VRLA or GEL Batteries in sealed environment 2

[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. 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.

 

[alehman]

Interesting question. I don't think the normal H2 vented from VRLA batteries would be a concern unless the enclosure is very small. Those numbers are readily available (at least "typical" type numbers). It would be a simple matter to calculate the H2 concentration based on the ventilation rate.

I can understand you might not get an answer about gassing from a 'catastrophic' incident with the batteries. Why is that a particular concern? That would not normally be covered under the definition of hazardous locations.

Keep in mind pressurized and purged enclosures are acceptable code-wise only if 'approved for the location' and provided with clean air.

 

[itsmoked]

mango,

In normal situations the GEL Cell batteries don't generate any appreciable gases. Otherwise they would dry out and fail. So the out gassing comes with overcharge situations.
This is why you are getting null responses from suppliers. The batteries essentially don't out gas unless they are being hammered. Gas evolution is a function of HOW much abuse is being dished out. So they have no way of guessing the gas evolution.

I suspect any purging is plenty but check the calcs.

We enclosed a group of Gates cylindrical cells in a light stainless steel box, potted them in epoxy and ran them thru a highly incendiary environment by the dozen daily.

Two important points:
1) The units were never charged in the environment.
2) The batteries had tygon ribbon running from the
venting area out of the potting in case charging generated any gas.

 

[mango1107]

Thanks for the quick response! Our concern is exceeding the mere requirements of the reg to put a perfect system together. I think GEL is the way to go vs. VRLA. I just worry that the charger could malfunction and in an overcharging situation, I'm not sure if I should be monitoring H levels inside or what.

Do you think the fact that they will be cycled down twice daily and charged in the sealed environment adds to the minimal if any H concerns in the system? It's about 18" x 24" x 12" with an upper minor enclosure 18" x 19" x 4"

 

[itsmoked]

mango, Are you really going to charge this puppy in the hazardous environment?

And since these kind of devices can cause BIG LOUD energenic problems, could you clarify what you mean by "cycle down"?

And next you say, "charged in the sealed environment", but you said this was a "purged" environment not a sealed environment. I'm confused. Sorry.

 

[mango1107]

I like to refer to the purged environment as sealed since we'll only be operating the purge at the minimum pressure (we have an on-board air tank) and I have concerns about the true air movement and replacement, especially if there were a large release of H2 due to some failure of the battery.

By cycle down, basically discharging the battery most of the way.

Gel battery reps and VRLA reps keep green lighting charging in the hazardous sealed environment, and I keep reading the label off of their battery and MSDS sheet that says "wll-ventilated" and all I keep getting back is resounding "duhs" - I'm finding it hard to quantify the difference between truly sealed and purged as far as the TRUE continual air movement & replacement in the unit. It has a standard mechanical purge vent on top.

 

[stevenal]

1.27 * 10^-7 M^3/s H2 can be produced per ampere per cell at 25 degees C at standard pressure. See IEEE 1187.

 

[mpparent]

If you're that concerned about off-gassing, why don't you install an exhaust system? It shouldn't take much.

Mike

 

[mango1107]

The system is pressurized, so air from the mobile tank inside is at a premium, and the operation of the pressurized system would be compromised. Does that make sense?

 

[alehman]

As itsmoked suggested, there is always a slight risk that a battery being charged can explode. Irrespecive of the H2 release, this could conceivably breach the purged enclosure.

Using lead-acid batteries in non-purged hazardous locations is not without precedent. For example,

http://www.hubbellonline.com/wiring/killark/pdf/7SecL/SecL138-L139.pdf

Perhaps the fixture is sufficiently rugged that a battery explosion is not a concern.

As a side note, you mention Gel and VRLA as two types of batteries being considered. Technically gelled electrolyte cells are actually a type of VRLA. I assume you are thinking of Gel vs. absorbed electrolyte?

 

[mango1107]

Alehman - correct we have considered AGM and GEL VRLA batteries. Due to the rigorous cycling of the battery - to be discharged fully several times daily, we have determined that we'll probably get more out of a GEL.

Thanks again to everyone for their responses. I've read code over and over and over, and that's good to a point, since every company likes to provide a certain standard of system - in our case we are looking for a perfect system (or close to), due to the extreme hazardous environment these will be used in.

 

[alehman]

I think your best bet would be totally sealed in a Cl 1, Div 1 enclosure. If the battery does fail violently, at least it will be contained to the enclosure and shouldn't ignite the surrounding hazardous environment.

 

[mango1107]

I think we are on to something. I have a two explosion-proof reps coming in. We'll put the battery in there, and the rest of the items in the purged/pressurized enclosure.

Of course, in all reality the probability of violent battery failure is rare, we just want to play it on the extra safe side...

 

[DRWeig]

Good analysis, Mango.

Extra safe is extra good.

Old Dave