Minimum Inlet Pressure; Maximum Inlet Pressure - Grundfos CM 1 Manual De Instrucciones

11.5 Minimum inlet pressure

The minimum inlet pressure "H" in metres head
required during operation to avoid cavitation in the
pump can be calculated from the following formula:
H = p x 10.2 - NPSH - H
b
p = Barometric pressure in bar.
b
(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 22 to 24 at the highest flow the
pump will be delivering).
H = Friction loss in suction pipe in metres
f
head.
H = Vapour pressure in metres head.
v
See fig. 18, page 25.
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.
Example
p = 1 bar.
b
Pump type: CM 3, 50 Hz.
3
Flow rate: 4 m /h.
NPSH (from fig. 14, page 22): 3.3 metres head.
H = 3.0 metres head.
f
Liquid temperature: 90 °C.
H (from fig. 18, page 25): 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 should
always be lower than the maximum system pressure.
- H - H
f v s
indicates the
b
- H - H [metres head].
f v s
17