Grundfos CR Serie Instrucciones De Instalación Y Funcionamiento página 16

9.1.1 Specification and calculation of inlet
pressure
Hf
H
Pb
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:
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
Appendix with NPSH
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.
16
NPSH
Hv
curves.
tm
(°C)
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
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.
3
Flow rate: 15 m /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
NPSH curves.
Hv
(m)
126
100
79
62
45
40
35
30
25
20
15
12
10
8,0
6,0
5,0
4,0
3,0
2,0
1,5
1,0
0,8
0,6
0,4
0,3
0,2
0,1
Appendix with