This article is the third of four excerpts from a new book, Class A Foam - Best Practice for Structure Firefighters , by Dominic J. Colletti. Larry Davis is the technical editor of this 240-page educational textbook (© 1998 by Lyon's Publishing, Royersford, PA). Ordering information is available...
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Low-expansion finished foams produced by an air-aspirating nozzle, particularly on the lower end of the low-expansion range, are excellent for exterior structure fire attack. Each air-aspirating nozzle has its own gpm flow rate. For structure firefighting, be sure to choose the nozzle with the appropriate flow rate (such as 125 gpm). Many air-aspirating nozzles are made for low-flow wildland firefighting usage (such as 20 gpm).
Air-aspirating nozzles can be awkward for use in interior fire attack. Depending upon their length, they can be difficult to maneuver into position while advancing down a tight hallway or when making a sharp turn through a narrow doorway.
Medium-expansion air-aspirating nozzles are not suitable for structure fire attack because of their short discharge distances. Since these nozzles use most of the fire stream energy to create finished foam, little energy is left to propel the stream. However, in fire situations such as overhaul, where discharge distance is not an issue, medium-expansion foam nozzles produce a drier finished foam that works well to fill wall cavities and other inaccessible or poorly ventilated fire compartments such as basements, attics, and pipe-chases. The foam produced by these nozzles also works well to blanket burning rubbish and tire piles to cut off oxygen while wetting and cooling the fuels. Medium-expansion foam is excellent for the overhaul and mop-up of structure fires.
Caution! A misapplication of Class A foam that has occurred often is one in which firefighters use air-aspirating nozzles to apply foam to burning fuels such as baled hay. While the fuel is blanketed with foam, there is little penetration. As a result, the fire continues to burn under the foam blanket. We then hear "the foam did not work," when, in fact, the application was incorrect. In deep seated fires the use of a smooth-bore nozzle provides the fastest penetration and extinguishment.
Variable-gallon/constant-flow and automatic fog nozzles. One option used by many departments just beginning the implementation of Class A foam, is foam application through their existing fog nozzle. This is ideal in that it means little change in firefighting technique; essentially the same delivery rates and application methods used for water application are used for foam applications.
Fog nozzles aspirate foam solution because their streams draw in air after leaving the nozzle. Depending upon nozzle design and stream pattern, these nozzles produce low-expansion, sloppy foams with expansion ratios of about 2:1 to 4:1. These nozzles work well for a structure fire attack as the foam they produce clings to and then quickly drains out to penetrate gypsum wallboard and other interior surfaces.
Most firefighters are familiar with the idea of a fog stream directed through a window to ventilate an area during overhaul. Inside the building, a narrow fog stream sprayed through an exterior window moves air to remove smoke and heat from the interior. When we use a fog nozzle to deliver Class A foam, the same spray pattern acts to draw air into the foam-solution discharge creating a low-expansion, sloppy foam.
Diagram Courtesy of Ansul
This diagram shows how air interacts with Class A foam solution streams from air-aspirating nozzles (foam tubes) and non-aspirating (variable-gallon and automatic fog) nozzles.
High-expansion foam generators. The ever-increasing number of fires occurring in warehouses, storage buildings, basements and tunnels has firefighters coming across fire situations where the use of traditional tactics - the advancement of hoselines to deliver water or foam streams - are not effective because hose teams have trouble reaching the seat of the fire. Often, in these fires, the combustibles are Class A and B types, with hazardous materials sprinkled into the mix. In these situations, high-expansion foam generators offer an alternative fire extinguishing procedure that, when properly deployed in many instances, will automatically seek out and extinguish the fire at the source. The high expansion foam blanket acts to transport the water to the fire, suffocates and cools it, while suppressing escaping vapors and encapsulating particulate.
High-expansion foam generators deliver expansion ratios between 200:1 and 1,000:1. The foam is capable of totally flooding large rooms and enclosures allowing it to reach the source and effectively extinguish horizontal and vertical (three dimensional) fires. After the foam dissipates, there is minimal water damage to the fire compartment and contents because of the foam's low water content.