Above ground installation & Bending
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 and it is important to consider / take into account even 1°C increase in temperature as this has an effect on the lifetime of the pipeline.
At lower temperatures, between 20°C and 0°C, the pressure handling capability does increase but it is recommended that his be ignored. With low temperatures, it is very unlikely for water freezing inside an HDPE pipe, 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 0.2mm per meter per °C rise or fall in temperature. A 30°C temperature rise will therefore cause a 36mm expansion of a 6-meter pipe.
Effect of temperature change
The standard design temperature for HDPE pipes is 20°C and working pressures are usually quoted for this temperature. HDPE pressure pipes function perfectly well below 20°C right down to freezing point and can in fact, with stand higher pressures that those quoted at 20°C.
As can be seen from the stress regression lines, the creep rupture strength diminishes with increasing temperature and working pressures must be down-rated if the same factors of safety are too be maintained.
Sub Zero Temperatures
Water has been known to freeze in HDPE pipes without causing fractures, but permanent strain can result, leading to severe reduction in the working life of the pipe. Hence HDPE Pipes – like other pipe – should be protected against sub-zero temperatures.
Expansion and Contraction
All plastics have a high co-efficient of expansion and contraction, several times those of the metals. This must be allowed for in any installation by the use of expansion joints, expansion loops etc.