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Basics of Centrifugal pumps


Centrifugal pumps basically consist of a stationary pump casing and an impeller mounted on a
rotating shaft. The pump casing provides a pressure boundary for the pump and contains
channels to properly direct the suction and discharge flow. The pump casing has suction and
discharge penetrations for the main flow path of the pump and normally has small drain and vent
fittings to remove gases trapped in the pump casing or to drain the pump casing for maintenance.

Figure 1 is a simplified diagram of a typical centrifugal pump that shows the relative locations
of the pump suction, impeller, volute, and discharge. The pump casing guides the liquid from
the suction connection to the center, or eye, of the impeller. The vanes of the rotating impeller
impart a radial and rotary motion to the liquid, forcing it to the outer periphery of the pump
casing where it is collected in the outer part of the pump casing called the volute. The volute
is a region that expands in cross-sectional area as it wraps around the pump casing. The purpose
of the volute is to collect the liquid discharged from the periphery of the impeller at high
velocity and gradually cause a reduction in fluid velocity by increasing the flow area. This
converts the velocity head to static pressure. The fluid is then discharged from the pump
through the discharge connection.
Centrifugal pumps can also be constructed in a manner that results in two distinct volutes, each
receiving the liquid that is discharged from a 180o region of the impeller at any given time.
Pumps of this type are called double volute pumps (they may also be referred to a split volute
pumps). In some applications the double volute minimizes radial forces imparted to the shaft and
bearings due to imbalances in the pressure around the impeller. A comparison of single and
double volute centrifugal pumps is shown on Figure 2.
Some centrifugal pumps contain diffusers. A diffuser is a set of stationary vanes that surround the impeller. The purpose of the diffuser is to increase the efficiency of the centrifugal pump by allowing a more gradual
expansion and less turbulent area for the liquid to reduce in velocity. The diffuser vanes are designed in
a manner that the liquid exiting the impeller will encounter an everincreasing flow area as it passes through the diffuser. This increase in flow area causes a reduction in flow velocity, converting kinetic energy into flow pressure.
Impeller Classification
Impellers of pumps are classified based on the number of points that the liquid can enter the impeller and also on the amount of webbing between the impeller blades. Impellers can be either singlesuction or double-suction. A single-suction impeller allows liquid to enter the center of the blades from only one direction. A
double-suction impeller allows liquid to enter the center of the impeller blades from both sides simultaneously. Figure 4 shows simplified diagrams of single and double-suction impellers.
Impellers can be open, semi-open, or enclosed. The open impeller consists only of blades
attached to a hub. The semi-open impeller is constructed with a circular plate (the web) attached
to one side of the blades. The enclosed impeller has circular plates attached to both sides of the
blades. Enclosed impellers are also referred to as shrouded impellers. Figure 5 illustrates
examples of open, semi-open, and enclosed impellers.
The impeller sometimes contains balancing holes that connect the space around the hub to the
suction side of the impeller. The balancing holes have a total cross-sectional area that is
considerably greater than the cross-sectional area of the annular space between the wearing ring
and the hub. The result is suction pressure on both sides of the impeller hub, which maintains
a hydraulic balance of axial thrust.
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