Mad About Science: Managing fire

By Brenden Bobby
Reader Columnist

Fire has been humankind’s greatest ally and adversary for at least 300,000 years.

It’s easy to believe that dousing a runaway flame with water will extinguish it, but this isn’t always the case. We have discovered that fire behaves differently in the presence of certain chemicals, and extinguishing one form of fire will not work for another type — in fact, adding water may make certain kinds of blazes far worse.

Fire requires three things in order to exist: oxygen, fuel and heat. Think of each of these as the legs of a tripod. So long as all three of these legs are standing, they support each other and the fire on top. If you can kick out just one of these legs, the fire won’t survive for long. To help identify how best to sweep the legs out from under a fire, people have developed classes to help manage ways to extinguish them. There are five classes of fire: A, B, C, D and K.

Courtesy photo.

Class A fires are the most common blazes, which are created when fuel sources like wood, paper, fabric or certain plastics feed the flames. This type of fire at a small scale can be doused with water. The water soaks the fuel source and creates a barrier between the fuel and the oxygen while also reducing the amount of heat present. Larger blazes may prove to be too much for water to handle. In the case of major structure fires and wildfires, the primary goal of firefighters shifts from stopping the fire to preventing it from getting larger. If firefighters can block the fire from accessing more fuel, it will rapidly burn itself out on the fuel that it’s already consuming.

In the case of a large-scale wildfire, there is simply too much fuel present to douse with water. Instead, firefighters move to cut off the fuel source through brute force, by cutting fire lines through the forest, using the topology of the wild terrain to their advantage. If a fire is moving up a mountain that’s jagged and rocky on the opposing side, the crews can force the fire to run up to the edge, run out of fuel and starve itself into self-extinguishing, effectively kicking the “fuel” leg out from the giant tripod.

Class B fires are trickier to manage. These fires are fueled by oil and petroleum products and cannot be controlled by water. Adding water to a gas fire will actually make this fire much worse. Oil and water repel one another, so spraying water on an oil fire will just make it spread more quickly as the oil moves on top of the water, similar to the Leidenfrost effect you see on a hot pan (that is, when water droplets skip around on the pan surface and are slower to evaporate than if exposed to lower temperatures). 

In some rare cases, militaries around the world have used explosives to stop blazes, particularly Class B fires in oil fields and refineries. While the heat is still present from the blast of the explosion, certain types of munitions will create a vacuum effect that momentarily sucks away all of the oxygen from a localized area, instantly snuffing the flame. The Swedish Air Force attempted this recently on a wildfire and, while it worked, it was understandably not a technique that you would use near human habitation.

Class C fires are a unique challenge of their own, as they are caused by electrical equipment. Dousing this with water could cause the electricity to travel through the conductive water and kill whomever is attempting to quench it. These fires are put out similarly to Class B fires, utilizing a blast of carbon dioxide to push out the oxygen and replace it with CO2 to suffocate the flame. Preferably, first responders can cut the power to whatever malfunctioning device is causing the fire, or it can start up again as oxygen begins to push out the carbon dioxide.

The typical red fire extinguisher you see is often an ABC Extinguisher, filled with monoammonium phosphate that coats fuel in a nonflammable material and separates the fuel and the oxygen. This extinguisher will not work on a Class D fire, however.

Class D fires are chemical fires triggered by certain metals like magnesium, titanium, sodium and potassium. Magnesium fires burn white, and if you see a pure white flame then you know it’s time to get out and call help. Metal fires burn very hot and very fast, and Class D extinguishers produce a toxic gas when reacting with the burning metal. This is most commonly seen in machine shops where tiny flecks of metal “dust” coat surfaces and can be ignited by rogue sparks. This can also happen in trucks and train cars that are carrying these types of materials during a crash.

Finally, Class K fires are from oil and grease and most frequently occur in commercial kitchens. Similar to a Class B fire, spraying water on a grease fire spreads out the grease and oil, heats up the water and causes the blaze to literally explode. Class K extinguishers have a special chemical that quickly combines with acids, creating a foamy layer on the surface of the grease to choke out the flame.

If you’re looking for some ways to guard your home from wildfires this summer, you should stop by the Sandpoint library on Saturday, March 25 from noon-1 p.m. as we host Sean Mitzel of Integrity Fire Protection. He will share some tips for landscaping for wildfire mitigation to keep your home safe. The present is the best time to plan for the future.

Stay curious, 7B.