# CENTRIFUGAL PUMPS

The hydraulic machines which convert the mechanical energy in to hydraulic energy are called pumps. The hydraulic energy is in the form of pressure energy. If the mechanical energy is converted in to pressure energy by means of centrifugal force acting on the fluid, the hydraulic machine is called centrifugal pump.

The centrifugal pump acts as a reversed of an inward radial flow reaction turbine. This means that the flow in centrifugal pumps is in the radial outward directions. The centrifugal pump works on the principle of forced vertex flow which means that when a certain mass of liquid is rotated by an external torque, the rise in pressure head of the rotating liquid takes place.

The rise in pressure head at any point of the rotating liquid is proportional to the square of tangential velocity of the liquid at that point.

(i.e. rise in pressure head = 𝑉2/2𝑔 or r2w2/2𝑔)

Thus the outlet of the impeller, where radius is more, the rise in pressure head will be more and the liquid will be discharged at the outlet with a high pressure head. Due to this high pressure head, the liquid can be lifted to a high level.

## The following are the main parts of a centrifugal pump.

1) Impeller.

2) Casing.

3) Suction pipe with foot valve and a strainer

4) Delivery pipe.

## 1. Impeller:

The rotating part of a centrifugal pump is called impeller. It consists of a series of backward curved vanes. The impeller is mounted on a shaft which is connected to the shaft of an electric motor.

## 2. Casing:

the casing of a centrifugal pump is similar to the casing of a reaction turbine. It is an air tight passage surrounding the impeller and is designed in such a way that the kinetic energy of the water discharged at the outlet of the impeller is converted in to pressure energy before the water leaves the casing and enters the delivery pipe. The following three types of the casing are commonly adopted.

a) Volute

b) Vortex

### a) Volute Casing:

It is the casing surrounding the impeller. It is of a spiral type, in which area of flow increases gradually. The increase in area of flow decreases the velocity of flow. The decrease in velocity increases the pressure of the water flowing through the casing. It has been observed that in case of volute casing, the efficiency of the pump increase slightly as a large amount of energy in lost due to the formation of eddies in this type of casing.

## b) Vortex Casing:

If a circular chamber is introduced between the casing and the impeller, the casing is known as vortex casing. By introducing the circular chamber, the loss of energy due to the formation of eddies is reduced to a considerable extent. Thus the efficiency of the pump is more than the efficiency when only volute casing is provided.

### c) Casing with guide blades:

In this type of casing the impeller is surrounded by a series of guide blades mounted on a ring known as diffuser. The guide vanes are designed in which away that the water from the impeller enters the guide vanes without shock.

Also the area of guide vanes increases thus reducing the velocity of flow through guide vanes and consequently increasing the pressure of the water. The water from the guide vanes then pass through the surrounding casing, which is in most of the cases concentric with the impeller.

## 3. Suction pipe with a foot valve and a strainer:

A pipe whose one end is connected to the inlet of the pump and other end dips in to water in a sump is known as suction pipe. A foot valve which is a non-return valve or one-way type of valve is fitted at the lower end of the suction pipe. The foot valve opens only in the upward direction. A strainer is also fitted at the lower end of the suction pipe.

## 4. Delivery pipe:

A pipe whose one end is connected to the outlet of the pump and the other end delivers the water at the required height is known as delivery pipe.

## Work done by the centrifugal pump on water:

In the centrifugal pump, work is done by the impeller on the water. The expression for the work done by the impeller on the water is obtained by drawing velocity triangles at inlet and outlet of the impeller in the same way as for a turbine.

The water enters the impeller radially at inlet for best efficiency of the pump, which means the absolute velocity of water at inlet makes an angle of 90with the direction of motion of the impeller at inlet. Hence angle 𝛼 = 90and 𝑉𝑤1 = 0 for drawing the velocity triangles the same notations are used as that for turbines.

## HEADS OF A CENTRIFUGAL PUMP:

It is the vertical height of the centre line of centrifugal pump, above the water surface in the tank or sump from which water is to be lifted. This height is also called suction lift.

The vertical distance between the centre line of the pump and the water surface in the tank to which water is delivered is known as delivery head. This is denoted by.

Manometric head is defined as the head against which a centrifugal pump has to work. It is denoted by 𝐻 .

## 5. Efficiencies of a Centrifugal Pump:

In a centrifugal pump, the power is transmitted from electric motor shaft to pump shaft and then to the impeller. From the impeller, the power is given to the water. Thus the power is decreasing from the shaft of the pump to the impeller and then to the water. The following are the important efficiencies of a centrifugal pump:

a) Manometric efficiency, 𝜂(𝑚𝑎𝑛)

b) Mechanical efficiency, 𝜂(𝑚) and

c) Overall efficiency, 𝜂(0).

### a) Manometric Efficiency 𝜼(𝒎𝒂𝒏):

The ratio of the Manometric head to the head imparted by the impeller to the water is known as The power at the impeller of the pump is more than the power given to the water at outlet of the pump. The ratio of power given to the water at outlet of the pump to the power available at the impeller is known as Manometric efficiency.

### b) Mechanical Efficiency 𝜼:

The power at the shaft of the centrifugal pump is more the power available at the impeller of the pump. The ratio of the power available at the impeller to the power at the shaft of the centrifugal pump is known as mechanical efficiency.

### c) Overall Efficiency 𝜼:

It is defined as the ratio of power output of the pump to the power input to the pump.

## SPECIFIC SPEED OF A CENTRIFUGAL PUMP 𝑵 :

The specific speed of a centrifugal pump is defined as the speed of a geometrically similar pump, which would deliver one cubic meter of liquid per second against a head of one meter. It is denoted by ′𝑁𝑠′.