Page 54 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

• Equal margin: The set point 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 require maintenance and are 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.

Vibration: Vibration is a good indicator of problems in compressors,
and accelerometers are mounted on various parts of the
equipment to be logged and analyzed by a vibration
monitoring system.

Speed governor If the compressor is turbine driven, a dedicated speed
governor handles the fuel valves and other controls on the
turbine to maintain efficiency and control rotational speed.
For electrical motors this function is handled by a variable
speed drive.

The final function around the compressor itself is lube and seal oil handling.
Most compressors have wet seals, which are traps around shafts where oil
at high pressure prevents gas from leaking out to atmosphere or other parts
of the equipment. Oil is used for lubrication of the high speed bearings. This
oil gradually absorbs gas under pressure and may be come contaminated.
So it needs to be filtered and degassed. This happens in smaller reboilers
much the same way as for the glycol reboilers described earlier.

4.3.4 Gas treatment

When gas is exported, many gas trains include additional equipment for
further gas processing to remove unwanted components such as hydrogen

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