Combustion pressure for explosion proof enclosure

[Rattletrap]

Hi All,

I need to find the resulting pressure of an explosion of a 50% Acetylene and 50% oxygen explosion in an enclosure. The idea is the resulting pressure as to be able to design an enclosure which will sustain such a blast.

I found the

http://en.wikipedia.org/wiki/Heat_of_combustion

value of acetylene and used it along with Q=mcΔt to determine the final temperature. Is this method reliable for this solution?

According to the above the temperature only raised by about 35°. Considering that acetylene burns at 3500°C I am doubtful of my conclusion here. Can anyone help me in the right direction please?

 

[pmover]

what is going on here?

is the pressure rise due to combustion, spontaneous ignition, electrical spark, etc.?

what type of enclosure? how big-volume? materials of construction? thicknesses of material?

dynamic pressure increase is way different than a gradual pressure increase.

not sure how else to help, but this may spur further analysis.

good luck!
-pmover

 

[Rattletrap]

Hi pmover,

Yes it is ex rating for an enclosure. The gas is ignited by a spark. The volume is undetermined because it will depend on the design, it will be less than a 1000 CC anyway but as there is an escape path the volume will not be constant. However I guess that initially the volume will be constant as the flow path is controlled in order to pass the heat on to the enclosure.

Thanks

 

[Latexman]

Here's a good reference:

http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1805/ch14-18.pdf

There are several NFPA standards that may be helpful. Google "deflagration containment", "explosion containment", "compression due to combustion', etc.

Good luck,
Latexman

 

[psafety]

Page 62 of NFPA 68

 

[StoneCold]

When you buy electrical equipment rated for Class 1 Div1 Group B.
The equipment must have been tested in some way to prove to be "explosion proof". So there must be a standard you can look at. Maybe talk to Appleton. They make a ton of stuff. On that note there is not a lot of equipment rated for Acetylene. Most group B equipment is purged.
Anyway I hope this helps.

Regards
StoneCold

 

[berkshire]

As a result of an experiment as an apprentice, I can tell you with good authority that 4 cubic feet of an acetylene oxygen mixture 50/50 when ignited in the center of the building, will creat enough over pressure to remove the end windows from a 5000 sq ft Nissan hut.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[DRWeig]

Acetylene is Group A. I've seen very few enclosures rated for Group A. Here's an example, though, if you want to look (Dwyer 1950G air differential pressure switch):

http://www.kele.com/templates/product.aspx?id=1923

If there's any way possible, make your work intrinsically safe instead. Acetylene is too scary for me.

Also, I'm not a chemical guy, but is 50/50 the ideal explosive mixture for acetylene and oxygen? (I'd like to know). The concern in the field and the standards will be percentage by volume of air, not oxygen, so you may want to factor that into your calculation. Or, it might be easier to just take a stab and start testing.

Your UL or FM test will involve igniting the ideal explosive mixture of acetylene and air inside your enclosure while it is inside another enclosure that also contains the ideal explosive mixture of acetylene and air. Yours will have to contain the explosion and cool the escaping gases enough to prevent igniting the surrounding mixture.

I bet they do this test outdoors.

Let us know how it goes!





Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[IRstuff]

Your usage of a steady state equation to model an extremely transient event is highly suspect.

TTFN
faq731-376

 

[CastMetal]

Depending upon the size of the enclosure a bunker might be more appropriate testing facility...

DRWeig

EX enclosures I have dealt with depend on eliminating the flame path with very smooth and flat mating flanges <.002". Exhaust gasses will still be hot but not currently undergoing combustion when leaving the enclosure. Cooling is secondary as the gasses only need to fall below the auto ignition temperature of the working fluid (acetylene ~580F).

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations

 

[Rattletrap]

Yes DrWeig, exactly. That is it. Although the enclosure will not work in the envirnoment it is tested in it is tested like this to ensure is is a safe enclosure.

Irstuff, That is why I ask the question. I realize that in my mixture I will have about 11 Mj of energy. How to get this energy released and absorbed in such a way that there is not too much force on the enclosure. That is the question!

Thanks anyway, I am looking at some standards now, thanks Latexman.

 

[IRstuff]

11 MJ sounds a bit high; acetylene's combustion energy density is about 48 MJ/kg, but 1 liter of of 50% acetylene gas would only be about about 1/2 gm of mass, so there would only be about 24 kJ of available energy, unless I'm misunderstanding the problem.

In any case, the critical missing factor is either the time or the velocity. With either, you can potentially synthesize a pressure. Say the explosion front velocity is 2270 m/s for acetylene. If you use the aerodynamic drag equation:
1/2(v^2)[&rho;], you get a pressure of 3.3 MPa, which is comparable to the 3+ MPa shown here:

http://www.gexcon.com/handbook/6_6.gif

TTFN
faq731-376

 

[psafety]

Group B is hydrogen.

Acetyene with 50% O2 has a fundamental burning vel of 1140 cm/sec. I think Pmax ( maximum pressure of a confined explosion) can be calculated.

 

[IRstuff]

Burning velocity is not the same as the explosion velocity. The supersonic shock wave accelerates the flame front, which is what sustains the explosion until all the fuel is gone. Acetylene can explode, and it should be clear that the explosion is not running at the burning velocity.

That's why FAE's work when dispersed over a large volume.

TTFN
faq731-376

 

[psafety]

If supersonic, it has transistioned to a detonation. Acetylene & O2 will need to start at an elevated pressure to reach that type pressure. Fundamental burning velocity is valid in a gas deflagration, as this mixture will per the OP.