By Sharon James, Rockwell Automation
Head, performance curve and laws all contribute to the of
umps are generally grouped into two broad displacement pumps and dynamic (centrifugal) pumps. Positive displacement pumps use
a mechanical means to vary the size of (or move) the
chamber to cause the to On the other hand, centrifugal pumps impart momentum to the by rotating
impellers that are immersed in the The momentum
produces an increase in pressure or at the pump outlet.
Positive displacement pumps have a constant torque
characteristic, whereas centrifugal pumps demonstrate variable torque characteristics. This article will discuss only centrifugal pumps.
in the pump casing, some of the kinetic energy is converted
to pressure energy. It is the resistance to the that
is read on a pressure gauge attached to the discharge line. A
pump does not create pressure, it only creates Pressure
is a measurement of the resistance to
In Newtonian (true) (non-viscous liquids, such as
water or gasoline), the term head is the measurement of the
kinetic energy that a pump creates. Imagine a pipe shooting
a jet of water straight into the air. The height that the water
reaches is the head. Head measures the height of a liquid
column, which the pump could create resulting from the
kinetic energy the pump gives to the liquid. The main reason
A centrifugal pump converts driver energy to kinetic energy
in a liquid by accelerating the to the outer rim of an
impeller. The amount of energy given to the liquid corresponds to the velocity at the edge or vane tip of the impeller.
The faster the impeller revolves or the bigger the impeller,
then the higher the velocity of the liquid at the vane tip and
the greater the energy imparted to the liquid.
Creating a resistance to the controls the kinetic
energy of a liquid coming out of an impeller. The resistance is created by the pump volute (casing), which catches
the liquid and slows it down. When the liquid slows down
Figure 1. Centrifugal pump
128 SEPTEMBER 2012
Figure 2. Representation of static discharge head, static suction lift and total static head