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.
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
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
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 sufficient 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.
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 first time.
•A new actuator motor has been installed.
4 – 2 | VSG/VSSG • Installation, Operation and Maintenance Manual •Vilter/Emerson • 35391SSG |
