Explosion Proof Enclosure Design

What is the application?

What is the content?

It is a Analyser with exploded gases inside for Sampling purpose.

Actually we have a Enclosure which is Explosion Proof, we are now reducing its size for a small varient, but the content will remain same as it is.

The new Enclosure is 60% of the Old one. But the FEA we done is on same Pressure, (eventhough we reduced its Volume); due to this it fells in the test for that much Pressure.

I just want to know did i need to keep the Pressure as it is or to reduce it as per the Volume.

If I understand your post, the FEA is failing due to the decreased volume. That doesn't seem consistent with my experience where larger volume boxes failed more easily in physical tests (as performed by notified bodies).

Did you also reduce the wall thickness? By how much?

What is the general shape?

Can you provide details about the general dimensions and the pressures you're using?

The current reduced Enclosure size is 14 x 13 x 10 Inch (Inside Volume 1235 Cubic Inch); earlier it was 2010 Cubic Inch. The input Pressure Value is 42.6 Bar (4.26MPa), for both FEA the Input Pressure is same.

I have not reduced the Thickness, it is as it is (22 MM) only size is reduced.

My Q. is did there is any Interrelation between the Volume of Enclosure & the Pressure Value for Designing the Explosion Proof Test.

Hello Sachin Gore:

The thing that first comes to mind is the cages used to store portable gas canisters. Such as you would see in most Garages. It would be a good base to start from. You know they work as they designed for holding gas containers.

Our local one is full to the brim now and I often wondered how they test those cages. Do the put one gas tank in. Or try it full and explode one or two, which will most likely mean a chain reaction? I have wondered for years!

It does not mean to say something better cannot be made, does it?

Good luck.......

There is no such relation between volume and the explosion pressure. Explosion Pressure depends even up on the placement of inside components. Thus, you have to test it actually at test laboratory.... At ERTL(E)/ CMRI for India

if you are exploding the same mass, I expect it will try to accupy the same volume V at same pressure p. Reduce the volume and the gas will no doubt assume a higher pressure. The constitutive equation for a 'perfect gas' says that pV=RT.

My guess is that explosions are closer to adiabatic, than any other idealised process -> you can find the solution p=p(V,T) for adiabatic processes in any textbook on thermodynamics. For this, you need to look up the R constant for your specific gas (assuming it behaves more or less like a 'perfect gas')

Hole this helps. Alas, I can offer no further insight, as I am not expert in gas explosions.

Enclosed FEA show that if dimensions encrease as you stated then for same pressure the von Mises equivalent stress will also go up. Not as much as the scale but near to.

The 2 situations have been computed according to given dimensions which lead to an other volume as mentioned for the small box.

Now there are 2 possibilities:

- the box is filled with a gas which has when burning same amount of energy per volume unit, in this case the pressure will be the same

- the box is fillet with same amount of explosive stuff which will give the same amount of hot gases but in smaller box, then the maximal pressure is not the same in both cases.

Results for smaller box with 4.26 M Pa

Results for bigger box.

If your computations give different results you should check them.

I accepted for a simplified modell that no hole is present and this allowed to make the modell with only 1/8 of the boxes.

Hope it will help.

Hi Guru,

Thanks for the Trial Run. But what i observe is; the Von Mises Stresses get increase when the Internal Volume is reduced.

One point you have mentioned that, the Maximum Pressure will not be same for both type of Volums: thats what i am looking for whats the exact factor.

Not true vM cannot grow up when the volume decreases.

For the small box the max is 197 and for the bigger 265 M Pa.

If your computations give a different result check you boundary conditions they may be wrong.

Sachin Gore,

I ran an FEA and got pretty much the same results that Nick Name did, so then I reread your post. Are you asking about the test pressure for different volumes as required by a testing lab? Can you tell us which notified body you are dealing with, what standard you are trying to meet, and how you configured the FEA?

And, finally, are there internal supports or ribs that are scaling/not scaling as you change volume?

Assuming that you have same mass of stuff in both cases you get same amount of energy. In the smaller box the pressure will be higher since the volume for the hot gases is smaller but you load same amount of energy.

Though what you say "same mass of stuff in both cases will same energy", mass will be less in smaller box, so the energy and effectively the pressure rise will be of the same order. You are not filling the explosive gasses in the box, they just enter the box at atmospheric pressure.

I think it is necessary to explain better the thoughts:

you assume that the box with gases at atmospheric pressure I considerd 2 possible cases since it was not clear to me as it is for you

- one possibility is as you assume then the energy per volume unit is the same in both cases

- an other possibility is that some exposive material is exploded in the box and if the mass to be exploded is the same in the 2 boxes the energy per volume unit in the small box is higher and the pressure will be as well bigger.

Now you understand?

WHEN WE TALK ABOUT THE EXPLOSION PROOF ENCLOSURE AS PER IEC 60079, NO EXPLOSIVE GASES ARE FILLED IN INTENSIONALLY. THEY GET ENTERED THOUGH THE GAPS (WHICH YOU CAN NOT AVOID). THUS THE SMALLER BOX WILL HAVE SMALLER VOLUME OF EXPLOSIVE GASES INSIDE, THAN THE BIGGER BOX. ENERGY OF EXPLOSION WILL ALSO BE PROPORTIONAL. BUT PRESSURE SHOULD BE OF THE SAME ORDER.

WITH HIGHER ENERGY IN BIGGER BOX, THE PRESSURE ACTING ON BIGGER WALL AREA WILL NEED MORE THICKNESS THAN THE SMALL BOX FOR PRESSURE OF THE SAME ORDER. THIS IS CONSISTANT WITH POST 3 OF TVP45.

BUT POST 5 OF SACHIN INDICATES SOMETHING DIFFERENT, WHICH IS NOT CLEAR. IS THAT INDICATE SOME INTENTIONALLY FILLING OF EXPLOSION GASES?

Hello gsuhas:

I am also a little confused here.

It seems you are talking of explosive gases entering from the outside of the box.

Where the originator of this thread, is putting an explosive inside the box to test the strength?

The smaller box is 40% smaller.

So whould the gas need to be reduced by 40% as well?

IE............10g of explosive in the large box.......6g of explosive in the small box?

Conventionally, you, me and whole world calls a box "Explosion Proof Enclosure", when there is no explosive intentionally kept inside the box. There is big set of standards IEC60079, out of which IEC 60079:1 is for Explosionproof Enclosures Protection type "d". But this is not applicable for the enclosure, where in you keep some explosives inside intentionally.

Idea of above standard is to make the box safe in case of entry of the gas inside the box from outside explosive atmosphere, and when there is some spark or heat inside the box, which creates explosion inside the box. Box is so designed that in case of such unintensional explosion inside the box, the hot gases inside the box will be sufficiently quinched / cooled before they escape through the various gaps (which are designed as per standard), so that there will not be any explosion of the gas in the atmosphere around.

In above case, the explosion pressure will be almost of the same order irrespective of the volume of the vessel, though the energy release will be proportional to volume of the enclosure.

But if Sachin wants to put the explosives inside the enclosure, the energy release will propotional to the quantity of explosive and pressure will be inversely proportional to the volume of the vessel.

Hello gsuhas:

It does sound to me as if he is exploding something in the box, and rightly or wrongly, that is his chosen method of testing. It sounds to me to be confused. I am not meaning any insult whatsoever ok?

How would you test those boxes then gsuhas?

I did not get you. What insult you are talking about. I am sure, we are only discussing technically. I do not see any sort of insult in any of the posts.

The boxes are tested in notified body laboratories, where they fill the enclosure with explosive gas mixtures appropriate for the area classification... zone... class etc. etc, and put the box in explosive atmosphere (in closed chamber). Now they creat a spark inside the box under test and creat explosion inside the box. If box is properly designed, explosive atmosphere around does not explode. If not properly designed, the explosive atmosphere outside the box, inside the chamber explodes. (If the box is too weak, the box itself may get damaged).

Properly designed box is supposed to get no damage as well as no explosion of explosive atmosphere in the chamber should happen

Hello gsuhas:

I did not get you. What insult you are talking about. I am sure, we are only discussing technically. I do not see any sort of insult in any of the posts.

Forgive me for confusing thing even more with my remarks aimed at the thread writer. I know there was no insult, but I thought it a courtesy to say as much. That is all.

I thank you for the rest of your explanation.

I assume that with the small box 40% smaller, it will have 40% less internal explosive mixture, which is 1 atmosphere.

Take care.............