VACUUM / SUCTION PIPELINES
Under-pressure (vacuum) might develop in the following cases:
- When a pipe is installed in vacuum-feeding pipelines.
- When a pipe is installed in a steep inclination, causing rapid free flow.
- Extreme temperature changes of the transported liquid.
If a PE-X pipe collapses, it results in an oval deformation.
Note that when a PE-X pipe collapses due to vacuum, it can be returned to its original round shape by applying internal pressure.
The amount of vacuum that a PE-X pipe can withstand depends on the pipe’s SDR. A pipe with sufficient wall thickness must be selected in order to resist the collapsing forces generated by the vacuum.
Allowable external pressure:
The maximum allowable external pressure Pc is calculated for a pipe having uniform cross-section, applying a safety factor of 1.5, which includes the influence of 3.5% pipe
out-of-roundness.
The maximum allowable external pressure Pc (in psi) can be calculated from the following Equation 9
Where: PE-X is in psi and E (psi) is the long-term modulus
Table n° 25: Long Term Modulus
| Temperature (°C) | Long Term Modulus [psi] |
|---|---|
| 20 | 3510 |
| 40 | 2436 |
| 60 | 1755 |
The following external pressures were calculated for a design temperature of 20ºC.
- For PE-X SDR 17, Pc = 8.86 psi
- For PE-X SDR 13.5, Pc = 21 psi
- For PE-X SDR 11, Pc = 36.3 psi
- For PE-X SDR 9, Pc = 71 psi
- For PE-X SDR 7.3, Pc = 145.24 psi
- For PEX SDR 6, Pc = 290 psi
Underground Pexgol pipe under vacuum or external pressures
Vacuum, or external pressures, creates hoop stresses in the pipe wall which are combined with the external pressures of the soil.
In extreme cases, these stresses can cause the pipe to collapse. Therefore, when a Pexgol vacuum pipeline is installed underground, the vacuum stresses have to be added to the total static and dynamic loads exerted by the soil and all the stresses must be considered. In this case, make sure that the soil around the pipe is compacted.
When designing a vacuum pipeline at recommended vacuum conditions, please contact our engineer team for consultation regarding installation of vacuum breakers.