In gas systems, plastic pipe is used to deliver gas at lower pressures and where external pressures are limited. Pipe size and type vary depending on various design considerations such as flow rate, pressure, and environmental factors. Key factors in specifying plastic pipe include geometry, strength, and crack resistance.
Geometry: As with steel pipe, pipe diameter and wall thickness are important to determine how much gas and what pressure a plastic pipe can carry. These are typical ranges for geometry of plastic pipe in gas distribution systems:
Strength: The strength of plastic pipe is determined by geometry and the pipe material. Almost all the plastic pipe installed today is one of two grades of polyethylene — either medium density called MDPE or high density called HDPE. High density has a greater strength but is a more expensive material. Some older plastic pipe that still exists in the field is of other material no longer used in new installations. The MAOP associated with plastic pipe is determined by the pipe’s long-term hydrostatic strength, which is specified by the manufacturer. The pipe must also be able to withstand environmental pressure from the surrounding soil including crushing, buckling, or deformation.
Crack resistance: Limiting punctures and slow crack failure over time is important to avoid leaks in plastic pipe. Pipe thickness beyond what is needed for design pressure is often used to provide protection against punctures from construction equipment or other external forces. Rock impingement and other stresses can cause initiation of cracks that over time propagate down the pipe. Each pipe material is classified for its capacity to resist the initiation and propagation of slowly growing cracks when subjected to a sustained localized stress.