WHAT IS CENTRIFUGAL PUMP?

WORKING MECHANISM OF A CENTRIFUGAL PUMP

ADVANTEGAES AND DISADVANTAGES OF CENTRIFUGAL PUMPS

Centrifugal pump works on the principle that when a certain mass of liquid is made to rotate along the impeller from the central axis of rotation, it impresses a centrifugal head. It causes the water to move radially outwards at higher velocity and causes the water to rise to a higher level. The motion of water is restricted by casing of pump, it result into pressure build up. In addition , the change in angular momentum of liquid during its fiow reults into increase in pressure head.

The steps involved in operation of centrifugal pump are as follows :

1.The delivery valve is closed.

2.The priming of the pump is carried out. Priming involves the filling the liquid in suction pipe and casing upto the level of delivery valve so that no air pockets are left in the system. if any air or gas pockets are left in this portion of pump, it may result into no delivery of liquid by the pump.

3. The pump shaft and impeller is now rotated with the help of an external source of power like a motor or any other prime mover.

The rotation of impeller inside a casing full of liquid produces a forced vortex which is responsible in imparting the centrifugal head to the liquid. It creates a vaccum at the eye of impeller and causes liquid to rise into suction pipe from the sump.

4. The speed of impeller should be sufficient to produce the centrifugal head such that it can initiate discharge from delivery pipe.

5. Now the delivery valve is opened and the liquid is lifted and discharge through the delivery pipe due to its high pressure.

Thus the liquid is continously sucked from the sump to impeller eye and it is delivered from the casing of pump through the delivery pipe.

6. Before stopping the pump, it is necessary to close the delivery pipe otherwise the back flow of liquid may take place from the high head reservoir.

:- In case the priming of

pump is not done and the pump is not done and the pump is not under the operation, the water present in the pump and suction pipe will flow back to the sump. The space occupied by water will be filled by air.

:- The operation of filling the casing , impeller and suction pipe and the portion of delivery pump upto delivery valve is called priming.

:- If the pump is now started, the air pockets inside the impeller may give rise to vortices and cause the discontinuity of flow. Under these condition, the flow of fluid does not commence and the pump runs dry. It causes the rubbing and seizing of the wearing rings, increases noise level and vibrations and finally may cause the serious damage to pump.

:- The priming in resiprocating pumps is not required since the pumping is done by positively moving the fluid out of the cylinder by the piston. Hence, the air will be displaced from the casing when the pump starts and it will get a suction pressure which will draw the fluid from the sump.

Priming for small pumps is done by hand

:- A foot valve is essential at the bottom of suction pipe

:- A funnel or priming cup is provided to fill the water by hand. An air vent is provided in the casing of pump. When the water is filled, the air escapes through the air vent. It is closed once the priming is completed

Convert the mechanical energy into hydraulic energy by centrifugal force on the liquid

Constitute the most common type of pumping machinery

Used to move liquids through a piping system

Has two main components:

1. Stationary componets, casing, casing cover and bearings2. Rotating components, impeller and shaft

Classified into three categories ; Radial Flow, Mixed Flow, Axial Flow

Simplest piece of equipment in any process plant

Energy changes occur by virtue of impeller and volute

Liquid is fed into the pump at the center of a rotating impeller and thrown outward by centrifugal force

The conversion of kinetic energy into pressure energy supplies the pressure difference between the suction side and delivery side of the pump

Liquid flow path inside a centrifugal pump

Advantages

Simple in construction and cheapHandle liquid with large amounts

of solidsNo metal to metal fitsNo valves involved in pump

operationMaintenance costs are lower

Disadvantages

Cannot handle highly viscous fluids efficiently

Cannot be operated at high headsMaximum efficiency holds over a

narrow range of conditions

•The power supplied to the pump:

Where m=mass flow rate, kg/s H= total discharge head, N.m/kg =efficiency

…………(1)

•The power delivered to the fluid:

…………(2)

•From equations (1) and (2) efficiency:

nq is the specific speed for a unit machine that

is geometric similar to a machine with the head Hq = 1 m and flow rate Q = 1 m3/s

43q H

Qnn

qs nn 55,51

Three basic components:

Volute, casing, bodyor Diffuser

Impelleror impellers

Driver (motor)

Centrifugal Pumps

Vanes

Direction ofrotation

Centrifugal Pumps

Single suction impeller

Centrifugal Pumps

• Open

• Semi-open

• Closed

- Single suction

- Double suction

• Non-clogging

• Axial flow

• Mixed flow

Maximum efficiency lies in the range:

2000<NS<3000High head, low capacity

pumps:500<NS<1000

Low head, large capacity pumps:

NS>15000

Centrifugal Pumps

Rotation

Impeller

Blades

Vt

V

r

V

s

Vr = Radial Velocity

Vt = Tangential Velocity

Vs = Vector Sum Velocity

Centrifugal Pumps

Suction Eye

Cutwater

Arrows represent thedirection of water flow Discharge

Nozzle

Centrifugal Pumps

Series 1510Series 1510

• 26 sizes– To 4400 gpm– To 520 ft TDH– To 150 HP To

8”x10”– General

Purpose Motor

Centrifugal Pumps

Bell & GossettSeries HSC

Bell & GossettSeries HSC

– 37 sizes– To 12500 gpm– To 840 ft TDH– To 1000 HP– To 14”x18”– General

Purpose Motor

Centrifugal Pumps

Typical Split Case Pump- Section ViewTypical Split Case Pump- Section View

Centrifugal Pumps

Series VSCSSeries VSCS

Series VSCSeries VSC

– 17 sizes– To 8000 gpm– To 400 ft TDH– To 600 HP– To 12”x14”– General Purpose

Motor

Standard construction, any impeller

Mechanical sealInternally flushed, seal

cavityVariety of seal materials

Stuffing box construction, any impeller

External flush linesCompression packing

ringsSingle flushed

mechanical seal

Centrifugal Pumps

Standard construction, any impellerMechanical sealInternally flushed, seal cavityVariety of seal materials

Centrifugal Pumps

Temperature of the flushing water.225ºF to 250ºF

System pH.7 to 9 for ceramic sealsup to 11 for tungsten carbide seals

Concentration of dissolved solids.TDS less than 1000 ppm

Concentration of suspended solids.Less than 20 ppmSilica, less than 10 ppm

Centrifugal Pumps

Stuffing box construction, any impellerExternal flush lines

Internal or External FluidCompression packing ringsSingle flushed mechanical seal

Centrifugal Pumps

ImpellerSingle suction or double

suction.Volutes

Base mounted or in-line.Internally flushed or

stuffing box.Single stage or multi

stage.Packing, seal or wet rotor.

Centrifugal Pumps

Centrifugal Pumps

5 dia.

1. Pipe supported2. Length of

suction piping allows even impeller loading

RIGHT WRONG

1. Pipe weight hangs on pump flange.

2. Short suction pipe results in uneven impeller loading.

Centrifugal Pumps

Single SuctionImpeller

The amount of fluid

The properties of the fluid

Type of power supply

Cost and mechanical efficiency of the pump