TECHNICAL
DYNAMIC (FLOW) PRESSURE VS STATIC PRESSURE
Dynamic (flow) pressure is the resultant pressure felt
at the end of a pipe installation after the total value of
friction loss has been accounted for over the length of
the pipe run.
Since dynamic pressure is what is experienced at the
fixture points, it is strongly linked to the performance
of an installation as well as customer satisfaction and
user comfort.
Flow rate
Flow rate is the volume of water passing through a
pipe or terminal fitting in a given period and is usually
expressed in litres per minute or litres per second.
Example
One litre per second × (60 seconds)
= 60 litres per minute × (60 minutes)
= 3 600 litres per hour (3.6 cubic metres per hour),
and so on.
IMPORTANT TO NOTE
Manufacturers of fittings such as taps, mixers, shower
heads, and flush valves, usually publish the expected
flow rate and performance of their fittings at various
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dynamic pressures, or the minimum dynamic pressure at
which they will render adequate performance.
WHAT DOES SANS SAY?
The flow rate required for different terminal fittings
to work effectively is specified by the re levant
equipment manufacturer. If this information is not
available, a general guideline is given within SANS
10252-1: 2004 (table 3 section 4.2.2.) In these
tables you will see that there are three different
flow rates given; that is likely maximum flow rate,
likely minimum flow rate, and design flow rate per
fitting. The design flow rate per fitting is usually used
for calculations but must be checked against the
manufacturer’s specification, if available.
The minimum value of dynamic (flow) pressure
required for a specific terminal fitting to perform
properly is given by the equipment manufacturer.
If such information is not available, then a general
guideline is also given in SANS 10252:1 2004
section 4.2.2, but care must be taken to ensure
that this information concurs with that given by the
manufacturer, since many high-performance fittings
available today require very specific values to work
adequately. See Figure 3.
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October 2018 Volume 24 I Number 8