Siphonic roof drainage: What’s on the horizon?
BY JERRY CARSON
Looking to the east from my office I can almost see into the far distance — the Ivory Towers of the Wise Siphonic Roof Drainage Men. The paths leading to them are little used and overgrown, far from what they had expected. If I turn about and listen closely I can hear the Medicine Man in the West, “The building’s not sick, it doesn’t need a new solution,” he murmurs. Of course the Young Braves, keen to make their mark, are singing the praises of “new technology”; but they are still in awe of the thinking of the Elders who tell them, “For generations we’ve worked with open channel flow and Manning’s formula. Our technique is simple to understand and easy to apply using tables that dictate the flow rates under given situations. Change can be good but what we know is better.” So say the Elders.
Is change inevitable?
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, so what’s the fuss about?
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.
SRD utilizes 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 drain[s] 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 depressurized 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), 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, (see “Anatomy of a system” below) 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 an “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 “tail pipe,” — consisting of a vertical section and a horizontal section — that links each roof drain to a lateral collector pipe. In the majority of systems, multiple tailpipes connect to the collector pipe(s), which ultimately branch into a vertical downpipe. In any siphonic roof drainage 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 while 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 minimize 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. While 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"). While 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 vapor 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 feet water column below atmospheric — at sea level. The effect of pressure also should be considered in relation to the choice of pipe material. While 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. Safety first! For cast iron systems the 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.
Siphonic roof drainage 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. The huge debt should be acknowledged to those Wise Siphonic Roof Drainage Men for their great efforts in identifying and exploiting the potential of this technology. They have taken the concept around the world and have proved both the technique and the technology beyond doubt.
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, I designed a siphonic solution to drain the roof of a large distribution warehouse. The design took a morning to produce and the client realized a saving of $240,000. That’s really what the fuss is about!
Jerry Carson is vice president of Siphonix — MIFAB's UK siphonic roof drain partner.