NFPA 497 -- electrical equipment distance from below-grade Div. 1 locations?

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Hi all,

I'm putting together electrical area classification drawings for a facility that handles ethylene oxide. We've figured out all of the trenches that are affected; however, I've been left with a question: What distance does non-explosion proof equipment have to be from below grade Div 1 areas in order to be considered safe? We may have to move some pumps away from the trenches, but we have no idea how far out they need to be.

Thanks!
 

rbalex

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Trenches are a problem. One of the worst industrial explosions that ever occurred was from a trench that carried flammable material almost a mile from the original source.

On top of that, ethylene oxide is a particularly nasty flammable material; it is a Group B material with many characteristics similar to hydrogen without being lighter than air.

That said, typically a pit or trench will act as a source at ambient pressure. It will have an envelope 10'-15' above and from all edges. If it is significantly heavier than air, (usually, vapor density > 3) it will have an additional envelope 1.5 - 2' high, 25' beyond the edges.
 
Thanks for the reply! So just to clarify -- when you say "envelope", you mean that the additional classified area created by the trench will be 10-15 feet on all sides?
And just in case I get asked, do you have a particular source for this, or is it based on personal experience alone?
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Thanks for the reply! So just to clarify -- when you say "envelope", you mean that the additional classified area created by the trench will be 10-15 feet on all sides?
And just in case I get asked, do you have a particular source for this, or is it based on personal experience alone?
Basically, an open-air Division 1 location must have an adjacent Division 2 location. See Section 500.5(B)(2)(3).

With sufficient time, effort and a bit of experience, classification of trenches can be gleaned from NFPA 497, especially the text (not just the diagrams) of Chapter 5. Properties of the gas[es], quantity of material, prevalent winds, physical confinement, etc. all play a part in the evaluation. ANSI/API RP 500 is a reference document in both the NEC and NFPA 497. It's Appendix D has a fairly analytical approach.
 
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