Grundfos NB Series, Installation And Operating Instructions Manual

Page: 22 / 52

22
13. Calculation of 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
b
x 10.2 – NPSH – H
f
– H
v
– H
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. The calculated "H" must be present dur-
ing operation.
Example:
p
b
= 1 bar.
Pump type: NB 50-200/219, 2-pole, 50 Hz.
Flow rate: 70 m³/h.
NPSH (read on page 477): 2.35 metres head.
H
f
= 3.0 metres head.
Liquid temperature: +90°C.
H
v
(read on page 483): 7.2 metres head.
H = p
b
x 10.2 – NPSH – H
f
– H
v
– H
s
[metres head].
H = 1 x 10.2 – 2.35 – 3.0 – 7.2 – 0.5 = –2.85 metres head .
This means that a suction head of 2.85 metres is required during
operation.
The inlet pressure calculated in bar: 2.85 x 0.0981 = 0.28 bar.
The inlet pressure calculated in kPa: 2.85 x 9.81 = 28 kPa.
p
b
Barometric pressure in bar.
(The barometric pressure can be taken as = 1 bar.)
In closed systems p
b
indicates system pressure in bar.
NPSH
Net Positive Suction Head in metres head (to be read
from the NPSH curve on page 477 at the highest flow
the pump will be delivering).
The maximum flow rate must not exceed the values
stated for the individual pump on page 463.
H
f
Friction loss in suction pipe in metres head.
H
v
Vapour pressure in metres head, see page 483, where
t
m
= liquid temperature.
H
s
Safety margin = min. 0.5 metres head.