In traditional gravity-based rainwater drainage systems, water flows together with air inside the pipe. Horizontal pipes must be given slope and the entire pipe cross-section does not fill with water. This prevents pipes from operating at full capacity.
Siphonic systems, on the other hand, prevent air from entering the pipe system while water flows thanks to specially designed roof filters. Thus, pipes operate at 100% fullness, ensuring full cross-section flow. As a result, it becomes possible to draw water at high speed with the vacuum effect created, and there is no need for slope in horizontal pipes.
When rainfall begins, the system initially behaves like conventional; that is, at low flows, water can flow partially full (air+water mixed) in pipes.
As rainfall intensity increases, the vortex-breaking baffle plates of the siphonic filters on the roof restrict air entry into the downstream pipe system. When pipes fill to full cross-section, the water column formed in the vertical downpipe creates a negative pressure (vacuum) inside the pipe as it flows downward.
Since there is no slope in horizontal pipes, water can also flow horizontally with the vacuum effect, thus making it possible to direct water to the desired point inside the building.
When rain stops and water level drops, the system takes air again and returns to normal pressure level and remains until the next rainfall. In summary, when rainwater exceeds a certain flow rate, the siphonic effect comes into play: Thanks to the completely full flow of pipes and negative pressure, the limits of traditional gravity drainage are exceeded and accumulated water on the roof is quickly sucked and removed.
Thanks to completely full (pressurized) flow, the pipe diameters required to carry equal flow are significantly smaller compared to traditional systems. Effective use of pipe cross-section provides benefits in terms of material savings and aesthetics.
In a siphonic system, usually only one vertical downpipe may be sufficient for each gutter/drain. Since air entry is prevented, a single vertical pipe can carry the water of a large roof area. This eliminates the need to use many columns/downpipes inside the building. As a result, space is gained in interior spaces, architectural design flexibility increases and infrastructure costs around columns especially decrease.
In siphonic systems, horizontal collection pipes can be installed at 0% slope. Since water can be carried along the horizontal line with the vacuum effect, pipes can cover long distances near the ceiling. This provides more freedom in architectural design and water collected on the roof can be conveyed to the desired point inside the building without slope concerns.
Since siphonic systems operate with completely full and high-speed flow, the risk of sediment, leaf, etc. accumulation inside pipes is minimal. The flow regime creates a dragging effect on pipe inner surfaces, cleaning possible accumulations. Thanks to this self-cleaning feature, the probability of clogging is very low.
In traditional systems, multiple downpipes are connected to separate manholes or collector lines at the building foundation. In siphonic systems, water is transferred outside the building from one or a few points through combined horizontal lines inside the building. Thus, there is no need to establish a wide rainwater network inside the building.
The combination of the above advantages positively reflects on the total system cost. Thanks to fewer pipes, less labor and infrastructure, siphonic systems generally offer economical solutions. In international applications, it is stated that a properly designed siphonic drainage can provide cost savings compared to an equivalent gravity system.
Siphonic rainwater drainage systems are especially preferred in structures with large roof areas.
Industrial facilities, factory and warehouse buildings
Shopping centers
Stadiums
Aircraft hangars
Buildings with large interior spaces such as atrium, hall
Projects with large and flat or low-slope roofs
In general, siphonic systems can be used in any type of structure where roof area is large, rainfall load is high and drainage is difficult with traditional methods.
The design of siphonic rainwater systems requires expertise as it is different and more complex than traditional systems. Compliance with international and national standards during the design phase is extremely important.
European Standard - regulates the calculation of roof drainage systems in buildings.
British standard - a comprehensive guide document on the design, application and maintenance of siphonic roof drainage systems.
It is determined according to intensity-frequency-duration curves obtained from State Meteorology data. The system is calculated to transition to completely full flow within a maximum of 1 minute according to the determined design rainfall.
It is done based on Bernoulli equation and energy conservation principles. The appropriate diameter and length of each pipe segment and continuity of flow under slope-free conditions are ensured.
The system's filling time, internal pressure fluctuations and allowed water accumulation height on the roof surface are critical design criteria.
The design and application of siphonic systems must be carried out by engineering teams specialized in this field.
Use of standard-compliant materials, correct hydraulic calculation and careful installation are the three fundamental components of system success.
An incorrectly designed or improperly applied siphonic system may fail to create the expected vacuum effect, leading to water accumulation on the roof.
It is recommended that the system be tested with water and leak-tightness and performance checks be performed before commissioning after installation.
In conclusion, siphonic rainwater drainage systems are a high-performance rainwater discharge method based on engineering calculations. When properly designed and applied, they can discharge more water faster with less material compared to conventional drainage systems, providing flexibility and economy to building design.