Three-Dimensional Finite Element Analysis of Pipe Flange Connections: The Case of Using Compressed Asbestos Sheet Gasket

The most important factor for the leakage problem of pipe flange connections is considered to be contact pressure distribution at the gasket bearing surface in service. In this study, the mechanical behaviors of the pipe flange connection are evaluated using FEM as a three-dimensional contact problem, in which a gasket is modeled as a nonlinear one-dimensional gasket element. Here, the contact pressure distributions at the gasket bearing surface and the variations of the bolt stress are estimated under uniform bolt preloads or nonuniform ones due to the elastic interaction during bolting up. The numerical procedure proposed here can successively deal with the processes of bolt-up, applying inner pressure and applying bending moment. The analytical objects are pipe flanges specified in JIS B 2238 with compressed asbestos sheet gaskets being inserted. The validity of the numerical method is ascertained by experiment.

Stress Analysis and the Sealing Performance Evaluation of Pipe Flange Connection With Spiral Wound Gaskets Under Internal Pressure

This paper deals with the stress analysis of a pipe flange connection with a spiral wound gasket using the elasto-plastic finite element method taking account the hysteresis and the non-linearity in the stress-strain curve of the spiral wound gasket, when an intemal pressure is applied to the pipe flange connections with the different nominal diameters from 2″ to 20″. The effects of the nominal diameter of the pipe flange on the contact stress distributions at the interfaces are examined. Leakage tests of the pipe flange connections with 3″ and 20″ nominal diameters were conducted and measurement of the axial bolt force was also performed. The results by the finite element analysis are fairly consistent with the experimental results concerning the variation in the axial bolt force. By using the contact stress distributions and the results of the leakage test, the new gasket constants are evaluated. As a result, it is found that the variations in the contact stress distributions are substantial due to the flange rotation in the pipe flange connections with the larger nominal diameter. In addition, a method to determine the bolt preload for a given tightness parameter is demonstrated.

Comparison of Leak Rates Predicted by ROTT Constants With Experimental Data From a 16″ and 24″ Pressure Vessel

Leak rates on two pressure vessels were experimentally determined. The first vessel was tested with a pair of 24″ flanges class 150 raised face weld neck flanges, the second vessel had a pair of 16″ class 300 RFWN. Comparisons between the experimental data and the predictions made by the ROTT gasket constants were made.

Requirements for Numerical Flange Calculations According to the German Nuclear Code

In Europe as well as in other countries a lot of effort is invested into developing new codes and standards for bolted joints under various loading conditions. The standardization of gasket factors and the improvement of calculation methods with respect to these factors characterize the last couple of years in this area. In Germany the nuclear code (KTA) is also influenced by this development. So, the leak rate dependency of gasket factors and the results of a research program on metal-to-metal contact type flanges were introduced into the new approach of the code for Class 2 and 3 components. Herein flange calculations can be performed for various flange types, floating type and metal-to-metal contact type. Generally, the calculations to be performed can be separated into a design step and the proof of sufficient tightness and strength of flange, bolts and gasket for the various operating conditions according to the chosen bolting method. In this step the stiffness of all components (flanges, bolts and gasket) and its influence on the mechanical behavior of the entire system is considered. Besides, influences caused by thermal effects, e.g. different thermal expansion between the flanges and the bolts, or seating effects in the gasket must be regarded. In cases, where the allowable stress values are not satisfied by performing code calculations or in cases, where the applicability of the code is not given, e.g. due to geometric facts, Finite-Element analyses often replace code calculations to show code compliance. Therefore, numerical Finite-Element analyses, performed according to a special code, e.g. KTA, must also fulfill the requirements of the code with respect to considered load cases, bolting condition, allowable stresses etc., to get an adequate testimony for a certain flange joint. In this paper certain methods for numerical calculations will be presented and compared to the results of the code for various flange types and dimensions.

A Simplified Modeling of Gasket Stress-Strain Curve for FEM Analysis in Bolted Flange Joint Design

Semi-metal and non-metal gaskets (spiral wound, joint sheet, etc.) are well known that their stress-strain curve shows strong nonlinearity and hysteresis. These characteristics should be considered, when we analyze the behavior of a bolted flange joint. Regarding the analysis of bolted flange joints, authors had proposed a simplified modeling method of gasket stress-strain curve for FEM analysis in the previous PVP conferences. The method approximates the nonlinearity of gasket stress-strain relation using two different linear elastic moduli in loading and unloading, respectively. This paper provides that the comparison of computed results due to the simplified analysis and the ones due to accurate nonlinear analysis that uses a nonlinear-hysteresis gasket model. This paper also proposes the guidance to determine the two linear elastic moduli for the simplified modeling. Since the simplified analysis gives a good approximation, we conclude that the method is very useful for the analysis of bolted flange joints, especially in design stage of pressure vessel flanges or piping flanges.

Simplified Sealing Test Procedure of Gaskets Based on Compressive Strain

With the recent increase of safety and environmental concern, the estimation of leak rate of a gasketed flanged connection in piping systems is an important subject to be studied. In order to estimate the leak rate, the sealing behavior of a gasket must be known. Currently, evaluation methods of the sealing behavior are proposed in the North America and Europe independently. One of the problems is that the representations of the sealing behavior are rather complex in both the methods. Thus, gasket tests take a long time to perform and cost much. The authors have carried out investigations on the sealing behavior of compressed fiber sheet gaskets and have shown that the leak rate is uniquely determined by the compressive strain of gasket. This fact makes the test procedure much easier eliminating complex loading-unloading sequences. A simplified leak testing procedure, in which a simple one-way loading is employed, has proposed. Furthermore, an equation for the sealing behavior of gasket, in which the effect of dimensions of gaskets is taken into consideration, has proposed. In this study, investigations on the sealing behaviors are overviewed first. Then, it is shown that the conventional sealing behavior based on the gasket stress is easily obtained from the sealing behavior based on the gasket strain and the stress-strain diagram of gasket. Both the sealing behavior and the stress-strain diagram of gasket are expressed by equations in this method. Finally, it has also shown that the sealing behavior obtained by the simplified test can be fully converted to the ROTT (ROom Temperature Tightness Test) forms.

Systematical FE Analysis of Bolt Assembly Process of Pipe Flange Connections

In bolt assembly process of pipe flange connections, repeated bolt-up operations are conducted to achieve uniform preloads. Nobody, however, knows how many times of bolt-up operation is required to achieve uniform preloads. It is important for practicing workers to know the number of bolt-up operations. The number of bolt-up operation required to get uniform bolt preloads depends on the nominal diameter of pipe flange, the number of bolts, the types of gasket, etc. In this study, systematical finite element analyses of bolt-up operation are conducted for JPI class 150in and 300in flanges with a compressed asbestos sheet gasket. The number of bolt-up operations required to achieve uniform preloads is estimated for each pipe flange connection.

Finite Element Simulation of the Disassembly Process of Pipe Flange Connections

Elastic interaction between bolts occurs in pipe flange connections during not only the bolt assembly process but also the disassembly process. In this study, the mechanical behaviors of pipe flange connections during the disassembly process have been systematically examined by using finite element analysis. As a result, it is shown that the increase in the rating class and nominal size of pipe flange increases the maximum value of bolt loads occurred during the bolt removing process. The effects of the bolt removing sequence are also analyzed in order to estimate the redistributions of bolt loads and the variations of contact pressure distributions on the gasket bearing surface. Furthermore, it is simulated how a pipe flange connection behaves when the clamping forces of some bolts are lost due to failure or loosening.

An Experimental Investigation on Frictional Properties of Bolted Joints

By using an approach developed to determining the torque-tension relationship for bolted joints, frictional properties of several typical bolted joints were studied experimentally. The approach allows for the direct determination of the thread friction and the bearing friction between the nut and its bearing surface independently. Detailed studies were made on the influences of the size and shape of the hole, the use of a slot in a bolted joint, contact area and position, and other factors such as turning speed, coating, and the use of wax on the bearing surface. The contact area and position of the washer have a marginal effect on the bearing friction. The organic coating on the nuts reduces the bearing friction significantly. Nuts with organic coating over a washer with zinc finish provide the smallest and the most consistent bearing friction. Experiments on thread friction shows that prevailing torque nuts with distorted threads and nylon inserts provided trivial benefits for preventing “self-loosening” of the nut. Repeated tightening-loosening generally increases frictions in a bolted joint. It was noted that the data scatter of the experimental results of frictions in a bolted joint may overshadow the influence of size, speed, and contact positions. The results from the experimental investigation will help to better design bolted joints.

Determination of Gasket Stress Levels During Thermal Transients

The leakage of bolted flange joints at high temperature or during transient thermal shock is a well recognised problem. However, the present pressure vessel design codes do not address the effects of temperature on the integrity of the bolted joint, other than material properties. A research project currently being conducted at Ecole Polytechnique is intended to provide designers with an analytic approach for establishing the effects of thermal loading on the joint sealing ability. This paper is the fourth to be published as part of this research project. The presented analysis method enables the determination of the temperature response of the joint components to a transition in internal fluid temperature. Using this data, the worst case operating scenario may be selected and calculations performed to determine the impact of the temperature transition on the gasket stress levels. The presented analytical method is verified by comparison to finite element analysis and experimental measurement.