Technical Note 814-TN
Engineering Considerations for Temperature Change
Like most materials, polyethylene is affected by temperature change. However, polyethylene’s response to temperature change is significant and unique when compared to other “traditional” piping materials. Polyethylene pipe design for thermal change may be significantly different compared to other piping materials.
Polyethylene pipe can be installed and operated in sub-freezing conditions. Ice in the pipe will restrict or stop flow, but not cause pipe breakage. Care must be taken during installation to avoid impact and suddenly applied high stress. In response to changing temperature, unrestrained polyethylene pipe will undergo a length change. Anchored or end restrained pipe will develop longitudinal stresses instead of undergoing a change in length. This stress will be tensile during temperature decrease, or compressive during temperature increase. If the compressive stress level exceeds the column buckling resistance of the restrained length, then lateral buckling (or snaking) will occur. While thermal stresses are well tolerated by polyethylene pipe, anchored or restrained pipe may apply stress to restraining structures. The resulting stress or thrust loads can be significant and the restraining structures must be designed to resist the anticipated loads.
The PlexCalc® II program is available from Performance Pipe to aid in performing many of the calculations in this technical note. PlexCalc® II is located on the Performance Pipe CD-Rom.
Unrestrained Thermal Effects
The theoretical change in length for an unrestrained pipe placed on a frictionless surface can be determined from Equation 1.
ΔL = Lα ΔT (1)
where:
ΔL = length change, in
L = pipe length, in
α = thermal expansion coefficient, in/in/°F
ΔT = temperature change, °F
The coefficient of thermal expansion for DriscoPlex® high density polyethylene pipe material is about 9.0 x 10-5 in/in/°F. This coefficient results in an approximate expansion for pipe of 1/10/100, that is, 1 in for each 10° F change for each 100 ft of pipe. This is a significant length change compared to other piping materials and should be taken into account in piping system design. A temperature rise results in a length increase while a temperature drop results in a length decrease.
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