Bolted flange joints are prone to leakage when exposed to high temperature. In most cases this is due to relaxation that takes place as a result of material creep. One way to solve this problem is to use Belleville spring washers or longer bolt with spacers. However, there is practically no reliable analytical model that can evaluate the exact number of washers or length of the bolts required to reduce relaxation to a target minimum level. This paper describes an analytical model based on the flexibility and displacement interactions of the joint different elements including the axial rigidity of the flange and bolts, used to evaluate relaxation. The developed analytical flange model can accommodate either Belleville spring washers or longer bolts with spacer tubes to reduce the bolt load loss to a maximum target value. This model is validated by comparison with the more accurate FEA findings. Calculation examples on a bolted flanged joint are presented to illustrate the suggested analytical calculation procedure.

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