Engineering manual - SAB 355 Screw compressor unit (including ATEX)
004100 en 2020.10
43/156
Technical description
4.1.8 No pump oil system
The screw compressor unit is designed to be self-lubricating. Oil supplied to the compressor from
the oil separator is at system head pressure. In the compressor, oil from all lubricating points is
returned to a point in the compressor body where the pressure is lower than the compressor dis-
charge pressure. During normal compressor operation, the compressor unit essentially operates
as its own oil pump. All oil entering the compressor is moved by the compressor rotors out the
compressor outlet and back to the oil separator.
For normal high-stage operation an oil pump is not required.
4.1.9 Demand pump oil system
This system is designed to provide adequate compressor lubrication when the oil differential pres-
sure across the compressor suction and discharge side is low. It applies to high-stage and booster
applications.
On start-up, the control system calculates the differential pressure between the compressor dis-
charge and the main oil injection port. If this differential pressure is less than 2.5 bar [35 psi], the
demand pump will turn on and continue to run until obtaining a 3 bar [45 psi] differential
pressure.
Note: For alarm descriptions and shutdown or cut-out parameters, see separate control system
manual.
4.1.10 Flow security
To ensure compressor lubrication before start-up, an oil level transmitter (LIT) is placed in the oil
separator reservoir. The transmitter registers the oil level and is connected electrically to a tem-
perature sensor, which will only let the compressor start when both limits are fulfilled.
4.1.11 Compressor oil separation system
The compressor is an oil flooded screw compressor. Most of the oil discharged by the compressor
separates from the gas flow in the oil charge reservoir. Some oil, however, is discharged as a mist
which does not separate readily from the gas flow and is carried past the oil charge reservoir. One
or more coalescer elements then coalesce the oil mist into droplets which fall to the bottom of the
coalescer section of the oil separator. The return of this oil to the compressor is controlled by an
automatic throttling valve on both high-stage and booster applications.
Note: Automatic oil return: Open service valves completely to keep coalescer end of separator
free of oil.
The sight glass in the valve indicates flow variations.
Note: The normal operating oil level is halfway in the top sight glass which is located midway
along the oil separator shell.
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