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 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(Automatic Control Valves) 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.

With respect to possible reasons why the compression ratio on a centrifugal compressor seems to be low we have seen everything from demister pads jammed in the suction to fouled (or mechanically damaged) rotors and diaphrams. My emphasis previously was to insure that the data was consistent in order to draw a conclusion.

There is nothing at this point to suggest that any plant data is bad, on the otherhand there is no reason to shutdown early to check the compressor for damage.

The surging is a seperate issue. Can you get speed on control by using extra spillback (manual with backup)?

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This is my own view of the operation if it helps. NOTE: I am speculating about your system as most of my refrigeration experience is with self regulating variable area condensing (as Decasto's looks to be). There is nothing in your sketch to indicate a liquid level in the finfan.

Speed control is normal in a variable area condensing case as it regulates the system so that the compressor is handling only the flow needed for the refrigeration load and the refrigeration temp is also controlled (vaporizing pressure).  Your system runs at constant speed so suction ressure floats. My speculation is that the condensing pressure is set by the finfan air flow (and temp) and exchanger area (assumes area fixed=no liquid level in finfan). Consider the the fixed position finfan control valve pipe as extra pipe resistance since the position is fixed. There is a pressure (around 5barg) at which the required flow will be completely condensed over the available cooler area- i.e. if pressure were lower not all would condense and pressure would rise, if discharge pressure were higher then all would condense and the pressure would drop.  The system in your constant speed case becomes self regulating through small (but important) suction pressure changes against the refrigerent condenser. If I am correct about the condensing pressure at a given refrigerent load being fixed by the finfan area and air flow, it may not ever be possible to also control the suction pressure. Such a case seems over specified since refrigerant flow (process heat load), inlet (speed controller) and outlet pressure (finfan area) are all set independently.

The net result of this to efficiency is that I think you should be conceptually wondering why the suction pressure is higher for the constant speed case rather than asking yourself why the discharge pressure is low. This was my confusion about what the "only 80% of polytropic head" really met. I know that you believe the suction pressure is low, but this would be one of the measurements that I would field check as it wouldn't need to be that much off to make everything ok. In addition my estimate of dew pt temp for your vapor mix at 0.105 barg was about 5C higher than your data, suggesting maybe you really do have a lower suction pressure (on the otherhand it may mean nothing but that my vapor pressure properties are a bit off).

Thanks for the sketches and all. I am sure that someone can give a better explaination about the speed vs pressure profile than I gave. If I am wrong then I would like to know what you think sets the compressor discharge pressure in your case. Anyway don't give up on your compressor theory, just try and put it into the context of the process (refrigeration loads and condensing conditions).

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