by Grant Lobdell
During the second draft meeting for the upcoming 2021 edition of the National Fire Protection Association (NFPA) Standard for Low-, Medium- and High-Expansion Foam (NFPA 11), the technical committee approved the addition of the following in Annex A 188.8.131.52(second revision no. 51):
“The foam concentrate proportioning equipment and discharge devices produce finished foam with certain qualities, including expansion ratio and 25 percent drainage time. The testing conducted by the Fire Protection Research Foundation showed that expansion ratios of 7 to 10 were critical for SFFFs’ ability to extinguish fires.”
The testing conducted by the Fire Protection Research Foundation, which was compiled into a research paper titled Evaluation of the fire protection effectiveness of fluorine free firefighting foams, can be reviewed online for free at nfpa.org.
Why is SFFF expected to have a higher expansion ratio than AFFF?
Finished foam produced from aqueous film forming foam (AFFF) can certainly have an expansion ratio below 7 and still be deemed acceptable. The United States Department of Defense, for example, allows expansion ratios of finished foam from AFFF to be as low as 5.0 according to standard MIL-PRF-24385F. NFPA 11, though, is suggesting that SFFF will need to demonstrate a much higher expansion ratio to have an effective level of fire protection. To understand why that is, let us examine how each of these foam types works to extinguish a fire.
Both AFFF and SFFF work by blocking an ignitable fuel from an ignition source using the finished foam blanket. What happens after the foam blanket is applied, however, is different between the two. When a foam blanket made by SFFF drains, the entire drained solution will fall below the surface of the fuel simply because the water that makes up the bulk of the solution is denser than the fuel. When a foam blanket made by AFFF drains, however, some of the solution can remain on top. This solution that remains on top of the fuel is referred to as a film. Like the foam blanket above it, the film is another barrier to isolate the fuel from an ignition source. It’s an added layer of protection. With AFFF, both the foam blanket and the film are working to isolate the fuel from the ignition source. With SFFF, though, the foam blanket is the only barrier between the ignitable fuel and the ignition source. As a result, it needs to be more robust. There is not a film to aid in the smothering of the fuel.
Why is AFFF able to form a film but SFFF cannot?
One way to prevent the drained solution from a foam blanket from falling below the fuel is to lower the solution’s surface tension. If the fuel’s surface tension is higher than the surface tension of the solution and the interfacial tension between the two, the thin film will not fall through it. Instead, it will spread across it. This is exactly what occurs with AFFF. Currently, fluorosurfactants are the only surfactants known to lower the surface tension of a foam solution enough to form a film. Because SFFF do not contain fluorosurfactants by definition, they lack the film forming ability of AFFF.
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