Showing papers in "Journal of Pressure Vessel Technology-transactions of The Asme in 2011"

Mechanics of Pipes Conveying Fluids—Part II: Applications and Fluidelastic Problems

Abstract: This paper is the second part of the two-part review article presenting an overview of mechanics of pipes conveying fluid and related problems such as the fluid-elastic instability under conditions of turbulence in nuclear power plants. In the first part, different types of modeling, dynamic analysis and stability regimes of pipes conveying fluid restrained by elastic or inelastic barriers were described. The dynamic and stability behaviors of pinnedpinned, clamped-clamped, and cantilevered pipes conveying fluid together with curved and articulated pipes were discussed. Other problems such as pipes made of viscoelastic materials and active control of severe pipe vibrations were considered. The first part was closed by conclusions highlighting resolved and nonresolved controversies reported in the literature. The second part will address the problem of fluidelastic instability in single- and twophase flows and fretting wear in process equipment, such as heat exchangers and steam generators. Connors critical velocity will be discussed as a measure of initiating fluidelastic instability. Vibro-impact of heat exchanger tubes and the random excitation by the cross-flow can produce a progressive damage at the supports through fretting wear or fatigue. Antivibration bar supports used to limit pipe vibrations are described. An assessment of analytical, numerical, and experimental techniques of fretting-wear problem of pipes in heat exchangers will be given. Other topics related to this part include remote impact analysis and parameter identification, pipe damage-induced by pressure elastic waves, the dynamic response and stability of long pipes, marine risers together with pipes aspirating fluid, and carbon nanotubes conveying fluid. DOI: 10.1115/1.4001270

Thermomechanical Analysis of a Pressurized Pipe Under Plant Conditions

Abstract: This paper is concerned with the development of a methodology for thermo-mechanical analysis of high temperature, steam-pressurised P91 pipes in electrical power generation plant under realistic (measured) temperature and pressure cycles. In particular, these data encompass key thermal events, such as ‘load-following’ temperature variations and sudden, significant fluctuations in steam temperatures associated with attemperation events and ‘trips’ (sudden plant shut-down), likely to induce thermo-mechanical fatigue damage. An anisothermal elastic-plastic-creep material model for cyclic behaviour of P91 is employed in the transient FE model to predict the stress-strain-temperature cycles and the associated strain-rates. The results permit characterisation of the behaviour of pressurised P91 pipes for identification of the thermo-mechanical loading histories relevant to such components, for realistic, customised testing; this type of capability is relevant to design and analysis with respect to the evolving nature of power plant operating cycles, e.g. associated with more flexible use of fossil fuel plant to complement renewable energy sources.Copyright © 2011 by ASME

Optimal Design of Onshore Natural Gas Pipelines

Abstract: The optimal design level for onshore natural gas pipelines was explored through a hypothetical example, whereby the pipe wall thickness was assumed to be the sole design parameter. The probability distributions of the life-cycle costs of various candidate designs for the example pipeline were obtained using Monte-Carlo simulation. The life-cycle cost included the cost of failure due to equipment impact and external corrosion, and the cost of periodic maintenance actions for external corrosion. The cost of failure included both the cost of fatality and injury as well as the cost of property damage and value of lost product. The minimum expected life-cycle cost criterion and stochastic dominance rules were employed to determine the optimal design level. The allowable societal risk level was considered as a constraint in the optimal design selection. It was found that the Canadian Standard Association design leads to the minimum expected life-cycle cost and satisfies the allowable societal risk constraint as well. A set of optimal designs for a risk-averse decision maker was identified using the stochastic dominance rules. Both the ASME and CSA designs belong to the optimal design set and meet the allowable societal risk constraint.

Fluidelastic Instability in a Normal Triangular Tube Bundle Subjected to Air-Water Cross-Flow

Abstract: This paper presents the results of tests on the vibration of a normal triangular tube bundle subjected to air–water cross-flow. The pitch-to-diameter ratio of the bundle is 1.5, and the tube diameter is 38 mm. The tubes were preferentially flexible in one direction. Both the lift and the drag direction were tested. A wide range of void fractions and fluid velocities was tested. Fluidelastic instabilities and tube resonances were observed. The resonances induced significant vibration amplitudes at high void fractions in the lift direction. The results are compared with those obtained with a rotated triangular tube bundle. They show that the normal triangular configuration is more stable than the rotated triangular configuration.

Buckling of Thin-Walled Long Steel Cylinders Subjected to Bending

Abstract: The present paper investigates structural response and buckling of long unstiffened thin-walled cylindrical steel shells, subjected to bending moments, with particular emphasis on stability design. The cylinder response is characterized by cross-sectional ovalization, followed by buckling (bifurcation instability), which occurs on the compression side of the cylinder wall. Using a nonlinear finite element technique, the bifurcation moment is calculated, the post-buckling response is determined, and the imperfection sensitivity with respect to the governing buckling mode is examined. The results show that the buckling moment capacity is affected by cross-sectional ovalization. It is also shown that buckling of bent elastic long cylinders can be described quite accurately through a simple analytical model that considers the ovalized prebuckling configuration and results in very useful closed-form expressions. Using this analytical solution, the incorporation of the ovalization effects in the design of thin-walled cylinders under bending is thoroughly examined and discussed, considering the framework of the provisions of the new European Standard EN1993-1-6.

Accurate Determination of Plastic Collapse Loads From Finite Element Analyses

Abstract: When computational modeling is used to evaluate the true strength of an imperfect elastic-plastic shell structure, the current European standard on shell structures requires that two reference strengths are always determined. the linear bifurcation load and the plastic limit (plastic collapse) load. These two loaas are used in more than one way to characterize the strength of all imperfect elastic-plastic systems. Where parametric studies of a problem are being undertaken, it is particularly important that these two loads are accurately defined, since all other strengths will be related to them. For complex problems in shell structures, it is not possible to develop analytical solutions for the plastic collapse strength, and finite element analysis must be used. Unfortunately, because a collapse mechanism often requires the development of very extensive plasticity involving large local strains, and the collapse load is simply at the end of a slowly rising load-deflection curve, it is sometimes difficult for the analyst to accurately determine this plastic collapse strength. This paper describes two methods, based on modifications of the Southwell plot, of obtaining very accurate evaluations of the plastic limit load, irrespective of whether a fairly complete plastic strain field has developed or not. These two methods allow plastic collapse limit loads to be reported with great precision.

Plastic Buckling of Cones Subjected to Axial Compression and External Pressure

Abstract: The paper provides details about buckling tests on six steel cones and the corresponding numerical estimates of failure load (asymmetric bifurcation and/or collapse). Test models were machined from 250 mm billet. The wall thickness was 2 mm, small-end radius was 74.0 mm and the large radius end was 100 mm. The semi-cone angle was 14 deg. Cones had substantial, and integral top and bottom flanges. Experimental failure loads were obtained for: (i) the first two cones subjected to axial compression, (ii) subsequent two cones subjected to external pressure, and (iii) the remaining two models subjected to combined action of external pressure and axial compression. The magnitude of test pressure was about 5 MPa, and the axial failure load was approximately 230 kN. Good repeatability of experimental failure loads was obtained. Numerical estimates of failure loads were obtained for elastic perfectly plastic, engineering stress-strain, and true stress–true strain modelling of steel. Apart from axisymmetric modelling of shells, true geometry with true wall thickness distribution was adopted in calculations. Some of the numerical estimates of buckling loads are close to test data but other are not. The reasons for these discrepancies are highlighted in the paper.Copyright © 2011 by ASME

A Tetraparametric Metamodel for the Analysis and Design of Bolting Sequences for Wind Generator Flanges

Abstract: This paper presents a metamodel that enables estimation of the elastic interaction that occurs in the bolted joints of a wind generator tower during the tightening sequence. In this kind of joint, there is a gap between the contact surfaces of the flanges. The metamodel is composed of four parameters, which are enough to simulate the response of the flange under the tightening loads of the bolts. Even though the behavior of the joint is nonlinear because of the gap, the parameters are obtained from two simple linear elastic analyses of a finite element (FE) model of the flange. The corresponding loss of load in the bolts has been estimated for various sequences with minimum computational cost. Thus, there is no need for costly experimental measurements or nonlinear FE simulations.

Buckling of Thin Cylindrical Shells Under Locally Elevated Compressive Stresses

Abstract: Thin cylindrical shells used in engineering applications are often susceptible to failure by elastic buckling. Most experimental and theoretical research on shell buckling relates only to simple and relatively uniform stress states, but many practical load cases involve stresses that vary significantly throughout the structure. The buckling strength of an imperfect shell under relatively uniform compressive stresses is often much lower than that under locally high stresses, so the lack of information and the need for conservatism have led standards and guides to indicate that the designer should use the buckling stress for a uniform stress state even when the peak stress is rather local. However, this concept leads to the use of much thicker walls than is necessary to resist buckling, so many knowledgeable designers use very simple ideas to produce safe but unverified designs. Unfortunately very few scientific studies of shell buckling under locally elevated compressive stresses have ever been undertaken. The most critical case is that of the cylinder in which locally high axial compressive stresses develop extending over an area that may be comparable with the characteristic size of a buckle. This paper explores the buckling strength of an elastic cylinder in which a locally high axial membrane stress state is produced far from the boundaries (which can elevate the buckling strength further) and adjacent to a serious geometric imperfection. Care is taken to ensure that the stress state is as simple as possible, with local bending and the effects of internal pressurization eliminated. The study includes explorations of different geometries, different localizations of the loading, and different imperfection amplitudes. The results show an interesting distinction between narrower and wider zones of elevated stresses. The study is a necessary precursor to the development of a complete design rule for shell buckling strength under conditions of locally varying axial compressive stress.

Use of Functionally Graded Material Layers in a Two-Layered Pressure Vessel

Abstract: This work explores the possibilities of using functionally graded material (FGM) layers to reduce normal and shear stress gradients due to internal pressure and thermal loadings at the interface of a two-layered wall pressure vessel. The two walls are made of an internal thin metallic layer (titanium used as a liner to avoid a chemical/physical reaction between the gas and the external layer) and an external thick layer (carbon fiber used as a structural restraint). Two main geometrical elements are investigated: a cylindrical shell and a spherical panel. The shell analysis has been made by referring to mixed layerwise theories, which lead to a three-dimensional description of the stress/strain fields in the thickness shell direction; results related to the first order shear deformation theory are given for comparison purposes. It has been concluded that it is convenient to use FGM layers to reduce shear and normal stress gradients at the interfaces. Furthermore, the FGM layers lead to benefits as far as buckling load is concerned; lower values of in-plane shear and longitudinal compressive stresses are, in fact, obtained with respect to a pure two-layered wall.

High Temperature Design and Damage Evaluation of MOD.9Cr-1Mo Steel Heat Exchanger

Abstract: High temperature design and evaluation of creep-fatigue damage for sodium-sodium heat exchanger, DHX (Decay heat exchanger) in a sodium test loop have been conducted. The DHX is a shell- and tube-type heat exchanger with outer diameter of 21.7mm, thickness of 1.65mm and effective length of 1.73m. The DHX shell and tube materials were Mod.9Cr-1Mo steel. The temperatures of shell inlet and shell outlet in the DHX are 510°C and 308°C, respectively, while the temperatures of tube inlet and outlet are 254°C and 475°C, respectively. Three dimensional finite element analysis was conducted for the DHX and evaluation of creep-fatigue damage at several critical locations of the heat exchanger was carried out according to the elevated temperature design codes of the ASME Section III Subsection NH and RCC-MR. Evaluations on the integrity of the DHX and code comparisons were carried out for the critical locations of the DHX.Copyright © 2011 by ASME

Equivalent Elastic Constants of Truss Core Sandwich Plates

Abstract: A plate structure of a triangular truss core sandwiched by two panels is treated as an equivalent homogeneous laminated plate by obtaining equivalent anisotropic elastic constants The equivalent elastic constants are obtained by considering generalized Hook’s law of a three dimensional elastic body with no a priori assumption and the equilibrium of a segment deformed by bending moments To verify the accuracy of the equivalent elastic constants, a linear static analysis of sandwiched aluminum plates subjected to lateral pressure is carried out The results of the finite element analysis applied to the equivalent laminated plates are compared with those of a NASTRAN analysis of the original structural layouts The results are also compared with a closed-form solution, which simplifies the sandwiched plate as a homogeneous orthotropic thick plate continuum (Lok and Cheng, 2000, “Elastic Stiffness Properties and Behavior of Truss-Core Sandwich Panel,” J Struct Eng, 126(5), pp 552–559) As the maximum deflections of three analyses agreed closely, one has assurance that the method of the homogeneous plate with equivalent elastic constants is valid and useful

The New Framework for Shell Buckling Design and the European Shell Buckling Recommendations Fifth Edition

Abstract: This paper outlines key aspects of the new European Standard on the Strength and Stability of Metal Shells EN 1993-1-6 with its extended commentary and expansion in the fifth edition of the European Recommendations on Shell Buckling. This European design standard is the first to be strongly oriented toward numerical analyses in design, with clear distinctions between different classes of both analysis and fabrication. It presents a different style of standard: Each limit state is defined in a separate chapter, but all shell geometries are treated and all analysis types are used within each chapter. The strength evaluation criteria differ according to the calculation that has been made. This new structure, with its new paradigm that permits generalization of the design procedure for all thin shells, geometries, load cases, boundary conditions, and qualities, represents a major step forward. It also offers the opportuniy for future research studies of shell structures to be undertaken within a coherent conceptual framework that is completely general. The EN 1993-1-6 standard goes a long way toward bridging the gap between the computational engineering mechanics and structural engineering design communities. Unfortunately, this European standard EN 1993-1-6 has a complex and extensive background that cannot be stated within the document so the European Recommendations on Shell Stability, now published in its fifth edition, gives an extensive commentary, many expanded rules, and many additional geometries and load cases that are not formally presented within the standard itself. The development of both EN 1993-1-6 and the recommendations has been the work of the Eurocode shell structures development committee CEN/TC250/SC3/PT4 and the European Convention for Constructional Steelwork committee (ECCS) TWG 8.4. It is presented here by the convener of these two committees. This paper explains the reasoning behind several particular choices that have been taken in developing the standard, occasionally running counter to traditional views. It also identifies several tricky issues that have not been addressed well in the shell buckling literature but that have arisen through the attempts to achieve completely general rules and which need imaginative answers to ensure a fully consistent treatment of all systems. It is hoped that this paper will assist researchers and designers to understand the rules and recommendations and will encourage researchers to undertake and present their work in a manner that permits its rapid adoption into the new standardized design procedures.

Burst Pressure Determination of Vehicle Toroidal Oval Cross-Section LPG Fuel Tanks

Abstract: This study addresses the prediction of the burst pressures and burst failure locations of the vehicle toroidal liquefied petroleum gas (LPG) fuel tanks using both experimental and finite element analysis (FEA) approaches. The experimental burst test investigations were carried out hydrostatically in which the cylinders were internally pressurized with water. The FEA modeling processes of these LPG fuel tanks subjected to incremental internal uniform pressure were performed in the nonlinear field. Two different types of nonlinear models, plane and shell, were developed and evaluated under nonuniform and axisysm-metric boundary conditions. The required actual shell properties including weld zone and shell thickness variations were also investigated and used in the computerized modeling processes. Therefore, the results of the burst pressures and their failure locations were predicted and compared with experimental ones.

Synchrotron Radiation Study on Alloy 617 and Alloy 230 for VHTR Application

Abstract: High-energy synchrotron radiation has proven to be a powerful technique for investigating fundamental deformation processes for various materials, particularly metals and alloys. In this study, high-energy synchrotron X-ray diffraction (XRD) was used to evaluate Alloy 617 and Alloy 230, both of which are top candidate structural materials for the Very-High-Temperature Reactor (VHTR). Uniaxial tensile experiments using in-situ high-energy X-ray exposure showed the substantial advantages of this synchrotron technique. First, the small volume fractions of carbides, e.g. ∼6% of M6 C in Alloy 230, which are difficult to observe using lab-based X-ray machines or neutron scattering facilities, were successfully examined using high-energy X-ray diffraction. Second, the loading processes of the austenitic matrix and carbides were separately studied by analyzing their respective lattice strain evolutions. In the present study, the focus was placed on Alloy 230. Although the Bragg reflections from the γ matrix behave differently, the lattice strain measured from these reflections responds linearly to external applied stress. In contrast, the lattice strain evolution for carbides is more complicated. During the transition from the elastic to the plastic regime, carbide particles experience a dramatic loading process, and their internal stress rapidly reaches the maximum value that can be withstood. The internal stress for the particles then decreases slowly with increasing applied stress. This indicates a continued particle fracture process during plastic deformations of the γ matrix. The study showed that high-energy synchrotron X-ray radiation, as a non-destructive technique for in-situ measurement, can be applied to ongoing material research for nuclear applications.Copyright © 2011 by ASME

Energy Based Modeling of Ultra High-Pressure Waterjet Surface Preparation Processes

Abstract: Ultra high-pressure waterjets (UHP-WJ) have been emerging as a viable method for surface texturing, cleaning, and peening of metallic materials. Previous experimental studies have suggested that removal of material can be related to the energy density of the waterjet impinging upon the workpiece, rather than the net energy. The net energy transferred to the workpiece is a function of four key process parameters, namely, (i) orifice diameter, (ii) orifice geometry, (iii) supply pressure, and (iv) traverse rate. The energy density also incorporates jet spreading as well as flow rate and impulse pressure distributions within the waterjet. In this paper, a novel representation of the power distribution within the waterjet is presented, as well as a relationship governing jet-material interaction. Empirical validation on a Ti-6Al-4V titanium alloy is presented, with good correlation noted between the predicted and experimental results.

Study on the Effects of Switching Temperature on the Thermal Fatigue Life of the Shell-to-Skirt Junction of Coke Drum

Abstract: In spite of the fact that coke drums are subjected to cyclic thermal and mechanical loads, generally, they are not designed for cyclic loads. Thus, their operational life is much shorter than other pressure equipment in refineries. Due to information developed from surveys, it was determined that the major typical location of failure due to thermal fatigue in coke drums is the shell-to-skirt junction area. This paper focuses on temperature and stress characteristics and also the thermal fatigue life of the junction area. The main objective of this paper was to explore effect of the switching temperature on thermal fatigue life of the junction area. Four coke drums, currently in service have been considered in the analyses, named drums A, B, C, and D, identical in dimensions and with an operating cycle period of 48 h. Operational temperatures and strains have been measured and collected every minute. The number of measured cycles of coke drum A, B, C, and D were 52, 53, 53, and 54 cycles, respectively. Thus, a total of 212 cycles have been analyzed. The operational temperatures and strains were examined. Finite Element Method (FEM) analyses have been performed on the selected cycles in order to find the most severe location in the junction area. The strain history and FEM results were used to assess thermal fatigue life. The thermal fatigue lives were calculated based on low cycle fatigue properties using engineering steels for high temperature components issued by National Institute for Materials Sciences (NIMS) in Japan. The number of cycle to fracture versus switching temperature for the coke drums was then plotted. The curve best fitting criteria was then used to develop an equation relating the number of cycle to fracture as a function of switching temperature. The results show that the switching temperature strongly affects the number of cycle to fracture. These results can be used to provide the necessary information to operate coke drums safely in order to extend their useful lives.

Time Domain Simulation of the Vibration of a Steam Generator Tube Subjected to Fluidelastic Forces Induced by Two-Phase Cross-Flow

Abstract: A computer simulation of the vibration of a tube bundle subjected to fluidelastic forces induced by two-phase cross flow is carried out. Two fluidelastic instability models are compared in the simulations: the Connors model and the quasi-steady model. In the quasi-steady model, the fluid forces are expressed in terms of the quasi-static drag and lift force coefficients and their derivatives which are determined experimentally. The consideration of the angle of incidence induced by the relative tube displacement with respect to the fluid introduces a velocity dependent term in the fluid force expression. The simulation has been done using ABAQUS. The ABAQUS user subroutine VUEL provides the required interface and information to calculate and apply the fluid forces to the structure. The fluidelastic forces applied to each element are calculated using the element displacement and velocity, the tube instantaneous frequency, a flow retardation parameter and the fluid damping and stiffness components. A typical U-tube in a steam generator subjected to a non-uniform two-phase flow was considered in the simulations. The tube support contact was modeled using the ABAQUS contact pair algorithm. The Anti-Vibration-Bars (AVB) limiting the tube vibration in the out-of-plane direction in the U-bend region were also included in this model. As the simulation was nonlinear because of the loose supports, a Fast Fourier Transform technique was used to estimate the tube instantaneous frequency. The quasi-steady fluidelastic force was compared to the fluidelastic force of the Connors model. The instability growth rate of the Connors model was found to be higher than that of the quasi-steady model. The impact forces at the supports and the AVB are also extracted. These forces can be used to calculate the work rate and estimate the tube wear.Copyright © 2011 by ASME

Permeability Measurement of Graphite Compression Packings

Abstract: In this work, we address the issue of the sealing performance of ring-shape valve compression packings. Our analysis is focused on the characterization of the permeability of the rings made of die-formed exfoliated graphite. Because of the tight character of the material, significant Klinkenberg effects are expected. In addition, due to the manufacturing process, permeabilities kz and kr as well as Klinkenberg coefficients bz and br in the respective axial and radial directions are markedly different and strongly dependent upon the applied stress. A specific experimental device based on pressure pulse decay of nitrogen through the material was designed for the measurement in each direction under a controlled axial compression. Determination of kz and kr and bz and br is performed on the basis of a nonstationary gas flow model in the radial and axial directions using an inverse procedure applied to the pressure decay signal. Our results confirm the efficiency of the method developed here. They clearly show the anisotropic character of the material (kz is roughly one order of magnitude larger than kr) and the dependence upon axial compression. The present analysis is the key step before further quantification of the leak rate that may result from the permeation through the material as envisaged here as well as through interfaces between the housing, the packings, and the stem.

Identification of the Cause of Variability of Probability of Failure for Burst Models Recommended by Codes/Standards

Abstract: Failure probability of oil and gas pipelines due to external corrosion defects can be estimated using corrosion growth model and the evaluation of remaining strength Codes/standards have been developed for the assessment of the remaining strength of corroded pipeline The remaining strength and the operating pressure were considered to develop the limit state equation and consequently the failure probability of the burst models recommended by codes/standards In the present paper, comparative analyses of the failure probability estimated by the codes/standards were conducted, using Monte Carlo simulation and first order second moment methods The analysis revealed that the failure probability of the burst models recommended by codes/standards varies significantly for the same defects size The study further explored the cause of variability in failure probabilities The study observed that different defect shape specifications (rectangular, parabolic, etc) and different stress concentration factor derivations (different contributions of l) for burst pressure estimation are responsible for high variability in the probability of failure It is important to reduce variability to ensure unified risk-based design approach considering any codes/standards © 2011 American Society of Mechanical Engineers

Numerical Investigation on Heat Transfer and Residual Stress in a Butt Welded Plate

Abstract: It is a well known fact that during welding, the metal at the welding zone gets melted and then solidifies, which results in shrinkage in all directions. Residual strain and stress distributions coming from shrinking are largely influenced by the nature and configuration of the welding process, metallurgical characteristics of weld, and the geometrical shape of the weld joint. The residual stress mainly depends on the thermal history cycle through which the specimen undergoes in the welding process. So these thermal history cycles are to be known in order to get a better knowledge of the welding phenomenon and to minimize the risk of failures. In this work, a detailed analysis has been carried out for predicting the heat flow pattern and stress distribution in an aluminum alloy plate during welding. In this study, the modified double ellipsoidal heat source distribution pattern is modeled and considered for the weld pool design. Elastic-plastic material properties at various temperatures are also considered for simulation. A detailed finite element analysis is carried out to predict the welding residual stress. In this, thermal analysis is carried out for actual variable welding speed and these transient thermal histories at various locations were numerically predicted and compared with experimental results. Further, these thermal results are used to predict the residual stress on the weld plate using finite element method.

A New OCTG Strength Equation for Collapse Under External Load Only

Abstract: The influence of manufacturing technology and imperfections on casing collapse strength is not considered in API bulletin 5C3. In the meantime, the current casing true collapse strength cannot be predicted accurately by API bulletin 5C3. For these cases, the joint API/ISO work group ISOTC67 SC5 WG2b has proposed the current API bulletin 5C3 and presented a new collapse strength model with manufacturing imperfections, such as ovality, eccentricity, residual stress, etc., which improves the casing strength calculation accuracy and benefits much more for casing strength design rather than just using API bulletin 5C3. The study on the new ISO collapse model has found that it cannot be used to predict the collapse strength of all pipe sizes, and the current study results are greatly different from actual conditions, which is not the last goal to propose API bulletin 5C3 for the joint API/ISO work group ISOTC67 SC5 WG2b. So, based on the new ISO collapse model, a new oil country tubular goods (OCGT) strength equation, “new equation” for predicting all OCTG sizes collapse strength under external load only has been presented in this paper. Numerical and experimental comparisons show that there is a safety margin appropriate to the desired target reliability level between the predictive values calculated by the new equation and test collapse data, and the new equation calculation accuracy is higher than that of API and ISO, which will make great improvements in the casing design of deep and superdeep wells on the basis of guaranteeing casing material safety.

Damping of Heat Exchanger Tubes in Liquids: Review and Design Guidelines

Abstract: This paper addresses the question of damping of multispan heat exchanger tubes with liquids (mostly water) on the shell side. The different energy dissipation mechanisms that contribute to damping are investigated. The available experimental data from the literature and from our own measurements are reviewed and analyzed. Three important energy dissipation mechanisms emerge. These are viscous damping between the tube and liquid, squeeze-film damping in the clearance between the tube, and support and friction damping at the support. Viscous damping only accounts for approximately 25% of the total damping of a typical tube. Thus, about 75% of the damping energy is dissipated at the support. Squeeze-film damping appears to be the most important energy dissipation mechanism. Squeeze-film damping is related to the support width and is inversely proportional to the tube frequency. Damping is formulated in terms of tube and tube-support parameters. Semi-empirical formulations for damping of heat exchanger tubes in liquids are recommended for design purposes.

The Effects of Residual Stress Distribution and Component Geometry on the Stress Intensity Factor of Surface Cracks

Abstract: The stress intensity factor estimated by the appropriate modeling of components is essential for the evaluation of crack growth behavior in stress corrosion cracking. For the appropriate modeling of a welded component with a crack, it is important to understand the effects of residual stress distribution and the geometry of the component on the stress intensity factor of the surface crack. In this study, the stress intensity factors of surface cracks under two assumed residual stress fields were calculated. As residual stress field, a bending type stress field (tension-compression) and a self-equilibrating stress field (tension-compression-tension) through the thickness were assumed, respectively. The geometries of the components were plate and piping. The assumed surface cracks for those evaluations were a long crack in the surface direction and a semi-elliptical surface crack. In addition, crack growth evaluations were conducted to clarify the effects of residual stress distribution and the geometry of the component. Here, the crack growth evaluation means simulating increments of crack depth and length using crack growth properties and stress intensity factors. The effects of residual stress distribution and component geometry on the stress intensity factor of surface cracks and the appropriate modeling of cracked components are discussed by comparing the stress intensity factors and the crack growth evaluations for surface cracks under residual stress fields.

Modeling Hydraulically Expanded Tube-to-Tubesheet Joint Based on General Stress-Strain Curves of Tube and Tubesheet Materials

Abstract: The strength of tube-to-tubesheet joints is crucial for the joint integrity and reliability of the tubular heat exchangers. The joint strength measured by residual contact pressure is affected by several design parameters, such as the yield strength and strain hardening of the tube and tubesheet materials, initial radial clearance between the tube and tubesheet hole, and the magnitude of the expansion. It is very important to determine the expansion pressure and the residual contact pressure in designing and manufacturing tube-to-tubesheet joints by the hydraulic expansion process. In this paper, a general strain-hardening material model and analytic expressions for calculating the expansion pressure and the residual contact pressure, considering the effect of the initial clearance and the material strain hardening, have been derived. The results predicted by the present model have been compared with the results predicted by elastic perfectly plastic model, linear strain-hardening model, and the nonlinear finite element analysis results. The comparison results show that the present analytic expressions can model the effects of strain-hardening of the materials and the clearance well. The models, such as elastic-perfect model, linear strain-hardening model, and power strain-hardening model, are the special cases of the present model. The parameters needed in the present model are determined by curve fitting of the actual tensile stress-strain data of tube and tubesheet materials, respectively.