FOAM CONCENTRATES– THEIR IMPACT ON FIRE FIGHTING

WHAT IS FOAM?  Foam is a low specific gravity homogeneous mass made up of tiny gas or air-filled bubbles.  Due to its low specific gravity, it is capable of floating on flammable liquids when applied correctly and in sufficient quantity, the floating foam layer acts as a blanket on the surface of flammable liquids and prevents the emission of flammable vapours and also preventing the atmospheric air from reaching the liquid, thereby helping the fire to extinguish.

HOW IS FOAM PRODUCED? For producing foam, 2 things are required, which together makes up a foam concentrate solution

1. 1. Foam concentrate
2. 2. Water

Foam is then produced by mechanically mixing air or gas to a foam compound solution (i.e., the Foam concentrate) in water.  This process of mixing provides mechanical aeration of Foam concentrate solution which results in the foam getting expanded due to the formation of air/gas bubbles. Depending upon the expansion ratio (The expansion ratio is the volume of finished foam produced versus the volume of premix solution supplied. The more air is introduced, the higher expansion ratio is produced) the foam is categorized as

1. 1. Low expansion foam concentrate – This provides expansion up to 20 times.
2. 2. Medium expansion foam concentrate – This provides expansion up to 20-200 times.
3. 3. High expansion foam concentrate – This provides expansion of more than 200 times.

 

HOW DOES FOAM STOP FIRE?  When the foam is applied to the hydrocarbon fuel/polar solvent surface, the following things happen

• • The foam acts like a blanket/protective layer on the fuel surface which helps in smothering the fire by stopping the ignition sources from the fuel surface- SMOTHERING EFFECT
• • Due to the presence of water, the foam cools the fuel and also the adjacent metal body which is heated up – COOLING EFFECT
• • The foam blanket suppresses the release of flammable vapours that can mix with air and results in fire propagation- STARVATION EFFECT

TYPES OF FOAM CONCENTRATES

There are different types of foam concentrates available. Depending upon the requirement and nature of the fire the foam concentrate is selected. 

• • PROTEIN FOAM(PF) — A foam concentrate that has a hydrolysed protein base with stabilizing additives. Foams generated by these type of foam concentrates are suitable for Class A and B fires.  These protein foam concentrates are dark/reddish-brown

• a. Only suitable for fighting hydrocarbon fuel fires.
• b. Not suitable for fighting polar solvent fires.
• c. Used as only low expansion foam concentrate

• • AQUEOUS FILM FORMING FOAM(AFFF) —A foam concentrate that has a fluorinated surfactant with stabilizing additive and synthetic surfactants. Foams generated by these types of foam concentrate are suitable for Class B hydrocarbon fuel fires such as benzene, kerosene, oil etc.  These are amber coloured clear transparent liquids

• a. Only suitable for fighting hydrocarbon fuel fires.
• b. Not suitable for fighting polar solvent fires.
• c. Used as only low expansion foam concentrate

• • ALCOHOL RESISTANT - AQUEOUS FILM FORMING FOAM(AR - AFFF) —A foam concentrate that has a fluorinated surfactant along with hydrocarbon- surfactant which is effective in fighting  Class B, polar solvent and hydrocarbon fuel fires. The concentrates can effectively fight Class A fires due to excellent wetting properties and may be used in conjunction with dry chemical agents to provide even greater fire suppression performance.

• a. Suitable for fighting hydrocarbon fuel fires and polar solvent fires.
• b. Can be used in low, medium and high expansion foam.

• • FILM FORMING FLUORO-PROTEIN FOAM(FFFP) — A foam concentrate that is protein base with one or more fluorinated surfactant additives. Foams generated by these types of foam concentrate are suitable for Class A and B fires. 

• • SYNTHETIC FOAM (SF) — A non-fluoro surfactant base concentrate non-hydrolyzed protein. Foams generated by these types of foam concentrate are suitable only for Class A fire.

• a. This is suitable for medium expansion foam and high expansion foam

SELECTION OF FOAM CONCENTRATES

There are two types of B Class fires on which the foam concentrates to be selected while fighting the fire.

• 1. Hydrocarbon fuel fires:  Basic conventional Foam concentrates like Fluoro protein foam (PF), Aqueous film forming foam  (AFFF), Film-forming fluoro protein foam(FFFP)  are suitable  for fighting  these  hydrocarbon fires that are insoluble in water (like diesel, petrol, kerosene, oils etc), 
• 2. Polar solvent fires:  Water-soluble polar solvents like alcohols, ketones, certain aldehyde fires destroy basic conventional Foam concentrates like Fluoro protein foam (PF), Aqueous film forming foam  (AFFF), Film-forming fluoro protein foam(FFFP)making them ineffective, hence Alcohol resistant – Aqueous film forming foam (AR-AFFF) are formulated to overcome this problem while dealing with polar solvent fires,  Alcohol resistant foams form a tough polymeric membrane on the surface of the fuel, which supports the blanket created by the foam. The formation of polymeric membrane between foam blanket layer and polar solvent is the main reason for AR -AFFF foams to effectively fight alcohol fires.

TYPES OF FOAM CONCENTRATES

The foam concentrates are mixed with water in three different ratios to create finished foams of required properties and these go by three types

• 1. Type 1—Used with 99 per cent water and 1 per cent foam concentrate.
• 2. Type 3 — Used with 97 per cent water and 3 per cent foam concentrate.
• 3. Type 6 — Used with 94 per cent water and 6 per cent foam concentrate.

PROPERTIES OF FINISHED FOAM

The most important properties which greatly affect the quality of finished foam which in turn affects firefighting are

• • Flame knockdown ability: The ability of the foam to quickly knock down the flames and control the fire.
• • Burn back resistance: The ability of the foam to stay intact on the fuel without getting destroyed due to heat from the fire.
• • Foam fluidity: The ability of foam to freely flow over the surface of the fuel
• • Fuel tolerance: The ability of foam to resist mixing with the fuel so to avoid contamination and breakdown of the foam.

• • Edge sealing:  The ability of foam to seal against hot metal surfaces.
• • Foam blanket stability: The ability of the foam to retain its liquid content so that the blanket it forms remains stable 

Article by PJ Mohan

Sr.Faculty, NIFS