Compressor Blocked Suction with Open Discharge Scenario

[Sawsan311]

Dear All,

I was discussing with one of the Engineers and he raised an example of an overpressure scenario experienced in a compressor stage. The compressor is discharging to a gas pipeline with a non-slam check valve provided downstream its discharge SDV.
The scenario he is proposing is that upon compressor shutdown due to any process trip or power failure, the inlet SDV would close but the outlet SDV would FAIL TO CLOSE upon demand leading to potential reverse flow through the compressor's anti-surge recycle line. Therefore, he is proposing to consider:

- either fully rating the suction scrubber of the compressor stage to the discharge design pressure of the compressor stage (quite unreasonable because conventionally, compressor suction equipment are designed for 1.05* settle out pressure)

- OR consider sizing the PSV on the suction scrubber for this reverse flow scenario.

In the first place, I would like to ask for your views whether this scenario is credible. The Compressor shutdown is concurrent with the opening of the HGBV (which is typically in most of the designs) and which precedes the reverse flow through the ASV line. Additionally, do you think that the large volume of the gas pipeline network would impose such risk on similar compressor designs despite having one or even two dissimilar non-slam check valves which quickly close upon any differential pressure change in reverse direction. Additionally, I believe the compressor when it shuts down there will be settle out across its internal labirath and balance piston / division wall, therefore, would the scenario of external reverse flow would still be valid or what I am referring to as pressure equalization within the compressor seal is a long term phenomena?

Looking forward for your views.

regards,

 

[georgeverghese]

The primary protection device for this reverse flow from the pipeline would be the check valve upstream of the discharge SDV. You could add a 2nd check valve here if the suction side hydrotest pressure is still lower than the max normal pipeline pressure to account for this case.

 

[rotw]

Would you elaborate why it would fail to close?
In case it occurs, volumes at suction and discharge (more specifically, in the pipe and any equipment between ASV, discharge check valve and compressor) could lead to surge. But whether likely or not would depend on:
- trip duration (inertia, driver type)
- ASV proper sizing
- presence of HGBV

Furthermore, isolation valves should not be located inside the ASV circuit but outside.

Also depending on driver type temperature increase at suction is a consideration.

 

[Sawsan311]

Thanks georgeverghese and Rotw for your feedback,

there is a check valve downstream the discharge SDV in addition to the check valve downstream the antisurge valve tapping of the recycle line. However, the scenario assumed by the Engineer is hat despite the SDV is Fail in close position (i.e. normally energized and then de-energize to close), however a mechanical failure caused to stuck in open position. I have never considered this reverse flow scenario from downstream network in sizing the suction KO drum PSV considering there is always two dissimilar non-slam type check valves downstream the compressor's anti-surge recycle line tapping. However, in the system being addressed by the Engineer, there is only one single non-slam type check valve downstream the SDV which is following the recycle line tapping (and not upstream the discharge SDV).


Is accordingly, the location of the check valve is to be re-evaluated considering the need to located one at least directly downstream the anti-surge valve tapping?

Thanks

regards,

 

[rotw]

I think a basic flow diagram locating the valves as per current set up and depicting the proposal for a change would be helpful for review purpose.

If you plan an escape, you must succeed as if you fail, you will be punished for trying. Never say or write down your plan. Heart is the only place where secrecy is granted.

 

[georgeverghese]

"there is a check valve downstream the discharge SDV in addition to the check valve downstream the antisurge valve tapping of the recycle line."

That should be good enough - 2 check valves before reverse flow occurs through FO ASV to suction side.