76
TECHNICAL
Basic theory of fluid mechanics
for plumbing (Part 3)
In Part 2 of this series, which featured in the September edition
of Plumbing Africa, we discussed static pressure in plumbing
systems. We now consider what happens to pressure when water
is caused to flow through a pipe; that is, dynamic (flow) pressure.
By Chris Kyle
About the author
Chris Kyle is a qualified
commercial pilot and flying
instructor, accredited CETA
trainer and assessor, CPD
course writer and presenter
to the architectural fraternity,
and professional plumbing
industry licensed plumber.
Chris has plotted his course
in the building industry from
his early days as national
specifications manager for
Cobra Watertech, to where
he is today as the general
manager of Calafrica.
To revise, static pressure is the pressure available
in a system when no water is being drawn off from
any fixture points, such as taps, flush valves, and
appliances. If you were to install pressure gauges on
a horizontal line at, say, intervals of 20m apart, all of
the gauges would read the same pressure; that is,
static pressure — no flow (see Figure 1).
How can there be a loss of pressure energy if the
laws of physics state that energy cannot be created or
destroyed? It can be changed into another form, though.
In this instance, there are essentially three energies
involved: pressure energy, kinetic energy (due to motion),
and frictional energy.
Figure 1: Static pressure.
As soon as any fixture point or points are used, water
flows through the pipework to feed these points and a
few things start happening. Let’s take a look at what
happens and how the performance of the plumbing
system can be affected. The sum of which equals a constant; thus, if one or
more of these energies are increased, then one or more
must decrease. Kinetic energy has increased and so
has frictional energy; therefore, pressure energy must
decrease proportionately to maintain the equation.
FRICTION LOSS (OR HYDRAULIC GRADIENT)
As water flows through a pipe, and associated fittings
the water molecules in contact with the wall of the
pipe experience some resistance to their motion. This
resistance is called friction loss or hydraulic gradient
(also sometimes referred to as ‘wall drag’). This friction
loss causes an ‘apparent’ loss in pressure as the water
travels through the length of a reticulation system. Figure 2 shows the apparent pressure loss on the
pressure gauges.
Friction loss is expressed in kPa per metre (or metres
per metre — remembering that one metre of head is
equal to 10kPa) and can be determined for various pipe
materials by making reference to the ‘friction loss tables’
as presented in the SANS 10252:1 document – Annex D.
Friction loss increases exponentially with an increase
in the speed of the water, roughly by the square of the
speed; that is, if the water speed doubles, friction loss
will be roughly four times as much.
Figure 2: Apparent
pressure loss on a
pressure gauge.
October 2018 Volume 24 I Number 8
Different piping materials have different frictional loss
characteristics due to the wall thicknesses of the various
materials, as well as the smoothness of the internal
surface, known as the ‘roughness factor’.
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