1. Siphonic roof drainage systems: no space to breath
Marc Buitenhuis
Hydraulic research engineer Akatherm International BV, Panningen, The Netherlands
23-11-2009
Abstract
This article describes the main difference between conventional and siphonic roof drainage
systems, ie: the prevention of air ingress in siphonic roof drainage systems. The airbaffle of
the roof outlet is the most important part to eliminate the air from the system. The airbaffle
consists of a disc to prevent direct air ingress, but also fins to prevent the occurrence of
vortices which can drag in air.
The above mentioned phenomena and their prevention will be discussed in detail in this
article.
1. Introduction Special precautions that have to be taken to
avoid air ingression are discussed below
Siphonic roof drainage systems are
designed to create full bore flow. Expelling 2. Avoiding air ingress into siphonic
air from the system leads to a roof drainage systems
depressurized system that sucks the water
from the roof at high speeds. Full bore As stated above there are two ways for the
flow can only occur when air is eliminated air to enter the system. The direct way is
from the system. This means that air not the inflow of air that is dragged in by the
only has to be removed from the system, friction at the water surface. This is easily
but air ingress also has to be prevented. To solved by separating the water surface
prevent air from entering the system one from the water flowing into the roof outlet.
has to concentrate mainly on the inlet of A disc, called air baffle, directly over the
the system. When water flows in an open roof outlet opening will do the job from the
system the friction at the water surface will moment the disc is fully covered by water.
drag the air along with it. Thus when the Beware that this is not when the overall
water is flowing into the inlet of a roof water level is at the same level as the disc
outlet air is being dragged in as well. height, since the suction pressure in the
Even when the water fully covers the roof vicinity of the roof outlet will gradually
outlet opening, air can still enter the decrease the water level as the flow is
system through a vortex. The suction accelerating towards the outlet.
pressure in the siphonic system can be so
strong that a vortex is being developed that
is able to drag in air (like the well known
bath tub vortex with its slurping sound).