On the Effect of Scattering of Bolt Forces on Gasket Stress

Abstract

To guarantee integrity and tightness of bolted flanged connections a set of gasket characteristics (for tightness e.g. QMIN(L), QSMIN(L), tightness class L) has to be known. These characteristics are determined in simplified standardized tests with uniform gasket stress levels, in which the effect of the scattering of the bolt force is neglected. In this paper, results of Finite Element studies with real flange models are presented in which the local distribution of the gasket stress was determined in dependence of the scatter of the bolt forces. In the calculations, different models of the material characteristics are evaluated. A linear elastic approach, a nonlinear elastic approach and a non-linear elastic-plastic approach of the gasket behaviour is analysed. The results of the FEM studies demonstrated that elastic-plastic material laws are necessary for a realistic description of the material behaviour. The calculations in which the elastic-plastic gasket behaviour is considered give a nearly uniform gasket stress distribution along the gasket circumference despite a large scatter in the bolt force. This means that a non-uniform load transmission into the BFC is “damped” through the flange rings to a large degree and the gasket is compressed almost uniformly along the circumference. In radial direction an increase of the gasket stress from the inner to the outer diameter can be observed. Depending on the material law used, a lift-off of the flange ring from the gasket at the inner diameter (caused by the flange rotation) is obtained (linear elastic approach). Using a non-linear elastic or elastic-plastic approach for the gasket material the gasket stays in contact with the flange surfaces over the entire width of the gasket. These experiences can only partly be considered in analytical calculation codes such as EN 1591-1 (1). This is one reason why there are often discussions about the tightening factors of different mounting procedures and therefore difficulties in stress and tightness analysis can occur.