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TECHNICAL
Basic theory of
fluid mechanics for
plumbing (Part 4)
In Part 3 of this series, we discussed dynamic pressure (flow
pressure) and friction loss (hydraulic gradient) in plumbing
systems. We now consider the speed (water velocity) at
which water will probably flow through a piping system or
pipe segment and the resultant effect thereof.
By Chris Kyle
About the author
Water velocity = distance/time
Have you ever watched a leaf being swept along the current
of a river and how it speeds up when it passes through a
narrowing of the stream and then moves more slowly at a
point where the stream widens out again? (See Figure 1.)
Why so? Because of the ‘equation of mass flow continuity in
incompressible fluids’, meaning that to get the same number
of water molecules to pass through the narrowing in the
stream, the speed of each molecule must increase to allow
the same mass of water made up of the molecules to pass
through the narrowing in the stream (Bernoulli’s principle).
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.
Water velocity is the speed at which water passes
through a tube and is usually expressed in metres
per second; that is, the time that it takes one
molecule of water to travel one metre of distance.
Figure 1.
Thus, in a piping system, the water velocity at a given flow
rate will be higher in a smaller pipe and lower in a larger
pipe. (See Figure 2.)
Water velocity is dependent on system pressure, flow rate,
and the diameter of the pipe.
Why is it important for water velocity to be kept at
acceptable levels in plumbing systems?
Damage and high maintenance
Water can be considered to be an incompressible fluid (only
slightly compressible at extremely high pressure ranges
that fall way above those possible in plumbing systems).
So, water in a pipe can be likened to a steel rod within a
pipe which, once moving, has the potential to do damage to
piping systems and fittings.
Much like travelling in a motor car, if you hit a brick wall at
2km/h, you will only slightly damage your car, but if you do the
same at 120km/h, it’s game over and everything is destroyed.
The higher the velocity, the higher the momentum of the
water, which means that the potential for damage to the
plumbing system is increased when taps are opened and
closed and directional changes are made at tees, bends,
and the like — Newton’s laws.
Noise
As we have already learnt, there is friction between the
water molecules and the pipe walls, and the higher the
water velocity, the higher the friction. More friction, more
noise transmitted by the flow of the water through the pipes,
just like a car on the highway: low speed, low tyre and wind
noise; high speed, high tyre and wind noise.
Figure 2.
February 2019 Volume 24 I Number 12
Remember, friction increases by the square of the velocity
— noise will increase by roughly the same proportion.
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