3.2. Limiting functions
3.2.1 Introduction
Unisab III includes a number of capacity limiting functions (in the following referred to as lim-
iters).
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 ca-
pacity means decreasing 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, capacity 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 capacity 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 relay 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 and HPC 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)
Compressor control and surveillance
84/319
Engineering manual - Unisab III 1.10
001930 en 2014.09
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