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Blown & Injected
10-31-2004, 12:29 AM
Lets say that a large metal container that only has an internal volume of about 2,000 sq in has about 7000 grains of PETN (high explosive) in it, and it is then filled with water through a 3/8" hole. The PETN is detonated and the container manages to contain the blast. About how much pressure, in PSI, is generated?

Thanks

antechinus
10-31-2004, 01:57 AM
This sounds like a joke question, but hey...

7000 grains is 50mg x 7000 = 350g of PETN

C5H8N4O12 -- > 2N2 + 4H2O + 2CO + 3CO2

The oxygen balance of PETN is a bit low so CO is formed.

Moles of PETN = 350/316 = 1.11 moles

So 11 x 1.11 moles of gas formed, which is ~270L at STP.

I assume you mean there is no gas inthe container to start with. You have 270L of gas, once the gas cools down that has no where to go. Assume the products are at STP - if you want you can calculate the final temperature, but you would need to know the initial temperature.

The water will compress by a few percent, so you will have 40 sq feet of volume for the 270L to compress into. A good amount of the gas will dissolve into the water.

Since I do not know what a square foot is in litres, you will have to calculate the last bit.

Blown & Injected
10-31-2004, 07:29 AM
Not a joke, just curious. Was at a "Dynamic Safe Opening" class last week and the techs could not open a 100+ year old safe. It is an explosion proof safe called a Cannonball.

When all that force was contained in the safe, I could not help but contemplate the enormous pressure peak.

Thanks for the solution - I gotta link to a good conversion table somewhere in my favorites.

Squink
10-31-2004, 09:32 AM
internal volume of about 2,000 sq inThe trouble will not be solved with a conversion table, that's a figure for area, not volume. Did you perhaps mean to say 2,000 cu in? That'd be a 10X10X20" internal volume, or 32.77 liters.

Blown & Injected
10-31-2004, 09:35 AM
Sorry - yeah that should be CUBIC INCHES, it was about 10x10x20

Squink
10-31-2004, 09:57 AM
Assuming the water fills the safe completely, the only volume for gases from the explosive to expand into will be that left vacant by the PETN itself. With a density of 1.5, 350 grams of PETN will take up 0.233 liters.
270 liters of gas in 0.233 liters will have a pressure of 1159 atmospheres, or 17,347 psi. As antechinus mentioned, the water will dissolve some of the gases, and also compress a little, so the final pressure won't be quite that high.

drachillix
10-31-2004, 10:26 AM
270 liters of gas in 0.233 liters will have a pressure of 1159 atmospheres, or 17,347 psi. As antechinus mentioned, the water will dissolve some of the gases, and also compress a little, so the final pressure won't be quite that high.

And for comparison, it only takes about a 5-7 PSI pressure differential to blow out the walls of a typical wood frame house (albiet over a much larger volume of gas).

Tough lil safe.