1. Operation of Centrifugal pump
Pump World
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.
Pump World
Pump World

2. Pump World http://www.pumpworld.com/contents.htm

3. Typically constant speed electrical motors operate pumps.
This means that for a given pump there is a discrete set of operating speeds
Pairs
60 Hz
50 Hz of
Poles
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

4. For a GIVEN pump operating at a given speed we can plot a
Performance Curve
The shutoff head the maximum head
that can be provided—the pump can lift water to
this height BUT water can not flow (Q = 0) Q
h, P h
The free-delivery This is the maximum
flow through the pump. It can only
be achieved if no pipe is attached to
the pump ( hP = 0).
Pump Efficiency

5. For a GIVEN pump operating at a given speed we can plot a
Performance Curve
Power
And define the Best Efficiency Point BEP Q
h, P
See Fig 14.9 in Book h
We attempt to size our pump so that the head required is as close as possible
To the BEP

6. Operating Point
In addition to the head provided by the pump we also have the
the system curve which says how the head required changes with Q Q h
Operating point: the head that
a given speed pump will operate at h
We attempt to size our pump so that the operating point is as close as possible
To the BEP

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

8. Dimensionless Pump Performance –see Fig 14.106 CH 1
Head Coef.
Power Coef.
Capacity Coef. h
CP and efficiency CH CP CQ
DH 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 (w radians per second)

9. 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
Solve by Iteration
Head required is = 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 =
Find f from Moody
hL = 6.7
14.9  Q = 0.23 m3/s

10. !4.22
If the pump in Fig 14.9 and Fig is operated at 1500 rpm what is
Discharge when the head is 150 ft
From Fig 14.9 we know D = 37. 1cm = ft
Fig  CQ = 0.122
Q = 5.5 ft3/s

11. 14.25
Fig pump has a D = 0.4 m and operates at 25 rps
What is discharge when head is 50 m
14.10  CQ=0.13
Q = m3/s

12. Specific speed
In selecting a pump a rule of thumb
An axial Flow pump is suited for low heads and high discharge
Radial 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 ns
Radial Mixed Axial
NOT dimensionless
This value can be used to select the correct style of pump (radial, mixed or axial) (see Fig 14.14)

13. 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).
(see example 14.7 on page 596)
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 ; cfs = 449 gpm = m3/s
1 m3/s = 15,854 gpm

14. 14.29
What is the suction specific speed for the pump of Figure 14.7—page 586
When the discharge is Q = 0.22 m3/s and velocity 2.21 m/s (answer of problem 4.13).
Is it safe.
D=.356
n = 11.5 rps, N = 690 rpm
Q = .22 m3/s = 3488 gpm
From Figure 4.13 (axial flow pump)
< 8500 so OK

15. 14.32
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
Use Fig and definition of specific speed
Fig  Radial Flow

16. Steps in Pump sizing –see Lab
For a given pump motor frequency and type and
Design discharge we can
Identify a pump speed N that will not cause cavitation h CQ CH 6
Could meet
Design by using
Pumps in series
or parallel see
Fig 14.9 (601)
With Known N use dimensionless
Pump performance curve
(supplied by manufacturer)
From definitions
Can calculate diameter of pump and head delivered
May require some iteration and use of pumps in parallel to meet
Design Head required