A simplified analysis of flange under thermal transients

Thermal transients such as startup, shutdown can cause temperature difference (thermal lag) between bolt and flange. The temperature difference results in differential thermal expansion of bolts and flange. This may lead to excessive overloading of bolts (or loss of pretension in the bolts) and excessive rotation of the flange. Discretely located bolts along with bolt holes in the flange call for 3 dimensional thermal analysis to get the temperature field in and around the bolt. Three dimensional thermal analysis not only becomes complicated and time consuming but structural analysis for the thermal lag becomes still more difficult. A simplified approach can replace the 3 dimensional thermal analysis modelling to a 2-dimensional axisymmetric simulation. In this paper a method for 2-dimensional axisymmetric thermal analysis is presented. Effect of important parameters like contact resistance (between threads or washers), air conductivity, convective heat transfer on external surfaces etc. on the thermal lag is analysed by using the simplified 2-dimensional approach. Effects of these parameters on bolt stresses is also brought out. (orig.)