What are firefighting foams?
The National Fire Protection Association (NFPA) 11 – Standard for Low, Medium and High Expansion Foam defines firefighting foam as “…an aggregate of air-filled bubbles formed from an aqueous solution which is lower in density than flammable liquids. It is used principally to form a cohesive floating blanket on flammable and combustible liquids, and prevents or extinguishes fire by excluding air and cooling the fuel. It also prevents reignition by suppressing formation of flammable vapours. It has the property of adhering to surfaces, whichprovides a degree of exposure protection from adjacent fires.”
Or put more simply… foam is used for the suppression of fire and can extinguish flammable liquid or combustible liquid fires in four different ways:
- separates the flames from the fuel surface
- retards vapour release from the fuel surface
- cools the fuel surface and any surrounding metal surfaces
- excludes oxygen from the flammable vapours
How is foam made?
Finished foam is a combination of a foam concentrate, water, and air. When these three components are brought together in the proper proportions and mixed, foam is produced.
To be effective, good foam must contain the correct blend of physical characteristics:
Knockdown Speed and Flow – this refers to the time required for the foam blanket to spread across a fuel surface or around obstacles in order to achieve complete extinguishment
Heat Resistance – foam must be able to resist the effects of heat from any remaining fire from the liquid’s flammable vapour or heated objects
Fuel Resistance – effective foam minimises fuel pick-up so that the foam does not become saturated and burn
Vapour Suppression – a vapour-tight blanket must be capable of suppressing the flammable vapours and minimise the rise of reigniting
Alcohol Resistance – foam blankets are more than 90% water. Because of this, foam blankets that are not alcohol resistant will not last very long
Foam Classifications and Types
Each type of firefighting foam has its applications ranging from wildland and structural firefighting to industrial high-hazard, high-risk applications found in Aviation, Chemical, Defence, Energy, Marine, Mining, Oil & Gas, Petrochemical, Pharmaceutical, Pipeline and Solvents & Coatings industries.
Firefighting foams are divided into two classifications Class A and Class B.
Class “A” Foam
Developed in the mid-1980’s, Class A foam was predominately used for wildland fires but as their popularity grew throughout the 1990’s the use of Class A foam was expanded for use on structural fires. Class A fires consist of ordinary combustible materials, such as paper, cloth, wood, and plastics. These types of fuels require the heat-absorbing (cooling) effects of water or water solutions.
Class A fires consist of two types: flaming combustion involving gases which result from the thermal decomposition of the fuel.
The second type is deep-seated or glowing. This type represents combustion within the mass of the fuel and has a slow rate of heat loss and a slow rate of reaction between oxygen and fuel.
- mop-up: 0.25%
- initial suppression: 0.5%
- fire brake: 0.75%
- protection : 1.0%
Note: the minimum admixture rate on other Class A fuel types are 0.1%.
As a synthetic based foam concentrate, Class A foam is applied at low concentrations ranging from 0.1% to 1.0%. Cooling and wetting are the primary extinguishing mechanisms. The use of Class A foam makes “water wetter” on average increasing the effectiveness of water tenfold.
These application rates make the use of Class A foam a cost effective means of combating fires because smaller amounts of foam concentrate can be used to make effective foam and is biodegradable and non-toxic, so it is environmentally sustainable. Class A foam is deployed through a variety of portable and fixed appliance devices ranging from firefighters’ backpacks, brush and fire apparatus, to rotary and fixed wing aircraft.
Class A Foam Mop-up Use Wild-land Class A Foam Structural Firefighting
Class “B” Foam
There are several types of available foams. Each foam concentrate is developed for a specific application. Some firefighting foams are thick and form a heavy, heat-resistant covering over a burning liquid surface. Other types of foams are thinner and because of that, they will spread much more quickly over the fuel surface. Still, other types of foams will generate a vapor sealing film on the surface
of the fuel. Additional foam concentrate types, such as medium and high expansion foams, can be used in applications requiring large volumes to flood surfaces and fill cavities within the hazard.
Firefighters Train on a Simulated Industrial Complex
Evolution of Class B Firefighting Foams
Firefighting foams have been on the market for almost 100 years in various types. Firefighting foams started with chemical foams, at each of these steps along the way, performance and safety of these various agents improved on the prior foam agent types/concentration.
Early 1900s – The first firefighting foams were chemical foams. They functioned by a chemical reaction from mixing two or more chemicals at the time of use, which created the foaming. More effective than water, but difficult to use and transport. Also, there was always the risk of improper mixing at the time of use.
1930s – Protein foams were a major improvement. They were chemically stable and effective on Class B fires. At that time, they quickly became the industry standard. Their major drawback was limited shelf life and limited storage temperatures, issues that are still problems for protein based products today. Also, protein foams work best when they are discharged through air aspirating equipment which creates a thick foam blanket, but depending on the type of foam discharge equipment, can negatively affect the discharge range compared to non-aspirated equipment.
1960s – Synthetics (AFFF and AR-AFFF) entered the market. They have the ability to readily spread over a fuel surface, are very forgiving during a fire, they can be discharged through all types of nozzles, and have an extended shelf life. Fluorinated synthetic foams are the mainstay of the foam fire protection industry, and have only recently been under review, not for their firefighting performance, but due to their environmental impact. Current synthetics are capable of extinguishing hydrocarbon and water soluble fuels, and can be discharged through air aspirating and non-aspirating nozzles, allowing maximum flexibility during use.
Mid-1960s – Fluoroprotein foam was introduced after the synthetics, mainly as a market response by the protein foam manufacturers. Adding fluorosurfactant to standard protein foams allowed the protein foams to more readily spread on the fuel surface. This step improved the performance of the protein based foams to somewhere between protein only foam and the fluorinated synthetics.
2005/Present – As environmental regulations on fluorinated synthetic foam concentrates increase around the world, a new generation of environment friendly foam concentrates is being developed. These concentrates are environmentally sustainable fluorosurfactant and fluoropolymer-free firefighting foam used to effectively extinguish Class B fuels with no environmental or toxic breakdown. They are synthetic based foam technology designed to replace traditional AFFF and AR-AFFF foam concentrates and older fluoroprotein foams.
As I mentioned at the start of this article, firefighting foams have been used for the extinguishment of fire for almost 100 years in various types. Firefighting foams started with chemical foams, and at each of the advancement steps along the way, performance and safety of those agents improved on the prior foam concentrate type. Despite the importance and need for firefighting foam today, the last significant development efforts occurred in the 1960’s with the commercialization of AFFF for Class B foams, and the 1980’s for Class A foam, now some fifty and twenty years ago respectively. So what will the future hold? Surely the developments in induction systems for Class A foam (and perhaps the agent chemistry) will continue to advance as it has over the past decade. But as for Class B foam concentrates, agent chemistry development efforts seem to have been frozen in time with reliance on exiting base technologies. Only since the introduction of environmental regulations over the past five years or so on fluorine containing AFFF’s have the manufacturers of firefighting foam taken the development challenge seriously. These fluorine-free products (some first generation and others second or third generation) will continue to evolve in both agent chemistry and firefighting performance with the goal of achieving high performance on flammable and combustible liquids, improved burn-back resistance for firefighter safety and provide for many additional years of shelf life over foams derived from protein.
This article has been written by Solberg Foam.