Centrifugal Pump Write Up.pdf

Lecture 7 Centrifugal pump.

CENTRIFUGAL PUMP  The word pump is a general term for any fluid machine that adds energy to a fluid.  The increase in fluid energy is usually felt as an increase in the pressure of the fluid to a sufficiently high value so that the liquid flows from the pump placed at lower level to the higher discharge end through the connecting pipeline.  A turbine converts the hydraulic energy into the mechanical energy, whereas, hydraulic pumps or simply pumps are devices meant to convert mechanical energy to hydraulic energy.  The principle on which the turbo machines work is that the torque on the runner is equal to the rate of change of moment of momentum of the fluid during its passage through the runner. This is known as the Euler’s principle  The time rate of angular momentum of a mass equals the moment of the force acting on the mass, which is equal to the torque. (

̅) =

 Centrifugal or radial pumps are simple and versatile constructions with a wide range of impeller design. The motion of water in it is from centre towards the periphery. The motion is caused by the centrifugal force created in the pump as a result of the revolving motion of the working wheel.  The centrifugal pumps have radially outward flow. The pressure increases with increase in radial distance due to centrifugal action. The pump is driven by power from an external source, usually an electric motor.  The main components of the pump are the volute casing and the rotating impeller. The impeller at the centre of the pump and is mounted on a shaft and is enclosed in a volute

chamber. The shaft is usually coupled to an electric motor. A suction pipe whose one end is connected to the inlet of the pump and the other end dips into sump. A foot valve, which is a non-return valve, is fitted at the lower end of the suction pipe above the strainer, which opens only in the upward direction. It serves to fill the pump with liquid before it is started, and prevents back flow when the pump is stopped. A delivery pipe whose one end is connected to the outlet of the pump and the other end to the over head tank.  Water enters at the centre of the impeller. The water passes between the vanes and is pushed radically towards the casing and then onwards through the discharge nozzle. The vanes create an increase in both water velocity and pressure. Thus, in order for the pump to function efficiently, it is necessary that the casing is full of water and no air is entrapped.  The housing that encloses the rotating element and seals the pressurized liquid inside commonly called as Casing. The casing is of spiral shape and it terminates in a delivery pipe.  The centrifugal pump usually works well within a head of 1 to 100 m and a discharge of up to 40-50 m3/s. The efficiency is usually high and up to 90%.  Centrifugal pumps are available in different varieties, handling different quantities of water at different heads. Three parameters usually important in selecting centrifugal pumps are discharge to be delivered, head to be delivered, and rotating impeller speed.

Delivery Over head tank/

Delivery Im Cas

Delivery

hd Suction

Delivery hs

H

Suction

Su

Foot valve

Figure 6.13 A typical centrifugal pump ( From: Chandramouli, et al., 2012)

Water exit Impeller vanes

Impeller Discharge nozzle

Water entry

Volute casing

Figure 6.14: A typical section of Impeller ( From: Chandramouli, et al., 2012)

 Centrifugal pumps find wide applications in the field of industry, agriculture, water supply, etc., they are available, in various sizes, to suit the particular discharge, and head.  The centrifugal pump works on the principle of forced vortex 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 the tangential velocity of the liquid at that point. That is, the rise in pressure head is given by,

=

 At the outlet of the impeller where the 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.

Classification of the centrifugal pumps. According to type of casing  Volute casing: It is also known as Spiral casing as the impeller is surrounded by a spiral casing. In the volute type of pump the liquid enters the impeller radially through a central opening and flows radially outward around the entire circumference into a progressively expanding spiral casing. This spiral casing provides a gradual increase in the area of flow, which decreases the velocity of water and correspondingly increases the pressure.  Volute casing with a vortex chamber: It is an improved version of volute casing, wherein the spiral casing is combined with a circular chamber (called vortex or whirlpool chamber). the formation of eddies are reduced to a considerable extent and an increased efficiency is obtained.

 Diffuser pump or turbine pump: In this type of casing, there are guide blades surrounding the impeller. These guide blades are arranged at such an angle, so that the water enters without shock and forms a passage of increasing area, through which the water passes and reaches the delivery pipe. The ring of the guide blades is called diffuser and is very efficient. According to relative direction of flow through impeller Based on the direction of flow of the liquid through the impeller, the centrifugal pumps are usually classified into following three general categories:  Radial flow: the pressure is developed wholly by centrifugal force upon the liquid which enters the impeller axially at the centre and flows radially to the periphery  Mixed flow: Here, the pressure is developed partly by centrifugal force and partly by the lift of vanes of the impeller on the liquid. Flow through a mixed flow impeller is a combination of axial and radial flows.  Axial flow: Here, the pressure is developed by the propelling or lifting action of the vanes of the impeller on the liquid. Axial flow pumps have a very large discharge and as such they are best suitable for irrigation purposes. According to Working head: Centrifugal pumps may be classified according to the head developed as:  Low head pumps : A low head pump is capable of working against a total head up to 20 m,  Medium head pumps: a medium head pump, is capable of working against a total head more than 20 m but up to 60m.

 High head pumps: A high head pump is one which is capable of working against a total head above 60m.