As compared to other firefighting agents, water is considered the most common and best due to its plentifulness and inexpensiveness of resources. Many believe that water is also an effective means of combating heat and flame in fire zones. Despite all the advantages, water does have its limitations, too.
For instance, in many areas water is not that easy to obtain or have access to. Most of the time we waste more water than needed when fighting fires because it is rare to produce accurate penetrating streams that can always reach the desired target. Besides, too much preparation and hard work are needed before you can obtain adequate water for effective applications.
Firefighters have long been figuring out how to make firefighting tasks more efficient since the early 20th century. Experiments with wide variety of substances have been conducted to find a substitute for water or something to mix with it, so that fighting Class A fires (those caused by ordinary combustibles) or Class B fires (those caused by flammable/combustible liquids) can be more efficient.
According to a report published in 1999 by the National Institute of Science and Technology’s Building and Fire Research Laboratory, seven agents met US Forest Service specifications for fighting Class A fires. The important report concludes that these agents, added to water, can improve their exposure protection on vertical wood surfaces, making it easier and more quickly to extinguish Class A and piled fires. Meanwhile, the amount of mixed solution (foam) needed to extinguish test fires is only one third of plain water.
Foam is a solvent made up of water, concentrate, and air when mixing in specific proportions. Among these three ingredients, the temperatures or quality of water seem not to affect the firefighting capability of foam except that it tends to hamper foam generation and affect concentrate’s long-term effectiveness. Most firefighters would preload the fire truck’s water tank by adding foam concentrate (batch mixing) or batch mix the foam into the portable tanks directly.
For Class A solution, standard firefighting nozzles (smooth-bore or combination nozzles) can be used because air is added in smaller proportions to create a wet form providing rapid target penetration and protective fog streams best suited for fire attacks and overhaul on a fire scene.
Air-aspirating nozzles can also be added to increase the foam expansion ratio because the drier foam created can work much better for fire attacks and exposure protection, except that it might cause shortened nozzle reach. On the other hand, the foam with greater expansion ratio might be more easily affected by winds on the fire scene.
Low-expansion foam nozzles provide little protection from radiant heat, causing safety concerns while conducting interior attacks or Class B attacks.
Medium-expansion nozzles, though used mainly for overhaul and exposure protection, can blanket the hazard when the foam must be discharged a short distance for has low expansion. For applications of compressed air foam system (CAFS), a full-flow ball valve or smooth-bore nozzle can get the job done because larger discharge usually causes drier foams.
Detailed information on Class A foams can be obtained from NFPA 18, 18A, and 1145, which further specify diverse aspects of the agents on the market as well as their applications.
