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[Butterfly]

Hello

I have a centrifugal compressor in a 3x50% configuration. Load sharing and anti-surge control are present. Compressors have speed control.

Gas is coming from the export gas compressors of Gas Plant A. This compressor compress the gas to a discharge pressure of 1200 psig.

Gas is then sent to the centrifugal compressors of my problem (and there is a pressure drop through the pipeline, more than 200 psig) , to be sent to a export gas pipeline. I need 1000 psig of discharge pressure.

Gas from the centrifugal compressors (3x50%) is discharged into a common header. A line comes out this header (to send the gas to the export pipeline). There is a PIC in this line to control discharge pressure. The PID diagram doesn't show any valve, signal from the PIC goes to the Load Sharing panel. There is also a Compressor Control Panel shown in the PID, which receives signals from the Load Sharing panel and from PITs located at each compressor's suction and discharge line.

General philosophy is that tha master control of the compressors is the PIC located at discharge (there is another PIC at suction header, but only as secondary control, for the case suction pressure falls too much), and this control will modify compressor speed to acomodate compressor load.

And here comes my question: I always draw in my mind the compressor performace curve for better understanding. It seems that I have a deltaP fixed for the compressor, and that by modifying the speed i can move compressor curve to acomodate for the gas flow coming from Plant A. But the working point also needs to intersect the system curve... I see many things fixed: discharge P, gas rate from plant A,... I cannot understand well how is this control working

Would be grateful if you could help me.