Flow patterns

In order for the considerable benefits offered by self priming syphonic systems to be fully and safely realised, a total approach needs to be adopted in their design, construction and maintenance. Self priming syphonic systems represent a higher level of technology than gravity systems and, therefore, require a correspondingly higher degree of understanding and expertise.

The flow regime within the self priming syphonic system develops through a cycle as the rainstorm events unfold. Initially, the flow through the syphonic system will be as it would in a gravitational system, resulting in a partially filled pipe. Please click icon to view film.

This may be transformed into full bore flow as the storm intensity rises. Air is excluded from the system as the water level within the outlet approaches the anti vortex plate, a key element of the outlet. Syphonic action is then initiated within the pipe network as the flow velocity increases and full bore flow established causing any remaining air to be entrained within the flow as a bubbly mixture and purged from the pipework.


Flow Pattern 1
Flow Pattern 1 - Gravity Flow
Flow Pattern 2
Flow Pattern 2 - Plug Flow
Flow Pattern 3
Flow Pattern 3 - Bubble Flow
Flow Pattern 4
Flow Pattern 4 - Full Bore Flow

As this priming process progresses to the full bore flow condition the flow capacity, and hence the quantity of water discharged from the roof or gutter, will be increasing. The syphonic system may be thought of as accelerating towards its design capacity.

If the rainfall intensity is able to satisfy the flow capacity of the self priming syphonic system, the syphonic action will be sustained. As the rainstorm begins to dissipate there will be insufficient rainwater to support the capacity of the syphonic system. This will result in falling water levels, allowing air to be drawn into the piping network and breaking the syphon.

The syphonic system therefore fluctuates from a gravitational flow regime to full syphonic action during any rainstorm. The period of time spent at full syphonic action will increase as the rainstorm intensity approaches the design condition. This ability to match flow capability to requirement is unusual in a system with no moving parts. The syphonic system has a low capacity when it needs one early in the storm event, yet can automatically increase its capacity up to its ultimate flow condition should it need to do so when presented with increasing storm intensity.

The priming action is, therefore, a significant factor contributing to the performance of the syphonic system. The speed of removal of air from the pipework depends on the air entraining properties of the flow and increases as the velocity and turbulence of the water is increased. The special outlet design prevents air entering the pipework and increases the velocity of water as it flows into the system.