Misconceptions about extinguishing with foam

To a layman most fires may look the same, but a professional firefighter will know that different types of fires must be extinguished using different types of extinguishing agents. 

Generally, fires are classified into six categories: 

Class A fire: combustible carbon-based solids such as paper, wood or textiles

Class B fire: flammable liquids such as paraffin, petrol, diesel or oil (but not cooking oil)

Class C fire: flammable gases such as butane, propane or methane

Class D fire: burning metals such as aluminium, lithium or magnesium

Class E fire: electrical equipment (also indicated by an electric spark symbol)

Class F fire: fats and cooking oils.

Classification of fires

Foam is used as an extinguishing method on Class A and Class B fires. Foam, also for firefighting purposes, is made from three components: water, foam concentrate and air. Water and foam concentrate are mixed in a very precise ratio. This mixture will make up a premix, which will generate foam once it is aspirated with air. Firefighting foam will extinguish by the combined mechanisms of cooling, separating the surrounding oxygen from the product surface, suppressing vapours and smothering by covering the flammable liquid or solid with foam.

For Class D fires a special kind of foam, CAF (Compressed Air Foam) can be used, mainly due to its excellent adhesive properties on surfaces.

Using extinguishing foam is unfortunately regularly seen as a complicated firefighting method and there are various misconceptions due to inexperience when using this extinguishing agent. Nonetheless, with the use of modern technology, its use is easy, effective and can assist in decreasing the environmental effect a fire and the extinguishing efforts may have.

Increasing the proportioning rate to make a thicker mixture allegedly extinguishes the fire more rapidly

Foam concentrates are engineered products and are intended to be used at the mixing rate they are designed for. Changing the proportioning rate may influence the extinguishing results. A mixing rate that is too low will create a foam that will lose its extinguishing properties and firefighting will get difficult. A 3% foam concentrate may still be acceptable at 2.7% but anything lower is risky. A mixing rate that is too high generally should not be a problem, but the foam may lose its capability to flow if it gets too rich. In addition, the foam concentrate supply will get used up a lot faster. 

Using water is equally effective as using foam

When dealing with Class A fires, water may be used as the extinguishing agent, but generally you will need more water for the same extinguishing result. Best would be the use of wetting agent. Wetting agent is a foam concentrate that is mixed with the firefighting water in a greatly reduced proportion rate (generally 10% of the recommended foam proportioning rate). This will reduce the surface tension of the water to allow it to migrate into the carbon-based solids such as paper, wood or textiles, resulting in quicker extinguishing and reducing the risk if reignition.

Most Class B fires in contrast cannot be extinguished with water 

Hydrocarbons are generally lighter than and do not mix with water. When you put water on a hydrocarbon fire, the water will sink below the hydrocarbon. This way it will spread the fire, migrate into the ground or instantaneously evaporate due to the heat of the fire, carrying fuel particles with it, that will spread the fire.  

For polar liquids such as alcohol on the other hand, water may be used for fighting a fire, but foam should be the extinguishing agent of choice. The water in this case may not actually extinguish the fire but may cool the system down and dilute the flammable liquid to an extent, where it cannot burn any longer. 

Fluorine-free foam concentrates (FFF) are drop-in replacements for fluorinated foam concentrates such as AFFF

The transition to fluorine-free foam concentrates does not come as a drop-in replacement for any fluorinated foam concentrate. 

First you must ensure, that the new foam concentrate is tested and certified for use with your specific fuels. Unfortunately, most fluorine-free foam concentrates are limited to specific fuels, unlike the general use of AFFF.

Besides having to look at application rates and extinguishing performance, the technology and technical properties of the firefighting system used must be evaluated. Can my system handle the required water flow? Is my proportioner compatible with the new foam concentrate (viscosity, proportioning rate, water flow)? Are my discharge devices suitable (expansion ratio, water flow)? These are some of the questions that need to be answered. 

The same foam concentrate can be used for extinguishing all fires 

Generally, not every foam is equal and not every foam can be used for extinguishing all fires. 

All foam concentrates are tested and certified for use with specific material or fuel fires. They should only be used accordingly. Even though the extinguishing system may be making a good-looking foam, it may not be designed to tackle the task at hand, i.e. when using a 3×6 foam concentrate designed for a hydrocarbon fire at a proportioning rate of 3% with a burning polar solvent, the foam will be partially destroyed by the solvent and will not extinguish the fire. But when used at 6% it can be used successfully. A regular 3% foam designed for hydrocarbon fires will not be suitable for burning polar solvents at all. 

The quality of finished foam depends on the type of foam concentrate used

Although the foam quality may depend on the type of foam used, the use of the correct equipment such as foam concentrate proportioners and discharge devices will have the main effect on the quality of finished foam.

Air in the foam concentrate is not a problem

With any low-viscosity foam concentrates, also known as Newtonian fluids, such as AFFF, air entrapment is not a problem. Any air bubbles that are introduced into the fluid can migrate to the surface without effect. 

With higher-viscosity foam concentrates, also known as pseudoplastic fluids, such as AFFF-AR or the now more commonly used FFF, the situation is different. Any air bubbles that may be induced during transport or refilling, will most likely remain in the liquid. The high viscosity will stop the bubbles from migrating to the surface. On one hand this will have an influence on the quantity of foam agent stored: 10% air entrapment means a 10% increase in volume, so that your complete foam agent may not fit into your tank. On the other hand, depending on what type of proportioning system you use, the entrapped air may have a negative effect on the efficiency of your proportioned.

The use of foam is not environmentally friendly

Foam concentrates are chemicals and as such may be harmful to the environment. But the intelligent use of chemicals can have a great effect by reducing the chemical load induced into the environment, conserving resources like water and protecting nature and assets by speeding up extinguishing results. 

Compared to water, foam or wetting agent most likely will extinguish Class A and Class B fires better and faster. This reduction in time will minimize the environmental impact of the fire by putting it out quickly while using the smallest amount of foam concentrate and water possible. Whether you use water or fluorine-free foam to extinguish a fire, the clean-up requirements are generally the same. The only difference if you use water only is that it will take you longer to put out the fire, and you are likely to have a much higher amount of run-off that needs to be cleaned up and will migrate into the ground and wash pollutants, caused by the fire, into the soil, ground water and rivers.

More efficient extinguishing means less collateral damage and environmental load.

Foam concentrates containing fluorine will be forbidden

AFFF (aqueous film forming foam) and the alcohol-resistant AFFF-AR foam concentrates contain fluorine surfactants which, thanks to their film-forming properties, provide excellent extinguishing properties when fighting large-scale liquid fires. However, fluorine surfactants are persistent in the environment and very harmful to the health of people and animals.

For some time, foam concentrates containing C8-fluorine have virtually been banned worldwide. Now there is a lot of discussion about restricting further fluorine compounds. 

In Europe the following restrictions have been put in place:

Since 4 July 2020: According to EU Regulation 2019/1021 (POPs Regulation) and Regulation (EU) 2017/1000, foam concentrates containing more than 25ppb (0.025mg/kg) of PFOA or one of its salts or more than 1ppm (1mg/kg, 1,000ppb) of individual PFOA precursor compounds must no longer be marketed in the EU.

Between 1 January 2023 and 3 July 2025: Use is permitted only if the extinguishing water can be completely collected and disposed of in accordance with the law after use.

From 4 July 2025: Use is no longer permitted.

Legislations in various parts of the world have started to put restrictions and/or bans on the use of specific fluorine components in firefighting foams. The industry has reacted by developing less harmful, non-fluorinated foam concentrates. The suitability of individual foam concentrates for all foreseeable extinguishing scenarios is continuously being tested and to prove their suitability. Approvals by the regional authorities and institutions are ongoing. 

Foam is only used for special applications

Foam is commonly used for Class B fires. Outside of that it is still not used widely by all fire departments. In some cases, foam is only seen as a special extinguishing agent for special situations. And some critical fire chiefs consider the hype surrounding foam to be an industry-driven trend that they do not want to follow. However, the data and the results of research into foam extinguishing agents are clear: the use of foam and wetting agent generally leads to faster extinguishing as well as the use of less water and foam concentrate, compared to only water. 

I have got sufficient foam reserves to fight my fires

The quantity of foam concentrate stored must be sufficient to allow extinguishing of the largest protected object, or of the objects to be protected simultaneously as a minimum. NFPA11 has recommendations for this. Generally, sufficient quantities for an initial firefighting attempt will be on hand, but often the quantities will not be sufficient if things do not go as planned. Especially if only the minimum requirements are stocked and surplus recommendation from NFPA11 are not considered.

The tested and approved application rate and proportioning rate (1% or 3%) will dictate the quantity of the foam concentrate required. Generally, it is important not to mix different foam concentrates or foam concentrates with different proportioning rates as this can lead to unstable foam formation. Following is an example calculation for the foam demand according to NFPA 11 for a crude oil storage tank. 

Tank surface x specific extinguishing water quantity (application rate) x proportioning rate of foam concentrate x requested minimum extinguishing time 

In the case of a crude oil tank of diameter 60m, NFPA 11 recommends an application of 6.5l/(min*m²) when using AFFF for an extinguishing time of 65 min. When using a 3% foam concentrate, this results in a minimum amount of approx. 36,000 litres of foam concentrate and a required extinguishing water flow rate of approx. 18,000 l/min. 

NFPA 11 recommends stocking additional foam concentrate for the dyke area of about the same amount. In addition, a safety factor of two is recommended to compensate for foam losses during extinguishing caused by, for example, wind and other factors. This results in a stock of 144,000 litres of 3% foam concentrate. If alternatively, a 1% foam concentrate is used the total storage quantity would amount to only 48,000 litres, requiring smaller storage tanks and set-up space. The choice of foam concentrate and proportioning rate is dictated by the fluid to be extinguished.

I can rely on my technology and do not have to test my proportioning rate 

As firefighting foam concentrate becomes more complex, especially with FFF, the cost rises. NFPA and FM highly recommend annual foam proportioning rate testing. 

With many systems the proportioning rate can only be measured after the premix of foam is generated. This will result in cost for refilling the spent foam agent and the disposal of the produced premix or foam. With its water-motor-driven piston pump technology FireDos achieves this testing by measuring the flow rate of the foam concentrate, which due to system design, can be returned to the foam storage tank. The water flow in the system should be known or it can be approximated by measuring the number of revolutions of the water motor.  With these two values the proportioning rate can be precisely calculated. This method works without admixing foam concentrate to the extinguishing water, so no premix or foam must be generated.

As indicated above, a wrong proportioning rate may lead to a foam that does not have 100% of its extinguishing capabilities or may deplete the foam storage faster than anticipated.

Beside the above there are numerous other misconceptions that you may have come across. If you have a question related to foam and foam concentrate proportioning, let us know. We will be happy to answer your questions and clear up misconceptions.

For more information, email info@firedos.de or go to www.firedos.de