Centrifugal Pumps working, types, specific speed and applications - Engineeringforu

What is centrifugal pump

A centrifugal pump is a machine which converts the kinetic energy  of the water into  pressure energy before the water leaves its casing. The flow of water leaving the impeller is free vortex.  The impeller of the centrifugal pump may have volute casing, vortex casing and volute casing with guide blades.
Centrifugal pump consists of three parts:
  • An impeller that rotates and imparts velocity to a liquid.
  • A casing that captures the velocity generated by the impeller and transforms that velocity into a stable flow.
  • An assembly of mechanical components that makes it possible for the impeller to be rotated within the pump casing.

Centrifugal pump working:

As the electric mortar starts rotating, it also rotates the impeller. The rotation of the impeller creates suction at the suction pipe. Due to suction created the water from the sump starts coming to the casing through the eye of the impeller.

From the eye of the impeller, due to centrifugal force acting on the water, the water starts moving rapidly outward and towards the outer of casing.
Since the impeller is rotating at high velocity it also rotates the water around it in the casing. The area of the casing increasing gradually in the direction of rotation, so the velocity of the water keeps on decreasing and the pressure increases. At the outlet of the pump the pressure is maximum. Now from outlet of the pump  the water goes to its desired location through delivery pipe.

Centrifugal pump calculations:

1. The manometric head is the actual head of water against which a centrifugal pump has to work. It may be obtained by using the following relations i.e
Manometric head = Workdone per kg of water - Losses within the impeller
                             = Energy per kg at outlet of impeller - Energy per kg at inlet of impeller
                             = Suction lift + Loss of head in suction pipe due to friction+ Delivery lift Loss head in delivery pipe due to friction + Velocity head in the delivery pipe

2. The discharge Q of the centrifugal pump is given by
Q = π D.b.Vf
where D = Diameter of impeller at inlet
            b = Width of impeller at inlet and
          Vf = Velocity of flow at inlet
3.The manometric efficiency of  centrifugal pump is defined as the ratio of the manometric head to the energy supplied by the impeller.
4. The mechanical efficiency of  centrifugal pump is defined as the ratio of energy available at the impeller to the energy supplied to the pump by the prime mover.
5. The overall efficiency of  centrifugal pump is defined as the energy applied to the pump to the energy available in the impeller.
6. The efficiency of  centrifugal pump will be the maximum when the blades are bent backward.
7. The power required to drive a centrifugal pump is given by
P = w.Q.Hmo
where w = Specific weight of water in kN/m3
           Q = Discharge of the pump in m3 /s
        Hm = Manometric head in metres and
          ηo = Overall efficiency of the pump.

Types of Centrifugal Pump:

There are many types of centrifugal pump. let us discuss some of them i.e,  multistage centrifugal pump, cantilever pump and magnetically coupled centrifugal pump.
Multistage Centrifugal Pump:
The Multistage Centrifugal Pump are those which have two or more identical impellers mounted on the same shaft or on different shafts. They are used to produce high heads or to discharge a large quantity of liquid. In order to obtain a high head, a number of impellers are mounted in series or on the same shaft while to discharge a large quantity of liquid, the impellers are connected in parallel.
Cantilever Pump:
They utilize a unique shaft and bearing support configuration that allows the volute to hang in the sump while the bearings are outside the sump. These are also known as vertical centrifugal pumps. These type of pump uses no stuffing box to seal the shaft but instead utilizes a throttle brushing.
Magnetically coupled centrifugal pump:
In magnetically coupled centrifugal pumps the mortar is coupled to the pump by magnetic means rather than by direct mechanical shaft. They are also known as magnetic drive pumps.

Specific Speed of  Centrifugal Pump:

The  specific speed of  centrifugal pump is defined as the speed of  an imaginary pump, identical with the given pump, which will discharge 1 litre of water, while it is being raised through a head of one metre. Mathematically, specific speed is given by
NS = N√Q / Hm3/4
The ranges of specific speeds for different types of pumps are as follows:
Type of Pump Specific speed in r.p.m
Slow Speed with radial flow at outlet 10 - 30
Medium speed with radial flow at outlet 30 - 50
High speed with radial flow at outlet 50 - 80
High speed with mixed flow at outlet 80 - 160
High speed with axial flow at outlet 160 - 500

Model testing and Similarity of the Pumps:

In order to know the performance of the prototypes, the models of centrifugal pumps are tested.
When the ratio of all the corresponding linear dimensions of the model and the prototype are equal, then they are said to have geometric similarity. In other words, geometric similarity is said to exist between the model and the prototype, if both of them are identical in shape but differ only in size.
When the ratio of the corresponding velocities at corresponding points are equal, then the model and the prototype are said to have kinematic similarity.
When the ratio of the corresponding forces acting at corresponding points are equal then the model and the prototype are said to have dynamic similarity.

Problems with centrifugal pumps:

  • Corrosion inside the pump caused by the fluid properties.
  • Leakage along rotation shaft.
  • Overheating due to low flow.
  • Centrifugal pumps must be filled with the fluid to be pumped in order to operate.
  • Impeller can be worsened by suspended solids.
  • Cavitation