Foam, foam and more foam
Published: 01 September, 2006
Dr. Roger Klein examines the rapidly changing commercial landscape in firefighting foams and the need for considering alternative extinguishing methodologies for fire brigade personnel.
On the other hand, a large Class B incident at a petrochemical or chemical processing facility should, in general, be much better contained through the use of bunding or a protected foul water drainage system. Unless the bunds fail or the foul water storage capacity is exceeded, the run-off should be contained for subsequent removal and treatment. Nature may, however, upset the best laid plans. Some years ago an incident involving a hazardous waste treatment plant in the west of England had been brought under control whereupon the River Severn flooded not only the incident site but all the farmland surrounding it!
USING FOAM FOR VAPOUR SUPPRESSION:
Other than extinguishing fire, foam may also be used to suppress volatile vapours, This is an important property of foam when it is used to extinguish large surface-area fires in the petrochemical or chemical process industries, inhibiting the phenomenon of ‘burn-back’. Vapour suppression or mitigation is, however, also crucial in preventing the release of toxic or flammable vapours from volatile chemicals at Hazmat incidents, where there may be a pool or open tank of the material, as a means reducing the risk to people or of the vapour cloud igniting. Particularly difficult to control are the highly polar liquids such as iso-propyl alcohol (IPA), methyl-ethyl-ketone (MEK), or methanol. Typical vapour suppression foams, usually applied as a 20-30 cm layer, include Ansul’s Target7 VMNA (vapour mitigation neutralising agent) or Arctic Foam’s alcoholresistant RF series. Class A foams which contain detergents are normally 100% biodegradable in the long term but do have significant acute toxicity to biological systems. Class B foams are currently almost all fluorosurfactant-containing, with some exceptions. Irrespective of whether the fluorosurfactant was manufactured using the older PFOS process, essentially no longer a problem as manufacture has ceased and stocks are being depleted especially after incidents like Buncefield, or the newer fluorotelomer process, the perfluorinated tail of the surfactant is so stable chemically that it is doubtful, based on the scientific evidence available, whether the perfluorinated degradation products breakdown at any significant rate in groundwater. PFOS-based foams were abandoned on the basis of environmental concerns which included extreme persistence, bioaccumulation and toxicity of the degradation product PFOS, of the degradation product PFOS, i.e., an unfavourable PBT profile. Fluorotelomer foams are now known, based on work from Jennifer Field’s laboratory, to give rise to the 6:2 fluorotelomer sulphonate (6:2FtS), also known as H-PFOS, which is at least as long lived as PFOS in groundwater. There are strong structural similarities between PFOS and 6:2FtS (H-PFOS) which would indicate a high level of suspicion that their biological behaviour such as toxicity and bioaccumulation might show similarities. Both are known to be extremely persistent but the jury is still out at present on whether 6:2FtS shows a biological profile similar to PFOS as the research results are just not available.
What about the ecosystems?
One consideration, therefore, in choosing between a Class A and a Class B foam concerns the potential impact of firefighting operations on the environment. Class A foams may be acutely toxic, for example, to water courses but this effect in most cases will be short-lived and the ecosystems will recover rapidly. Fluorosurfactant-containing Class B foams, on the other hand, also have short-term acute toxicity to animal systems because of their detergent content but also have the major disadvantage that the fluorosurfactants degrade to extremely environmentally persistent degradation products, known to be toxic and to bioaccumulate for PFOS-based foams, but of as yet unknown toxicity and bioaccumulative potential for the fluorotelomer foams. It would also be environmentally irresponsible to use a Class B foam, even diluted, for training purposes. For this reason the industry continues to invest considerable effort into finding a fluorine-free replacement for the AFFF foams with acceptable Class B performance characteristics.
The foam conundrum:
Can or should one type of foam concentrate be used for both Class B and Class A fires as a ‘one-stop’ solution? Apart from the environmental issues highlighted above, the simple answer is ‘no’ based on effective firefighting and on recent official UK guidance.. A recent report from the Canadian Research Council, however, has suggested that a properly proportioned Class A CAFS system may approach the performance of standard Class B AFFF for hydrocarbon pool fires under test conditions. Class A foams and additives are formulated so as to ensure than the water penetrates the carbonaceous fuel rapidly thus reducing the temperature. Fluorosurfactant-containing Class B foams are film-forming, hence the acronym AFFF = aqueous film-forming foams, based on the hydrophobic (water-hating) and oleophobic (oilhating) properties of their perfluoro-tail. Class B AFFF foams simply do not penetrate carbonaceous fuels as well as Class A foams!
Getting to the chemistry:
At a meeting of the German Environment Agency (Umweltbundesamt) held at Dessau in January 2006, Thierry Bluteau of BioEx explained - based on a novel test procedure - how it was that Class A foams penetrate a carbonaceous fuel such as wood whereas Class B AFFF does not.
Fluorosurfactants, although more expensive on a weight basis than hydrocarbon surfactants, are much more efficient at reducing the surface tension at the air-water interface (to 16-19 mN/m) than their hydrocarbon counterparts (FS = fluorosurfactant; AlkS = alkyl sulphate; AES = alkylether sulphate; AOS = alkyl-olefin sulphonate; AS = alkyl betaine). For this reason, fluorosurfactants have a positive spreading coefficient, Sair/fuel = sfuel - (sair + sfoam/fuel) > 0 or > 3 mN/m for MIL specifications, on hydrocarbon fuels resulting in film-formation, something that hydrocarbon surfactants are incapable of, and they can be used at low concentrations making the unit costs comparable to hydrocarbon foams.
One should not confuse a surfactant’s ability to reduce the surface tension at the air-water interface and to have a positive spreading coefficient at the air-fuel interface, with the ability to wet and to penetrate a carbonaceous matrix. Liquids only wet surfaces effectively when the ‘wetting angle’ or ‘contact angle’ is less than 90°, with efficient penetrants having wetting angles very close to zero. Wetting or contact angles are determined not only by the properties of the liquid but also by the properties of the solid. For example, water wets glass or paper but mercury does not. On the other hand, water does not wet a Teflon surface. A penetrant’s wetting characteristics are largely responsible for its ability to fill a surface void.
Two physical conditions must be met for a liquid to spread over a surface: (i) the surface energy of the solid-gas interface must exceed the combined surface energies of the liquid-gas and solid-liquid interfaces, and (ii) the surface energy of the solid-gas interface must be greater than that of the solid-liquid interface. It is absolutely clear that the alkyl-ether sulphate (AES) and alkyl sulphate (AlkS) surfactants are far more efficient at enabling penetration of the wood matrix through wetting than either fluorosurfactant (FS) or the other hydrocarbon surfactants (AS or AOS).
For this reason, it is not efficient to use a Class B AFFF in place of a properly formulated Class A foam for carbonaceous fuel fires. The message for operational and procurement officers is clear. You should choose your fire fighting foam so that it is fit for purpose, having considered all the advantages and disadvantages of using it. There is no ‘one-stop’, fit-all solution that can be used on Class B hydrocarbon fires, structural Class A fires, for training or for large uncontained wildland fires.
The correct choice of foam or other extinguishing agents is an imperative for any municipal fire authority. It makes little sense for a municipal fire brigade to equip itself with foam meant for fighting large hydrocarbon fires, and the necessary equipment, unless they are likely to have to do this without expert assistance in highly trained personnel and materiel from industry and unless the chances of a fire occurring on their patch are high.
Most municipal fire brigades would also be unable to sustain the level of training necessary for such incidents. All petrochemical or chemical process sites will have both the equipment and large stocks of foam on site for such an eventuality. In many countries this is required by law and the quantity and type of foam concentrate to be held on site may be mandated. Assuming a large petrochemical site has the required equipment and foam stocks available, there are key questions which should have been answered at the emergency planning stage - when the ‘worst case’ scenario was considered and the ‘what if’ principle applied. Is there sufficient foam concentrate available so that an effective fire fighting attack can be mounted immediately, and is it fit for purpose?
Can additional supplies be obtained rapidly? Where is the foam concentrate stored and mixed for use? Is it stored sufficiently far away from the risk to withstand a major incident? Are all firefighting monitors, pipework and pumping facilities sufficiently protected from blast and radiant heat? These are all questions which must be resolved in the emergency plan. Essex Fire Brigade operating a WF&HC Six Gun ‘knocked down’ the majority of the tank fires, oneby-one.
BUNCEFIELD – IN A NUTSHELL:
The major fire at the UK’s Buncefield petroleum storage facility near London in December 2005 brought into focus not only the need for specialist firefighting foams for use with extremely large hydrocarbon fires, currently only satisfied by film-forming products, but also serious environmental issues. This includes the contamination of groundwater and drinking water aquifers, as well as major logistic problems resulting from the substantial reduction of fuel supplies to one of the largest international airports, London Heathrow.