Legacy aqueous film-forming foams (AFFF) used in firefighting contain per- and polyfluoroalkyl substances (PFAS). The leading, commercially available PFAS-free foams (PFFs) do not form a film like legacy foams and rely solely on the foam blanket as a “mechanical” means for smothering a liquid fuel fire. Most foam approval test standards use air-aspirating discharge devices to apply foam to the test fire. The current military specification (Mil-Spec) for approving AFFF uses an air-aspirating foam discharge nozzle during the fire performance approval tests and does not consider or try to replicate the foam quality that will be produced by the various fielded discharge devices. Neither does the draft land-based Mil-Spec currently under development.
Since the new PFFs rely solely on the foam blanket to smother/extinguish the fire, foam quality is a key consideration/parameter in actual applications, which is being overlooked when considering these new PFFs as AFFF alternatives.
The objective of this project was to develop the link between small-scale approval test results and actual versus expected fielded capabilities. To make this link, the foam quality produced by the various discharge devices used by Department of Defense (DoD) needed to be characterized, and an understanding of the capabilities of these new PFF as a function of foam quality needed to be developed.
This project consisted of two tasks; a demonstration and characterization of the foam quality produced by the various discharge devices used throughout the DoD, and a demonstration of the capabilities of these new PFFs as a function of foam quality and flow rate using the Mil-Spec fire tests as the basis of this demonstration.
The focus of the analysis and discussion was based on the expansion ratio (i.e., the degree of aspiration). All of the commercially available PFFs are “foamier” (i.e., produce better expanded foam solutions) than legacy AFFF when discharged through the same device. The foam quality produced by over 30 discharge devices was characterized during this program.
The firefighting capabilities (extinguishment and burnback resistance) of the PFF were determined using a 28 ft2 pool fire test as a function of foam quality (expansion ratio). These tests were conducted using the exact same equipment and test personnel that typically perform the MIL-PRF-24385F approval tests.
The firefighting capabilities of each PFF were quantified over a range of expansions (2-9 expansion ratios) against two different test fuels (legacy unleaded zero alcohol gasoline and Jet A) with two foam solution flow rates 2 gpm and 3 gpm. The degree of foam solution aspiration was varied by partially blocking the aspiration opening on the standard Mil-Spec nozzle.
The new PFF were “foamier” than legacy AFFF. Specifically, the expansion ratios of the PFF were typically 1-2 expansion points higher than AFFF when discharged through the same device, under the same pressure.
The critical expansion ratio appeared to be about 4 to cover the two fuels used during this assessment. Gasoline fires were very difficult to extinguish using a PFF if the expansion ratio of the foam solution was below 4. Consequently, any situation where the design application rate was less than 0.11 gpm/ft2 and the foam expansion was below 4, was potentially problematic and warranted additional consideration/assessment, especially if the hazard was gasoline. If the hazard was a kerosene-based fuel like Jet A/JP-8/F-24, the critical expansion ratio was about 3 for typical design application rates.
For fixed fire suppression systems, potential areas of concern included systems that used straight orifice type discharge nozzles in general and deflector plate discharge nozzles if the hazard being protected was gasoline.
The general consensus of the large-scale tests as it pertains to foam quality was consistent with the findings of this program. It appeared that after an initial attack on the fire with a straight stream, the next steps would be to slightly increase the spray pattern angle to extinguish any remaining fires and to blanket the fuel surface with foam with a higher expansion ratio. An assessment of various discharge approaches, tactics, and potential hardware/nozzle modifications has been proposed and will be conducted as the third phase of the large-scale test program.
The data set provided a tool to estimate the capabilities of legacy AFFF systems and hardware during the transition from AFFF to these new PFF. Additional areas of research included specific nozzle selection and/or development if a legacy system/discharge device was determined to be inadequate during this program. With respect to manual firefighting, there was a need to explore enhancements to delivery equipment and/or improvements to application techniques and tactics to compensate for deployment of less effective agents.
Methods of improving the performance of PFF as they relate to foam quality and spray characteristics have been proposed as Phase III efforts: