Investigation of Flexibility Factors and Stress Indices for Class 1 Curved Piping

In the analysis of piping, consideration of the behaviour of bends and elbows is essential. Due to the ovalization phenomenon, the bends exhibit more flexibility under bending moment loading than straight pipes, while generating comparatively higher stresses. This has been recognized in the ASME Code for Class 1 piping, where flexibility factors and stress indices have been given for curved pipes. It has been understood that the values of stress indices given in the Code are conservative. As a result, extensive research has been performed to obtain lower values, as allowed by the Code. However, these studies have been performed separately from the flexibility factors, where research is generally lacking. The Code recognizes that the given flexibility factors have limited scope of use and more work on this issue has been proposed. It is evident that a more realistic assessment of pipe bends has to be based on revised stress indices and flexibility factors. In this paper, a number of compound bends are studied, where on one hand the stress indices are conservative and on the other the flexibility factors of the Code may not be applicable. For this purpose, the behaviour of these bends is simulated using 3D finite element models. Using these models and by comparison to the flexibility and stresses in straight pipes, flexibility factors and stress indices for the bends are obtained. A comparison to the values in the literature and discussion of different factors contributing to these values are presented.© 2010 ASME