Specifications

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Fig.14.4 centrifugal Pump action
The centrifugal pump converts 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 due to two main parts of the pump, the impeller and the volute or casing.
The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or
casing is the stationary part that converts the kinetic energy into pressure energy.
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 rotates or the bigger the impeller is, then the higher will be the velocity of the liquid at the
vane tip and greater will be the energy imparted to the liquid. This kinetic energy of a liquid
coming out of an impeller is controlled 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.
14.4 Friction Head
As water moves through the pipe its contact with the pipe wall creates friction. As flow (or
velocity) increases, friction also increases. Thus the greater energy is required to push it through.
Friction or Friction Head is defined as the equivalent head in feet of liquid necessary to overcome
the friction caused by flow through a pipe and its associated fittings. The suction head allowable
for a pump is very limited and therefore every foot saved is precious. By providing a larger
diameter for the pipe the velocity of flow and therefore friction losses are reduced.