Pump is adding energy to a fluid. The energy addition is
achieved by means of centrifugal force in centrifugal
pump.When fluid enters into the impeller eye (Center of the
impeller where vacuum exists because of pumping), the fluid
will be thrown out of impeller eye by a centrifugal force
which adding the velocity to the fluid. The volute
surrounding the impeller guides the fluid to the discharge
and it converts the velocity enegy to pressure energy
(head).when u keep the dischage valve closed, it will
attain the shut off head. once the discharge valve is
opened the flouid will start to flow and head will start
decreases since the energy addition is shared by pressure
energy and velocity energy.

centrifugal pumps work by converting the kinetic energy of
a liquid into pressure energy.it does this by means of two
major components,i.e the impeller and the diffuser.as the
impeller is rotated by means of a motor or engine,a low
preesure region is created at the impeller eye causing the
liquid to be sucked into the eye.as the impeller rotates
the sucked fluid is thrown out with force through the
periphery of the impeller.now the diffuser comes into
action.the diffuser due to its shape of varying xsectional
area causes the liquid to slow down and from bernoullis
principle, a reduction in kinetic head will be compensated
by an increase in pressure head.thats basically how it
works.

fluid is enter in center of the impeller,and is picked up by
impeller, fluid move radially due to centrifugal force.No
demand of fluid in centrifugal pump so doesn' t need
pressure relief valve.

In this used only High volume & low pressure condition.

The centrifugalpump works on principle of forced vortex
flow, which means that when certain mass of liquid is
rotated by an external torque,the rise in pressure head of
rotating fluid takes place.

Fluid entering a centrifugal pump is immediately directed
to the low pressure area at the center
or eye of the impeller. As the impeller and blading rotate,
they transfer momentum to incoming
fluid. A transfer of momentum to the moving fluid increases
the fluid’s velocity. As the fluid’s
velocity increases its kinetic energy increases. Fluid of
high kinetic energy is forced out of the
impeller area and enters the volute.
The volute is a region of continuously increasing cross-
sectional area designed to convert the
kinetic energy of the fluid into fluid pressure. The
mechanism of this energy conversion is the
same as that for subsonic flow through the diverging
section of a nozzle. The mathematical
analysis of flow through the volute is based on the general
energy equation, the continuity
equation, and the equation relating the internal properties
of a system. The key parameters
influencing the energy conversion are the expanding cross-
sectional area of the volute, the higher
system back pressure at the discharge of the volute, and
the incompressible, subsonic flow of the
fluid. As a result of the interdependence of these
parameters, the fluid flow in the volute, similar
to subsonic flow in a diverging nozzle, experiences a
velocity decrease and a pressure increase.

It works on the principle of forced vertex flow and
mechanical energy is converted into hydraulic energy ie
pressure energy.Mainly the centrifugal pump has two
components ,Impeller & shaft.when the impeller is moved in
the liquid,there is centrifugal force is applied on rotating
fluid.so we can define it as the pump lifts the fluid with
aid of atmospheric pressure and pressurizes it and dicharges
from casing with high velocity.

Working Mechanism of a Centrifugal Pump
A centrifugal pump is one of the simplest pieces of equipment in any process plant. Its purpose is to convert energy of a prime mover (a electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.
Note: All of the forms of energy involved in a liquid flow system are expressed in terms of feet of liquid i.e. head.

Generation of Centrifugal Force
The process liquid enters the suction nozzle and then into eye (center) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same one that keeps water inside a bucket that is rotating at the end of a string.

Liquid flow path inside a centrifugal pump
Conversion of Kinetic Energy to Pressure Energy
The key idea is that the energy created by the centrifugal force is kinetic energy. The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.
This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle.
Therefore, the head (pressure in terms of height of liquid) developed is approximately equal to the velocity energy at the periphery of the impeller.

This head can also be calculated from the readings on the pressure gauges attached to the suction and discharge lines.
One fact that must always be remembered:
A pump does not create pressure, it only provides flow. Pressure is a just an indication of the amount of resistance to flow.
Pump curves relate flow rate and pressure (head) developed by the pump at different impeller sizes and rotational speeds. The centrifugal pump operation should conform to the pump curves supplied by the manufacturer. In order to read and understand the pump curves, it is very important to develop a clear understanding of the terms used in the curves. This topic will be covered later.

A centrifugal pump is one of the simplest pieces of
equipment in any process plant. Its purpose is to convert
energy of a prime mover (a electric motor or turbine) first
into velocity or kinetic energy and then into pressure
energy of a fluid that is being pumped. The energy changes
occur by virtue of two main parts of the pump, the impeller
and the volute or diffuser. The impeller is the rotating
part that converts driver energy into the kinetic energy.
The volute or diffuser is the stationary part that converts
the kinetic energy into pressure energy.
Note: All of the forms of energy involved in a liquid flow
system are expressed in terms of feet of liquid i.e. head.

a centrifugal pump has a casing,an impeller mounted on a shaft,an inlet and an outlet pipe.the shaft is connected to a motor,which causes the impeller to rotate.this produces a low pressure area at the eye of the impeller(the center of the impeller).the pressure difference causes the fluid to be sucked up into the casing.the curved vanes of the impeller rotates the fluid,imparting kinetic energy to the fluid due centrifugal force(hence the name centrifugal pump).energy is imparted to the fluid by this means.this fluid now enters the volute,which is a part whose cross sectional area increases gradually.as per Bernoulli's law and equation of continuity, the velocity decreases as the area of cross section decrease.the drop this kinetic energy is compensated by an increase of pressure energy(pressure head),which can be used to transport the liquid to a greater height.basically,a pressure difference between the inlet and the reservoir sucks the fluid into the casing,and the energy imparted in the form of kinetic energy is converted to pressure head