Classification of Fires

There are different types of fuels that can react in many different ways, not all of them good. For example, if you have a grease fire on the stove and try to put it out with water, the oil will splatter and spread the fire. For this reason, it is important to know the classifications of fires. The following fire classifications have their basis, on the type of fuel that is involved:

A.     Class A: Ordinary Combustible Materials

Class A fires contain combustible materials, such as paper, cloth, wood, rubber, trash and plastics, which we can find almost anywhere. We can extinguish this class of fire by cooling the fuel below the ignition temperature, by using water, or other extinguishing agents.

B.      Class B: Flammable Liquids

Class B fires contain flammable liquids, which burn at room temperature and combustible liquids, which require heat in order to ignite. Some examples include oils, petroleum grease, tar, solvents, oil-based paints, alcohols, and flammable gases. This class has a high fire hazard, in which water may not extinguish a fire. We can extinguish this class of fire by creating a barrier between the fuel and oxygen. For example, we can use a layer of foam on top of the fuels.

C.      Class C: Electrical

Class C fires contain the same ordinary fuels that are in Class A and B fires; however, they also contain an electrical component. We cannot extinguish this type of fire with any type of water-based product, as water conducts electricity. The best thing to extinguish this fire would be to cut the source of the electricity off and fight the fire as you normally would for Class A or B fires. Firefighters also use carbon dioxide or some type of chemical agent to fight this type of fire if they cannot cut off the source of electricity.

D.     Class D: Combustible Metals

Class D fires contain combustible metals such as potassium, lithium, sodium, magnesium, and titanium. This class of fire requires a high amount of heat to combust metals. For example, this class of fire may occur when sawdust near machinery catches on fire and then ignites the fine metal shavings surrounding the piece of machinery. The best way to put this class of fire out is to use dry chemical extinguishing agents.

E.      Class K: Kitchen Fires

Class K fires contain some type of combustible cooking medium, such as animal and vegetable, oils and fats. They normally occur within well-insulated cooking appliances. For instance, you have a pan of oil on the stovetop that is extremely hot. If you pour water into it, the water sinks to the bottom of the pan and splatters the hot grease all over the curtains, creating a fire. To extinguish this class of fire, wet chemical fire extinguishers work quite well.

The Stages of Fire

The stages of a fire are an important part of knowing how to protect yourself, your family, and your property, if the time comes. The image to your right represents how the stages of fire behave during a fire, in which the placements of the tags follow the graph up and down depending on the temperature that occurs during the fire. 

A.     Ignition

Ignition is the first stage of any type of fire. During this stage, fuel, heat, oxygen, and a chemical reaction combine and produce flames. The fire does not influence the fire compartment's environment very much at this stage. With little heat, a fire extinguisher would be able to control and extinguish the fire.

B.      Growth

The second stage of a fire is the growth; the initial flame is a heat source that ignites additional fuels. Convection and radiation ignite additional fuels, increasing the size and temperature of the fire. At this point, the fire is actually influencing the environment. Hot gases and smoke form layers at different areas of a fire, which we refer to as thermal layering. The hotter and lighter gases rise to the ceiling, while the heavier gases fall to the ground. As the fire burns and oxygen comes into play, it heats up all nearby fuels until they reach their combustion temperature. Within an enclosed environment, a flashover is imminent and is a transition to the next stage of fire.

C.      Full Development

The third stage of fire is full development and is the most intense stage of any type of fire. The fire spreads over most of the available fuels left and the temperatures are at their maximum, which results in heat damage. At this stage, a fire also consumes oxygen very rapidly, creating an unstable situation.

D.     Decay/Burnout

Fire Weather Factors

The weather is the most variable element of the fire environment and is dependent on the time of day and the air mass at that current time. The following weather factors can vary with space and time.

A.     Temperature

Temperature is one of the three most important factors of fire weather. For example, if the sun is out and the temperature is 100 degrees Fahrenheit, it can dry out the fuels making it easier to ignite. The cold can affect fire weather in the same manner. Extreme cold can dry fuels out, making it easier to ignite a fire.

B.      Relative Humidity

Relative humidity is the second of the three most important factors of fire weather. In fact, they go hand in hand. If you have low relative humidity and high temperatures, you almost have the perfect combination of weather factors to have a fire start.

C.      Wind Speed and Direction

Wind speed and direction is the third of the three most important factors of fire weather. Wind speed and direction goes beyond just temperature and relative humidity. If the winds are high, the temperatures are high, and the relative humidity is low, you have the perfect combination of weather factors to start a fire and keep it burning.

D.     Atmospheric Stability

Atmospheric stability can also affect the way in which a fire behaves. For instance, if the atmospheric ceiling is high, it will allow the smoke to go very high up and disperse in the upper atmosphere. However, if the atmospheric ceiling is very low, the smoke will stay close to the ground, impeding sight and making it more dangerous to fight a fire.

E.      Precipitation

Precipitation can affect the manner in which a fire behaves, by saturating the fuels in an area. On the other hand, a lack of precipitation can allow the fuel to dry out; in which there is a higher chance of fire if there has been a drought.

Structure Fires

As mentioned previously, the four stages of fire include ignition, growth, full development, and decay. The progression of the stages of fire is common in fuel-controlled fires, such as a grass fire. However, this is not always the case in ventilation-controlled fires, such as a structure fire in a well-sealed building. The structures, like homes, the fire reaches a point in the growth stage, in which ventilation can be a driving force of the development of the fire. Although heat is present, the available oxygen dictates the rate of burning and growth of the fire.

When we know the true nature of fire, we can prepare our families and ourselves if the occasion arises. Therefore, it is crucial that you understand the basic characteristics of fire. Fire burns very quickly in a well-sealed environment. Within two minutes, a small flame can become a fully developed fire, with thick black smoke filling the building. You will not have time to grab material items, just your family and yourself should be a consideration.

The smoke and heat from a fire can be more dangerous than the flames themselves. Structure fires tend to produce poisonous gases that can disorient or make you and your family drowsy. This is a dangerous situation, as you can fall into a deeper sleep, in which you or your family members may not awaken. In this case, asphyxiation is the leading cause of fire deaths.

According to a 2010 to 2014 report, U.S. firefighters responded to an estimated average of 358,300 structure fires. House fires cause an estimated average of 2,560 civilian deaths, 12,720 civilian injuries, and 6.7 billion dollars during the years of 2010 to 2014. Cooking appliances, smoking, and heaters were the leading causes of these house fires.

Wildland Fires

We can define wildland fires as an uncontrolled fire fueled by the wind, weather, and dry fuels. Wildland fires behave very differently than house fires. Wildland fires only occur in the wildland, where there are no structures. Wildland fires can burn acres of land, and consume all of the fuels in its path, in a matter of minutes.

Wildland fires occur all around the world, but in the U.S., they are most prominent in the Western part of the country. The reason for so many wildland fires is due to the heat, drought, and frequent thunderstorms, which create perfect wildland fire conditions. California, Colorado, Idaho, Montana, Oregon, Washington, and Wyoming have some of the worst wildland fires in the United States.

As mentioned previously, a wildland fire needs three elements in order for it to burn, fuel, oxygen, and a heat source. In terms of the fuel load, the greater the fuel load of an area, the more intense the wildland fire will burn. A wildland fire requires a heat source to spark a fire and bring the fuel temperatures high enough to ignite. Sufficient heat sources include campfires, cigarettes, lightning, or hot winds.

Even though most wildland fires start from humans, nature is also happy to oblige. Having dry weather and droughts can transform green vegetation into dry, flammable fuels. Warm temperatures and strong winds that spread fire very quickly over the wildland combine to encourage combustion. A wildland fire only needs a spark in the form of lightning, campfires, or a cigarette to set a fire. This can ignite a fire that can burn thousands of acres and last for weeks.

Even though wildland fires can be destructive to people and property, naturally occurring wildland fires play an essential role in nature. Naturally occurring wildland fires, return nutrients to the soil, removes disease and harmful insects. Wildland fires also allow sunlight to reach the forest floor, by burning through brushy undergrowth and thick canopies. This allows a new generation of trees and other beneficial plants to have a chance to grow.

There are many factors to consider while fighting a wildland fire. There are also many ways, in which firefighters fight a wildland fire. The main way is to deprive the wildland fire of one or more of the fire triangle components. This may include dousing the fire with water or use some type of fire retardant.

Following are important factors about wildland fuels, and the way they influence a wildland fire:

A.     Fuel Load

The fuel load is the amount of available fuels in an area. If there is a high amount of fuels located in an area, and a wildland fire breaks out, it will be more difficult to extinguish.

B.      Chemical Content

The chemical content of wildland fuels is very important to know when fighting a wildland fire. For example, in the southeast United States, wax myrtles are a very popular ornamental shrub, which also grow in the wildland areas. Wax myrtles are highly flammable due to the amount of oil contained within the leaves. Since they contain such a high amount of oil in the leaves, when they burn, they tend to splatter the oil and fire onto other fuels, intensifying and progressing the wildland fire.

C.      Size and Shape

The size and shape of the wildland fuels located in an area can also determine the behavior of the wildland fire. For instance, large dead logs will smolder and burn for one hundred hours, which we refer to as "100-hour fuels". On the other hand, wildland leaves, and other light debris will only burn for an hour, which we refer to as "1-hour fuels".

Spontaneous Combustion or Chemical Reaction Fires

Spontaneous combustion or a chemical reaction fire is a byproduct of unprompted heating. This occurs when the temperature of a material increases without drawing heat from its surroundings. Oily rags, hay, and other agricultural products are examples of materials that are prone to spontaneous combustion.

The garage of a residence is a very common area of origin for a spontaneous combustion to take place and oily rags, and improperly stored chemicals are the leading cause for these fires. Spontaneous combustion fires also occur commonly in laundry or dry cleaning businesses, with oily rags, and improperly stored chemicals being the leading causes of these fires. Most spontaneous combustion car fires begin in the trunk, with flammable liquids or gases as the leading cause of the fire due to improper chemical storage or leaks.

There are a couple of ways to prevent spontaneous combustion fires. You should properly store all chemicals and make sure they are away from other ignition sources. You should keep oily rags in a well-covered metal can and make sure they are thoroughly dry before transport or collection. You can prevent a spontaneous combustion fire with agricultural products by controlling the moisture level. Regular temperature checks, proper drying, and adequate airflow will limit the heating of the agricultural products.