An increase in rainfall rate and the acceleration down the vertical creating suction leads to the water being pulled into plugs that fill the pipe for short lengths.
These plugs also push out pockets of air entrapped between the full bore plugs as shown below. As the rainfall rate intensifies, the water almost fills the whole of the pipe and any remaining air is carried out in bubbles entrained in the water flow.
As the design rainfall rate is achieved, no air remains in the pipework, leaving the pipes fully primed to run full bore. At full-bore flow with high, self-scouring velocities, the HydroMax system is now utilizing the full height of the building to draw water efficiently off the roof. As the air is removed from the piping system the whole system changes from the empty pipe system to a fully primed system with the downpipes also full.
The total of all of these energy forms, according to Bernoulli, is balanced even though the individual pressure, velocity or potential energy conditions may change. In mathematical form, BernoulliOs theory is expressed as Equation 2 here. Obviously, the frictionless assumption of Bernoulli is made to demonstrate a basic principle of energy conservation.
However, real fluid flow in a pipe experiences energy loss due to viscous effects, according to the Darcy-Weisbach Equation shown in Equation 3. People often make the incorrect statement that a siphonic system is not a gravity system. Traditional gravity drainage operates at atmospheric pressure throughout the pipe system and relies on the physical gradient of horizontal piping to induce flow. While a siphonic system operates under different physical conditions, it still works under the influence of gravity, as the Disposable Head term on the left side of Equation 5 shows.
Without gravity, a siphonic system does not work. The key to a siphonic system, apart from the siphonic drain, is the downpipe. Once the piping is filled with water, the column of water in the downpipe wants to fall.
The water in the horizontal collector pipe is pulled to the downpipe to replace this water. As a result, the water pressure in the horizontal collector falls below atmospheric. Water on the roof is literally sucked or siphoned into the connected drains. If it happens to rain at a rate well below the design DRI, the system simply acts like a traditional gravity system with an air-water mixture at atmospheric pressure.
The drainage rate in the collector pipe, even when partially full and with no pitch, is surprisingly good. Once the system is fully primed, there is an abrupt increase in system velocities that serve to clean the system of any debris that may have been ingested. Siphonic drainage flow patterns are a subject of much intensive research. This author had the opportunity to observe the largest test rig in the world at the University of Sheffield in the United Kingdom.
Clear sections of piping in the system allow visual observation of the flow patterns. Since sub-atmospheric pressures generated at the top of the downpipe induce water flow, there is no need to pitch horizontal collector piping. In fact, this pipe can be installed perfectly flat. The flows induced by water falling down the downpipe allow the system to operate at flow velocities higher than can be achieved in atmospheric gravity systems.
This "full-bore" high velocity flow allows the engineer to specify smaller pipe diameters than would have been prescribed by plumbing codes for traditional gravity systems. Because siphonic piping does not have to be pitched like traditional open channel drainage, the piping system requires no more space than other pressurized mechanical piping systems. This makes it easier to provide adequate drainage in buildings with challenging architectural limitations with respect to space.
Fewer downpipes means fewer chances of interfering with architecture. Because locating downpipes in a siphonic system is more flexible, the design can accommodate not only architectural demands, but also site drainage limitations. Syphonic systems are the most efficient method of doing this, and whilst they can be applied to most industrial roofing designs you must always ensure that your system is installed by competent syphonic drainage contractors make sure your system runs properly.
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