Creep analysis of bolted flange joints

In the petrochemical and nuclear industries, the difficulty in assessing the effect of creep on the tightening load of bolted flanged connections is recognized. Under high temperatures, the leak tightness of bolted joints is compromised due to the loss of the bolt load as a result of creep of not only the gasket and bolt materials but also the flange material. Apart from acknowledgment of this effect, there exists no established design calculation procedure that accounts for creep. This is because the relaxation of the bolt load and the corresponding loss of the gasket contact stress are not easy to assess analytically. The main objective of the work is the development of a simple analytical solution to the creep-relaxation problem encountered in bolted flange connections of the float type. Particular emphasis is put towards relaxation caused by the flange and bolt material creep.The validation of the methodology is carefully checked against the more complex 3D numerical FE method using different size flanges. Based on the creep constants of the different joint elements (bolt, flange and gasket), the implemented analytical method is shown to predict the bolt and gasket load relaxation with reasonable accuracy. In some flange cases, up to 70% of bolt load relaxation is found with both methods depending on the flange creep material and size.