THE AFFECTS OF ASSEMBLY TORQUE AND POLYETHYLENE PIPE INSERTION DEPTH ON SERVICE HEAD ADAPTER PERFORMANCE

摘要

Polyethylene (PE) pipe has been used as a proven means of conveying and distributing fuel gases since the 1960s.1 In the United States and Canada, PE pipe accounts for over 90% of the natural gas distribution system and has become the de facto industry standard. Although PE pipe is so prevalent, industry codes and regulations do not allow plastic pipe to be installed within a structure as a suitable means to convey fuel gas. Therefore, a coupling or fitting is used to transition from the exterior PE pipe to the metallic pipe associated with distribution within a structure. There are various manufacturers that make fittings with a standard National Pipe Thread (NPT) connection to transition the PE gas line into a metallic pipe. This type of fitting is commonly referred to as a service head adapter (SHA). There are numerous styles and variations of SHAs, but all share a common design factor in that they employ mechanical compression to form a gas-tight, leak free connection. Fuel gas leaks resulting in explosions and fires have occurred due to leaks at the SHA/PE tubing connection. Often these leaks do not manifest themselves during the initial pressure tests performed following installation, but instead occur months or years later. Several factors may contribute to these failures, which include: improper installation, manufacturing and design defects, geotechnical issues, and environmental conditions. A study was conducted using common ASTM D2513 Category 1i SHAs, both old and new, from various manufacturers. Tests were performed to specifically explore failure modes associated with the assembly of SHAs. Two variables, assembly torque and insertion depth of the PE pipe, were varied during testing to assess their individual affects on SHA connection strength. Each SHA assembly, following an initial leak test, was subjected to an axial tensile test while under pressure. Testing was performed until a failure of the assembly was detected. Yielding of the PE tubing, pullout of the PE tubing, and/or loss of integrity (i.e. leaking) all constituted a failure thereby completing a test.