Siphonic drainage explained
Basic principles
Akatherm siphonic roof drainage systems are engineered on the concept of a full bore (a fill rate of 100%). This implies that rainwater flows at high speed through small diameter pipe work, at normally zero gradient.
This siphonic effect is created by the (kinetic) energy derived from the hydraulic head, caused by the difference in height between the roof outlet and the discharge point in a building. Specialist roof outlets prevent air being sucked into the system.
The engineering principle of siphonic roof drainage design is based upon the Bernoulli energy equation for a steady flow of an incompressible fluid with constant density. In order to balance the equation, and to quarantee the required siphonic effect according to the rainfall's intensity, the ideal pipe dimensions per flow path need to be determined.
- Determining the number of roof outlets/flow paths
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The total volume of rainwater that has to be evacuated by the system can be calculated using this equation:
Having calculated the total volume of rainwater that has to be drained, the number of roof outlets can be calculated using:
The volume flow per roof outlet has to be limited to 85% of the drainage capacity of the outlet to be able to balance the system at a later stage in the design. Determining the number of roof outlets one has to take in account the structural details of the building like firewalls, roof construction and other (small) roofs that drain their rainwater onto the calculated roof surface. On each lowest point of the roof construction a roof outlet has to be placed. The maximum distance between 2 outlets is 20 m.
From the product range of roof outlets the correct roof outlet can be chosen depending on roof construction, roof membrane or heating element. Please use our roof outlet selector to determine the best roof outlet for you.
- Calculation principles
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A roof from which rainwater is drained by means of a siphonic systemgenerally contains several roof outlets that are collected into a single down pipe. The Bernoulli equation needs to be applied to every flow path from roof outlet (entry point) to the transition to partial filling (exit point).
The purpose of the calculation is to keep the static residual pressure at the exit point of every flow path within ±100 mbar. The static residual pressure of a flow path is equal to the available pressure difference created by the height difference between the entry point and the exit point (ha) minus the pressure loss
caused by the pipe friction in the
auxiliary sections of the system.The available pressure difference is calculated as following:
Pressure loss is calculated as following:
For a detailed calculation example and further requirements for a siphonic system please check the download area.
At the heart of the siphonic roof drainage system, the roof outlet responds to negative pressure conditions in the system by streamlining the fluid flow, whilst preventing the entry of air. For every level of rain intensity, type of roof construction and roof membrane, you can choose a dedicated Akasison roof outlet.
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