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In the design of piping systems, there are many options for transitioning between HDPE and metallic piping. One common option is the use of flanged joints. As a result of the visco-elastic nature of HDPE, the use of HDPE-to-metallic flanged joints requires special design considerations. When HDPE-to-metallic flanged joints are used in ASME Class 3 systems, the design is further complicated by the requirements provided in the ASME B&PV Code, Section III for flanged joint analysis. This paper examines the differences between HDPE piping flanged joints and metallic piping flanged joints, including consideration of industry guidance and available industry testing results. The paper provides a proposed methodology for evaluating ASME Class 3 HDPE-to-metallic flanged joints and HDPE-to-HDPE flanged joints, including the determination of required bolt torque values and the determination of the maximum internal pressure that the joint can resist without experiencing leakage.

The design of buried piping systems requires special considerations. In the evaluation of seismic loads on buried piping, the associated stresses are typically considered to be secondary stresses as the piping is assumed to move with the soil during a seismic event and inertial seismic loads are considered to be negligible. During a seismic event, buried pipes are subject to relative displacement-induced strains, induced primarily by seismic wave passage. Typically, the areas of highest stress are found at offset locations as a result of applied moments (primarily due to axial loads into an elbow or tee) and/or at transition locations near entry into buildings or subgrade vaults as a result of seismic anchor movements and/or differential settlement. This paper examines the relative influence of the number of diameters of straight piping between offsets and the number of diameters of straight piping in between building/vault entry and the first support on resulting seismic piping loads.

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.