Maximum System Pressure And Permissible Liquid Temperature; Minimum Inlet Pressure; Maximum Inlet Pressure - Grundfos CM Montage- Und Betriebsanleitung

11.4 Maximum system pressure and permissible liquid temperature

Material variant
Cast iron
(EN-GJL-200)
Stainless steel
(EN 1.4301 / AISI 304)
Stainless steel
(EN 1.4401 / AISI 316)
* 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:
H = p x 10.2 - NPSH - H
b
p = Barometric pressure in bar.
b
The barometric pressure can be set to 1
bar.
In closed systems, p
system pressure in bar.
NPSH = Net Positive Suction Head in metres
head. To be read from the NPSH curves
on pages 381
the pump will be delivering.
H = Friction loss in suction pipe in metres
f
head.
H = Vapour pressure in metres head.
v
See fig. 9, page 384.
t = liquid temperature.
m
H = Safety margin = min. 0.5 metres head.
s
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.
14
Shaft seal Permissible liquid temperature*
-20 to 40 °C
AVBx
41 to 90 °C
AQQx -20 to 90 °C
-20 to 40 °C
AVBx
41 to 90 °C
-20*** to 90 °C
AQQx
91 to 120 °C**
-20 to 40 °C
AVBx
41 to 90 °C
-20*** to 90 °C
AQQx
91 to 120 °C**
- H - H
f v s
indicates the
b
to 383 at the highest flow
(-4 to 104 °F)
(105.8 to 194 °F)
(-4 to 194 °F)
(-4 to 104 °F)
(105.8 to 194 °F)
(-4 to 194 °F)
(195.8 to 248 °F)
(-4 to 104 °F)
(105.8 to 194 °F)
(-4 to 194 °F)
(195.8 to 248 °F)
Example
p = 1 bar.
b
Pump type: CM 3, 50 Hz.
3
Flow rate: 4 m /h.
NPSH (from fig. 5, page 381): 3.3 metres head.
H = 3.0 metres head.
f
Liquid temperature: 90 °C.
H (from fig. 9, page 384): 7.2 metres head.
v
H = p x 10.2 - NPSH - H
b
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.
Maximum system
pressure
10 bar (145 psi)
6 bar (87 psi)
10 bar (145 psi)
10 bar (145 psi)
6 bar (87 psi)
16 bar (232 psi)
10 bar (145 psi)
10 bar (145 psi)
6 bar (87 psi)
16 bar (232 psi)
10 bar (145 psi)
- H - H [metres head].
f v s