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PLUMBING CONNECTION
SPRING 2016
INSTALLING BACKFLOWPREVENTERS
W
ith the use and installation of
backflow prevention devices
being commonplace now
since the mid-90s, you’d think the
industry would have a good handle on
the installation requirements and the
hydraulic aspects of how they work.
However, based on some recent and
recurring issues I’ve encountered, it
seems that this may not be the case.
HYDRAULICS
Backflow prevention devices are very
simple mechanical valves that stop the
reversal of flow of drinking water. Each
device is marked with the direction of
flow as they can only operate in one
direction.
The backflow prevention device uses
force-closed, spring-loaded check
modules that close when the incoming
water pressure drops below the spring
tension setting. The very fact that the
incoming water has to overpower a
spring means there will be a pressure
loss across the check valve. The degree
of this pressure loss will depend on the
type of device installed.
The Double Check Valve (DCV) has
two independently operating check
valves within the same body. To
comply with the Australian Standards
the spring settings must exceed a
differential across each check valve
of 7Kpa. This means the minimum
pressure drop across a DCV should
be 14Kpa. In reality, 14Kpa is not
achievable due to the tolerances in
springs and the flow passages through
a backflow preventer. Experience
indicates that a DCV has a pressure
drop in the vicinity of 25Kpa, but this
also depends on the manufacturer and
the age of the device as springs tend to
go soft over time.
The Reduced Pressure Zone device
(RPZ) also has two independently
operating check valves within the same
body but has a differential pressure
relief valve that opens to atmosphere
so that water will be drained from the
device if a backflow incident occurs.
To ensure the relief valve operates
as it should, the spring tensions in the
checks must be different enough to
provide a safety margin, otherwise
the device would continually drop
water from the vent due to supply
main pressure fluctuations. This extra
safety comes at the expense of a
higher pressure drop across the device
as the spring tension on the first check
must exceed 35Kpa and the tension
on the second check must exceed
7Kpa. Again, experience shows that
this 42Kpa is not achievable and the
pressure drop across an RPZ is more
likely to be above 65Kpa.
Protection of the drinking water
comes at the expense of the available
pressure, but a mechanical device
is still often a better option than a
complete break and repressurisation.
INSTALLATION GUIDELINES
The RPZ is only approved for
horizontal installation. This is because
the vent to drain the water away must
be the lowest point of the installation
and this cannot be guaranteed in a
vertical installation.
The DCV can be installed in both
vertical and horizontal applications
as long as there is adequate spacing
around the device for maintenance,
testing and repair.
PETER MCLENNAN
DISPELS SOME MYTHS SURROUNDING THE TWO MAIN BACKFLOW PREVENTION DEVICES SEEN
TODAY: THE REDUCED PRESSURE ZONE DEVICE (RPZ) AND THE DOUBLE CHECK VALVE (DCV).
BACKFLOW PREVENTION
PETER MCLENNAN
It is good practice to support the device so that its weight doesn’t cause any
strain on the pipe work.