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⎪
Pump systems, pipes, valves and seals
⎪
Mechanical Technology — September 2015
17
company literature. You will find most
literature states ‘metres of head’ without
specifying water or any named liquid. The
specific gravity becomes important as
you convert metres of head into pressure
in kPa. The specific gravity is important
as you size the motor to the pump.
Head is a measure of specific energy.
The units of ‘head’ are metres (m).
head = specific energy
= energy (E)÷weight (w)
And if you divide the units of energy
(kg.m
2
/s
2
) by those of weight (kg m/s
2
)
you get:
kg m
2
/s
2
÷kg m/s
2
= m
The liquid’s weight or specific gravity
(density relative to water) is not a com‑
ponent of the term ‘head’.
Let’s go back in time a few centuries.
The scientist Aristotle had theorised that
the acceleration of gravity was propor‑
tional to an object’s density. Aristotle
said a 2.0 kg mass would accelerate
toward earth twice as fast as a one
kilogram mass.
In 1589, Galileo purportedly dropped
two similar balls together from the
Leaning Tower of Pisa in Italy, a height
of 56 m. The two balls were of different
density (weight). The balls
fell toward earth together and
struck the ground at the same
instant. As we now know, the
acceleration and time of de‑
scent is independent of mass.
With this experiment 426
years ago, Galileo proved that
gravity’s rate of acceleration
is a constant, defined today
as 9.8 m/s
2
. With time and
technology, gravity’s accelera‑
tion is clocked slightly faster at
the earth’s poles compared to
the acceleration at the equator.
This is called centrifugal relief.
So, in the same way as ac‑
celeration due to gravity is in‑
dependent of an object’s mass,
the specific energy required to
elevate a liquid against gravity
is also independent of the liquid’s mass
or density relative to water. Pump compa‑
nies call this ‘head’. ‘Head’ is a measure
of specific energy. The units of energy are
‘metres’ of head against gravity. And for
centrifugal pumps, head is determined
by two principal factors: the impeller’s
speed (rpm); and the impeller’s diameter.
When a pump company shows metres
of head on a pump curve, the liquid’s
specific gravity (or density relative to
water) is not important. But when you
read a pressure gauge or measure the
kilowatts (or amps drawn) by an electric
motor, the liquid’s specific gravity, density
and weight are very important.
q
On a typical pump characteristic curve, while the total head (specific
energy) is independent of the fluid’s density. But the actual pressure
produced and power required to produce that pressure both depend
on specific gravity or the density of the fluid being pumped.