Selection Of Diaphragm Tank And Setting Of Precharge Pressure And Pressure Switch - Grundfos SQE Montage- Und Betriebsanleitung

7.2 Selection of diaphragm tank and set-
ting of precharge pressure and pres-
sure switch
The installation must be designed for the
maximum pump pressure.
As the pump has a built-in soft starter giving a run-up
time of 2 seconds, the pressure at the pressure
switch and diaphragm tank during starting will be
lower than the pump cut-in pressure set on the pres-
sure switch (p ). This lower pressure is called
cut-in
minimum pressure (p ).
min
p is equal to the desired minimum pressure at the
min
highest tap + head and head loss in the pipe from the
pressure switch and diaphragm tank to the highest
tap (p = B + C), see fig. 12.
min
Fig. 12
H
Pressure switch
C
B
A
Q
max.
Diaphragm tank
p
min
0.6 0.8 1 1.2 1.5
[m]
25 8 8 18 18 18 18 24 33 33 50 50 50 50 80 80 80 80 80
30 8 8 18 18 18 24 33 33 50 50 50 50 80 80 80 80 80
35 8 18 18 18 18 24 33 33 50 50 50 80 80 80 80 80
40 8 18 18 18 18 24 33 50 50 50 80 80 80 80 80
45 8 18 18 18 24 33 33 50 50 50 80 80 80 80
50 8 18 18 18 24 33 50 50 50 80 80 80 80
55 18 18 18 18 24 33 50 50 50 80 80 80
60 18 18 18 18 24 33 50 50 80 80 80 80
65 18 18 18 24 24 33 50 50 80 80 80 80
1 m head = 0.098 bar.
10
A:
B:
C:
Note: Make sure that the selected pump can deliver
a pressure higher than p
p
pre
p
min
p
cut-in
p
cut-out
Q
Using p
size, precharge pressure and pressure switch set-
tings can be found in the guideline table below:
Example:
p
min
On the basis of this information, the following values
can be found in the table:
Minimum diaphragm tank size = 33 litres.
p
pre
p
p
cut-out
cut-in
p
cut-in p
cut-out
p
min.
p
pre.
Q
Q [m³/h]
max
2 2.5 3 3.5 4 4.5 5 5.5
Diaphragm tank size [litres]
Head + head loss from dynamic water level
to diaphragm tank.
Head + head loss from diaphragm tank to
highest tap.
Minimum pressure at highest tap.
+ A.
cut-out
: Precharge pressure of diaphragm tank.
: Desired minimum pressure.
: Cut-in pressure set on pressure switch.
: Cut-out pressure set on pressure switch.
: Maximum flow at p .
max min
and Q , the minimum diaphragm tank
min max
= 35 m head, Q = 2.5 m³/h.
max
= 31.5 m head.
= 36 m head.
= 50 m head.
6 6.5 7 7.5 8
p p p
pre cut-in cut-out
[m] [m] [m]
22.5 26 40
27 31 45
31.5 36 50
36 41 55
40.5 46 60
45 51 65
49.5 56 70
54 61 75
58.5 66 80