TECHNICAL
69
The function of an RPZ is to prevent the backflow of
polluted water by way of the two check valves, but most
importantly, to dump any backflow that may find its way
past the downstream check valve via the discharge
port to a safe point of discharge, thus preventing
contamination of the upstream water supply. No rocket
science.
Figure 2: An illustration of an RPZ valve cross section
WHAT IS A REDUCED PRESSURE ZONE BACKFLOW
PREVENTER? B/A TYPE
Commonly referred to as RPZs, they usually consist
of a DZR brass or bronze body and have an upstream
and downstream high-quality spring-loaded check
valve. They comprise an operating chamber, a reduced
pressure zone (hence, the valve’s name RPZ), a
diaphragm system that divides these two zones, a
spindle and discharge port and seat, as well as a
spring. It also has three test ports on top of the valve
used to monitor and test for proper operation (not
shown in Figure 2).
even swimming pools must be filled with water that
meets the SANS 241 specification. Complete off-
grid uses of alternate water will only be permitted
for large consumers such as big residential estates
and corporates, where very expensive and costly in
upkeep filtration systems are installed and maintained
at prescribed and regular intervals to ensure that
the quality of water provided meets the standards
of SANS 241. In all of the above instances, reduced
pressure zone backflow preventers must be installed.
Figure 3: Flow conditions.
HOW THEY ARE INSTALLED
All RPZs must be installed with a positive shut-off
isolating valve (that is, an approved shut-off valve that
will ensure no leakage when closed) upstream and
downstream of the RPZ. This is required not only for
servicing purposes but is needed for testing purposes,
too. A line strainer must be installed between the
upstream isolating valve and the RPZ.
Should the municipal water pressure exceed 600kPa,
then it would be advantageous to install an adjustable
pressure-reducing valve with a pressure gauge
between the line strainer and the RPZ. Do some
research on SANS 10252:1 to find out what the
regulation says about supply pressure considerations
7.1.1.2. Remember that one of the primary methods
of reducing water usage and system maintenance is to
www.plumbingafrica.co.za
HOW DO THEY WORK?
By applying the formula for pressure — that is,
Pressure = force/area — and Newton’s Laws, and
following these simple sketches, you will be able to
determine how they work.
Figure 4: No flow conditions.
It is crucial that all of us take
adequate measures to protect
our precious water resources
and act responsibly.
Continued on page 70 >>
April 2018 Volume 24 I Number 2