Fire-fighting foams are generally used to extinguish liquid spill fires. When the flammable liquid is water soluble, it will be diluted rendering the flammability to be nothing, but water alone cannot be used for many reasons. If the spilled liquid is lighter than water and insoluble in water, it will float on top spreading the fire. Water will steam and boil causing an eruption increasing the acceleration of the fire's burning rate if the flammable liquid is oil or fat. Water being added to a grease fire is a prime example of boiling eruption. Dilution with just water is not ideal for deep spills because the dilution extinguishment could take just as long as or longer than using foam. When the nature of the spill is unknown, it is highly advised to use foam right away to avoid any issues caused by water.
Foam is a mass of bubbles formed from an aqueous solution that is a specially formulated foaming agent. These bubbles are produced by many different methods. The foam produces an air excluding, cooling, vapor sealing, water bearing material that immobilizes or inhibits combustion with a continuous layer over the spill. Deterioration and vaporization of the foam happens when it exposed to heat and flames. The deterioration and vaporization help contain the heat and spread. Foam is applied to a fire so that the volume compensates for the deterioration and vaporization along with producing a residual foam layer. Although foam breaks down when exposed to flame and heat, the addition of chemical or mechanical forces can ruin its fire-fighting ability.
Definitions of Foams
This section provides details about the fire-fighting foams that are used in the oil and gas industry. Each type of foam will be described based on its properties and application. All information was pulled from Section 17 of the NFPA Fire Protection Handbook, Twentieth Edition. The foams are in the same order as they appear in the handbook.
Aqueous Foam Filming Foams (AFFF):
AFFF is composed of synthetically produced materials. On the surface of hydrocarbons, AFFF is capable of forming a water solution film. Low viscosity gives AFFF fast spreading and leveling characteristics when covering a fire. The film produced by AFFF excludes air and halts vaporization by acting as a surface barrier. The continuous layer (film) produced on top of the spilled liquid suppresses the combustible vapors and cools the spilled liquid. AFFF is considered to be a self-healing after a mechanical disruption. It is self-healing because it will cover the exposed area up again after disrupted. Self-healing only takes place if there is a reservoir of AFFF present. The effectiveness of AFFF is reduced when it is used on hot surfaces and aromatic hydrocarbons. The film dexterity and fluidity of AFFF make it very suitable for use on jet fuel fires. AFFF's low surface tension makes it useful in for fire situations that involve a mix of combustible classes.
The foam can be found in 1%, 3%, or 6% concentrations. The water can be either freshwater or seawater. When applied, AFFF should cover the entire affected area so that it can take full effect. Dry chemical agents can be used in conjunction with AFFF, although mixing with other foams will decrease the efficiency. The normal ambient temperatures for application should be between 35??F and 120??F.
The composition of fluoroprotein foam is very similar to that of protein foams with the addition of fluorinated surface active agents. These agents provide the foam with a fuel shedding property. This property makes fluoroprotein foam particularly effective when fuel coats the foam. Deep spills of crude oil or other hydrocarbons are effectively fought by fluoroprotein foam because of the fuel shedding property. Fluoroprotein foam possesses superior vapor securing and burnback resistance compared to regular protein foams.
After dilution, the foam is biodegradable and nontoxic. Fluoroprotein foams are available in 3% and 6% concentration like many other types of foam. The solution can be made with freshwater or seawater. The compatibility of dry chemical agents and fluoroprotein is better than regular protein foams. The normal ambient temperatures for application should be between 20??F and 120??F.
Film-Forming Fluoroprotein (FFFP) Foams:
FFFP is a protein agent with film-forming fluorinated surface- active agents. The film-forming fluorinated surface- active agent gives this foam the abilities to form water solution films on flammable hydrocarbons' surfaces and provides the foam with the fuel shedding property. Like many other foams, air foams produced using FFFP solutions have fast spreading and leveling characteristics, and acts as a surface barrier to eliminate air and inhibit vaporization. Similarly to AFFF, FFFP produces a self-healing continuous floating film on the surface of hydrocarbon fuels that helps suppress combustible vapors. FFFP has less of a burnback resistance compared to regular protein foams and drains much more rapidly too.
The foam can be found in 3% or 6% concentrations and can be mixed with either freshwater or seawater. When applied, the foam blanket must cover the entire fuel surface area. FFFP can be used with dry chemical agents without any compatibility problems.
Protein foam uses an aqueous liquid concentrate proportioned with water to be produced. It contains a high-molecular-weight natural protein like polymers. These polymers are derived from the chemical digestion and hydrolysis of natural protein solids. The protein like polymers also provides the foam with elasticity, water retention, and mechanical strength capabilities. The polymers are assisted in their bubble strengthening capability by dissolved polyvalent metallic salts. The salts provide the assistance when the foam is introduced to heat and flame.
The concentrations are found in 3% and 6% and use freshwater or seawater for the solution, like many other types of foam. When applied, the protein foam is dense and viscous with high stability, high heat resistance, good burnback resistance, but less resistant to break down because of fuel saturation than AFFF and fluoroprotein foams. After dilution the foam is nontoxic and biodegradable. The normal ambient temperatures for application should be between 20??F and 120??F.
Low Temperature Foams:
These foams are meant to be used in low temperature areas. They can be used in low temperatures because the freezing point depressants that have been added to the foam. They may be used at ambient temperatures of -20??F or higher. The concentrations are found in 3% and 6% and use freshwater or seawater for the solution. The base for low temperature foams can be either AFFF or protein foam.
Alcohol Resistant Type Foams (AR):
Ordinary foam agents are subject to rapid breakdown and lose effectiveness when used on fires that involve water soluble, water miscible, or polar solvent fuels. Examples of these fuels are as follows: alcohols, lacquer thinners, acetone, ethyl acetate, the amines and anhydrides. Small amounts of the fuels mixed with hydrocarbons may cause rapid breakdown for ordinary foams. Alcohol resistant foams have been made to solve this exact problem.
The base foam for these foams can be of the protein, fluoroprotein, or aqueous film-forming types. The most common base is the aqueous film-forming type. The AR-AFFF variety of foam is very suitable for a spill or a deep spill that contains either a hydrocarbon or a water-miscible flammable liquid. When used on hydrocarbons specifically, the characteristics of an AFFF foam shows up. The foam creates a floating mass of gel for the foam to buildup on water miscible liquids. The normal ambient temperatures for application should be between 20??F and 120??F.
Medium and High Expansion Foams:
Medium and high expansion foams are groups of bubbles generated by aspiration or a blower. This is same as regular foams, but stretched out due to the blower. The expansion rate of these foams ranges from 20:1 to 1000:1. Tests have shown that when used with automatic sprinklers, expansion foams provide more control and extinguishing power than just by themselves. Expansion foams are meant to be used indoors or in confined spaces, so the outdoors application is very limited. When it comes to the outdoor application of expansion foams, the weather is the biggest factor.
Using the air outside of the structure or away from the burning material is the wisest decision when using expansion. If the air used to generate the expansion foam is from inside the structure or from near the material, problems can arise. Using the air from the burning material or structure has been shown in tests to have adverse effects on the volume and stability of the foam. Products of combustion reduce the volume and increase the drainage rate. The high temperature of the air breaks the foam down as it is produced, rendering the foam useless. The foam is also physically disrupted due to the solid and vapor particles that introduced into the foam. With a higher rate of production, these issues can be compensated. A higher rate can be good or bad for the foam, so it needs to be done with caution.
Expansion foams are used to fight class A and B fires in confined spaces. The blower used to produce the foam pushes air through a net or screen that has been wetted by the foam solution. The efficiency of the foam is all based on application: the rate of application, the stability of the foam, and the expansion of the foam during application. Expansion foams can be found in 2% concentration solutions. Medium expansion foams may also be produced from 3% or 6% concentration solutions of fluoroprotein, protein, or AFFF foam types.
If someone was to enter an area that expansion foam had been recently used in, there are some specific guidelines that they must follow. They are as follow: said person must be wearing a self-contained breathing apparatus (SCBA) since air is limited; a guide rope must be used because the senses of sight and sound become severely impaired when in the foam. These are in place to keep firefighters, or anyone who enters the foamed area, safe.
Expansion foam can be used on more than regular flammable liquid fires. It can also be used on Liquefied Natural Gas (LNG) fires. The needed expansion rate for foam to properly subdue a LNG spill is 500:1. Expansion works on LNG spills in a similar fashion to how AFFF works on liquid spills. The water in the foam drains on to the LNG forming a thin layer of ice. This thin layer then floats on the LNG spill giving support to the foam blanket.
Expansion foam has specific effects on fires and they are as follows:
1. With enough volume, the foam can prevent the air that is necessary for combustion from reaching the fire
2. The water in the foam turns into steam when introduced to heat or flame. The steam reduces the oxygen available to the fire by diluting the air.
3. When the water turns into steam, it absorbs heat from the fuel. The foam will continue to breakdown as long as it is exposed to a hot object. This exposure will create more steam and cause the surrounding area to cool down
4. The solution present that is not converted to steam tends to penetrate Class A materials. It does because of the low surface tension that is present.
5. When expansion foam accumulates in depth, it prevents fire spread by providing an insulation barrier to unaffected materials or structures.