Limit load evaluation of inlet pigtail pipe bends with ovality under in-plane bending

Abstract

The prediction of limit load on inlet pigtail pipe bends employed in a hydrogen reformer is one of the unresolved engineering tasks in chemical industries. This is because of the uncertainty and required extrapolation of the applicable data. In practice, the cross section of the inlet pigtail pipe bend becomes non-circular due to the bending process. This irregularity in shape extends when pipes are subjected to in-plane bending moment during plant operation. Ovality is the shape imperfection in a pipe bend considered for the present study. It affects the load carrying capacity and life cycle of the pipe bend. A three-dimensional finite element method is used to model and analyze a stand-alone, long radius inlet pigtail pipe bend. The pipe bend is modeled with a shape irregularity for which the percent ovality varies from 0 to 20. By considering ovality, it is shown that there is a significant effect in limit load due to in-plane bending moment. Also, the stress induced in the pipe bend geometry increases with the percentage of ovality. The allowable levels are obtained from finite elements analysis, considering various bend factors, and the closed form limit load solution is proposed, including the effect of ovality in pipe bend.