Designing the pipe configuration for optimum performance

One of the major benefits of a syphonic system is that the horizontal pipe runs do not have any fall, minimising the space required to accommodate the system. This provides the designer with freedom to route the pipes to any location at high level, before dropping to ground level. The high suction in the system reduces the pipe diameters and number of vertical drops needed compared to a gravity system, providing a reduction in cost for most installations.

The major advantage of a syphonic system is that drainage can be taken to the end of the building, removing the need for almost all under slab drains.

To ensure an efficient system is created the following design criteria must be followed:

a) Prime time – All roof drainage design in the UK is based on a 2-minute storm, so if the system does not operate fully in less than 1 minute, there is a risk the building will flood. As buildings have got larger, so have syphonic systems, and unless the prime time is considered, systems can be created which will not function properly.

b) Balance - Syphonic systems should be carefully balanced, usually by changing pipe sizes, to ensure the correct amount of water enters each outlet. The balance is achieved by trading off the available suction pressure against the pipe and fitting energy losses. Also try to balance the flow rate into each outlet, although it is not critical for them to be exactly equal but large difference will result in air being pulled into the low flow rate outlets.