Standard Limiters - Johnson Controls SABROE Unisab III Control Engineering Manual

Compressor control and surveillance

3.2.2 Standard limiters

The standard limiting function is based on the user selected high/low shutdown limit, high/low
alarm limit and the limiting zone (Lz). In the particular case of the high motor current/kW lim-
iter, the value of Lz is 5% for both screw and reciprocating compressors. In all other cases, Lz
is calculated as half the difference between the shutdown limit and the alarm limit. Fig. 17
illustrates how a standard limiter works with the high discharge pressure limiter as example.
Screw compressors (all limiters) and
reciprocating compressors (motor current
limiter)
Discharge press. [bar]
Fig. 17: Limiting sequence
Pos. numbers
1. Limiter is watching. Normal control, capacity can increase or decrease. If compressor
is not in operation, it can be started.
2. Limiter changes from watching to passive.
3. Limiter is passive. Capacity cannot be increased. Capacity is not reduced by limiter,
but possibly by regulator. If compressor is not running, it cannot be started and will
be in Pause state.
4. Limiter becomes active. Capacity is reduced at once and timer begins to count.
5. Limiter is active. At each timeout, capacity is reduced and timer restarted, until com-
pressor stops at low capacity. If compressor is not running, it cannot be started and
will be in Pause state.
6. Limiter changes from active to passive.
7. Limiter changes from passive to watching.
8. Compressor has stopped immediately (shutdown).
In the example, the shutdown limit for high discharge pressure has been set for 16 bar, while
the alarm limit is 14 bar. As indicated, the size of the limiting zone (Lz) is then (16-14) / 2 =
1 Bar. The limit of the limiting zone is thus alarm limit - Lz = 14 -1 = 13 bar.
Note: The limitation is calculated in °C/R. Therefore, never set the low shutdown for suction
pressure to -1 bar as -1 bar equals -273°C/R when using ammonia.
Example:
Low alarm -0.9 bar ~ -71.7 °C/R.
Low shutdown -1 bar ~ -273 °C/R.
In this case the limitation starts at -71.9 + (273-71.9)/2 = 28.65°C/R.
Obviously, the suction pressure can never reach that value and the plant will never start! As
shown in Fig. 17, most reciprocating compressor limiters differ from the corresponding screw
compressor limiters in the way that when the measured value is within the limiting zone, the
state of the limiter depends on whether the limiter has been watching or active. If the limiter
has been watching, it will remain watching in the limiting zone. If the limiter has been active,
it will become passive for as long as it remains within the limiting zone.
Engineering manual - Unisab III 1.10
001930 en 2014.09
Reciprocating compressors
(All limiters except motor current
limiter)
Discharge press. [bar]
SHUTDOWN
16
ALARM
14
Limiting zone
13
Time
85/319
Time