System Sizing - Grundfos CU 321 Installation And Operating Instructions

System limits
Even though the CU 321 is controlling the pressure
within ±3 psi, larger 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 ±7 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. 3.
Pressure
Stop
+7 psi
A
Controlling
Start at
±3 psi
setpoint
0 8
Fig. 3 Pressure as a function of the flow
Note: The pressure may fluctuate up to 20 psi below
the setpoint when the pump is started in a high-
demand situation. A larger diaphragm tank will mini-
mize this effect.

1.4 System sizing

To ensure the correct function of the system, it is
important that the pump is of the right type.
During operation, the CU 321 controls the pump
speed within the range from 1,500 to 3,600 rpm,
see fig. 4.
H [ft]
Pump curve
at 1,500 rpm
Fig. 4 Pump curve (sizing)
Dynamic
variations
±7 psi
Flow
gpm
Pump curve
at 3,600 rpm
Q
]
rated flow Q [gpm
Recommended guidelines for system sizing:
The following must be fulfilled:
• Min. head at no flow < static head + system
pressure.
Comment: If this is not fulfilled, the pressure may
exceed the pressure set on the CU 321.
• Max. head at rated flow > dynamic head + system
pressure.
Comment: If this is not fulfilled, the pressure may
fall below the pressure set on the CU 321.
Max. head at rated flow and min. head at no flow can
be found in the table below:
3 hp:
Min. head
at no flow,
Pump type 1500 rpm
[feet]
16S30-24 128
25S30-15 80
40S30-9 45
75S30-5 30
5 hp:
Min. head
at no flow,
Pump type 1500 rpm
[feet]
16S50-38 200
25S50-26 105
40S50-15 75
75S50-8 45
Max. head
at rated flow,
3600 rpm
[feet]
490
305
185
105
Max. head
at rated flow,
3600 rpm
[feet]
825
530
310
175
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