TECHNICAL
Nothing worse than a noisy plumbing system in
a nice building!
Water hammer
The rapid closing and opening of valves and
terminal fittings at high water velocities can
easily cause a ‘pressure shock wave’, which
propagates through the plumbing system,
causing noise and banging, with the potential of
bursting pipes and damaging fittings.
I am sure you have all been confronted with this
sort of complaint before, especially on single-
lever or ceramic disc taps and mixers, which can
be closed very quickly.
Cavitation
At rapid water velocities, water can be brought
to its vapour pressure as it is squeezed through
small orifices in fittings or as it is forced around
bends forming tiny ‘bubbles’ of water vapour
that implode downstream of the fitting or bend
as pressure is recovered. The forces caused by
the implosion of the vapour ‘bubbles’ impinge
on the metallic surfaces of fittings and pipework,
causing pitting in the material which can
ultimately lead to noise and mechanical failure.
Ever heard of damage caused by cavitation to
the propeller blades of a ship or boat? Similar
principle. Google it to find out more!
WHAT ARE ACCEPTABLE WATER
VELOCITIES IN PLUMBING INSTALLATIONS?
For all the above reasons, also remembering that
system pressure has a large bearing on water
velocity, SANS 10252:1 states:
59
HOW TO DETERMINE WATER VELOCITY IN
VARIOUS PIPE SIZES
Firstly, you need to determine the expected flow
demand (Q), meaning the flow rate on the pipe
run in question.
How do you do this? There are many ways!
SANS 10252:1 ((4.2.2 Probable (or design) flow
demand)) gives reference to one method. For
our example:
Step 1 — Given
A pipeline feeding 10 high-pressure toilet
flushing valves (Cobra Junior Flushmasters)
located in a male ablution block of an upmarket
office development. The only possible routing
of the piping is at the soffit over a boardroom
where any noise must be kept down. Selected
piping material is SANS 460-0 Copper Tubing.
Step 2 — Calculate
Obtain the manufacturer’s technical data (see
Figure 3 on page 79 of the October issue of
Plumbing Africa). Each of these valves require
a flow rate of 65 litres per minute. There are
10 valves, but it is highly improbable that all 10
could ever be flushed at once.
Most probable and likely would be two being
used simultaneously, resulting in an assumed
and reasonable probable flow demand (Q) of
130 litres per minute. This concurs roughly with
SANS 10252:1, as well as simultaneous usage
flow demand tables given by the manufacturer
for this product and the project type, for example
an office block.
It must be noted that the calculation of (Q) is
always based on reasonable assumptions and is
not an exact science.
7.2.1.1 Unless otherwise required:
a) The design static pressure at any terminal
fitting other than a fire hose reel or fire hydrant
shall not exceed 600kPa.
7.2.1.2 Unless otherwise required, the velocity
of the flow of water in any pipe shall not exceed:
a) 3.0m/s (metres per second) for buried
pipelines and for pipelines installed above
ground or where noise is not a critical factor, or
b) 1.5 to 2.5m/s depending on the nature of the
installation and the noise levels allowed. Step 3 — Determine water velocity for
pipe size
Use suitable hydraulic gradient flow charts,
which are usually available from the
manufacturer of the selected piping material,
in this case SANS 460-0 Copper Tubing. Such
graphs are also readily found in SANS 10252:1
Annex D (informative) from which a selection of a
fit-for-purpose pipe size can be made.
These are excellent guidelines to follow, but
the designer or installer can alter these criteria
fittingly, for example the size of pipe crossing in
the ceiling space of, say, a home theatre or luxury
bedroom could be increased as to reduce the
water velocity to, say, 1.0m/s or even less. See Figure 3. Enter the graph from the right at the
130 litre per minute mark, using a horizontal line
move across the graph to the left, note where this
line intercepts the various pipe size lines. At these
intercept points, move upwards to interpolate the
water velocity for each pipe size. Based on this
information, make a selection of pipe size that will
meet the requirements for the job.
Continued on page 61 >>
www.plumbingafrica.co.za
February 2019 Volume 24 I Number 12