Relative Influence of Soil Stiffness and Elbow Geometry on Buried Piping Thermal Stresses

Abstract

The design of buried piping systems requires special considerations. Historically, buried piping was evaluated for thermal expansion and contraction using simple hand calculations considering the piping to be fully-constrained by the surrounding soil. With the development of analytical software, more advanced analysis of buried piping is possible considering detailed piping routing and the stiffness of the surrounding soil and of the piping itself (in cases of more flexible piping materials). Typically, the areas of highest thermal stress occur at changes in direction (i.e. elbows, etc.) due to the applied moments, and the relative stress magnitude is influenced by the stiffness of the surrounding soil. Due to the relatively high coefficient of thermal expansion of polyethylene, stresses in buried piping due to thermal expansion and contraction are of particular note for high density polyethylene (HDPE) piping. This paper examines the relative influence of the analytical representations of a variety of HDPE piping elbow geometries (e.g. mitered elbows, molded elbows, etc.) and corresponding soil restraint. The study demonstrates that total longitudinal stress calculated in a finite element analysis may be reduced using minor to moderate efforts of refinement.