Oil and gas production handbook An introduction to oil ... - ABB Group
Oil and gas production handbook An introduction to oil ... - ABB Group
Oil and gas production handbook An introduction to oil ... - ABB Group
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• Set point adjustment: If rapid variations in load cause surge valve<br />
action, the set point will be moved <strong>to</strong> increase the surge margin.<br />
• Equal margin: The set point is adjusted <strong>to</strong> allow equal margin <strong>to</strong><br />
surge between several compressors.<br />
• Model based control: Outside the compressor itself, the main<br />
parameter for the surge margin is the <strong>to</strong>tal volume from the surge<br />
valve <strong>to</strong> the compressor suction inlet, <strong>and</strong> the response time for the<br />
surge valve flow. A model predictive controller could predict surge<br />
conditions <strong>and</strong> react faster <strong>to</strong> real situations while preventing<br />
unnecessary recirculation.<br />
Since compressors require maintenance <strong>and</strong> are potentially expensive <strong>to</strong><br />
replace, several other systems are normally included:<br />
Load management: To balance loading among several compressors in a<br />
train <strong>and</strong> between trains, the compressor control system<br />
often includes algorithms for load sharing, load shedding<br />
<strong>and</strong> loading. Compressors are normally purged with inert<br />
<strong>gas</strong>, such as nitrogen during longer shutdowns, e.g., for<br />
maintenance. Therefore, startup <strong>and</strong> shutdown sequences<br />
will normally include procedures <strong>to</strong> introduce <strong>and</strong> remove<br />
the purge <strong>gas</strong>.<br />
Vibration:<br />
Vibration is a good indica<strong>to</strong>r of problems in compressors,<br />
<strong>and</strong> accelerometers are mounted on various parts of the<br />
equipment <strong>to</strong> be logged <strong>and</strong> analyzed by a vibration<br />
moni<strong>to</strong>ring system.<br />
Speed governor: If the compressor is turbine driven, a dedicated speed<br />
governor h<strong>and</strong>les the fuel valves <strong>and</strong> other controls on the<br />
turbine <strong>to</strong> maintain efficiency <strong>and</strong> control rotational speed.<br />
For electrical mo<strong>to</strong>rs this function is h<strong>and</strong>led by a variable<br />
speed drive.<br />
The final function around the compressor itself is lube <strong>and</strong> seal <strong>oil</strong> h<strong>and</strong>ling.<br />
Most compressors have wet seals, which are traps around shafts where <strong>oil</strong><br />
at high pressure prevents <strong>gas</strong> from leaking out <strong>to</strong> atmosphere or other parts<br />
of the equipment. <strong>Oil</strong> is used for lubrication of the high speed bearings. This<br />
<strong>oil</strong> gradually absorbs <strong>gas</strong> under pressure <strong>and</strong> may become contaminated. It<br />
needs <strong>to</strong> be filtered <strong>and</strong> de<strong>gas</strong>sed. This happens in smaller reb<strong>oil</strong>ers, in<br />
much the same way as for the glycol reb<strong>oil</strong>ers described earlier.<br />
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