Compressor surge during emergency shutdown

[rQuestionEngineering]

How is it possible that the centrifugal compressor can surge during emergency shutdown ?
All the anti surge valves are fully open.

 

[rotw]

It can surge, principally due to the "dynamic part" of the problem.

If the volume at the discharge of the compressor is relatively important, it takes a while to get rid of the pressure downstream.
Opening the anti-surge valve fully may be insufficient in some cases. While the compressor descellerate, there may be enough energy downstream to cause flow reversal/surge. A hot gas by-pass is generally designed / prescribed for that specific purpose, that is to relieve the system pressure much quicker thereby maintaining the compression path (during ramp down) at safe distance from surge line.

Moreover if the driver, for example a turbine, does not permit to reach a stop / stand still in a short while and compressor effectively undergoes surge during shutdown, that surge phenomenon cannot be considered limited anymore which includes potential consequences on machinery (damages).

 

[georgeverghese]

Presume you have a high tech antisurge controller for each stage, for example CCC, and that there are no programming faults
Some solutions may be:
a) If the ASV is slow, check that the volume booster near the I/P convertor is working okay - some times the instrument air line size may be too small and choking the air supply to the volume booster
b) If the air supply is not choked and volume boosters are working okay, you may need to install a hot gas bypass for each compression stage.
c) For ESD shutdown, the antisurge controller should force open the recycle valve without waiting for the compressor to reach near the surge line - is this is case in your facility?
d) Check that you have a high integrity non slam check valve on the discharge of each stage ( Mokveld is the only type I have seen in this service), else you can get surge also(where there is low rotational inertia in the compressor bundle - driver train), no matter how fast the ASV is.