The system is based upon the principles of intermittent transport and immediate access to vacuum.
When the interface valve is opened, the difference in pressure between the atmosphere and the main pipeline pushes the volume of water and normally several times that volume of air through the service connection into the main pipeline. This creates a large local pressure differential which accelerates the water in the vicinity. As the pressure is equalized and the air is rushing through the system, it sequentially accelerates several independent masses of water that have accumulated in the low points of the pipework. This movement of water takes place in both directions, but the slope has a directional effect on the water flow.
After a number of repeated accelerations of water slugs, the air has lost most of its kinetic energy and cannot create any more pumping action. For indoor systems, the transportation length is normally within the reach of every interface unit. For longer systems, the interface units have to interact to provide the necessary pumping action.
For a vacuum drainage system, it is necessary to sequentially generate high acceleration and self-cleansing velocities without the use of tremendous amounts of energy.*
*Reference: "British Standard - EN 12109 - Vacuum Drainage Inside Buildings"
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