Emerson Vilter Vission 20/20, Operation And Service Manual

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6 – 11
Section 6 • Compressor Control
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
After the 12th step (running at 25 psig), 48 hours will
have elapsed, and the new setpoint becomes 20 psig,
achieving the 20 psig setpoint after 48 hours. After the
pulldown setpoint is equal to or is less than the control
setpoint, the pulldown feature will disable itself.
Pulldown:
• This checkbox enables the Pulldown feature. If this
box is unchecked, the Pulldown setpoints are ignored
and the operator is not allowed to edit them.
Initiate Pulldown at Next Start:
• This checkbox, when enabled, turns on the Pulldown
process at the next start cycle, and Pulldown opera-
tion will work in the following manners:
• Pulldown only works when Control mode is
Suction Pressure 1.
• If not started in Suction Pressure 1 then
Pulldown process will not run until stopped and
restarted in Suction Pressure 1.
• If started in Suction Pressure 1 and changed
after start, then Pulldown process will be sus-
pended and restarted once Control mode is
changed back to Suction Pressure 1.
Initiate Pulldown at Every Start:
• This checkbox, when enabled, turns on the Pulldown
process at every start cycle.
• The Pulldown feature will not disable itself
when the stop pressure setpoint is achieved and
this checkbox is enabled.
Step Pressure:
•
This setpoint defines the step decrements at which
the suction pressure value will be controlled at.
Delay Per Step:
•
This setpoint defines the time increment at which the
compressor will be controlled at each step, in hours
and minutes. Minimum range of 1, maximum of
1259.
Stop Pressure:
•
This setpoint defines the suction pressure value at
which the Pulldown operation will get completed.
When the suction pressure value is equal to or goes
below this setpoint, the Pulldown feature disables it-
self. Also “Pulldown” and “Initiate Pulldown at Next
Start” checkboxes will be automatically deselected as
normally this is a one time use feature.
Auto Cycle Differential:
•
This setpoint defines the offset pressure values for
Auto Cycle Start Pressure and Stop Pressure from
the Suction Pressure setpoint. The Auto Cycle Start
Pressure setpoint will be the Suction Pressure set-
point increased by this setpoint pressure value, while
the Auto Cycle Stop Pressure value will be the Suction
Pressure setpoint decremented by this setpoint pres-
sure value.
When the Pulldown feature is enabled, the Pumpdown
checkbox is automatically grayed out. Similarly when
the Pumpdown feature is enabled, the Pulldown check-
box is automatically grayed out and hence, the user will
not be able to operate the Pulldown feature. This is done
to keep the Pulldown & Pumpdown features mutually
exclusive.
Active Control Mode
This drop down box gives the operator the ability to
change the type of Active Control Mode such as suction
pressure, process control or discharge pressure. The op-
erator can also switch from setpoint 1 and setpoint 2 for
each control method. What is available in this dropdown
box depends on the number and type of control selected
in the configuration screen, see Figure 6-12.
Load Anticipating
The purpose of the load anticipating algorithm is to re-
duce the amount of overshoot of the capacity slide posi-
tion while the compressor attempts to meet the control
setpoint. This advanced feature of the Vission 20/20
closely monitors the rate of change of the process vari-
able and compares it to the control setpoint. If the pro-
cess variable is changing in the direction of the control
setpoint at the specified rate or greater, then the normal
command to move the capacity slide is interrupted. The
rate is calculated between time intervals set in the pro-
portional control section of this screen.
Enable Load Anticipation Algorithm:
• Allows the operator to choose if the load anticipation
algorithm runs.
Rate Deadband:
•
Defines the rate at which the capacity slide move -
ment will be interrupted. This value is an absolute val-
ue of the process variable. For example, the default
value is 0.25. If the control mode is suction pressure,
then this value is 0.25 Psig or if process temperature
is the control mode then the value would be 0.25°F.