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This change was first made in 1954 at the Fire Service Institute at Iowa State University by Keith Royer and Bill Nelson. These two men created a formula based upon two scientific facts. The first fact is that the expansion ratio of liquid water to steam is 1/1,700 (1/1.700) at 212° F (100° C). At 212° F (100° C) liquid water is transformed into a gas (steam), and one cubic foot (cubic meter) of liquid water expands instantly to 1,700 cubic feet (meters) of steam` This blast of steam smothers the fire if the fire is confined by a ceiling or roof. Depriving the fire of oxygen immediately stops combustion, hence it controls or extinguishes the fire.
How does this process work to cool a fire? This physical process of transforming a liquid to a gas is an endothermic (heat absorbing) process. In fact, for water, steam absorbs a tremendous amount of heat—968 btus per pound of water (2.257 J/g (Jules/gram)). This is far greater than that of any other substance that could be used to fight a fire. This heat must be retained by steam, otherwise it condenses back to liquid water. Also it is important to remember that this process does not raise the temperature of steam above 212° F (100° C). The capacity of water to absorb heat to create steam is called the enthalpy of vaporization of water.
Firefighters can operate in such a steamy atmosphere fully protected without being harmedSteam is highly effective in fighting fires because the temperature of 212° F (100° C) is well below the temperatures needed to start a fire. The ignition temperatures of hydrocarbon fuels range from 400° F (204° C) to 500° F (260° C) and upward. After ignition fire temperatures range upward to 1,000° F (537,7° C) at flashover and on upward. When steam is created in a confined fire, the temperature rapidly drops in less than one minute to around 300° F (148,8° C). This temperature is below ignition temperatures but still above 212° F (100° C), the temperature at which steam is created. Firefighters can operate in such a steamy atmosphere fully protected without being harmed.
Royer and Nelson created the confined structure fire formula by starting with the liquid water to steam ratio of 1/1,700 (1/1.700) cubic feet (meters) of steam, that is, one cubic foot (meter) expands to 1,700 cubic feet (meters) of steam. Since one cubic foot contains 7.48 gallons, dividing
| 1,700/7.48 = 227 |
This is the number of cubic feet of steam produced by one gallon of water. Royer & Nelson rounded 227 down to 200 to allow for a 90% conversion rate of liquid to steam. Thus the Royer-Nelson r.o.f. formula is:
| Gpm x t = (Vol/200) |
Let’s use this formula to find out how much water is needed for a confined structure fire. A small room on the average contains about 1,000 cubic feet (28.300 L or 28,3 cubic meters)
| Gpm x t = (1,000/200) | Gpm x t = 5 |
Just five gallons (18,9 liters) is needed to fill this fully involved 1,000 cubic feet (28.300 L) room full of steam. At a r.o.f. of 30 Gpm (113,5 Lpm), this fire can be controlled in a time of
| 30 x t = 5 | 113,5 Lpm x t = 18,9 |
| t = 5/30 | t = 18,9/113,5 |
| t = 1/6 = 0.166 | t = 1/6 = 0.166 |
One sixth of a minute equals 10 seconds (60/6). Thus the information that we have is that a r.o.f. of 30 Gpm (113,5 Lpm) applies 5 gallons (18,9 liters) of water in 10 seconds. This can be done easily using a fog nozzle and the combination method of attack. It is easy enough to use this formula to calculate how much water is needed for a 2,000 cubic foot (28,3 cubic meters) or for any other size fire.
