Power Failure Compressor Surge

[Sawsan311]

Dear All,

1) I am currently analyzing why power failure is one of the governing extreme scenarios driving the compressor into surge and dictating the size of the anti-surge valve and the requirement of hot gas bypass. Is the root cause that upon power failure, the gas will be imposing the inertia on the compressor which has come into halt/stop upon power failure? or is it attributed to closing the inlet and the outlet valves when the compressor stops?
The trajectory of the operating point is that it tends to be driven to the left of the SLL and if an adequately sized ASV is in place, it will be driven back to the right of the SLL.

2) Additionally, when the compressor stops by ramping down, do you agree that as we reduce the speed, the operating point would be more shifted closed to the SLL and hence rundown surge during shutdown shall also be checked.
I have discussed with one of the compressor control manufacturers about the surge issues during a power dip in the plant and they agree that when there is a sudden dip, the point will be dragged at lower speeds to be fall in the zone near SLL. Do you agree?

Thanks

Regards,

 

[rotw]

Single answer : dynamic effects.

On power failure, compressors trips and the deceleration in mainly dictated by train inertia (speed is not a parameter we can control). As the compressor decelerates the pressure decreases. At the same time, the anti-surge valve is supposed to be in fully open position. So these are the forces acting on the system:

a/- Downstream system pressure is now overcoming pressure at the discharge of the compressor -> cause for surge
b/- Compressor is decelerating so the item a/ is evolving dynamically
c/- Anti-surge valve is relieving the downstream system pressure so that a/ can be copped with

How these three things (mainly) interact depends essentially upon, the equipment and pipe volumes, the sizing of the anti-surge valve and its dynamic response, the inertia of the train, and generally the power involved.

Only a dynamic study could simulate what will be going on, yet it the results are valid on the paper only as the problem is complex. The outcome can be no surge, limited surge or serious surge. Surge duration and at which speeds occur surge also matter before to come to a final assessment. To be on safe side or often just by policy, a hot gas by pass is included - but an emergency shutdown dynamic study is recommended.


Life is not about waiting for the storm to pass. It's about learning dance in the rain.

 

[pmover]

avoiding surge is all about reducing the system resistance across the compressor. so upon a driver shutdown, the system resistance needs to be reduced/minimized and this is done w/ a properly size anti-surge valve that rapidly opens. a proper anti-surge system can be tuned to avoid the dynamics occuring upon drive shutdown or otherwise.

 

[rotw]

pmover,
In some cases, anti-surge valve cannot be sufficient, even if it is sized large enough to choke the compressor. But that was something (I have only seen) resulting from dynamic study, which like I said is valid only on paper...