FAQs

If you can’t find the answer to your question below please do not hesitate to call our Technical Manager Martyn Bramhall; Tel: 0114 247 3655 or e-mail m.bramhall@uk.fullflow.com.

What would be the result if an outlet were to block?

It has been shown through testing, that if a small number of syphonic rainwater outlets within a system were to block, the remaining outlets would accept the flow that was apportioned to the blocked outlets. However, the required working head of water around the operational outlets would increase in order to accept the additional flow.

The capacity of a syphonic system is determined by the pipe work dimensions and not necessarily by the outlets. For this reason it is possible to achieve high flow rates through outlets, providing that an allowance is made for the additional working head of water. Should an outlet become blocked the capacity of the system is not affected as the pipe dimensions remain the same. It is highly unlikely that the blocked outlet will allow air to enter the system, as the outlet rapidly becomes submerged due to the increased requirement for additional working head of water.

I’ve heard the pipes may compress when using a syphonic system?

During the operation of a syphonic system sub atmospheric (negative) operational pressures are generated within the pipework system.

Pipes subjected to internal vacuum or positive external pressures are likely to flatten (Buckle) or fail in a catastrophic manner.

The allowable vacuum operating pressure should therefore represent one of the design limitations. There are however other conditions present within syphonic systems, which have a impact upon the permissible vacuum pressure, notably cavitation and the spontaneous vaporisation of water into a vapour or gas.

Commonly acceptable boundaries of -8mWc are employed with appropriate dimensional limitations. The preferred material for construction of syphonic systems is high density polyethylene (HDPE). This materials has the advantage of being available in a wide range of diameters and the ability to be flattened without rupture or discharging of the pipe contents. However the material has time dependant properties, which affect its load carrying capacity.

Pipe work design codes and information are more commonly directed towards the ability of HDPE to withstand internal positive pressures for periods of more than 50 years without suffering failure. When vacuum pressures are addressed on 50 year loading period, it becomes clear that these loading conditions are not present within a syphonic rainwater drainage system.

The acceptable operational pressure varies from piping material to piping material. The ability of any pipe to withstand internal pressure or vacuum is dependant upon its dimensions and material properties.

By their very nature syphonic rainwater systems often operate at sub atmospheric pressures, although this is not always the case. The early system designers often neglected the need to install pipe capable of operating under sub atmospheric pressure, and utilised typical uPVC drainage pipe. Light and easy to work with, uPVC is also scratch and notch sensitive, which means that any damage to the surface produces an inherent weakness within the pipe. The inherent weakness combined with the brittle nature of the material in some cases caused pipes to fail when subjected to sub atmospheric pressures. Unfortunately, this occurred when a rainstorm was at it’s most intense. As designers became more aware of the required strength of material, piping systems were constructed using a more robust material known as high-density polyethylene (HDPE). The choice of this material also requires careful consideration as the pipes overall strength is dependent upon the wall thickness. 

How does a syphonic system interface with underground drainage?

In order to correctly size the underground pipe work associated with a syphonic system we must first understand how the syphonic action within a system is initiated. Priming of the syphon is achieved by ensuring that the velocity of the water is sufficient that the air in the pipes becomes entrained within the water. I.e. that bubble flow forms. Experiments undertaken by May et al (1991) have confirmed that air entrainment velocities are within 2 – 3 m/s.

Due to the hydrodynamic shape of the Primaflow rainwater outlet, the waters velocity is rapidly increased over a short distance. This rapid increase in the waters velocity ensures that the tail pipes of the individual outlets prime independently of the main system, ensuring that the syphonic action begins during the early part of a rainstorm. If the rainstorm is of sufficient capacity to allow the air entrainment velocities to further develop within the main collector pipes, almost full syphonic action will ensue.

By understanding the priming process and the need for air entrainment velocities, it follows that in order to break the syphonic action the water velocity must be reduced. The velocity of the water is calculated through dividing the flow rate by the area of the pipe, therefore, by increasing the pipe diameter the velocity of the water will reduce. It is common practice to install underground pipe work to gradients (or slopes), which when combined with an increase in diameter will, along with the inflow rate allow to pipes to flow as an open channel with an air / water interface. i.e. flow part full. For each down pipe of a syphonic system a flow rate and velocity is shown on the installation drawing, this figure is within 10% of the maximum achievable flow rate. Therefore, the underground pipe work should be dimensioned in such a way as to flow part full at this given flow rate. The recognised method of calculating flow rates within pipes of varying diameters and gradients is through tables such as "Tables for the hydraulic design of pipes, sewer and channels, H.R.Wallingford". It is recommended that the underground pipe discharges into an inspection chamber prior to joining the main sewer system.

Theoretically there is no need for any further requirements with regard to the design of underground pipe work, however, most underground systems are designed to surcharge (completely fill) occasionally. In order to accommodate the downstream surcharging it is necessary for the underground pipe to enter an inspection chamber with a vented grate. The vent (with an open area equal to, or greater than the area of the syphonic system discharge pipe) will allow the syphonic system to discharge (albeit at a reduced rate) even if the underground pipe work down stream of the inspection chamber were to surcharge.

If a system is installed with polyethylene pipe, how is the issue of expansion dealt with?

Once a system has been designed, analysis will show the operating parameters. It is vitally important that the pipe used within the design is of an adequate wall thickness and correctly supported. The pipe supports also add strength to the pipe and restrict linear expansion. Fullflow Limited utilise a closely toleranced pipe clip and rail system in order to achieve not only an aesthetically pleasing system, but a system that is practical in its installation and operation, which in turn leads to customer confidence.

Tell me more about the occurrence of Self-Cleansing Velocities?

As discussed in the Underground sizing / Breaking of the syphonic action section, there is a requirement for high water velocities within the pipe work in order to achieve the priming of the syphon. Coincidentally, the priming velocities are far in excess of those recommended as self-cleansing. BS 8301: 1985 code of practice for Building Drainage recommends that self-cleansing velocities should be maintained under normal working conditions. During most rainstorms self-cleansing velocities are achieved for varying periods of time as the type of flow fluctuates between patterns 1, 2 & 3 highlighted below.

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 a result of achieving high velocities within a syphonic system there is no requirement for rodding access. Furthermore, the inclusion of rodding access could cause areas of possible air ingress to the system resulting in a reduction of system performance.

Additionally, as with all types of rainwater system, recommended maintenance schedules play a key role in the continuing operation of a syphonic rainwater system. Fullflow Limited issue a Clients Maintenance Package with all installed systems and by adhering to this document the risk debris collecting in the pipe work and/or any outlets becoming blocked will be kept to a minimum.

Is the system available in material other than HDPE?

Yes, Fullflow can provide the system in a range of materials. We have provided systems in Cast Iron, Galvanised Mild Steel, Copper and Stainless Steel.

It is feasible for the system to be produced in material other than those mentioned providing the material has the necessary qualities (as determined by Fullflow) for the system to function effectively. Please contact Fullflow for further details. E-mail: info@uk.fullflow.com / info@fullflow.es or call 0114 247 3655 / +34 666 538 331.