Thermal expansion: Water's unique properties

Water does not behave like other liquids. It is the only liquid to expand when it changes phase, taking up more space when it freezes. But people are not always aware that it expands again in its liquid phase, at temperatures above 4 ℃. This can lead to movement or "thermal expansion" of the piping system. We show you what you need to keep in mind.

The expansion of the pipework can obviously present problems when the pipe systems that transport it are exposed to freezing conditions. However, in many commercial heating systems, it is the expansion of both the water and the pipe material in their normal phase that can cause serious problems. In particular, this happens where the difference between the ambient temperature and the hot water operating temperature is significant. In such circumstances, the differential tends to be substantial, normally in the order of 50-60 ℃.

A small degree of thermal expansion can cause big problems

Different metals expand to differing degrees, but all of an order that can be significant when pipe runs are very lengthy. To be more specific, by this we mean pipe runs generally in excess of 10m, where the expansion can be of the order of 10 mm.

So if we imagine a continuous pipe running in a straight line for about 50 metres, maybe in a corridor or a services shaft, this could expand by 50 mm. This sounds like a minuscule amount, but changes of even this magnitude can have significant effects if the pipe is fixed at both ends. The expansion forces of the pipe are far greater than most fixings can accommodate, so the pipe will either break the fixing or bow and bend where it can.

This in itself may not cause a problem, but pipe runs of this length may have branches and valves in the line. This can lead to two potential issues:

  • If there is a valve that has a connection to the pipe, and it is then subject to a bending moment, these forces may well exceed the material’s ability to withstand them. Consequently, a stress fracture in the material may manifest itself in the connecting piece, leading to a leak.

  • If the pipe expands and there is a branch, say into a flat, the branch pipe will experience a bending moment. The issues arise when either the branch pipe is restrained, or when there are valves into which the bending forces are then transferred. If the leg length of the branch is insufficient in length to accommodate the bending action, then the force is transferred directly into the valve, which again may develop a stress fracture at the point of connection.

The importance of a well-designed piping system

This expansion of pipes can and should be accommodated by sensible design. There are three design principles that can be adopted.

The first is to use axial compensators or bellows, which are like a spring within the pipe. These can compress and expand to reflect the movement of the pipework. For these to work in the manner in which they are intended, they must be installed correctly. This, in practice, means the compensators must be fixed at either end by a suitable anchor, and the pipe about 300mm from the compensator must also be fixed. This is to ensure the expansion is driven in a linear manner and does not cause the compensator to buckle. In addition to this, a decision will need to be made as to whether or not the compensator needs to be fixed to one side of the fitting. This ensures the movement is driven into the compensator and not further down the pipework.

It is not uncommon for many compensators to be used in a long run of pipework in order to accommodate the amount of expansion. It is also important to note that compensators are a serviceable item; access to these fittings should be allowed in the ceiling voids.

The second way is to allow the free expansion of the pipes in a controlled manner that does not impact on any other components of the system. This is typically done by creating a loop in the pipework. The lengths of the loop are calculated to accept the potential bending moment, and the fixings are not clamped tight on the tube, allowing movement to occur. Alternatively, if the pipe bends around the corner, then the pipe supports can be spaced such that they allow the pipe to expand in one direction and bend in the other.

Finally, if pipes are buried in soil or concrete or pass through solid walls, there must be sufficient allowance for movement. This is normally provided by the use of pipe lagging.

The wider impact of expansion

When considering the consequence of expansion, it is also important to consider the other components of the system, and how they are supported. For example, some components, particularly when the pipe diameter is large, become very heavy. These will require support in their own right, and it should not be presumed that the pipe supports are sufficient to fix the component.

In addition, heavy valves can sometimes require large forces to operate them. If the valve is not sufficiently anchored, these forces can be transferred to the fittings which connect them to the pipework. As mentioned above, fittings are designed to connect items together, not to provide or withstand structural or significant bending forces.

Where to go for advice

CIBSE and IPHE, as well as manufacturers of components and pipework systems, all provide extensive guidance and documentation on this issue. SANHA have a comprehensive design service which can recommend the frequency and type of compensators, along with a project take-off and specifications. It is always good practice to double-check the design of expansion, and the use of such services can provide valuable access to expert advice.