Another important piece in the foam delivery system is understanding the application rates of foam. This is specifically geared towards fuel spills that have ignited. In order to blanket the spill so that the foam blanket is not destroyed from the heat, we have to deliver enough foam at once.
To prevent confusion of various foam related vocabulary, let's take a look some definitions that may cause confusion.
Ratio is meant to describe the amount of concentrate to water. For example, a 3 percent ratio means that we are adding 3 gallons of concentrate and 97 gallons of water to create a foam solution that will be discharged out of the nozzle.
Application rate is the amount of foam solution that must be delivered to a fuel fire to readily extinguish it. The application rate must be able to deliver enough agent to absorb the heat being given off (otherwise the foam blanket will break down) and also provide the smothering effect discussed in previous articles. If the application rate is not delivered, the foam attack may fail and all that concentrate will have been wasted. We'll talk about the logistics associated with the foam attack later on, but for now it is important to understand that any attack must be able to deliver the needed amount of agent over a specific area within a specific time frame. The application rate differs for different types of fuels. This is an important part of the application equation, so know the specifics of your concentrates.
Like structural fire attack, a minimum flow rate is recommended. Even at a spill without fire, it is still highly recommended that the standard application rate be applied. The reason for this is based on our safety. If the spill ignites, we need maximum firepower to control the unexpected! Also, like in structural fire attack, have a backup hoseline in the event you lose foam during the incident. Remember, a backup line should flow equal to or greater than what the attack line flows. This may seem like overkill, but if it prevents one firefighter from being horribly burned and disfigured, then it was worth it!
Duration is the amount of time that the application of foam has to be delivered to create a finished foam blanket that will provide a barrier and prevent reignition. The length of time that this barrier will last before a reapplication is necessary is also dependent on the environment (such as wind), the temperature of the spilled product (hotter fuels will degrade a foam blanket faster), and type of fuel that you are interacting with. NFPA 11 identifies a minimum of a 15-minute application of foam at the required rate as a minimum typical for a spill (IFSTA Principles of Firefighting Foam covers this point in great depth.)
The amount of foam that's required is measured in terms of gallons per minute, per square foot. Each manufacturer has set minimum application rates for specific types of fuels. A good rule of thumb to follow is fires involving hydrocarbons requires between 0.10 gpm/ft2 to 0.15 gpm/ft2 and fires involving polar solvents can range between 0.10 gpm/ft2 0.24 gpm/ft2 . These numbers are very subjective though! Each manufacturer has very specific numbers for certain types of fuels and certain types of foam concentrates. Check the manufacturers web site for the concentrate that your department uses and see the listed application rates.
In addition, each type of foam concentrate has specific ratios with which it has to be mixed with water. You could have a foam concentrate that is rated as a 1 percent x 3 percent AR-AFFF. At 1 percent (1 gallon of concentrate mixing with 99 gallons of water), a major manufacturer's brand says that an application rate of 0.10 gpm/ft2 is required on a hydrocarbon spill, yet this same concentrate can also be used on the polar solvent Ethanol at a 3 percent ratio (3 gallons of concentrate to 97 gallons of water) and requires an application rate also of 0.10 gpm/ft2 .