For many years Geberit International has felt that the way forward has often been to examine previous research and development for knowledge that can be applied to new, sophisticated and exciting products.

Although not a product in the usual sense, the well-established principle of siphonic roof drainage offers an example of this ‘test and re-test’ approach.

When the system was developed a couple of decades ago the big question was whether it would work under extreme rain run-off. Time has shown the answer to be a resounding yes.

Most in the industry did not question whether the system should be examined under minimal run-off – if it could handle peak loads then everything else would be all right.

However, Geberit International engineers have conducted a research program to examine the system’s low run-off performance, and they presented a paper to the CIB W062 2006 Symposium held in Taipei, Taiwan.

Building services and plumbing design personnel are well aware of the many difficulties in achieving a safe, efficient and economical roof drainage system.

A system with very large gutters and down-pipes may be the answer in many instances but is unacceptable in many modern buildings due to aesthetics, space or cost restraints.

As industrial, commercial and supermarket projects continue to grow in scope, alternative solutions are being applied.

About 20 years ago Geberit International was at the forefront of developing a siphonic roof drainage system relying on a designed head pressure partly dependent on the use of relatively small drain lines. Early systems also used the roof design to create a substantial head and volume of water over the surface outlet point. Another feature related to the size of the vertical drain conveying water from the under-roof space to the stormwater drains. Pipelines in the under-roof space were intentionally installed at a flat grade.

These features separately and in combination resulted in reduced space requirements and removed rainwater from roof to underground drains safely efficiently and economically. The greater the rainfall the more effectively the system worked.

Under minimal rainfall the system did not reach its full capability until the roof outlet and horizontal drain lines were filled in order to achieve a siphoning condition. This low flow was evident most of the time, as heavy downpours are the exception rather than the rule. The part-load condition led to air suction and mixing, which created flow activity characterized by a multi-phase phenomenon.

The Geberit researchers’ contention is that the behavior of the water-air mixture should be considered when designing a system, particularly where an accurate estimate of working pressures under part-load conditions is required.

Additionally, study of the mixed flow is important when there is a need to provide for dynamic and acoustic characteristics.

The research was based on a tower with a single outlet fitted to the top level, utilizing pipes of three sizes and two vertical drop lengths. Differing rainfall intensities were simulated with a variable-speed pump. The pipeline configuration replicated as near as possible a typical installation.

The pipe sizes were DN56, DN63 and DN75. The vertical drop lengths were 10m and 5m. Flow rates in the pipelines, expressed in liters per second, were: 2.6, 6.1, 9.0, 11.9 and 13.5. Each pipe size was tested on both vertical drops.

The installation was fitted with a siphonic roof drain (SRD) outlet connected to a short length of vertical pipe which, by use of a 90° bend, connected to a short section of clear tube of the desired diameter. Flow in this section of pipe was observed and filmed using a high-speed camera (640 frames per second).

Transducers were fitted to the vertical line just below the SRD and results relayed to a computer via a pressure sensor.