A Closer Look at How Water Is Used in Fire Suppression
It’s more complicated than just “putting the wet stuff on the red stuff”
By Shannon Pieper

Globalization isn’t just a trend that affects businesses and trade patterns; it’s affecting the fire service as well. As the exchange of knowledge and information becomes easier, we are seeing more international sharing of ideas about fire suppression. One group dedicated to “firefighting without borders” is the International Fire Instructors Workgroup (IFIW), a loosely organized group of renowned fire instructors and researchers from the U.K., Sweden, Australia, Ireland, the United States and other countries.

Several members of the IFIW are teaching at FDIC this week, among them Stefan Sardqvist, PhD, a fire protection engineer with the Civil Contingencies Agency in Sweden. His class, “Water for Fire Suppression,” held earlier today, challenged some of the traditional U.S. ideas about fire behavior and fire suppression tactics. Prior to FDIC, I caught up with Dr. Sardqvist via e-mail to find out what ideas are being called into question and how U.S. firefighters can use information and research from abroad to experiment with their tactics.

Shannon Pieper: What’s wrong with the traditional (American) understanding of the fire tetrahedron?
Stefan Sardqvist: The four aspects of the tetrahedron are valid for a fire to start, but suppression is a different and simpler process: it is about heat and heat removal to a temperature low enough. This thermal model has been validated in many situations. The thermal model works like this: By adding an extinguishing agent to a flame, the agent is heated, thereby cooling the flame. The flames go out when the temperature falls below a certain level. When it has fallen enough, radicals can’t exist and the chain reaction is broken. This is why air can act as an extinguishing gas at over-ventilated fires, and fuel can act as a thermal ballast when there’s insufficient air. Any surplus gas in the flame acts as a thermal ballast, cooling the flame, and at sufficiently low temperatures, radicals can’t exist and the flames go out.

According to the tetrahedron model, a flame would go out at a specific concentration of fuel and oxygen. However, it takes about 30% carbon dioxide to suppress a flame, but 40% of nitrogen, giving different concentrations of fuel and oxygen at extinction. The thermal model can explain this, since carbon dioxide can absorb more energy than nitrogen.

SP: Why should firefighters believe that, in some cases, what they’ve always been taught is wrong?
SS: What you learned in the old days was right at that time, but the world has changed. Your recliner doesn’t look the same as your grandparents’ recliner, and they will burn in different ways. A fire looks quite different in a new room in comparison to a legacy room. Also, the tool box has changed due to technological development—it’s up to you whether you want to use the old tools or the new, safer and more efficient tools. Finally, the demands have changed. A key responsibility of the modern firefighter is to get back to his family after work.

SP: Briefly explain the fog vs. solid stream debate.
SS: When we use water on a fire, the water evaporates, absorbing heat. There are two common ways of doing this. The first is by cooling the fuel surfaces, shutting off the gas supply to the fire. However, this requires a direct hit from the nozzle with only a small amount of water. The problem is that you have to reach the target, meaning that the firefighters have to come close to the fire. Another problem is if you use too much water, you will get steamed.

The second technique is to cool the smoke. A properly applied water spray may cool smoke enough to prevent or even knock down a flashover, which then enables firefighters to approach and reach the fuel surfaces. When water converts to steam, it expands by a ratio of 1,700:1, creating pressure in the room. The interesting thing with cooling smoke is that if you evaporate water in hot smoke, the shrinking effect of the cooling is greater than the expansion the smoke, so the increase in pressure is nullified and may even be negative. You will notice that the smoke contracts and the heat impact is less.

The focus has for a long time been on how much water the nozzle delivers. However, as the heat absorption in the fire gives the suppression capacity, a more interesting question would be, how much water  actually evaporates in the fire? I ask this because the rest is a waste. Water on the floor has used, at the most, 10% of its suppression capacity. By increasing the share that evaporates, you can decrease the flow in the hose while having the same suppression power. By using a spray pattern and smaller droplets, rather than a solid stream, the water is more easily evaporated in the gases and is more evenly distributed over a fuel surface. Besides, it gives a hose more maneuverability.

Using the resources in the most effective way, regardless of technique and tools, the flow rate given from fire department statistics is around 9–15 l/m²min (0,2-0,4 gal/ft²min), measuring the horizontal surface of the fire area.

SP: Most firefighters and fire departments in the U.S. aren’t going to be ready to just accept dramatic changes in their traditional tactics. Have you sensed resistance to these ideas? How are you attempting to educate and change behavior?
SS: The starting point in any change is asking yourself: What am I doing, why am I doing it and is it good enough? After that, you can ask if anyone out there has solved a similar problem in a good way. I would ask myself if the methods that we use and teach give the best result in terms of minimizing the damage from the fire and from the firefighting.

When you discuss the “best” method, you meet resistance. However, if you discuss how things work, and if you can explain unforeseen phenomena, most people can find pros and cons with different methods for themselves. New ideas should be questioned, but the important thing is to maintain a constructive debate. There are progressive departments in the U.S. asking why they do things the way they do. In fact, the use of PPA and a lot of the tactical thinking in Sweden originally comes from U.S. departments.

Shannon Pieper is senior deputy editor for FireRescue magazine.

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