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Pump Hydraulics

Gauge Pressure is related to atmospheric pressure and is the amount by
which the stated pressure exceeds atmospheric pressure.
e.g. Absolute Pressure = Atmospheric Pressure + Gauge Pressure

Vacuum is also relative to atmospheric pressure, but it is that amount by
which the stated pressure is less than atmospheric pressure.
e.g. Absolute Pressure = Atmospheric Pressure- Vacuum Gauge Reading

Differential Pressure is the difference in pressure between two points in
a system. More specifically, it is used to identify the difference between
the suction and discharge pressures of a pump when referred to the
same datum and expressed in feet of head.

50 p.s.l..

6 Ins.
I

12 Ins.

Figure 2.2: Differential pressure measurement

2.1.2 Pumping pressure
To relate this information more closely to the pump operation, consider
three identical pumps with the same impeller diameter, and running at
the same speed. Each pump is handling a liquid of different density to
the others.
9 the Kerosene has a Specific Gravity of 0.8%

9 the Water has a Specific Gravity of 1.0
9 while the Sulphuric Acid has a Specific Gravity of 1.8
This means that the Kerosene is 20% lighter than Water, while
Sulphuric Acid is about 80% heavier than Water. In spite of this, all 3
pumps will develop the same Head of 100 feet when running at the
same speed. The Discharge Pressure of the pumps is quite different
however, because of the different densities of the various liquids.

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