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Technical appendix
How to select a centrifugal pump
The selection of 0 U.S. a centrifugal g.p.m. 100 pump should 150 be 200made 250 according 300 to the 350 actual
40
characteristics and conditions of the plant.
130
The required data for a correct A Ø 168selection
are the following:
Flow Q
Quantity of fluid delivered by the pump in the unit of time, generally expressed
in m3 35
71
η 72%
110
/h.
B Ø 158
72
H m
30
Total manometric head Hmt
69
It is considered as the sum between the geometric head exsisting between 90
67
the fluid levels and the head loss due to frictions from the fluid 65 passage in
25
the pipework, into the pump and relevant hydraulic accessories. 80
The expression is the following:
70
20
3 0 m /h 20 40 60 80
0 200 400 600 800 1000 1200
Q
Hmt = Hg + pc mt l/min fluid column
0 l/s 4 8 12 16 20
Hg = geometric 8 head inlet (Hga) + geometric head outlet (Hgp)
A
pc = sum of head loss of the plant calculated from the following data:
6
B
- Diameter, length and material of the suction and delivery piping (see 6
4
table
no. 2).
4
- Number and 2type
of elbows in the piping and hydraulic accessories 2 such
as foot valves, gate valves, non-return valves and strainers etc. (see
0
0
table no. 1).
- Type, temperature, 6 viscosity and density of the fluid (if different from that of
water)
15
P kW
NPSH m
4
2
65
67
69
Pay attention to the manometric suction lift Hga + pc asp, which should be
5
compared with 0the
suction capability of the pump.
0
72.019
0 3 Q m /h 20 40 60 80
This suction capability or NPSHr is defined as net positve suction head and its
value is obtained from a curve in accordance with the flow.
0
0 3 Q m /h 10 20 30 40 50
0
72.020
60
For this purpose, 0 U.S. once g.p.m. the 50 pump has 100 been selected 150 according 200 to 250 the required flow and 0 U.S. head, g.p.m. where 100possible 150 at 200 the middle 250 of 300 the curve, 350 check the
following simplified 90 formula:
70
A Ø 250
45
C
10 mt ± Hga - pc asp. > NPSH required + 0.5 mt
80
Hga is the difference in B Ø height 230 between the free surface of the water, and 240 its value is negative if the pump is installed above the free water 57 surface.
pc asp is the 70sum
of the remaining distributed (piping) and concentrate (valves, bends, 50 etc.) suction E Ø 190head
loss
55
H m
C Ø 217
If the final result is negative, it is often possible to adjust flow via a gate valve on the delivery side, in order to restore correct pump operating 140 condi-
60
200
40
tions, without cavitation.
For fluid temperatures 50 higher than the average of about 20°C, the pumps loose their suction 30 capability.
160
Such changes, referred to pumps with suction capability of 7 meter at normal temperature, are shown on table no. 3.
CHARACTERISTIC DATA OF THE PUMPS
Once the flow (Q) and total manometric head of the installation (Hmt) are established, the pump absorbed power N should be calculated through the
following formula:
N = Q x H x A
C
B 20
D
20
C
E
10
10
in kW
10
10
367 x p where:
P kW
Q = Flow in m3/h
H = Head in mt
= Fluid density (water = 1 kg/dm3 6
4
)
p = Pump efficiency (Ex. Pump efficiency 68% = ➩ p = 0.68)
NPSH m
The pumps are 0normally
connected to electric motors, which operate at 2900 0 rpm with 2-pole 0 motors at 50Hz, or at 1450 rpm with 4-pole motors at 50Hz. 0
72.021
72.036
However, they can 0 3
run at 10any
other 20 speed 30 within the 40limits
of 50 design. 60
0 3
Q m /h
Q m /h 20 30 40 50 60 70 80 90
Therefore, when changing the number of revolutions, the pump performance will change according to the following rules:
The flow in proportion to the number of revolutions: Q2 = Q1 x
The head, in proportion to the square of the number of revolutions: H2 = H1 x ( ) 2
The absorbed power, in proportion to the cube of the number of revolutions: N2 = N1 x ( ) 3
71
120
100
10
8
10
5
H ft
P HP
NPSH ft
60
50
40
0 U.S. g.p.m. 50 100 150 200 250
65
64 120
30
100
3 0
Q m /h
0 l/min
10
200
20
400
30
600
40 50
800
60
1000
0 l/s 2 4 6 8 10 12 14 16
12
10
A 15
8
6
B
10
4
2
0
6
4
2
A Ø 200
B Ø 188
50
55
60 62 64 65
50 140
80
40
3 0
Q m /h
0 l/min
10
200
20
400
30
600
40 50
800
60
1000
20
3 0
Q m /h
0 l/min
20 30
500
40 50 60
1000
70 80 90
1500
0 l/s 2 4 6 8 10 12 14 16
20
0
2
50
53
NM 50/16
55
NM 50/25
η 59%
57
n2
n1
280
260
220
180
26
0
20
15
10
5
H ft
P HP
NPSH ft
379
n2
n1
H m
P kW
NPSH m
H m
P kW
NPSH m
60
0 l/s
10 20
20
0
8
6
4
2
n2
n1
Ø 218
D Ø 203.5
52 55
57 59 61 η 62%
NM 50/20
61
η 66%
NM 50M
59
52
180
160
140
5
0
20
15
10
220
200
180
160
120
100
26
0
25
20
15
10
5
H ft
P HP
NPSH ft
H ft
P HP
NPSH ft
49