Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations The Obturator on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Chemical explosive equivalent 2

Status
Not open for further replies.
WARose

WARose

Structural
Mar 17, 2011
5,594
I'm working on a project where I've got some blast loading to consider. They've got a variety of chemicals (storage barrels of methanol, ethyl alcohol, etc)....and I have no clue how to convert that into a equivalent TNT charge. (Which is what I am use to starting a project with.)

After checking my sources, I see no means to figure that. (All those resources are structural publications. Books like 'Design of Blast Resistant Buildings in Petrochemical Facilities' and so on.) Everywhere I have worked, a chemical/process engineer has come up with the equivalent charge.

So I guess my question is: is there a means to come up with this charge weight that a structural guy can do.....or is it indeed best left to ChemEs?

Thanks.
 
Replies continue below

Recommended for you

Sort by date Sort by votes
Can't you figure it from the heat of combustion for the chemical (E=mHc) and then convert the blast energy to the mass of TNT required to achieve that energy? Alternatively, there's sometimes a documented RE factor which describes the relative power of an explosive compared to TNT. That may not apply to your alcohols though.

Not my area of expertise and I'll be interested to see other answers; this was just my addled recollection of some high school chemistry.
 
Upvote 0 Downvote
@rather_be_riding: Don't know the formulas you are posting.....so I should probably stay away.

@Retrograde: Thanks. It doesn't have all the chemicals I have.....but that is good.
 
Upvote 0 Downvote
Yes you will need a ChemE involved. Issues such as min/max explosive limits for the vapors, sensitivity to detonation, and velocity of flame all enter into this.
We stored such things (and worse) in a very small room to minimize the volume of fuel and air that could be involved in an incident. Fresh air was forced in by a low voltage fan, and the vent had a flammable gas sensor in it. If we detected a leak we call the fire dept, evacuated that end of the building, and flooded the room with nitrogen (we had two bulk liquid tanks on site for process). The roof did have a blowout panel in it.
We did have one chemical that could spontaneously decompose with force. It was stored in a shed about 250yd from anything else.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
Upvote 0 Downvote
Thanks Ed. I suspected it was too far out of my line.....and now I'm sure.
 
Upvote 0 Downvote
Sure, just use the heat of combustion and calculate how many pounds of TNT contains the same energy. It should be easy to calculate the destructive power of a 2x4 of southern yellow pine as pounds of TNT equivalent.[pipe]
 
Upvote 0 Downvote
Out of curiosity, the heat of combustion approach can only be conservative right (particularly if you decide to take it to the extent of calculating TNT equivalence of non-explosive substances!)? As noted previously, not my area of expertise but I am curious to know if the rough principle is completely off base. It's not often these days, I get to think about much chemistry at all beyond a little corrosion prevention. Not quite as entertaining as blowing things up.
 
Upvote 0 Downvote
I found one resource that gets the number by comparing the mass specific energy [kJ/kg] of [whatever] chemical and comparing it to TNT. In other words: if you have (say) RDX (Cyclonite), which has a mass specific energy of 5360 kJ/kg, and you compare it to TNT (4520 kJ/kg).....you have a ratio of 1.185. In other words, 100 kg of RDX apparently is the equivalent of 118.5 kg of TNT.

Only problem is: I don't know if I trust any number I would find or calculate for these compounds......there may be other variables here that I am not aware of that affect that specific energy number. (I.e. the environment it is in and so on.) So I still don't feel confident doing it without ChemE input. But at least I have a ballpark idea now as to what is going on.

 
Upvote 0 Downvote
Considering that grain dust explosions are rather violent it doesn't seem like a simple "how much energy" is enough to make a comparison. Many explosives have unstable bonds - the "tri-nitro" configuration is one of them. Others are putting oxidizers in close proximity to things that can be oxidized - such as ammonium nitrate enhanced with fuel.

How big the boom you get from a stable fuel alone is going to depend on how much oxidizer you are planning to mix into it.

Then too is the amount of containment. Plenty of basic explosives only burn vigorously when left in the open; C-4, for example. Or, like when that bomb disposal unit decided to destroy some fireworks and blew up their bomb disposal containment vessel. Remember folks, leave this to the trained professionals. "injured 17 people — including 10 LAPD officers, one ATF agent and six civilians — and damaged or destroyed 13 businesses, 22 residential properties and 37 vehicles, police have said." Those aren't amateur numbers. If an amateur did all that they would be in prison.

I do like the thunderfoot video on explosions - (it covers a more than just energy; this is at the timecode for the explosives part.)

I think thunderfoot was the one who figured out why sodium in water explodes so violently when there is nominally insufficient surface area for the reaction.

The Royal Institute has another one - more fun to watch though not as quantitative.
Explosive science with Chris Bishop
Nitrogen triiodide (a perennial favorite)
 
Upvote 0 Downvote
Just a update: I have a outside consultant on board [a chemical engineer] and they are working on it. They are considering a number of scenarios (including a vapor type explosion). Will let you know how it turns out.
 
Upvote 0 Downvote
Good, fuel air explosions are the real limiting case.
Under worse case conditions (semi-confined) 20 gal of gasoline is roughly 175 sticks of dynamite.
Make other car risks pale in comparison.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
Upvote 0 Downvote
Status
Not open for further replies.

Part and Inventory Search

Sponsor

Back
Top Bottom