The nomogram provides a guid to friction losses that can be expected when using clean HDPE pressure pipes with clean water at 20oC. No account has been taken of any possible fittings in a line.
EFFECT ON PRESSURE
Pressure de-rating factors should be applied to HDPE pipes when operating temperatures rise above 20°C. The de-rating factors below are applicable to HDPE.
At lower temperatures, between 20 °C and 0 °C, the pressure handling capability does increase but it is recommended that this be ignored. In the unlikely event of water freezing inside an HDPE pipe damage is unlikely to occur. Nonetheless it is recommended that the pipeline system be protected against freezing to obviate flow restrictions.
EFFECT ON DIMENSIONS
Due to the relatively high co-efficient of expansion and contraction it is necessary to make allowance for this in any design and installation which is exposed to wide variations of temperature. HDPE pipes will expand or contract by 0.2 mm per meter per °C rise or fall in temperature. A 30 °C temperature rise will therefore cause a 36 mm expansion of a 6 meter pipe.
Above Ground Installation
For exposed above-ground pipework proper anchorage and support is essential. It must cater for thermal stresses or movement over the ambient temperature range to which the pipe system will be exposed.
Above-ground HDPE systems should preferably be installed at or near maximum operating temperature. This will ensure that the pipe is thermally expanded when clamps or supports are bolted into position and the pipe will be prevented from contracting. Tensile stresses will develop as the pipework cools, and the pipeline will therefore remain straight between supports. When reheated to installation temperature, any sagging will be minimized.
When suspending HDPE pipes the recommended centre distance between supports at various temperatures is given by the following graph. The graph is based on PE 63 material and SDR 11 and SDR 17.
As a rule of thumb the following adjustments can be made for variations in temperature and class:
• Reduce the centre distance by 3% for every 10 °C increase in temperature.
• Increase the centre distance by 2.5% for every 10% increase in wall thickness above above/below SDR 17.
Pipe clamps used for anchorage and support should have flat, non-abrasive contact faces, or be lined with rubber sheeting and should not be over-tightened. The width of support brackets and hangers should normally be either 100 mm or half the nominal pipe diameter, whichever is the greater. Support brackets should allow free axial movement.