|
There are few
real mysteries remaining about the mechanisms at play in building drainage and
vent systems. This has been well understood from the beginning of modern
sanitary engineering at the end of the 19th Century. The description
of Building drainage and vent system operation is best understood in the
context of engineering science in general and fluid mechanics in particular.
Early
researchers in the field were well aware of this and many examples of the
application of sound fluid mechanics are available as evidence. Much research
has been carried out since the end of the World War II, where, particularly in Europe, extensive reconstruction work prompted the quest
for more efficient approaches to drainage and vent system design.
At the center of
the system’s integrity is the water trap seal, which stops foul air from
entering a habitable space from the sewer. The water trap seal is usually 1½ or
2 inches in depth depending on the fixture it is protecting.
It comes as a
surprise to many that the flow of air is as important, if not more important, than
the flow of water, to the safe operation of the drainage system. This air flow
is ‘induced’ or ‘entrained’ by the flow of water. The unsteady nature of the
water flows causes pressure fluctuations (known as pressure transients) which
can compromise water trap seals and provide a path for sewer gases into the
habitable space.
Transients can
be dealt with by a combination of careful design and the introduction of
pressure relief devices as close to the area of concern as possible. Long vent
pipes can be an inefficient way of providing relief due to friction in the
pipe. Distributing air supply inlets using AAVs around a building provides an
efficient means of venting and it reduces the risk of positive transient
generation. AAVs do not cause positive pressure transients, they merely respond
to them by closing, and hence reflect a reduced amplitude wave.
In tall
buildings parallel vent pipes can only provide a small relief path for a
positive pressure transient (approx 1/3 if the vent pipe is the same diameter
as the main vertical stack) thus a wave will still propagate throughout the
rest of the system that could compromise water trap seals. The introduction of
a positive air pressure transient alleviation device provides a means to ‘blow
off’ pressure surges as close to their source, thereby protecting water traps.
Attenuation of up to 90% of the incident wave can be achieved, thus protecting
the entire system. There is little that can be done for a system experiencing a
total blockage, generating excessive static positive pressures in the drainage
system. In such circumstances the lowest water trap seal will ‘blow’ providing
relief for the whole system. This will occur regardless of the method of
venting employed.
In validated
test simulations air admittance valves (AAVs) have been shown to provide as
least as good protection for water trap seals as a fully vented system, and in
tall buildings in some circumstances, even better. The fully engineered
designed active control system utilizing AAVs for negative pressure relief and Positive
Air Pressure Transient Attenuators (PAPAs) for positive transient relief is
shown to be an effective method for balancing the need for safety and
efficiency while maintaining functionality
invisible to the user.
To see the complete article, download the .doc file here.
|
