Constant-Pressure Control System; Cu 321 Control Unit; Description Of A Constant-Pressure System; Function - Grundfos CU 321 Installation And Operating Instructions

1. Constant-pressure control system

1.1 CU 321 control unit

The CU 321 control unit is designed for use only with
certain Grundfos SP submersible pumps and motors.
Configuration and operation parameters are opti-
mized for the Grundfos pump and motor sets in the
table below.
3 hp
16S30-24
25S30-15
40S30-9
75S30-5
For more information, see section 6. Technical data.
1.2 Description of a constant-pressure
system
The system maintains a constant pressure within the
maximum pump performance in spite of a varying
water consumption.
The CU 321 is designed for wall mounting only.
Fig. 1 Example of a system using the CU 321
Pos. Description
1 CU 321 control unit
2 Diaphragm tank (4 gal.)
3 Pressure sensor
4 SP pump
6
5 hp
16S50-38
25S50-26
40S50-15
75S50-8

1.3 Function

The pressure is registered by the pressure sensor,
which transmits a 4-20 mA signal to the CU 321.
To maintain a constant pressure, the CU 321 adjusts
the pump performance accordingly by changing the
pump speed.
When does the pump start?
The pump starts as a consequence of a low pres-
sure. 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. 2.
Pressure
Low flow
Fig. 2 Pressure in relation to a high and
a low flow
The pump will start when the pressure has dropped
below the pressure setting.
The pressure is boosted to 7 psi above the pressure
setpoint before the pump is stopped, thus precharg-
ing the tank in preparation for the next start cycle.
The pump will stop if 7 psi above the pressure set-
point is not reached within 2 minutes.
Flow detection
During pump operation, i.e. when water is con-
sumed, the CU 321 will adjust the pump speed to
maintain a constant pressure. The CU 321 performs
flow detection continuously in order to stop the pump
when no water is consumed.
The flow detection is based on the actual power con-
sumption compared to a no flow power for the actual
pump.
The system will operate in pressure switch mode
±7 psi around the setpoint when the flow is less than
10% of the pump's max. rated flow.
Time