CENTRIFUGAL PUMP
Centrifugal pump is so named
because the pressure head is generated by centrifugal action. The impeller is made up of a number of
curved vanes, which are supported on both sides by plates known as shrouds. It
rotates inside a casing or volute. Flow enters the pump through the centre or
eye of the impeller. Energy is given to the liquid as the blades of the
impeller transport it outwards in a radial direction.
The volute is usually shaped
in the form of a spiral to form a gradual increase in flow area so that the
velocity energy at exit from the impeller is converted to additional pressure
energy.
The centrifugal pump is
initially primed wherein the suction pipe, casing of the pump and the portion
of the delivery pipe up to the delivery valve are completely filled with the
liquid to be pumped. With the delivery
valve closed, the impeller is made to rotate. As a result a forced vortex is
developed which imparts a centrifugal head to the liquid. Simultaneously the
angular momentum is changed resulting in an increase of the liquid pressure.
When the delivery valve is opened the liquid is forced to flow in an outward
radial direction thereby leaving the vanes of the impeller at the outer
circumference with high velocity and pressure. The high pressure of the liquid
leaving the impeller enables the liquid to rise to a high level. This action is
a continuous process because the eye of the impeller is continuously supplied
with replacement liquid from the sump as a result of the pressure gradient in
the suction pipe (a partial vacuum
exists at the eye of the impeller and the liquid in the sump is at atmospheric
pressure). The high absolute velocity at the outlet of the vanes is converted
to useful pressure energy by shaping the casing such that the liquid flows
through a gradually expanding passage.
In summary, it may be stated
that a centrifugal pump lifts the liquid to a higher level as a result of a
modification of the hydraulic gradient caused by centrifugal action and change
in angular momentum. This is in contrast to a positive displacement pump
wherein lifting action is due to pushing in a confined space.
It may also be noted that
the action of a centrifugal pump is the reverse of a radially inward flow
reaction turbine.
The main advantages of a
centrifugal pump vis-à-vis a positive displacement pump is that its discharge
capacity is much greater, it can be used to pump highly viscous liquids also,
it can be operated at high speeds with less danger of separation and
cavitation, and its maintenance requirements are low. However, it cannot
build-up pressures as high as those that can be built up by reciprocating
pumps.
The performance of a pump at
a fixed/ variable speed may be represented as follows:
Let, Inlet pressure, m =
p1
Discharge
pressure, m = p2
Flow rate, m3/s = Q
Datum, m = Z2
(Here datum is
the distance of the centre of the pressure gauge connected in the delivery line
from the flange.)
Total head across pump H =
(p2- p1) +Z2 m
For obtaining the output of
the motor (input of the pump) attached to the pump, a swinging arm field
dynamometer is provided.
Torque T = (load x arm
distance)
Input power P = (2p x speed in r.p.s. x T) watts
Water power
(Where r is the mass density of the liquid being pumped).
Water Power
Efficiency h% = ¾¾¾¾¾ x 100
Input Power
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