compressors. The alarm is only active when the oil pump is running and then the alarm will be
delayed with a preset delay Oil pump low alarm, see Table 49: Timer values - screw compres-
sors.
2.2.36 Oil pump low pressure shutdown
SGC compressors (SAB 120→151, SAB 157 HR and SAB 193→355)
This shutdown is issued if the pressure difference between oil pressure and discharge pres-
sure is less than a preset value, see Table 8: Measured and calculated pressure levels - screw
compressors. The shutdown is only active when the oil pump is running and then the shut-
down will be delayed with a preset delay Oil pump low shutdown, see Table 49: Timer values
- screw compressors.
2.2.37 Liquid slugging alarm
This alarm protects against possible liquid in the compressor. The alarm is issued by a sudden
decrease in discharge temperature which is greater than default 5°K for a fixed 5-second peri-
od. The 5°K limit is set in Set point 1 for Discharge gas superheat.
For reciprocating compressors, the alarm will be suppressed in 60 seconds after a capacity
step has been engaged to avoid false alarm. This is because the discharge temperature will
drop due to the increased capacity when a capacity step is engaged.
2.2.38 Liquid slugging shutdown
This shutdown protects against possible liquid in the compressor. The shutdown is issued by a
sudden decrease in discharge temperature which is greater than default 10°K for a fixed 5-
second period. The 10°K limit is set in Set point 2 for Discharge gas superheat.
For reciprocating compressors, the shutdown will be suppressed in 60 seconds after a capacity
step has been engaged to avoid false shutdown. This is because the discharge temperature
will drop due to the increased capacity when a capacity step is engaged.
2.2.39 High separator flow (screw compressor)
This function limits the compressor capacity to a calculated limit at certain running conditions.
When the difference pressure across the compressor is too low, the capacity is limited in order
to protect the unit against oil carry-over to the system.
2.2.40 VSD backspin
SABflex and SABCube (SAB 109→159) compressors
For SABflex this shutdown indicates that the discharge pressure, after compressor stop and
after the timer Backspin delay has run out, is higher than the Internal suction pressure.
For SABCube this shutdown indicates that the drive during the compressor stop sequence is
rotating backwards with a speed of more than 5,000 rpm.
The shutdown will activate the POV valve to avoid the risk of backspin, i.e. that the compres-
sor starts rotating backwards. This might be caused because the SSTV valve did not close cor-
rectly at compressor stop.
2.2.41 Network connection to VSD alarm
Compressors with VSD and Modbus drive communication
This alarm (“VSD link missing”) indicates that the Modbus communication between Unisab III
and the VSD drive has been shortly disrupted, for 5 seconds.
Alarms and shutdowns
Engineering manual - Unisab III 1.10
001930 en 2014.09
53/319
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