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11
11.4 Maximum system pressure and permissible liquid temperature
* At liquid temperatures below 0 °C (32 °F), higher motor outputs may be needed due to increased viscosity,
for instance if you have added glycol to the water.
** 120 °C applies only if the pump has an AQQE shaft seal.
*** CM pumps for pumping liquids at temperatures below -20 °C are available on request. Please contact
Grundfos.
11.5 Minimum inlet pressure
You can calculate the minimum inlet pressure "H" in
metres head required during operation to avoid
cavitation in the pump from the following formula:
If the calculated value of "H" is positive, the pump
can operate with a maximum suction lift of "H"
metres.
If the calculated value of "H" is negative, a minimum
suction head of "H" metres is required during
operation to avoid cavitation.
Example
p
b
= 1 bar.
Pump type: CM 3, 50 Hz.
Flow rate: 4 m
3
/h.
NPSH (from fig. 5, page 15): 3.3 metres head.
H
f
= 3.0 metres head.
Liquid temperature: 90 °C.
H
v
(from fig. 9, page 18): 7.2 metres head.
H = p
b
x 10.2 - NPSH - H
f
- H
v
- H
s
[metres head].
H = 1 x 10.2 - 3.0 - 3.3 - 7.2 - 0.5 = -3.8 metres head.
This means that a suction head of 3.8 metres is
required during operation.
Pressure calculated in bar: 3.8 x 0.0981 = 0.37 bar.
Pressure calculated in kPa: 3.8 x 9.81 = 37.3 kPa.
11.6 Maximum inlet pressure
The actual inlet pressure plus the pressure when the
pump is operating against a closed valve must
always be lower than the maximum system pressure.
Material variant Shaft seal Permissible liquid temperature*
Maximum system
pressure
Cast iron
(EN-GJL-200)
AVBx
-20 to 40 °C
41 to 90 °C
(-4 to 104 °F)
(105.8 to 194 °F)
10 bar
6 bar
(145 psi)
(87 psi)
AQQx -20 to 90 °C (-4 to 194 °F) 10 bar (145 psi)
Stainless steel
(EN 1.4301 / AISI 304)
AVBx
-20 to 40 °C
41 to 90 °C
(-4 to 104 °F)
(105.8 to 194 °F)
10 bar
6 bar
(145 psi)
(87 psi)
AQQx
-20*** to 90 °C
91 to 120 °C**
(-4 to 194 °F)
(195.8 to 248 °F)
16 bar
10 bar
(232 psi)
(145 psi)
Stainless steel
(EN 1.4401 / AISI 316)
AVBx
-20 to 40 °C
41 to 90 °C
(-4 to 104 °F)
(105.8 to 194 °F)
10 bar
6 bar
(145 psi)
(87 psi)
AQQx
-20*** to 90 °C
91 to 120 °C**
(-4 to 194 °F)
(195.8 to 248 °F)
16 bar
10 bar
(232 psi)
(145 psi)
H=p
b
x 10.2 - NPSH - H
f
- H
v
- H
s
p
b
= Barometric pressure in bar.
The barometric pressure can be set to 1
bar.
In closed systems, p
b
indicates the
system pressure in bar.
NPSH = Net Positive Suction Head in metres
head. To be read from the NPSH curves
on pages 15 to 17 at the highest flow the
pump will be delivering.
H
f
= Friction loss in suction pipe in metres
head.
H
v
= Vapour pressure in metres head.
See fig. 9, page 18.
t
m
= liquid temperature.
H
s
= Safety margin = min. 0.5 metres head.
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