Pressure Vessel Components Research Articles

Application of submerged arc welding technology with cold wire addition for drum shell long seam butt welds of pressure vessel components

Submerged arc welding is most widely used method for welding thick section in down hand position being a highly economical method for fabrication. As products must meet more stringent requirement of quality, requiring more flexible processes to reduce cost, further improvements is studied. Quality control aspects are also becoming more important to meet regulation and monitoring and control of welding processes, and the standardized testing of joints will meet all the requirements. This paper describes the experimental work carried out to study the effect of process parameters and weld metal property of thick section welds in SA 299 material using cold wire addition technology in tandem submerged arc welding for boiler drum shell fabrication. Improved results of mechanical and metallurgical properties obtained in this study are reported.

Effect of Bolt Spacing on the Circumferential Distribution of the Gasket Contact Stress in Bolted Flange Joints

Abstract Bolted flange joints are part of pressure vessel and piping components and are extensively used in the chemical, petrochemical, and nuclear power industries. They are simple structures and offer the possibility of disassembly, which makes them attractive to connect pressurized equipment and piping. In addition to being prone to leakage, they often require maintenance while in operation in which case the bolts are either retightened as in hot torquing or untightened to be replaced. Although costly shutdowns are avoided, such maintenance work exposes the operator to a potential risk because the bolt load alteration can produce a gasket load unbalance, which results in the local gasket contact stress to drop below some critical value, causing major leak and hence jeopardizing the life of the worker. This paper addresses the issue of the contact stress level unbalance around the flange when the bolts are subjected to initial tightening. The study compares the contact stress distribution variations, an analytical developed model based on the theory of rings on elastic foundation, to those given by the finite element model and the simple beam on elastic foundation model developed by Koves (2007, “Flange Joint Bolt Spacing Requirements,” Proceedings of PVP2007, ASME Pressure Vessel and Piping Division Conference). This study is developed for the purpose of helping limit the degree of load increase in hot torquing or the maximum number of bolts to be replaced at a time and identify those flanges for which the bolt cannot be replaced in service.

Assessment of Overlapped Internal and External Volumetric Flaws in p-M Diagram

Abstract Assessment of overlapped internal and external volumetric flaws is one of the most common problems related to pressure vessel and piping components. Under the current fitness for service rules, such as those provided in ASME, BS, and so on, the procedures for the assessment of these flaws have not yet been defined. In this paper, a reference stress, incorporating the decrease in the effective cross section as a function of flaw depth and flaw angle in a cylinder, has been proposed in order to assess the flaws using the simple p-M (pressure-moment) diagram method. Numerous finite element analyses for a cylinder with overlapped internal and external flaws were conducted to verify the proposed procedure. There is good agreement among them.

Evaluation of Residual Stress Measurements Before and After Post-Weld Heat Treatment in the Weld Repairs

Welding repairs are increasingly a structural integrity concern for aging pressure vessel and piping components. It has been demonstrated that the residual stress distribution near repair welds can be drastically different from that of the original weld. Residual stresses have a significant effect on the lifetime performance of a weld, and a reduction of these stresses is normally desirable. The aim of this paper is to investigate residual stresses in various weld repair arrangements using the non-destructive neutron diffraction technique. This research is focused on characterization of the residual stress distribution: (i) in the original weld; (ii) in a shallow toe weld repair; and (iii) after conventional post-weld heat treatment. The focus of the measurements is on the values of the subsurface strain/stress variations across the weld.

Multiaxial fatigue life assessment of welded structures

Welded structures, such as welded pressure vessel components subjected to multiaxial cyclic loading, are particularly susceptible to fatigue damage. In this paper, a new path-length-based effective stress range is proposed to assess the fatigue life of weld joints under multiaxial fatigue loading. The path-length measure, a function of both normal and shear components on a critical crack plane, has a solid root in classic fracture mechanics and its application is validated by correlating nominal fatigue data including pure-bending, pure-torsion, in-phase, and out-of-phase loading. Path-Dependent Maximum Range (PDMR), a unique general-purpose fatigue life assessment package for multiaxial variable-amplitude loading, is introduced in this paper. Finally, the application of PDMR to multiaxial fatigue life assessment of complex loading cases is also discussed.

A new test method for determining the fracture toughness of concrete materials

The Spiral Notch Torsion Test (SNTT) determines the intrinsic fracture toughness (KIC) of structural materials by applying pure torsion to cylindrical specimens having a notch line that spirals around the specimen at a 45° pitch. KIC values are obtained with the aid of a three-dimensional finite-element computer code, TOR3D-KIC. The SNTT method is suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments, as well as ceramic and graphite materials. One important characteristic of SNTT is that neither a fatigue precrack nor a deep notch is required for evaluation of brittle materials, significantly reducing the sample size requirement. Results are reported for a Portland cement-based mortar demonstrating applicability of the SNTT method to cementitious materials. The estimated KIC of the tested mortar samples with compressive strength of 34.45 MPa was found to be 0.360 ± 0.017 MPa √m.

Simplified Stress Categorization Using a Single Linear Elastic Analysis

The ASME Boiler and Pressure Vessel Code (code) provides guidelines for the classification of linear elastic stresses into primary, secondary, and peak stresses. Although these guidelines cover a wide range of pressure containing components, the guidelines are sometimes difficult to employ for three-dimensional components with complex geometry. This paper uses the mα-tangent method, an assessment of constraint in the component, based on limit load multiplier estimates as a stress classification tool. The method is applied to several practical pressure vessel components from simple to relatively complex geometric configurations. The results compare well with those obtained by conventional techniques, e.g., inelastic finite element analysis.

An innovative technique for evaluating fracture toughness of graphite materials

Spiral notch torsion fracture toughness test (SNTT) was developed recently to measure the intrinsic fracture toughness ( K IC) of structural materials. The SNTT system operates by applying pure torsion to uniform cylindrical specimens with a notch line that spirals around the specimen at a 45° pitch. The K IC values are obtained with the aid of a three-dimensional finite-element computer code, TOR3D-KIC. The SNTT method is uniquely suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments, including others such as ceramics, their composites, and concrete.

Corrosion Fatigue Crack Growth Rate for Petroleum Refining Pressure Vessel Materials (2.25Cr-1Mo Steel)

Abstract The 2.25Cr-1Mo steel is used for pressure vessel components and has sometimes suffered from hydrogen embrittlement. Therefore, it is important to investigate the effect of hydrogen on the embrittlement for 2.25Cr-1Mo steel. Under corrosion fatigue conditions, the effect of hydrogen embrittlement (HE) on corrosion fatigue crack growth rate (CFCGR) appears in different ways depending on the applied stress wave forms. This behavior has been clarified by previous theoretical and experimental analyses. In this paper, based on the cited theoretical analyses, the effect of stress waveform on CFCGR for 2.25Cr-1Mo steel was investigated and the sensitivity to hydrogen embrittlement is examined for this material.

Study of Elasto-Plastic Anisotropic Response of Stainless Steel Weld Metal Using Neutron Diffraction

Current analytical techniques for simulating the generation of residual stresses within welded structures make approximations and simplifications relating to the mechanical properties of the weld metal. This is due to the paucity of knowledge on the anisotropic nature of weld metal. Consequently, the study of elastic and plastic anisotropy, within metals has been a subject of considerable interest to both experimentalists and modellers alike. This study investigates the elastic and plastic response of crystal lattice planes within single-pass and multi-pass austenitic steel weld metal using time-of -flight (TOF) neutron diffraction. As this material is often used for the fabrication of high temperature, pressure vessel components, it is important to understand the evolution of lattice strains within each set of planes during elastic and plastic loading. Neutron diffraction measurements were carried out for both single and multi-pass weld metal subjected to in-situ uni-axial tensile loading. The elastic response of individual reflections was recorded by diffraction measurements. All measurements were carried out at the ENGIN-X facility at ISIS, UK. The objective of the measurements was to determine the Young's modulus and characterise the elastic and plastic anisotropy of the weld metal. Measurements indicate a significant variation between the Young's modulus for the two materials, with values close to 210 GPa for the singlepass weld metal and 90-130 GPa for the multi-pass welds. These results are explained in terms of the texture of the welds.

Recent advances in residual stress measurement

Until recently residual stresses have been included in structural integrity assessments of nuclear pressure vessels and piping in a very primitive manner due to the lack of reliable residual stress measurement or prediction tools. This situation is changing the capabilities of newly emerging destructive (i.e. the contour method) and non-destructive (i.e. magnetic and high-energy synchrotron X-ray strain mapping) residual stress measurement techniques for evaluating ferritic and austenitic pressure vessel components are contrasted against more well-established methods. These new approaches offer the potential for obtaining area maps of residual stress or strain in welded plants, mock-up components or generic test-pieces. The mapped field may be used directly in structural integrity calculations, or indirectly to validate finite element process/structural models on which safety cases for pressurised nuclear systems are founded. These measurement methods are complementary in terms of application to actual plant, cost effectiveness and measurements in thick sections. In each case an exemplar case study is used to illustrate the method and to highlight its particular capabilities.

The Design Master S-N Curve in ASME Div 2 Rewrite and its Validations

A master S-N curve approach has been adopted in the new 2007 ASME Div 2 as an alternative method to the existing ASME fatigue evaluation procedure for fatigue assessment of welded components through a rewrite effort over the last few years. The master S-N curve has been developed based on two key technical advances in fatigue research over the last decade: (1) a robust structural stress method that offers good mesh-insensitivity in stress concentration calculation; (2) an equivalent structural stress based single master S-N curve that represents a large amount of fatigue test data from various joint types, loading modes, plate thicknesses, etc. Based on the statistical characterization of over 1000 weldment fatigue tests, a design master S-N curve has been proposed to provide both technically sound and reasonably conservative fatigue evaluation for pressure vessel and piping components. In this paper, two new series of recent fatigue tests on Div 2 vessels and related weld details will be used to further validate the design master S-N curve proposed. The emphasis will be placed upon its relative safety margins in comparison with those inherent in the traditional ASME fatigue procedure in lights of these recent tests.

Burst Pressures for Torispheres and Shallow Spherical Caps

Abstract: Burst pressures are computed for shallow spherical caps and torispheres loaded by uniform static pressure. They are defined as maximum pressures at which proposed plastic strain criteria are met. The criteria are taken from true stress–true strain curves of uniaxial tensile tests. Experimental tests, carried out on pressure vessel components made from aluminium alloy 6061 and mild steel, provide experimental results against which the proposed criteria for burst pressures are benchmarked.

General principles of the use of safety factors in design and assessment

Any structure or component can be made to fail if it is subjected to loadings in excess of its strength. Structural integrity is achieved by ensuring that there is an adequate safety margin or reserve factor between strength and loading effects. The basic principles of ‘allowable stress’ and ‘limit state’ design methods to avoid failure in structural and pressure vessel components are summarised. The use of risk as a means of defining adequate safety is introduced where risk is defined as the product of probability of failure multiplied by consequences of failure. The concept of acceptable ‘target’ levels of risk is discussed. The use of structural reliability theory to determine estimates of probability of failure and the use of the reliability index β are described. The need to consider the effects of uncertainties in loading information, calculation of stresses, input data and material properties is emphasised. The way in which the effect of different levels of uncertainty can be dealt with by use of partial safety factors in limit state design is explained. The need to consider all potential modes of failure, including the unexpected, is emphasised and an outline given of safety factor treatments for crack tip dependent and time dependent modes. The relationship between safety factors appropriate for the design stage and for assessment of structural integrity at a later stage is considered. The effects of redundancy and system behaviour on appropriate levels of safety factors are discussed.

Numerical simulation of mechanical stress relieving in a multi-pass GTA girth welded pipe–flange joint to reduce IGSCC

Welding of piping components produces highly non-uniform residual stresses in the weldment which consequently affect material structural response under loading and also expedite origination/propagation of defects such as stress corrosion cracking and brittle fracture. For welding-induced residual stresses, mechanical stress relieving (MSR) is one of the mitigation techniques to improve the service life of pressure vessel components. However, its application to piping systems is rarely reported in the literature. This paper presents a two-dimensional axisymmetric finite element model of a pipe–flange joint subjected to a multi-pass girth welding followed by an MSR process. Sequentially coupled non-linear transient thermo-mechanical analysis for multi-pass gas tungsten arc welding is first performed to calculate welding residual stresses. Subsequently separate parametric studies for three different types of MSR load including internal pressure, external pressure and axial pull are performed on the pre-stressed model, and stress relieving behaviour is studied. It is concluded that both the internal pressure and axial pull have a significant effect on residual stresses on the inner surface of the joint.

Crack initiation and propagation close to the interface in a ferrite–austenite joint

The safety assessment of welded structures and joints still remains an important industrial problem. In this study, a simple diffusion bonded bi-material joint has been made in order to analyze the plastic mismatch effect. It consists of an assembly of ferritic and austenitic steels which are representative of nuclear pressure vessel components. Tests were performed on various specimens including smooth and notched tensile bars, Charpy V-notch specimens and single-edge notch bend specimens. Homogeneous and bimaterial specimens were tested. On deeply notched specimens, the effect of the distance between the notch root and the interface on fracture initiation and crack propagation direction was studied. Tests were modeled using elasto-plastic finite element simulations. These simulations were post-processed in order to determine crack initiation based on the Rice and Tracey criterion according to the “local approach of fracture” procedure.

A New Approach to Evaluate Fracture Toughness of Structural Materials

A new method, designated as Spiral Notch Torsion Test (SNTT), is developed recently to measure the intrinsic fracture toughness KIC of structural materials. The SNTT overcomes many of the limitations inherent in traditional techniques and makes it possible to standardize fracture toughness testing. It is uniquely suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments, including others such as ceramics, their composites, and concrete. The SNTT system operates by applying pure torsion to uniform cylindrical specimens with a notch line that spirals around the specimen at a 45 deg pitch. The KIC values are obtained with the aid of a three-dimensional finite-element computer code, TOR3D-KIC, developed at Oak Ridge National Laboratory (ORNL). The fundamental mechanism of SNTT approach is also described in the paper.

Fracture Mechanics Treatment of Residual Stresses in Defect Assessment

Over the last decade, as more in-depth understanding of weld residual stresses is being achieved, particularly of their characteristic distributions in pressure vessel and piping components, the residual stress effects on stress intensities at welds are becoming better understood. In this paper, some of the important residual stress characteristics are first identified in the form of either “bending” dominated or “self-equilibrating” dominated types for girth welds. The applicability in other joint configurations in welded structures is then discussed, with a collection of validated residual stress distributions. The characterization of both “bending” and “self-equilibrating” types in residual stress distributions provides a consistent framework for stress intensity factor considerations in either fracture and fatigue assessment. The contribution of weld residual stresses to stress intensities at welds are shown to be in the form of Ksolutions under displacement controlled conditions. The “bending” type residual stresses provide a longer range of influence than “self-equilibrating” type in K solutions. The contribution of “self-equilibrating” type residual stresses to stress intensities is shown to be dominant when crack size is small, while the contribution of the “bending” type dominant for crack size up to a much larger size with respect to wall thickness.

Simulation of ratcheting and low cycle fatigue

Fatigue failure in the extremely low cycle and low cycle regimes is characterised by plasticity and high strains. Ratcheting as a secondary phenomenon of cyclic plasticity can accelerate fatigue damage or even act as the failure mechanism itself. The combination of the two damage phenomena and their interaction are of special interest. This paper will focus on the design by analysis of welded and non-welded pressure vessel components with respect to combined ratcheting and low cycle fatigue damage mechanisms. A combination of the local strain approach with a ratcheting check based on the non-linear kinematic constitutive law of Ohno and Wang is proposed. The method is verified by numerical fracture mechanics based on the cyclic J-integral. An algorithm for the determination of component fatigue curves is proposed.

Life Estimation for Components of Pressure Vessels and Pipelines Due to Corrosion under Stress

Life Estimation for Components of Pressure Vessels and Pipelines Due to Corrosion under Stress