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Negative-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.

Note that when a Pexgol pipe collapses due to vacuum, it can be returned to its original round shape by applying internal pressure.

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 51.1 shows maximum rates of vacuum supported by Pexgol pipes of different classes and different design temperatures. These values are empirical.

Table 51.1: Service under Vacuum
Pipes Class SDR Temp. 1 Year 50 Years
30 SDR 6 up to 80ºC Full Vacuum *
24 SDR 7.4 up to 80ºC Full Vacuum *
15 SDR 11 up to 80ºC Full Vacuum *
12 SDR 13.6 20ºC Full Vacuum *
12 SDR 13.6 60ºC -0.9 -0/8
10 SDR 16.2 60ºC -0.65 -0.6
8 SDR 21 60ºC -0.52 -0.5
6 SDR 26 60ºC -0.45 -0.4

* Tested under full vacuum conditions: -1 bar (g) 0 bar (a)

The values in the table are in bar (g) (Bar gauge). For example:
-0.8 bar (g) is equal to 0.2 bar (a) or Bar absolute.
Pexgol pipe class 10 is not recommend for use under full vacuum conditions.

Allowable external pressure

For pipe of uniform cross-section, applying a safety factor of 1.5 which includes the influence of pipe ovality, the maximum allowable external pressure Pc in bar can calculated from the following equation:

Pc = 2618/ (SDR-1)3

  • For Pexgol class 10 SDR 13.6 Pc = 0.75 bar
  • For Pexgol class 12 SDR 13.6 Pc = 1.0 bar
  • For Pexgol class 15 SDR 11.0 Pc = 2.5 bar
  • For Pexgol class 19 SDR 9.0 Pc = 5.0 bar
  • For Pexgol class 24 SDR 7.3 Pc = 10.0 bar
  • For Pexgol class 30 SDR 6.0 Pc = 21.0 bar
Underground Pexgol pipe under vaccum 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 for consultation regarding installation of vacuum breakers.

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