Steam Explosion and TNT equivalance

[morninglori]

Good Afternoon,

I am looking for any references that I can use to support that the pressure developed in a steam explosion (water on a molten metal pool) does not create the overpressure which develops when a true explosive is detonated.

A am trying to develop pressure mitigation hardware for a VAR furnace. and the engineer before me has (very conservatively, IMO) used SFPE Ideal Overpressure versus scaled distance curves, converting the energy of the steam expansion to TNTequivalent mass to determine the effects on the room structure, and then to define dificult parameters for which to design safing hardware.

Basically are there a references that state that steam explosions are not detonations, so that I can remove the conservatism from the analysis and redesign the safing hardware

Thanks!

 

[flamby]

I can blow a balloon using my mouth and explode it. Does it count?

Without going into numbers and quantities, I do not think there can be an answer to your question (which itself is not very clear to me).

Ciao.

 

[SincoTC]

There are some pressure pulse figures resulting from various molten metal/water explosions in this paper:

http://www.jstage.jst.go.jp/article/jnst/39/8/845/_pdf

It may be a starting point for further investigations.

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

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[btrueblood]

" any references that I can use to support that the pressure developed in a steam explosion (water on a molten metal pool) does not create the overpressure which develops when a true explosive is detonated."

I would agree with you that equating the energy of steam to equivalent energy of TNT will overstate the blast pressure and blast damage potential of the steam.

Predicting peak pressure of TNT blast is a bit problematic, but you can start with:

"The velocity of detonation is approximately 22,300 fps.
(from ref.

http://www.globalsecurity.org/military/systems/munitions/explosives-nitroaromatics.htm)

Detonation velocity is driven by the speed of sound of the reaction products behind the detonation wave, so the temperature of the reaction is thus known, if you know the reactant gas species.

It may be simpler to just compare the blast wave speed - an explosion can accelerate debris no faster than the speed of sound behind the shock wave, which for a steam explosion, you can predict from knowledge of the steam pressure and temperature. Compare the two speeds, the TNT will be an order of magnitude or so higher....

Or you can use your pressure argument (two ways of saying the same thing). This paper gives both detonation velocities and pressures for a variety of high explosives:

http://www.vti.mod.gov.yu/ntp/rad2004/34-04/dzin/dzin.pdf

A range of 150 to 250 kbar (roughly 2 to 3 million psi) is given for TNT detonation pressures.

All of this is predicated on the fact that a detonation will develop pressures and temperatures that are highly dependent on the way that the charge is confined, how it is detonated (single or multi-point), etc. etc. which determine how efficiently the available chemical energy is converted to mechanical energy. But the numbers from the above should give you ample information to suggest that equating a steam explosion to TNT based on energy content is a poor comparison.

 

[morninglori]

Thank you for your help. Sorry if I was confusing and didn't state my question very well.

I appreciate the references.

 

[cb92]

I agree with btrueblood, that it is a poor comparison. Steam has push; detonations happen very quickly, and then they're gone. Great for shattering concrete. Steam would push the concrete away, it would try to push all the walls down. Detonation would just shatter everything. That is what I see as the difference.

 

[IRstuff]

Blast pressure is a function of the amount of material and distance from the center. A TNT explostion scaled to the same energy as a steam explosion will not have the same impact as a larger TNT explosion.

Just consider the difference between an M80 and a single firecracker. One potentially can kill you outright, the other will simply damage your hand, yet the instantaneous pressure wave velocities at center of detonation are probably identical. A firecracker's explosive material, when not confined in the firecracker simply results in a rather satisfying flash, yet, contains the identical energy stored in a normal firecracker.

TTFN

FAQ731-376

 

[btrueblood]

The thing about a steam explosion is that you are usually talking about a boiler failing, and dropping its load of superheated water across the floor of a boiler room, where it then flashes to steam... Lots of surface area, a huge volume of vapor produced (rapidly compared to human's ability to outrun it, slow relative to a TNT blast) at relatively low pressure...enough to knock down/blow out walls, but it won't cut through steel like a properly focussed TNT blast. The secondary hazard of steam is that it is hot, and produces scalds as it re-condenses to water against someone's skin (or throat and lungs if they try to breathe within the cloud), this happens at a fairly large radius from the "epicenter". Please do not interpret anything in my first post as minimizing the hazard of a steam explosion, just analyze it for what it is, and that is definitely NOT a TNT blast.

 

[cb92]

I studied steam for a while, made some models,read some books. The qreatest fear in steam is a boiler explosion. That happens when all the water is gone, and the boiler overheats and blows. More pressure is created than the safety valves can release before the boiler ruptures violently. It was a big problem in the early days, taking out buildings, steamboats, probably even locomotives. Safety is highly stressed to modelers, and builders of small steam powered boats have contingency plans to manually pump water into the boiler, dump heat, and release as much pressure as possible if necessary. I prefer tube boilers, no large vessel to blow up.
To stay on topic, I would like to see a comparison of a steam explosion, black powder, and tnt. I think you would see an increase in speed. Imagine an old cannon using all three. Steam probably wouldn't push as far as black powder, and tnt would probably blow up the cannon.

 

[rmw]

When I read the original post, the first thing that popped into my mind with respect to steam explosion with respect to water over molten metal was water disassociation and a hydrogen explosion. If that occurred, then you are in fact in a detonation situation. Detonations are by definition at supersonic speed. I can't picture steam expansion (without disassociation) getting to that velocity (without the aid of a nozzle such as in a steam jet.)

rmw

 

[vims]

Hydrogen explosion was the first thing that came into my mind to when I read molten metal and water.

 

[unclesyd]

Here is a paper on explosions in VAR furnaces.

http://www.tms.org/pubs/journals/JOM/0005/Poulsen-0005.html

Here a couple of leads to other furnace explosions.

There was a molten metal vs water steam explosion in Japan several years ago. I haven't been able to locate any information so far.
Also many years ago there was an electric furnace explosion and fire at ACIPCO and there was a debate at the time whether cooling water had started the process.

Anecdotal:
My personal experience with a furnace explosion, hydraulic oil, was with a rocking furnace that I had just finished inoculating. When I reached the end of the charging platform a second I signaled the operator to restart the furnace it blew up. I caught two slugs of molten brass in the back.

 

[morninglori]

Thank you all again.

I have gone back to look at he prior engineers analysis and he did not include the hydrogen that is generated from the reaction of water with molten Ti.

So back to the "original" question....even with the hydrogen disassociation from the metal, would this explosion still behave like a chemical TNT explosion? The pressure would be the same, but the velocity of the shock wave will be decreased, right?

Thank you for the JOM reference...I have found that one, but no other furnace explosion references, or articles on this type of reaction (lots on molten metal droplets into water, but no water on a molten metal pool)

The system I am looking at is very small relative to the furnaces referenced in the JOM article. the crucible diameter is 6 inches.

Thank you all again for your comments.

 

[IRstuff]

I think it really depends on how that's modeled. A steam explosion, by its very nature does not propagate from a point source; it'll be distributed across the actual wetting area. Likewise, if you treat the equivalent TNT energy as a distributed phenomenon over the same wetting area, you might find little difference in the pressure wave velocity.

TTFN

FAQ731-376

 

[unclesyd]

You might need to get hold of this guideline for melting Ti. If none of the companies can help you might try the Linda Hall Library

The paper, “Safe Design of Melting Systems for Titanium,” was authored by Eldon Poulsen, Timet; Steven C. Stocks, Oremet; Steve Giangiordano, RMI: Eric Jarvis, IMI; and Jim Silvas, Titanium Melt Division, Teledyne Allvac.

http://www.lindahall.org/

 

[IRstuff]

Not there, but there is:

http://www.tms.org/pubs/journals/JOM/0005/Poulsen-0005.html

Note:

When water leaks into furnaces, it causes a two-stage explosion. The first is a steam explosion, which is then followed by a hydrogen explosion. In one of the early industry explosions, it was calculated that the combined explosion was the equivalent to a 500 pound bomb or 200 pounds of TNT—not a good thing to have in a melt shop.

TTFN

FAQ731-376

 

[btrueblood]

Sigh. I love quotes like that, with no further information to evaluate the validity of the comparison. Usually the comparison is made on an energy equivalance. "200 pounds of TNT" certainly conveys a sense of danger greater than "75 psi steam" or "400 MJ of energy".

Also, by no means is hydrogen evolution from the puddle going to be the same as a TNT detonation either. The H2 will evolve without a stoichiometric level of oxygen being present, so it will have to mix/diffuse into surrounding air before it can detonate, reducing the blast pressure and velocities obtainable. Again, not to minimize the very real hazard, but trying to point out that equating the energy of a TNT detonation to anything other than high explosives is useless as means of analysis.

 

[MotorVib]

Just a Side Note 1 ton of TNT is about 3,965,667 BTU's or 4.184e+009 Juole's or energy.

Chris

 

[ivymike]

at a LHV of 120MJ/kg, that would require 39kg of H2, or the amount you get from 351kg water, yeah?

 

[maytag]

I have no experience with a "VAR" furnace but 18 years experience with electric arc furnaces-the water on top of the molten steel does not present the problem that the water under the molten pool does. We have been very blessed to never have a serious accident. One of our sister plants had a failure of this type-blew the furnace roof and evacuation ductwork ( about 40,000 lbs.)completely off the furnace-landed severla feet away-the figures I was given about water flashing to steam was expansion by 1700 i.e. a cubic foot of water suddenly wants to occupy 1700 cubic feet. I have no resources to say otherwise-hope this may help.
Maytag