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Siphonic solutions the way forward

The US-based Mifab* is promoting siphonic roof drains, which represent the latest technique in drain large roofs with minimal pipework. Jerry Carson of Mifab Siphonix elaborates on the system.

01 November 2009

TRADITIONAL thinking and techniques have determined not only drainage strategies but also how roofs have been designed for generations. Yet the size and complexity of buildings continues to increase.

So it is surely good engineering practice to actively examine more flexible and appropriate solutions to draining roofs. Siphonic roof drainage (SRD) is such a solution.

How does it work?
Put simply, SRD is the application of a technique to drain large roofs with minimal pipework comprising vertical pipes and lateral pipes without grade.
It utilises the potential energy inherent in the collected rainwater on a roof as it descends to the point of discharge. As the water falls through the roof drains and into the pipework, potential energy is converted to kinetic energy. As the water flows through the pipe system, the pressure drops. This lower or negative pressure will, in turn, draw into the piping a homogenous mixture of water and air through the roof drains above it.
If, however, air is prevented from entering into the system, the negative pressure can only draw in more water, further reducing the volume of air entrained in the pipework. The pressure within the pipe system will reduce until a point is reached where all air has been evacuated and only water remains, completely filling the pipes. The stage of what is termed ‘full bore flow’ has now been reached and the system is working at its maximum capacity. During full bore flow, the system is depressurised and so there is no need for inclined pipes to induce the flow of water to the discharge point.
Clearly, a siphonic system will not work at full capacity every time it rains. Under light rainfall or at the beginning of a storm, water will enter the roof drains and, since water will always find the point of least energy (and take the path of least resistance), it will flow slowly along the pipes draining the roof in the traditional manner. The smaller pipe size and absence of inclination in the pipes initially limits the flow rate allowing the rainwater to build up around the roof drain. However, once the water level reaches the air baffle, the flow rate through the roof drain increases dramatically as siphonic action begins.

Anatomy of a system
Siphonic roof drains are a little more sophisticated than conventional types. The significant difference is the inclusion of the ‘air baffle’ located just above the top of the siphonic roof drain and extending some way beyond the throat of the drain. Crucially, a siphonic roof drain must be thoroughly tested to ensure that it operates in a predictable manner and its hydrodynamic characteristics are fully understood and documented.
Connecting to each siphonic roof drain is a ‘tailpipe’, consisting of a vertical section and a horizontal section, which links each roof drain to a lateral collector pipe. In the majority of systems, multiple tailpipes connect to the collector pipes, which ultimately branch into a vertical downpipe. In any SRD system, it is the tailpipes that first operate siphonically.
The increased flow rate generated by the siphonic action in the tailpipes forces air out of the lateral collector and downpipe and facilitates siphonage throughout the whole system. It is important to note that whilst identical roof drains may be used at each drainage point on the roof, the capacity of individual drains may well be completely different. Drainage capacity is determined not only by the roof drain but also by the pressure in the connected tailpipe.
For the whole system to function correctly it is important that, once fully-filled (primed), each tailpipe continues to operate siphonically. If one tailpipe has insufficient water to maintain its primed state, it could allow air to enter the pipework and break the siphonic action throughout the entire system. It is for this reason that each drain must be “balanced” to minimise the head difference.
In general terms, the collector will have a greater diameter than tailpipes and increases in diameter as more tailpipes are connected to it. At no point should the collector decrease in diameter along its length though it is possible that in some designs the collector may be of a single diameter along its entire length. The downpipe should never have a greater diameter than the collector at its maximum and it is commonplace to reduce the diameter of the downpipe as it descends. Whilst this may seem alien compared to traditional drainage solutions, it is an accepted and extremely effective method of controlling the pressure in the system and hence its capacity.
Controlling the pressure within a siphonic system is often the most difficult aspect of the design phase. If large negative pressures are allowed to be generated in the tailpipes, extremely high fluid velocities may be experienced in the small diameter pipes (typically 3 inches). Whilst high flow velocities are rarely a real problem, it is preferable to control them.
Of far greater concern is where the pressure is allowed to drop too low (below the vapour pressure) and bubbles are generated in the water, a condition known as cavitation. At this stage, the water is effectively boiling and as the steam bubbles reach regions of higher pressure, they implode, generating significant noise and energy. Once initiated, cavitation may continue even as the pressure rises, producing noisy systems with unpredictable results. For this reason, the pressure in siphonic system pipework should be limited to 29.5 ft water column below atmospheric, at sea level. The effect of pressure should also be considered in relation to the choice of pipe material.
Whilst the pressure regime in operational siphonic systems tends to be negative, there are often instances (particularly in high-rise installations), where positive pressures may require attention both under drainage conditions and in the theoretical (albeit unlikely) case of a total blockage.
For cast iron systems, pressure is hardly ever an issue. For plastic pipes, the thickness of the pipe is the critical factor. It must be noted that pressure ratings for pipe are always stated for positive pressure, the consequences and performance under negative pressure are quite different.

Conclusion
SRD differs from conventional systems in that a design must be engineered for each and every installation.
Nevertheless, it is still only a technique for draining rainwater from roofs. To design and implement a siphonic roof drainage solution certainly requires more thought and effort than the “old methods” but it can provide exceptional rewards. Recently, Mifab designed a siphonic solution to drain the roof of a large distribution warehouse. The design took a morning to produce and the client realised a saving of $240,000.
* Mifab, established in 1982, manufactures commercial plumbing products. Operating as a family business, its factories are located in Des Plaines, Illinois and North Battleford, Saskatchewan. The company’s head office is located in Chicago, Illinois.
Mifab’s products have many features and benefits that have made them the choice of many specifying engineers and contractors. For example, a complete range of stainless steel drains and cleanouts that eliminates dishing, discoloration and corrosion normally associated with drain tops made of other materials are offered at the same cost as the industry standard, nickel bronze. Its products are distributed through a network of more than 70 representatives throughout the US, Canada, Middle East and other parts of the world.

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