Luc Marchand

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

The Effect of Bolt Size on the Assembly Nut Factor

This paper details recent testing that was performed as an extension of earlier work on nut factor and high temperature breakout performance of selected anti-seize products. Comparison is made between results obtained using bolt diameters from 3/4 inch to 2 inch, two different anti-seize products (Molybdenum and Nickel) and two different bolt materials (ASTM A193-B7 & ASTM A193-B8M). In addition, common equations used for the determination of achieved bolt load from a given torque are examined and compared from a practical perspective in light of the nut factor test results. The test methods that were used are designed to closely mimic actual bolt assembly in a process plant environment. The paper, therefore, presents useful information that will enable more accurate assembly of bolted flanged joints on pressure vessels and piping in any process plant environment.Copyright

Bolt Anti-Seize Performance in a Process Plant Environment

This paper details recent testing that was performed as an extension of earlier work on nut factor and high temperature Break-out performance of selected anti-seize products. Comparison is made between previous results obtained on C-Si bolts (ASTM A193 B7) with more recent tests on 316 Stainless Steel bolts (ASTM A193 B8M). The bolt nut factor versus temperature and the required Breakout torque after one week at elevated temperature are detailed. In addition, common theories regarding the unsuitability of anti-seize containing graphite on stainless steel and the use of milk of magnesia as an anti-seize are quantitatively tested by comparison with standard anti-seize products. The test methods used are designed to closely mimic actual bolt assembly in a process plant environment. The paper, therefore, presents useful information that will enable more accurate assembly of bolted flanged joints on pressure vessels and piping in any process plant environment.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.

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.