Grundfos CU 301 - Page 4

4
When does the pump start?
The pump starts as a consequence of...
• a high flow or
• a low pressure or
• a combination of both.
To ensure that the pump is started when water is
consumed, a flow detection is required. The flow is
detected via pressure changes in the system. When
water is consumed, the pressure will drop according-
ly depending on the size of the diaphragm tank and
the water flow:
• at a low flow, the pressure will drop slowly.
• at a high flow, the pressure will drop quickly.
See fig. 3.
Fig. 3
Note: When the pressure is dropping 0.1 bar/s or
faster, the pump will start immediately.
If a diaphragm tank of 8 litres is used, the pump will
start at a flow rate of approx. 0.18 m³/h.
Note: If a larger tank is used, the flow must be
higher before the pump starts.
Consumption up to 0.18 m³/h:
The pump will start when the pressure has dropped
to 0.5 bar below the pressure setting.
The pump will run until the pressure is 0.5 bar above
the pressure set.
Flow detection:
During pump operation, i.e. when water is con-
sumed, the CU 301 will adjust the pump speed to
maintain a constant pressure. In order to stop the
pump when no water is consumed, the CU 301 per-
forms flow detection every 10 seconds.
The pump speed is reduced until a small pressure
drop is registered. This pressure drop indicates that
water is consumed and the pump speed is resumed,
see fig. 4.
If the pump speed can be reduced without any pres-
sure drop being registered, this indicates that no
water is consumed. The diaphragm tank will be filled
with water and the pump will be stopped.
Fig. 4
System limits:
Even though the CU 301 is controlling the pressure
within ±0.2 bar, bigger pressure variations may occur
in the system. If the consumption is suddenly
changed, e.g. if a tap is opened, the water must start
flowing before the pressure can be made constant
again. Such dynamic variations depend on the pipe-
work, but, typically, they will lie between 0.5 and
1 bar.
If the desired consumption is higher than the quantity
the pump is able to deliver at the desired pressure,
the pressure follows the pump curve as illustrated in
fig. 5.
Fig. 5
A = Pressure set
TM01 8545 0400
Pressure
Time
Low flow
High flow
TM01 8546 0400TM01 8634 0500
Pressure
Time
Flow detection
10 s10 s
Controlling
±0.2 bar
Dynamic
variations
±0.5 bar
Pressure
Start
-0.5 bar
A
Stop
+0.5 bar
m³/h
0.18
F
low
Flow