Pipe Expansion and Contraction

Plastic has a much higher coefficient of expansion (LCoE) than metal. As can be seen in the example table below, UPVC is almost 5 times as high as Mild Steel.

Example of difference in linear coefficient of expansion - PVC and Steel

The higher LCoE means that plastic pipe, such as PVC and ABS, will expand and contract more with a change in temperature than metal pipe. As a result, plastic pipe must be clipped/supported differently to metal pipe. Even small changes in temperature will result in the pipe wanting to expand or contract.

If plastic pipe is gripped hard and fixed in place, it is unable to move when it expands and contracts. Laws of physics mean that it HAS to expand or contract, we cannot stop it. If we restrict a direction of expansion/contraction, the plastic will expand and contract in a different direction. This can lead to issues such as stress fractures in the pipe and fittings, leaking joints, and general weakening of the system which may adversely affect the pressure rating of the system.

In ALL scenarios, plastic pipe should only be gently held/supported in such a way that if it needs to expand or contract it can move freely through the pipe clip.

It is also important to “support” pipes gently if there is any risk of "water hammer" from the sudden start up of a pump or opening/closing a valve. This gives the vibration somewhere to dissipate.

Almost all instances of system damage with expansion and contraction occur when the pipe is clipped too hard. This is an easy mistake to make with rubber lined pipe clips, where the temptation is to screw them down tightly so they grip the pipe firmly.

1. The primary purpose of a pipe clip is to support the pipe and stop it bowing under the combined weight of the pipe itself and the material being carried inside.
2. The secondary purpose is to support either side of heavier pipe system fittings like valves and filters, which will put more strain on the pipe due to their increased weight.
3. Clips are also used to hold the pipe in place and prevent movement.

There are several choices of pipe clip for plastic pipes. Some clips are made from rigid plastic and are designed specifically to grip the pipe gently so it can slide in between the clips when it expands or contracts. Rubber lined pipe clips are also often used as many installers find them more flexible in terms of the distance you can stand them off a surface. When rubber lined clips are used, they should only ever be fitted loosely so as to allow the pipe to move between them. They should never be fully tightened down.

An expansion loop can refer to a single elbow designed into the system with thermal expansion in mind or a purpose made loop consisting of four or more elbows. When designing a pipe system, thought should always be given to how and where the pipe will be able to expand and contract. A good explanation and calculation can be found here: PVC Pipes - Expansion Loops.

If you would like more information on this topic, please feel free to contact us at Plastic Pipe Shop.