Why doesn't cold water cool down a gasoline fire?

It’s canonical that you should never use water to put out a gasoline (or oil) fire because doing so spreads the oil around, and the oil floats to the surface of the water and keeps burning.

What I don’t get is why the coldness of the water doesn’t put out the fire by taking away its heat (assuming we’re talking a very thin film of oil, not some foot-thick sludge of oil). Doesn’t that cold negate heat somewhere?

While the cooler temperature of the water can’t hurt … it just doesn’t make very much difference.

What is burning in an oil fire are the volatile fractions which have evaporated off the oil surface. Some of these fractions have low boiling points and to an extent are insulated from the surface temperature by the film of less volatile oil.

Water quenches fire because 1) it cools and 2) it smothers.

Because the oil is on top of the water it’s supply of oxygen to the fire isn’t interrupted.

Water doesn’t stop combustion by “cooling” it; it smothers the ability to the ability of the substance on fire to combine with air. Solid substances burn by off-gassing volatiles which energetically combine with oxygen in the air, and a substance like wood or cloth is saturated with water it prevents it from off-gassing. In the case of gasoline, oil, or other substances that don’t readily mix with water, putting water on it while burning causes the fluid to sputter off or float to the top and keep burning, even spreading the burning substance farther.

I was once in a situation where I had a cast-iron pan of burning oil and no lid or baking soda (don’t ask) and carried it outside and dropped it on an almost freezing, lightly mist-covered concrete patio, (fortunately not catching me or anything else on fire). Despite the water being a slushy layer, the cold water didn’t put the fire out; the oil just spread it out into a big sheet that completely consumed itself almost instantaneously, leaving no oily residue nor doing any damage or leaving a mark on the patio due to the Liedenfrost effect.

Water is used by municipal fire trucks because it is readily available and good for putting out house fires, but you’ll notice that airport fire trucks us fire-suppressive foam or gel because they are most concerned about burning jet fuel, because the foam or gel mixes with the kerosene and prevents it from allowing it to volatilize.


Interesting, I see…

I had always thought the purpose of pouring water onto a wood or paper fire was to cool it down, hence I couldn’t understand why oil flames were “exempt” from being cooled down this way.

Fire authorities propagate simplistic rules that they believe to be of widest general application. It is simply incorrect that water cannot put out oil or gasoline fires.

The Kuwait oil fires after the first gulf war were extinguished with nothing but sea water.

When I was in my late teens my best friend’s vintage car suffered a gasoline fire when it burst a fuel line outside my parents’ house. It had an electric fuel pump. The burst line caused the engine to stop and my friend and I jumped out, not realising that he had left the ignition on. In our panic, by the time we realised that the loud whirring from under the hood was the fuel pump, and why it was still running, flames had got through the firewall and it was no longer safe to reach back into the car to turn off the key. Consequently, the electric fuel pump just kept pumping fuel into the inferno till it eventually melted.

When the fire brigade arrived they put it out - by then a raging gasoline fire - using water hoses. They also told us that if we had acted quickly, while the fire was small, we could have snuffed it out using just a garden hose set to a wide spray.

Water does work on an oil or gas fire by cooling and cutting off air, but you need a lot of it in a wide spray - sufficient to snuff the whole thing at once (or effectively sweep across it) so it doesn’t keep relighting itself. And the danger is if you try this without sufficient water to snuff the fire, you will just spread the oil or gas and make it worse.

I was taught how to put out an oil fire using water. In a confined area, ship’s engine room. You start on one side and push the fire away from you, you spray the fire and keep pushing it away and into a corner. It has to be done skillfuly and carefully. But it takes a lot of water. If you do not use enough water then the oil can just float on top of the ware and it will spread.

Think about sea battles. When a ship is sunk in gun fire the oil in the fuel tanks gets out of the tanks and floats on the top of the ocean and can be set afire. that is why there was so many burn sailors at Pearl Haarbor. The ended up in oily water that was burning. Think of the the WWII Merchant Sailors that had their ships sunk by u boats.

You can put out an oil fire with water if applied right. Ig applied wrong it can just spread the fire.

The coolness of the water isn’t remotely “cold” enough to put out a fire. You can pour flammable liquid on ice and still light it.

For a related, fun and dangerous example of heat vs cold, I refer you to: Toaster vs Freezer.

It’s not the coolness it’s the specific heat.

When you hit a fire with water, the water doesn’t go away till it absorbs sufficient heat to boil and turn to steam and fark off. And as we all know water is dense and the amount of energy required to turn water into steam is known in scientific terms as a “Metric Farking Buttload”.

When you light a fire on ice the fire is not heating up - or cooled by - the ice beneath it (much). When you spray water on a fire, the spray is falling on the fire and may well absorb its heat and cool it below the temperature at which combustion will continue.

Remember, fire needs three things to continue burning:

  • Oxygen (usually)
  • Heat
  • Fuel

Removing any one of those will extinguish a fire.

But, as noted, oil/gas floats on water so mostly the water will just go under the gasoline and do nothing.

That said, I think when you see firemen spraying water in a wide fan they are attempting to lower the temperature.


Versus this:

There seems to be a fair bit of not-quite thereness to this…

I was an airport firefighter for a long, long time, and now I’m a fire protection engineer.

Flammable liquid firefighting using water based agents (plain water or foam) is concerned with doing two things: vapor suppression and cooling. Water is useless for vapor suppression on nearly all flammable and combustible liquids, as they nearly all have specific gravities less than 1 (they all float). The water sinks to the bottom, and gives the flammable liquid a much nicer surface to travel on.

Water does cool the liquid down. If you can cool a flammable or combustible liquid to below its flash point, the fire will go out. Diesel, kerosene, and Jet A all have flash points in the 105-120F range. Put a spray of water across the surface, the fuel cools down, and the fire shuts off (the same thing happens with a solid fuel - cool to below its ignition temperature, pyrolysis ends, and the fire goes out). This isn’t quick, and the larger/deeper the fire the more difficult it is to make it work. As stated above, water absorbs a metric buttload of heat.

If you direct a straight stream of water into a liquid fire, it churns the fuel up and lifts it into the air, giving the burning fuel more surface area and the fire gets bigger.

Gasoline has a flash point of -44F. You can’t spray water on it to cool it down enough. Water will not put out a gasoline fire (you can sometimes push the fire off the surface of a small gasoline fire, but it usually reignites).

Foam (specifically aqueous film forming foam [AFFF]) is a mixture of water and a surfactant. The surfactant allows the liquid foam solution to maintain a bubble structure when its air aspirated. The surfactant also electrically polarizes the water so that it floats on the surface of the fuel, but as it’s an electrical thing at a molecular level, it’s a very thin layer of water. When this bubble matrix is laid down on the burning fuel, the water leaks out of the bubbles and floats on the fuel beneath.

Foam works in a few ways. It cools like water does (because its 97% water [well, its 94% to 99%, but that’s another thread). More importantly, the floating layer of water suppresses vapors (it puts enough weight on the fuel that it overcomes the fuel’s vapor pressure) and/or separates the fuel from the oxygen in the air. The aqueous layer of water travels super quickly ahead of the bubbles, and gives crazy fast knockdown of the fire. The rapid knockdown is why foam is the agent of choice for airport fire departments.

The downside of the foam layer is that the bubbles lose their trapped water in 3 to 5 minutes. The water that collects on the surface of the fuel, once the layer gets too thick for the electrical thing to hold up, falls through the fuel and acts like normal water. In those 3 to 5 minutes, there isn’t enough of a layer to suppress vapors anymore. Now you need to add more foam.

Extinguishment gets more complicated based on the surface the fuel is on. Oddly, fuel floating in water is the easiest. Pavement isn’t bad. Fuel on soil is horrifyingly difficult to extinguish.

Depth is also a factor. The deeper the fuel is, the longer it generally takes to extinguish. There is also an autorefrigeratiom effect that takes place at the surface. As the fuel vaporizes, it pulls some of the heat from the fuel just below the surface, so fuel temperature below the surface tends to stay pretty stable. Under certain circumstances, fuels in storage tanks can get a heat wave that travels downwards and can cause other problems.

The fluorinated “forever chemical” crisis is part of the surfactant thing for AFFF. Newer fluorine free foams (F3) don’t seem to do the liquid layer thing, and rely solely on the blanket of bubbles to suppress vapors and separate from oxygen. Interestingly, the new foams are working the same as the old protien foams from pre 1970, which were made from fish guts and cow hooves (seriously). Whats old is new again.

As an interesting effect, you can keep all three and still extinguish a fire, by stopping the reaction. That’s what Halon does.

As a relative n00b to fire service (I am a volunteer ff, got my FF1 a little over a year ago), may I ask you a question?

As you may know, there is a push now to replace the AFFF due to it’s horrifically bad for you cancer causing effects.

And for a while, the suggested replacements weren’t as good, but you indicate that the new F3 stuff is decent. Was there any known health effects to the pre-AFFF stuff you allude to that was protein based?

Thank you!

Protein foams, theoretically, were non toxic. They were made from animal parts, just not quite food-grade. The stink was indescribable. And if you got it on your gear, it was never coming out (my first coat - neoprene liner with a cotton duck shell and corduroy collar) met its demise from a foam pail mishap.

To keep the concentrate from rotting away, there were some sort of preservatives in the mix. I have never found what exactly they were. It gave the concentrate a shelf life, in an unopened container, of about 5 years. For comparison, AFFF was more than 20 years. If you put protein foam in a truck’s tank, it would turn into gelatin in a year or so. Any time it was used, you had to flush the foam system with fresh water for a long time to clear the foam out. Airport fire trucks still have a flush mode for that functionality.

Protein foam eventually shifted into fluoroprotein foam. If it doesn’t do what you want, add fluorine and it works great. I never got to use fluoroprotein, my department jumped from protein to AFFF in the early 1990s. I understand it was good for subsurface injection in vertical storage tank fires, but I have had zero experience with it.

Protein was a cast-iron you know what to apply. It was wicked finicky, and it did not tolerate any mistakes. You needed a perfect blanket of bubbles, any holes and the fire came back through (and would eat the rest of the blanket). AFFF is extremely forgiving, and super fast.

The new F3 foams are passing the ICAO C foam performance test, which is pretty impressive. To my knowledge, none have passed the MIL-F24385 mil-spec test yet, which is what the FAA requires for airports. That’s the platinum standard for foam, and only AFFF was able to do it. Give it another year or two, I think there will be some mil-spec F3 foams.