Grundfos CRE Instrucciones De Instalación Y Operación página 50

The advantages of enabling the "Low-flow stop function" are the
following:
• no unnecessary heating of the pumped liquid
• reduced wear of the shaft seals
• reduced noise from operation.
The disadvantages of enabling the "Low-flow stop function" may
be the following:
• The delivered pressure is not completely constant as it
fluctuates between the start and stop pressures.
• The frequent starts/stops of the pump may in some
applications cause acoustic noise.
The impact of the above disadvantages very much depends on
the setting selected for the stop function.
The "High-comfort mode" setting minimizes pressure fluctuations
and acoustic noise.
Select "Energy-optimal mode" if the main priority is to reduce the
energy consumption as much as possible.
Possible settings of the stop function:
• "Energy-optimal mode"
The pump automatically adjusts the parameters for the stop
function so that the energy consumption during the start-stop
operation period is minimized.
In this case, the stop function uses the factory-set values of
the minimum flow (Q ) and other internal parameters. See
min1
fig. 70.
• "High-comfort mode":
The pump automatically adjusts the parameters for the stop
function so that the disturbances during the start-stop
operation period are minimized.
In this case, the stop function uses the factory-set values of
the minimum flow (Q ) and other internal parameters. See
min2
fig. 70.
• "User-defined mode" ("Customised operating mode"):
The pump uses the parameters set for ∆H and minimum flow
(Q respectively for the stop function. See fig. 70.
min3)
H
Stop pressure
H
Start pressure
Q
min2
Fig. 70 Difference between start and stop pressures (∆H) and
minimum flow rate
In start-stop operation, the pressure varies between the start and
stop pressures. See fig. 70.
In "User-defined mode" ("Customised operating 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 changes to start-stop operation if the flow becomes
lower than the minimum flow.
The minimum flow is set in % of the nominal flow of the pump
(see the pump nameplate).
In "User-defined mode" ("Customised operating mode"), the
minimum flow has been factory-set to 10 % of nominal flow.
50
Q
Q
min1
Q
min3
Factory setting
See section 10. Technical data.
"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 for flow switch.
2. A flow switch connected to one of the digital inputs.
1. Low-flow detection function:
The pump checks 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 is increased until
the stop pressure (actual setpoint + 0.5 x ΔH) is reached and
the pump stops. When the pressure has fallen to the start
pressure (actual setpoint - 0.5 x ΔH), the pump restarts.
– If the flow is higher than the set minimum flow, the pump
returns to continuous operation at constant pressure.
– If the flow is still lower than the set minimum flow (Q
pump continues in start-stop operation until the flow is
higher than the set minimum flow (Q
higher than the set minimum flow rate (Q
returns to continuous operation.
2. Flow switch:
When the digital input is activated for more than 5 seconds
because there is low flow, the speed is increased until the
stop pressure (actual setpoint + 0.5 x ΔH) is reached, and the
pump stops. When the pressure has fallen to start pressure,
the pump restarts. If there is still no flow, the pump quickly
reaches the stop pressure and stops. If there is flow, the pump
continues operating according to the setpoint.
Operating conditions for the low-flow stop function
You can only use the stop function if the system incorporates a
pressure sensor, a non-return valve and a diaphragm tank.
Always install the non-return valve before the
Caution
pressure sensor. See figs
Pressure sensor
Non-return
valve
Fig. 71 Position of the non-return valve and pressure sensor in
system with suction lift operation
Diaphragm tank
Pressure sensor
Pump Non-return valve
Fig. 72 Position of the non-return valve and pressure sensor in
a system with a positive inlet pressure
), the
min
). When the flow is
min
), the pump
min
71 and 72.
Diaphragm tank
Pump