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Centrifugal Compressor Operation

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#1 Zauberberg


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Posted 27 May 2007 - 11:17 AM

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 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.

Can you please throw some light on this.
Thanks in advance.

#2 JoeWong


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Posted 29 May 2007 - 05:55 AM

Just wanna to drop some points...if good just take.

You mentioned that your system is at "minimum alkylation unit capacity", and antisurge valve in operation (52% open) when the speed is at 7000 RPM, when you drop the compressor speed to 6800 RPM, the antisurge valve open futher (56% open)...

I guess...your compressor is operate at recycle condition and the anti-surge controller is working to push the operating point to surge control line (point A in attached sketch when you are in 7000RPM & AS CV is at 52% open).

Attached File  Comp_curve.xls   896.5KB   291 downloads

If you force the speed drop to 6800 RPM with same head (same suction and discharge pressure), then theoritically it will move to point B in sketch. The movement from point A to B should result the compressor flow reduce from Q1 to Q2'. However, B is in surge region, then the anti-surge controller will ask the anti-surge control valve to move point B to point C and compressor flow increase from Q2' to Q2. Hence there are extra flow (Q2-Q2') needs to recycle via the anti-surge control valve. So the anti-surge valve will open more. Another point is lower speed, lesser flow (Q2 vs Q1) through the compressor (but not the anti-surge valve in your case).

As for the first query, difficult to answer without looking at your datasheet, compression curve and operating point.

Hope other Che Jedi can point out if i make mistake in explaining above.


#3 Zauberberg


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Posted 29 May 2007 - 08:59 AM

Hello Joe, thanks for participating.

Actually, there is nothing wrong with your explanation - it just doesn't match our system. Remember, when lowering compressor RPM on manual mode (and it is only a small change at the beginning, 10-15RPM lower than maximum), if same polytropic head is going to be achieved, the compressor will have to pump less gas.
Now look into your chart. For smaller RPM, minimum suction flow required to prevent surging also decreases. Having the same polytropic head developed by refrigerant compressor, and having the process gas flow unchanged, it means that spillback flow is going to be reduced. But for such small change in RPM, a reduction of spillback stream flowrate actually pushes the compressor away from surging - at least according to compressor ACFM-Hp charts.

And now the best part: if there is really so little process gas and 52% antisurge valve opening is required to prevent surging (which means that compressor operates at only 10% higher suction flow - 10% away from surge point, at given/maximum RPM), why is there so dramatically reduced compressor discharge pressure, if suction pressure and temperature are at design conditions? It should be 6.46barg, and in real operation it is only 5.12barg! Compressed gas composition is as designed, and it has been checked in the laboratory (GC analysis). Refrigerant compressor is delivering much lower polytropic head than it should do, and consequently much lower discharge pressure (measured just at the compressor discharge outlet). Since process conditions, in my opinion, are not the cause of abnormal compressor operation, I was wondering if someone may have a clue what is going on. Also, when running the compressor in manual mode, the whole system operates smoothly; as soon as we switch to automatic suction pressure control (PC connected to turbine motive steam valve), the compressor starts surging. At all times, steam is condensed under vacuum and there are no significant changes of steam condensate outlet parameters.

#4 djack77494


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Posted 29 May 2007 - 06:06 PM

Hello Zauberberg,
Yours is a perplexing problem and I have more questions than answers. The one thing I would say about your compressor is that you should check to confirm that it is indeed operating on its curve. That will immediately give you a clue regarding many of the possibilities. If the compressor is operating on its curve, preferably confirmed at several speeds, than you can rule out machine problems.

As you describe relatively stable manual operation, but unstable automatic operation, I get very skeptical about your control system. Perhaps you have a problem there, such as a bad input or just a problem with tuning. In any case, I do feel you have some problem with your control system, since it should not fall short of manual operation.

Regarding valve position vs compressor speed, keep in mind that as the speed drops, so does the head. While it is true that you need less flow to avoid surge, you also have less dP to push your gas through the minimum flow valve. (Still, I would think the net effect would be in the opposite direction.)

Good luck in solving your problem, and don't feel shy about supplying additional details if you feel it could elicit a more helpful response. If you do, please describe the routing of recycle gas - Does it go directly to the compressor suction?, Is it cooled?, etc.

P.S. I liked your idea of retirement in the Virgin Islands.

#5 Zauberberg


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Posted 30 May 2007 - 03:37 PM

Hello Doug,

Recycle (spillback) stream enters the compressor suction trap without previously being cooled, but after mixing with sub-cooled liquid refrigerant (from liquid refrigerant accumulator drum) in special vapor-liquid mixing nozzle. The bottom line: suction temperature is not changing by time, as well as suction pressure - with compressor operating in manual mode (fixed RPM). So generally, as you can conclude, it is very much stable process.

There is a sketch of refrigeration section available as an attachment of this message, or at: http://www.imagehost...ection.jpg.html

Note that all downstream control valves maintain resistance to flow practically unchanged: LIC10 is in manual mode, PDIC (vapor bypass) is on automatic mode. Everytime when LIC10 is closed by only 0.2% of valve controller output, compressor starts surging. In my opinion, for LIC10 to operate in the automatic mode, automatic operation of compressor suction pressure controller needs to be provided; otherwise, small changes in LIC10 valve % opening affects compressor load (required polytropic head).

As you can see, there is no backpressure PC valve at the compressor discharge section. Downstream pressure is "fixed" by condenser outlet temperature and pressure drop of compressor downstream equipment. I did not notice any significant changes in this particular pressure over time (PR10 on the sketch). For emergency purposes, or in the case of propane accumulation in the system, there is HCV connected to flare system (the pump for liquid C3/C4 stream transport to depropanizer is currently out of service).

My question regarding compressor operation is: if compressor operates at maximum RPM on manual mode, and if inlet flow is equal to surge point flow + 10% safety margin, will it develop the same polytropic head at the compressor discharge nozzle, regardless of changing downstream resistance? Or not? And why?

My question regarding instruments tuning and antisurge control: what causes unstable operation when switching from manual to automatic mode of suction pressure control, if process is stable while operating in manual mode?

#6 djack77494


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Posted 01 June 2007 - 02:07 PM

Hi Zauberberg,
I have looked over the sketch you provided but still have no more insight into your problem than before. I remain of the opinion that you have a control system problem, though there may be other problems that cannot be totally ruled out. Some part of your system is definitely unstable (duh!), and the control system is the prime candidate. It is not normal that the small changes you describe should result in the system responses you see.

To address your direct questions, the developed head is a function ONLY of your hardware and the throughput. The downstream pressure is irrelevant except as for its impact on the machine's throughput. Assuming no mechanical damage to the machine, it MUST operate on its curve. As part of your troubleshooting exercize, I urge you to confirm that the compressor is/is not operating on its curve.

It is difficult to answer your question about the cause of unstable operation when the controls are put in operation. There are many possible causes. Could be bad inputs, unsuitably slow responses, bad tuning, poor programming, any number of things. Being stable in manual mode just means that you have at least one stable point; that does not imply that automatic operation will be successful.

I'm sorry that i can't be of more assistance, but you've pushed the limits of my ability to infer solutions. Hopefully someone more experienced in such things will offer you some possible solutions.

#7 Zauberberg


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Posted 03 June 2007 - 04:06 AM

Hello Doug,

We have an interesting discussion about refrigerant compressor at Eng-Tips forum. Maybe you can find additional information in some of the messages we posted there:

Thanks a lot for your assistance. It was of much help to me.

Regards, friend

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