n
100%
50%
25%
12,5%
50%
100%
80%
80%
e
7
5
h
a
2
0
8
.1
0
P o w er ~n
3
P r essur
e ~n
2
Fl o w ~n
Illustration 7: Affinity Laws for Centrifugal Pumps
Flow:
Q1
Q2
=
n1
n2
Pressure:
H1
H2
=
n1
n2
2
Power:
P1
P2
=
n1
n2
3
Assuming an equal efficiency in the speed range.
Affinity Laws
Q=Flow
P=Power
Q
1
=Flow 1
P
1
=Power 1
Q
2
=Reduced flow (or increased flow)
P
2
=Reduced power (or increased power)
H=Pressure
n=Speed regulation
H
1
=Pressure 1
n
1
=Speed 1
H
2
=Reduced pressure (or increased pressure)
n
2
=Reduced speed (or reduced speed)
4.3.2 Valve Control versus Speed Control of Centrifugal Pumps
Valve control
As the demand for process requirements in water systems varies, the flow to be adjusted accordingly. Frequently used methods for
flow adaptation are throttling or recycling using valves. A recycle valve that is opened too wide can cause the pump to run at the
end of the pump curve with a high flow rate at a low pump head. These conditions do not only cause a waste of energy due to the
high speed of the pump, but can also lead to pump cavitation resulting in pump damage.
Throttling the flow with a valve adds a pressure drop across the valve (HP-HS). This can be compared with accelerating and pulling
the brake at the same time in an attempt to reduce the car speed. As shown in
, throttling makes the system curve turn
from point (2) on the pump curve to a point with significantly reduced efficiency (1).
AJ300847815559en-000101 / 130R0337 | 23
Danfoss A/S © 2020.09
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