Under-pressure (vacuum) might develop in the following cases:
1. When a pipe is installed in vacuum-feeding pipelines.
2. When a pipe is installed in a steep inclination, causing rapid free flow.
3. Extreme temperature changes of the transported liquid.
If a Pexgol pipe collapses, it results in an oval deformation which increases progressively. Note that when a Pexgol pipe collapses due to vacuum, it can be returned to its original round shape by applying brief internal pressure at high temperature.
The following drawing shows an above-ground situation schematic of a Pexgol pipe under vacuum. The pipe’s cross-section is slightly flattened because of the vacuum inside the pipe.
The amount of vacuum that a Pexgol 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.
Table 33.1 shows maximum rates of vacuum supported by Pexgol pipes of different classes and different design temp.These are values empirical.
maximum rates of vacuum supported by Pexgol pipes

Table No. 33.1: Service under vacuUm

Pipe Rating Service Duration
  Day 1 Month 1 Year 1 Year 50
Class 24
Tested under full vacuum
conditions -1 bar (g) 0 bar (a)
Class 15
SDR 11
Class 12
SDR 13.6
    0.9 0.8
Class 10
SDR 16.2
  0.75 0.65 0.6
Class 8
SDR 21
60°C   0.55 0.52 0.5
Class 6
SDR 26
60°C   0.5 0.45 0.4

Above ground Pexgol pipe under vacuum

When selecting Pexgol pipes for vacuum conditions above the ground, it is recommended to use the following criterion—

when DvH/Do is below 1.2, a Pexgol pipe laid above ground is considered safe against vacuum failure.

Underground Pexgol pipe under vacuum

Vacuum, which is a negative pressure, 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 for consultation regarding installation of vacuum breakers.