Industrial Systems
Thermal Expansion and Thermal Stresses
It is important to consider thermal expansion when designing a system with Corzan® pipe. Most thermoplastics have a coefficient of thermal expansion which is significantly higher than those of metals. The thermal expansion of a piping system subject to a temperature change can therefore be significant, and may need compensation in the system design. The expansion or contraction of thermoplastic pipe may be calculated from the following formula:
Thermal Expansion Formula |
|
| Where: | ΔL = Change in length due to change in temperature (in.) |
| Lp = Length of pipe (in.) | |
| C = Coefficient of thermal expansion (in./in./°F) = 3.8 x 10-5in./in./°F for CPVC |
|
| ΔT = Change in temperature (°F) |
The thermal expansion and contraction of CPVC and other piping materials is displayed below.
Expansion Loops and Offsets
As a rule of thumb, if the total temperature change is greater than 30°F (17°C), compensation for thermal expansion should be included in the system design. The recommended method of accommodating thermal expansion is to include expansion loops, offsets, or changes in direction where necessary in the system design.
An expansion loop schematic is presented here.
Modulus of Elasticity and Working
Stress for CPVC |
||
| Temperature (°F) | Modulus, E (psi) |
Stress, S (psi) |
| 73 | 423,000 |
2000 |
| 90 | 403,000 |
1800 |
| 110 | 371,000 |
1500 |
| 120 | 355,000 |
1300 |
| 140 | 323,000 |
1000 |
| 160 | 291,000 |
750 |
| 180 | 269,000 |
500 |
Expansion loops and offsets should be constructed withstraight pipe and 90° elbows which are solvent cemented together. If threaded pipe is used in the rest of the system, it is still recommended that expansion loops and offsets be constructed with solvent cement in order to better handle the bending stresses incurred during expansion. The expansion loop or offset should be located approximately at the midpoint of the pipe run and should not have any supports or anchors installed in it. Valves or strainers should not be installed within an expansion loop or offset.
Thermal Stresses
If thermal expansion is not accommodated, it is absorbed
in the pipe as an internal compression. This creates a compressive
stress in the pipe. The stress induced in a pipe which is restrained
from expanding is calculated with the following formula:
| S = EyΔT | |
| Where | S = stress induced in the pipe |
| E = Modulus of elasticity at maximum temperature | |
| y = coefficient of thermal expansion | |
| ΔT = total temperature change of the system |
Because the coefficient of thermal expansion of steel is five times lower than that of CPVC, dimensional changes due to thermal expansion will be five times less. However, as can be seen by the equation above, the stresses induced in the piping system due to restrained thermal expansion are dependent on the material’s modulus as well as its coefficient of thermal expansion. Because the modulus of steel is approximately 80 times higher than that of CPVC, the stresses resulting from restrained expansion over a given temperature change will be approximately 16 times higher for steel than for CPVC.
For instance, restrained expansion over a 50°F temperature change will produce approximately 600 psi of stress in a CPVC system, but 9800 psi of stress in a steel system. CPVC’s relatively more flexible nature will usually allow it to absorb its lower stresses in a buckling or snaking of the line if necessary. Because steel piping is too rigid to buckle, its higher stresses are often transferred to surrounding structures, resulting in damaged supports, anchors, or even abutting walls.
