Firefighting Foams Explained – Foam action
The chemical reaction of combustion takes place when a combustible substance or fuel is oxidised by an oxidiser in the presence of an energy source. The four classes of combustible substances are:
- Class A (wood, cardboard, paper, textile etc),
- Class B (petrol, oil, white-spirit etc),
- Class C (gases such as propane, butane, acetylene, hydrogen, methane etc),
- Class D (finely divided metals such as powdered aluminium, steel wool, magnesium etc).
Examples of oxidising agents include oxygen from the air, chlorine, oxygenated water, nitric acid, chlorates, perchlorates, whilst energy sources come from mechanical friction, electric (lightning or static electricity), chemical, biochemical, solar etc.
Fire-fighting foam acts in two phases, insluation and cooling. Insulation stops the airflow and prevents the combustible from evaporating, taking away the oxidising agent. Cooling removes or decreases the energy that combustion requires as the water in the bubbles decants and evaporates.
Low, Medium and High Expansion
The ratio between the Foam Volume obtained and the Volume of Foaming Solution used to produce it is called expansion. For example, 100 litres of water + foam concentrate pre-mix gives 1000 litres of foam. Expansion is therefore 1000/100 = 10. Expansion increases as more air is introduced. The three types of expansion are, depending on the equipment used: Low Expansion, Medium Expansion and High Expansion.
Low Expansion is usually used on widespread hydrocarbon fires, for example on storage tanks and holding tanks. Its higher density allows the use of long range jets using hoses or monitors. Wind or rain, Low Expansion foam is the least sensitive to atmpspheric conditions. It is stable and provides a strong cover. Its high water content guarantees significant additional cooling.
Medium Expansion is used for smaller surfaces such as solvent storage and cellars, as a rule in closed or partially closed areas of which the walls limit the spread. Medium Expansion foam can be sprayed up to about ten metres. Because of its low density it is sensitive to wind and bad weather.
High Expansion is preferably used on dry product fires or on premises on which there are mixed risks like stores and warehouses. As the range of High Expansion form is almost nil, meaning the spreading orifice must be brought to the immediate area of the fire.
Expansion is strongly dependent on the type of foam concentrate and the type of hose used. Thus, protein foam concentrates are usually suitable for Low and Medium expansion. Synthetic foam concentrates are suitable for Low, Medium and High Expansion.
Direct and Indirect applications of an AFFF or AR form concentrate on class B fires
To extinguishing a fire a suitable foam concentrate, and a controlled application are a necessity.
The European (EN1568) and international (ISO7203) standards recognise two application nodes.
In direct application, the fire fighting teams directly spray the foam into the heart of the liquid using a hose or a monitor in the “solid jet” position. This gives them the ability to fight the origin of the fire from a distance and not be so exposed to intense heat.
Static installations are concerned in the case of long distance static monitors, or classic headed sprinklers. The jet of spray strikes the liquid directly. This type of spray can only be applied to class B fires of the hydrocarbon type (water-immsicible liquids), using AFFF foam concentrates (Aqueous Film Forming Foam).
Indirect application is essential for all polar solvent liquid fires (miscible with water). The foam is sprayed onto a vertical surface and then runs onto the burning liquid to spread evenly on its surface without contaminating it. The foaming agent therefore strikes a surface before coming into contact with the polar product. This is the case for most fire brigade interventions and for mobile resource operating tactics.
The sprayed foam can also be applied directly onto the liquid but using Medium or High expansion. This is the case for almost all static installations (crown with ME foam box on a holding tank, HE flooding, foam head sprinkler…).
The purpose of indiract application is to prevent the foam from being directly absorbing by the polar solvent, and this process is therefore mandatory. Hydrocarbon type fires (water-immiscible liquid) can also be extinguished using this method of application.
Thus, two actions have the same extinguishing effectiveness – chemical action with direct application, and mechanical action with indirect application.
- Chemical action with direct application
The foam generated by an AFFF foam concentrate has the property of quickly draining thus allowing the quick formation of a thin fluorine compound film at the surface of the hydrocarbon.
The hydrocarbon vapours are prevented from igniting thanks to the barrier ceated by this heat resistant film.
- Mechanical action with indirect application
A mattress forms through the accumulation of foam to isolate the burning liquid. This heat resistant thickness of foam creates a barrier that prevents the solvent vapours from igniting.
In all cases the foam concentrate makes it possible to break “the fire triangle” by isolating from the air and a cooling action of the fire’s environment.
For example ECOPOL, the 1st fluorine free foam concentrate in the world, is AR and therefore perfectly effective on all class B fires.