Q.Why only low expansion foam is allowed for the fixed foam fire fighting system on tankers ?

Foam is made up of three things

• Foam concentrate
• Water
• Air

Foam concentrate is also defined by proprtioning ratio the foam is required to be used. So when we have 6% Foam concentrate, it is most effective when 6 parts of foam concentrate is mixed with 94 parts of water to form foam solution.

Expansion ratio of the foam is the ratio in which foam is generated from the foam solution.

So if the expansion ratio of the foam is 1:10, this means that 1 m3 of foam solution can form 10 m3 of fire fighting foam. Out of these 10 m3 foam, 1 m3 is foam solution and rest 9m3 is air.

Low expansion foams have the expansion ratio of less than 1:20

High expansion foams have the expansion rario of more than 1:200

Now which foam will be heavier ? Foam generated from high expansion foam concentrate or low expansion ?

You got it right !! Fire fighting foam generated from low expansion foam concentrate will be heavier as it has less air compared to high expansion foam.

Now if we use high expansion foam (Lighter) on open deck areas, the foam blanket will break very easily with the wind. So the high expansion foams will be ineffective for deck fires.

For this reason, for deck fires, it is low expansion foam that is used for fire fighting.