Johnson Controls SABROE Unisab III Control, Engineering Manual

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Engineering manual - UniSAB III 1.10.8
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001930 en 2021.06
Alarms and shutdowns
2.1.9 Suction gas superheat, shutdown
The suction gas superheat shutdown is a shutdown which in many cases will protect the compressor
against liquid slugging. However, many aspects may affect superheating and therefore this shutdown
must not be considered full protection against liquid slugging.
If the superheat falls below the set value, the compressor stops. However, on R717 pump circulation
plants with a short distance between liquid separator and compressor, the suction gas superheat may
drop down to 0 K during normal operation. In such cases adjust
Low shutdown to 0.0 thus making it
inactive.
2.1.10 User input 1 (4-20 mA)
The shutdown and alarm limits are activated when Ext. Input Signal has been selected under Setup /
Configuration/External input .
2.1.11 Oil pressure alarm (SABflex and SABCube)
The alarm indicates that the oil pressure for a SAB 109-159 is below the alarm limit curve
Y [bar] = Y
1
+K
1
x P
suction, abs
under normal operation
Y’ [bar] = Y
1
+K
1
x P
suction, int, abs
when the start-up limiter is active
Y [bar] = Y
1
+K
1
x P
suction, abs
when the start-up limiter is active, but where only the SSSTV valve is limited
(i.e. closed)
Where Y1 is the low alarm limit from Table 9, and the constant K1 is the low alarm coefficient also found
in Table 9.
P
suction, abs
is the external (plant) suction pressure, and P
suction, int, abs
is the internal suction pressure, both
measured absolute.
The alarm is only active when the compressor is running and the Start unload timer has run out.
2.1.12 Oil pressure shutdown (SABflex and SABCube)
The shutdown indicates that the oil pressure for a SAB 109-159 is below the shutdown limit curve.
Z’ [bar] = Z
1
+K
1
x P
suction, int, abs
under normal operation
Z’ [bar] = Z
1
when the start-up limiter is active
Z’ [bar] = Z
1
+K
1
x P
suction, int, abs
when the Start-up limiter is active but where only the SSSTV valve is
limited (i.e. closed).
Where Z
1
is the low shutdown limit from Table 9 and the constant K
1
is the low alarm coefficient also
found in Table 9.
The shutdown is only active when the compressor is running and the Start unload timer has run out. As
long as the start-up limiter is active, the shutdown limit curve (where the limit is a function of the internal
suction pressure) is not activated. In this case the shutdown limit is equal to the low oil pressure shut-
down limit, i.e. the constant Z
1
in above equation.
2.2 Other alarms and shutdowns
2.2.1 High oil differential pressure (SMC Mk 4)
The alarm is delayed 300 seconds during start. It is released if the oil filter differential pressure becomes
too high.
2.2.2 Oil system error (screw compressors)
The reason for the shutdown is either that the oil flow switch is not activated within the time set in the tim-
er
Oil flow during prelubrication or that there is a drop-out on the oil flow switch for a longer period than
the one set in the timer No oil during operation.