Designing for Thermal Movement

Designing for Thermal Movement

A common barrier to the use of plastic pipes for hot and cold water services is the perceived problematic issue of thermal movement experienced within these systems. However, if the issue of expansion and contraction is factored in at the design stage, the pipework system will operate effectively and offer enhanced benefits, compared with metal materials, over the lifetime of the system.

Although metal pipework also suffers from thermal movement, it is widely acknowledged that plastic materials expand and contract to a greater degree compared with metals. This can sometimes contribute to traditional metal systems being specified instead of plastics. Thermal movement can be easily managed within plastic water networks if the system is designed in a way to cater for it, but with so many options available it can often be difficult to know what the best solution is.

The issue of expansion is generally only experienced with hot water systems, but it is crucial that both expansion and contraction are addressed at the design stage of a project. Plastic pipework should always be allowed to expand and contract freely otherwise significant problems can occur, which could include at best unsightly snaking pipes, or, at worst, stress on pipe joints, potentially causing leaks in the system over time.

If it is not catered for, expansion of a pipework system can cause compression of the pipe, resulting in buckling and the risk of deformation. At the other end of the spectrum, contraction of the pipe can cause it to be tensioned again, developing tensile loads in the pipe system.

It is important to note that different plastic materials expand at different rates. With hot and cold water systems, the greatest single change in temperature occurs when the pipework is first put into service, particularly if it is installed in cold conditions. For hot and cold water lines, a C-PVC system expands / contracts by 0.65mm per metre per 10°C change in temperature, therefore; if the ambient temperature is 10°C and the working temperature is 60°C, the expansion on an installed length of 10m C-PVC pipe would be approximately 32.5mm.

As you would expect, chilled water systems work in the opposite way so while the greatest temperature change still occurs when the pipe is put into service, this is evident to a greater degree during the warmer months in the spring and summer. For chilled water lines, an ABS system expands / contracts by 1mm per metre per 10°C change in temperature, therefore; if the ambient temperature is 30°C and the CHW flow runs at 6°C, the contraction on an installed length of 20m pipe will be 48mm.

The simplest and most cost effective method of catering for thermal movement is to utilise the natural flexibility of the pipework system by building in changes of direction to the network design. The direction of pipe movement can be controlled by the use of anchor points at strategic positions.

Where there are no changes of direction, for example on long, straight runs, expansion loops can be installed by using 90° elbows. Another cost effective solution, expansion loops utilise the system’s components, avoiding the need for additional mechanical fittings. However, changes in direction and expansion loops are not always a practical solution and could be discounted due to the building design.

For applications that require pipework to be installed within restricted spaces, for example in long straight runs along narrow corridors, utilising the flexibility of the pipework will sometimes not be possible and linear expansion devices will need to be used. In cases such as these, contractors and consultants should always look for a pipework system that incorporates an expansion device, as ranges that offer a fully integrated solution will be more cost effective than purchasing a pipework system and expansion fittings separately.

If separate expansion devices are needed, the main options are expansion compensators, bellows and flexible hoses. These need to be monitored at regular intervals so must be accessible, it should be possible to inspect, dismantle and replace parts in them without disturbing other elements of the system.

Typically, flexible hoses are used on smaller diameter pipework and it is critical that the expansion joint is not subject to twisting during installation or operation in order for it to remain effective. Another alternative is purpose-designed expansion compensators dedicated to particular plastic pipe systems. This type of fitting needs to be fixed in position so as to take up any expansion and contraction utilising a sliding component within the body of the fitting.

The most costly option, bellows are generally used on larger diameter sizes and, available in a variety of different materials, it is important that the selected fitting is compatible with the specific plastic pipe material it is being connected to.

Thermal expansion of plastic pipework systems should not be a barrier to their specification, simply designing for movement at the outset will ensure pipework does not suffer the effects of expansion or contraction. Technical advice from the pipework manufacturer should always be sought to ensure the most appropriate solution for each individual application is achieved.

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