5 replies to this topic

[mykid]

Hi need some advise here.
I would like to know if we need to size the PSV on compressor suction scrubber for failed open of antisurge or capacity control valve for a centrifugal compressor loop apart from fire?

The suction scrubber is designed for teh compressor settled out pressure.
Thanks

[JoeWong]

Mykid,

Normally yes as the compressor discharge (downstream of recycle tap) check valve can fail when anti-surge or capacity control valve in operation.

[djack77494]

I would like to know if we need to size the PSV on compressor suction scrubber for failed open of antisurge or capacity control valve for a centrifugal compressor loop apart from fire?

For a your compressor try to picture what would happen if you closed the discharge isolation valve (or JoeWong's check valve failure) while the compressor was running. Of course, the ASV would open and you'd be recycling gas. But would the pressure build or would you just be running your gas in a loop? I can see the possibility of both situations, depending on the system particulars. If you just recirculate the gas, then there is no "new" gas entering that would increase the total system pressure, and so a separate PSV should not be necessary. I think this is the more common situation, as I'd like to design for each compressor being pretty much self-contained; i.e. having its own scrubber with ASV return going to that scrubber. Another comment: If you've designed for a blocked in firecase, considering that condensed liquid may be in the scrubber, chances are you have a decent size PSV already.

[mykid]

Thanks both for the quick reply.
If we assume the antisurge valve/ capacity control valve fails under normal operation, i.e. there will be recycle gas + incoming gas entering the scrubber, the pressure will rise due to the increase in volume, can we take into account of high pressure trip in the compressor loop which will eventually tripping the compressor upon high pressure. Therefore the entire system will be at its settle out condition, and no overpressure scenario to be considered for the suction scrubber PSV?

[JoeWong]

Relying on single tripping function...

If your system is designed for API 14C, please check. I don't think it will pass API 14C.

Probability of Failure on demand (PFD) for single high pressure trip is very high and normally it wouldn't pass the Instrumented Protective Functionality (IPF) accessment.

In many event, it wouldn't pass the HAZOP as well.

[djack77494]

If the ASV fails (open), then some of your gas flows in a loop back to the Scrubber. I assume some will continue flowing forward to the normal destination. Now, will the pressure in the Scrubber begin to build? It depends on the rest of the system and its controls. If you have a source of higher pressure gas that feeds this system, then the answer may well be "yes"; if not, the answer is probably "no". These same answers may apply even if there is no forward flow.