Michel Derenne

Analytical modeling of the contact stress with nonlinear gaskets

Gasket contact stress and its variation through the gasket width is caused by the rotation of the flange and has an influence on the leakage tightness behavior of bolted flange joints. The future implementation by the ASME of proposed design rules is based on new gasket constants obtained from the ROTT (room temperature tightness) tests conducted on rigid platens. The gasket contact stress distribution needs to be addressed for the purpose of better joint tightness predictions. This paper presents a comprehensive analytical method that predicts the gasket contact stress distribution taking into account the nonlinear mechanical behavior of the gasket material. Based on the flange rotational flexibility, the proposed analytical model that is implemented in the SuperFlange program is supported and validated by numerical FEA and experimental analyses on flange rotations, radial distribution of gasket contact stress, and joint leak tightness.

Predicting Gasket Leak Rates Using a Laminar-Molecular Flow Model

The tightness characterization of gaskets used in static seal applications, such as bolted flanged connections, is achieved by performing leakage tests with a single fluid, usually a gas like helium. Attempts made in the past to predict gasket leakage with other gases had limited success unless the leak flow regime through the gasket was predominately laminar, which is not the case with most of the gaskets. In this work, a new gasket leak flow model that combines both molecular and laminar flow regimes is developed to predict the gasket leak rate under different pressures and with different gases. The Laminar-Molecular Flow (LMF) model is first constructed around a reference pressure for which the fraction of the total leakage that occurs through laminar flow channels is established. This fraction is computed using a simple leakage test performed with one gas and at least two different pressures. The model is then tested against experimental leak data obtained from two different gaskets and four gases and is shown to produce accurate predictions.Copyright

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.Copyright

Determination of gasket effective width based on leakage

The concept of the effective gasket width is used in the ASME code for flange designs to account for flange rotation. Its origin has never been revealed and it is suspected that only limited experimental testing was conducted for verification. In addition, with the new design rules based on the new gasket constants obtained from ROTT tests conducted on rigid platens, there is a need to revise the effective gasket width formulas. This paper presents some work development conducted under the auspices of the PVRC and related to the investigation of the effect of the non-uniform gasket contact stress due to flange rotation on the leakage behavior of sheet gasket materials. A experimental investigation coupled with some numerical work were conducted to determine the effective gasket width and its variation with gasket type, flange flexibility and load.Copyright

Tube-to-Tubesheet Joints: Maximum Tensile Stress and Contact Pressure Due to Thermal Loading and Temperature Cycling

Service reliability and durability of tubular heat exchangers and steam generators are much dependent on the proper response of the tube-to-tubesheet joints to the operating conditions. In this paper a 2-D axisymmetric finite element model is proposed and compared to a 3-D finite element solution for the purpose of predicting the temperature effect on the residual contact pressure and maximum tensile residual stresses of such joints. A parametric study using the finite element results shows that, although thermal loading and temperature cycling have a negligible effect on the maximum tensile residual stresses, the room-temperature initial residual contact pressure may be completely relieved following the initiation of plastic deformation in either the tube or the tubesheet during thermal loading. A comparison between the results of the proposed finite element model and those obtained from the literature shows good agreement. A simplified analytical approach, which may be used for the design of tube-to-tubesheet joints, is also proposed to predict the joint behavior at the operating conditions.© 2002 ASME

Service Temperature Characterization of Polytetrafluoroethylene-Based Gaskets

Gaskets based on polytetrafluoroethylene (PTFE) are used extensively in bolted flanged connections, especially in difficult chemical process plant applications where blowouts due to excessive bolt load loss are of major concem. The determination of their service temperatures requires the characterization of their short- and long-term creep relaxation resistance. An experiment-alanalytical procedure has been developed to determine a recommended service temperature for PTFE-based gaskets. Based on an improved version of the Hot Blowout Test (HOBT) in which thermal cycling has been incorporated, the procedure quantifies the short-term hot relaxation resistance, and the margin of safety against an in-service blowout. The improved HOBT test provides an excellent tool for the selection of blowout-resistant PTFE gaskets for difficult service. Currently, the procedure estimates the cooldown load loss due to the thermal contraction difference between the gasket and the flange, based on a rough estimate of the gasket thickness and the coefficient of thermal expansion and their vanation with temperature. The proposed procedure provides not only a better estimation of the gasket thickness but also accounts for thermal ratcheting and the temperature lag between flange and bolts.

Development of Test Procedures For Fire Resistance Qualification of Gaskets

Among the elevated temperature performance of gaskets, their mechanical and tightness integrity in flammable services due to fires is a key consideration necessary to provide gasket users with a safe gasket. This is particularly true since, of the numerous non-asbestos products now on the market, some would appear to have questionable integrity at elevated temperature as compared to that of the traditional asbestos products. Therefore, fire exposure survivability is an important aspect of gasket qualification for many process applications and the evaluation of the fire resistance is a key property to be considered for the acceptance of new products.

On the validity and limits of the gasket effective width concept

In the current ASME code procedure for the design of the gasketed bolted joints, the concept of the effective width is introduced to take into account the effect of the non-uniform gasket contact stress distribution. The code fixes a threshold value of the gasket width above which an adjustment of the gasket contact width is introduced. The validity of this threshold has never been verified. Under the normal operating conditions of flanges used with flat gaskets, the definition of this threshold is independent of the bolt load, the average gasket stress, the internal pressure and the flexibility of the joint. In this paper a study on the new limits of the concept of the effective gasket width is conducted. A new approach for the calculation of the effective gasket width, based on the results of an experimental investigation and undertaken in parallel to a numerical study, is presented. This approach takes into account the non-uniform distribution of the gasket stress, the flexibility of the flanges, the mechanical behavior and the sealing performance of the assembly.Copyright

Recent Developments In Elevated Temperature Gasket Evaluation

This paper dicusses a recently developed approach for evaluating sheet gasket performance at elevated temperature that is based on gasket weight loss during thermal exposure. Using simple testing devices, strong correlations are shown to exist between weight loss and the change of the most important gasket properties during aging, namely: thickness change, tensile strength, tightness, and relaxation of the initial compression stress. These correlations permit a better understanding of the fundamental mechanisms involved in the degradation process of elastomer bound sheet gasket materials and have lead to the development of an aging parameter (Ap), based on on weight loss, that succesfully combines the time and temperature effect.