Siphonic rain water collection - how does it work?
Rainplus® is defined as a siphonic rainwater drainage system because it is based on the same principle as a siphon. The siphon is in general a reversed U shaped pipe used to pour a fluid from one container to another located in a lower position. When the pipe is full, the fluid contained in the longer stretch of pipe tends to fall due to its weight causing the suction of the liquid in the shorter section, which is lighter in weight. This process only starts when the pipe is completely full and continues until a balance between the two containers is reached: either when the two containers reach the same level or when the fluid level in the higher container goes below the pipe inlet section. The driving force that causes this effect is a result of the difference in height of the two containers: the bigger the difference, the stronger the driving force and, as a consequence, the greater the flow velocity in the pipe.
The performance of the Rainplus® siphonic drainage system is therefore decidedly better than a conventional system where the driving force is generated exclusively by the amount of water that accumulates on the roof. When the siphonic drainage system runs at full capacity, the “syphon effect” is triggered resulting in a driving force that is proportionate to the height of the roof and the end of the circuit, which is typically located at ground level. Such power generates levels of positive and negative pressures in specifi c points of the circuit such as to rapidly increase the velocity and, as a consequence, the flow rates of the system.
Rainplus® vs. conventional systems
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 a vacuum system or full section system. Both use the force of gravity but in a decidedly different manner, resulting in differences in performance, design and calculation.
A conventional drainage system can be designed for large surface areas 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 of 80% or 67% of the pipe section.
In conventional roof drainage, the outlets are simple “funnels” installed on the roof covering and connected to the downpipes which are as high as the building and the water collectors which require a gradient of at least 1%, are dimensioned for a maximum filling factor of 70%. When the 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.
Flow in a conventional outlet
In conventional roof drainage systems, the outlet does not incorporate any sort of insert or device, hence the flow entering the system in characterized by a vortex that pulls air into the pipes.
The Rainplus® siphonic drainage system is made up of special outlets that incorporate an anti-vortex plate that prevents air entering into the pipes. The outlets are connected via 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) untill it reaches connection with the downpipe. The down pipe drops into the drainage line which is buried in the ground and conveys the water straight into a collection tank or the municipal stormwater mains.
The absence of air in the system allows it to run 100% full of water making use of the entire pipe section and vastly increasing fl ows that are 10 times faster when compared to conventional drainage systems.
Flow in a Rainplus® outlet
With the Rainplus® siphonic drainage system, at design flow values, the roof outlets prevent air from entering and forming a 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).