Effects of Scatter in Bolt Preload of Pipe Flange Connections With Gaskets on Sealing Performance

It has been well known that a scatter in axial bolt forces of pipe flange connections tightened by the torque control method is substantial. It is necessary for evaluating the sealing performance of the pipe flange connections with the gaskets subjected to intemal pressure to know the contact gasket stress distributions due to the scatter of the axial bolt forces in the connections tightened by the torque control method. This paper deals with the leakage of the pipe flange connections with a spiral wound gasket and that with a compressed sheet gasket tightened by the torque control method. The scatter in the axial bolt forces was measured in the experiments. The contact gasket stress distributions at the interfaces of the pipe flange connections with the gaskets were calculated under the measured axial bolt forces by using elasto-plastic finite element method (FEM) taking into account hysteresis and non-linearity in the stress-strain curves of the gaskets. The effects of the scatter in the axial bolt forces tightened by the torque control method on the gas leakage were also examined by using the actual pipe flange connections. As the result, a difference in an amount of gas leakage measured was found to be substantial between our study and PVRC procedure. By using the calculated contact gasket stress distributions under the internal pressure and the results of the leakage tests, the sealing performance was evaluated. It is found that the sealing performance is worse in the actual pipe flange connection than that evaluated by PVRC procedure.

Bolt Load Changes Due to Internal Pressure in Gasketed Flange Connections

Bolt load changes due to internal pressure are very important in order to evaluate the integrity of gasketed flange connections in the sealing performance point of view, because its gasket stress which dominates leak rate changes according to the bolt load changes. For establishing a connection possesses high reliability and sufficient integrity, it is necessary to clarify the mechanics ofgasketed flange connections. For this purpose, authors carried out experimental pressurizing tests for 3B and 20B gasketed flange connections clamped by various bolt preloads and measured the bolt load changes with increasing internal pressure up to 5MPa. Also a load factor, which is defined as the ratio of axial bolt force increment to pressure thrust force, was calculated using the test results. The test results indicate that 3B and 20B flange connections with the spiral wound gaskets have a constant load factors under sufficient initial clamping forces in assemble.

Finite Element Analysis of the Thermal and Mechanical Behaviors of Bolted Joint

Mechanical and thermal behaviors of the bolted joint subjected to thermal load are analyzed using axisymmetric FEM, where the effects of thermal contact resistance at the interface and heat flow through small gaps are taken into account in order to accurately evaluate the variations of bolt preloads. It is expected that the numerical procedure proposed here provides an effective means for estimating the strength of such critical structures as internal combustion engines, pressure vessels, steam and gas turbines, etc. An empirical equation that can compute the thermal contact coefficient at the interface composed of common engineering materials has been proposed in the previous paper. In this study, a simple equation for evaluating the amounts of heat flow through small gaps is shown by defining apparent thermal contact coefficient. A finite element approach has been established by incorporating the aforementioned thermal contact coefficients into the finite element formulation. By use of the FE code, it is shown that among various thermal properties, coefficient of linear expansion has dominant effects on the variations of bolt preloads. The validity of the numerical approach is demonstrated by experimentation.

On the Use of Dual Kriging Interpolation for the Evaluation of the Gasket Stress Distribution in Bolted Joints

The leakage behavior of bolted joint is very much dictated by the gasket contact stress. In particular, the non-uniform distribution of this stress in the radial direction caused by the flange rotational flexibility has a major influence on the leak tightness of some gasket types. The current ASME flange design rules and the new ASME proposed design rules addresses this effect by introducing the concept of gasket effective width for which the validity of the suggested values has not been verified. This paper presents a simple comprehensive analytical approach based on the dual kriging interpolation technique to predict the gasket contact stress distribution in floating type bolted joints. The kriging methodology is shown to be very efficient when nonlinear modeling such as gasket material mechanical behavior is involved. Together with the flange rotational flexibility, this technique implemented in the “SuperFlange” program is supported and validated by numerical FEA conducted on different flange sizes and gasket materials combinations.

Calculation of Bolted Flange Connections of Floating and Metal-to-Metal Contact Type

In Europe, in 2001 the new standard EN 1591 for strength and tightness proofs of bolted flange connections (BFC) of floating type flanges was released. In addition, the German nuclear code was revised regarding the calculation of BFC. With this standard not only the floating type but also the metal-to-metal contact type of flanges (MMC) can be treated. Additionally, the ASME code is the basis for the flange calculation in the European standard EN 13445, which is the standard for unfired pressure vessels. In compliance with the goal of the calculation, the different calculation codes can be used. There must be a differentiation between the design of the components, the determination of the prestress values for assembly, the stress analysis and the tightness proof of the BFC. First, all parameters which influence the function of the bolted flange connection are considered. In a second step, the range of use of the different standards and the calculation algorithm are discussed.

Web-Enabled Database for Gasket Parameters

The available technique of a Web-enabled material database was adapted to a new Gasket Data Bank (Gasket-DB) in the frame of an European research project PERL (Pressure Equipment, Reduction of Leak rate: gasket parameters measurements). Main content are gasket characteristics determined on all gasket types defined in the new European gasket standard series EN 1514, part 1 to 7: • actual gasket parameters in ENV 1591-2 (public access), • proposed gasket parameters to define more reliable values (restricted access), • certified manufacturers gasket data (restricted access), • gasket parameter ranges (public access). Data retrieval is providing intelligent user guidance. Data entry is carried out by Joint Research Centre (JRC) Petten, NL, by means of standardized format (Excel file with different sheets, reflecting the table structure of the database). The prototype database and the user interface can easily be extended and customized to the needs and requirements of the engineering gasket society, e.g. link to customer documentation, databases, services like design calculation tools.

Achieving a Leak Free Start Up: Impossible Dream or Achievable Project?

Large petrochemical plants are shutdown periodically for maintenance and inspection. During the shutdowns a large number of gasketted joints are dismantled in order to gain access to the equipment, and subsequently remade at the end of the shutdown. In order to ensure the integrity of the pressure envelope, the plant systems are leak tested with high-pressure nitrogen prior to the introduction of hydrocarbons. If leaks from gasketted joints are discovered during this process, it becomes necessary to de-pressure the systems in order that the joints can be remade. During the 1990 Shutdown of the Huntsman Olefins6 plant at Wilton, approximately 200 gasketted joints were required to be remade during leak testing. This caused significant and costly delays to the start up of the plant. Later analysis showed that the leaks were the result of poor standards in the making and checking of the gasketted joints. A new joint making procedure was written for the 1993 and 1997 shutdowns, and this included a requirement for joints to be traceable by a tagging system to the technician who made the joint. The number of remakes reduced dramatically, to approximately 10 in each of the two shutdowns. In preparation for the 2002 shutdown engineers at Wilton set a target of achieving ZERO remakes during leak testing after the shutdown. This was a great challenge, especially given that approximately 7000 joints were to be remade by 400 technicians. The project was developed in partnership with Hedley Purvis Limited, who supplied design services for calculating residual bolt loads, and Klinger UK Limited, who supplied gaskets for use during the shutdown. The Olefins Joint Making procedure was further developed, requiring full traceability and the use of controlled tightening of joints to a pre-defined load. Great emphasis was placed on the role of people in effective joint making, and in ensuring that each technician was carefully briefed and validated in his understanding of the importance to the business of high standards in joint making. A training video was prepared to help in the process of gaining the understanding and involvement of the technicians and their supervisors. Hedley Purvis and Klinger supported Huntsman in the development of the video, and in the delivery of the training and validation exercise. The project was a great success, and resulted in the first “leak free” start up of the Olefins6 plant. The paper will described the project in detail, and will include a showing of the training video.

The Design of Flanges Based on Flexibility and Tightness

From the leakage tightness consideration, the behavior of gasketed joint relies very much on the actual gasket load and its variation during the different operating conditions. With the future implementation by the ASME of new design rules based on tightness, a more meaningful and precise evaluation of the gasket load is useful for the purpose of improved prediction of joint tightness. A new design procedure based on the traditional code design philosophy that also accounts for the flexibility of all elements of the joint as well as tightness and gasket creep is suggested. In this paper a general comprehensive method based on the gasket-bolt-flange elastic interaction is presented for the analysis of the joint. The proposed analytical model that is implemented in the “TightFlange” program will be supported by a numerical FEM and experimental investigations to assess the effect of the mechanical and thermal loading including pressure, external loads, thermal expansion and gasket creep on the leakage behavior of gaskets.

FEM Stress Analysis and Sealing Performance in Pipe Flange Connections With Gaskets Subjected to External Bending Moment: Case Where Internal Fluid is Liquid

The leakage evaluation when gas is used is more severe than that when liquid is used in pipe flange connections. In a practical design, it is also necessary to examine the leakage in the connection under liquid internal pressure. This paper deals with the contact gasket stress distributions in the pipe flange connections with a spiral wound gasket and a compressed sheet gasket by using elasto-plastic finite element method (FEM) taking account hysteresis and non-linearity in the stress-strain curves of the gaskets, when bending moments as well as internal pressure are applied to the connections. In the FEM calculations, the effects of the gaskets and the initial clamping bolt force (bolt preload) on the contact gasket stress distributions are examined. The leakage tests for the connections under bending moments were also conducted by using liquid (water). By using the results of the leakage tests and the calculated contact gasket stress distributions, the sealing performance of the connections is evaluated. It is found that the sealing performance of the connection under the bending moment can be estimated when internal fluid is liquid (water).

Standardization Related to the Pressure Equipment Directive With a Focus on Bolted Joints

Standardization related to the European Pressure Equipment Directive 97/23/EC (PED) is of considerable importance. European Member States must permit the placing on the market of pressure equipment and assemblies at the date of application of this Directive since 29 May 2002, and permit such equipment and assemblies to be put into service since this date. The economic significance of the pressure equipment sector for the common European market is made clear by the fact, that approximately every tenth subject presently dealt with by the European Standardization Committee (CEN) is directly or indirectly linked to the PED. On a European level, a total of 19 Technical Committees in CEN and 9 Technical Committees in ECISS (European Committee for Iron and Steel Standardization) are involved in standardization relating to the Directive 97/23/EC. Three committees are dealing with flanges, gaskets, bolts and valves. On a international level two technical committees standardize flanges and bolts.