54
TECHNICAL
Basic theory of
fluid mechanics for
plumbing (Part 5)
In Part 4 of this series, we discussed the speed (water
velocity) at which water will probably flow through
a piping system or pipe segment, the resultant effect
thereof, and how to select pipe sizes accordingly.
By Chris Kyle
However, it should be understood that water velocity
is not the only criterion that must be considered in
the selection of pipes of adequate size: Friction loss
or hydraulic gradient is also an essential factor to
be considered. One must ensure that there will be
adequate flow pressure at terminal fittings for them
to offer acceptable performance.
About the author
To recap: 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.
Friction loss is expressed in kilopascal (kPa) per metre
(or metres per metre — remembering that 1m of
head is equal to 10kPa) and can be determined for
various pipe materials by referring to the ‘Friction Loss
Tables’ as presented in SANS 10252:1 – Annex D.
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.
May 2019 Volume 25 I Number 3
www.plumbingafrica.co.za