Syphonic Drainage System
London City Hallâ€™s drainage system is designed by Fullflow. â€˘ Like most of the the design of the City Hall, the drainage system of this building is also quite simple but efficient. Syphonic system operates at full capacity, when water is sucked or syphoned from the roof down into the drain at high velocity. The Fullflow system allows architects, consultants and contractors alike to specify a lower number of roof outlet drains and have them flow into a single downpipe. The syphonic drain incorporates an anti-vortex plate that acts as a baffle, allowing only water to be drawn of the roof. During heavy rainfall the outlet drain fills to above the anti-vortex plate, cutting
off air flow into the pipe. This lack of air, coupled with the downward pull of the water creates a vacuum.
The water pressure in the system falls below atmospheric pressure and water on the roof is sucked into the connected drains. On reaching the ground, the water travels into a vented manhole or inspection chamber where it is discharged at atmospheric pressure into the storm sewer. Source: http://www.fullflow.com