Grundfos CRE Series Installation And Operating Instructions Manual page 27

H
Stop pressure
H
Start pressure
Fig. 31 Difference between start and stop pressures (∆H) and
minimum flow (Q )
min
In start/stop operation, the pressure will vary between the start
and stop pressures. See fig. 31.
In "User-defined" mode, ∆H has been factory-set to 10 % of the
actual setpoint. ∆H can be set within the range from 5 to 30 % of
actual setpoint.
The pump will change to start/stop operation if the flow becomes
lower than Q .
min
Q is set in % of the rated flow of the pump (see pump
min
nameplate).
In "User-defined" mode, Q has been factory-set to 10 % of
min
rated flow.
Low-flow detection
Low flow can be detected in two ways:
1. A built-in "low-flow detection function" which is active if none
of the digital inputs are set up for flow switch.
2. A flow switch connected to one of the digital inputs.
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.
– If the flow is higher than the set minimum flow (Q
pump will return to continuous operation at constant
pressure.
– If the flow is still lower than the set minimum flow (Q
pump will continue in start/stop operation until the flow is
higher than the set minimum flow (Q
higher than the set minimum flow (Q
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 restart. If there is still no flow, the
pump will quickly reach the stop pressure and stop. If there is
flow, the pump will continue operating according to the
setpoint.
Q
Q
min
), the
min
), the
min
). When the flow is
min
), the pump will
min
Operating conditions for the low-flow 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
Caution
before the pressure sensor. See figs
Pressure sensor
Pump
Non-return
valve
Fig. 32 Position of the non-return valve and pressure sensor in
system with suction lift operation
Pressure sensor
Pump Non-return valve
Fig. 33 Position of the non-return valve and pressure sensor in
system with a positive inlet pressure
Set min. flow
Set the minimum flow (Q ) in this display. This setting
min
determines at which flow rate the system is to change from
continuous operation at constant pressure to start/stop operation.
The setting range is 5 to 30 % of rated flow.
Factory setting: 10 %.
32 and 33.
Diaphragm tank
Diaphragm tank
27