centrifugal pump.pptx

8/10/2019 centrifugal pump.pptx

http://slidepdf.com/reader/full/centrifugal-pumppptx 1/19

THE ENERGY ABSORBING

MACHINES

The Hydraulic PUMPS



PUMPS

A PUMP is a device that converts the MECHANICAL ENERGY from external

sources (e.g. motors) into equivalent useful HYDRAULIC ENERGY mainly in the form of

pressure energy.

Main purposes of using pumps are-

1. To raise liquids from lower level to a higher level.

2. To add energy to the fluid to boost its pressure head.

CLASSIFICATION OF PUMPS:

Depending upon the Principle of operation:

1. Rotodynamic Pump Or Centrifugal Pumps

2. Positive Displacement Pump



Rotodynamic PUMPS

These pumps simply add momentum to the fluid/liquids by means of fast

moving blades or vanes. The momentum or the velocity of the fluid also increases

while moving through suction pipe and most of these velocity head is converted to

pressure energy while moving through the blade passages and the diffuser vanes.

Construction of a centrifugal pump:

The main parts of a centrifugal pump are:

1. Impeller 3. Suction pipe with foot valves and strainer

2. Casing 4. Delivery pipe 5. Guide vanes (optional)



Centrifugal PUMP



Impeller:

The rotating part of a centrifugal pump is called Impeller . It consist of series of curved vanes

mounted on a shaft which is driven by an electric motor.

Casing:

This is an airtight passage surrounding the impeller and designed in such a way that the

kinetic energy of the water delivered at the outlet of the impeller is converted to pressure

energy.

Three types of casing are commonly used-

1. Volute Casing 2. Vortex casing 3. casing with guide vanes.

Volute casing:

It is a spiral casing in which area of flow increases gradually due which velocity decreases. The

decrease in velocity increases the pressure of water flowing through the casing. In a pump

with volute casing the efficiency of the pump decreases slightly as significant amount of

energy is lost due to formation of eddies.

Construction of Centrifugal Pump



Vortex casing:

If a circular chamber is introduced between casing and impeller, the casing is known as vortex

casing. By introducing the chamber the loss of due formation of eddies can be reduced

considerably.

Casing with guide vanes (Diffuser vanes):

In these casing the impeller is surrounded by series of guide vanes mounted on a ring. The

guide vanes are designed in such a way that the water from the impeller enters the guide

vanes with same velocity and the increase in are of the guide vanes reduces the velocity and

consequently increases the pressure of water.

Construction of Centrifugal Pump



Working Principle of Francis Turbine

A centrifugal pump works on the principle of forced vortex flow which

means when a certain fluid mass is rotated by an external torque, pressure rise of thefluid takes place.

As the impeller rotates a vacuum is created at the impeller eye and liquid is

drawn into the blade passages through the suction pipe. Since the suction pipe axial,

the liquid enters the impeller with very little whirl. As the fluid mass is rotated by the

revolving impeller blades, a centrifugal force is generated and the fluid flows out in

the direction of the blades. The liquid gains momentum as it flows over the revolvingblades and is discharged with increased pressure and velocity head. Often Diffuser

blades (guide vanes) are provided at the impeller outlet which converts the kinetic

energy into pressure energy. From the diffuser blades fluid moves into the volute

casing that has a gradually increasing area and also serves to convert the velocity

head of the liquid into pressure head.



Velocity Triangle and Work done on the water:



Workdone per second on the rotor wheel by the fluid,

Since Whirl at the inlet is negligible,

So,

Workdone per sec, per unit weight of the water per sec,

= Eulers Equation

11w22w uVuVQsec/workdone

0V 1w

22w uVQsec/workdone

g/uV 22w

Consult class notes for the derivation:



Heads Of Centrifugal Pump

1. Suction Head (hs): The Height of the centerline of the pump above the liquid

surface. It is also known as Suction Lift.

2. Delivery Head (hd): The vertical distance between the centerline of the pump

and the liquid surface of the reservoir to which liquid is delivered.

3. Manometric Head (Hm): It is the total head developed by the pump.

Different expressions for manometric head—

a) Hm= Total head at outlet – Total head at inlet

b) Hm= Head imparted by the impeller to the water – Head loss in

pump casing and impeller

c) Hm= hs + hs + hfs + hfd + velocity head at delivery side



Priming:

Priming is the process of filling up of the suction pipe of the pump casing and

the portion of the delivery pipe with water before starting the pump.

Without priming the pump will be running in air, and thus the suction headgenerated will be in terms of air. As the density of the air is very low in comparison to

water, the suction head will be very negligible and as a result water may not be sucked in.



Multi Stage Centrifugal Pump :

Centrifugal pumps containing two or more impellers connected to a single shaft are

known as multi stage pumps. These pumps are used essentially when a large pressure

head is to developed.

Water from the suction pipe passes over the 1st impeller and then the water withincreased pressure is discharged at the inlet of the next impeller as shown in figure.



Pumps in series:

The large head requirement can also be achieved by connecting several single

stage centrifugal pumps in series where the delivery pipe of the first pump discharges at

the inlet pipe of the next pump.

Pumps in parallel:

If the requirement is to discharge a large a large quantity of liquid, then severalcentrifugal pumps are connected in parallel. Here all the pumps draw liquid from the

same sump and delivers to a common delivery pipe.

Consult class notes for the diagram:



Fig: Pumps connected in parallel.



Specific speed of centrifugal pump:

Specific speed of a centrifugal pump is the speed of a geometrical similar pump, which

would deliver 1 m3 of liquid per second (i.e. unit discharge) against a head of 1 meter.

4/3m

sH

Q N N

Pump similarity relationships:





CAVITATION :

It is the process of formation of vapour bubbles when pressure in any region falls below

the vapour pressure of the liquid at the existing temperature. This vapour bubbles creates

vapour locks to the flow and the flow is restricted. When these bubbles are carried awayby the incoimng liquid to a region of higher pressure these vapoure bubbles collapses and

cavities are formed. The surrounding liquid then rushes in to fill these cavities creating

very high pressure pulses, which can ultimately lead to serious damage in the form of

erosion to the solid surface of the impeller.

Note: (Most important for GATE and interviews)1. Cavitation is also present in turbine.

2. In pumps cavitation occures at the inlet (eye) while in turbine chances are more

at the outlet of the runner as the pressure at these regions are the lowest.



Net positive Suction Head: (NPSH)

Net positive suction head (NPSH) is defined as the total suction head required to make the

liquid flow through the suction pipe to the impeller.




Effect of blade angle:


centrifugal pump.pptx

Documents

Transcript of centrifugal pump.pptx