Classified Location Question 1

[biofueler]

OK, a class I group D area is defined as an area where a flammable vapor/air mixture can exist under normal operations. If this mixture is contained within a vessel, is the area around the vessel classified, or only if a flammable vapor/air mixture exists under normal operations in the atmosphere outside the vessel?

 

[itsmoked]

Seems to me (someone will pipe in that actually knows) if you have a furnace with a perfect flammable mixture in it you don't need the room to be classified. I have one in my floor behind me.

 

[xchnauzer]

The immediate area around the vessel should also be classified, albeit a different classification level. Consult relevant standards for more information.

 

[biofueler]

well it is designated as class I, group D, div II where a flammable mixture exists under abnormal conditions (leak, etc). just not sure if it is div I a certain distance from the vessel.

 

[rbalex]

It will not be necessary to classify it Division 1 around the vessel; in fact, if it weren't for avoiding "boundary seals" most of the Division 2 would disappear too.

In general, “closed” systems do not need classification except in the immediate vicinity of “… valves, fittings, flanges, and similar accessories…”

Even in those cases it is usually easy to justify very limited hazard radii. The problem is that it leaves you with a bunch of Division 2 “bubbles” that used to create a “boundary seal” nightmare. It was easier to just connect all the “bubbles” into one big one to avoid those seals. Over the last 15 years or so the NEC Code Panel in charge of Hazardous (Classified) Locations has begun to recognize more and more justifications to eliminate Division 2/Unclassified boundary seals - almost (but not quite) to the point you could eliminate them altogether.

Bottom line - unless you have reason to believe the vessel will “leak” under normal operation, no Division 1 is necessary.

BTW; per the NEC, “Classes” are designated by “Roman” numerals and “Divisions” are by “Arabic.” The significance can be significant legally, since specifying equipment by “Division II” is technically meaningless.

 

[itsmoked]

Good stuff there rbalex!

Hey what specifically/physically do you mean by a "boundary seal"?

And I'm not clear on your answer "Bottom line" [unless you have reason to believe the vessel will “leak” under normal operation, no Division 1 is necessary.]

Are you saying the adjacent area doesn't necessarily need to be classified at all or just not Division 1?

 

[rbalex]

"Boundary Seals" are those installed under [2005] Section 501.15(B)(2) or, in the case of Division 1, 501.15(A)(4).

If you read the Division 2 requirements carefully you will find enough exceptions to the general rule that most boundary seals aren't necessary. This is NOT to say all are unnecessary. Most of the exceptions were developed over the 10-15 years; but historically, it was easier to just create one large Div 2 area to avoid a seal at each boundary.

The NEC is an installation standard. Except for the few specific applications in Arts 511 to 516, it does not actually tell you HOW to classify an area. You have to go to another document such as NFPA 497 or API RP500 for general applications. If you use those documents as the basis of electrical area classification, you would find many currently classified locations that are actually over classified. But, as I said, to avoid Div 2/unclassified boundary seals it used to be simpler to just lump the locations together in one “mega Division 2.”

Personally, I believe the time is ripe to eliminate them as the general rule and specify the few limited places where they are necessary. I am in the process of submitting a formal Proposal to do just that this coming November.

 

[itsmoked]

Well I hope you can make it clearer in your proposal because you didn't answer either of my questions!

 

[rbalex]

I think I did answer what a boundary seal is. But specifically, it is a seal mandated by Section 501.15(B)(2) at the boundary of a Division 2/unclassified location.

With regard to your second question: it definitely doesn’t need to be Division 1 for any distance around the vessel there is a pressure relief device mounted on the vessel. In that case, there would both be a small Division 1 location centered on the relief port and a slightly larger Division 2 location also centered on the port. It’s unlikely that the maximum hazard radius would exceed 5’ inclusive of both unless the pressures involved were exceptionally high.

Otherwise, there probably doesn’t need to be Division 2 either. It may. It depends on several operating parameters, such as pressure, volume and flow rate. None of them were specified, so it’s difficult to make any but the most general statement and refer to NFPA 497 for more specific guidance.

 

[itsmoked]

Thanks for the reattempt rbalex and your patience. Still I'm not getting the... place hand on, real..um thing. Is the seal a rubber gasket or a wall or calculated region? What physically con-notates these types of seals.

I understand #2 now. Thanks!

 

[rbalex]

Ah...now I think I understand your question. The "seals" I refer to are conduit seals. They serve two purposes as described in the Fine Print Note No.1 of Section 501.15:

Seals are provided in conduit and cable systems to minimize the passage of gases and vapors and prevent the passage of flames from one portion of the electrical installation to another through the conduit. Such communication through Type MI cable is inherently prevented by construction of the cable. Unless specifically designed and tested for the purpose, conduit and cable seals are not intended to prevent the passage of liquids, gases, or vapors at a continuous pressure differential across the seal. Even at differences in pressure across the seal equivalent to a few inches of water, there may be a slow passage of gas or vapor through a seal and through conductors passing through the seal. See 501.15(E)(2). Temperature extremes and highly corrosive liquids and vapors can affect the ability of seals to perform their intended function. See 501.15(C)(2).

Note they “prevent the passage of flames” but only “minimize the passage of gases and vapors.”

The “basic” physical characteristics of these seals are described in Section 501.15(C). It’s a relatively long Section with six (6) Subsections. Conceptually, the ones I think you may be most interested in are:

(1) Fittings. Enclosures for connections or equipment shall be provided with an integral means for sealing, or sealing fittings listed for the location shall be used. Sealing fittings shall be listed for use with one or more specific compounds and shall be accessible.
(2) Compound. The compound shall provide a seal against passage of gas or vapors through the seal fitting, shall not be affected by the surrounding atmosphere or liquids, and shall not have a melting point of less than 93°C (200°F).
(3) Thickness of Compounds. In a completed seal, the minimum thickness of the sealing compound shall not be less than the trade size of the sealing fitting and, in no case, less than 16 mm (5?8 in.).

Most of the “listed” sealing compounds are literally a cement-like material.

 

[itsmoked]

Oh wow. Okay I get it now.

Many thanks for clearing that up rbalex.