Residual Deformation of a Hard-Coated Valve Seat Subjected to Thermal Shocks

Most safety related valves in EDF’s nuclear plant must prove their ability to sustain thermal shocks of approximately 240K amplitude. This paper evaluates the simulation of a globe valve tested for thermal shocks. Since the physical test campaign showed inadequate internal sealing, the simulation focuses on the residual deformation of the hard alloy, planar seat, welded on successive body designs. This deformation is the result of the thermal loadings first induced by the welding process, then by fluid flow inside the valve. A chain of 3D simulations successively computes: a welding temperature transient in the body, the resulting strain hardening — especially in the seat vicinity —; temperature transients in the flow and the valve parts, and the resulting strains in the body causing a bump deformation of the seat surface. This end result agrees with measurements on the tested valve specimen. We show that inaccurate results are obtained on simpler assumptions, such as no welding, and we give insights on the dominant effect of the first hot, cold, hot transient over other profiles. Finally, the agreement we obtain on deformation predictions is toned down by an unsatisfactory sealing prediction, as well as the complexity and duration of the simulation chain compared with physical testing.

Stress Concentration at the Root of Bolt Thread

The bolt strength is determined based on the concentrated bolt stress at the thread roots. The allowable stress is determined so that the thread root will not yield by the pretension and the external loads, using the stress concentration factor obtained as 3 to 5 from experiments. However, the concentration factor is not clear so far, as it is quite difficult to measure the stress at such a localized region. On the other hand, structural analysis, namely finite element analysis, has the possibility to provide the most-likely stress at the thread root. In this paper, a special technique, a.k.a. submodelling, is used to calculate the stress distribution at thread surfaces very precisely. The result will be useful to solve any stress related problems.

Evaluation of Leakage Probability of Non-Asbestos Fiber Sheet Gasket at Elevated Temperature Based on Percolation Theory

A long-term life prediction method for a compressed fiber sheet gasket under a high-temperature environment is studied. Non-asbestos compressed fiber sheet gaskets are now being used as a substitute for asbestos in the bolted flange joint, for instance petrochemical factories. Consequently, there is a real need for a technology to predict the lifetime of non-asbestos compressed fiber sheet gaskets quantitatively. In this report, the facing surface of the gasket and flange is visualized with scanning acoustic tomography (SAT). Voids were observed on the facing surface of the gasket and increased with the increase in exposure time at high temperature. If a leakage path for inner fluids is created by the increasing number of voids, the leak occurs on the facing surface of the gasket. The probability of a leak due to voids and the lifetime of this gasket are predicted by applying the percolation theory, which describes the connectedness of clusters.

A Note on Hydraulic Leaks for Mobile Equipment

Leaks in general and hydraulic leaks in specific are one of the major failure modes for mobile equipment. In-plant leak rates may range from 1 to 25%, while field leaks may range from 1 to 13% depending on the complexity of the system, assembly process, and parts quality. In this paper, the main potential leak causes are discussed and the performance of different types of fittings is compared; including JIC 37 Flare Fittings, O-Ring Boss Seal, and O-Ring Face Seal. Several potential improvement actions related to assembly process, torque specifications, and quality of manufactured fittings are recommended to reduce leaks. Among the many benefits leak reduction will result in are customer safety and satisfaction, assembly down time reduction, cost and warranty reduction, and environmental impact reduction.

Determination of CANDU End Fitting Jacking Limits Using Elastic-Plastic Finite Element Analysis

In one CANDU reactor unit in Ontario, the west end fitting is designed to connect to the end shield via a stop collar. The outboard end of the stop collar is welded to an attachment ring which shrink-fits on the end fitting body. The east side end fitting is supported by inboard and outboard journal rings resting on their respective bearing sleeves which allow the ‘free’ axial movement of the channel. In support of some maintenance activities, the west end fitting is required to be jacked to get certain clearance for accommodating the operating tools. The previous elastic calculation got the jacking limit of 0.35″ while did not provide enough clearance for tooling. In this paper, an elastic-plastic finite element analysis following ASME B&PV code Section III, Division 1, Subsection NB is performed to increase the jacking limit. The finite element analysis is carried out using ANSYS and validated by an ABAQUS model. In the elastic-plastic finite element analysis, the following effects are considered: strain hardening of stop collar material, stress concentration in stop collar weld, notch effect on stress concentration and fatigue in stop collar. Cyclic jacking loads as displacement controlled loading are applied in the analysis. Considering the time to the end of unit life, the maximum anticipated end fitting jacking cycles are 8. The higher jacking limit is achieved with an acceptable plastic deformation and fatigue damage at the stop collar, which is the weakest part during the end fitting jacking. The results show that the end fitting can be jacked at west side End-face with 1.17″ for 1–3 cycles, 1.15″ for 4 cycles, 1.03″ for 5 cycles, 0.95″ for 6 cycles, 0.85″ for 7 cycles and 0.80″ for 8 cycles. The jacking limits achieved in this paper provide enough clearance for the required maintenance operations.

Numerical Simulation of Fatigue Crack Propagation in Round Bar Using S-FEM and Experimental Verification

In this paper, a methodology for a three-dimensional simulation of fatigue crack growth behavior in a round bar is presented. This method is based on linear elastic fracture mechanics and S-version FEM (S-FEM) is employed for numerical analysis. Plasticity induced crack closure effect is considered by integrating a crack opening model into simulation method. This crack opening model is obtained from a result of a cyclic elastic-plastic finite element analysis. The accuracy of the solution for stress intensity factor (SIF) is checked by comparing with the available solutions in literature. For verification, experiments are conducted on round bars with single cracks and two cracks. A simulation of single crack propagation is conducted and the crack shape change prediction is compared with the experimental result. Two cracks problem is qualitatively investigated by experiments. In simulation, the shape change and SIF are shown. Results give acceptable predictions comparing to Experimental results. It is notable that the behavior of two cracks on the different plane can be easily simulated. The interaction of two cracks is discussed.

Limit Load Analysis of Crack Contained Thick Walled Cylinders

Reliable limit load estimations for thick walled pressurized cylinders containing defects are required for the assessment of integrity of structures that experience significant plastic deformation prior to failure. Analytical and finite element analyses of limit load in thick walled cylinders containing defects are presented in this paper. FE analyses were conducted to obtain estimates of the limit state of loading for a range of combined loading schemes and loading sequences for open-end and closed-end cylinder. Part through shallow and deep hoop cracks in the cylinder for uniform radial, uniform axial and combined loading were examined. The results suggest that adjustments to the estimates of limit loads obtained from conventional methods reported in literature are needed in order to reflect the role of material response, crack configuration and boundary conditions on the limit loads of defected thick walled pipes and cylinders. These findings are very important and should be noted carefully, especially in the context of treatment of hoop and axial residual stresses in the integrity assessment of pipelines containing part through cracks.

Shakedown Analysis Combined With the Problem of Heat Conduction

This paper deals with the computation of the shakedown load of engineering systems subjected to varying loads. In particular, we focus on thermal loading and the resulting heat conduction problem in combination with shakedown analysis. The analysis is based on the lower bound shakedown theorem by Melan. The calculation is carried out by use of an interior-point algorithm. Emphasis is placed on the presentation of theoretical derivations whereas numerical aspects are out of scope and will be presented elsewhere. The methodology is illustrated by the application to a simplified model of a tube sheet in heat exchangers.

Development of Modelling Guidelines for Stress Analysis of Pressure Vessel Bolted Closures

This paper describes some of the outcomes of the development of finite element modelling guidelines for the stress analysis of bolted joints in pressure vessels and piping. The modelling methods originally developed at Rolls-Royce typically used 2D axisymmetric models as this was deemed adequate at the time. However, computing software and hardware improvements have subsequently been made which enable more realistic 3D bolted joint models to be solved where a greater level of geometric detail is required. For example the bolts, nuts and perforated flanges can now be represented more realistically reducing the degree of geometric abstraction that is required. Also, modern finite element codes such as ABAQUS and ANSYS now offer gasket elements which enable the initial compression, in-service performance and unloading of the joint to be modelled more realistically. Additionally, contact techniques can also be used to simulate the axial and radial distribution of thread load in the joint which will affect the stress distribution remote from the threaded region. Consequently, the modelling guidelines have been updated and provide guidance for stress engineers to decide which degree of model complexity is warranted.

A Proposed Model for Predicting Residual Clamp Load in Gasketed Bolted Joints

An improved mathematical model is proposed for predicting the residual clamp load in gasketed bolted joints, taking into consideration gasket creep relaxation behavior, bolt stiffness, and joint stiffness. The gasket creep relaxation behavior is represented by a number of parameters which has been obtained experimentally in a previous work. An experimental procedure is developed to verify the proposed model using a single-bolt joint. The bolt is tightened to a target preload and the clamp load loss due to gasket creep relaxation is observed over time under various preload levels. The experimental and analytical results are presented and discussed. The proposed model provides a prediction of the residual clamp load as a function of time, gasket material and thickness, bolt stiffness, and joint stiffness. The improved model can be used to simulate the behavior of creep relaxation in soft joints as the joint stiffness effect is considered. Additionally, a closed form solution is formulated to determine the initial clamp load level necessary to provide the desired level of a steady state residual clamp load in the joint, by taking the gasket creep relaxation into account.