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Points on Sizing a Siphonic Roof Drainage System

In sizing a siphonic roof drainage system, much of the design centers on the balance between the energy available to the system and the energy lost from each drain to the point of discharge. Energy within the system is lost as a result of viscous flow through the piping system in a full-bore steady state condition to achieve the specified drainage rate. For example, the height of the building represents the available “potential energy” to the system and is akin to a centrifugal pump performance curve. With a given roof surface area to be drained at specific rainfall intensity, the total design flow, including the flow to each drain, is the designer’s starting point. The pipe system diameters, bends, increasers and even drains are then selected and analyzed for energy loss to a point where the energy balance is satisfied to achieve the design flow, requiring fairly precise calculations.

The required resistance coefficients for common pipe components like elbows and increasers are available from engineering references. Siphonic roof drains have their own specific resistance coefficient and a maximum flow capacity. This data is also required. The data for siphonic roof drains is available in ANSI/ASME A112.6.9 “Siphonic Roof Drains” and the pending ASPE Standard on Siphonic Roof Drain. Smith’s 1005 and 1605 Siphonic Roof Drains meet both of these standards. For assistance on sizing, see the Specifier’s Guide.

The hydraulic principles of siphonic roof drainage are quite simple and no different than any other piping system, but the calculations are easier to resolve with the assistance of computer software to facilitate in the repetitive calculation process and enhance accuracy of the results.

For more information about calculation software, training, and engineering support, contact:

Rainwater Management Solutions (RMS)
1260 West Riverside Drive
Salem, VA 24153
Telephone: (540) 375-6750
Fax: (540) 375-6751

E-mail: siphonic@rainwatermanagement.com

http://www.rainwatermanagement.com		 Codes and Standards

Siphonic Roof Drains Codes and StandardsIn August of 2002, the American Society of Mechanical Engineers (ASME) Committee A112 “Plumbing Materials and Equipment” formed A112 Support Team PT 6.9 (PIR 02- 6) for the purpose of drafting a standard for siphonic roof drains. On March 15, 2004, this proposed standard (designated A112.6.9 “Siphonic Roof Drains”) was approved. It was then submitted to and approved by the American National Standards Institute (ANSI) on July 8, 2005. This standard establishes the testing procedures for siphonic roof drains used to determine the product resistance coefficient, flow range and response to varying flow rates.

Hydraulic tests were performed at CRM Laboratory, Bury, United Kingdom, under the supervision of HR Wallingford, on Smith’s siphonic roof drains. The tests were performed in a test rig conforming to the ASME Standard A112.6.9-2005. The test covered the following aspects: observation of water patterns and effect of debris on capacity, determination of rating curve and single resistance value, and response to sudden inflow. All of Smith’s siphonic roof drains were tested successfully. A copy of the certificate is available upon request.

The test procedures for our drains are based on the exact tests used by drain manufacturers in Europe where the siphonic drainage method has been successfully used for many years. All manufacturers of siphonic roof drains offering products for sale in the United States will need to certify their product(s) as compliant with this standard in the same way other plumbing fixtures and equipment must comply with ASME standards. Smith siphonic roof drains are at the forefront of product certification.

There are no U.S. testing laboratories equipped to test siphonic roof drains. CRM Laboratory has the test facility, expertise and experience to perform and certify siphonic roof drain testing not only to satisfy European Standards but also the ASME Standard A112.6.9-2005 Siphonic Roof Drains.

Always dedicated to the enhancement of technical expertise in the evolving community of plumbing engineering, Jay R. Smith Mfg. Co. is actively involved in the American Society of Plumbing Engineer’s (ASPE) publication of their technical standard for the design of siphonic roof drainage systems. This technical standard is based upon the hydraulic principles common to all mechanical piping design and catered to the specific parameters of siphonic roof drainage. The standard promises to be the “gold standard” in siphonic roof drainage design and will provide the design professional with everything necessary to validate engineering calculations.
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