ME 1102

Study of Pumps

MD. ZAHIRUL ISLAMB.Sc. In Mechanical Engineering ,BUET.Lecturer ,AUST

Pump

The hydraulic machines, which convert the

mechanical energy into hydraulic energy, is

called pump. Hydraulic energy is in the

form of pressure energy.

Classification of Pumps

Rotadynamic pump (Centrifugal pump )

Centrifugal pumps are the machine which employ centrifugal force to lift liquids from a lower level to higher level by developing pressure.

Main Components1. Impeller .2. Casing .3. Suction pipe .4. Discharge pipe .

Different Parts of a Centrifugal Pump

ImpellerThe wheel fitted with a series of backward curved vanes is known as impeller.

Different Parts of a Centrifugal Pump

Types of Impeller:

1. Shrouded or closed impeller ( fig A)

2. Semi open impeller (fig. B)

3. Open impeller (fig. C )

Casing

The casing of a centrifugal pump is an air tight chamber covering the impeller.

Volute type Casing.

The cross-sectional area of the casing is gradually increased .

Kinetic energy of fluid is converted into pressure energy .

Priming

Obviously an impeller running in air would produce only a small head.

The first step in the operation of a centrifugal pump is to fill the pump with the liquid to be pumped.

This process is called the priming of the pump.

Priming is done by pouring liquid into the funnel provided for this purpose.

Cavitation is defined as the formation of bubbles and vapor filled cavities in a flowing fluid as a result of reduction in fluid pressure.

Vapor cavities are formed when the pressure at any point in a flow field falls to the vapor pressure of the liquid at that temperature.

The negative pressure at any point in a pump should not exceed the limiting pressure given by

Hn = Ha – Hv

Hn limiting negative pressure head

Ha atmospheric pressure

Hv vapor pressure head

Cavitation

Effects of Cavitation

1. The normal flow pattern is changed as the cavitation occurs. This disturbs the smooth flow.

2. The hydraulic machines indicate a sudden loss in efficiency as soon as cavitation occurs.

3. As cavitation occurs, it may cause vibration. It may further lead to fatigue stresses and excessive wear.

4. Cavitation is usually accompanied by noise.

5. The cavitating parts in the flow phenomenon cause an increase in the drag force.

6. Some of the boundary material may be eaten away at the points where cavitation occurs. It is known as pitting.

Net Positive Suction Head

NPSH is defined as the net head in meters of liquid that is required to make the liquid flow through the suction pipe from the sump to the impeller.

NPSH requiredIt is a function of the pump design. This is positive head in meters absolute required at the pump suction to overcome pump internal head losses. Pump manufacturer generally provide this information.

NPSHa must always be greater than NPSHr or

damage to the pump will occur due to cavitation.

NPSH availableIt is available head at inlet of the pump.

• Static Head:

• Static head is the difference of elevation between the liquid surface in the sump and that in the reservoir to which the

liquid is delivered.

• Hs = hs + hd

• hs suction head

• hd delivery head

Manometric Head: The manometric head is the

head developed in the pump. It is equal to the energy given to the liquid by the impeller minus the losses in the pump.

Hm = Energy Given – Losses in the pump

Hm = Hs + Losses in the pipe + Vd

2/g

Efficiencies

Mechanical EfficiencyIt is defined as Power developed by the impeller to the power supplied by the motor.

Manometric EfficiencyIt is defined as ratio of power developed by the pump to the power supplied by the impeller.

Overall efficiency

Various Losses in Pump

20

Shaf

t Po

wer

Imp

elle

r Po

wer

Wat

er P

ow

er

Stat

ic

Pow

er

Mechanical Losses

Hydraulic Losses

Losses in Pipes

Reciprocating Pump

Single Acting

Types of Reciprocating Pump

Two types1. Single Acting Reciprocating pump .2. Double Acting Reciprocating Pump .

Double Acting

Positive Displacement Pumps

24

Positive displacement pumps always produce a fixedamount of liquid to flow per revolution unlike the nonpositive displacement or dynamic pump

Works with high pressure comparing the dynamic pump Small and compact sizeGood performance

But Why we use centrifugal pumps ? For low pressure and high volume flow generally used for

supplying purpose

For which reason the Positive Displacement Pumps ? For high pressure and fixed volume purpose , generally

used for fluid power