Page 36 -

Pump

hydraulics

2.1 The pressure-head relationship

In considering the amount of pressure energy required from a pump, all
the various aspects of energy in the system on both sides of the pump
must be considered. As these energy levels are customarily identified in
'pressure' terms (such as pounds per square inch) or in 'head' terms
(such as feet of head) it is important to be comfortable with the
relationship between these two important terms.

All pressures can be visualized as being caused by a column of liquid
which (due to its weight) would produce a pressure at the bottom of
that column.
To exert a pressure of one pound per square inch at the base of a
column of water at 60 ~ Fahrenheit, with a specific gravity of 1.0, that
column must be 2.31 feet high. To exert a pressure of 14.7 pounds per
square inch at its base, that column must, therefore, be 34 feet high.
This assumes that there is no external pressure being exerted on the top
of that column of water.

Therefore, it can be assumed that if a tank of water at the same
temperature is open to atmosphere at sea level, it will have a pressure on
its surface of 14.7 p.s.i, or, in other terminology, 34 feet of head.
Therefore, in that same tank of water at the same temperature, the
pressure existing at any point in the liquid will be the sum of the weight
of the liquid above that particular point, plus the pressure on the free
surface of the water.

In other words, the total head being exerted at the bottom of a storage
tank of water, 15 feet deep, and open to atmospheric pressure at sea
level, will be 15 feet depth plus 34 feet of atmospheric pressure. This
equals a total head of 49 feet.