Pulsing the Knob A Technique to Help Prevent FLASHOVER
You are on the nozzle of first hose line to enter a burning house. You and your partner are crawling down the hallway, staying low to avoid the heat. There is an open door at the end of the hall, and there is a fire dancing around in that room. The closer you get to the fire room, the hotter it gets. You feel heat soaking through your coat, and a very intense itching/burning sensation through your nomex hood. Suddenly, everything around you is orange. You have just been caught in a flashover. Instinctively, you open up and drown the fire, ending the flashover. However, you and your partner are now steam burned, and any chance of survival for civilians has just dropped dramatically. How could you have prevented the flashover? In this article, I will discuss a technique to cool the gases in a fire compartment using a fog nozzle. At the same time, when this technique is used properly, the smoke level will be raised allowing us to see better, heat will be reduced and fresh air will be increased at the cooler levels, all giving any trapped civilians a better chance to survive
First, some quick review. Fires all start small. They grow exponentially as time goes on. And, they put off a lot of gases, also called smoke. The smoke gases are actually unburned products of combustion. For the purposes of this article, I will refer to smoke as fire gases. These gases, of course, are extremely hot. For whatever reason, they did not get completely burned in the fire.
As we all know, heat rises. When it hits a ceiling, then it spreads laterally, carrying heat with it. When the fire is in a closed compartment, this lateral travel will stop, and then the gases will start banking down. Meanwhile, the fire keeps growing and producing even more heat. The hotter gas layers will be at the ceiling, and the cooler gas layers will be at the bottom of the plume. As the gases spread and drop, everything they contact becomes heated. Over time this heating can & does cause anything combustible to reach it’s ignition temperature.
As the fire continues to grow, areas away from the main fire will experience sporadic ignitions of the gases. This is called rollover, and it can last for a few seconds at a time. These gases can also ignite at the entrances to the fire compartment. This is because the hot gases ( now remember, there is heat and fuel, two of the three sides to the fire triangle, already present ) have now found a fresh supply of oxygen.
Ok, now, how do we prevent flashover? All we have is our charged line and a fog nozzle! I won�t go into hydraulics, flow settings, pump pressures, or anything like that here. Hopefully your set up will be within normal operating procedures that meet NFPA guidelines.
You may think this is crazy. I know I did when I first heard it. I advocate using short pulses, putting a fine mist into the gases. I first heard of this while surfing the web one night in the fall of 1999. I was at www.firetactics.com and read about it. It seemed ridiculous at first. Putting short bursts of mist into the hot gases. C�mon, that�s almost no GPM at all! Everyone knows that big fires need big water, and a flashover condition is about to become a bigger fire, so why use a small flow? Well, it does work. I kept on reading, and I�ll summarize it for you.
Putting a mist into the gases allows the hot gases to absorb the water. As the water is heated, it will reach it�s boiling point. It is at this boiling point that the water gets converted from a liquid form to a gaseous form. This conversion is where the bulk of the heat gets absorbed by the water. Put a pulse of water mist into the hot gases about once a second.
To make the pulse, just barely crack the nozzle. As soon as it is open, close it again. The pulse should be a quick one, meaning no more than a half of a second, no more. Remember, when a fog nozzle is flowing, the stream itself will bring in air currents from behind the nozzleman. These air currents are frequently fed from the hot gasses. This will crash the hot thermal layers down around you.
Try, at all costs, to NOT get the water on the hot surfaces (walls, ceilings, objects in the room, etc.). It is of utmost importance to only put the water mist into the gases. The water hitting the hot surfaces will cause additional, non-productive steam to be formed. (Remember that 1,700:1 expansion ratio?) This non-productive steam will collapse the thermal layers down on your position, cause painful steam burns, and generally make a good firefighting operation go sour for a while.
By pulsing a mist into the hot gases, the steam production will be minimal, and will be productive. Keep misting for a few seconds, and the temperature of the hot gases will start to reduce. Cooling these hot gases will keep them from lighting up. Makes sense, doesn�t it? Ok, keep pulsing, and start to move in, pulsing all the while. This will continue to cool the gases. This will make your approach to the main body of fire safer. Once you get to the seat of the fire, use your usual methods to extinguish it. Pulse on approach, blast on arrival! I love straight streams!
As an added benefit, as the gases cool, they also contract. It�s true! I tried this at a practice fire in April of 2000. We let it get cooking, to the point of flashover, and started pulsing the heck out of the fire. Not only did the rollovers stop and the flashover never happened, but the smoke level rose about eight (8) inches higher than it was when we started pulsing. We were then better able to see the base of the fire and extinguish it. So, we not only cooled the gases, PREVENTING FLASHOVER, we raised the level of the smoke approximately 8 inches. This allowed us to SEE BETTER. And, we weren�t as hot as we were a minute ago. AND by raising the gas levels, we just gave any civilians still inside more fresh air, allowing them a BETTER CHANCE TO SURVIVE!.
Pulsing has been researched heavily in Sweden and in the UK since the mid-1980�s. They have found, through scientific research and through actual combat firefighting that pulsing works, and works well. Here in America it is not that popular — YET! I think that if we are truly committed to fulfilling our mission of saving lives and property from fires, then we, as the American Fire Service, need to research and implement pulsing. It prevents flashover and provides a cooler atmosphere for firefighters to get in to attack the base of the fire. And that equates into a safer operation!