cylinders, from worn valves, or from faulty conditions
in the pipe connections external to the pump.
When a body, or a liquid, is made to revolve or
whirl around a point, a force is created that impels the
body or fluid to move outward from the center of
rotation. This phenomenon is called CENTRIFUGAL
FORCE. It is from this force that the centrifugal pump
got its name.
The basic centrifugal pump has only one moving
part-a wheel or impeller that is connected to the drive
shaft of a prime mover and rotates within the pump
The design, or form, of the impeller varies
somewhat. However, whatever its form, the impeller
is designed to impart a whirling or revolving motion to
the liquid in the pump. When the impeller rotates at
relatively high speeds, sufficient centrifugal force is
developed to throw the liquid outward and away from
the center of rotation. Thus the liquid is sucked in at the
center or eye of the impeller (center of rotation) and
discharged at the outer rim of the impeller. Note that
by the time the liquid leaves the impeller, it has
acquired considerable velocity. In this connection, a
fundamental law of liquid physics states, in part, that as
the velocity of a fluid increases, the pressure or
pressure head of that fluid decreases. Therefore, the
liquid discharge from the impeller has a high velocity
but low pressure. Before the liquid can be discharged
from the pump, an INCREASE in pressure is
necessary. In other words, the primary concern in
practically all pumping systems is to maintain the
discharge pressure so liquid can be distributed
effectively throughout the system. In centrifugal
pumps, a device is required to decrease the velocity of
the impeller discharge and thereby increase the liquid
pressure at the discharge outlet.
One method of increasing the discharge pressure
of centrifugal pumps is by providing additional
impellers. Pumps with only one impeller are SINGLE
STAGE. Pumps with two or more impellers are
MULTISTAGE. In multistage pumps, two or more
impellers are placed on a common shaft (within the
same pump housing) with the discharge of the first
impeller being led into the suction of the next impeller,
and so on. As the liquid passes from one stage to the
next, additional pressure is imparted to it. In this
fashion, the final discharge pressure of the pump can
be increased considerably.
TYPES OF CENTRIFUGAL PUMPS.
Centrifugal pumps are also HORIZONTAL or
VERTICAL, depending upon the position of the pump
shaft. Generally, large, multistage, high-capacity
pumps are horizontal. Most other pumps are vertical.
The impellers used on centrifugal pumps may be
SINGLE SUCTION or DOUBLE SUCTION. The
single-suction impeller allows liquid to enter the eye
from one direction only; the double-suction type
allows liquid to enter the eye from two directions.
Impellers are CLOSED or OPEN. Closed
impellers have sidewalls extending from the eye to the
outer edge of the vane tips; open impellers do not have
these sidewalls. Most centrifugal pumps in the Navy
have closed impellers.
In the VOLUTE type of centrifugal pump shown in
figure 6-15, the impeller discharges into a volute or
gradually widening channel in the pump casing. As the
liquid passes into the expanding neck of the volute, its
velocity is considerably diminished; and, with this
decrease in velocity, the pressure increases.
Another variation is the DIFFUSER or VOLUTE
TURBINE type of centrifugal pump shown in figure
6-16. In this pump, the impeller discharges into
Figure 6-15.Volute pump.
Figure 6-16.Diffuser-type centrifugal pump.