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Exploding Fire Extinguisher


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I work on fire extinguishers, and I've just had something happen I can't explain. A customer of mine had a pretty bad fire and the fire extinguisher mounted to a forklift (which burned) exploded--blew out the side of the extinguisher.

 

Fire extinguishers are pressurized with nitrogen to 195 psi. Assuming that the o-rings on either the valve stem and/or the neck did not melt first and allow the pressure to escape, and knowing that there is no "escape valve" on a fire extinguisher, and assuming that the aluminum head did not melt prior to the steel shell warping and allowing the pressure to escape--all of which do not seem to have happened......the question is: How hot would a fire extinguisher have to get for the nitrogen inside to expand sufficiently to rupture the cylinder?

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It's a difficult question.

You can calculate the pressure at any given temperature quite easily, but the steel will weaken a lot as it gets hotter. That second factor is difficult to guess.

 

What you need is an expert in failure analysis.

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Curves exist for many steel composition giving the proof stress versus the temperature. More often so for steel designed to work hot. A typical Fe-Mn-C alloy looses much strength at 500°C, well attained within a good fire over some time.

 

The same reason lets steel buildings collapse during a bad fire.

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In very rough terms, at 500C the thing will be red hot, the pressure will have trebbled and the steel will have lost something like 20% of its strength.* The aluminium will also have weakened considerably, but it might not have been working so near it's limits in the first place.

 

Based on this

http://ro.uow.edu.au/cgi/viewcontent.cgi?article=1345&context=engpapers

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