Oil and gas production handbook - Process Control and ...

The operating characteristics are defined by the manufacturer. In the above diagram
the blue lines mark constant speed lines. The maximum operating limits are set by
the orange line as described above. The surge domain is the area to the left of the red
surge curve.
The object of the compressor performance control is to keep the operating point
close to the optimal setpoint without violating the constraints, by means of control
outputs, such as the speed setting. However gas turbine speed control response is
relatively slow and even electrical motors are not fast enough since the surge
response must be in the 100 mS range. The anti surge control will protect the
compressor from going into surge by operating the surge control valve. The basic
strategy is to use distance between operating point and surge line to control the valve
with a slower response time starting at the surge control line. Crossing the surge trip
line will control a fast response opening of the surge valve to protect the compressor.
Operation with recirculation wastes energy (which could result in unnecessary
emissions) and wear, particularly of the surge valve. Each vendor supplies several
variants of compressor control and anti surge control to optimize performance, based
on various corrective and predictive algorithms. Some strategies include:
• Setpoint adjustment: If rapid variations in load cause surge valve action, the
setpoint will be moved to increase the surge margin.
• Equal margin: The setpoint is adjusted to allow equal margin to surge
between several compressors.
• Model based control: Outside the compressor itself, the main parameter for
the surge margin is the total volume from the surge valve to the compressor
suction inlet, and the response time for the surge valve flow. A model
predictive controller could predict surge conditions and react faster to real
situations while preventing unnecessary recirculation.
Since compressors are relatively maintenance intensive and potentially expensive to
replace, several other systems are normally included:
Load management:
To balance loading among several compressors in a train
and between trains, the compressor control system often
includes algorithms for load sharing, load shedding and
loading. Compressors are normally purged with inert gas,
such as Nitrogen, during longer shutdowns, e.g. for
maintenance. Therefore, startup and shutdown sequences
will normally include procedures to introduce and remove
the purge gas.
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