Explosion Pressure

I'd be focused on a path to channel any blast to a relatively safe area. your grade and thickness of stainless, the choice of welding methods (and inspection of the finished seams and welds will all play a role in your overall design. I assume this is a cylinder or sphere of some type. ₂(g) + O₂(g) → 2 H₂O(l) + 572 kJ (286 kJ/mol) <<<that's how fast hydrogen burns

Thanks,

We are not really accustomed to these units of measure. Can this be converted to PSI?

The enclosure is box shaped and the environment always has hazardous gases present. The box has to contain the explosion. The flame is prevented from exiting the box by a "flame path", which starves the flame of oxygen. The flame path is simply two high tolerance machined surfaces, with very small gap.

I don't understand:

"The flame is prevented from exiting the box by a "flame path", which starves the flame of oxygen. The flame path is simply two high tolerance machined surfaces, with very small gap."

The flame is on the gas supply side so it will pass through the gap. In the Davy Lamp, the gas goes through the mesh to get to the flame; the flame cannot get through the mesh to the gas because the mesh robs it of the heat needed for combustion.

If the two plates do not remove the heat needed for combustion, it will re-flame on reaching oxygen.

You are right in the case of Davy Lamp there is no gas over pressure and as you wrote the screen cools the hot gas so much that the temperature after the screen is under the dangerous threshold. In the case of a box the gas mixture inside will burn and the pressure will increase due to the generated heat. This over pressure leads to a flow through the narrow gap due to its expansion (due to the velocities the expansion is adiabatic) the gas is cooled. The choice of the gap as length and section is very important: if it is too small the over pressure grows too much and if it is too large the gas temperature is too high!

'H₂(g) + O₂(g) → 2 H₂O(l) + 572 kJ (286 kJ/mol) <<<that's how fast hydrogen burns'

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That doesn't say anything about the speed with which hydrogen burns. It is the heat of reaction or alternately enthalpy of formation (standard enthalpy change of formation, if you have a stick up your posterior).

Let me start out by saying, even with the few specifics provided, it is clear the task you are considering is not trivial.

Unless I am grossly misunderstanding what you have provided in the OP, the questions you are asking indicate it is time to hire someone who specializes in this type of work. This is not a simple task, and the downside can be severe.

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Perhaps some more detail with make things more clear?

This enclosure for hazardous areas: are you proposing something on the order of a building, or a cabinet?

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What will be the atmosphere in the area prior to the hydrogen leaking in?

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I am assuming the enclosure will be at atmospheric pressure typically, with negligible chance of pressure building due to hydrogen leaking in?

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Re-reading this....I think my first take was....a gross misunderstanding, and that what you are inquiring about is not the endeavor I originally assumed....

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I guess that will become clear with a little additional info.

And, the concentration is?

Gases - Explosive and Flammability Concentration Limits
First time doing this?

Something like this?

http://www.whatpowersupply.com/db/images/Zone%201%20Battery%20Box.pdf

or more like this....

http://www.stolarhorizon.com/PDFs/XP-catalog.pdf - sorry, link no longer available

I can't imagine that you can engineer AND get an agency approval on an EXP enclosure for a fraction of what you can buy one for, even if they do cost an arm and a leg, unless the available sizes don't match your requirements.

Firms like Adalet have been doing EXP enclosures for years, and have recently added EXP in stainless. Their home page has this graphic with a link to their cast SS EXP enclosures, that come with various agency approvals.

Adalet brochure:
http://www.adalet.com/uploads/14751334670029.pdf

There are other firms with similar offerings.

Simple. Conserve energy within the container while carrying out the explosion, then apply the Ideal Gas Equation to the system before and after.

Next!

The objective of enclosures in a hazardous area is to PREVENT and explosion, not to contain it. If there is a is a possibility that and explosion will occur with the Ex'D' (creating a flame path so that a fault within the enclosure does not ignite the gasses outside) technique you are employing, you should consider Ex'M' (potting the components to exclude gasses from the fault) or Ex'Ia' (limiting the amount of energy in the hazardous area to a level where a fault will not ignite he gasses). N.B. Different approval authorities and recent changes in terminology may use other names for the classifications quoted.

If the above options are not feasible, rather than designing the enclosure to contain the explosion you should install a bursting disc to direct the path of the expanding gasses to a non-threatening location. The busting disc will limit pressure so obviate the need for you to calculate internal pressures. The size of bursting disc is related to the volume of the enclosure. Check local regulations but the vessel design should be capable or withstanding a minimum of twice the bursting pressure.

One final comment. This is a job for experts because it will have to be approved by the local inspectorate (including all the calculations) before the equipment can be used.

Hydrogen mixed with oxygen from air tends to detonate with very high overpressures. The pressure wave would hit the walls long before any appreciable temperature increase in the enclosure metal. I suggest that you vent the enclosure rather than try to contain a hydrogen detonation. Prevention of this scenario is key. Elimination of oxygen is one way. Gas detection may help. Please consult a professional.

If you can't design the box to meet the electrical area classification, consider a "Z" purge.