Siphonic System Works

Our siphonic system setup comprises of high quality stainless steel siphonic roof outlets, high density polyethylene (HDPE) pipes, fittings and a professional support package, the siphonic rainwater drainage system that allows the complete drainage of water from the roof without the requirement of a slope in the pipework, thus saving valuable space, set-up time and money. Our siphonic rainwater drainage system has been used in both the industrial and commercial sectors. This includes flat (concrete deck) roofs and any form of roof shape, which drains into a box gutter or eave gutter. Our siphonic rainwater drainage systems implemented so far include Industries, Warehouses, Shopping Malls, Office buildings, Apartment buildings, Sporting facilities, Multi-story car park decks and we efficiently undertake the Renovations of existing drainage systems too. The innovative component of our siphonic rainwater drainage system is the specially designed roof outlet which controls the pattern of discharge, preventing air from entering the pipework. By minimizing the intake of air into the system, the water creates a siphoning effect, maximizing the suction of the water into the outlet.

The concept of the siphon has been recognized since ages. A siphon is formed by a tube or other type of channel filled with the fluid to be siphoned, thereby creating a continuous and closed path. In a siphon mechanism, the release end of the channel will usually be lower than the level of the fluid at the source. Atmospheric pressure at the reservoir surface develops the driving force in pushing the fluid through the tube to the point of discharge at the lower end. The roof drain has an air baffle that promotes a full-bore flow. Flow is induced by natural hydraulic action of siphoning. When system primes, the piping depressurizes. Atmospheric pressure pushes the water into the drains with a considerable force. Despite the name, siphonic roof drainage does not work as a siphon; the technology makes use of gravity induced vacuum pumping to carry water horizontally from multiple roof drains to a single downpipe and to increase flow velocity Metal baffles at the roof drain inlets reduce the injection of air which increases the efficiency of the system. One benefit to this drainage technique is reduced capital costs in construction compared to traditional roof drainage. Another benefit is the elimination of pipe pitch or gradient required for conventional roof drainage piping. However this system of gravity pumping is mainly suitable for large buildings.

Differences between Traditional Drainage and Siphonic Roof Drainage system

A Traditional System consists of a number of roof drains connected by open outlet to a vertical downpipe. The pitch in the piping allows rainwater to flow to a discharge point. This configuration necessitates relatively large diameter stacks which connect into a larger underground drainage network. A traditional rainwater piping system is sized and pitched to be at atmospheric pressure throughout. Since pressure is constant from inlet to outlet, the only thing inducing flow is the pipe pitch.

In a horizontal pipe segment, water cascades along the invert of the pipe. About ½ of the pipe cross section is used to convey water and the remaining ½ is air at the maximum expected rainfall rate. Since conveyed water contains air, it works at only a fraction of its design capacity.

A Siphonic System induces flow by creating a full-bore continuous path of water making pitch unnecessary. The full-bore flow is achieved through natural hydraulic action and is not produced by any sort of moving part, special fitting or control in the piping network. Siphonic systems do not require any special installation kit or procedure. The pipe materials and fittings used with siphonic roof drains are the same as those required for traditional drainage systems

With a flat, level design, long horizontal runs above overhead ceilings are possible; this reduces the amount of buried pipe and the associated costs with trenching, bedding, and backfilling within the building’s footprint.

A siphonic roof drainage system is one of the most effective technologies offered for capturing rainwater from a building roof top to aid in implementing rainwater harvesting.

To make a siphonic rainwater system work correctly; several factors have to be considered:

  • Rainfall Intensity Rate
  • Use of specially designed roof outlets
  • Calculated method for sizing the pipework
  • Materials specification for the pipework
  • Designing the pipe configuration for optimum performance
  • Rainfall Intensity Rate

To ensure the building is adequately protected from water ingress, the rainwater system must be designed to remove all the water that falls on the roof quickly.

Siphonic outlets are designed to reduce the entry of air into the system, if air reaches more than 40% of the volume of the pipe the siphonic action will break. To reduce the amount of air entering the system a baffle plate is usually fitted over the orifice of the outlet, this not only reduces the amount of air being pulled into the outlet opening, it also stop a vortices forming that will draw air into the system rapidly. One of the major benefits of a siphonic system is that the horizontal pipe runs do not have any fall, minimising the space required to accommodate the system. This provides the designer with freedom to route the pipes to any location at high level, before dropping to ground level. The high suction in the system reduces the pipe diameters and number of vertical drops needed compared to a gravity system, providing a reduction in cost for most installations. The major advantage of a siphonic system is that drainage can be taken to the end of the building, removing the need for almost all under slab drains.

Theory of Siphonic system by Bernoulli’s equation:

Bernoulli's equation may be applied to a siphon to derive the flow rate and maximum height of the siphon.

Example of a siphon with annotations to describe Bernoulli's equation

Let the surface of the upper reservoir be the reference elevation.

Let point A be the start point of siphon, immersed within the higher reservoir and at a depth −d below the surface of the upper reservoir.

Let point B be the intermediate high point on the siphon tube at height +hB above the surface of the upper reservoir.

Let point C be the drain point of the siphon at height −hC below the surface of the upper reservoir. Bernoulli's equation:

=fluid velocity along the streamline

=gravitational acceleration downwards

=elevation in gravity field

=fluid density

Apply Bernoulli's equation to the surface of the upper reservoir. The surface is technically falling as the upper reservoir is being drained. However, for this example we will assume the reservoir to be infinite and the velocity of the surface may be set to zero. Furthermore, the pressure at both the surface and the exit point C is atmospheric pressure. Thus by applying Bernoulli's equation to point A at the start of the siphon tube in the upper reservoir where P = PA, v = vA and y = −d By applying Bernoulli’s equation to point B at the intermediate high point of the siphon tube where P = PB, v = vB and y = hB By applying Bernoulli's equation to point C where the siphon empties. Where v = vC and y = −hC. Furthermore, the pressure at the exit point is atmospheric pressure. Thus as the siphon is a single system, the constant in all four equations is the same. Setting equations 1 and 4 equal to each other gives: Solving for vC: Velocity of siphon:  The velocity of the siphon is thus driven solely by the height difference between the surface of the upper reservoir and the drain point. The height of the intermediate high point, hB, does not affect the velocity of the siphon. However, as the siphon is a single system, vB = vC and the intermediate high point does limit the maximum velocity. The drain point cannot be lowered indefinitely to increase the velocity. Equation 3 will limit the velocity to a positive pressure at the intermediate high point to prevent cavitation. The maximum velocity may be calculated by combining equations 1 and 3:

 Setting PB = 0 and solving for vmax:

Maximum velocity of siphon:

The depth, −d, of the initial entry point of the siphon in the upper reservoir, does not affect the velocity of the siphon. No limit to the depth of the siphon start point is implied by Equation 2 as pressure PA increases with depth d. Both these facts imply the operator of the siphon may bottom skim or top skim the upper reservoir without impacting the siphon's performance. Note that this equation for the velocity is the same as that of any object falling height hC. Note also that this equation assumes PC is atmospheric pressure. If the end of the siphon is below the surface, the height to the end of the siphon cannot be used; rather the height difference between the reservoirs should be used.

Maximum height

Although siphons can exceed the barometric height of the liquid in special circumstances, e.g. when the liquid is degassed and the tube is clean and smooth, in general the practical maximum height can be found as follows. Setting equations 1 and 3 equal to each other gives: Maximum height of the intermediate high point occurs when it is so high that the pressure at the intermediate high point is zero; in typical scenarios this will cause the liquid to form bubbles and if the bubbles enlarge to fill the pipe then the siphon will "break". Setting PB = 0: Solving for hB:  

General height of siphon:

This means that the height of the intermediate high point is limited by pressure along the streamline being always greater than zero.

Maximum height of siphon: 

This is the maximum height that a siphon will work. Substituting values will give approximately 10 meters for water and, by definition of standard pressure, 0.76 meters (760 mm or 30 in) for mercury. The ratio of heights (about 13.6) equals the ratio of densities of water and mercury (at a given temperature). Note that as long as this condition is satisfied (pressure greater than zero), the flow at the output of the siphon is still only governed by the height difference between the source surface and the outlet. Volume of fluid in the apparatus is not relevant as long as the pressure head remains above zero in every section. Because pressure drops when velocity is increased, a static siphon can have a slightly higher height than a flowing siphon.