Atmospheric PSV and Blowdown Venting

[Djack63]

My company deals with gas transmission. We have several compressor stations in which the PSVs and blowdowns vent to the atmosphere. Many of them have older style CS flapper valves at the top of the vent to prevent water and debris accumulation. The industry and API discourage the use of these flappers due to the potential for rust accumulation and the possibility that the flappers may stick closed or open. Our internal standard calls for plastic fitted caps and/or weep holed in the discharge piping to prevent water accumulation. Unfortunately, at many of our compressor stations the vents are not easily accessible (safely, even with a manlift) and weep holes (due to piping design) are impractical. Plastic caps would not be practical, since they would have to be replaced on a fairly frequent basis. We are considering some non-ferrous based or SS flappers to reduce the possibility of rust, increase reliability and reduce the frequency of testing and inspection of the flappers. Has anyone else encountered this situation? What have others done for this? Does anyone have aluminum flappers on their vent stacks? Any suggestions would be appreciated.

 

[georgeverghese]

We've not used flappers on local PSV / BDV vents - only a decent sized drain line at the low point of the exit stack which is fitted with one or 2 liquid drainers in parallel and/or a valved bypass line. You could orient the exit line tip at 45degrees if you want to reduce rain ingress, but then the dispersion/heat radiation profile in the safety study should take this deflected exit stream into account.

 

[MortenA]

weep hole at the 90ºbend?

 

[Djack63]

Thanks georgeverghese and MortenA for the feedback. Weep holes or drains in the low point of the exit stack is our standard design now. Unfortunately, these compressor station (1960s vintage) vents were designed such that the exit stacks are pocketed underground, so a conventional drain system is impractical, but not impossible. We can also re-pipe the stacks or build a walkway along side the stacks at a considerable cost, but if we can find a lower cost option that is safe and works, we'll go that direction. We do like to use some sort of cover as we have had issues with birds entering the stacks in the remote stations.