Technical appendix
How to select a centrifugal pump
The selection of a centrifugal pump should be made according to the actual
characteristics and conditions of the plant.
The required data for a correct selection are the following:
Flow Q
Quantity of fluid delivered by the pump in the unit of time, generally expressed
in m
3
/h.
Total manometric head Hmt
It is considered as the sum between the geometric head exsisting between
the fluid levels and the head loss due to frictions from the fluid passage in
the pipework, into the pump and relevant hydraulic accessories.
The expression is the following:
Hmt = Hg + Δpc mt fluid column
Hg = geometric head inlet (Hga) + geometric head outlet (Hgp)
Δpc = sum of head loss of the plant calculated from the following data:
- Diameter, length and material of the suction and delivery piping (see table
no. 2).
- Number and type of elbows in the piping and hydraulic accessories such
as foot valves, gate valves, non-return valves and strainers etc. (see
table no. 1).
- Type, temperature, viscosity and density of the fluid (if different from that of
water)
Pay attention to the manometric suction lift Hga + Δpc asp, which should be
compared with the suction capability of the pump.
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.
60
50
40
30
12
10
8
6
4
2
0 U.S.g.p.m. 50
100
150
200
250
6
4
2
Hga is the difference in height between the free surface of the water, and its value is negative if the pump is installed above the free water surface.
Δpc asp is the sum of the remaining distributed (piping) and concentrate (valves, bends, etc.) suction head loss
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 condi-
tions, without cavitation.
For fluid temperatures higher than the average of about 20°C, the pumps loose their suction capability.
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
72.020
60
0
For this purpose, once the pump has been selected according to the required flow and head, where possible at the middle of the curve, check the
following simplified formula:
10 mt ± Hga - Δpc asp. > NPSH required + 0.5 mt
the following formula:
where:
N = Q x H x γ in kW
367 x η
p
Q = Flow in m3/h
H = Head in mt
γ = Fluid density (water = 1 kg/dm
3
)
ηp = Pump efficiency (Ex. Pump efficiency 68% = ➩ ηp = 0.68)
The pumps are normally connected to electric motors, which operate at 2900 rpm with 2-pole motors at 50Hz, or at 1450 rpm with 4-pole motors at 50Hz.
However, they can run at any other speed within the limits of design.
Therefore, when changing the number of revolutions, the pump performance will change according to the following rules:
n
2
The flow in proportion to the number of revolutions: Q2 = Q1 x n
1
n
2
The head, in proportion to the square of the number of revolutions: H2 = H1 x
(
n
1
)
2
n
2
)
The absorbed power, in proportion to the cube of the number of revolutions: N2 = N1 x
(
n
1
3
0
Qm
3
/h
10
20
0
l/min
200
400
30
40
50
800
600
0
l/
s
2
4
6
8
10
12
14
16
0
0
0
0
Qm
3
/h 10
20
30
40
50
η 66%
180
160
140
120
100
60
1000
15
10
5
20
15
10
5
A Ø 200
NM 50/20
B Ø 188
50
55
60
62
64
65
65
64
N
P
S
H
m
P
k
W
H
m
N
P
S
H
f
t
P
H
P
H
f
t
A
B
491
