Firefighting foam
Published: 01 September, 2006
Class B foams are used to fight fires involving liquid hydrocarbon fuels and may also need to be alcohol-resistant (AR) for polar solvents, such as methanol or MEK. Aqueous film-forming foams (AFFF) containing fluorosurfactants revolutionised this area of fire service operations when they were first introduced in the 1960s.
Class B foams are used to fight fires involving liquid hydrocarbon fuels and may also need to be alcohol-resistant (AR) for polar solvents, such as methanol or MEK. Aqueous film-forming foams (AFFF) containing fluorosurfactants revolutionised this area of fire service operations when they were first introduced in the 1960s.
Initially the fluorosurafctants used in AFFF were of the PFOS-type manufactured by 3M but since withdrawal of this chemistry from the marketplace in 2000 because of environmental concerns to do with the PBT profile of PFOS (perfluorooctanyl sulphonate), we are now left with fluorosurfactants manufactured by the fluorotelomer process.
Contamination issues
Fluorotelomers appear to breakdown in groundwater to yield the fluorotelomer sulphonate - predominantly 6:2 FtS which contains a total of eight carbon atoms, six of which are fluorinated - rather than to perfluorohexanoic acid or other perfluoro-carboxylates, although this process would be expected to occur in activated sewage sludge based on the available scientific evidence.
There are strikingly structural similarities between PFOS and 6:2 FtS, whose other acronym is H-PFOS, pointing to a high index of suspicion that there may be similarities in their PBT profiles.
We know both are extremely persistent (vP) in the aquatic environment, with half-lives probably of the order of decades, but do not yet have firm evidence on the toxicity or bioaccumulation of 6:2 FtS as compared to PFOS, which is classified as vPBT.
No realistic decisions on the potential environmental impact of 6:2 FtS (H-PFOS) can be taken with any confidence until this information has been obtained from independent sources.
Fighting tank fires
At the present time it is only the AFFF(AR) or fluoroprotein foams that have adequate performance for the very demanding and highly specialised task of extinguishing large hydrocarbon tank fires.
This is an area in which the petrochemical and chemical process industry have accumulated a wealth of experience and training.
Major tank fires of this kind are probably best left to industrial fire brigades, not really being the domain of municipal fire services who have neither the equipment, foam stocks, expertise or training, and who are better providing logistic and Command and Control support at large incidents of this type, such as the Buncefield storage depot fire.
Alternative foams
There are currently a number of alternative technologies available for Class B applications, other than in the petrochemical and chemical process industries, which do not involve fluorosurfactants.
These include fluorine-free foam (FFF) formulations, such as the re-healing RF-series of foams available from 3M Australia and its alcohol-resistant (AR) variants from Solberg Scandinavian, or the fluorine-free product made by Bio-Ex.
Fluorine-free foams are currently undergoing extensive testing up to and including the LastFire test, to show under what circumstances they can compete effectively with fluorosurfactant AFFF products under practical fire conditions.
The products mentioned have all passed the international test requirements. There have been nteresting developments in compressed air foam technology (CAFS), including the demonstration in Canada that, under certain circumstances, a properly-proportioned Class A CAFS system can perform comparably with Class B foam on a hydrocarbon test fire.
This provides an attractive alternative for municipal fire brigades as appliances can be modified for CAFS fairly readily without huge financial implications.
Water is most effective
One major reason that water is so effective as an extinguishing agent is that each litre or kilogram of water at ambient temperature (20°C) requires 2.59 MJ (620 kcal) to turn it into to steam at 100°C, thus extracting heat from (i.e., cooling) the fire and lowering the combustion temperature.
Water is non-inflammable unlike, say, alcohol which also has a latent heat of vaporisation less than half that for water. If one were able to discharge 20 m3 (20 tons) of water per minute onto the seat of the fire with 100% efficiency, for example, by using a large monitor, it would theoretically be possible to extract the heat from a fire of around 800 MW!
Unfortunately efficiencies of 5% or less are more realistic and, moreover, one is unlikely to have either a large monitor or sufficient water supplies available on-site at the average wildland fire.
On the other hand, fire retardants have a definite place in such situations. These are usually mixtures of inorganic salts, often highly coloured, e.g., purple or orange, for good visibility, which are non-flammable and non-volatile, discharged onto the fire by ‘bombing’ and capable of covering the carbonaceous fuel in a retardant layer which delays or inhibits combustion.
Manual clearing or foam barriers
Wildland fire spread can also be inhibited physically, for example, by using trenches cut by bulldozers, fire-breaks made by clearing fuel sources manually or using explosives, or with foam barriers.
A double line of foam 2-3 metres apart is particularly effective in preventing the spread of low brush fires.
In Australia there is a huge problem with fires involving stands of eucalyptus as these trees produce a highly volatile and inflammable oil-mist on being heated, which is capable of travelling considerable distances and giving rise to vapour cloud fires and explosions.
Moreover, the candle gum, for example in Tasmania, sheds its friable bark in fires and these burning fragments can be blown for up to 1-2 km windward of the advancing fire front, greatly accelerating its advance.
Compacted carbonaceous fuel, such as forest or wildland litter, peat deposits set alight after the now prohibited (at least in the UK) procedure of stubble burning at harvest time, straw bales, car tyres, or even collapsed structural woodwork, needs the extinguishant to be able to penetrate the fuel mass.
Fighting carbonaceous fires
Firefighting foams and additives intended for Class A fires are formulated so as to maximise penetration of carbonaceous fuels. As any surfactant will function as a wetting agent it is essential to distinguish between ‘wetting’ and ‘penetration’.
As discussed recently by Thierry Bluteau of Bio-Ex at a meeting held by the German Environment Agency (Umweltbundesamt) in Dessau near Berlin, properly formulated hydrocarbon surfactant mixtures penetrate carbonaceous fuels far more efficiently than fluorosurfactants, as shown by a specially developed test procedure, even though fluorosurfactants are more effective at reducing the surface tension of water.
Thus it is extremely misleading to imagine that fluorosurfactants can be used efficiently in formulations for Class A foams.
Moreover, the application of Class A foams in wildland fire incidents involves a high risk of environmental exposure, since almost by definition containment of fire-water run-off is all but impossible, with a very high potential for environmental contamination by extremely stable fluorine-containing breakdown products if fluorosurfactants are being used.
Fluorotelomer persistence
Although hydrocarbon surfactants are somewhat more toxic to the aquatic environment on an acute time-scale, this will be short lived and the ecosystems will recover quickly as these surfactants are highly biodegradable.
On the other hand, the fluorine-containing fluorotelomer surfactant degradation products are stable for a decade or more in groundwater and are, as yet, of unknown biological toxicity and bioaccumulative potential.
All these arguments mitigate against using any foam, particularly a fluorosurfactant foam, as a ‘one stop’ answer for both Class A and Class B fires.
The properties required for effective and efficient extinction, thus minimising the amount of foam and water used, are quite different for Class A and Class B fires. There are also potential legal implications of using a fluorosurfactant foam unnecessarily, and without express permission, for an environmentally-sensitive incident when alternatives are available, as discharge of organohalogens into groundwater is prohibited.
This is because they are List 1 compounds under the Groundwater Regulations (1998), derived from the EC Groundwater Directive.
