Methodology to Determine Design Life of Assemblies of PE Pipe, Heat Fusion Fittings, and Heat Fusion Joints

Papers # 2014 Chicago

The scope of this paper is to provide information for the plastic piping industry on a test methodology that could determine the design life of heat fusion fittings and joints at the end user field conditions of operating stress, operating temperature and desired design life. This methodology is useful not only for design life based on internal pressure, which is the primary load, but is also useful to determine the effects of secondary loads such as bending, rock impingement, squeeze-off and deflection.

Improvements in the long-term performance of PE piping compounds have created interest in the use of higher operating pressures for gas distribution piping systems. In Canada, this has culminated in the approval of PE 100 pipe with an operating pressure of 145 psig for SDR 11 pipe, and the approval of PE 2708 PLUS and PE 4710 PLUS compounds that qualify for a 0.45 design factor for gas applications. In the US, the AGA (American Gas Association) has submitted a petition to PHMSA to increase the design factor from 0.32 to 0.40. When the 0.40 design factor is approved, SDR 11 PE 4710 pipe with an ASTM D 2837 HDB of 1600 psi can be operated up to 125 psig (calculated value is 128 psig, but DOT limit for PE is 125 psig) for gas applications in the US. It is clear from these initiatives that there is increasing interest in the safe operation of high performance PE 2708 and PE 4710 gas piping systems at higher system operating pressures.

However, gas companies recognize that the ASTM D 2837 pressure-rating method is for pipe only. Their concern is that their piping systems also include heat fusion fittings and joints such as polyethylene (PE) socket fusion, saddle fusion, butt fusion and electro-fusion fittings. These gas companies would like a methodology to determine the design life for their entire system (PE heat fusion fittings that are joined to PE pipe) that will operate at these higher pressures. For the purposes of this Paper we are using the term “design life” to mean the minimum desired time that the assembly will last at its maximum operating stress and average annual operating temperature based on just the internal pressure. With this methodology, we can also consider possible effects of additional secondary loads, such as bending, deflection, rock impingement, etc. This methodology can, therefore, predict the lifetime (service life) of the installed piping system under in-service conditions.

This paper summarizes a methodology that can determine design life for polyethylene (PE) heat fusion fittings joined to PE pipe used in gas distribution applications. All types of mechanical fittings whether made of metal or plastic are excluded. This methodology determines the design life for the assembly of PE fittings joined by heat fusion to PE pipe.

Related keywords : stress, temperature, fusion, heat, fittings, methodology, factor, pressure.
Please note that the whole article content is available on PPCA website onlySource : 2014 Chicago


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