Pumps are the most widely used industrial machineries worldwide, next only to
electric motors. They are used in multitude of services and applications, in
various industries. Among industrial pumps, centrifugal pumps are the most
popular and widely used. This creates an ever increasing demand for students,
employees, professionals, and end-users to gain familiarity and working
knowledge of pumps, as well as the need to keep pace with recent advances in
pump technology. The EMA Project is created in response to the need for
reliable and accurate technical information and know-how relating to the pump
Centrifugal pumps are mechanical equipment that imparts kinetic energy to a
liquid to increase its pressure by means of centrifugal force and move it from
one point to another in a controlled and confined environment.
Centrifugal pumps running in reverse rotation perform the reverse function of
recovering energy from a liquid by reducing its pressure. They are called
hydraulic power recovery turbines (HPRT), or pump as turbine (PAT); their
design, selection and operation are different from those of standard centrifugal
The term liquid is used (instead of fluid) because the word pump is most
generally used to refer to equipment that handles liquid, whereas one that
handles air, gas, or vapor (collectively to be called gas for simplicity) is aptly
referred to as air pump, vacuum pump, compressor, blower, or fan. This
distinction is important because a typical pump is not intended to handle gas –
its presence in liquid, even in small volumetric amount, is unwanted and impairs
the pump performance and operation.
Although many liquids, such as boiler feedwater, go through deaeration or
degasification to remove gas from the liquids, pumps will invariably be put into
service to handle liquids with some amount of dissolved or entrained gas.
Typical examples are self-priming and froth pumps. In recent years modern
centrifugal pumps have been designed to handle dense carbon dioxide (CO2) -
defined as CO2 in gaseous state but is exhibiting properties of a liquid. One of
the challenges facing pump engineers is to develop design innovations to
mitigate the adverse effects of entrained or dissolve gas in pumped liquids.
The design of modern centrifugal pumps goes beyond the basics of moving and
adding pressure to the pumped liquid - it also need to address the increasing
demand for higher efficiency to conserve energy and the need for controlling
emission and mitigating noise propagation to protect the environment. The need
to control costs means that pumps have to be designed to run at higher speeds,
or at wider performance envelope. Advances in metallurgy result in better
materials more resistant to corrosion, cavitation, fitting and erosion. These
modern design considerations result in longer and more efficient life cycle for
Types of pumps
There are various types of pumps and it is likely that a pump belongs to more
than one group. Pumps may be classified according to their hydraulic design,
mechanical design, compliance with specific industry standard, function, special
One major type is according to the manner energy is imparted to the liquid – (1)
by impulse energy exemplified by positive displacement pumps, or (2) by kinetic
energy exemplified by rotodynamic pumps.
Positive displacement pumps include reciprocating pumps (piston, plunger, and
diaphragm) and rotary pumps (gear, lobe, screw, sliding vane, and cam).
Rotodynamic pumps include centrifugal pumps of radial and mixed flow design.
Radial pumps impart energy purely by the centrifugal action of their impellers
(also called runners) whereas mixed flow pumps use a combination of centrifugal
force and the lifting action of the impellers. Axial pumps impart energy purely by
the lifting action of the impellers so they should not be considered a sub-set of
centrifugal pumps (See discussions on pump specific speed.)
The advantages of centrifugal pumps are that they can operate over a wide
range of flow rates and pressures. They have high efficiencies, and deliver
uniform flow. They are readily available and commercially competitive. A big
disadvantage is that their performance deteriorates more with significant
reduction in head and efficiency when handling highly viscous liquids.
The advantages of positive displacement pumps are that they deliver constant
volume with varying range of pressures. They are more suitable for low specific
speed application and for handling highly viscous liquids. A big disadvantage is
that they have narrower flow range; their flow tends to be pulsating and may
require flow dampeners.
The discussions in this article are mainly about industrial centrifugal pumps but
many of the concepts and practices apply to other types of pumps as well.
The next part of this article discusses the different types of centrifugal pumps.
Introduction to centrifugal pumps
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