4/12/2006: Ellsworth AFB foam test

[FireFightingMan]

Ok, to get the obvious out of the way, this is really funny, especially the images of the foam releasing out of the hangar.

Now for the serious part: I've been a firefighter for...a while, and I joined the Cellar just so I could reply to some of the crap on this post. As somebody else pointed out, the foam is AFFF, typically used to suppress ignition of combustible fluids (more on that in a moment), especially JP and gasoline, where the vapors are explosive and heavier than air (thus they can obtain a vapor density sufficient to be combustible). The goal of AFFF is NOT to combine with (i.e. emulsify) the fluid but to sit on top of it and smother the fire by dispersing the vapors and limiting atmospheric esposure to the liquid, which limits evaporation, which means it can't burn (I'm skipping the Junior Chemistry on exothermic oxidation reactions ). The surface tension of the bubbles further suppresses other chemical reactions which can allow spontaneous oxidation and combustion (in other words, some emulsification is a good thing). Of course, the mass of the foam and the relative temperature differential also plays a significant part in limiting oxidation as well. So, for those of you who just had the lightbulb go off in your head, yes AFFF is also effective at suppressing class "A" fires. The reason it isn't more widely used for class "A" fires (structure fires, in particular) is a complicated issue, although apparatus manufacturers (most notably Pierce) has been trying to convince more fire companies of the significant advantages of fighting all class A and B fires with AFFF. My recollection (although someone else can correct me on this) is that AFFF is NOT electrically conductive and therefore can also be used on class "C" fires. Again, I don't know that for sure. Class "D" fires (combustible metals) are not candidates for suppression with AFFF.

Anyway, AFFF is typically fractioned at 1-4%, depending on the situation and application, although some fractioners only disperse AFFF at one concentration for ease of use. Can you suffocate in this stuff? Hell yes. You are in soap suds, but dense soap suds, not like the stuff on the top of your bubble bath. As you can see from the runoff from the hangar, and the dispersal pattern from the nozzles, there is a significant density to the solution, which means significant air displacement, and thus significant fire suppression and cooling. I have never been in AFFF of a depth even approaching 1m (espcially in a confined space), so I can't tell you what the differential is between the top and bottom. At the depth of the solution in the hangar it would be impossible to carve out a significant air pocket to ensure survival. The good news is that you are a lot more likely to survive an AFFF near-drowning than you are a flashover.

This system was surely modified after this test to limit the total flow to ensure that a situation like the one illustrated for the test ever occurred, in case it was impossible for personnel to evacuate the hangar, or to release the foam.

Oh, and one other thing: yes, AFFF is essntially dish soap, although I believe it also contains a surfactant to ensure consistent bubble size. Being cheap by nature (AFFF is expensive), most of us in our department have acquired old Class "A" fire extinguishers (water cans) and have filled them with homemade AFFF (check the net for a recipe that suits you - I use a general purpose one because I don't know if I'll be first-on-scene at a car fire with entrapment or a brush fire).

Anyway, to repeat, the pictures are damn funny.

[xoxoxoBruce]

Hey thanks, FireFightingMan.
If I do the net recipe and stick it in my water extinguisher, what makes it foam when it comes out, just being forced through the nozzle at speed?

Welcome to USAvetUSAFspouse, too.

[FireFightingMan]

Originally Posted by xoxoxoBruce

If I do the net recipe and stick it in my water extinguisher, what makes it foam when it comes out, just being forced through the nozzle at speed?

Hey, Bruce. You will get some foaming action on contact just by the force of the straight stream. However, I prefer to shake my can several times (usually while running to whatever I'm trying to put out) to ensure that I have some bubbles already in the mix.

Two things on this point: 1) You don't have an aerating nozzle on a water can. You have a straight-tip nozzle. Therefore you are not going to get the same quality foam you would if you were able to introduce air to the system as you go (the system that Pierce recommends, called CAFS introduces compressed air to the mix in your truck as it comes out of your pump, meaning that you already have foam started in the hand line, even though you might have a conventional straight-tip or combination nozzle instead of a foam nozzle). You can improve your foam by modifying your straight tip on your can to be a venturi, although I don't recommend that. 2) Just for illustration on how effective just adding soap and/or surfactant can be without modifying the nozzle, most Essentials classes (the first firefighting class someone takes) include a session where a class B fire is started in a large pan (about 6x6), and the firefighter is given a water can and ordered to extinguish the fire. Most people can extinguish the fire with one or two cans with just water (although keeping the fire from flaring up is difficult, and some trainees take three or four cans to keep the fire out). HOWEVER, with the addition of a foaming agent (AFFF, or insert your home brew here), almost all trainees are able to extinguish the fire with ONE can, AND the mix becomes extremely difficult to reignite for the next trainee.

So the REALLY long answer to your question is...you might not get "good" foam, but it appears to be good enough, and considering how cheap it is to obtain an "old" can from a fire extinguisher dealer (everyone who I know that has asked has been given one for free), fill and pressurize it (you can do it yourself since you have all the ingredients and a water can is charged with an average air hose), and the volume of agent that you can put in a can compared to your average dry chem extinguisher it's a great way to go, at least for the people I know who have seen a fire or two.

[xoxoxoBruce]

I'll leave the image of you shaking your can, to the ladies.

Class B fires, flammable liquids and gases, solvents, oils, greases (excluding cooking oils/greases in depth) tars, oil-based paints and lacquers, (yeah, I had to check) keep flaring up because the fumes are reignited by what's still burning? Do they stop flaring once the flames are completely extinguished?
I guess the AFFF would suppress the flammable vapors from reforming while you get the flames beaten down.

[FireFightingMan]

Originally Posted by xoxoxoBruce

Class B fires keep flaring up because the fumes are reignited by what's still burning? Do they stop flaring once the flames are completely extinguished?
I guess the AFFF would suppress the flammable vapors from reforming while you get the flames beaten down.

First a CORRECTION: AFFF is NOT appropriate for Class C (which makes sense).

On to the question: Please understand that I am not 100% certain on the physics, only about 70% certain. Once you get ANY combustible burning, regardless of its class, there is a certain period of time where the surrounding environment is still hot enough to vaporize the material, which can then oxidize and thus reignite - remember, if it ain't evaporating, it ain't burning...unless it's a class D, but I'm not going to go there. This situation is especially problematic for volatile liquids because they are constantly evaporating, even at room temperature, and typically have flashpoints that are close to their evaporation point. So, once you get a diesel/gas mix burning, you are in deep crap. The gas gets the diesel hot enough to ignite. Then when you put it out, you are still at just below the flash point of the diesel...which is well above the flashpoint of the gasoline. So, it is common for the pan fire to be extinguished, only to reignite two seconds later.

The students who are successful in putting the pan fire out quickly start in the middle, then force the fire into a corner while spraying back and forth across the surface that they have already extinguished. After the fire is "out", they continue to spray back and forth across the entire surface for several seconds longer. This continued spraying 1) displaces the gasoline vapors that are continuing to evaporate so that they cannot reach a density sufficient to combust and 2) cools the surface so that less vapor is released.

In the real world this type of thing does not always happen. If the fire is in an area where the liquid is in a low spot and the sides are raised, without significant air disturbance, then the danger is real. A hangar is a great example of a setup where such reignition would be expected.

[xoxoxoBruce]

Thank you, Sir.
Now all the Cellar members are qualified for Jr. Firefighter Badges. But remember folks, if you work the siren, you can't ring the bell on the same run.

[FireFightingMan]

Originally Posted by xoxoxoBruce

if you work the siren, you can't ring the bell on the same run.

Actually, the "Siren" is called a "Cue".

[fredengle]

well what if instead of the nozzle i made the hose into a venturi?????

[fredengle]

you can get an aerating nozzle at fireade source .com

[xoxoxoBruce]

Holy crap, 523,000 views on this thread!

Welcome to the Cellar, fredengle.
Fireman?