Thursday, April 03, 2014

Halon Extinguisher

This video is sort of adorable, and I think it may be as close as I'm going to get to my VP's request of a video showing a Halon fire extinguisher in action. Finishing the job with an obviously capped bottle of water is a great touch. It was probably made for the same reason that I'm searching for on: it would be nice to know in advance how long the unit will function, what the chemical looks like coming out, and the efficacy we can expect in using it. Halon is restricted under the Montreal Protocol, to which Canada is signatory, so you can't just go out behind the hangar and discharge expired bottles for training purposes. Expired units are returned to the manufacturer and the Halon is recovered and recycled.

I'm pretty sure the colourless Halon gas is virtually invisible while being discharged. The information on the can is sufficient to calculate that the contents are not in excess of what will produce an average 2% concentration in the air of the aircraft cabin. But how long will it take to be exhausted, and how big a fire is too big for it? You don't want to use the whole bottle on half the fire, but perhaps you want to be sure you have put out one part of a fire before moving on. But it lacks information on the stream rate, discharge rate, time to be fully discharged, whatever one might call it. I am, however, finding plenty of interesting things about Halon.

There are two types of Halon: 1211 versus Halon 1301. (That isn't the interesting part yet. I might also be overselling "interesting".) I just knew that the handheld extinguishers use 1211 and the installed setups like in computer rooms and cargo areas use 1301. I hadn't even thought about what "halon" meant, but it's a hydrocarbon with halogens replacing some of the hydrogens. Halon 1211 is chlorodifluorobromomethane. The number isn't just some kind of catalogue number, but a description of the molecule. The first digit specifies the number of carbon atoms: unsubstituted methane is CH4. the second digit is the number of fluorine atoms, the third is the chlorine atoms, and the fourth is the number of bromine atoms. If there happens to to be iodine on board they'll add a fifth digit for that, and presumably a sixth digit could be added for astatine, but an unstable radioactive halogen wouldn't be a good choice for a fire extinguisher. (If you didn't find an interesting part in there, sorry. It was interesting to me. I love knowing how things got their names).

Something I didn't know, and actually believed the opposite of, is that Halon 1211 is not appropriate for burning metal fires. Fortunately my aircraft does not have magnesium components. But perhaps I should double check that.

And then, just as I thought I had milked the Internet dry of useful (or interesting) information on Halon fire extinguishers, I found this study. Really if you're nerdy enough about aircraft firefighting chemicals to have read this far, you should just go and roll around in that link. Sure, it's a PDF of a typewritten document, but what do you expect from 1986? Six percent of cabin fires during the period they sampled were from "smoking materials". i.e. people lighting fires on board aircraft for recreational purposes. (There's no breakdown of tobacco versus other). The FAA built a wind tunnel out of sheet steel, a Cessna 210 and a couple of jet engines. It's not crystal clear from the description but it appears that this test rig was designed and constructed specifically for testing fire extinguishers. And damnit, they did. There are sixty pages in this document, with graphs and tables and all the data I could ask for, including the fact that a 2.5 lb Halon fire extinguisher takes ten to fourteen seconds to fully discharge. Thank you, American taxpayers.


Jeremy said...

I seem to remember that metal fires (class D) can only be extinguished by "dry powder" extinguishers and nothing else. (This is different from the normal "dry chemical" used on AB(E) fires.)

The class I was taking was for marine fire-fighting - their solution was never to have combustible metals on board! (I suppose technically aluminium is combustible but that seems to be not to be a big concern for aluminium hulled boats.)

Are FM200 fire extinguishers available for aviation purposes, as a replacement for halon? Apparently these are installed in some marine engine rooms these days.

Unknown said...

Hmm. "Damnit", not "dammit", you say...

(Yes, the halon information is interesting, btw.)

Jack L. Poller said...

Many years ago, someone mistakenly pulled the release on my halon fire bottle in my race car. The result was a cloud of vapor, as the halon comes out so cold that it condenses the water in the air.

Unfortunately for the poor soul, his arm was directly under one of the the nozzles. He now sports a silver-dollar sized frostbite mark!

Aviatrix said...

Thanks, Jack! That's a super interesting piece of information that I had not considered. Wow, I sound like a spambot.

David, you may be right as to the conventional spelling. I figure if I say damn, then I must be saying damnit. I guess I should spell it damn it.

Majroj said...

I was an aircraft (as well as structural) firefighter when Halon made its military debut. We didn't know it ate up the ozone layer then but it was beaucoup expensive and in one areas as effective and seemingly less toxic than CO2. So, Halon in computer rooms and in planes, CO2 in the big fifty lb tanks on the ramp.

Halon is very close to Freon, each a halogenated hydrocarbon. It disrupts the hydroxyl and other free radicals in the combustion process, so it won't work on Class D (burning metals). It won't penetrate layers of fuel or debris if not totally contained and flooded into a basement or an engine nacelle, so not so good for much else

A side effect of breathing straight Halon: lowered sperm count (or so they told us).

Halon passing through high-enough combustion temps breaks down into some nasty byproducts.

If you have an electrical fire, you can knock it down and even cool it off, but if the juice is still connected, it will return.

Frank Van Haste said...

Dear Trix:

On behalf of all of us US taxpayers, you're welcome! It was entirely our pleasure.

And thank you!!, for the image arising from "roll around in that link". First smile of the day, there.



Anonymous said...

When I was in secondary school, the school computer room had a couple of halon 1211 fire extinguishers. Teenagers being what they are, I experienced the discharge of one of them at close range. I think there was a slight cloud of mist (but less than I'd expect from a carbon dioxide extinguisher) and I had a headache for the rest of the day.

Majroj said...

I got my laugh for the day by watching it full screen. The "fire" is a black box with an orange light and fog puffer, and the
halon" stream doesn't disturb the leisurely path of the fog.
So effective it kills fire even when it's EMPTY.

nec Timide said...

"...people lighting fires on board aircraft for recreational purposes..." What a wonderful phrase.

Sarah said...

Yes, nec Timide, I giggled at that phrase too. What a strange concept, "lighting fires on aircraft for recreation".

I'm old enough to remember flying in airliners when smoking in them was allowed. It was like traveling in an aluminium ashtray.

Unknown said...

In my Navy days (about 15 years ago), we used to use expired fire extinguishers to hunt rats on board.
It was halon 1301, and it would freeze the rat so you could "dispose" of it. Just don't ask what happened if a frozen rat was dropped.

I also remember something about not breathing to high a concentration of the stuff for fear of major lung damage (Oedemas?). I think the problems started at 8% in air.