:10.1115/PVP2007-26305.

In the 1960s and 1970s when the surveillance programs for currently operating commercial nuclear reactors were established state of knowledge limitations resulted in the use of Charpy-V notch (CVN) specimens rather than fracture toughness specimens. Reasonable success has since been achieved in correlating CVN and fracture toughness parameters. Such correlations provide an important part of the technical basis for both current regulations and ASME codes. These correlations imply that trends manifest in CVN data must also appear in fracture toughness data even though empirical evidence demonstrates that this is not always true. For example, the temperature dependence of CVN energy (CVE) in transition is thought to be a unique feature of each specific sample of ferritic steel that is tested, a view in sharp contrast with the now widely accepted view of a “Master Curve” for transition fracture toughness (KJc ). Also, effects of product form on CVE temperature dependence and property correlations are widely reported despite the fact that product form effects are absent from KJc properties. These observations suggest that the mapping of CVE behavior onto fracture toughness implicit to correlation-based regulations and ASME codes may produce erroneous trends in estimated values of fracture toughness. In this paper we investigate the hypothesis that the apparent differences between CVE and fracture toughness arise due to differences in how the temperature dependence of CVE and KJc data have historically been modeled. Our analysis shows that when CVE data are analyzed in a manner consistent with KJc data (i.e., transition and upper shelf data are partitioned from each other and analyzed separately rather than being fit with a continuous tanh function) the apparent differences between CVE and toughness characterizations are minimized significantly, and may disappear entirely. These findings demonstrate the differences between CVE and fracture toughness data to be an artifact of the tanh analysis method rather than an intrinsic property of CVE.

Commentary by Dr. Valentin Fuster
2007;():319-323. doi:10.1115/PVP2007-26386.

The inlet branch pipes connected the aggregate pipes and furnace tubes of a reformer, the outlines of them extended as “S” shapes, and they were made of austenitic steel 1Cr18Ni9Ti. After about 60,000 hours service, a lot of cracks were found near the elbows of inlet branch pipe and joints with the furnace tubes, many of them propagated through the pipe walls, and lead to leakage of raw material. In order to explore the failure causes, the microstructure of cracks and corrosion ingredients remained on fracture surface were analyzed by scanning electron microscopy (SEM), chemical composition, metallographic and mechanical properties of the branch-pipe steels were examined. The test result showed that the cracks propagated from the outside to inside of the pipe wall, the fracture surfaces were almost flat, there was no obvious deformation existed at the fracture section. Certain percent of chlorine and sulphur elements could be found and the secondary cracks could be observed on the microstructure fracture surfaces. The inlet branch pipes were elongated when they were connected with the aggregate pipes and furnace tubes, the stress were analyzed by finite element method (FEM) under the tensile deformation, the results showed that the stresses near the elbow were high. It could be concluded that the crack of the inlet branch pipe were stress corrosion cracks accelerated by the aggressive medium rich of CI and S ion and high stresses. Based on the causes analysis results, the corresponding countermeasures and improvements were suggested.

Commentary by Dr. Valentin Fuster
2007;():325-328. doi:10.1115/PVP2007-26388.

The stress corrosion cracking causes of the reboiler served in desulfuration system of a refinery were analyzed. According to visual inspection, chemical composition and metallographic examination, microstructure and ingredients of the fracture analysis by scanning electron microscopy (SEM), it was found that there was no obvious plastic deformation near the fracture, the chemical composition of the base material and welds were within the range specified in standards, the metallographic examination showed normal microstructure of this type of steel, intergranular brittle fracture and secondary cracks were found on the microstructure fracture. It could be concluded that the crack of the reboiler cylinder was stress corrosion cracking accelerated by the aggressive environment rich of H2 S and high welding residual stress. Based on the causes analysis results, the corresponding countermeasures and improvements were suggested.

Commentary by Dr. Valentin Fuster
2007;():329-333. doi:10.1115/PVP2007-26534.

A discrete multi-layered explosion containment vessel (DMECV) consists of a thin cylindrical inner shell and helically cross-winding flat steel ribbons, which has advantages of convenient in fabrication and low in cost. The DMECVs generally fall into one of two categories: vessels designed for one-time-use and those for multiple-use. For a multiple-use DMECV, it is important to predict its remaining life under explosive loading for safe application. A method based on the growth of fatigue cracks in the inner shell is presented to predict the total number of explosive tests that a DMECV can withstand. By integrating the Paris law, an expression was obtained to calculate the crack growth in a single explosive. Then the expression was used as a recursion relationship to determine the new crack size for the next test, when the critical crack size is reached, the maximum number of explosive tests was obtained. Results are presented and discussed for an initial axial side-crack in the inner shell of the DMECV.

Commentary by Dr. Valentin Fuster
2007;():335-341. doi:10.1115/PVP2007-26549.

During a pressurized thermal shock (PTS) event, the overlay cladding on the inner surface of reactor pressure vessel (RPV) is subjected to high tensile stress compared to base metal because of the difference in thermal expansion coefficients between cladding and base metal. To calculate a stress intensity factor for a postulated crack considering the stress discontinuity with the plastic yielding of cladding, the scheme developed previously has been incorporated into the PASCAL code for the structural integrity analysis. Using the new scheme, conditional probabilities of crack initiation (PCI) were calculated for a typical RPV with a surface crack or under-clad crack under some PTS transients. The PCI values were quantitatively evaluated as a function of neutron fluence using the PASCAL code. It is concluded that the new scheme reduces significantly the PCI value for a surface crack as compared with the conventional method based on elastic stress analysis.

Commentary by Dr. Valentin Fuster
2007;():343-353. doi:10.1115/PVP2007-26599.

This work presents an investigation of the ductile tearing properties for an API 5L X60 pipeline steel using experimentally measured crack growth resistance curves (J-R curves). Use of these materials are motivated by the increasing demand in the number of applications for manufacturing high strength pipes for the Brazilian oil and gas industry including marine applications and steel catenary risers. Testing of the pipeline steels employed side-grooved SE(T) specimen with varying crack size to determine the J-R curves based upon the unloading compliance method using a single specimen technique. Recent developed compliance functions and eta-factors applicable for SE(T) fracture specimens are introduced to determine crack growth resistance data from laboratory measurements of load-displacement records. This experimental characterization provides additional toughness data which serve to evaluate crack growth resistance properties of pipeline steels using SE(T) specimens with varying geometries.

Commentary by Dr. Valentin Fuster
2007;():355-364. doi:10.1115/PVP2007-26611.

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.

Commentary by Dr. Valentin Fuster
2007;():365-371. doi:10.1115/PVP2007-26794.

In the stochastic mechanics community, the need to account for uncertainty has long been recognized as key to achieving the reliable design of structural and mechanical systems. It is generally agreed that advanced computational tools must be employed to provide the necessary computational framework for describing structural response. A currently popular method is the stochastic finite element method (SFEM), which integrates probability theory with the standard finite element method (FEM). However, SFEM requires a structured mesh to perform the underlying finite element analysis. It is generally recognized that the creation of workable meshes for complex geometric configurations can be difficult, time consuming, and expensive. This discrepancy is further exacerbated when solving solid mechanics problems characterized by a continuous change in the geometry of the domain under analysis. The underlying structures of these methods, which rely on a mesh, are cumbersome in treating moving cracks or mesh distortion. Consequently, the only viable option when applying FEM is to remesh during each discrete step of the model’s evolution. This creates numerical difficulties, even for deterministic analysis, and often leads to degradation in solution accuracy, complexity in computer programming, and a computationally intensive environment. Consequently, there is considerable interest in eliminating or greatly simplifying the meshing task. In recent years, a class of Galerkin-based meshfree or meshless methods have been developed that do not require a structured mesh to discretize the problem, such as the element-free Galerkin method, and the reproducing kernel particle method. These methods employ a moving least-squares approximation method that allows resultant shape functions to be constructed entirely in terms of arbitrarily placed nodes. Meshless discretization presents significant advantages for modeling fracture propagation. By sidestepping remeshing requirements, crack-propagation analysis can be dramatically simplified. Since mesh generation of complex structures can be far more time-consuming and costly than the solution of a discrete set of linear equations, the meshless method provides an attractive alternative to FEM. However, most of the development in meshless methods to date has focused on deterministic problems. Research into probabilistic meshless analysis has not been widespread and is only now gaining attention. Due to inherent uncertainties in loads, material properties and geometry, a probabilistic meshless model is ultimately necessary. Hence, there is considerable interest in developing stochastic meshless methods capable of addressing uncertainties in loads, material properties and geometry, and of predicting the probabilistic response of structures. This paper presents a new stochastic meshless method for predicting probabilistic structural response of cracked structures i.e., mean and variance of the fracture parameters.

Commentary by Dr. Valentin Fuster
2007;():373-380. doi:10.1115/PVP2007-26815.

Prevention of failure of pressurised and high-energy components and systems has been an important issue in design of all types of power and process plants. Each individual component of these systems must be dimensioned such that it can resist the forces or moments to which it will be subjected during normal service and upset conditions. Design by analysis is an important philosophy of modern design. The ability of now-a-days computers to numerically handle complex mathematical problems has inspired the use highly nonlinear material behaviour (including material softening) instead of classical linear constitutive theory for the materials. Under the influence of these developments, a fundamentally different type of modelling has emerged, in which fracture is considered as the ultimate consequence of a material degradation process. Crack initiation and growth then follow naturally from the standard continuum mechanics theory (called continuum damage mechanics). Numerical analyses based on these so-called local damage models, however, are often found to depend on the spatial discretisation (i.e., mesh size of the numerical method used). The growth of damage tends to localise in the smallest band that can be captured by the spatial discretisation. As a consequence, increasingly finer discretisation grids can lead to crack initiation earlier in the loading history and to faster crack growth. This non-physical behaviour is caused by the fact that the localisation of damage in a vanishing volume is no longer consistent with the concept of a continuous damage field, which forms the basis of the continuum damage mechanics approach. In this work, the Rousellier’s damage model has been extended to its nonlocal form using damage parameter ‘d’ as a degree of freedom. The finite element (FE) equations have been derived using the weak form of the governing equations for both mechanical force equilibrium and the damage equilibrium. As an example, a standard fracture mechanics specimen [SE(B)] made up of a German low alloy steel has been analysed in 2D plane strain condition using different mesh sizes near the crack tip. The results of the nonlocal model has been compared with experimental results as well as with those predicted by the local model. It was observed that the fracture resistance predicted by the local damage model goes on decreasing when the mesh size near the crack tip is refined whereas the nonlocal model predicts a converged fracture resistance behaviour which compares well with the experimentally determined behaviour.

Commentary by Dr. Valentin Fuster
2007;():381-388. doi:10.1115/PVP2007-26837.

A system-integrated modular advanced reactor is being developed for multi purposes such as electricity production, sea water desalination and so on in Korea. While ASME Codes provide simplified design and operation procedures to determine allowable loadings for pressure retaining materials in components, the procedures are applicable when a temperature change rate associated with startup and shutdown is less than about 56°C/hr. If the procedures are applied to a rapid temperature change, results would be overly conservative. The objective of this research is to assess an applicability of the simplified design procedures to reactor coolant system of the integrated modular reactor with the change rates of 56°C/hr and 100°C/hr. To investigate effects of cooldown rate, heatup rate and surface crack location, systematic three-dimensional finite element analyses are carried out. The resulting pressure-temperature limit curves are compared with those obtained from the ASME Sec. XI operating procedure as well as Sec. III design procedure. Thereby, it was proven that the specific design features significantly affect the safe design region in the pressure-temperature limit curve to prevent a nonductile failure.

Commentary by Dr. Valentin Fuster
2007;():389-396. doi:10.1115/PVP2007-26865.

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.

Commentary by Dr. Valentin Fuster
2007;():399-404. doi:10.1115/PVP2007-26158.

Buckling analysis of simply supported functionally graded cylindrical shells under mechanical loads is presented in this paper. The Young’s modulus of the shell is assumed to vary as a power form of the thickness coordinate variable. The shell is assumed to be under three types of mechanical loadings, namely, axial compression, uniform external lateral pressure, and hydrostatic pressure loading. The equilibrium and stability equations are derived based on the first order shear deformation theory. Resulting equations are employed to obtain the closed-form solution for the critical buckling load. The influences of dimension ratio, relative thickness and the functionally graded index on the critical buckling load are studied. The results are compared with the known data in the literature.

Commentary by Dr. Valentin Fuster
2007;():405-413. doi:10.1115/PVP2007-26170.

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.

Commentary by Dr. Valentin Fuster
2007;():415-421. doi:10.1115/PVP2007-26191.

The inner tube of the double-tube reactors used in some chemical process units must be designed to resist buckling. When the inner tube is operated at a higher temperature and at a lower pressure and outer tube is operated at a lower temperature and at a higher pressure, the inner tube will be subjected to combined thermal loads and external pressure. ASME Code Sec. VIII Div. 1 provides a design procedure for shells, based on a B-chart, to ensure against buckling under external pressure, however, additional consideration should be made where plastic deformation may occur due to very large longitudinal thermal loads. In this study, nonlinear finite element analyses were performed to investigate the collapse of thick-walled cylindrical shells subjected to combined thermal loads and high external pressures. Two nonlinearities, a material nonlinearity (elastic-plastic behavior) and a geometric nonlinearity (large deformation), were considered in these analyses. The effects of initial imperfection in the shells (out-of-roundness) as well as of thermal loads were studied. The results showed that the longitudinal thermal loads reduce the plastic collapse load especially when the thermal loads are tensile. It was also shown that the loading sequence has a large effect on the collapse load especially when the thermal stress was larger.

Commentary by Dr. Valentin Fuster
2007;():423-430. doi:10.1115/PVP2007-26227.

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.

Commentary by Dr. Valentin Fuster
2007;():431-438. doi:10.1115/PVP2007-26298.

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.

Topics: Deformation , Nozzles
Commentary by Dr. Valentin Fuster
2007;():439-446. doi:10.1115/PVP2007-26426.

In this paper based on the multiplicative decomposition of the deformation gradient, the plastic spin tensor and the plastic spin corotational rate are introduced. Using this rate (and also log-rate), an elastic-plastic constitutive model for hardening materials are proposed. In this model, the Armstrong-Frederick kinematic hardening and the isotropic hardening equations are used. The proposed model is solved for the simple shear problem with the material properties of the stainless steel SUS 304. The results are compared with those obtained experimentally by Ishikawa [1]. This comparison shows a good agreement between the results of proposed theoretical model and the experimental data. As another example, the Prager kinematic hardening equation is used. In this case, the stress results are compared with those obtained by Bruhns et al. [2], in which they used the additive decomposition of the strain rate tensor.

Commentary by Dr. Valentin Fuster
2007;():449-456. doi:10.1115/PVP2007-26246.

Pressure drop has been used for more than half a century to control resonant pulsation in reciprocating compressor piping. Although avoiding these resonances is the preferred method, this is not possible in many high-speed/variable-speed installations. In these cases, resonant pulsation is often managed by using orifice plates to dampen the response. Helmholtz absorbers are an old technology, used to improve the acoustics of ancient Greek theaters and modern recording studios alike. Although their application in the field of piping acoustics has been well documented, this paper presents new ways in which they have not yet been applied. In this paper, experimental data is shown for a self-tuning Helmholtz absorber, or Side Branch Absorber (SBA) used to cancel a piping length resonance, and for a Virtual Orifice that is used to reduce cylinder nozzle pulsation. These devices open up new doors for controlling pulsation with reduced horsepower costs in reciprocating compressor installations.

Commentary by Dr. Valentin Fuster
2007;():457-463. doi:10.1115/PVP2007-26676.

This paper details methods of interpreting maximum surge pressures in LNG pipelines due to valve closures and other transient events. The standard methodology for determining the onset of surge events and the pressure transients involved uses explicit integration; this method of analysis produces inherent “noise” in the solution results due to the integration method. The paper discusses methods of filtering data obtained through explicit integration and demonstrates which filters provide the best results for these analyses. Filtered and unfiltered results are presented for an actual LNG unloading facility subjected to a number of transient events, with discussion provided on determining the maximum peak pressures, their duration and the frequency content of secondary pressure waves.

Topics: Surges
Commentary by Dr. Valentin Fuster
2007;():465-471. doi:10.1115/PVP2007-26809.

Estimation of point loads on a span for a vibrating pipe has been done with the help of boundary measurement of slope data. The approach is based on the theory of inverse force identification for hyperbolic systems. Depending on the form of the forcing function, which also satisfies some continuity conditions an exact determination, is possible. Though restrictive in its formulation some applications are still possible. A typical case for harmonic excitation of a pulsating reciprocating compressor has been shown as an illustration.

Commentary by Dr. Valentin Fuster
2007;():473-480. doi:10.1115/PVP2007-26823.

The Second West-East Gas Pipeline (WEGP) is planed be constructed trans-China within recent years. The Pipeline is about 7,000 kilometers with outside diameter 1219mm, operation pressure 12MPa, and steel grade API-X80 [1], all of which is the first time in China. Both safety and reliability of the Pipeline are the most important issues which should be thought over by the gas company. In this paper, the method of limit state analysis is used to study the effect of strength, toughness and allowable flaw size on the safety and reliability of the Pipeline. The calculation results suggest that the overmatched weld has the advantages of improving limit load and maximum allowable defect sizes of the pipelines. When Ms (Ms is the ratio of yield strength of weld σSW over that of base metal σSB , i.e. MS = σSW / σSB ) is lower than 0.9, there is much possibility for the happening of fracture initiation in the weld zone. When Ms is higher than 1.2, the limit load will not increase with the increase of weld strength. Although the Second WEGP will transmit sweet gas, there is still possibility that sour gas releases to the Pipeline in case of operation accident. As to the high-grade pipeline, in order to reduce the sensitivity of cold crack, hydrogen induced crack (HIC) and stress induced corrosion crack (SCC), etc., However, chemical composition and hardness of both base and weld metal can influence the resistance to HIC and SCC. It can be concluded that the best range of strength mismatched ratios Ms is 0.9 ∼ 1.2 for the Second WEGP. The critical defect sizes proposal is 10.0mm of maximum allowable defect length and 0.95mm of maximum allowable defect depth at the weld mismatch.

Commentary by Dr. Valentin Fuster
2007;():483-492. doi:10.1115/PVP2007-26099.

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.

Commentary by Dr. Valentin Fuster
2007;():493-498. doi:10.1115/PVP2007-26357.

ASME Code stress assessment of pressure vessels in the power generation industry is usually done by finite element analysis using one of the two approaches. In the first, “shell-element” approach, vessels are modeled out of shell elements; primary plus bending and primary plus secondary stresses are taken directly from the finite element analysis results and the alternating stresses are based on primary plus secondary stresses prorated by respective stress concentration factors. The strength of the “shell-element” approach is its simplicity; its weakness is problematic modeling of the stress concentration and some modeling difficulties (varying wall thickness, nozzle/vessel connectivity, pressure applied to the mid-surface instead of to the inner surface.) In the second, “solid-element” approach, vessels are modeled out of solid elements; “linearized” stresses can not be taken directly from the finite element analysis results, first they must be linearized, and only then, can be compared against their allowable counterparts; the alternating stresses can be based directly on the outer/inner-surface-node-stresses, provided that the mesh of the model is fine enough to account for the stress concentration effect. The strength of the “solid-element” approach is its high accuracy; its weakness is the time consuming, sometimes ambiguous, stress linearization process. This paper proposes a modification of the “solid-element” approach, in which the time consuming linearization process is replaced by a modification of the original model. To do so, a vessel must be modeled out of quadratic 20 node solid elements; the mesh density of the model (on its surface and through thickness) must be adequate for stress concentration representation and the mesh lines in the thickness direction must be more or less normal to the surfaces. The results from this original model can be taken directly for fatigue evaluation. To obtain the “linearized” stresses the original model must be slightly modified, specifically the number of elements through thickness must be reduced to one, and the reduced integration technique is recommended. For such a modified model, the nodal stresses are equivalent to the “linearized stresses” of the original model. The equivalence is discussed on a model of a circular nozzle attached to a cylindrical vessel. The vessel loads are pressure and thermal expansion.

Topics: Stress
Commentary by Dr. Valentin Fuster
2007;():499-504. doi:10.1115/PVP2007-26774.

Fiber-reinforced polymer composite pipe may provide superior performance in terms of weight and corrosion resistance compared to structures made from conventional engineering materials. Using advanced winding techniques and adhesive bonding, composite tubes of high quality can be manufactured and joined in a cost effective manner. However, the current understanding of the damage mechanisms and long-term behavior of the tubes and joints is limited, thus the prediction of composite pipe performance is currently inadequate. Prior to developing modeling and failure prediction methodologies, it is imperative to identify and describe the types and evolution of damage associated with these structures. The present study is concerned with the monotonic and cyclic damage behavior of adhesively bonded glass-fiber reinforced epoxy polymer tubes. Experimental analyses on small-scale and large-scale specimens were conducted, and results revealed characteristic damage modes for the pipe body and the joint area. It was observed that damage modes and their severity depended on the applied biaxial pipe stress ratio and the type of loading (monotonic or cyclic).

Topics: Fibers , Pipes , Polymers
Commentary by Dr. Valentin Fuster
2007;():505-515. doi:10.1115/PVP2007-26784.

The ASME Boiler and Pressure Vessel Codes and Standards used for designing pressure vessel and piping provide guidelines to classify the linear elastic stresses into primary, secondary and peak categories. Although these guidelines cover a wide range of pressure components, they are sometimes difficult to apply to the three-dimensional components with complex loading and geometries. The concept of “reference two-bar structure” is used in this paper to categorize the stresses in pressure components and structures, using linear elastic finite element analyses. The method is applied to a number of components and structures from simple to relatively complex geometric configurations. The results compare well with those obtained from commercial finite element codes.

Topics: Stress
Commentary by Dr. Valentin Fuster
2007;():519-528. doi:10.1115/PVP2007-26081.

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.

Commentary by Dr. Valentin Fuster
2007;():529-535. doi:10.1115/PVP2007-26168.

At an incomplete mixing area of high and low temperature fluids, fluid temperature fluctuation often occurs. It induces cyclic thermal stresses in the wall, which may result in fatigue crack initiation. Kasahara et. al. proposed the thermal fatigue evaluation method based on power spectrum density (PSD) in PVP05. This method generalizes the evaluation procedure by preparing PSD charts of fluid and frequency transfer functions of stress for various kinds of plant components. From design point of view, however, this method is too complicated due to the inverse Fourier transform and wave decomposition procedures named Rain Flow Cycle Counting (RFC). In this paper, simplified damage evaluation method for thermal fatigue is proposed by directly evaluating fatigue damage from PSD of stress. Since analytical treatment for evaluation of fatigue amplitude distribution based on PSD is difficult due to complicated procedure of RFC, direct evaluation method for RFC amplitude distribution from PSD is newly proposed. This method gives fatigue damage evaluation with safety margin. This paper shows the dependency of safety margin on geometry of PSD. Finally, application to design for thermal fatigue will be shown. Since PSD of stress in the wall near temperature fluctuation can be easily evaluated using Kasahara’s method, the proposed method will make thermal fatigue damage evaluation far easier.

Commentary by Dr. Valentin Fuster
2007;():537-544. doi:10.1115/PVP2007-26414.

Thermal fatigue strength tests subjected to sinusoidal fluid temperature waves were performed by the SPECTRA test facility, where frequencies were 0.05, 0.2, and 0.5Hz. Cracks were observed on the inner surface of cylindrical test pieces after testing. 0.05Hz’s wave caused a greater number of and deeper cracks than 0.5Hz’s wave under the same fluid temperature range and the same fatigue cycles. The crack initiation region of the 0.05Hz’s wave was larger than for the 0.5Hz’s wave. Estimated fatigue failure cycles based on the frequency transfer functions were compared with test results. Frequency-dependency in failure cycles was observed through these test results, and frequency transfer functions could estimate this dependency. The test results supported the fatigue damage evaluation method with frequency transfer functions.

Commentary by Dr. Valentin Fuster
2007;():545-551. doi:10.1115/PVP2007-26782.

Towers are applied in the wide range of the petrochemical industry. The flow condition and the temperature distribution in the tower are the focus of the people’s attention, which would affect function of the tower and could result in unstable operation of the tower. In this paper, the flow field in a quench oil tower is simulated based on CFD method. The DPM (Discrete Phase Model) is used to calculate and analyze flow distribution and heat transfer between gas and liquid. The numerical results such as temperature and velocity distributions below lower tray in tower are obtained. According to CFD results, modification method of improving the flow distribution is proposed.

Commentary by Dr. Valentin Fuster

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