There are two types of rainwater drainage systems: conventional systems that are incorrectly called gravity systems, and the Rainplus® siphonic system, which is also known as vacuum system or full section system.
A conventional drainage system can be designed for large surfaces but does not cut off air flow into the pipe. For this reason pipes are sized for filling ratios of 20% or 33% (depending on national or local standards and regulations) allowing considerable amounts of air into the pipes up to 80% or 67% of the pipe section.
In conventional roof drainage, outlets are simple “funnels” installed on the roof covering and connected to the downpipes which are as high as the building and water collectors, which require a gradient of at least 1%, are dimensioned for a maximum filling factor of 70%.
When water collectors are very long and it is not possible to provide the minimum slope necessary due to the limited space available, the only solution is to increase the size of the pipes with a consequent rise in installation costs.
Both use the force of gravity but in a decidedly different manner, resulting in differences in performance, design and calculation.
The Rainplus® siphonic drainage system is made up of special outlets incorporating an anti-vortex plate that prevents air from entering the pipes. The outlets are connected through short pipes of relatively small diameters to the horizontal water collector which is located just under the building roof.
The collector pipe, generally installed at the highest possible position, runs horizontally (no fall angle is required) until it is connected to the downpipe. The downpipe drops into the drainage line which is buried in the ground and conveys the water straight into a collection tank or into the municipal stormwater mains.
The absence of air in the system allows it to work 100% full of water, making use of the entire pipe section and vastly increasing flows, that are 10 times faster when compared to conventional drainage systems.
In conventional roof drainage systems, the outlet does not incorporate any sort of insert or device, hence the flow entering the system is characterized by a vortex pulling air into the pipes.
Flow in a Rainplus® outlet
With the Rainplus® siphonic drainage system, at design flow values, the roof outlets prevent air from entering and forming vortex, thus ensuring the system works at full capacity; in such conditions design can be based on the equations of fully developed flow rates operating at positive or negative pressures (Bernoulli’s energy conservation principle).