Engineering manual - SAB 193-233-283 S A-frame (including ATEX)
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4.8
Oil injection valve
In order to control the amount of oil being led to the rotors, a hand expansion valve (pos. 124)
is installed in the main oil injection line.
Variable speed drive (VSD) units running high-pressure applications are equipped with a mo-
torised valve in this line. The motorised valve closes with the rotational speed and ensures that
the amount of oil in the compressor does not get too high when lowering the speed.
Adjustment of the hand expansion valve and the motorised oil injection valve is described in
the operating manual.
4.9
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 com-
pressor discharge pressure. During normal compressor operation, the compressor unit essen-
tially 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.
4.10
Demand pump oil system
This system is designed to provide adequate compressor lubrication when the oil differential
pressure 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
discharge 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] differen-
tial pressure.
Note:
For alarm descriptions and shutdown or cut-out parameters, see separate control sys-
tem manual.
4.11
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
temperature sensor, which will only let the compressor start when both limits are fulfilled.
4.12
Compressor oil separation system
The compressor is an oil flooded screw compressor. The oil charged to the compressor for lu-
brication, sealing and cooling of the compressor block is mixed with the refrigerant gas and led
to the oil separator collectively.
Oil and gas are separated in the oil separator and the gas, which is almost free of oil, leaves
the compressor unit through the discharge connection. The system is described in detail
below.
Note:
Automatic oil return: Open service valves completely to keep coalescer end of separator
free of oil.