Thermal Movement

The coefficient of expansion of plastic materials, are generally much greater than those of metals. Particular care should be taken in the design of pipe work layouts above ground, or where there is considerable fluctuation in temperature, to ensure adequate allowance for expansion and contraction.

Plastic materials have reduced impact strength at low temperatures and this should also be taken in to account in the siting of pipe work to avoid damage under these conditions.

Higher temperatures, either environmental or in the fluids conveyed, soften the materials and thus reduce the internal pressure at which the pipes may safely be used; this should be allowed for in the design of systems. Higher temperatures generally reduce the resistance of plastic to chemicals.

Expansion and contraction can be calculated using the formula.

where ?
L = Change in length in millimeters a
a = 0.08mm/ m/oC
L = Original length of pipe in meters ?
t = Total temperature range in oC.

Calculation of expansion and contraction should take account of the minimum and maximum foreseeable temperature conditions. It is normally possible by correct bracket arrangement to direct movement in such a manner that, this is accommodated by directional changes in the line.

Expansion bellows may be used to accommodate excessive movement but in such instances the pipes so connected must be restrained against possible separation.


Support and spacing requirement for the piping system should be designed in to the system to allow for increase temperature as temperature increases, the tensile strength of the pipes decreases, so the pipe and associated fixtures must be well supported.

Horizontal piping system should be supported on uniform centers which are depending by maximum operating temperatures. These spacing apply to uninsulated lines either in a building or exposed to the atmosphere. The formula use to determine the spacing data takes in to account the heating effect of the sun on low temperature lines.

Do not clamp or anchor the pipe so that it is held absolutely rigid or constricted. Some slight axial movement is necessary.