1. SG is specific gravity, the ratio of a fluid's density to that of water at standard conditions;
2. Head is proportional to the potential energy of a liquid raised to a height h;
3. Why is the term head used, why not just use pressure? You could; pressure is a fundamental unit of fluid mechanics.
Head means water at a height. To get water to a certain height you have to spend energy.
Water at a height has potential energy. You can recover that energy by letting the water flow
down from the height and activate a turbine for example. The pump manufacturer's use the term head because
a pump with sufficient energy to get water to a given height can be said to have a head corresponding to that height.
Since many applications for pumps involve moving water upwards this is convenient shorthand for sizing a pump.
The figure above shows a tank full of water, the pressure at the bottom of the tank will be proportional to the height of the water level
and the density of the fluid; i.e. p(psi) = dens. (lbf/ft3) x h(ft)/144 or in terms of specific gravity SG, p(psi) = SG x h(ft)/144/62.34.
On the right we see a pump pushing water upwards as far up as it can, this is known as the shut-off head of the pump, and this condition
occurs at zero flow. Since the pump can move the liquid to the same height as the water tank level it can be said by analogy to have the
same head that the water tank produces.
Any pressure or pressure difference can be converted to head or head difference. This is very useful when dealing with pumps;
see a more comprehensive explanation.
5. This app is intended to be used for educational purposes, the author takes no responsibility for the use of these results.
J. Chaurette June 6, 2018
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