70
TECHNICAL
How to prevent
backflow (Part 2)
Patrick Gordon, general
manager at CalAfrica
since June 2019, started
in the plumbing retail
business in 1985 with
the Boumat Group.
In 2000 he moved
from the retail to the
manufacturing sector
when he joined Cobra
Watertech. He remained
with the company
for 19 years, rising
rapidly in various roles
from product support
manager / product
development manager,
a role which was
expanded in 2005 to
national product support
manager with, latterly,
more than half his life
spent on technical
training. He has also
qualified as a facilitator
and assessor. He is the
featured personality in
the August 2019 issue of
Plumbing Africa.
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In the previous article we discussed what backflow is and
why we must protect against it. But how do we prevent
backflow?
By Patrick Gordon
There are many ways to minimise the risk of
backflow and with various degrees of efficiency.
The first and most common way is to use a
check valve; also known as a non-return valve.
But how effective is it?
As you will know there are two different types of
check valves:
1. The one is the flap or swing type that can only be
used in ascending lines so that the flap defaults to
the closed position. The sealing faces of this valve
are both metal and not forgiving when any dirt is
present and so this valve should not be used for
domestic installations.
2. The spring-loaded check valve has a spring
assisted plunger that will default to the closed
position no matter the angle of the installation. The
one side of the sealing face is made of rubber and
There is product available on the market that does
just that all in one unit. This unit, although more
efficient, is by no means foolproof. So how do you
know when the seals are not closing properly?
This is where the Reduced Pressure Zone Backflow
Preventer (RPZ) valve comes in. it has a chamber
between the two check valves that defaults open
to the atmosphere when the mains pressure fails.
So, if there is any backflow leaking past the check
valve it will enter this port and be directed out to
the atmosphere. The valve must thus be plumbed in
such a way that the discharge port is visible so that
if water is exiting the port it will immediately raise an
alarm with passers-by and attention can be given to
the malfunction. This will also mean that any water
that does flow past the check valve will not return to
the municipal supply and backflow into the supply will
be prevented.
This is a very secure way to prevent backflow into the
municipal supply but in some instances, it is still not
viewed as enough and an air gap is required so that
there is absolutely no chance of reverse flow taking
place. This can be achieved by piping the municipal
supply into a tank via a float valve and then taking
an outlet from the tank and supplying the installation
from there. If the tank is not high enough to supply
enough head of water a pressure pump can be fitted
to boost the pressure.
Inlet zone
Reduced pressure zone
Downstream zone
Downstream check valve
Relief port
RPZ Diaphragm
Relief seal
At the end of the day, you as the installer need to
make sure that you have applied the correct backflow
prevention equipment for the relevant risk of the
installation. I will be covering the risk categorisation
in a follow-up article. PA
Reduced Pressure Zone Backflow Preventer (RPZ) valve.
www.plumbingafrica.co.za
this is more forgiving of irregular sealing areas.
This valve is more suited for use in domestic
installations to prevent any fluid flow in the
reverse direction. But the valve still has its
short comings in that dirt can get lodged on the
sealing face and prevent the reverse flow from
being successfully prevented. So why don’t we
just put two check valves after each other in the
installations, the second one being less likely to
also be jammed open at the same time?
@plumbingonline
@plumbingonline
@PlumbingAfricaOnline
October 2019 Volume 25 I Number 8