Fire Development, Growth and Spread

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Introduction

Fire plays a pivotal role in the lives of human beings. It’s used for lighting, heating, cooking and in some cases protection. As such, humankind acknowledges that fire is an essential component to any ecosystem. However, despite its uses and benefits, fire poses a serious danger to nature, people and their property. Considering this fact, equipping ourselves with adequate knowledge regarding fire is not only important, but also crucial towards safeguarding us and our environment against the dangers posed by fires. To this end, this essay shall set out to explore documented facts regarding the development, growth, and spread of fires. This shall aim at elaborating the importance of equipping ourselves with knowledge about these stages of fire. Such knowledge is essential since it ensures development of proper and safer methods of handling fires wherever and whenever they occur.

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Importance of studying fires

Phillips and McFadden contend that there are various environmental factors that come into play in order for a fire to occur (26). The authors further state that these factors include oxygen, heat, fuel and a chain reaction. Collectively, these factors form a fire tetrahedron (13). Scientifically fires are classified into four categories depending on the type of fuel that leads to their occurrence. Fuel is the main variable since the other three factors remain fairly constant. As such, understanding these classes of fire go along way towards facilitating fire experts devise specific fire intervention methods for each class of fire. As a result, fires are controlled with minimum casualties and damages.

Quintiere asserts that for a fire to occur, it has to follow sequential stages that are dependent on each other (43). These are the developmental, growth, and spread stages. People in the fire protection business require knowledge of these stages in order to design systems and guidelines of approaching fire. Arguably, by knowing specific stages of fire, specialists can precisely judge when and when not to engage a fire.

In addition, knowledge about fire and how it occurs enables proper design of structures that are able to protect occupants from injury whenever a fire occurs. These designs entail appropriate location of emergency firefighting kits. At the same time, room for entry and exit (emergency exit) are designed and located efficiently, if the process of fire development is well understood. This knowledge contribute towards the strategic location of fire warning systems, emergency kits and fire fighting equipment that alert any occupant whenever a fire hazard is eminent (Redsicker and O’Connor 65). It therefore goes without saying that knowledge of the fire development process is essential to individuals and firefighters since it provides crucial information to these factions on the cause and origin of fires. As a result they are better placed to design proper fire prevention systems to counter and control fires whenever and wherever they are spotted.

Fire Development, Growth, and Spreads

Many contributing factors affect fire development. At its initial stages, fire development is mainly influenced by fuel supply and the availability of oxygen, which is pivotal to the process of combustion (Quintiere 12). According to Quintiere, oxygen gas takes approximately 21 percent of the total air composition (75). This backs the fact that it is in abundance in our environment. In principle, once a fire starts, it follows natural paths that offer the least resistance as it moves up and away simultaneously from its point of origin. Studies conducted over the decades indicate that all fires have a common pattern that they follow (Quintiere 44). This pattern includes burning of combustible materials around the fire as it spreads while following a path with the least resistance. With time, and as fire spreads around, other environmental factors such as building design, the quantity of additional fuel accessible, materials used in the construction of structures, and the quantity of oxygen gas in the immediate area play a major role in influence the rate and extent to which fire will spread.

Classification of Fires

As stated earlier, fire is categorized based on the kind of fuel or substance that is burning. As such, firefighting specialists and other organizations with similar interests have managed to classify fires into the following tabulated types depending on the fuel type that starts the fire.

A table showing classes of fire and their fueling factors.

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Type of Fire Fuel/Material consumed
Class A Ordinary burnable materials or those substances that generate coals, ash or glowing embers when burnt. For instance rubber, wood, paper and cloth
Class B Burnable or flammable liquids. For example alcohol, gasoline liquid, fuel oil and common kerosene
Class C Energized electrical machines
Class D Flammable metals such as titanium and magnesium.

Stages of Fire: Fire Development

Fire begins instantly when the four constituents of the fire tetrahedron merge. In the initial stages of fire development, generated heat goes up and creates a plume of hot gas. Quintiere states that fire spreads as a result of the initial heat spreading from the original heat source to other flammable materials in that environment (59). In open spaces, fire plume moves up unhindered, and atmospheric air is drawn in as the fire cloud moves up. Air drawn into the flame is at lower temperature than fire gases and as a result, gases above the fire are cooled. The spread of fire in an open area is chiefly due to heat energy that is transmitted from the plume to nearby fuels. The rate at which fire spreads outside is propelled by a sloping terrain, amount of oxygen being drained in, and wind, which facilitates preheating of the uncovered fuel.

The mechanism through which fire spread in a compartmented or partitioned area is multifaceted, unlike in open spaces (DeHaan 31). In this paper, a compartment is considered as an enclosed space or room in a given building. As the term suggests, fires that arise in compartments are normally determined by the amount of oxygen and fuel that is accessible. When the quantity of oxygen and accessible gas is inadequate, the environment is deemed as being ventilation controlled. On the other hand, when the quantity of fuel accessible to burn is inadequate, the environment is said to be fuel controlled.

In recent times, there have been attempts to categorize compartmental fires based on the transformation phases or stages that take place as the fire expands. These fire development phases beginning from the initial stages which include ignition, growth, flashover, fully developed and decay. These transformational stages are aimed at illustrating the multifaceted reaction that take place as fire develops, grows and spreads. It should be noted that the stages are an attempt to describe the multifaceted reaction that occurs as a fire expands in an open space where no attempts are made to contain it from spreading. The incipient stage is a contribution of many variables. Therefore, each fire calls for special evaluation so as to accurately describe it. This would explain why it is often difficult to explain the incipient and development stages of compartment fire in general. Information concerning fire has been gathered to give an insight into fire as a lively event that heavily depends on surrounding factors for its development and expansion.

Ignition (Incipient Stage)

At ignition stage, four constituents of the fire tetrahedron, come together to facilitate combustion. For compartmental fires, we have two types of ignitions namely; a piloted ignition, where fire is caused by flame or spark, and a non-piloted ignition where a material attains ignition temperature because of self-heating (National Fire 29). A good example of a non-piloted ignition is impulsive ignition. As such, before a fire starts, whether inside a compartment or in an open space, it has to be as a result of either piloted or non-piloted ignition. At the ignition stage, the fire is small and is often restricted to the matter that ignited first.

Growth (Free Burning Stage)

Immediately after ignition, a fire plume forms above the burning fuel and spreads depending on the surface covered by the fuel. As it expands it starts to draw in air from the surrounding environment into the plume. Initially, the fire has a growth that resembles that of an unconfined fire. This growth is a function of the fuel burning. However, due to the confining effects of the ceilings and walls of the space occupied, the fire plume growth is affected. The initial effect results from the entrapped air, which is cooler, compared to the hot gases produced by the fire. This air brings about a cooling effect on the fire, resulting in temperature reduction. The amount of air entrapped is determined by the location of the fuel package in respect to the compartment walls. As a result, this affects the magnitude of cooling that occurs.

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Less air is entrapped in situations where we have fuel packages near walls leading to very high plume temperatures. This has a considerable effect on the temperature of the developing hot-gas layer over the fire. With time, the hot gases rise and start spreading to the compartment walls resulting in an increase in the depth of the gas layer. The change in temperature within the compartment is a function of the amount of air entrapped, the total heat being conducted in the walls and ceiling and the position of the fuel package. It has been demonstrated through research that there is a consistent decrease in gas temperature with increase in distance from the center of the plume (National Fire 78). This growth stages is mainly influenced by the fuel source. The overall temperature inside the compartment and of the gas layer at the ceiling level increases as the fire continues to grow.

Flashover

The transition between the growth and the completely developed fire stages is called the flashover stage. Unlike the ignition stage, a flashover is not a single specific occurrence. Owing to the transition from the materials initially ignited, to combustion involving all the exposed combustible elements in the compartment, conditions inside the compartment are subject to rapid change.

At the ceiling, in the growth phase, hot-gas layer develops leading to radial combustion of materials that are away from the point of origin of the fire. When a flashover occurs, the radiant energy (heat flux) emitted from the hot gas layer surpasses 20kilowatts/mass (squared) resulting in pyrolysis of combustible matter within the compartment. This radiant heat, heats the generated gases (during flashover) to their ignition temperature.

Flashover has been defined in many ways but most scientists base their definition on the temperature within the compartment that leads to concurrent ignition of all combustible matter in the area. In such cases a temperature range of between 900 degrees F to 1,200 degrees F (483 degrees C to 649 degrees C) has been agreed upon (Zabetakis 41). This temperature range corresponds to the temperature of ignition for carbon monoxide (CO); one the gases that is a result of pyrolysis. Before a flashover occurs, several processes are usually underway; combustible gases are being produced from the fuel, temperatures are rising and there is an involvement of more and more fuel packages.

As the flashover process continues there is progressive combustion of gases that are produced during pyrolysis, and of any combustible matter within the compartment. This brings about full room involvement. 10,000kW or more of heat is released. The room and any people occupying it at this moment are unlikely to come out alive. At the same time, this puts the firefighters at extreme risk even if they are wearing protective gear.

Fully Developed (Post-Flashover) Stage

A fire is developed to the maximum when all the matter that is burnable in the compartment is involved in the fire. At this juncture, the fuels that are aflame in the room emit the highest amount of heat possible from the fuel packages present. This results in massive quantities of fire gases. Depending on the quantity and size of aeration, there is a variation in the amount of heat and fire gases produced. With time, the fire progression is aeration dependent and as a result there is production of massive quantities of unburned gases. At this time, these unburned gases find their way into adjoining compartments where they encounter plenty of air. As result, they ignite in this area.

Decay (Smoldering Stage)

The rate at which heat is produced gradually decreases as the fuel present is consumed by the fire. Once more, the fire becomes subject to amount of fuel available for combustion leading to dying of the fire and concomitant decrease in temperature. Instances occur when temperatures remain high after the fire has died. This is due to embers that burn slowly for a long time maintaining elevated temperatures within the room.

Fire Spread

The spread of a fire is dependent on the heat transmitted. As mentioned earlier, fires can be brought about by super heated pipes, girders, walls or even floors. This leads to a situation whereby fire spreads to other compartments through conduction (Bowes 75). Furthermore, fire spreads through convection (upward) and radiation (downward) trends that result from the motion of superheated gases. Other factors also contribute to the spread of fires. Factors such as air movement promote convectional spread to other fuel rich areas. Air movement can lead to maintenance of old fires. Similarly, air movement can also lead to the promotion and progression of new fires. Crumpling of internal structures may lead to further spread of fires (Zabetakis 148). Essential factors that may result in further spreading of fires include:

  1. Combustion rate
  2. Amount of fuel
  3. Speed and bearing of spread
  4. Combustion pattern
  5. Color of flame and resultant smoke

Combustion rate

Evidently, oxygen is a key factor in determining the speed at which matter is combusted. When the supply of oxygen is unlimited, the rate at which matter is combusted is high, resulting in production of large volumes of heat and combustion of all matter. The ignition point of a slow combusting fire is exhibited by homogenous wall and ceiling destruction. Surfaces such as wood or painted surfaces show a baked picture at the ignition point and there is progressive charring and stains of smoke on doors, windows and window glasses. On the other hand, the starting point for a fast combusting fire shows extensive overhead destruction with defined combustion patterns on walls, and pronounced charring on wood especially the parts that come in direct contact with the flame (National Fire Protection Association 83). A discrete line close to the start point of the fire separates the areas that have been burnt and those that have not. There is also a distinct line between the burned and unburned regions on doors and windows.

Amount of fuel (fuel load)

Amount of fuel, also known as the fuel load, is a measure of the latent severity of a fire. The relationship between fire severity and fuel load was the foremost technique put forward to foresee the severity of a fire in a given compartment to the occupants (Quintiere 77). The predicted maximum amount of heat that would be emitted if all available combustible matter in an area is burned is called the fuel load.

Maximum Heat release=weight of Combustible matter× its heat of combustion.

In a common structure, the fire load comprises of the combustible matter, floor finishes, interior finishes and the structural framework. Fuel load is usually given in terms of the average fire load;

The fire load = combustible weight/area in square feet.

When NIST tests were analyzed, they showed an approximate correlation between the Fuel load and an exposure to a fire severity equivalent to the standard time temperature curve.

The weight per square foot (m2) of ordinary combustibles such as wood, paper, and comparable materials with a heat of combustion of 7,000 to 8,000 Btu per pound (16,282 to 18,608 J/Kg) was related to hourly fire severity, as described in Section7/chapter 5/ Table 7-5a/page 7-79 of the NFPA’s Fire Protection handbook.

Speed and bearing of spread (Avenues of Travel and Rate of Spread)

Air is heavier than flames and superheated gases. When air comes in contact with them, the flames and the superheated gases tend to rise. An exposed fire emerges to the surface as a cloud of hot gases (plume). This results in the fire preheating any combustible matter or fuel above the start point of the fire. As soon as the material gains enough heat, it ignites and substantially increases in volume and leads to spread of the flame and heated gases. As a result, cool air is drawn in at the base of the flame from all directions. There are three directions in which fire travels:

  • Vertical (upward): This occurs in situations where the specific structure in which the fire has occurred only allows upward movement. Such places include open stairways, pipe shafts, elevator shafts, and the gap between exterior and interior walls of a structure. Any flammable matter within the surrounding of the fire may become hot enough to cause these fuels to vaporize and start burning. In cases where there is obstruction of the upward movement of burning vapors, there is fanning of these vapors in all directions. They move on the side of the ceiling until they meet another obstruction like a wall. If the wall is devoid of openings, the combustible matter accumulates and is forced to take downward movement on the side of the wall. This is known as the mushrooming effect.
  • Horizontal: Products of the mushrooming effect may take on a horizontal direction if they encounter an opening, or move towards an unaffected area. Any combustible matter in the way of the flame is ignited and the fire continues to spread horizontally at the ceiling level; thereby leaving the walls undamaged. Extensive open areas like churches and supermarkets facilitate rapid spread of fire at or near the level of the ceiling. The hot gases accumulate in the uppermost parts and when optimum conditions are achieved, they ignite and start burning. Horizontal spread is very rapid and fire can spread over a large area in a very short time. It is widely accepted that the fire that occurred in the MGM Grand Hotel’s casino in Las Vegas in 1080 may have spread by this method.
  • Downward: Downward movement of a fire occurs when burning embers drop from an upper area to a lower area especially in balloon-frame construction. Fire can also travel downwards along walls especially if combustible materials such as paint, varnish, or flammable paneling cover these walls. This is usually a slow process and is responsible for minor dissemination of fire. Fire follows a slope on which highly combustible liquid has been poured. An example is when gasoline is poured down a staircase. Initially fire burns in a downward direction following the flowing gasoline, thereafter it starts burning in an upward direction once the gasoline vaporizes.

Combustion pattern

Distinctive and defining patterns are created as super heated gas and flame pass through a structure in an upward direction. The “V” pattern is commonly associated with the vertical movement. The start point is represented by the apex of the “V”. Fire investigators employ these patterns to determine start points of fires.

Color of flame and resultant smoke

The visible manifestation of partial combustion is called smoke. It comprises of solid, liquid, and gaseous materials that are unburned. The type of matter burning can be determined by observing the color of the flame and resultant smoke. The temperature of the flame can also be approximated from the color of the flame. This is only feasible in the early stages of combustion. However, in advanced stages, is becomes difficult to determine and estimate the temperature of the fire or the burning matter. As mentioned earlier, fire always moves in the direction of low resistance. In cases where many petroleum materials are involved the laws of physics are not obeyed and fire spreads in all directions (National Fire Protection Association 39).

Table showing Smoke and Flame Colors for Certain Fuels. (Source: Redsicker and O’connor 48).

SMOKE COLOR FLAME COLOR FUEL
Gray to Brown Red to yellow Wood/paper/cloth
Black Red to white Gasoline
White to gray Yellow to white Benzine
Black to Brown Yellow to white Turpentine
Black Dark red to orange yellow Kerosene
Black Blue white to white Naphtha

Table Showing Flame Colors and Temperature (oF) Ranges. (Source: Redsicker and O’connor 43).

FLAME COLOR TEMPERATURE FLAME COLOR TEMPERATURE
Light Red 900 – 1000 Salmon 1600 – 1700
Light Red 1000 – 1100 Orange 1700 – 1800
Dark Cherry 1100 – 1200 Lemon 1800 – 1900
Medium Cherry 1200 – 1300 Light Yellow 1900 – 2100
Light Cherry 1300 – 1400 White 2150 – 2250
Bright Cherry 1400 – 1500 Bright White 2500 – over

Conclusion

The knowledge about fire is crucial for proper handling and engagement of fire. In the fire protection field, this knowledge plays a central role for fire investigators by helping them ascertain the origin and progression of fires. Point of origin of fires can be determined with accuracy by studying shapes of burned areas. This assists fire investigators to pinpoint the origin and links to possible sources of the fire. The apex of a “V.” represents point of origin as indicated earlier. Similarly, the rate of progression of fires can be determined by characteristic burn marks that occur on burnt surfaces. This is of importance since from this; investigators are able to determine type of fuel involved.

In the fire protection field, prevention is better than cure. As such, knowledge about systematic process of fire progression is essential in formulating proper guidelines to be followed in emergencies. These guidelines are core when it comes to safe evacuation of compartments under fire. In addition, reporting of occurrence of fire is important for timely intervention. Maximum protection against fire is achieved when the fire protection team fully understands proper locations to install fire-warning systems such as alarms and fire stimulated cold-water sprinklers. Such systems can only be designed when the designers fully understand the fire development sequence.

Prediction of probable damage is a core component in the fire protection field. Having knowledge regarding the stages of fire development (from the incipient to decay stage), one is able to critically assess the magnitude of damage that can occur when a structure comes under fire.

Experts in the fire protection field recommend educating the public on causes of fires, their risks and proper safety measures that can be taken in order to avoid injury. Providing detailed information to the public and building occupants on the weak points, escape routes and various fire codes is essential for overall fire protection mechanism.

Works Cited

Bowes, Peter. Self-heating: evaluating and controlling the hazards. New York: Elsevier, 1984. Print.

DeHaan, John. Kirk’s Fire Investigation. Upper Saddle River, New Jersey: Pearson Education, Inc., 2002. Print.

National Fire. Principles of Fire Protection Chemistry and Physics. Quincy, MA: National Fire Protection Association, 1998. Print.

National Fire Protection Association. Fire Protection Handbook. Quincy, MA: National Fire Protection Association, 1997. Print.

Phillips, Calvin and David McFadden. Investigating the Fire Ground. USA: PennWell Publishing Company, 1996.

Quintiere, James. Principles of Fire Behavior. New York: Cengage Learning, 1998. Print.

Redsicker, David and John, O’Connor. Practical fire and arson investigation. New York: CRC Press, 1997. Print.

Zabetakis, Michael. Flammability and Characteristics of Combustible Gases and Vapors. Washington, D.C: U.S.Bureau of Mines, 1965. Print.

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