bluuebunny/crossref_metadata_2025 huggingface dataset:
A Proposed Design Criterion for Vessel Lifting Lugs in Lieu of ASME B30.20
Authors: Williams Dennis K.
Published: August 2008

This paper describes a method for evaluating the structural adequacy of various lifting lugs utilized in the erection and up righting of large pressure vessels. In addition, the analysis techniques are described in detail and design guidelines for vessel lifting are tendered. The statutory and provincial regulations in both the United States and the province of Alberta, Canada are also reviewed and discussed with respect to the too often utilized phrase “factor of safety” (FOS). The implied implications derived from the chosen FOS are also outlined. A discussion is presented as to the applicability of the ASME safety standard B30.20 [1] entitled, “Below the Hook Lifting Devices” and as to the severe shortcomings of the safety standard in its attempt to delve into the design of lifting devices, especially when applied to lifting lugs on large and heavy-weight pressure vessels. Exemplar lugs on vessels are defined and the finite element analyses and closed form Hertzian contact problem solutions are presented and interpreted in accordance with the proposed design criteria. These results are compared against the very limited design information contained within ASME B30.20 [1]. Suggestions for the revision and applicability of the Below the Hook Lifing Devices safety standard and presented and discussed in light of the examples and technical justification presented in the following paragraphs. In addition, the silence of the referenced safety standard on the very large contact stresses that are well known to exist between a lifting pin and clevis type geometry is also discussed. Due to the limited number of repetitive loading cycles that vessel lifting lugs acturally experience during the service life of a vessel, a recommendation is made to either clearly exclude vessel lifting lugs from the scope of ASME B30.20 [1] or to specifically include a separate design and analysis section within the referenced stardard to properly address the mechanical and structural design issues applicable to pressure vessel lifting lugs.
Residual Stress of Thin-Wall Pipe Subjected to Axisymmetric Plastic Expansion
Authors: Chiba Norimasa , Ishida Yuji , Ogasawara Nagahisa , Ito Hiroshi , Enomoto Kunio , Kimoto Hiroshi
Published: August 2008

A straight thin-wall pipe was plastically expanded at one end in the radial direction by inserting a rigid disk. The residual stress measured after withdrawal of the disk at the inner surface in the hoop and in the longitudinal direction shows a strong tensile peak beyond the region where the pipe was directly expanded by the insertion of the disk. The reason why the residual stress reaches its peak at the location far inward of the pipe, not in the region where the pipe was directly expanded, is discussed. From the FE analysis, it is concluded that the residual stress reaches its tensile peak on the inner surface at the plastic region front that was developed during the pipe expansion, and a simple formula for the tensile peak location is proposed. The similarities and differences between the residual stress distribution of the thin-wall pipe and the thick-wall pipe are discussed.
ASME B&PV Code Section VIII Pressure Vessel Design: A Comparison — Division 1 Versus Division 2
Authors: Smith Dwight V.
Published: August 2008

Historically, the ASME B&PV Code, Section VIII, Division 2, Alternative Rules for Construction of Pressure Vessels (Div.2), ASME [1], was usually considered applicable only for large, thick walled pressure vessels. Otherwise, ASME B&PV Code, Section VIII, Division 1, Rules for Construction of Pressure Vessels (Div. 1), ASME [2], was typically applied. A case can also be made for the application of the Div. 2 Code Section for some vessels of lesser thicknesses. Each vessel should be closely evaluated to ensure the appropriate choice of Code Section to apply. This paper discusses some of the differences between the Div. 1 and Div. 2 Code Sections, summarizes some of the main design requirements of Div. 2, and presents a ease for considering its use for design conditions not usually considered by some, to be appropriate for the application of Div. 2 of the ASME Code.
Contact Simulation in Finite Deformation — Algorithm and Modelling Issues
Authors: Sauve´ R. G. , Morandin G. , Khajehpour S.
Published: August 2008

Many problems, involving finite deformation, necessitate an accurate treatment of sliding interface boundaries and impact conditions. Examples of this are the accidental impact of radioactive waste transportation packages, crashworthiness, fluid-structure interaction and various manufacturing processes including metal forming and cutting. In terms of general use and applicability, large scale nonlinear simulations involving arbitrary contact continues to pose a number of challenging issues. Faster computing processors and parallel processing strategies have helped to mitigate the impact of some of these issues on modelling. However, there is room for overall improvement in accuracy, efficiency and user-friendliness of contact algorithms used in the modelling of multiple body impact/contact, eroding surfaces where new free contact surfaces are created, large interface motions of initially unconnected surfaces and post-buckling behaviour of structures where surfaces fold onto themselves. For example, the benefits of using a faster processor with an iterative solver on an elliptic class of problem could be quickly neutralized if the number of iterations rise due to inaccuracies in the contact algorithm. In this paper, salient features of state-of-the-art general three-dimensional contact algorithms for use in finite element software based on explicit and/or iterative solution techniques are reviewed and some of the more complex modelling issues faced by users of such algorithms are addressed.
The Meshless Dynamic Relaxation Techniques for Simulating Atomic Structures of Materials
Authors: Pan Li , Metzger Don R. , Niewczas Marek
Published: August 2008

Traditionally, Molecular Dynamics combined with pair potential functions or the Embedded Atom Method (EAM) is applied to simulate the motion of atoms. When a defect is generated in the crystalline lattice, the equilibrium of atoms around it is destroyed. The atoms move to find a new place where the potential energy in the system is minimum, which could result in a change of the local atomic structure. The present paper introduces new Dynamic Relaxation algorithm, which is based on explicit Finite Element Analysis, and pair or EAM potential function, to find equilibrium positions of the block of atoms containing different structural defects. The internal force and stiffness at the atoms (nodes) are obtained by the first and second derivatives of the potential energy functions. The convergence criterion is based on the Euclidean norm of internal force being close to zero when the potential energy is minimum. The damping ratio affects the solution path so that different damping ratios could lead to different minimum potential energy and equilibrium shapes. The numerical responses and results by applying free boundary conditions and certain periodic boundary conditions are presented. The choice of scaled mass of atoms, proper time step and damping appropriate for the efficient and stable simulation is studied.
Subcycled Hourglass Control for Explicit Time Integration of Dynamic Relaxation Equations
Authors: Metzger D. R. , Gao S.
Published: August 2008

Explicit methods, such as the central difference operator, rely on the economical evaluation of internal forces at each time step of a transient dynamic problem. One-point quadrature applied to the spatial discretization provides the greatest efficiency, but hourglass control is required to eliminate spurious zero energy modes. Computationally practical hourglass control methods involve considerable approximation in evaluating the internal force. Thus, a small additional approximation due to an alternative temporal integration of the hourglass force may not seriously affect the accuracy of the analysis. In particular, the possibility of evaluating the hourglass terms on a larger time interval than the usual stable time step could provide significant efficiencies. The proposed approach of subcycling the hourglass terms is examined in detail with respect to stability and accuracy. Implementation into an explicit finite element program is demonstrated on a three-dimensional example that involves several hourglass modes, and the new method proves to be beneficial for noninertial problems where artificial damping is used.
Extended GDQ and Related Discrete Element Analysis Methods for Transient Analyses of Continuum Mechanics Problems
Authors: Chen Chang-New
Published: August 2008

The extended GDQ proposed by the author is used to develop solution algorithms for solving the discrete transient equation system of continuum mechanics problems. It is a direct integration approach. Two integration methods are developed. They are time-element by time-element method and stages by stages method. These two time integration algorithms can be used to solve generic discrete transient equation system of an originally discrete system or a discrete system resulting from the discretization of a transient system of continuum mechanics problems by using a certain discretization technique such as the DQEM, FEM, FDM,…, etc.
Inclusion of Local Shell Behavior of Tubes Into a Two-Dimensional Beam Approximation of Deformation in Nuclear Fuel Channels
Authors: Khajehpour S. , Sauve´ R. G. , Badie N.
Published: August 2008

A method has been developed to incorporate the local three-dimensional shell behavior of two concentric tubes in the two-dimensional beam modeling of the problem. The two dimensional modeling of fuel channels in CANDU pressurized heavy water nuclear reactors is used in lieu of a more accurate three dimensional finite element approach in order to reduce the on-line simulation time which greatly affects the SLAR (Spacer Location And Repositioning) maintenance operation cost during outage. However, effort must be made to include the three-dimensional shell behavior of these channels into the two-dimensional modeling. In recent studies a nonlinear force-dependent model for contact stiffness between the calandria tube and pressure tube has been developed. However, local deformation of calandria the tube at spacer locations due to in-reactor creep leads to settling of the spacer into the calandria tube that consequently reduces the gap between the two tubes. In this work, the effect of local deformation (elastic and creep) of calandria tubes on modeling of contact at spacer locations is assessed using a three dimensional finite element code. The result is incorporated into a two-dimensional beam model of the problem as a reduction in size of the spacers that separate the two tubes. It is shown that the proposed method increases the accuracy of prediction of contact time and the spacer. In general, the method described in this paper suggests a way to incorporate local shell deformation into beam models of slender shell structure.
Finite Element Analysis of Complex Corrosion Defects
Authors: Cronin Duane S.
Published: August 2008

Aging gas and oil transmission pipeline infrastructure has led to the need for improved integrity assessment. Presently, external and internal corrosion defects are the leading cause of pipeline failure in Canada, and in many other countries around the world. The currently accepted defect assessment procedures have been shown to be conservative, with the degree of conservatism varying with the defect dimensions. To address this issue, a multi-level corrosion defect assessment procedure has been proposed. The assessment levels are organized in terms of increasing complexity; with three-dimensional elastic-plastic Finite Element Analysis (FEA) proposed as the highest level of assessment. This method requires the true stress-strain curve of the material, as determined from uniaxial tensile tests, and the corrosion defect geometry to assess the burst pressure of corrosion defects. The use of non-linear FEA to predict the failure pressure of real corrosion defects has been investigated using the results from 25 burst tests on pipe sections removed from service due to the presence of corrosion defects. It has been found that elastic-plastic FEA provides an accurate prediction of the burst pressure and failure location of complex-shaped corrosion defects. Although this approach requires detailed information regarding the corrosion geometry, it is appropriate for cases where an accurate burst pressure prediction is necessary.
A Finite Element Analysis of the Residual Stresses Incurred During Bending of Pipes
Authors: Scott C. S. , Kozluk M. J.
Published: August 2008

This work illustrates the potential for finite element methods to be used in support of metal fabrication processes. The focus is an analysis of the residual stresses incurred during cold bending of small diameter pipes. The pipe was modeled using 3D constant strain elements. The mandrels used to support the pipe and apply the necessary bend forces were modeled using 2D rigid surfaces. Contact surfaces were defined on the outside of the pipe and the inside of the mandrels. The fabrication process was simulated by programming the nodes of one of the mandrels with prescribed velocities. The finite element analysis was performed using H3DMAP, proprietary software that includes a hybrid explicit/dynamic relaxation module. The technique is a quasi-static approach that discounts inertial effects. The finite element analyses are used to predict the residual stresses and plastic strain in the pipe. The studies involve a constant pipe size. Two stress/strain curves are used. The effect of using isotropic or kinematic material hardening models, compressive pre-stressing and differing bending procedures are considered, and results compared. The details of each simulation are shown to influence the calculated residual stress field.
Dynamic Response of Timoshenko Beam Structures Solved by DQEM Using EDQ
Authors: Chen Chang-New
Published: August 2008

The dynamic response of Timoshenko beam structures is solved by using the DQEM to the space discretization and EDQ to the time discretization. In the DQEM discretization, DQ is used to define the discrete element model. Discrete dynamic equilibrium equations defined at interior nodes in all elements, transition conditions defined on the inter-element boundary of two adjacent elements and boundary conditions at the structural boundary form a dynamic equation system at a specified time stage. The dynamic equilibrium equation system is solved by the direct time integration schemes of time-element by time-element method and stages by stages method which are developed by using EDQ and DQ. Numerical results obtained by the developed numerical algorithms are presented. They demonstrate the developed numerical solution procedure.
Threshold and Variable Amplitude Crack Growth Behavior in 350WT Steel
Authors: Rushton P. A. , Taheri F. , Stredulinsky D. C.
Published: August 2008

A number of models are currently available for the prediction of fatigue crack growth (FCG). Among them, Zhang and Hirt model has been identified as the most promising, based on the fact that it requires knowledge of only the basic mechanical properties of the material and the threshold stress intensity factor (Kth). To determine an appropriate value for Kth, an experimental program was designed to investigate the crack growth threshold behavior of 350WT steel. The resulting Kth was found to be similar to that suggested by other workers. However, our investigation showed that when used in conjunction with the Zheng and Hirt model, the experimentally determined Kth yields poor fatigue life predictions (FLP). An experimental testing program was also developed to investigate the fatigue crack growth behavior of center-cracked 350WT steel specimens when subjected to semi-random loading comprised of various combinations of intermittent tensile overloads and compressive underloads. Taheri et al. [1] proposed an exponential delay model for the prediction of crack growth behavior under the influence of intermittently applied tensile overloads. The current investigation proposes a modification to the exponential delay model to include the effects of not only overload ratio, but also stress ratios and overload/underload ratios. The modified delay model predictions proved to be in good agreement with the experimental data.
Piping System Structural Integrity Simulation for Post LOCA Water Hammer Loads
Authors: Manning B. W. , Stevens T. , Morandin G. , Sauve´ R. G. , Richards R. , Mantro S. , Czajkowski J.
Published: August 2008

The Canadian Nuclear Safety Commission (CNSC) required as part of the operating license for Ontario Power Generation’s Darlington Nuclear Generating Station, that the structural integrity of the piping following a loss of coolant accident (LOCA) be demonstrated. This is necessary to ensure that no subsequent pressure boundary failures will impede the ability to maintain fuel cooling. The injection of cold emergency coolant following a LOCA creates the potential for the occurrence of condensation-induced water hammers (CIWH) in the primary heat transport (PHT) system piping. Classical linear elastic piping analysis using the class 1 NB-3656 rules of the ASME Boiler & Pressure Vessel Code failed to demonstrate the adequacy of the piping and/or its supports that were designed using the linear elastic rules of subsection NF for nine of the twelve piping models that comprise the PHT system. A decision was made to undertake a state-of-the-art non-linear explicit analysis in order to qualify the piping. Strain rather than stress limits would be applied similar to those being developed by ASME for nuclear packaging undergoing accidental impact during transportation. In order to address the feasibility of this approach, a non-linear analysis was performed on a portion of one of the piping systems. The piping was modeled as shells and again as beam elements with and without detailed modeling of the supports. After these initial simulations, it was determined that the piping could be modeled with simplified beam elements, however, the supports would require a more detailed modeling in order to determine the extent of support damage and the effect the supports have on the integrity of the piping system itself. This paper addresses the non-linear modeling of the piping models and discusses the modeling details, assumptions and analysis results. This approach is shown to be a useful alternative for predicting the extent of structural damage that can be expected by a Level D event such as a condensation induced water hammer following a loss of coolant accident.
Finite Element Formulation for the Analysis of the Erosive Character of Oil-Based Drilling Fluids During Well Drilling Processes
Authors: Jenkins Jack , Nu´n˜ez Oswaldo
Published: August 2008

In this work we study the erosive character of oil-based drilling fluids, weighted with Barite and Orimatita, on equipment and tools during the drilling process. Using an experimental circuit, erosion is studied by means of a jet of mud impinging on a plate of the same material of the equipment and tools used in field. Simulations using the finite element method obtain the flow pattern of the jet of mud and the velocities and impact angles of the particles of Barite and Orimatita that produces wear on the plate. A disperse flow model was used to carry out the simulation and pertinent consideration are assumed to simplify the analysis, keeping the results with enough accuracy for practical purposes. Based on the morphology of the erosion tread carved on the 1020-Carbon Steel plate used in the experiments, the finite element results suggest a range of critical speeds and impact angles of Barite and Orimatita along with a range of speeds and impact angles that generate no significant wear on the material.
Roller Burnishing — A Cold Working Tool to Reduce Weld Induced Residual Stress
Authors: Martin John
Published: August 2008

The possibility of stress corrosion cracking (SCC) in regions of tensile residual stress introduced by weld deposited material has been a concern where environmental effects can reduce component life. Roller burnishing, a form of mechanical cold-working, has been considered as a means of providing for residual stress state improvements. This paper provides a computational evaluation of the roller burnishing process to address the permanent deformation needed to introduce a desirable residual stress state. The analysis uses a series of incrementally applied pressure loadings and finite element methodology to simulate the behavior of a roller burnishing tool. Various magnitudes of applied pressure loadings coupled with different size plates and boundary conditions are examined to assess the degree and depth of the residual compressive stress state after cold working. Both kinematic and isotropic hardening laws are evaluated.
Numerical Study of Mass Dispersion in Laminar and Turbulent Flows Through a Pipeline
Authors: Guzma´n Horacio Antonio Flo´rez
Published: August 2008

A computer code for solving the equations of mass diffusion has been developed and applied to study the molecular-level mixing between two fluids inside a pipe. First, one fluid occupies the entire volume within the pipe, and then a second miscible fluid is forced into the pipe, developing a mixing process through the interface between the fluids. This phenomenon occurs as the combination of molecular diffusion, variation of velocity over the cross-section and turbulence. The code developed for this study is based on the finite element method for domain discretization and standard finite difference schemes for temporal discretization. Comparison with experimental data shows that the code is able to reproduce the physical trends and gives good predictions for engineering applications. A grid independence analysis is presented for all computations.
Out-of-Plane Deflections of Nonprismatic Curved Beam Structures Solved by the Differential Quadrature Element Method
Authors: Chen Chang-New
Published: August 2008

The differential quadrature element method (DQEM) is used to solve the out-of-plane deflections of nonprismatic curved beam structures. The extended differential quadrature (EDQ) is used to discretize the governing differential equations defined on all elements, the transition conditions defined on the inter-element boundary of two adjacent elements and the boundary conditions. Numerical results obtained by DQEM are presented. They demonstrate the developed numerical solution procedure.
Computation of Ratchet Limits for Structures Subjected to Cyclic Loading and Temperature
Authors: Ponter A. R. S. , Chen H. , Habibullah M.
Published: August 2008

The paper discusses methods of evaluating the ratchet limit for an elastic/plastic structure subjected to cyclic thermal and mechanical loading. A recently developed minimization theorems by Ponter and Chen [2] provides a generalization of the shakedown limit theorems for histories of load in excess of shakedown. This allows the development of programming methods that locate the ratchet boundary in excess of shakedown. Examples of applications are provided including the performance of a cracked body subjected to cyclic thermal loading. Finally, the theory is used to discuss Kalnins’ [4] proposal that short cut finite element solutions may be used to assess whether a particular loading history lies within a ratchet limit.
Application of a Sixth Order Generalized Stress Function for Determining Limit Loads for Plates with Triangular Penetration Patterns
Authors: Gordon J. L. , Jones D. P.
Published: August 2008

The capability to obtain limit load solutions of plates with triangular penetration patterns using fourth order functions to represent the collapse surface has been presented in previous papers. These papers describe how equivalent solid plate elastic-perfectly plastic finite element capabilities are generated and demonstrate how such capabilities can be used to great advantage in the analysis of tubesheets in large heat exchanger applications. However, these papers have pointed out that although the fourth order functions can produce sufficient accuracy for many practical applications, there are situations where improvements in the accuracy of in-plane and transverse shear are desirable. This paper investigates the use of a sixth order function to represent the collapse surface for improved accuracy of the in-plane response. Explicit elastic-perfectly plastic finite element solutions are obtained for unit cells representing an infinite array of circular penetrations arranged in an equilateral triangular array. These cells are used to create a numerical representation of the complete collapse surfaces for a number of ligament efficiencies (h/P where h is the minimum ligament width and P is the distance between hole centers). Each collapse surface is then fit to a sixth order function that satisfies the periodicity of the hole pattern. Sixth-order collapse functions were developed for h/P values between .05 and .50. Accuracy of the sixth order and the fourth order functions are compared. It was found that the sixth order function is indeed more accurate, reducing the error from 12.2% for the fourth order function to less than 3% for the sixth order function.
Equivalent Solid Based Fatigue Analysis of Perforated Plates with Triangular Perforation Pattern
Authors: Reinhardt Wolf
Published: August 2008

The current fatigue assessment method of the ASME B&PV Code uses an elastic stress multiplier to determine the total stress in perforated plates with triangular pattern. This method contains several simplifications and shortcomings. The present paper shows that the perforation pattern symmetry imposes constraints on the total stress multipliers that can be used to simplify the elastic analysis, particularly of three-dimensional (non-axisymmetric) plates. For thin ligament conditions, the very conservative Code analysis can be replaced by a more accurate stress multiplier approach for which the stress concentration around the hole is derived from an elastic Finite Element analysis. This can result in a significant reduction of the calculated fatigue usage factor. Finally, the issue of strain concentration is addressed. When the total stress range at a location around the ligament exceeds 2Sy, the elastically predicted strain range is potentially unconservative. The elastic-plastic strain at this location can be estimated from the elastic result using simplified methods. The corresponding predictions are compared to elastic-plastic analysis results.
Some Achievements of the European Project LISA for FEM Based Limit and Shakedown Analysis
Authors: Staat M.
Published: August 2008

The load-carrying capacity or the safety against plastic limit states are central questions in the design of pressure equipment. The new European standard prEN 13445-3 reflects the fact that plastic limit states can be calculated simply by perfectly plastic limit and shakedown analyses. These methods can be derived from static and kinematic theorems for lower and upper bound analysis, respectively. Both may be formulated as optimization problems for finite element discretizations of structures. The problems of large-scale analysis have been solved in an European research project. The methods could be made more realistic by extension to other material models such as a two-surface plasticity model of kinematic hardening. Limit and shakedown analyses are briefly demonstrated with illustrative examples, which are chosen to show some limitations of the 3Sm criterion and the effect of kinematic hardening.
Limit Analysis of Shells With Random Pattern of Spread Pits
Authors: Porowski Janek S. , Osage David A. , Janelle Jeremy
Published: August 2008

Limit analysis is used to determine the strength of the shells weakened by the material loss due to pitting corrosion. The wall is modeled as a multi-layer structure. The outer layers include penetrations due to the pitting attack. The layers with pits are analyzed as the plane perforated sheets with the equivalent, uniform pattern of circular holes, subjected to the arbitrarily oriented biaxial loading. The results of these analyses have been used for the evaluation of the pitting corrosion in the American Petroleum Institute Recommended Practice for Fitness-For-Service API-579. Examples of evaluations are provided for the pressure vessel and storage tank walls.
Comparison of Limit Load Solutions with Results of Collapse Tests of Perforated Plates with a Triangular Penetration Pattern
Authors: Jones D. P. , Gordon J. L.
Published: August 2008

Limit load solutions obtained by elastic-perfectly plastic finite element analysis (EPP-FEA) are compared to results of tests of low-alloy steel perforated plate geometries loaded to full plastic collapse. Results are given for two plastic-collapse tests of flat circular disks with circular penetrations arranged in a triangular pattern and drilled normal to the surface of the plate. The ligament efficiency (minimum distance between holes divided by the distance between the centers of the holes) of the pattern is 0.32 and the plate thickness is 2.39 inches (60.7 mm). The tests were designed so that a transverse load generated plastic collapse in the outer row of penetrations due to a combination of transverse shear and in-plane bending. Limit-load solutions were obtained using EPP-FEA with small-strain, small-deflection linear geometry assumptions. Two FEA models are used: one where the perforated region is modeled using an equivalent solid plate (EQS) representation and another where each hole is explicitly modeled by FEA. The results presented in this paper demonstrate that the deformation patterns produced by the EPP-FEA solutions match exactly with the deformation patterns produced by the test. The EQS-EPP FEA solution is about 15% lower than the explicit-hole EPP-FEA solution. Using one-third the actual ultimate strength of the material as the strength parameter in the limit load calculation produces a calculated limit load that is greater than a factor of three less than the mean measured plastic-collapse load obtained in the tests. This paper adds to the qualification of the use of limit-load solutions obtained using small-strain, small deflection EPP-FEA programs for the calculation of the limit load for perforated plates.
Effect of Plane Strain, Plane Stress and Bending on Equivalent Solid Collapse Predictions for Perforated Plates
Authors: Reinhardt Wolf
Published: August 2008

An equivalent solid based method of predicting the plastic (limit) collapse of perforated plates has been developed in the recent literature. Higher order collapse surfaces with suitable symmetry properties are used for the analysis. The equation for the collapse surface contains a number of constants that are determined by comparison to a Finite Element analysis of an actual perforated periodic cell subject to selected membrane states of stress. Using a typical triangular perforation pattern, the present paper investigates the effect of simplifying assumptions that were made during the periodic cell analysis on the predicted collapse load of the actual cell geometry. For a finite thickness cell subjected to a membrane state of stress, the collapse load is expected to lie between that of the bounding cases of plane stress and generalised plane strain, which are compared here. The connection between the collapse of a cell subjected to a membrane state of stress and a cell of large thickness in bending is established.
Can Repeated Elastic Analyses Always Provide Exact Limit Loads?
Authors: Mangalaramanan Prasad
Published: August 2008

Past research showed that obtaining statically admissible stress distributions based on elastic moduli modification methods exactly correspond to stress distributions due to stress-strain power law, for beam bending and cylinder under internal pressure. This paper investigates shear stress redistribution based on modified elastic moduli, theoretical and inelastic finite element analysis methods.
Use of Structural Reliability Analysis for Uprating Above Ground Installations
Authors: Francis A. , Jandu C. S. , McCallum M. A.
Published: August 2008

In support of an extensive programme to increase the operating pressure of the UK National Transmission System (NTS) Advantica Technologies Limited have developed a structural reliability based methodology which is used to demonstrate the safe operation of Above Ground Installations (AGIs) at increased pressure levels. The approach is based on Advantica’s methodology for demonstrating the safe operation of pipeline sections at high design factors. It incorporates the effects of stress concentrations occurring at Tees and bends within complex pipework systems, and addresses the credible failure modes, including shakedown, corrosion and fatigue, taking account of pressure and thermal loadings. Particular attention is given to the time-dependent nature of the failure modes and the mitigating effect of the pre-service hydrostatic test and weld inspections is included.
Pro-Active Fitness for Service of 19 Meter Diameter NGL Surge Sphere
Authors: Walsh Michael P.
Published: August 2008

To minimize costly down time, pro-active in lieu of reactive FFS assessments offer distinct advantages since the acceptable/unacceptable defect geometries and limitations are determined prior to the actual inspection. General Corrosion, LTAs and pitting were detected in the lower accessible sections of this 2″ average thickness sphere designed to ASME Section VIII, Division 2. API-510 and API-579 Levels 1, 2 and 3 were utilized to evaluate the documented defects in a comparison study. The defect geometries were then extrapolated to unacceptable levels to be used as a go/no-go reference tool for the remainder of the sphere and future inspections. Excel spread sheet numerical calculations and FEA non-linear analyses are employed. A significant increase in service life and defect tolerance over API-510 is realized.
Fitness-for-Purpose Assessment of Encirclement Split-Tees
Authors: Wheat S. , Jandu C. S. , Bramley D. N. , Liu J. H.
Published: August 2008

Hot tap tees of the full encirclement split tee design are currently used in the UK gas industry to provide connections onto existing high pressure gas transmission pipelines and above ground installations. The fitness-for-purpose of this type of tee is not covered by the UK design code for above ground gas installations, IGE/TD/9. A fitness-for-purpose methodology has been developed by Advantica Technologies to determine the integrity of the fitting and attachment welds. The fitness-for-purpose assessment addresses the following: • The compliance of the fitting to plastic collapse, shakedown and fatigue design criteria. • The integrity of the attachment welds onto the carrier pipe. • The use of Engineering Critical Assessments in conjunction with existing procedures to ensure overall integrity.
Steam Piping Fitness-for-Service Evaluation
Authors: Vecchio Robert S. , Sinha Sri K. Sam
Published: August 2008

Recently, in the boiler room of a high-rise commercial building in New York City, a flash tank ruptured. As a result of dynamic loads generated during the explosion, steam piping in the boiler room was subjected to large displacements and severe vibrations. The building owner, concerned about the safety and integrity of the steam piping, requested an assessment of the piping and a fitness-for-service evaluation for continued operation. This paper presents a summary of the investigation performed including inspection of the piping, determination of operating and residual stresses, metallurgical evaluation and fitness-for-service assessment. A tolerable or critical pipe flaw size was established using the failure assessment diagram (FAD) method and, to the extent possible, the guidelines of API 579. Fracture mechanics evaluation and crack growth calculations were performed to establish re-inspection frequency for critical welds and pipe sections. Finally, nondestructive examinations were performed to determine which pipe sections and welds, if any, should be repaired or replaced.
Hydrogen Embrittlement Testing of 2.25 Cr-1Mo Steel Using Large Thick Specimen
Authors: Wada Yoru , Hasegawa Tatsuo , Inoue Hirokazu
Published: August 2008

A large 3.5 inch thick compact tension specimen (CT) was hydrogen pre-charged in an extra high capacity autoclave in order to introduce hydrogen into steel, then hydrogen cracking tests were conducted for an extended period of time in ambient air. The anticipated out gassing from the specimen was significantly less in the 3.5T-CT specimen than in the conventional 1.0T-CT specimen. The residual hydrogen after 2 weeks of exposure to ambient air was as much as 80% (≥2 ppm) of the original hydrogen introduced. The threshold stress intensity factor for the onset of cracking (= KIH) for the high toughness, recently manufactured 2.25Cr-1Mo steel was severely degraded to KIH = 42 MPa√m (46 ksi√in) under ultra slow strain rate (dK/dt = 0.005 MPa√m/sec. (0.006 ksi√in/sec)) and subcritical cracking continued over 100 hours. The crack growth rate was kept almost constant regardless of slow change of increase or decrease in K. On the other hand, the temper embrittled 60’s 2.25Cr-1Mo steel showed brittle, unstable fracture at very low stress intensity factor (KIC-H = 33 MPa√m(36 ksi√in)) with no subcritical crack occurring before the fracture event. The fracture point KIC-H turned out to be as low as 10% of the fracture toughness KIC. Finally, comparisons were made between 2.25Cr-1Mo and 2.25Cr-1Mo-1/4V steels by tests of small specimens, since the 2.25Cr-1Mo-1/4V steel substantially retains hydrogen for its lowest diffnsion coefficient. The degree of hydrogen embrittlement is higher at room temperature for both steels.
Flaw Growth Prediction and Fitness-for-Service Assessment of a PWR Nuclear Plant Steam Generator U-Tube
Authors: Vecchio Robert S. , Sinha Sri K. Sam
Published: August 2008

Primary water stress corrosion cracking (PWSCC) continues to be a dominant degradation mechanism affecting the service life of steam generators in several operating pressurized water reactor (PWR) nuclear plants. Recently, in one operating nuclear plant, a steam generator U-tube ruptured catastrophically while the unit was on-line. Although the plant operators were able to shutdown the reactor without significant release of radiation, the Nuclear Regulatory Commission (NRC) and the owner utility launched a full-scale investigation of the incident. The owner utility requested that a crack growth analysis and engineering evaluation of the tube rupture be performed, as well as assess the fitness-for-service of the generator for continued operation. This paper presents a summary of elastic-plastic finite element and fracture mechanics analyses performed for a steam generator U-tube, subjected to crack initiation at the inner diameter of the tube in the apex region. Residual stresses were computed from a finite element model of the tube simulating the mechanical bending process with the use of an anvil. Fracture mechanics and crack growth evaluations were performed to predict the time required for a pre-existing flaw at the inside diameter of the tube to propagate through-wall. Additionally, a fitness-for service assessment was performed in order to permit a degraded tube to remain in service, given an initial flaw size as determined by nondestructive examination.
Simulation of Degraded Repair Weld and Serviceability Assessment of Petroleum Pressure Vessel
Authors: Kayano Rinzo , Hasegawa Tatsuo , Inoue Hirokazu
Published: August 2008

In order to simulate temper and hydrogen embrittlement in 2.25Cr-1Mo pressure vessel steel in the laboratory, test specimen exceeding 100 mm (4 in.) in thickness containing repair welds made with the shielded metal arc welding (SMAW) process were exposed to hydrogen environment in an autoclave. By investigating the dilution, hardness and microstructure characteristics of the specimen repair welds, it was possible to determine a standard repair technique including minimum stainless steel overlay thickness. From the test results, it was concluded that a minimum of 3.0 mm (1/8 in.) residual overlay thickness was recommended as part of the repair technique by SMAW. In addition, tensile tests of the hydrogen exposed specimens confirmed the serviceability of recent and old generation 2.25Cr-1Mo pressure vessel steel repair welds. In particular, the effect of temper and hydrogen embrittlement on serviceability was examined by detailed observation of microstructure and fracture surface of the tensile specimens.
Creep Constitutive Equations for 1CrMoV Bolt Material
Authors: Ellis Fred V. , Zielke Robert L.
Published: August 2008

Creep constitutive equations were developed for 1CrMoV bolt material using the NRIM creep data at temperatures of 450°C, 500°C and 550°C. The creep constitutive equations were those used by the ANSYS FEM stress analysis program. Three types of creep equations were developed: (1) primary, (2) secondary, and (3) primary plus secondary. The primary creep equation had a power law in stress and time with an exponential inverse temperature dependence. The secondary models for log minimum creep rate had either log stress alone or stress and log stress terms. Two primary plus secondary models were used: (1) a power law primary and (2) the rational polynomial. For the rational polynomial, the constant p was estimated as nineteen over the time to the end of primary creep. The time to end of primary creep was defined by the attainment of a low value for the ratio of the primary creep rate to minimum creep rate. The primary creep rate was calculated using the power law primary component of the power law primary plus steady state creep equation. To evaluate the fits, comparisons of calculated and observed creep curves were made as well as plots of residual in log creep strain. Based on these comparisons and the statistical measures (R2 and standard deviation), the derived creep equations were judged to be a good description of the creep behavior.
IGSCC Failure of Refractalloy 26 Turbine Bolts
Authors: Ellis Fred V. , Tordonato Sebastian
Published: August 2008

Refractalloy 26 turbine bolt failures were discovered during refurbishment activities. Failure analysis was performed to determine the failure mechanism and to develop a comprehensive/plausible root cause of failure scenario. The failed bolts are used to attach the nozzle block to the nozzle chamber and were installed during the 1993/1994 overhaul of the turbine. The unit was retired in 1997 and had been in operational service approximately 40% of the time following the 1993/1994 overhaul. Fifty of the 53 broken bolts were located in the lower or bottom half of the turbine and a white deposit was found covering the nozzle block and two rows of the stationary blades adjacent to the nozzle block for the lower cylinder. Based on the fractographic and metallographic examinations, the damage features are: (1) numerous, branched, intergranular cracks that initiate at the stress concentrated regions of the transition radius from the head to uniform section of the bolt, and at the thread root, and (2) continuous, surface connected cracks that are filled with corrosion product. The failure mechanism was identified as intergranular stress corrosion cracking (IGSCC). The failures require contaminant ingress of typical boiler feedwater chemicals and carryover. Based on SEM and x-ray diffraction results, the probable contaminants are Na, P, S, and Cl and the failures result from carryover of either NaOH or di-sodium phosphate into the steam. Based on a review of operating history, two root cause failure scenarios were developed: (1) single acid contamination even followed by significant caustic corrective action, and (2) multiple upset events, termed “decreasing temperature excursions”, that typically occur during an increase in steam flow. Both scenarios require low steam flow (such as that found at low loads) in order to cause agglomeration of the contaminants at the bottom of the turbine. The actual events that caused the IGSCC damage are not definitively known. Nevertheless, corrective actions that eliminate carryover are expected to be effective in preventing any future SCC damage to the turbine.
Development of FFS Handbook for Refinery and Petrochemical Industries
Authors: Ideguchi Yoshiro , Kikuchi Tsutomu , Ikeda Koichi , Watanabe Tomohiro , Tahara Takayasu , Anderson Ted L.
Published: August 2008

Pressure equipment in refinery and petrochemical industries in Japan has been getting old, mostly more than 30 years in operation. Accordingly, there are many fitness for service evaluations required for pressure equipment. However, Japanese regulation still requires as the same quality level as the original fabrication for pressure equipment in service. In order to introduce the latest FFS methodologies to Japanese industries, a special task group was established at the High Pressure Institute of Japan and started study of API RP579 from its drafting stage as a member of TG579. And then, FFS handbook for refinery and petrochemical industries has been developed based on API RP579 with several modifications to meet Japanese pressure vessel regulations on April 2001. It is expected that FFS handbook will be introduced as an exemplified standard with Japanese regulations for practical maintenance.