Flovac vacuum mains

Vacuum mains are installed in narrow trenches in a stepped profile for level and uphill transport. Vacuum mains follow the natural surface for downhill transport. The stepped profile keeps the mains shallow and is designed to ensure that sewage will not block the pipe when the sewage rests. Lifts minimise trench depth. A lift is simply two 45-degree elbows and a straight piece of pipe. Sewage will lie at these low spots until other valves upstream open.


Vacuum lines are slightly sloped (0.2%) towards the collection station. Unlike gravity sewers that must be laid at a minimum slope to obtain this scouring velocity, vacuum mains can be laid with a flatter slope since a high scouring velocity is a feature of vacuum sewage transport.

Vacuum mains are of a smaller diameter than gravity mains for a given flow because they are designed to operate with higher velocities and under two phase flow conditions, this provides for more efficient fluid transport and also maintains the pipe in a much cleaner condition than gravity. The vacuum mains are PE PN 10 or PVC Class 9 or Class 12.

In general, a potential vacuum loss is associated with every lift. This limits the length of each vacuum line to about 3 to 5 km in flat terrain. Elevation changes can extend or reduce this range. Longer distances are possible depending on local topography and the design of the system.

The size of the vacuum mains, the shallow depth of construction and the speed at which installation occurs all reduces our impact on the environment and reduces our carbon footprint. If a break ever occurred in a vacuum main, no contamination occurs as the negative pressure does not allow any sewage to leak into the environment. A break will also be quickly alerted to the operators as the vacuum pumps running time will increase. Residents will not be inconvenienced as a rare break is easy to find and repair. If our pipework was 5-10 metres deep as with a gravity sewer this would be far more difficult.

Gone are the days when operators need to enter the sewer pipes or worry about gas buildups or rats or snakes in the system.