Thursday, 1 December 2011

Wet water, fire fighting foams....Firefighters user surface tension

My Father occasionally fought fires at a his work.  They had to fight the fires to protect the refinery.  The workers had to do it because the fire department which was just up the hill was not be quick enough or skilled enough to put out huge oil fires.  It got me thinking about the firefighting of different fires.  Is water is always the best substance to use on a particular fire?

The answer is 'no.'  Many different substances could be used to fight fires depending on the fire. 
"Wet water": Water into which a surface tension reducing agent has been introduced. The resultant mixture, with its reduced surface tension, is more able to penetrate burning product more deeply and extinguish deep-seated fire.  This material reduces the surface tension of plain water (to <33 dynes/centimeter).  Wetting agents like SILV-EX® concentrate improve the efficiency of water in extinguishing fires in combustibles like forest fires and burning tires.  Since refinery fires might involve jet fuel and kerosine another substance called Aqueous Film Forming Foam (AFFF) should be used.  The AFFF includes a fluorosurfactant that forms blanket to form across the liquid surface blocking flammable vapors.

Since the mixture of either of the substances above is mixed with air, by means of a discharge device, it produces a foam which is very fluid and can flow easily over liquid surfaces.  It allows the foam to do a couple of things to stop the fire:

1) Excludes air from the flammable vapors.
2) Eliminates vapor release from fuel surface.
3) Separates the flames from the fuel surface.
4) Cools the fuel surface and surrounding metal surfaces.

Firefighting foams have been commercially used since the early 1900’s.   The mixtures which are around 1-6% v/v water can be readily tested with a surface tension device to ensure that the wetting agent is doing its job keeping the refinery and my Father safe.

Example of how a refinery fire can easily break out of control.

Material referenced from :