compressor recycle valve failure

[korn]

I am trying to verify that a failure of a recycle control valve on the second stage discharge from a PD compressor will indeed overpressure the instage drum located on the suction of the second stage compressor. The first check valve located downstream of the recycle control valve is located downstream of several large process vessels who's volume far exceeds that of the interstage KO drum. In addition, the discharge pressure from the second stage is 1100 psig and the interstage drum MAWP is 600 psig. Can anyone explain to me why or why not the interstage drum would be overpressured?

Thanks

 

[djack77494]

It sounds to me as if a control valve failure could possibly result in large gas flowrates into your Interstage KO Drum. Withdrawal from that drum is at the fixed rate set by your PD Compressor (though the mass flowrate will increase some as pressure increases). If this scenario is true, then it is quite possible that the Interstage KO Drum MAWP could be exceeded. You may needed to incorporate a mitigation strategy (e.g. PSV on the KO Drum).
Good Luck,
Doug

 

[monaco8774]

Korn

If you have a fail open recycle valve on your PD compressor assuming the compressor trips on high suction pressure (usually have this to prevent rod loading problems etc) you will reach what is called settle out pressure in the suction. This is quite easiy to estimate, take the total mass in the system at the time the compressor trips and then assume it settles out to a final pressure with no loss in mass across suction and discharge.

The only hard bit is you need to get all your piping isos and vessel GA's out and get a good estimate of suction and discharge volumes. As you have mentioned dont forget location of chack valves.

If you go under the assumption that the compressor keeps running then you had better check the machine doesnt break a rod which is probably going to happen before you have any issues with your suction scrubber.

 

[Montemayor]

This post is extremely difficult to understand or interpret:

1) It is said that a recycle control valve “fails”. OK, but does it fail CLOSED or does it fail OPEN? It makes a big difference.

2) What is the significance of a downstream check valve? If the compressor is a PD, reciprocating piston compressor, it has “check valves” in each suction and discharge port. If the discharge gas is going back to suction conditions – regardless whether it is 1st or 2nd stage – it should make no difference since the flow is always from the higher to the lower pressure. Something seems to be missing here.

3) A recycle valve implies that the discharge gas is recycled back to the suction. But which suction – 1st stage or the 2nd stage? Or are there more than 2 stages to this compressor?

4) What is the “the instage drum located on the suction of the second stage compressor”? Is this the 2nd stage suction drum?

5) If the recycle control valve is discharging to “several large process vessels”, how does the gas make it back to the compressor – and at which stage?

If this is a 2-stage, reciprocating compressor with a capacity control valve that recycles 2nd stage discharge gas back to the 1st stage suction drum to keep the 1st stage suction pressure constant, then say so. Otherwise, please describe the operation appropriately to allow us to understand what is being described. If we can obtain an understandable description, we may be able to resolve the problem.

 

[korn]

Thanks monaco8774, you have understood and confirmed my thoughts on the scenario.

Thanks