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VSG/VSSG • Installation, Operation and Maintenance Manual •Vilter/Emerson • 35391SSG
Section 4 • Operation
Control System Calibration
Equipped for automatic operation, the screw compres-
sor unit has safety controls to protect it from irregular
operating conditions, an automatic starting and stop-
ping sequence, capacity and volume ratio control
systems.
Check all pressure controls with a remote pressure
source, to assure that all safety and operating control lim-
its operate at the point indicated on the microprocessor.
The unit is equipped with block and bleed valves that
are used to recalibrate the pressure transducers. To use
the block and bleed valves to recalibrate the pressure
transducers, the block valve is shut off at the unit and
the pressure is allowed to bleed off by opening the bleed
valve near the pressure transducer enclosure. The trans-
ducer can then be calibrated at atmospheric pressure (0
psig), or an external pressure source with an accurate
gauge may be attached at the bleed valve.
The discharge pressure transducer cannot be isolated
from its pressure source, so it is equipped with only a
valve to allow an accurate pressure gauge to be attached
and the pressure transducer calibrated at unit pressure.
Recheck the transducers periodically for any drift of cali-
bration, refer to maintenance/service interval table in
Section 5.
Starting, Stopping and Restarting the
Compressor
Starting
Before the screw compressor unit can start, certain
conditions must be met. All of the safety setpoints must
be in a normal condition, and the suction pressure must
be above the low suction pressure setpoint to ensure a
load is present. When the “ON/OFF” switch or “Manual-
Auto” button is pressed, the oil pump will start. When
suffi cient oil pressure has built up and the compressor
capacity control and volume ratio slide valves are at or
below 10%, the compressor unit will start.
NOTE
The amount of oil pressure that needs to be achieved
before compressor start is at least 6 psig above the
discharge pressure. For additional information on
Low Oil Pressure at Start, see Troubleshooting Guide -
General Problems and Solutions in Section 6.
If the compressor is in the automatic mode, it will now
load and unload and vary the volume ratio in response
to the system demands.
Stopping/Restarting
Stopping the compressor unit can be accomplished a
number of ways. Any of the safety setpoints will stop
the compressor unit if an abnormal operating condition
exists. The compressor unit “On-Off” or stop button
will turn the compressor unit off as will the low pressure
setpoint. If any of these conditions turns the compres-
sor unit off, the slide valve motors will immediately
energize to drive the slide valves back to 5% limit. The
control motors will be de-energized when the respec-
tive slide valve moves back below 5%. If there is a power
failure, the compressor unit will stop. If the manual
start on power failure option is selected (see appropri-
ate Microprocessor Instruction Manual), restarting from
this condition is accomplished by pushing the reset but-
ton to insure positive operator control. If the auto start
on power failure option is selected (see Compact Logix
PLC manual), the compressor unit will start up after
a waiting period. With both options, the compressor
slide valves must return below their respective 5% limits
before the compressor unit can be restarted.
NOTE
Wait a minimum of 20 minutes (to allow the
compressor unit to equalize to suction pressure)
between pre-lubing or pushing the start button.
Calibrate Slide Valve Actuators
Slide valve actuators must be installed prior to calibra-
tion. Refer to Slide Valve Actuator Installation proce-
dure. The following steps pertain to calibrating one slide
valve actuator. Repeat procedure to calibrate other slide
valve actuator.
WARNING
After stopping the compressor, allow the compressor
and surrounding components to cool down prior
to servicing. Failure to comply may result in serious
injury.
CAUTION
Do not calibrate in direct sunlight. Failure to comply
may result in damage to equipment.
Both the capacity and volume slide actuators should be
calibrated when one or more of these have occurred:
• Compressor unit starting up for the fi rst time.
• A new actuator motor has been installed.