Minimum Inlet Pressure - Npsh; Maximum Inlet Pressure - Grundfos CM Instrucciones De Instalación Y Funcionamiento

13.1 Minimum inlet pressure - NPSH

The minimum inlet pressure "H" in feet of head required during
operation to avoid cavitation in the pump can be calculated from
the following formula:
H = p – NPSH – H – H – H
b f v
p = Barometric pressure in feet absolute.
b
(Barometric pressure can be set to 33.9 ft)
In closed systems, p
b
feet.
NPSH = Net Positive Suction Head required in feet of head (to
be read from the NPSH curves in fig. 15 through fig. 20
at the highest flow the pump will be delivering).
H = Friction loss in suction pipe in feet of head.
f
H = Vapor pressure in feet of head, see fig. 13.
v
t = liquid temperature.
m
H = Safety margin = min. 2.0 feet of head.
s
If the calculated value of "H" is positive, the pump can operate
with a maximum suction lift of "H" feet.
If the calculated value of "H" is negative, a minimum suction head
of "H" feet is required during operation to avoid cavitation.
Example
p = 33.9 ft.
b
Pump type: CM 3, 60 Hz.
Flow rate: 20 gpm.
NPSH (from fig. 16): 14.4 ft head.
H = 10 ft head.
f
Liquid temperature: 194 °F.
H (from fig. 14): 24 ft head.
v
H = p – NPSH – H – H – H [ft head].
b f v s
H = 33.9 - 10 - 14.4 - 24 - 2 = –16.5 feet head.
This means that a suction head of equal to or greater than 16.5
feet is required during operation.
Pressure calculated in psi: 16.5 / 2.31 = 7 psi.
H
f
NPSH
H
p
b
H
V
Fig. 13 Minimum inlet pressure (NPSH)
s
indicates the system pressure in

13.2 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.
tm
(°F)
370
360
340
320
300
280
270
250
230
212
194
176
158
140
122
104
86
68
50
32
Fig. 14 Vapor pressure
Hv
(Ft)
413
328
259
203
148
131
115
98
82
66
49
39
33
26
20
16
13
10
6.6
4.9
3.3
2.6
2.0
1.3
0.9
0.7
0.3
11