I have experienced something that I consider strange in regular centrifugal compressor operation. I would be grateful if someone of forum members could clarify these issues:

Turbine-driven refrigerant compressor (isobutane) in alkylation unit is suffering from frequent surging when operating in automatic mode. Automatic operating mode means the following:
- antisurge flow controller is in automatic mode
- suction pressure controller is in automatic mode (suction vessel PC connected to high pressure steam servo assembly, which regulates the RPM of compressor by manipulating steam flow into the turbine; steam is condensed under vacuum)
- suction temperature is always constant, meaning that composition of the compressed gas is also unchanged

At minimum alkylation unit capacity, compressor operates at 95% of maximum RPM, developing polytropic head 80% of design value (?). Lowering the RPM pushes the machine into surge region and raises the suction pressure, so the operators found that it is better to run the compressor with almost maximum RPM in manual mode, in order to have relatively smooth operation of the plant. This somewhat causes suction pressure to vary with time, but with no significant consequences.
What surprised me the most is the following:

1) With this parameters I described, antisurge FCV is open 52%. Polytropic head is 80% of design value, as I said.
2) Lowering the RPM from 7000 to 6800 RPM does not affect suction and discharge pressure (?), but it causes antisurge valve to open further, up to 56%! Moreover, machine goes into surge cycles.
3) Switching from manual to automatic mode of RPM control (via suction PC), makes incredible changes in compressor operation: relatively smooth operation is turned into surging cycles, so the automatic operation is completely abandoned.

My questions are:

1) If actual gas composition, suction pressure and temperature are as designed, why cannot we achieve design polytropic head? Is it possible that there is so little process gas (compared to spillback stream), that antisurge flow (52% valve/Safety valves  open) pushes the compressor so much right off the curve, developing less head? Is it possible that machine is mechanically damaged, causing lower polytropic head at 95% of design RPM?
2) Why antisurge valve continues to open further when RPM is reduced, if suction and discharge pressures are unchanged? Isn't it contradictory, practically impossible? Less RPM should require smaller recycle stream (if being far enough from the surge point) in order to achieve the same head - that is what I (thought) I knew about centrifugal compressors.

Q: Is your "antisurge flow controller" a characterized antisurge system; just a PID minimum flow controller; or part of a strategy being used to control discharge pressure?

To analyze the second problem of percieved lower than design head, start with a work balance on the turbine side and compare wih compressor side. Verify that what you calculate to be the compressor flow is consistent with the horsepower input and discharge (T,P) conditions.

The primary benifit of the force balance is as a very simple consistency check that the compressor flow, inlet and outlet conditions are actually as believed. If any one of Q, Ti, Pi, To, Po is not as you believe (higher than expected recycle, partial blocked suction, etc), the force balance will not close.

As this is a clean system (regrigerant), the system can be easily analyzed. I think you can solve this mystery.

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