Extinguishing Properties of Water

Water has the ability to extinguish fire in several ways. The primary way is by cooling, which removes the heat from the fire. Another way water extinguishes fire is by smothering. Water has the ability to absorb large quantities of heat. Amounts of heat transfer are measured in terms of British thermal units (Btu) or in joules (lBtu = 1 055 J or 1.055 kJ).

Water is a compound formed of hydrogen and oxygen when two parts of hydrogen combine with one part oxygen (H₂0). At normal temperatures (32° to 212°F [0°C to 100°C]), water exists in a liquid state. Below 32°F (0°C) (the freezing point of water), it converts to a solid state in the form of ice. Above 212°F (100°C) (the boiling point of water), it converts into a gas called water vapor or steam. Water cannot be seen in this vapor form. When steam starts to cool, however, its visible form is called condensed steam.

For all practical purposes, water is considered to be incompressible, and its weight varies at different temperatures. Water's density, or its weight per unit of volume, is measured in pounds per cubic foot (kg/L). Water is heaviest (approximately 62.4 lb/ft¹ [1 kg/L]) close to its freezing point. Water is lightest (approximately 60 lb/ft¹ [0.96 kg/L]) close to its boiling point. For general purposes, ordinary fresh water can be considered to weigh 62.5 lb/ft' (1kg/L).

Complete vaporization does not happen the instant the water reaches the boiling point, because each pound of water requires approximately 970 Btu's of additional heat to completely turn into steam. When a fire stream is broken into small particles, it absorbs heat and converts into steam more rapidly than it would in a compact form because more of the water surface is exposed to the heat.

Another characteristic of water that is sometimes an aid to fire fighting is its expansion when converted into steam. This expansion helps cool the fire area by driving heat and smoke from the area. This steam, however, can cause serious burn injuries to firefighters and occupants. The amount of expansion varies with the temperatures of the fire area. At 212°F (100°C). a cubic foot of water expands approximately 1.700 times its original volume. The greater the temperature, the higher the amount of expansion. For example, at 500°F (260°C), the expansion ratio is approximately 2,400 times, and at 1,200°F (649°C), the ratio is approximately 4,200 times.

Visualize a nozzle discharging 150 gallons (600 L) of water fog every minute into a heated area of approximately 500°F (260°C) and the water fog being converted into steam. During one minute of operation, 20 cubic feet (0.57 m") of water will have been discharged and vaporized. The 20 cubic feet (0.57 m) of water will have expanded to approximately 48,000 cubic feet (1359 m^J) of steam. This is enough steam to fill a room approximately 10 feet high (3 m), 50 feet (15 m) wide, and 96 feet (30 m) long. In extremely hot atmospheres, steam expands to even greater volumes.

Steam expansion is not gradual, but rapid. If the room is already full of smoke and gases, the steam that is generated will displace these gases if adequate ventilation openings are provided. As the room cools, the steam condenses and allows the room to refill with cooler air. The steam produced can also be an aid in fire extinguishment by smothering when certain types of materials burn. Smothering is accomplished when the expansion of steam reduces oxygen in a confined space. The use of a fog stream in a direct or combined attack requires that adequate ventilation be provided ahead of the hose line. Otherwise, there is a high possibility of steam or even fire rolling back over and around the hose team, and the potential for injury is great.

Water may be used as a smothering agent when it floats on liquids that are heavier than water such as carbon disulfide. It can be used to smother fires in liquids that are lighter than water or with which water is usually miscible if a foam concentrate is added to the water. In some cases, water can be used to dilute a miscible liquid enough that it will be below its fire point. This can only be done if the container holding the liquid is large enough to hold all of the fuel and water.

Some of the observable results of the proper application of a fire stream are: the fire is extinguished or reduced in size, visibility may be maintained, and the room temperature will be reduced.

Several characteristics of water that are extremely valuable for fire extinguishment are as follows:

• Water has a greater heat-absorbing capacity than other common extinguishing agents.

• A relatively large amount of heat is required to change water into steam.

• The greater the surface area of the water exposed, the more rapidly heat will be absorbed.

Foams

Water alone is not always effective as an extinguishing agent. Under certain circumstances, foam is needed. Fire fighting foam (especially the low-expansion form) is especially effective on the two basic categories of flammable liquids: hydrocarbon fuels and polar solvents.

Hydrocarbon fuels, such as crude oil, fuel oil, gasoline, benzene, naphtha, jet fuel, and kerosene, are petroleum based and float on water. Fire Fighting foam is effective as an extinguishing agent and vapor suppressant because it can float on the surface of these fuels.

Polar solvents, such as alcohols, acetone, lacquer thinner, ketones, esters, and acids, are flammable liquids that have an attraction for water, much like a positive magnetic pole attracts a negative pole. Fire fighting foam is effective on these fuels, but only in special alcohol-resistant formulations (referred to as Alcohol Type Concentrate [ATC]).

Fire fighting foam is also used for acid spills, pesticide fires, confined- or enclosed-space fires, and deep-seated Class A fires. In addition to fire fighting foams, there are special low-expansion foams designed solely for use on unignited spills of hazardous liquids. These foams are necessary because unignited chemicals have a tendency to change the pH of or remove the water from fire fighting foams, thereby rendering them ineffective.

Foam extinguishes and/or prevents fire in several ways:

• Smothering: preventing air and flammable vapors from combining

• Separating: intervening between the fuel and the fire

• Cooling: lowering the temperature of the fuel and adjacent surfaces

• Suppressing: preventing the release of flammable vapors

In general, foam works by forming a blanket on the burning fuel. The foam blanket excludes oxygen and stops the burning process. Hazardous materials mitigation foam blankets the potential fuel and prevents vapors from escaping and being ignited.

How Foam Is Generated

Foams in use today are of the mechanical type, that is, they must be proportioned and aerated (mixed with air) before they can be used. Before discussing types of foams and the foam-making process, it is important to understand the following terms:

Foam Concentrate — The raw foam liquid as it sits in its storage container, usually a 5-gallon (20 L) pail, 55-gallon (220 L) drum, or an apparatus storage tank.

Foam Proportioner - The device that injects the correct amount of foam concentrate into the water stream to make the foam solution.

Foam Solution - The mixture of foam concentrate and water that is discharged from the proportioner and passed through the hoseline.

Finished Foam - The completed product after the foam solution reaches the nozzle and air is introduced into the solution (aeration).

Four elements are necessary to produce high-quality fire fighting foam: foam concentrate, water, air, and mechanical agitation (aeration).

All these elements must be present and blended in the correct ratios. Removing any element results in either no foam or a poor quality foam.

There are two stages in the formation of foam. First, water is mixed with foam liquid concentrate to form a foam solution.This is the proportioning stage of foam production. Second, the foam solution passes through the hose line to the foam maker, which is usually part of the foam nozzle. The foam maker aerates the foam solution to form finished foam.

Aeration produces an adequate amount of foam bubbles to form an effective foam blanket. Proper aeration also produces uniform-sized bubbles, which form a longer lasting blanket. A good foam blanket is required to maintain an effective cover over the flammable or toxic liquid.

Proportioners and foam nozzles (also called foam makers) are engineered to work together. Using a foam proportioner that is not matched to the foam maker (even if the two are made by the same manufacturer) can result in unsatisfactory foam or no foam at all. There are numerous appliances for making and applying finished foam.

Fire fighting foam is 90 to 99 percent water. Foams in use today are designed to be used at 1, 3. or 6 percent concentrations, although very strong polar solvents may require foam concentrations of up to 10 percent. In general, foams designed for hydrocarbon fires are used at 1 to 6 percent concentrations. Polar solvent fuels require 6 to 10 percent concentrates. Medium-and high expansion foams are typically used at 1.5, 2. or 3 percent concentrations.

To be effective, foam concentrates must also match the fuel to which they are applied. Foams designed for hydrocarbon fires will not extinguish polar solvent fires regardless of the concentration at which they are used. Many foams that are designed for polar solvents may be used on hydrocarbon fires, but this should not be attempted unless the particular concentrate being used specifically says this can be done. This is why it is extremely important to identify the type of fuel involved before beginning to apply foam.