Selection and sizing
CR, CRI, CRN, CRE, CRIE, CRNE
24
Minimum inlet pressure - NPSHA
Calculation of the inlet pressure "H" is recommended in
these situations:
• The liquid temperature is high,
• The flow is significantly higher than the rated flow,
• Water is drawn from depths,
• Water is drawn through long pipes,
• Inlet conditions are poor.
To avoid cavitation, make sure that there is a minimum
pressure on the suction side of the pump. The
maximum suction lift "H" in feet can be calculated as
follows:
H = p
b
– NPSHR – H
f
– H
v
– H
s
P
b
= Barometric pressure in feet absolute.
(Barometric pressure can be set to 33.9 feet.
At sea level. In closed systems, pb indicates
system pressure in feet.)
NPSHR = Net Positive Suction Head Required in feet.
(To be read from the NPSHR curve at the
highest flow the pump will be delivering).
H
f
= Friction loss in suction pipe in feet.
(At the highest flow the pump will be
delivering.)
H
v
= Vapor pressure in feet. (To be read from the
vapor pressure scale. "H
v
" depends on the
liquid temperature "T
m
").
H
s
= Safety margin = minimum 2.0 feet.
If the "H" calculated is positive, the pump can operate
at a suction lift of maximum "H" feet.
If the "H" calculated is negative, an inlet pressure of
minimum "H" feet is required.
Fig. 26 Minimum inlet pressure - NPSHR
Note: In order to avoid cavitation never, select a pump
whose duty point lies too far to the right on the NPSHR
curve.
Always check the NPSHR value of the pump at the
highest possible flow.
In case a lower NPSHR value is required, see Lists of
variants - on request on page 83.
T
M
02
77
29
39
0
3
66
49
39
33
26
20
16
13
10
6.6
3.3
2.6
2.0
1.3
0.9
0.7
0.3
4.9
250
230
194
212
176
158
140
122
104
86
68
50
32
Hv
(Ft)
tm
(°F)
300
270
280
82
115
148
131
98
320
340
360
370
203
259
328
413
Hf
Pb
NPSHR
Hv
H
