The two main components of a centrifugal pump are the impeller and the volute. The impeller produces liquid velocity and the volute forces the liquid to discharge from the pump converting velocity to pressure. This is accomplished by offsetting the impeller in the volute and by maintaining a close clearance between the impeller and the volute at the cut-water.

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Operation of Centrifugal pump

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Pairs 60 Hz

50 Hz

of

Poles Synchronous Induction Synchronous Induction

1 3600 rpm 3500 rpm 3000 rpm 2900rpm

2 1800 1750 1500 1450

3 1200 1160 1000 970

4 900 870 750 720

8 450 435 300 360

Typically constant speed electrical motors operate pumps. This means that for a given pump there is a discrete set of operating speeds

Q

h, PThe shutoff head the maximum head that can be provided—the pump can lift water tothis height BUT water can not flow (Q = 0)

The free-delivery This is the maximum flow through the pump. It can onlybe achieved if no pipe is attached tothe pump ( hP = 0).

P

Qhp Pump Efficiency

For a GIVEN pump operating at a given speed we can plot a Performance Curve

Q

h, P

For a GIVEN pump operating at a given speed we can plot a Performance Curve

Power

And define the Best Efficiency Point BEP

We attempt to size our pump so that the head required is as close as possibleTo the BEP

See Fig 14.9 in Book

Q

h

Operating Point

We attempt to size our pump so that the operating point is as close as possibleTo the BEP

In addition to the head provided by the pump we also have thethe system curve which says how the head required changes with Q

Operating point: the head thata given speed pump will operate at

The performance of Similar Pumps can be represented by A single Dimensionless Performance Curve see 14.10 which can be used to size the pump for the job

CQ

CH

CP

CP a

nd e

ffic

ienc

y 0

1

CH

0

6

Dimensionless Pump Performance –see Fig 14.10

3Q

35P

22H

nD

QC

,nD

PC

,g/nD

HC

Head Coef.

Power Coef.

Capacity Coef.

H head required, Diameter of impeller, P power n revolutions per second,

Note n rotational speed. Some times this is written as N –revs per MINUTE

If you are given an angular speed ( radians per second) 2/n

14.21

A pump defined by Fig 14.9 –pumps water from, 366 to 450 m through A 0.36 m steel pipe 610 m long. What is Q for Pipe

Head required is = 450-366 + head loss (hL) in Pipe—Guess 90 (hL = 6)

14.9 Q = 0.24 m3/s

V=Q/A=2.36 m/s Relative Roughness ks/D = 0.00012

g2

V

D

Lfh

2

L Find f from Moody hL = 6.7

14.9 Q = 0.23 m3/s

Solve by Iteration

!4.22 If the pump in Fig 14.9 and Fig 14.10 is operated at 1500 rpm what isDischarge when the head is 150 ft

217.5)60/1500(x217.1

2.32x150

g/nD

HC

2222H

From Fig 14.9 we know D = 37. 1cm = 1.217 ft

Fig 14.10 CQ = 0.122 33 217.1x)60/1500(

Q

nD

Q

Q = 5.5 ft3/s

14.25

Fig 14.10 pump has a D = 0.4 m and operates at 25 rpsWhat is discharge when head is 50 m

9.4)25(x4.

81.9x50

g/nD

HC

2222H

14.10 CQ=0.13 33 4.0x)25(

Q

nD

Q

Q = 0.208 m3/s

Specific speed

In selecting a pump a rule of thumbAn axial Flow pump is suited for low heads and high dischargeRadial flow for higher heads and lower discharge

We can be a little more precise in pump style selection by defining the specific speed obtained by combing CH and CQ to eliminate the size factor D

feet)in ΔH andmin per gallonsin is Q rpm, is N in USA, used (form

H

NQN,

Hg

nQn

4/3

2/1

s4/34/3

2/1

s

This value can be used to select the correct style of pump (radial, mixed or axial) (see Fig 14.14)

NOT dimensionless

ns

Radial Mixed Axial

Suction Limit and Cavitation

The Net Positive Suction Head NPSH is defined as the pump head on the suction side of the pump minus the vapor pressure (expressed as head).

v

2sabss p

g2

V)p(NPSH

(see example 14.7 on page 596)

feet)in NSPH andmin per gallonsin is Q rpm, s i (N NPSH

NQS

4/3

2/1

Note: Typically ps ~ atmospheric pressure. Pressure is ABSOLUTE

In US dimensioned system we can also define a suction specific speed

(SI dimensionless form given on page 597)

A value of S < 8500 is usually safe from cavitaion

Note in Roberson and Crowe S = Nss ; 1 cfs = 449 gpm = 0.02832 m3/s 1 m3/s = 15,854 gpm

14.29

What is the suction specific speed for the pump of Figure 14.7—page 586When the discharge is Q = 0.22 m3/s and velocity 2.21 m/s (answer of problem 4.13).Is it safe.

424.0356.x5.11

22.

nD

QC

33Q

D=.356 n = 11.5 rps, N = 690 rpm

From Figure 4.13 (axial flow pump) 5.1CH

NPSH

NQS

4/3

2/1

v

2sabss p

g2

V)p(NPSH ft8.33m30.10

81.9

34.2

81.9x2

21.2

81.9

101 2

Q = .22 m3/s = 3488 gpm

29088.33

3488x69075.

5.0

< 8500 so OK

14.32

4/34/3

2/1

sHg

nQn

Use Fig 14.14 and definition of specific speed

What type of pump should be used to pump at Q =0.4 m3/s

Under a head of 70 m if N = 1100 rpm

0864.070x81.9

4.0x)60/1100(4/34/3

5.0

Fig 14.14 Radial Flow

Steps in Pump sizing –see Lab

For a given pump motor frequency and type andDesign discharge we can Identify a pump speed N that will not cause cavitation

feet)in NSPH andmin per gallonsin is Q rpm, s i (N NPSH

NQS

4/3

2/1

CQ

CH

0

CH

0

6

With Known N use dimensionlessPump performance curve(supplied by manufacturer)

3Q

22H

nD

QC

,g/nD

HC

From definitions

Can calculate diameter of pump and head delivered

May require some iteration and use of pumps in parallel to meetDesign Head required

Could meetDesign by usingPumps in seriesor parallel seeFig 14.9 (601)