Steam is both the firefighter’s enemy and ally. It can burn us by penetrating anything that does not have a moisture barrier, such as our hoods, or by entering any gaps in our gear that expose skin. There is even research ongoing at NIST to determine if it can get past intact PPE, or be formed when our sweat vaporizes (http://www.nist.gov/pml/div685/grp01/firefighters-steam-burns-20150...). On the other hand, the tremendous amount of heat that can be absorbed when water transitions from a liquid to a gas - becomes steam - makes it an exceptionally efficient extinguishment agent, allowing for the application of a relatively small amount of water to significantly reduce the heat of a fire.
Anyone who has experienced the pain that ensues from steam exposure will take great care to avoid it in the future. It is not surprising, then, that causing burns from steam is a concern of those who question the wisdom of flowing water into a fire compartment from the exterior, one of the recommended methods of accomplishing the "C" (Cool from a safe distance) in the SLICE-RS game plan. While a real danger, I will review why the creation of steam is not a reason to delay extinguishment until interior hoselines can be positioned.
First, some myth-busting: Despite what many of us were taught in “fire school”, we do not rely on steam to “smother” a fire. Yes, water expands to 1700 times its volume when it vaporizes, which can decrease the rate of combustion by reducing the concentration of oxygen, especially in an enclosed compartment. But it is primarily through temperature reduction, the removal of the heat side of the fire triangle/tetrahedron, that it accomplishes extinguishment. That is why the European technique of “pulsing” fine water sprays into a hot smoke layer, which is already oxygen-deficient (or else it would be a “flame” layer), allows a crew to advance into a burning building, essentially cooling their way to the seat of the fire. More importantly, like every other gas, smoke contracts when its temperature is decreased. Therefore, despite the expansion of the steam that results from flowing water into a room on fire, the tremendous cooling that also occurs can result in a reduction of the volume of smoke (and steam) within. (For a detailed and expert discussion of the science behind these statements, I would recommend the blog of Ed Hartin, MIFireE, CFO at http://cfbt-us.com/wordpress/?p=1212)
True, steam is potentially more injurious than “dry heat”, with its ability to transfer that same energy absorbed from the fire back to any surface (like skin) upon which it condenses. (Bear in mind that our skin’s threshold for pain is 111F/44C, and it sustains permanent damage at 162F/72C, well beneath the condensation temperature of steam of 212F/100C.) Realistically, though, with temperatures inside a burning room and communicating areas reaching well over 1000 degrees Fahrenheit, the ill effects of humidity are likely negligible in comparison. Even our PPE cannot tolerate such extremes (the polycarbonate of our SCBA masks begins melting at 446F/230C, while our turnouts start to char above 572F/300C), and we can be driven out by even lower temperatures once our gear becomes heat-saturated and is no longer an effective insulator.
Finally, whatever fears and trepidations that may linger regarding steam, it is something we just need to deal with. A natural product of combustion even before we arrive and open our nozzles, it is an inevitable result of utilizing water as our fire extinguishing agent, and will occur whether we are operating streams from the interior or the exterior of the building. (I am aware of the mistaken belief that interior streams can effectively direct products of combustion away from uninvolved areas, and I will address that misconception in a future post, so hold those thoughts.) The best we can do is limit steam's contact with any occupants within a structure, including ourselves. This is accomplished by keeping out of flow paths, and the handiest tool for doing so is a door. Occupants need to have a closed door between themselves and the fire in order to remain protected, at least temporarily, from the products of combustion. This, of course, will need to occur long before our arrival in order to be effective. For our part, we need to close doors, at least most of the way, behind us as we enter, in order to avoid creating new flow paths, especially any we might find ourselves within.
Live fire experiments have repeatedly demonstrated the cooling/contracting effects of water flow on a fire. In no instance was there any increase in temperature that would have resulted from steam expansion "pushing" the products of combustion out of the area involved in fire. Admittedly, despite the dramatic interior temperature reductions demonstrated from water flows, initial effects fall short of creating survivable conditions for unprotected occupants, and merely allow a more rapid entry of firefighters to search and complete extinguishment. Steam, like other products of combustion, will follow a flow path from the seat of the fire to an exhaust site. Cooling those products and/or closing off that flow path are our best options for protecting victims, and the sooner we can accomplish that, the better.
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