Grundfos CU 301 Manual De Instrucciones página 6

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 accord-
ingly 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.
Pressure
Low flow
Fig. 3 Pressure changes in relation to flow
Note: When the pressure is dropping 1.4 psi/s or
faster, the pump will start immediately.
With a diaphragm tank of 2 gal., the pump will start at
a flow rate of approx. 0.8 gpm.
Note: If a larger tank is used, the flow must be higher
before the pump starts.
Consumption up to 0.8 gpm
The pump will start when the pressure has dropped
to 7 psi below the pressure setting.
The pump will run until the pressure is 7 psi above
the pressure setting.
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 and pressure is read.
A pressure drop indicates that water is being con-
sumed 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.
6
Pressure
Fig. 4
System limits
Even though the CU 301 is controlling the pressure
within ±3 psi, 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 pipework,
but, typically, they will lie between 7 and 14 psi.
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
the far right of fig. 5.
Pressure
Time
A
Fig. 5
A = Pressure setting
Flow detection
10 s 10 s
Flow detection every 10 seconds
during operation
Stop
+7 psi
Controlling Dynamic
Start
variations
±3 psi
-7 psi
±7 psi
0.8
Possible pressure variations during
constant-pressure operation
Time
Flow
Flow
gpm