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Information
about siphonic drainage
The first siphonic system (sometime spelled syphonic)
was installed in Scandanavia in the late 1960’s, and
since the 1980’s siphonic systems have been
installed throughout the UK, Europe and Worldwide.
They are without doubt the best way of draining
large roof areas, but unfortunately much of the
information available to specifiers in the past has
been fairly partisan, and in some cases misleading.
This section will explore the workings of a siphonic
system, explode some of the myths.
How it all works
Siphonic drainage is actually very simple in
principle and all systems work in exactly the same
way. Baffle plates inserted in the outlets restrict
air entering the top of the system which, when
combined with carefully sized pipework, causes the
system, horizontal and vertical, to run full. In a
very similar way to a simple tube siphon (such as
you would use to empty a fish tank), the action of
water dropping down the downpipe will cause a
negative pressure to form at the top. This negative
pressure can be harnessed to suck water along a
collector pipe installed horizontally connecting the
outlets at high level.
The benefits this gives are:
Each gutter will have only one or two downpipes,
and these can be located at the end of the building,
allowing free use of floor space by eliminating
downpipes and therefore reducing columns.
The horizontal collector pipe can be very close to
gutter, allowing full use of internal space.
Underground drainage can be eliminated internally
in building, and can be significantly reduced
externally, which can provide considerable cost
savings and enhance construction programme on all
sites, and particularly contaminated ones.
For sites with a requirement for SuDS, siphonic
drainage will allow water to be delivered at a
designated point at shallow depth, which can
significantly reduce the construction costs,
especially for pond based solutions.
What to specify
When specifying siphonic drainage there are a number
of key factors which must be covered. These are:
Rainfall intensity - The rainfall levels should be
determined from BSEN 12056-3:2000, using the
projected building life, and a suitable factor of
safety. The contents of the building should be
considered as well as building type. The more years
specified the lower the risk to the building, but
the more expensive the system, so it is always a
balance to suit the acceptable level of risk.
Filling time and gutter calculations - It is
vitally important that the siphonic contractor
provides calculations to show that the system will
fill within 1 minute, and that the gutter will
function correctly i.e. will not over-top. In the UK
the design rainfall event (the most intense period
of a storm) is 2 minutes, and so a siphonic system
must begin to function within half this time, or the
roof may flood. In the past some companies have
claimed that their systems do not need to fill to
operate, but this is simply not correct. Gutter
calculations should be to BSEN12056-3:2000, using
outlet data from a BBA certificate or other third
party source.
The majority of siphonic drainage systems in
Britain use high density polyethylene pipework. HDPE
can be connected using either electrofusion
couplings, which are heated by internal elements, or
by butt-jointing, where the cut ends of the pipe are
melted and then forced together under pressure to
make a joint. Butt-joints should only be made using
a machine incorporating a jig and control system to
monitor the temperature, time and pressure required.
Site butt jointing of HDPE should only be allowed in
a specification if 'factory' conditions are set up
on site so that consistent quality can be
guaranteed.
Metal pipe systems (cast-iron, galvanized or
stainless steel) can also be used for siphonic
drainage. The specification should detail that
installation should be according to their
manufacturers recommendations for negative pressure.
Don’t bother to specify a "self-priming outlet " :
they all self prime!
Where should siphonic systems be specified?
The answer is that almost any building can be
drained siphonically but with the following
provisos:
Large industrial, storage or retail buildings will
show much greater benefit gains over gravity
drainage - in fact it would be almost impossible to
drain some of these buildings by gravity.
The gutters or flat roof areas must be large
enough to accept a siphonic outlet, and must have
adequate access for maintenance. Gutters in
inaccessible locations may not be so suitable.
All drainage can produce unwanted noise. In areas
sensitive to sound, siphonic systems like gravity
pipework may need acoustic insulation.
What can go wrong?
In the 1990’s there were a number of high profile
failures of siphonic roof drainage systems, which
led to the technology becoming suspect in some
people’s eyes. This is however a great shame, as in
all cases poor design was the cause, not a failure
in the system. The key reasons for failure were:
One or two companies set up in the industry
without an adequate level of technical knowledge,
and designed systems where the negative pressure was
so great that pipework actually collapsed under the
pressure, causing serious flooding of the building.
This would not have happened if the pipework had
been designed using suitable software, such as is
used by all members of the Siphonic Roof Drainage
Association.
Many specifiers exploited a loophole in the
previous drainage standard and designed systems to
operate at 75mm/hr. These systems worked perfectly
well, but were overwhelmed by higher rainfall
events, which in south and east of England could
occur every year or so. It should be stated that
this was also a problem with gravity drainage, but
the link was not so obviously made. The new standard
BSEN12056-3:2000 closed this loophole, and so it is
no longer an issue with siphonic drainage, but
continues in some gravity systems.
What is the industry doing today?
The current siphonic roof drainage industry is a
more mature industry than the one existing in the
1980’s, with a wider spread of major companies. A
draft British Standard, dealing specifically with
siphonic roof drainage has been drawn up, and is
about to go to the BSI committee procedure. This
will help define good practice in an industry which
for so many years, in Britain, has been forced to
set its own procedures for good practice.
In another step towards raising the industry
profile, an industry association has been set up;
the Siphonic Roof Drainage Association (SRDA). The
Association has the goal of bringing together
companies to promote good practice. Members of the
Association must show that they have:
A suitable outlet and pipework and hanging system
A functional computer based flow-balancing program
All these systems should have third-party
accreditation.
In addition to these requirements, member companies
will be audited by the Association to determine, on
randomly chosen projects, whether their design,
installation and customer service meet the
Association’s standards.
With these stringent criteria for membership, any
specifier using an SRDA member can be sure that they
are placing this vital part of their project in the
hands of a company who have the expertise to do the
job properly.
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