VRLA/Gel Batteries in Class I Div I Hazardous Area

[mango1107]

Does anyone have any background with using VRLA or GEL style 12V batteries in a Class I Div I area? This NEMA 4x sealed mobile system is protected with a Y Purge and a non-incendive enclosure and contents and has its own mobile pressurized air supply. but I can't get a straight answer out of any battery makers on the outgas of the battery (excess of H production that can't recombine) in a scenario where it could possibly fail catastrophically, cause it's own ignition source for example, or there is a failure inside the system that causes a random spark.

It doesn't seem that the minimal air vented to the outside in a typical purge system would truly address venting the H to outside the unit, and I can't crank the air up with the limited on-board air supply.

It makes me think the only suitable application of a battery in such an environment is to apply it in its own explosion-proof (ugly) enclosure.

Thanks for any and all advice.

 

[DRWeig]

Mango,

Is it not possible to get the battery capacity you need with ni-cad dry cells? Some pretty potent ones, in suitably designed sealed plastic enclosures, are routinely found in hand-held devices that are listed as intrinsically safe.

Of course, you'd have to get your battery / case / connector assembly through the FM or UL process for intrinsic safety, or else you'd have to get the manufacturer to make a commitment to do so.

Just a thought.

The failure modes of gel or liquid electrolyte cells would make me too nervous in a non-explosionproof enclosure.

I don't think hydrogen generation is as big an issue as the internal failure modes. One study I've seen (by ATT I think) showed that wet cells can't possibly generate enough hydrogen in a room to reach the LEL (4%) as long as there's enough ventilation to keep them cool enough that they don't melt...

I have an interest in learning a definitive answer to this one myself, so I'll join you in asking for more opinions.

Best to ya,

Old Dave

 

[mango1107]

Thanks Old Dave!

Yeah, I'm researching NiCad as well as an option, I've even entertained some sort of NiCad UPS. We need to power 80-100 watts worth of stuff for 4-5 hours at a time. The standard VRLA or Gel we'd consider are the 70/80 amp hour batteries so I'm challenged to find the equivalent NiCad. Heck, I'd probably be able to get away with 2-2.5 hours.

Otherwise it may be back to our square one which is throw it into an ugly explosion proof box, but that's better than having a pretty enclosure explode and then look ugly.

 

[DRWeig]

That's a lot of power, you'd probably need to gang NiCads. Worth looking into though.

Say, Mango, if you wind up needing to go explosion proof and are concerned about looks, there's a small manufacturer I've found who does prettier ones than most -- they must take an extra step to polish the exterior. It's where I get my explosion proof stuff.

http://www.akronelectric.com

I bet they'd do something custom for you. Of course, it's still going to have a big fat flange and lots of bolts, or else a big honking screw-on lid.

Let us know what the end-all is!!

Old Dave

 

[stevenal]

From the NEC handbook:

In stationary installations, nickel-cadmium cells are generally of the vented type and will liberate hydrogen and oxygen during normal charging. Hermetically sealed nickel-cadmium cells are sometimes used, but they require special charging equipment to prevent gas emissions.

 

[mango1107]

Stevenal - that all sounds pricey - still, I'm pursuing it to see what the deal is. These systems typcially run $30K, so hiding $1K worth of stuff in it probably not too much of a problem.

Old Dave - thanks for the tip - I'm checking them out now. I like the simplicity of the explosion-proof enclosure.

 

[rbalex]

I’m making the possibly erroneous assumption that this is “Type X” pressurization per NFPA 496.

If the assumption is correct, then you need to review the definitions of “pressurization” and “purging.” As you are describing it, this is attempt to do both, because as least one ignitable material is already inside the enclosure.

Having said that, hydrogen off-gassing from batteries is not generally considered a basis for classifying a location. See NEC [2002] Art 480 for general battery applications and see NFPA 497 Section 1.1.4 for classifying hazardous locations under conditions “…that may involve catastrophic failure of or catastrophic discharge from process vessels, pipelines, tanks, or systems.”

 

[Kamalo]

mango1107,

I was using VRLA batteries for a substaion control room application and I ran into a similar problem. My client wanted me to use explosion proof fittings. However when I researched the problem, I found that the specific vrla batteries that I had used could not release any amount of hydrogen that would need an explosion proof room. However Enersys, is the company that I got the batteried from and they provided me with a calculated report on what the hydrogen emissions would be. I would suggest dealing with them next time.