Mechanical Technology September 2015

⎪ Pump systems, pipes, valves and seals ⎪

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

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

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.

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

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.

Mechanical Technology — September 2015

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