Line losses in gas breakthrough RV sizing case

[MarkESmith]

Hi all,

I am a recent process engineering graduate working on sizing a degasser vessel relief valve. I am looking at the case where we have gas breakthrough from upstream (due to fail-open of FCV) that could potentially overpressure the vessel.

My question: Is it usual to take into account line losses between the breakthrough gas source and the vessel to be protected? One of my bosses says yes and the other says no. I have looked at both cases and the more conservative 'No line losses upstream' case requires the RV outlet pipe to header to be increased. This raises substantial logistical problems with equipment layout. When line losses are taken into account, we do not have this problem.

Any help greatly appreciated.

 

[Chance17]

Both Bosses are right.
It triples the size of the job to include piping.
It is usual for the initial evaluation to include no piping - only the control valve limitations.
If the downstream safety valve has suficient capacity, there is no need to evaluate upstream or downstream pipe.
If the safety valve is too small it is usual to expand the work and include piping to choke off some flow.
If the safety valve is still too small, one would evaluate downstream liquid static head resistance.
The correct answer is control valve resistance plus piping resistance plus liquid head resistance.
If the safety valve continues to calculate too small, many folks woud consider a safety critical restricting orifice rather than swap out safety valves.

 

[MarkESmith]

Thank you for the reply.

As we are limited in the HP vent header diameter, and not being able to easily move equipment around, I incorporated line losses to provide justification for leaving the piping to header the same size.

Incidentally, after recieving vendor information on a desander unit upstream of this FCV, it was found that this unit lowers the applicable relief load by having a lower 'effective Cv' than the FCV.