Engineering manual - UniSAB III 1.10.8
90/346
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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