The deflections of gaskets after initial tightening largely affect the residual bolt forces and the tightness of bolted flanged connections. The bolt forces decrease significantly due to the deflections of gaskets after initial tightening in the cases where PTFE gaskets are used. The visco-elastic characteristic of gaskets is the main cause for the phenomenon. It has been shown that the tightness improved even if the bolt forces reduced in the long term. The reason was thought that gaskets became packed under a high gasket stress. Because of this effect, the leak rate decreased. It is important to estimate the thickness change of gaskets to evaluate the tightness of flanged connections.
In this paper, a model of a flanged connection, in which the flange, the bolts and the gasket are represented by spring constants, is proposed. Using the model, the relationship between the gasket deflection and the tightness are discussed.