Offshore Atmospheric Vent Stack

[chemtech1]

I am wondering if the vent system is allowed to be filled with air during normal operation as there is no continuous purge gas available. Hydrocarbon gas will only exist in the vent stack during venting and relief and will form a flammable mixture with air which will cause detonation or deflagration in the vent system if ignition present. Is purge gas is mandatory or can we design the system at higher pressure to withstand detonation without purge gas?
Anyone can give a clue?

 

[JLSeagull]

I have seen several examples on a platform piped to a high elevation. I have also seen start gas exhausts to the atmosphere with no inert pad but with a flame arrester. However, I don't know the current environmental requirements for such vents or release points.

 

[jwysmooth]

I'm not sure if there of regulations for vent stacks other than what is required for air permitting. There is a device called a "molecular seal" that is basically an inverted can inside a larger containment that mounts in line with the vent pipe. Normally it is used in a flare line to minimize purge gas rates. In principle it uses the density difference of the air and vent gas to seal the flow of air into the vent gas line. Its not perfect, but I beleive it helps when you don't have purge gas available.

If an ignition does occur, a Detonation Arrester is probably the best solution to prevent a flashback over a flame arrester. A simple internet search will tell you why. The location of the DA will be dependent on where you think the source of ignition is.
Cheers,
jwysmooth

 

[chemtech1]

Thanks for the replies.

 

[MortenA]

Molecular seal only works as a means to lower the purge rate. I think its just a fancy name for a ventury

And yes: vent to sagfe location is acceptale if allowed by company stadards. I see them on unmanned installations. I dont like flame arrestors. I think you introduce an obstruction in the vent path. They corrode and may also become a resting place for birds nets etc.

Best regards

Morten

 

[SJones]

ISO 23251/API 521

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[mbeychok]

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