Quote:
|
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.