Engineering manual - UniSAB III 1.10.8
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001930 en 2021.06
Compressor control and surveillance
3.2 Limiting functions
3.2.1 Introduction
UniSAB III includes a number of capacity limiting functions (in the following referred to as limiters).
The purpose of a limiter is to prevent shutdowns by limiting or even changing the compressor capacity
when the measured value exceeds the selected limits. In most cases, changing capacity means de-
creasing it. However, this is not the case with all limiters.
In general, a limiter can be watching, passive or active. When all limiters are watching, the compressor
start/stop and the capacity regulation will work normally.
While a limiter is passive or active, the compressor cannot be started and will be in Pause state. If the
compressor is operating, the capacity regulation will be partly disabled. Further, if a limiter is active, ca-
pacity will be changed actively.
All relevant limiters are permanently in operation. If necessary, they will intervene in any of the operating
modes manual, auto and remote. If the compressor is in manual mode and a limiter is active, the ca-
pacity slide will automatically return to its original value as soon as the limiter is watching.
When a limiter is passive or active, the light above the start key will flash yellow slowly and the alarm re-
lay will be activated. Further, the display will show when a limiter is passive or active. There is a standard
limiter for each of the following:
• Low suction pressure
• High discharge pressure
• High motor current
• Low process out (brine) temperature
• High process out (water) temperature
• High differential pressure P
diff
=P
disch
- P
suct
(HPO, HPC and HPX reciprocating compressors)
Furthermore, there is a special limiter for each of the following:
• High suction pressure
• Suction ramp
• High discharge temperature (reciprocating compressors)
• Separator velocity (screw compressors)
• Insufficient main oil pressure (screw compressors)
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