Grundfos CRT Series, Installation And Operating Instructions Manual, Page: 16 / 384

En
gl
is
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B)
16
9.1.1 Specification and calculation of inlet
pressure
Fig. 5 Schematic view of open system with a
CR pump
Calculation of inlet pressure
Calculate the maximum suction lift "H" in m head as
follows:
Fig. 6 Vapour pressure for water in m head
If the calculated "H" is positive, the pump can
operate at a suction lift of maximum "H" m head.
If the calculated "H" is negative, an inlet pressure of
minimum "H" m head is required. There must be a
pressure equal to the calculated "H" during
operation.
Example:
Pb = 1 bar.
Pump type: CR 15, 50 Hz.
Flow rate: 15 m
3
/h.
NPSH (see the appendix*): 1.1 m head.
Hf = 3.0 m head.
Liquid temperature: 60 °C.
Hv (see fig.
6 ): 2.1 m head.
H = Pb x 10.2 - NPSH - Hf - Hv - Hs [m head].
H = 1 x 10.2 - 1.1 - 3.0 - 2.1 - 0.5 = 3.5 m head.
This means that the pump can operate at a suction
lift of maximum 3.5 m head.
Pressure calculated in bar: 3.5 x 0.0981 = 0.343 bar.
Pressure calculated in kPa: 3.5 x 9.81 = 34.3 kPa.
*Link to appendix, see section
9.1.2 .
TM
02
01
18
38
0
0
H = Pb x 10.2 - NPSH - Hf - Hv - Hs
Pb = Barometric pressure in bar.
Barometric pressure can be set to 1 bar.
In closed systems, Pb indicates the
system pressure in bar.
(10 bar = 1 MPa)
NPSH = Net Positive Suction Head in m head, to
be read from the NPSH curve in the
appendix* (at the highest flow rate the
pump will be delivering).
*Link to appendix, see section
9.1.2 .
Hf = Friction loss in the inlet pipe in m head
at the highest flow rate the pump will be
delivering.
Hv = Vapour pressure for water in m head.
See fig. 6 . If the pumped liquid is not
water, then use the vapour pressure for
the liquid which is being pumped.
tm = Liquid temperature.
Hs = Safety margin = minimum 0.5 m head.
Hv
H
NPSH
Pb
Hf
TM0
2
744
5 3
5
0
3
20
15
12
10
8,0
6,0
5,0
4,0
3,0
2,0
1,0
0,8
0,6
0,4
0,3
0,2
0,1
1,5
120
110
90
100
80
70
60
50
40
30
20
10
0
Hv
(m)
tm
(°C)
150
130
140
25
35
45
40
30
160
170
180
190
62
79
100
126