In order to understand how foam suppresses fire, it is first necessary to understand the process of combustion.

Combustion is a process where fuel undergoes a rapid exothermic chemical reaction (release of heat) with an oxidizing agent, usually air, resulting in the formation of products of combustion and energy (fire).

A fuel is any material that can be oxidized; it can be a solid, liquid, or gas and is generally organic in nature, i.e., composed mostly of carbon, hydrogen, or oxygen. The products of combustion of an organic fuel (assuming complete combustion) are carbon dioxide and water. The energy released may be in the form of heat or light, or the combination of heat and light (fire).

The chemical reaction is not a simple one-step reaction, but is a chain reaction resulting in a number of interdependent chemical reactions. Figure 1-1 depicts the four requirements for combustion using the “fire tetrahedron.”

It follows that any method for extinguishing fire must involve one or more of the following techniques:

1. Remove heat at a faster rate than it is released.
2. Separate the fuel from the oxidizing agent.
3. Dilute the vapour-phase concentration of the fuel and/or oxidizing agent below that necessary for combustion.
4. Terminate the chemical chain-reaction sequence.

Fire fighting foam is an aggregate of gas-filled bubbles (Figure 1-2) formed from aqueous solutions of specially formulated, liquid agent concentrates. The gas used is usually air, but certain applications use an inert gas.

Since foam is lighter than flammable and combustible liquids, it floats on the fuel surface producing a continuous blanket that suppresses fire by separating flammable vapours and oxygen as shown in Figure 1-2. Because foam is a water-bearing material, it also cools the fuel surface.

There are two basic stages in the generation of foam. The first is when "foam concentrate" is added to water in a given concentration to form "foam solution". This stage is called proportioning. It is a very critical stage because it is vital that the correct percentage concentration is formed if the foam is to perform properly.

The next stage is when air is mixed with the solution to produce bubbles of foam or "finished foam".

Of course, producing bubbles is not all there is to a foam system. There are many different types of foam concent­rate available, many types of proportioners and many different types of foam generating and application equipment.

For every flammable liquid fire protection problem there is an optimum combination of equipment, concentrate and application technique. Many factors including cost and environmental impact must be taken into account. In Control Fire Protection can provide this optimum combination in every case.

To produce a reliable and cost-effective foam system, detailed hazard and fuel information is required and several vital decisions have to be made:

• What is the risk protected?

• Which foam is the best for the task?

• What proportioning system is most appropriate?

• What is the best application method?

• What standard of protection should be applied?

• What foam solution flow rate is required?

• How long should the system run for?

• How much foam concentrate is required?

General

Fire fighting foam is a collection of bubbles formed by the aeration of a solution of foam concentrate with water.

Foam is made up of three components – foam concentrate (a liquid produced by chemical manufacturers and supplied in drums or bulk) water and air. The finished product, foam, floats on a fuel surface to extinguish a flammable liquid fire by separating the fuel from oxygen.

Because of its high water content it also helps cool the fuel surface and any hot objects in the fire area. Foam expansion is classified into three headings – Low, Medium and High.

Expansion is defined as the ratio of the volume of foam produced to the volume of foam solution required to make it. Expansion ratios up to 20:1 are usually considered to be low expansion foams; 20:1 – 200:1 medium, and above 200:1 high. Each expansion ratio has different uses.

A well formulated foam applied correctly will exhibit a range of properties including stability, cohesion, rapid fire knockdown, heat resistance and vapour suppression that will ensure that a fire is extinguished efficiently and securely so that re ignition does not occur.

Foam concentrates are available from a number of manufacturers within the Tyco Group including an extensive line of ANSUL® premium foam concentrates for both Class A and B fires. We can also supply foam concentrates from the Towalex and Sabo range.

The product range includes Protein, Flouroprotein, and AFFF foams suitable for most types of flammable liquids including Polar Solvents.

Foam concentrates are available in 1%, 3% or 6% concentrations. We can also provide foam for vapour suppression / neutralising and where require we can provide training foam. Product Datasheets and Material Safety Data Sheets are available for all foam agents – see the links on each product page.

Foam Types

There are numerous types of foams that are selected for specific applications according to their properties and performance. Some foams are thick, viscous, and form tough, heat-resistant blankets over burning liquid surfaces; other foams are thinner and spread more rapidly. Some foams are capable of producing a vapour sealing film of surface active water solution on a liquid surface. Others, such as medium and high expansion foams, are used as large volumes to flood surfaces and fill cavities.

Chemical Foam

Foams have been classified in different ways over the years. The earliest foams were based upon a chemical reaction occurring between aluminium sulphate (Al2 (SO4)3) and sodium bicarbonate (NaHCO3). The energy used to create the foam bubbles came from the chemical reaction. This type of foam is now largely obsolete.

Mechanical Foams

Mechanical foam is produced by mixing a foam concentrate with water at the appropriate concentration, and then aerating and agitating the solution to form a bubble structure. Therefore, unlike chemical foams, the energy used to create the foam bubbles of a mechanical foam comes from an outside source. There are several types of mechanical foams: – Protein – Fluoroprotein – Film-Forming Fluoroprotein (FFFP) – Aqueous Film-Forming Foam (AFFF) – Alcohol-Resistant Concentrate (ARC) – Synthetic Detergent (High/Medium Expansion) The differences between these foam concentrates depend on: – whether the concentrate is based upon naturally occurring materials or synthetic chemicals. – whether the synthetic chemicals are fluorinated or nonfluoronated. – the type of fuel being protected. – the expansion ratio. – whether they will form an aqueous film on certain fuels.

Protein Foam

Protein Foam is derived from naturally-occurring sources of protein such as hoof and horn meal or feather meal. The protein meal is hydrolyzed in the presence of lime and converted to a protein hydrolysate which is neutralized and to which other components are added such as foam stabilizers, corrosion inhibitors, antimicrobial agents, and freezing point depressants. Foams derived from protein foam concentrates generally have very good heat stability and resist burnback, but are not as mobile or fluid on the fuel surface as other types of low expansion foams. Protein foams are susceptible to fuel pickup; consequently, care should be taken to minimize submergence.

Fluoroprotein Foam

Fluoroprotein Foam is derived from protein foam concentrates to which small amounts of fluorochemical surfactants are added. The fluorochemical surfactants are similar to those developed for AFFF foam agents but used in much lower concentrations. The addition of these chemicals produces an easier flowing foam. Because of these chemicals, fluoroprotein foams are said to be oleophobic (oil shedding) and are well suited for sub-surface injection.

Aqueous Film Forming Foam (AFFF)

Aqueous Film-Forming Foam (AFFF) is a completely synthetic foam. It consists of combinations of fluorochemical and hydrocarbon surfactants combined with high boiling point solvents and water. Surfactants are chemicals that have the ability to alter the surface properties of water. Fluorochemical surfactants alter these properties in such a way that a thin film (Figure 1-7) can spread on a hydrocarbon fuel (such as gasoline) even though the aqueous film is more dense than the fuel.

Film Forming Fluoroprotein (FFFP)

Film-Forming Fluoroprotein (FFFP) is a protein base foam concentrate to which quantities of fluorochemical surfactants (similar to those used in AFFF foam agents) are added. This improves the mobility of the foam to the point where it begins to approach the quick extinguishment that is characteristic of AFFF foam agents. On some fuels, it also forms an aqueous film like the AFFF foam agents. However, this reduces the burnback resistance that is characteristic of protein-based foams. Film-forming fluoroprotein foams tend to be a compromise between AFFF and fluoroprotein foam agents.

Synthetic detergent foam

Synthetic Detergent type foam agents are based on mixtures of non-fluorochemical, hydrocarbon type surfactants along with solvents and water. These foam agents do not form aqueous films or polymeric membranes. Instead, they function by forming an aggregate of foam bubbles on the surface of the fuel. They are used most frequently with high expansion foam generators yielding expansion ratios of 200 to 1000:1 (see Figure 1-9). The reduced water content of high expansion foams makes them suitable for use in total flooding applications and on cryogenic type fuels such as liquefied natural gas (LNG). Some of these foam agents are specially formulated to be used with low, medium, and high expansion foam hardware at different proportioning ratios and are referred to as multiple expansion foam agents.

Alcohol Resistant foams (AR)

Alcohol-Resistant Concentrate (ARC) produces a foam that is effective on fuels such as methyl alcohol, ethyl alcohol, and acetone which have appreciable water solubility or miscibility. Standard foam agents are mixtures of chemicals (natural or synthetic) whose bubbles collapse when applied to water soluble fuels. These fuels are said to be foam destructive. The early alcohol-resistant foams were based on mixtures of protein foams and chemicals called metal soaps. These chemicals are hydrophobic or water repellent. The most current alcohol-resistant concentrates are based on AFFF concentrates to which a water soluble polymer (polysaccharide) has been added. When these foam agents are applied to a water soluble fuel such as methyl alcohol, a polymeric membrane (Figure 1-8) is formed between the foam and the water soluble fuel. When this foam agent is used on a conventional (water insoluble) hydrocarbon fuel, it functions as an AFFF foam by forming an aqueous film at the fuel/air interface. Since the polymer is a naturally occurring chemical, small amounts of an antimicrobial agent are added to prevent biological degradation.

Hazardous material "HAZMAT" foams

Specialist foams for toxic vapour suppression of fuels and other chemicals are now available. Such foams often require resistance to chemical reaction. Usually they are applied at medium expansions to form a long lasting and deep blanket of foam on the chemical surface.

Depending on what type of foam concentrate you have, the typical shelf life will vary.Protein type concentrates have a shelf life of about 7 to 10 years.

The synthetic type concentrates, which are the AFFF, ARC type AFFF, Class “A” and High-Expansion concentrates, have a shelf lives of 20 to 25 years.

Click Here to download the Ansul Technical Bulletin on AFFF Shelf Life

If you’re not already aware, the European Community has passed laws stating that all materials containing PFOS (perfluorooctane sulfonates) will be banned. As a Tyco Authorised Distributor we have access to world experts in fire fighting foams and can help you meet your obligations.

Read the Full Article