Pumps

Dr. Mohsin Siddique

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Pumps

� Pumps convert mechanical energy to fluid energy

� A pump usually refers to a machine used for incompressible fluids (water, oil); fans, blowers

� Types of pumps

� Positive displacement

� Centrifugal pump

� Axial flow pump

� Mixed flow pump

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Pumps: Types

� Positive Displacement Pump

� These types of pumps displace fixed volumes of fluid during each cycle or revolution of the pump.

� No longer used for distribution system pumping in most water systems, but portable units may be used for dewatering excavations during construction.

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Pumps: Types

� Centrifugal Pump

� Frequently used in waterdistribution systems.

� Water enters the pump throughthe eye of the spinning impellerand goes outward from thevanes to discharge pipe.

� A centrifugal pump consists of: arotating element (impeller) andhousing which encloses theimpeller and seals thepressurized liquid.

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Pumps: Types

� Axial Flow pumps

� In axial-flow pumps, the flowenters and leaves the pumpchamber along the axis of theimpeller, as shown in Figure

� In mixed flow pumps,outflows have both radial andaxial components.

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Pumps: Types

� The pumps illustrated in Figure are both single-stage pumps, which means that they have only one impeller.

(a) Typical centrifugal pump installation. (b) Typical axial-flow pump installation.

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Pumps: Types

� In multistage pumps, two or more impellers are arranged in series in such a way that the discharge from one impeller enters the eye of the next impeller. These types of pumps are typically used when large pumping heads are required.

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TDH

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Head Developed by Pump

Head developed by a pump. In this case ps/γ is negative.

++−

++=−= s

ssd

ddsd z

g

Vpz

g

VpHHh

22

22

γγ

The net head h delivered by the pump to the fluid

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Performance of Pump

� Head and Capacity (Q)

� BHP (Brake Horsepower) and Capacity

� Efficiency and Capacity

� NPSH and Capacity

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Pump Curve vs System Curve

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With increasing discharge (Q) head losses (hL) in pipeline increase

and at the same time head provided (h) by pump reduce.

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Specific Speed

� For pumps, the commonly used definition of specific speed (also called shape number), ns , is given by

� where any consistent set of units can be used. In SI units, w is in rpm, Q in m3/s, g in m/s2, and hp in meters.

� It is common practice in the United States to define the specific speed, Ns, as

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Problem:

� It is desired to deliver 100L/s at a head of 270m with a single stage pump.

� (a). What would be the minimum rotative speed that could be used. Assuming that the minimum practical specific speed, Ns, is 10.

� (b). For the conditions of (a) how many stages must the pump (Ns=10) have if a rotative speed of 600 rpm is to be used.

( )( )

rpm

Q

hN

h

QN

ps

p

s

21061000/100

270102/1

4/3

2/1

4/3

4/3

2/1

==

=⇒=

ω

ωω

( )

stagepermh

N

Qh

p

s

p

6.50

1910

1.06002/12/1

4/3

=

===ω

Total Reqd. Stages=270/50.6=5.34

6 stage are required

a.

b.

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Problem:

� Determine the specific speed of a pump that is to deliver 125L/s against a head of 45m with a rotative speed of 600rpm.

( )( )

2.1245

1000/1256004/3

2/1

4/3

2/1

==⇒= ωω

p

sh

QN

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Total Dynamic Head

� To determine the size of the pump, one must know the total dynamic head that the pump is expected to provide.

� Total dynamic head (TDH) consists of

� The difference between the center line of the pump and the height to which water must be raised.

� The difference between the suction pool elevation and centerline of the pump

� Frictional losses in the pump and fitting

� Velocity head

� Mathematically, it can be written as;

TDH=Hs + HL + Hv

� Hs= Total static head (difference between elevations of pumping source and point of delivery

� HL = Friction losses in pipes and fittings

� Hv= Velocity head due to pumping

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Total Dynamic Head (Head Added by Pump)

� If a pump has been selected, Bernoulli’s equation can be rearranged to solve for the head added by a pump

� Where,

ha=head added by pump (TDH)

hf= head loss in attached pipe and fittings

P=Atmospheric pressure

V=velocity

Z=elevation

( ) fa hZZVVPP

h +−+−

+−

= 12

2

1

2

212

γγ

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Head Added by Pump

� A pump is being used to deliver 35gpm of hot water from a tank through 50feet of 1-inch diameter smooth pipe, exiting through a ½ inch nozzle 10 feet above the level of tank. The head loss due to friction in the pipe is 26.7 feet. The specific weight of water is 60.6 lb/ft3. Determine the head added by pump.

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50’10’

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27

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e

29

30

+−=

9.0Re

74.5

7.3

/log2

1 De

f

De /

De /

e

32

De /

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Cavitation

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Cavitation

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For any operating pump, a

cavitation parameter, σ, is defined

by the relation

where hp is the head added by the

pump.

For all pumps, there is a critical value

of the cavitation parameter, σc below

which cavitation in the pump can be

expected to occur.

This critical value of the cavitation

parameter is usually provided by the

pump manufacturer and generally

places a limit on the operating range

of the pumpValues of σc as a function of

specific speed

Net Positive Suction Head (NPSH)

• The absolute dynamic head at the pump inlet (suction) in excess of the vapor pressure

Cavitation

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e

Cavitation

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Calculation of the theoretical required

power of a pump

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Calculation for Pump Efficiency

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Pump Power and Efficiency

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Pumps in Series and Parallel

� Pumps can be arranged in serial or parallel to provide an additional head or flow rate capacity.

Pumps in Series

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� When two (or more) pumps are arranged in serial theirresulting pump performance curve is obtained by addingtheir heads at the same flow rate as indicated in the figure below.

Pumps in Parallel

� When two or more pumps are arranged in parallel theirresulting performance curve is obtained by adding theirflowrates at the same head as indicated in the figure below.

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Variable Speed Pump (Affinity law)

Thank You

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