Emerson Vilter Vission 20/20 Operation And Service Manual page 106

Section 10 • Compressor Sequencing
Fast Load Timer:
• If process pressure surpasses the fast load temp off-
set setpoint then this timer's value is used to make
periodic sequencing decisions.
Slow Unload Timer:
• If process pressure drops below the process pressure
control setpoint then this timer's value is used to
make periodic sequencing decisions.
Fast Unload Timer:
• If process pressure drops below the fast unload temp
offset setpoint then this timer's value is used to make
periodic sequencing decisions.
Discharge Pressure Control Setpoints
The compressor sequencing screen defines settings
those are used by the master compressor for sequenc-
ing. For Discharge Pressure Control Setpoints, see
Figure 10-5.
PRESSURE SETPOINTS
Start Offset:
• Defines the offset from the discharge pressure con-
trol setpoint to start the slave compressor. If dis-
charge pressure drops below the start offset setpoint
and master compressor's capacity has reached max
trigger setpoint then the sequencing algorithm
Figure 10-5. Compressor Sequencing Screen - Discharge Pressure Control Setpoints (Page 2)
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
allows the starting and loading of slave compressors
to cater for increasing load requirements.
Setpoint:
• The target setpoint is a read-only value here. This
setpoint can be changed by logging on to the
"Compressor Control" Screen.
Fast Load Pressure Offset:
• Defines the offset from the discharge pressure con-
trol setpoint to monitor compressor load. If the dis-
charge pressure drops below this setpoint's value
then sequencing decisions are made according to
Fast Load Timer.
Fast Unload Pressure Offset:
• Defines the offset from the discharge pressure con-
trol setpoint to monitor compressor load. If the dis-
charge pressure surpasses this setpoint's value then
sequencing decisions are made according to Fast
Unload Timer.
Capacity Load/Unload Timers
One of the following Capacity Load/Unload timers is used
to make sequencing decisions periodically. Discharge
Pressure setpoints are monitored to identify which one
of the following timers to be used.
Slow Load Timer:
• If discharge pressure drops below the discharge
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