9 replies to this topic

[musrey]

Most companies would only require a flame arrestor on an "open" vent. I believe there is a specific siting in Factroy Mutual literature.

[virat_chem05]

I didn't get a response to this in the relief devices forum, so I am trying again here. Hope that doesn't go too much against protocol.

I have a relief valve on the discharge of a compressor in a flammable gas system (60% methane, 40% carbon dioxide). The valves will relieve to atmosphere. Do I need a flame arrestor on the vent line? What document governs?

Thanks,

Mike

hi

[musrey]

Unfortunately, I can remenber the exact reference. It was in a section dealing with storage of flammable liquids.

[ShaunHill]

I would not normally put a flame arrestor on a PSV vent line.
If you have any potential for moisture condensing in the line, they are prone to plugging and especially freezing (if you are in a cold climate like I am). The dangers of over-pressuring the vent system seem to be more of a risk than accidental ignition of a PSV vent burning back to the source. Also the high CO2 content of the gas you specified (40%) will lower the heating value of this gas and also lower the likely hood of accidiental ignition.

With a flare system that has a continuous flame, I would normally put a purge gas on the flare system to prevent air ingress into the line, but for a vent system, I believe this is not required.

[djack77494]

I agree with Guest and Shaun in their inclinations to avoid a flame arrestor. In fact, I think it totally wrong to put one on the discharge of a high pressure (setpoint >15 psig) PSV. I'm sure they're not required since you can refer to many regulations and standards dealing with LPG, Ammonia, and, I think, LNG and never see "flame arrested" PSV's. Why would you want to introduce a tortuous path (flame arrestor) in a PSV outlet. Freezing, corrosion, and probably just the random deposition of atmospheric dust could put the entire safety system at risk by restricting the flow from the PSV. What are you protecting against? Flame propagation backwards through a sonic flow nozzle into a tank with a flammable mixture? Think about that and I think you'll agree that the hazards that the flame arrestor might guard against are essentially non-existant. Keep it simple - short discharge pipe direct to atmosphere but away from any ignition sources.
Doug

[rxnarang]

Please no obstructions on the PSV discharge. The entire purpose of the PSV will be defeated if that flame arrestor clogs up.

For concerns regarding fires at end of vent lines, please consult API 521. If I remember correctly this is not an issue at all. For proper dispersion of flamable vapors there is a reynolds number criteria to be met. I don't have API 521 with me right now, but I think this criteria is in there.