TECHNICAL
Potential energy
The amount of water pressure available in a system at a given point
depends on the potential energy; that is, the height of the column of
water above that point. In the plumbing industry, we refer to this as the
‘head’ of water above a point.
For example, a system being fed by a tank or reservoir that has a height
of 20 metres above a certain point will have a static pressure of 200kPa,
or 2 bar (see Figure 3). Or as another example, one having a head of,
say, 35 metres will have a static pressure of 350kPa, or 3.5 bar.
Static pressure
Static pressure is the pressure at a point in a system when no flow is
taking place and the water is standing still, meaning that no water is
being drawn off at any point. Something of importance to note: The
mistake is often made whereby it is perceived that for a given column
of water or ‘head’, pressure would be greater in large pipe sizes by
comparison to a smaller pipe.
For example, the pressure exerted by a column of water 20m high
(200kPa) will be the same in a 300mm pipe as in a 15mm pipe. Why so?
Remember the formula for pressure: P = F/A (force/unit area).
How can static pressure be determined?
• As mentioned above, by knowing the height of a supply tank
above the system to be installed.
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By simply using a suitable pressure gauge to measure the
pressure (gauge pressure).
PRACTICAL APPLICATION
Having this knowledge, you will be better equipped to determine the
following, among many other considerations:
• The static water pressure available from a tank or reservoir if you
know the height of the tank above the plumbing installation.
• Whether or not there is sufficient pressure available to feed a
specific installation efficiently or whether pumps are needed to boost
the water pressure so that you can advise your client accordingly.
Remember that if pumps are needed, they cannot be installed such
that they pump directly from a municipal main supply. A ‘break
pressure tank’ must be installed in this instance, from which the
pumps can deliver water to the system at a higher pressure.
• Whether or not the pressure in a service pipe feeding terminal fitting
does not exceed that as stated in SANS 10252:1, where it says in
7.1.1.2 (Supply pressure considerations): “If the supply pressure
could exceed 600kPa in any general or combined installation, and
unless otherwise required, a pressure-reducing or control valve shall
be incorporated in the service pipe upstream of the first terminal
water fitting, to limit such pressure to a maximum of 600kPa.”
• The class and pressure rating of piping materials to be installed.
In the next edition, we will discuss dynamic pressure and what happens
to pressure when water flows in a system. PA
September 2018 Volume 24 I Number 7