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In our single stage centrifugal compressor primary seal gas is ethylene. N2 is provided as backup in case C2 pressure drops due to any reason. What is the safe logic for seal changeover from C2 to N2? Primary Seal gas flow control valve and flow transmitter is in vendor package. There are individual on/off valves in C2 and N2 supply lines, these are in client's scope. Differential pressure transmitter is available across N2 supply on/off valve.
 
Answers
08/06/2015 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I am not controls person, especially compressor controls. I suggest you consult the compressor manufacturer. I would think you would trip the C2 closed on low pressure and open the N2, The C2 pressure would then have to rise some amount above the minimum before you switched back from N2 to C2 to avoid switching back and forth between systems very much. Your valve logic would need to include some interlocks and check valves to minimize risk of cross contamination of the N2 and C2 systems.