Expansion and Contraction

As with all materials, HDPE is subject to expansion/contraction caused by changes in temperature.  If unrestrained, an HDPE pipeline installed above ground will tend to move laterally as a result of temperature change, especially if the line is empty.  When lateral movement is restricted, expansion will take place in either length or diameter, or pipe will move vertically, whichever is less restrained.

In most conditions of installation, some restraint is automatically provided.  With pipes of 4 nominal or greater, simple burial under 24 of soil (above crown of pipe) usually provides ample restraint.  Under these conditions, expansion or contraction due to temperature changes does not occur and no design considerations are required to provide restraint.

The Coefficient of Thermal Expansion is approximately:

0.8 10-4 inch per inch per degree F.

Formula: ΔL = L C ΔT

Where:   ΔL = change in length, inches

L = original length, inches

C = coefficient of Thermal Expansion, in./in./F (0.8 10-4 inch per inch per degree F)

ΔT = maximum temperature minus minimum temperature, F

Example:  A 500 unrestrained, level, straight pipe run is subjected to a temperature fluctuation from 90F during the day to 65F at night. The difference in pipe length between night and day is: 

ΔL = 500 12 0.8 10-4 25

ΔL = 15 shorter due to the temperature drop[3]. 

[3] CSR PolyPipe, Design and Engineering Guide C1000, pg D1




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