Page 24 - Know and Understand Centrifugal Pumps

Basic Pump Principles

We’re more concerned with pressures above atmospheric pressure. For
example, a flat tire on a car still has 14.7 pounds of pressure inside it.
We would consider this to be a flat tire because the pressure outside the
tire is equal to the pressure inside the tire. We would say the tire has no
pressure because it would not be inflated and could not support the
weight of the car.
What is more important to us is the differential pressure inside the tire
compared to outside the tire (atmospheric pressure). For reasons such
as these, the world has adopted a second and artificial zero, at
atmospheric pressure as a reference point. This is why a simple pressure
gauge will read zero at atmospheric pressure.

Because simple pressure gauges are made with an artificial zero at
atmospheric pressure, this is why the term psig exists, meaning pounds
per square inch gauge. As mentioned earlier, the psig is equal to the
absolute pressure minus the atmospheric pressure.
Psig = Psia - ATM
Pressures less than atmospheric are recorded as negative pressures (-psi)
on a simple pressure gauge.
Technically speaking, negative pressures don’t exist. Pressure is only a
positive force and it is either present or absent.

Pressures inside the pump

Suction pressure
Suction pressure is the pressure at the pump’s suction nozzle as
measured on a gauge. The suction pressure is probably the most
important pressure inside the pump. All the pump’s production is based
on the suction pressure. The pump takes suction pressure and converts
it into discharge pressure. If the suction pressure is inadequate, it leads
to cavitation. Because of this, all pumps need a gauge at the suction
nozzle to measure the pressure entering the pump.

Discharge pressu re
This is the pressure at the pump discharge nozzle as measured by a
gauge. It is equal to the suction pressure plus the total pressure
developed by the pump.

Seal chamber pressure
This is the pressure measured in the stuffing box or seal chamber. This
is the pressure to be sealed by the mechanical seal or packing. The seal
chamber pressure must be within the limits of the mechanical seal. This

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