Creep analysis of bolted flange joints

In the petrochemical and nuclear industries, the difficulty in assessing the effect of creep on the tightening load of bolted flanged connections is recognized. Under high temperatures, the leak tightness of bolted joints is compromised due to the loss of the bolt load as a result of creep of not only the gasket and bolt materials but also the flange material. Apart from acknowledgment of this effect, there exists no established design calculation procedure that accounts for creep. This is because the relaxation of the bolt load and the corresponding loss of the gasket contact stress are not easy to assess analytically. The main objective of the work is the development of a simple analytical solution to the creep-relaxation problem encountered in bolted flange connections of the float type. Particular emphasis is put towards relaxation caused by the flange and bolt material creep.     

The modelling of bolted flange joints used with disc springs and tube spacers to reduce relaxation

Bolted flange joints are prone to leakage when exposed to high temperature. In several cases, the root cause is relaxation that takes place as a result of material creep of the gasket, the bolt and the flange. One way to overcome this problem is to make the joint less stiff by introducing disc springs or the use of longer bolts with spacers. Although widely used, these two methods have no reliable analytical model that could be used to evaluate the exact number of washers or length of the bolts required to reduce relaxation to a minimum acceptable level.This paper describes an analytical model based on the flexibility and deflection interactions of the joint different elements including the axial stiffness of the flange and bolts, used to evaluate relaxation. The developed analytical flange model can accommodate either disc springs or longer bolts with spacer tubes to reduce the bolt load loss to a maximum acceptable value. This model is validated by comparison with the more accurate FEA findings. Calculation examples on a bolted flanged joint are presented to illustrate the suggested analytical calculation procedure.     

Analysis of externally loaded bolted joints: Analytical,computational and experimental study

The behaviour of a simple single-bolted-joint under tensile separating loads is analysed using conventional analytical methods, a finite element approach and experimental techniques. The variation in bolt force with external load predicted by the finite element analysis conforms well to the experimental results. It is demonstrated that certain detailed features such as thread interaction do not need to be modelled to ensure useful results. Behaviour during the pre-loading phase of use agrees with previous long-standing studies. However, the pre-loading analysis does not carry over to the stage when external loading is applied, as is normally assumed and it is shown that the current, conventional analytical methods substantially over-predict the proportion of the external load carried by the bolt. The basic reason for this is shown to be related to the non-linear variation in contact conditions between the clamped members during the external loading stage.     

On the use of plate theory to evaluate the load relaxation in bolted flanged joints subjected to creep

The effect of load relaxation due to creep has a major impact on the tightness of bolted flanged joints. Several leak incidents have been reported in the petrochemical and nuclear power plants related to creep. It is clear that the current flange design code does not address adequately the effect of high temperature on the mechanical and leakage integrities of bolted flange joints used in pairs or with blind cover plates. It is possible to include the creep analysis in the design methodology.

This paper outlines an analytical approach based on plate theory used to determine the creep effect of blind cover plates and flanges of a small size on the bolt and subsequently the gasket load relaxation in bolted flanged joints. It details the theoretical procedure to predict the creep effect of those flanges considered as plates. The results from the theoretical model are verified by comparison with 3D finite-element results.     

The effect of material difference and flange nominal size on the sealing performance of new gasketless flanges

This paper deals with a new seal system between flange joints without using a gasket. This gasketless flange includes a groove and an annular lip that is machined in one of the flange rings which when removed being in contact with the other flange to form a seal line when the flanges are assembled. In this study, firstly, fundamental dimensions are examined for unplasticized polyvinyl chloride (PVC-U JIS) to obtain the best sealing performance. Then, the effects of material difference and flange nominal size upon the sealing performance of the new gasketless flange are investigated for two types of materials, 0.25% carbon steel (S25C JIS) and PVC-U. It is found that the critical internal pressure at which leakage appears is mainly controlled by the maximum stress at the annular lip for each material even if the flange nominal sizes are different. The gasketless flange made by PVC-U shows the higher critical internal pressure compared with the case of S25C if the same clamping forces are applied. The effect of stress relaxation for PVC-U on the sealing performance is also considered. Then, it may be concluded that this PVC-U gasketless flange as well as S25C has good sealing performance.     

A thermomechanical model for the analysis of disc brake using the finite element method in frictional contact

Abstract

In this work, a numerical simulation of the transient thermal analysis and the static structural one was performed here sequentially, with the coupled thermo-structural method using the ANSYS software. Numerical procedure of calculation relies on important steps such that the CFD thermal analysis has been well illustrated in 3D, showing the effects of heat distribution over the brake disc. This CFD analysis helped in the calculation of the values of the thermal coefficients (h) that have been exploited in the 3D transient evolution of the brake disc temperatures. Three different brake disc materials were selected in this simulation and comparative analysis of the results was conducted in order, to derive the one with the best thermal behavior. Finally, the resolution of the coupled thermomechanical model allows to visualize other important results of this research such as; the deformations, and the equivalent stresses of Von Mises of the disc, as well as the contact pressure of the brake pads. Following our analysis and results we draw from it, we derive several conclusions. The choice makes it possible to deliver the best brake rotor so as to ensure and guarantee the good braking performance of the vehicles.     

A coupled elastoplastic-transient hydrogen diffusion analysis to simulate the onset of necking in tension by using the finite element method

Hydrogen-enhanced localized plasticity (HELP) is an acceptable mechanism for hydrogen embrittlement which is based on the experimental observations and the theoretical computations. The underlying principle in the HELP theory is that the presence of hydrogen causes the localization of the slip bands which results in the decrease of the fracture strength. In a sample under plane-strain tensile stress, plastic instability can lead to either the concentration of plastic flow in a narrow neck or bifurcation from homogeneous deformation into a mode of an exclusively localized narrow band of intense shear. Recently, it has been demonstrated that the presence of hydrogen can indeed induce shear banding bifurcation at macroscopic strains. By using a steady-state equilibrium equation for hydrogen diffusion analysis, the effect of hydrogen on the bifurcation of a homogeneous deformation in a plane-strain tension specimen into a necking or a shear localization mode of deformation has already been studied. In the present research, using a transient hydrogen diffusion analysis and introducing a new constitutive equation accompanied by considering the reduction in the local flow stress upon hydrogen dissolution into the lattice, the effect of hydrogen on shear localization is investigated. In addition, progress has been made in that, the changes in the distribution of the total and trapping hydrogen concentrations through the loading time and particularly during the development of the necking event have been determined.     

Experimental study of low-cycle fatigue of pipe elbows with local wall thinning and life estimation using finite element analysis

Low-cycle fatigue tests were conducted using elbow specimens with local wall thinning. Local wall thinning was machined on the inside of the elbow in order to simulate metal loss from erosion corrosion. The local wall thinning was located in three different areas known as the extrados, crown and intrados. The elbow specimens were subjected to cyclic in-plane bending under displacement control without internal pressure. In addition, three-dimensional elastic-plastic analyses were also carried out using the finite element method. As a result, the crack penetration area and the crack growth direction were successfully predicted by the analyses. The fatigue lives estimated by the analyses were close to those obtained by the experiments.     

Three-dimensional finite element analysis to predict the effects of SAW process parameters on temperature distribution and angular distortions in single-pass butt joints with top and bottom reinforcements

Achieving adequate top and bottom reinforcement is important to minimize angular distortions in single-pass submerged arc welded (SAW) butt joints. This is achieved in the present work by using a reusable flux-filled backing strip and proper SAW process parameters without resorting to costly distortion mitigation techniques. The butt joints were made without edge (square butts) preparation. The process was also modeled by using three-dimensional finite element analysis by incorporating the top and bottom reinforcements into the modeling. Filler material deposition was also simulated. Temperature distributions and angular distortions obtained from the modeling closely matched with the experimental values. Thus, the cost effective experimental methodology established in the present work can be utilized for minimizing angular distortions in SAW square butts. The modeling methodology adopted can be used for predicting the angular distortions in SAW square butts with top and bottom reinforcements.     

变截面管道消声特性的边界元方法研究

针对管道消声器的消声特性,采用边界元方法研究了变截面管道对消声器消声特性的影响.分析了不同形式的膨胀室和最大扩张截面位置对管道消声特性的影响规律.计算结果显示:直线和圆弧模型的膨胀室消声器在850~1 000 Hz范围内的消声特性优于矩形膨胀腔,而膨胀室截面最大扩张位置对传递损失的影响是关于扩张段中心相对称的;当最大扩张截面位于消声器两端时,传递损失在频率大于1 100 Hz时变化不大.研究可为消声器的设计提供一定的参考.     

Improved polymer electrolyte fuel cell performance with membrane electrode assemblies using modified metallic plate: Comparative study on impact of various coatings

Bipolar plate is one of the key components of polymer electrolyte membrane fuel cell. In the present study, metallic plates are explored as bipolar plates in comparison to most generally used high-density graphite plates. Among various metals, stainless steel 316L is preferred due to its low cost, high strength, ease of machining and for its corrosion resistance characteristics. However, the challenges associated with metallic plates are high interfacial contact resistance due to passive oxide layer formation and possible corrosion product during operation in chemically harsh environments, which may contaminate the membrane electrode assembly. Three electrically conductive and corrosion resistant coatings namely Titanium Nitrides, Plasma Nitride, and Gold have been coated over the surface of stainless steel 316L metallic plate to overcome these challenges and to explore their impact on fuel cell performance using standard membrane electrode assemblies. These coatings are characterized by X-Ray Diffraction, Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy along with interfacial contact resistance measurements. Further, the coated SS plates have been tested in real time polymer electrolyte membrane fuel cell operation for their use as bipolar plates and their performances have been compared with the fuel cell comprising conventional graphite plates. A cell comprising Titanium Nitride, Gold and Plasma Nitride coated metallic plates exhibit a power density of 430, 720 & 268 mW cm⁻² respectively, at an operating fuel cell potential of 0.6 V. Gold coated metallic plate shows comparable polymer electrolyte membrane fuel cell performance in relation to conventional graphite plate.     

A study on the advanced performance of an absorption heater/chiller with a solution preheater using waste gas

A large amount of consumed energy is released to the environment as waste heat, which may be used directly in some applications for useful purposes. Thus, from the standpoint of energy conservation, it will be meaningful to investigate systems with waste heat utilization. It has long been indicated that absorption cycles may have good potential applications for enhancing energy conservation via waste heat recovery. As such they have often been identified as an appropriate subject for research and development. Along this line, this paper also examines the development of an absorption cycle with waste heat utilization. More specifically, this study investigates a double-effect LiBr–water absorption cycle which uses exhaust gases from the burner of the high-temperature generator to preheat the weak absorbent solution on its way from the absorber to the low-temperature generator. The overall performance of the absorption heater/chiller system is analyzed and discussed on the basis of experimental results.     

A study of the effect of compression on the performance of polymer electrolyte fuel cells using electrochemical impedance spectroscopy and dimensional change analysis

Compression plays an important role in the performance of polymer electrolyte fuel cells (PEFCs). In this study, dynamic compression is applied using a cell compression unit (CCU) to study the effect on performance of a membrane electrode assembly (MEA) with dimension change. The stress/strain characteristics of the MEA are observed to be dominated by the gas diffusion layer (GDL), with the GDL exhibiting a degree of plasticity. Electrochemical impedance spectroscopy (EIS) is used to delineate the effect of compression on contact resistance and mass transfer losses.     

A numerical study on the performance of PEMFC with wedge-shaped fins in the cathode channel

The gas flow field design has a significant influence on the overall performance of a proton exchange membrane fuel cell (PEMFC). A single-channel PEMFC with wedge-shaped fins in the cathode channel was proposed, and the effects of fin parameters such as volume (0.5 mm³, 1.0 mm³, and 1.5 mm³), number (3, 5, and 9), and porosity of the gas diffusion layer (GDL) (0.2, 0.4, 0.6, and 0.8) on the performance of PEMFC were numerically examined based on the growth rate of power density (GRPD) and polarization curve. It was shown that wedge-shaped fins could effectively improve the PEMFC performance. With an increase in fin volume, the distributions of oxygen mass fraction in the outlet area of the cathode channel were lower, the drainage effect of the PEMFC improved, and GRPD also increased accordingly. Similar results were obtained as the number of fins increased. The GDL porosity had a greater effect than the wedge-shaped fins on the improvement in PEMFC performance, but the influence of GDL porosity weakened and the GRPD of porosity decreased as the porosity increased. This study provides an effective guideline for the optimization of the cathode channel in a PEMFC.     

A study of thermal performance with frosting under forced convection on a flat plate: Prediction of frost growth using nonhomogeneous property

The purpose of this study is the evaluation of thermal performance of a heat exchanger with frosting and decision of optimal defrosting cycle. Because the increase of flow resistance is the principal factor of a drop of heat transfer performance with frosting, thermal performance characteristics were examined. Based on those experimental results, we proposed a one‐dimensional nonhomogeneous frost growth model, and compared it with the experimental data in time and space. In a nonhomogeneous model with the frost property distribution taken into account, density distribution similar to the experimental result can be predicted. It is possible to make prediction closer to the experimental result compared with the conventional homogeneous model with respect to the temporal variation of frost height. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 674–689, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10122     

Influence of multi-structure optimization on the comprehensive performance of micro-cylindrical combustor inserting with spiral fin by using grey relational analysis and analysis of variance

Targeted at improving the energy output of thermophotovoltaic system, a novel micro combustor with spiral fin is proposed. The multi-structure factors of spiral fin concluding spiral fin length, spiral fin pitch, spiral fin number and spiral fin opening size impact the thermal and energy performance of micro combustor are investigated in this study. The method of Taguchi experimental design is introduced to determine the testing cases, Orthogonal design table ${\text{L}}_{16}\left(4^4\right)$. Then, grey relational grade is adopted to obtain the influence of four factors on performance parameters. The evaluation indexes of each testing case include mean outer wall temperature, outer wall temperature uniformity and pressure drop, which are closely connected with the thermal and energy performance of micro combustors. Moreover, the results calculated by grey relational grade are verified by analysis of variance. Results show that the optimal combination of the structure is micro combustor inserting with spiral fin, in which the spiral fin is 16 mm, the spiral fin pitch is 1.57 mm, the spiral fin number is 8 and spiral fin opening size is 0.2 mm. Meanwhile, the spiral fin length has the greatest impact on the comprehensive performance of the micro combustor, with a contribution of 48.522%.     

A study on rotational augmentation using CFD analysis of flow in the inboard region of the MEXICO rotor blades

This work presents an analysis of data from existing as well as new full‐rotor computational fluid dynamics computations on the MEXICO rotor, with focus on the flow around the inboard parts of the blades. The boundary layer separation characteristics on the airfoil sections in the inboard parts of the rotor are analysed using the pressure and the skin friction data at a range of angles of attack. These data are used to gain insight on the relative behaviour of separated boundary layers in 3D flow compared with 2D flow. It has been found that separation on airfoils in rotating flows is different from that in 2D flows in two respects: (i) there is a chord‐wise postponement (or delay) of the separation point, and (ii) the angle of attack at which separation is initiated is higher in 3D compared with 2D. Comments are made on the mechanism of stall delay, and the main differences between the skin friction and pressure distribution behaviours in 2D and 3D rotating flows are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.     

A study on the design and analysis of a heat pump heating system using wastewater as a heat source

In this study, the compression heat pump system using wastewater, as a heat source, from hotel with sauna was designed and analyzed. This study was performed to investigate the feasibility of the wastewater use for heat pump as a heat source and to obtain engineering data for system design. This heat pump system uses off-peak electricity that is a cheap energy compared to fossil fuel in Korea. For this, the charging process of heat into the hot water storage tank is achieved only at night time (22:00–08:00). TRNSYS was used for the system simulation with some new components like the heat pump, which we create ourselves.As a result, it was forecasted that the yearly mean COP of heat pump is about 4.8 and heat pump can supply 100% of hot water load except weekend of winter season. The important thing that should be considered for the system design is to decrease the temperature difference between condenser and evaporator working fluids during the heat charging process by the heat pump. This heat pump system using wastewater from sauna, public bath, building, etc. can therefore be effectively applied not only for water heating but also space heating and cooling in regions like as Korea.     

A DFT study on the hydrogen storage performance of MoS2 monolayers doped with group 8B transition metals

The adsorption of hydrogen (H₂) molecules on MoS₂ monolayers doped with Fe, Co, Ni, Ru, Rh, Pd, Os, Ir or Pt was calculated via first-principle density functional theory (DFT). The H₂ was found to interact most strongly with the MoS₂ doped with Os with a higher adsorption energy of ?1.103 eV. Investigations of the adsorptions of two to five H₂ molecules on Os-doped MoS₂ monolayers indicate that there are at most four H₂ interacting stably with the substrate with a promising average adsorption energy of ?0.792 eV. Molecular dynamics simulations also confirmed that the four H₂ molecules can still be reasonably adsorbed and stored on the Os-doped MoS₂ monolayer with a comparable average adsorption energy of ?0.713 eV at 300 K. This study indicates that MoS₂ monolayer doped with Os is a promising substrate to interact strongly with H₂ and can be applied to effectively store H₂ at room temperature.     

A CFD model with semi-empirical electrochemical relationships to study the influence of geometric and operating parameters on DMFC performance

A three-dimensional computational fluid dynamics (CFD) model is developed to investigate the influence of geometric and operating parameters on performance of a direct methanol fuel cell (DMFC). Semi-empirical relationships are introduced to describe the electrochemical behaviors required in the CFD governing equations. Coefficients in these semi-empirical relationships are fitted using experimental data. Two geometric configurations with serpentine channels at the anode and cathode are considered in this work. Temperature, methanol concentration, and methanol flow rate are selected as the operating parameters. Due to the computational effort of CFD, an adaptive metamodeling method is developed to reduce the number of data-fitting iterations for obtaining the coefficients in the semi-empirical relationships. The effectiveness of the method is demonstrated by fitting the model using the experimental data collected from the first geometric configuration of the DMFC and comparing the predicted performance of the second configuration with its experimental performance. A commercial CFD system, Fluent 12.0, was used in this research.