r/ChemicalEngineering • u/Steamfitted • 1d ago

Design Question on hazardous areas

I am seeking some advice after waffling on the subject of hazardous areas for a few years. I feel as though I have an adequate understanding of defining hazardous areas (C1D1, C1D2, etc). However, what has always confused me is WHEN I need to start considering this. NFPA70 basically says when an ignitable or explosive mixture can exist. But what defines the volume where this starts? Is it milliliters, a 55 gallon drum, the MAQ per building code?

Any input or guidance to the right code (for the US) would be greatly appreciated!

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u/AICHEngineer 1d ago

Not volume, its concentration. For example natural gas is flammable between 5% - 15% by volume in air.

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u/Steamfitted 1d ago

Point taken- the LEL and UEL define when this could occur. But I suppose is it just a risk analysis then for a company? I can create a small short lived flammable environment with a shot of whiskey; but I’m not putting C1D2 lighting on my bar.

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u/AICHEngineer 1d ago

What matters is regulatory conformance and beyond that, personal risk tolerance. For example, to permit an LNG facility youll have to either design to 49.CFR.193 or NFPA59A. To get your permit to build it from the government who regulates these things, youll have to follow certain rules. Within NFPA59A, certain areas must be classified as hazardous under NFPA70 and youll have to spec equipment as such. Class 1 div 2 would be an area where flammables are present but normally contained, so electrical components like an electric switch enclosure for a pumping skid would have to be certified as NEMA 7 (explosion proof) or Z type, like a Nema 4x with Z purge (resistant to corrosion and Z purge is a continuous purge of the cabinet to prevent flammable mixture from being near the sparky sparky stuff).

Some scenarios may not require such risk mitigation, but your client or yourself may request out of caution.

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u/AICHEngineer 1d ago

For example, i have a system im working on that takes aqueous ammonia and vaporizes it and mixes that with recirculated hot flue gas air to make a dilute gaseous ammonia air mixture (~1% ammonia). Thats not explosive, but i guess it could be if the vaporizer was active and isolated and then briefly something explosive, since ammonia is explosive from 15-28% in air by volume.

Client made us specify the skid we are purchasing as class 1 div 2 since ammonia is present. Sure, ammonia can be flammable, we personally wouldnt worry about this system, but they do, so we do it.