Bolt Preload Reduction of Pipe Flange Connections After Plant Shutdown

Abstract

When pipe flange connections clamped with sheet gaskets are subjected to thermal load, the bolt preloads that tighten a pair of pipe flanges tend to decrease. It has been studied in the previous paper why and how the bolt preloads decrease, in which the loading and unloading curves of sheet gaskets at elevated temperature are expressed by means of simple equations. Meanwhile, it is also known that the bolt preloads are likely to decrease when the plant is shut down. In this paper, a numerical procedure, which can predict the amounts of bolt preload reduction occurred in the shutdown process, is proposed using axi-symmetric finite element method (FEM). The analytical objects are pipe flange connections tightened with aramid sheet gaskets. In the finite element formulation, stress-strain curves of the gaskets after plant shutdown are approximated by simple curves, considering that the hysteresis appeared in the curves is considerably small, comparing to that between the loading and unloading curves in the running condition. It is shown that the bolt preloads are decreased by as much as from 40% to 50% of those in the running condition. The validity of the proposed numerical method is confirmed by comparing the numerical results of the bolt preload reduction to experimental ones.