System Limits; System Sizing - Grundfos CU 301 Instrucciones De Instalacion Y Operacion

Flow detection
During pump operation, being when water is
consumed, CU 301 adjusts the pump speed to
maintain a constant pressure. In order to stop the
pump when no water is consumed, CU 301 performs
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. 8.
If the pump speed reduces without any pressure
drop being registered, this indicates that no water is
consumed. The diaphragm tank is filled with water
and the pump stops.
Pressure
Flow detection
10 s
Fig. 8 Flow detection every 10 seconds during
operation
If the pump goes into low flow shutdown
and it cannot reach the boost pressure, it
will time-out and stop to prevent it from
running with no flow.

System limits

Even though, CU 301 controls the pressure within ±
3 psi, bigger pressure variations may occur in the
system. If the consumption is suddenly changed, for
example if a tap is opened, the water must start
flowing before the pressure can be made constant
again. Such dynamic variations depend on the pipes,
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
fig. 9.
10
10 s
Time
Pressure
Stop
+ 7 psi
A
Controlling
Start
± 3 psi
- 7 psi
0.18
Fig. 9 Possible pressure variations during
constant-pressure operation
A = Pressure set

4.2 System sizing

To ensure the correct function of the system, it is
important that the pump is sized properly.
During operation, CU 301 controls the pump speed
within the range from 3000 rpm to 10,700 rpm. See
fig. 10.
We recommend that you follow the guidelines below:
H [m]
Pump curve at
3000 rpm
Fig. 10 Pump curves at 3000 rpm and 10,700
rpm
A: Minimum head at no flow.
B: Maximum head at rated flow.
The following must be fulfilled:
Dynamic
variations
± 7 psi
Flow
gpm
Pump curve at
10,700 rpm
Q
Q [gpm]
rated flow