Unlike a gravity drain system which mixes air and water by the vortex effect, the siphonic evacuation, which is also called depression evacuation, full section or induced vacuum, prevents the air to enter in its conducts as a consequence of the known "Coriolis effect".
BRIEF SYSTEM DESCRIPTION
This absence of air, which is achieved for relatively low thresholds of rainfall intensity, produces a physical suction phenomenon: the water is immediately "suctioned" from the downpipe and occupies 100% of its useful section, so the system must be calculated at full pipe. The siphonic design induces an hydraulic piston in the vertical or downpipe, which is the "natural motor" of the system.
This phenomenon is caused by the Bernouilli theorem, which proves the conservation of the total energy of the water in the downpipe. Thanks to this effect, the siphonic system can provide an extremely high drainage capacity by using pipes with a much lower diameter. This is due to the high speeds of evacuation achieved using the siphonic effect. That is, if the flow must always be equal to section per speed, it is clear that the same flow can be evacuated by different values of the two factors, and in the particular case of the siphonic system, a lower section implies higher fluid speed.
To induce siphonic action two basic conditions must be met:
- Surface of the liquid to be evacuated above the point of discharge.
- Avoid air entry into the pipe.
The role of anti-vortex device of outlets is to prevent the entry of air and thus it gets "speed up" the process of priming the system.
The self-priming outlet is therefore a key part of the system because it "sucks" the water in the guttering, preventing the entry of air. The pipes are sized considering they will work at 100% capacity, from the roof to the ground level, according to the current design, which gives an idea of the importance of the right choice of this parameter by the designer of the building.
Being the whole pipe full of water from the gutter to the collection point, the hydraulic piston is the full height of the building, opposite the spout height which would act as a conventional piston. A higher energy resulting in evacuation speed much higher than the traditional or conventional gravity system.
The full section flow allows to use all the building height as hydraulic charge where the negative pressure in the water column of the vertical pipe accompanies the siphonic function having its maximum point of depression in the upper side of the downpipe and gradually moving to experience positive pressures in the lower section once the vertical stretch finish.
The hydraulic load entails an increase in the speed of water higher than 7 m/s (1.5 m/s would be maximum for a gravity system) which results in a considerable reduction of the whole piping system.
Bernoulli Continuity Equation:
h1,h2 = height in which points 1 and 2 are placed.
ρ1/γ, ρ2/γ = pressure in points 1 and 2.
V21/2g, V22/2g = flow pressure
ΣH1-2 = hydraulic losses between points 1 and 2
It is deduced:
Δh1-2 ≥ ΣH1-2
The height difference between collection and discharge must be greater than the hydraulic lost.
Calculation of diameters using the Colebrook-White's formula
It is important to highlight that the fact of functioning with pressure also in the inferior collector section (normally buried or pending from slabs below the discharge elevation plants) has a substantial advantage, which is the absence of pending, and therefore saving in excavation costs or loss of size (or gauge) in case of devising by a lower plant.
This has particular impact in areas of high water tables or bedrock and also in large car parks in shopping centres or logistics buildings, for example.