For years, process plants have relied on non-metallic piping as a cost-effective alternative to stainless, alloy and other expensive metallic piping in applications posing high corrosion rates or requiring stringent cleanliness standards.
Plastic piping affords excellent resistance to attack by many chemicals, including most acids, alkalis and salt solutions. Such piping comes in schedule-40, schedule-80 and other common sizes, with wall thickness usually corresponding to that of steel piping. In addition, some plastic piping, e.g., PVC, is offered with standard dimension ratio (SDR) ratings, which mean the piping system will maintain a more-or-less uniform pressure rating at a specified temperature regardless of pipe diameter.
However, some issues — e.g., thermal movement and other thermal effects, and liquid hammer — demand more attention with plastic piping than with commonly used metallic piping. Most plastic piping materials exhibit a relatively high coefficient of thermal expansion. Elevated temperatures may seriously affect plastic piping; for some materials, pressure/temperature ratings drop substantially at temperatures above 50ºC. So, plastic piping should not be located near steam lines or other hot surfaces. When liquid flow in a piping system stops suddenly (for instance, because of a quick-closing valve), a pressure surge known as liquid hammer (or often water hammer) develops and can easily rupture a plastic piping system.