Grundfos CRE Serie Instrucciones De Instalación Y Funcionamiento página 21

9.3.8 Stop function
The stop function can be set to these values:
• Active
• Not active.
When the stop function is active, the pump will be stopped at very
low flows. The causes are the following:
• avoid unnecessary heating of the pumped liquid
• reduce wear of the shaft seals
• reduce noise from operation.
H
Stop pressure
ΔH
Start pressure
Fig. 24 Difference between start and stop pressures (H)
ΔH is factory-set to 10 % of actual setpoint.
ΔH can be set within the range from 5 % to 30 % of actual
setpoint.
Low flow can be detected in two different ways:
1. A built-in "low-flow detection function" which functions if the
digital input is not set up for flow switch.
2. A flow switch connected to the digital input.
1. Low-flow detection function
The pump will check the flow regularly by reducing the speed for
a short time. If there is no or only a small change in pressure, this
means that there is low flow. The speed will be increased until the
stop pressure (actual setpoint + 0.5 x ΔH) is reached and the
pump will stop. When the pressure has fallen to the start pressure
(actual setpoint - 0.5 x ΔH), the pump will restart.
When restarting, the pumps will react differently according to
pump type:
Single-phase pumps
The pump will return to continuous operation at constant pressure
and the pump will continue checking the flow regularly by
reducing the speed for a short time.
Three-phase pumps
1. If the flow is higher than the low-flow limit, the pump will return
to continuous operation at constant pressure.
2. If the flow is still lower than the low-flow limit, the pump will
continue in start/stop operation. It will continue in start/stop
operation until the flow is higher than the low-flow limit; when
the flow is higher than the low-flow limit, the pump will return
to continuous operation.
2. Flow switch
When the digital input is activated for more than 5 seconds
because there is low flow, the speed will be increased until the
stop pressure (actual setpoint + 0.5 x ΔH) is reached, and the
pump will stop. When the pressure has fallen to start pressure,
the pump will start again. If there is still no flow, the pump will
quickly reach stop pressure and stop. If there is flow, the pump
will continue operating according to the setpoint.
Operating conditions for the stop function
It is only possible to use the stop function if the system
incorporates a pressure sensor, a non-return valve and a
diaphragm tank.
The non-return valve must always be installed before
Caution
the pressure sensor. See figures
Non-return
valve
Fig. 25 Position of the non-return valve and pressure sensor in
system with suction lift operation
Q
Fig. 26 Position of the non-return valve and pressure sensor in
system with positive inlet pressure
Diaphragm tank
The stop function requires a diaphragm tank of a certain minimum
size. The tank must be installed immediately after the pump and
the precharge pressure must be 0.7 x actual setpoint.
Recommended diaphragm tank size:
Rated flow rate
of pump
3
[m /h]
0-6
7-24
25-40
41-70
71-100
If a diaphragm tank of the above size is installed in the system,
the factory setting of ΔH is the correct setting.
If the tank installed is too small, the pump will start and stop too
often. This can be remedied by increasing ΔH.
25
Diaphragm tank
Pressure sensor
Pump
Diaphragm tank
Pressure sensor
Pump Non-return valve
Typical diaphragm
CRE pump tank size
1s, 1, 3, 5
10, 15, 20
32
45, 64
90
and 26.
[litres]
8
18
50
120
180
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