No abstract

It is known that bolt forces reduce significantly after tightening bolted flanged connections in which expanded PTFE gaskets are used. Bolts are often post-tightened in practice after initial tightening to compensate for the reduction of bolt forces. The viscoelastic characteristic of expanded PTFE gaskets is thought to be the main cause for the phenomenon. Some gasket users are not confident whether the bolt forces remain above a minimum required gasket load until next maintenance. However, the long term characteristics of the residual bolt forces and the sealing performance of flanged connections have not been clarified yet. In this study, using flanged connections with expanded PTFE gaskets, the reduction of bolt forces and the change in the sealing performance were measured at room temperature for one year. Based on the test results, the residual bolt force and the sealing performance were studied. It was clarified that bolt forces converges to a certain level at early stage within almost 1000 hours. After that, the bolt forces were significantly affected by the ambient temperature change. This is because the thermal expansion of PTFE material is high and also the thermal characteristic of PTFE material has a temperature dependency. It was confirmed that although the bolt force reduced significantly at early stage, it is maintained in long-term. The sealing performance was improved due to the increased compression of gasket.

When flanged connections are used at a high temperature, one of the issues is the reduction of bolt forces, especially in the cases where PTFE-based gaskets are employed. Bolt forces significantly reduce when flanged connections are heated and re-torque is necessary in many cases. It is important to estimate the residual bolt forces under thermal loading to ensure the integrity of flanged connections. In this paper, the behavior of bolt force changes under thermal loading was experimentally examined using a flanged connection with expanded PTFE sheet gaskets. The flanged connection was heated and then cooled in an electric oven and the changes of the bolt forces were measured. It is found that the bolt forces reduce at the first thermal loading due to the gasket flow. Once the gasket is settled, the bolt force changes depend on the difference of thermal coefficients of the bolts and the gasket. It is also clarified that thinner gaskets and spiral wound gaskets are effective for elevated temperature applications.