The effect of ovality on the membrane stress in a 2" thick walled 90 deg steam pipe bend

Abstract

Pipe bends are an integral part of any pipe work system. Pipe bends are often seen as the weak point in a pipe work system due to the additional stresses that are imposed on a bend during operation as well as the defects that develop in a bend during manufacturing. Ovality is one such defect and causes significant changes not only to the stress distribution, but also the location and magnitude of the maximum stress in a bend. A need therefore exists, to better understand the change in stress distribution due to ovality. In this document, different bend manufacturing methods are considered. It also endeavours to explain the various stresses present in pipe bends, both thick-walled and thin-walled and different method to determine these stresses. With this information, an oval bend was simulated using two different FEA packages, namely PATRAN and ANSYS. A test rig was also designed and built that was used to measure the strain on an oval bend subjected to internal pressure. It was found that ovality can increase the stress in an oval bend by up-to 50% compared to a straight pipe. However, a correlation could not be found between the experimental analysis and computer simulation. The reason for this is explained in the report. The results were compared, analysed and differences were explained.