Oil and gas production handbook An introduction to oil and gas production

Figure 8. Compressor state diagram 
 
The operating characteristics are defined by the manufacturer. In the 
diagram above, 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 objective of compressor performance control is to keep the operating 
point close to the optimal set point 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 electric motors are not fast 
enough, since surge response must be in the 100 ms range. 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 cause a fast 
response opening of the surge valve to protect the compressor. 
 
Operation with recirculation wastes energy (which could result in 
unnecessary emissions) and produces wear and tear, particularly on 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: 
 

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•  Set point adjustment: If rapid variations in load cause surge valve 
action, the set point will be moved to increase the surge margin. 
•  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 become contaminated. It 
needs to be filtered and degassed. This happens in smaller reboilers, in 
much the same way as for the glycol reboilers described earlier.  

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4.4  Oil and gas storage, metering and export 
The final stage before the oil and gas leaves the platform consists of 
storage, pumps and pipeline terminal equipment. 
4.4.1 Fiscal metering