Emergency relief: flare vs. atmospheric release

[BlackSmoke]

I've gotten a new job in the wastewater industry and I've found that another engineer has specified relief valves that relieve sour methane gas to the atmosphere. The relief load is relatively small but the valve does not have a stack or anything to discharge it at a safe location, just a little rain cover.

I'm from the petrochemical industry and this seems very strange, as we're planning to install a process flare on-site already. I'm a younger engineer, but I have heard that this kind of atmospheric used to be a common practice in the past, even for toxic chemicals. Furthermore, in my reading of NFPA 820 for wastewater plants, mention is made of relief valves that discharge directly to atmosphere as a basis for the hazardous classification of certain equipment.

My question is: is there a standard governing when a combustible (or borderline combustible) gas can be discharged to the atmosphere during emergency reliefs, and is it industry specific? (i.e. API 520 & 521 might be overkill for the wastewater industry) I believe that the gas should be flared and will design it as such no matter what. However, it would help if I can point to a specific standard to justify the increased costs to the bean counters. Thanks for any guidance you can provide.

 

[SNORGY]

It's not so much Codes or Standards that will provide the acid test with respect to guidance on this topic as it is the Regulatory Authority. If you have a continuously lit flare on site, I would suspect it would be difficult o explain to a Regulatory Authority why you choose to discharge a sour service relief valve to atmosphere. The idea probably wouldn't fly.

 

[SNORGY]

Sorry, BlackSmoke...

By "...you..." it was meant figuratively. I duly note that YOUR position is to route it to flare, and that is the right thing to do.

 

[LittleInch]

This normally revolves around environmental restrictions or explosive atmosphere considerations, I forget thecode, but it's the electrical hazardous area calculation. Beyond a negligible flow, the size of your zone 1 area around the relief valve gets to be huge and starts to affect other equipment or extends beyond the property line. There are also personnel issues if the sour bit is h2s and a dispersion analysis would need to show levels below acceptable limits.

As a minimum the vent should be a vertical vent 2 to 3m high with a cone hat if required to aid dispersion, not discharging at what sounds like eye level!

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[don1980]

If this is a pop-acting relief valve, and it's discharging vapor vertically upward into an open area, then you have nothing to worry about with regard to flammable cloud / explosion hazards. The jet momentum of the gas entering the atm causes it to quickly dilute below the LFL. If the discharge is in an enclosed area, then that's different. Flammable vapors released from a PSV create a predictable flammable envelope, similar in shape to the flame from a gas burner. High mass rates simple mean it has a higher profile, but the risk of creating an explosive cloud is practically nil. You can prove this to yourself by using a rigorous gas dispersion analysis tool.

The only potential concern is the H2S. Whereas LFL values are in the percent range, toxicity limits are generally in the ppm range. The phenomenom mentioned above results in significant dilution of the toxic vapor, but the concentration might still be too high.

Since your relief valve is small, H2S toxicity probably isn't a problem either, but that requires a case-by-case evaluation.

 

[MortenA]

Hi don,

I dont think that its uncommon that the relief from a PSV cathes fire - see e.g. this picture:

http://www.livemint.com/Politics/lF...a-battles-flames-at-its-largest-refinery.html

Best regards,
Morten

 

[don1980]

Morten - I agree. If the vent is spraying into an area that has a potential source of ignition, then it can catch fire. Or, as shown in the posted photo, a plant fire can ignite the PSVs which are discharging in that area.

The greatest hazard from flammable materials is a vapor cloud explosion (VCE). Those are generally catasptophic for people and property. A PSV releasing flammable vapor can potentially ignite, but if it's directed vertically upward into an open area, the risk of VCE is practically zero. In the unlikely chance that ignition does occur for a vent pointed upward in an open area, the risk is radiant heat exposure. That's not inconsequential, but it's a risk that's generally low and manageable. That's the point I was trying to make.