purge and pressurize

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megloff11x

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NFPA 496 allows purged and pressurized enclosures in Class I Division 2, but there is no NEMA number requirement on the enclosure that I can find.

Some vendors of such systems swear that you must start with a NEMA 4 or NEMA 12.

I'd prefer to make my own enclosure. As long as it doesn't leak sparks, am I fine here? If not, where are the code requirements found that say otherwise?
 
Re: purge and pressurize

As long as it doesn't leak sparks, am I fine here? If not, where are the code requirements found that say otherwise?
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No. It is not that easy! Read 501.3(B) Class I, Division 2.

There are several conditions that must be met before you can use a "general purpose" enclosure.
 
Re: purge and pressurize

Are you sure you want to expose yourself to a huge liability and use a homemade enclosure in a hazardous area instead of a UL listed enclosure tested and approved for these environments?

I'd prefer to make my own enclosure. As long as it doesn't leak sparks, am I fine here?
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There's more to it than making sure it doesn't leak sparks.

[ November 22, 2004, 08:41 PM: Message edited by: tkb ]
 
Re: purge and pressurize

More detail is needed from me.

I haven't found a listed enclosure that fits. I'm asking about the requirements for building one, or for that matter buying the right one if I can find it.

NFPA 496 does not come out and say you must use a NEMA XX box to start with, or your lip must be so thick and so wide, you must use #YY screws every ZZ inches and engage AA threads, and your seal made of...

Do I need to have an NRTL certify the design, and if so, which one and what tests?

I once got an exemption from DOT to ship acetylene vessels on commercial and freight aircraft by proving that the vessel would not knock down the plane if ignited (they were very small vessels), so it can be done.

What I'm asking for is where in the electrical code does it give me specifications on a purged and pressurized box; the box itself, not how to purge it? The explosionproof has to be NEMA 7 and so stamped. But the code is ambiguous if I just purge it.

I would actually prefer it if they said, "meet NEMA 4 or NEMA 12 and purge it." Then I'd have a solid foundation.
 
Re: purge and pressurize

When I worked for an OEM years ago, we'd take a NEMA 4 or NEMA 6 enclosure and seal it up. We would have an air bleed inside and also an air timer activited by an air pressure switch. When the pressure hit the setpoint, the timer would start. After five minutes, it would hit another air to electrical switch to turn on the power inside. If at anytime it dropped below the preset pressure setpoint inside, the power would shut down, and the process would start all over again.

We had two regulators installed to control the input and output air flow. There was also a safety over pressure diaphram located in the enclosure so that you couldn't overpressure the cabinet and blow it up. Just a few ounces of pressure throughout a 48x36x8 cabinet can bow it considerably. They worked well and we shipped several of these throughout the country without any violations. However, it was an expensive setup with all the extras needed for it.
 
Re: purge and pressurize

The best answer I can think of is:

After reading 501.3(B) and making sure your enclosure complies with 312 also. The next hurdle would be to make sure the AHJ will approve your use of the enclosure before you begin construction. He may cite 90.7 or 110.3.
 
Re: purge and pressurize

Contact the manufacturer of your "air purge" system for recommendations. An air purge system may require a gasketed enclosure to provide the proper pressure differential without requiring an excessively large air source.

I had a chance to tour a picture tube plant once, natural gas every where to keep the glass molten. All of the enclosures were built with the bottom 1' nothing but a screen mesh. Each enclosure contained a lit "pilot flame" to burn off an stray gas and prevent exlosive concentrations. Very wierd looking.
 
Re: purge and pressurize

IIRC, the enclosure msut be gasketed. This would imply something like a NEMA12 enclosure.

Most people these days buy a purge controller because the purge system manufacturer has taken all the guess work out of what is needed and takes responsibility for the design. You only have to supply an appropriate Ul listed enlsoure.

if you want to do your own thing, I guess you can, but you won't gain much cost wise, and in fact its almost always mopre cost effective to buy enclosures than to try to make your won.
 
Re: purge and pressurize

Originally posted by megloff11x:
More detail is needed from me.

I haven't found a listed enclosure that fits. I'm asking about the requirements for building one, or for that matter buying the right one if I can find it.
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Many custom enclosure makers can build you a NEMA12 or NEMA4 enclousre just about any size and/or shape you want.

Since they are sold by the pound for all practical purposes, they often cost no more to get a custom enclousre made than to just buy an off the shelf unit.
 
Re: purge and pressurize

Most "UL 508" shops can build an enclosure any size/shape or refer you to a shop that can. Go to this web page and enter "NITW" as the "keyword." You can even narrow the search to a specific ZIP code if you wish.

If you still wish to build your own, you can apply to be a UL 508 shop or several NRTLs can offer field certification of the enclosure. You would need this as a minimum OSHA requirement. The basic standard you would need to follow, surprisingly enough, is UL 508A.

The NEC recognizes "Purged and Pressurized" as protection technique [500.7(D)] and defines it in 500.2. The referenced standard in the 500.2 FPN, NFPA 496, is primarily a system performance standard.

Neither the NEC nor 496 specifies an enclosure type. In addition to the specific performance standards of the various "Types," the installation must still meet the ordinary location requirements for Type "X" and "Z" protection or Division 2 for Type "Y" protection.
 
Re: purge and pressurize

The leaking sparks thing bothers me a lot because that is not the function of an explosion proof enclosuure. Explosion proof enclosure are certainly very tite so they dont "leak sparks" however the true function of the enclosure is to contain any explosion of vapor or combustible material within the enclosure and not ignite the surrounding atmosphere.
 
Re: purge and pressurize

Think about it this way - how many installations with "leaking sparks" would be permitted in an "ordinary location.?"
 
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