Surge Control

[RKJustice]

I am trying to get my head around one aspect of surge control:

System – Single stage centrifugal compressor with discharge cooler and suction scrubber. There is a PCV controlling the pressure downstream of the discharge cooler and there is a surge control recycle line downstream of the cooler. The compressor is fed from a separator where pressure is maintained via a PCV.

If the discharge cooler were to partially fail, say only provide half of the required duty, and the compressor is operating in partial recycle then hot gas would be delivered back to the compressor inlet. This would result in a lower compression ratio and the discharge PCV closing. As the discharge PCV closes the surge control opens the recycle valve to maintain flow. As the recycle valve opens, this maintains the compressor upstream pressure and the system settles at a higher recycle flow (with a higher upstream pressure maintained by the recycle). Is this correct?

 

[EmmanuelTop]

I would think of the following: if compressor starts recycling warmer gas, the actual suction flow would start to increase. This will result in less polytropic head developed by the compressor (at a fixed speed). If discharge pressure is fixed (e.g. controlled externally), the result would be an increase in suction pressure. The antisurge valve would then throttle to maintain the initial flow-head operating point. All this assuming the machine does not trip on high suction/discharge temperature.

Dejan IVANOVIC
Process Engineer, MSChE

 

[LittleInch]

The issue is that compressor surge control cannot really be separated from the overall system control and for me there are a few issues with your description.

PCV d/s of compressor and cooler and presumably d/s of the surge / recycle line? This will maintain a fixed pressure d/s of this valve?

Upstream inlet - fed from a separator where pressure is maintained. Where is pressure maintained? n the separator or the inlet to the compressor??

discharge cooler partially fail?? A bit difficult to work out how or why, but assume you mean that heat rejection is reduced.
Compressor in partial re-cycle. This would / should only be happening if the flow through the compressor would otherwise be less than the stall line - generally quite a low percent of max throughput hence the heat quantity to removed to maintain the correct discharge temperature is also reduced from the maximum.

Why would the discharge PCV start to close? this should only happen if your downstream flow is being further reduced.

why would the suction pressure go up? The A/S valve throttles the discharge gas back to the suction pressure. Depending on the control function of the separator PCV, the inlet flow will be affected and hence less "new" gas enters the system to match the outflow of gas at the discharge.

In full re-cycle the head across the compressor can become quite low and hence power input quite low, but without a cooler you will get incremental temperature rise.

Without grasping the process flow of the entire system it will be difficult to see the impact of one isolated thing.


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[RKJustice]

Thanks for replying, see responses below…

The issue is that compressor surge control cannot really be separated from the overall system control and for me there are a few issues with your description.

PCV d/s of compressor and cooler and presumably d/s of the surge / recycle line? Yes. This will maintain a fixed pressure d/s of this valve? No, this will maintain a fixed pressure upstream of the valve.

Upstream inlet - fed from a separator where pressure is maintained. Where is pressure maintained? n the separator or the inlet to the compressor?? The PCV maintains the separator pressure, any excess is sent to flare. I had better upload a picture.

discharge cooler partially fail?? A bit difficult to work out how or why, but assume you mean that heat rejection is reduced. The cooling medium flow rate is reduced because of partially closed manual valve, partial temperature control loop failure, control valve stuck etc

Compressor in partial re-cycle. This would / should only be happening if the flow through the compressor would otherwise be less than the stall line - generally quite a low percent of max throughput hence the heat quantity to removed to maintain the correct discharge temperature is also reduced from the maximum. Towards the end of field life when feed gas flow is very low the compressor will be operating continuously in partial recycle as part of normal operation.

Why would the discharge PCV start to close? this should only happen if your downstream flow is being further reduced. For a fixed suction pressure but increased suction temperature the compression ratio is reduced therefore to maintain the discharge pressure the discharge valve would begin to close.

why would the suction pressure go up? The A/S valve throttles the discharge gas back to the suction pressure. Depending on the control function of the separator PCV, the inlet flow will be affected and hence less "new" gas enters the system to match the outflow of gas at the discharge. This is probably the nub in my lack of understanding here, as an aside if the discharge PCV sp were to be increased the compressor moves back along its curve but at some point the surge control valve opens. When it opens either the suction pressure would need to increase via increased flow from the recycle or the system would be unable to achieve the discharge PCV sp and go into full recycle, I thought it was the former?

In full re-cycle the head across the compressor can become quite low and hence power input quite low, but without a cooler you will get incremental temperature rise.

Without grasping the process flow of the entire system it will be difficult to see the impact of one isolated thing.

 

[LittleInch]

you really need to sketch this out as currently it doesn't make sense.

Only then can we see what is going on, but A?S or recycle valves will throttle gas flow so you can maintain both discharge pressure and suction pressure s long as some flow is going out through the discharge valve.

Note that running compressors this way for a long time isn't good for them and is very inefficient, so maybe a better plan is to think about installing a smaller one as the field flow declines.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way