Many contributing factors affect fire development. At its initial stages, fire development is mainly influenced by fuel supply, which is oxygen. As we know, oxygen gas takes approximately 21 percent of the total air composition a clear indication of its abundance in our environment. In principle, once it starts, it will follow natural path of least resistance as it moves up and away simultaneously from its original point. All fires have a common pattern that they follow i.e. burning combustible materials around it as it spreads while following path of 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 structure constructing and the quantity of oxygen gas immediate area play a major role in influence the rate and extend to which fire will spread.
Classification of Fires
Fire is categorized basing on the kind of fuel or substance that is burning. NFPA has managed to categorize fire by fuel into the following tabulated types.
Fire Development (Stages of Fires)
Fire begins instantly when four constituents of fire tetrahedron merge. In the initial stages of fire development, heat generated goes up and creates a plume of hot gas. For fire to spread away from the first material, it is necessary heat generated from the first material to spread to other neighboring fuel packages so that this kind of cycle continues and in the process fire spreads.
In open space, fire plume moves up unhindered, and atmosphere air is drawn in as fire plume 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 of at which fire spreads outside is propelled by sloping train, amount of oxygen being drain in and wind that facilitates preheating of the uncovered fuel.
The mechanism by which fire spread in a compartment or partition is multifaceted unlike for open space. In this paper, a compartment is considered as an enclosed space or room is a given building. As the term suggests, compartment fire is normally proscribed by amount of oxygen and fuel accessible. When the quantity of oxygen gas accessible is inadequate, such state is referred to as ventilation controlled. On the other hand, when quantity of fuel accessible to burn is inadequate, the fire is fuel controlled.
In recent times, there have been attempts to categorize compartment fires basing on transformation phases or stages that take place as fire expands. These fire development phases beginning from initial stage are ignition, growth, flashover, fully developed and decay. These transformational stages are aimed at illustrating the multifaceted reaction that take place as fire the 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 when no attempts are made to contain it from spreading. The incipient stage is a contribution of many variables, a reason why we cannot easily explain the incipient, and development stages of compartment fire in general and therefore each fire calls for special evaluation so as to accurately describe it. Information concerning fire has been gathered to give an insight into fire as a lively event that heavily depends on surrounding factors for its expansion and development.
Ignition (Incipient Stage)
At ignition stage, four constituents of the fire tetrahedron, team up to to facilitate combustion. For compartment fire, we have to types of ignitions namely; piloted ignition where fire is caused by flame or spark and non-piloted ignition where once the material attains ignition temperature because of self-heating for instance impulsive ignition. At ignition stage, the fire is small and in general restricted to the matter ignited first. Before fire starts, whether inside a compartment or in an open space, it can be as a result of either piloted or non-piloted ignition.
Growth (Free Burning Stage)
Immediately after ignition, a fire plume forms above fuel that is burning expanding above the burning fuel. As it expands it starts to draw in air from the surrounding environment into the column. Initially the fire has a growth that resembles that of an unconfined fire. This growth is a function of the fuel burning.
Due to the confining effect of the ceiling 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 affecting the magnitude of cooling that occurs. 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 of the compartment is a function of the amount 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 centre of the plume. This growth stages are under the influence of the fuel. The overall temperature inside the compartment and of the gas layer at the level of the ceiling increases as the fire grows and grows.
Flashover
The transition between the growth and the completely developed fire stages is called the flashover. Unlike ignition, 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 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. This temperature range corresponds to the temperature of ignition of carbon monoxide (CO) one the gases that is a result of pyrolysis. Before flashover, 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 a fully developed the room and any [people occupying the room at this moment are unlikely to come out alive. At the same time, this puts the firefighter’s extreme risk even if they are wearing protective gear.
Fully Developed (Post-Flashover) Stage
The fire stage is developed to the maximum when all matter that is burnable in the compartment is involved in the fire. At this juncture, the fuels that are aflame in the room are emitting the highest amount of heat feasible for fuel packages present resulting in massive quantities of fire gases. Depending on the on the quantity and size of aeration there is a variation in amount of heat and fire gases produced. With time, the fire progression is aeration depended as a result, three is production of massive quantities of unburned gases. At this time these unburned gases find their way into adjoining compartments where thy 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 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 fire is dependent on transmission of heat. Fires as mentioned beforehand can be brought about by super heated pipes, girders, walls or even floors. This results in spread of fire to other compartments through conduction. Furthermore, fire spreads through convection (upward) and radiation (downward) as a result of motion of superheated gases. Other factors also contribute to the spread of fires. Factors such as air movement promote convection to other fuel rich areas. Air movement can lead to maintenance of old fires and promotion of progression of new fires. Crumpling of internal structures may lead to further spread of fires. Essential factors that may result in spread of fire are
- Combustion rate
- Amount of fuel
- Speed and bearing of spread
- Combustion pattern
- Color of flame and resultant smoke
Combustion rate
As we know oxygen is a key factor in determining the speed at which matter is combusted. When the supply of oxygen is in unlimited, the rate at which matter is combusted is high resulting in production of large volumes of heat and combustion of all matter. A slow combusting fire’s start point is exhibited by homogenous wall and ceiling destruction. Surfaces such as wood or painted surfaces show a baked picture and there is progressive charring and stains of smoke on doors, windows and window glasses. A fast combusting fire’s start point exhibits 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. A discrete line close to the start point of the fire separates the charred and uncharred areas. There is also a distinct line between the burned and unburned regions on doors and windows.
Amount of fuel (fuel load)
Amount of fuel usually referred to as fuel load is a measure of the latent severity of a fire. As per the NFPA handbook, the relationship fire severity/fuel load was the foremost technique put forward to foresee the severity of a fire in a given compartment to the occupants. 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 =combustibleweight/firearea ∈ squarefee
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 such that 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 column 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 three directions in which fire travels
- Vertical (upward). This occurs in situations where the specific structure in which fire is 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 an effect called the mushrooming effect.
- Horizontal. Products of mushrooming may take on a horizontal direction if they encounter an opening and move to an unaffected area. Any combustible matter in the way of the flame is ignited and there is the spread of the fire horizontally at the level of the ceiling sparing the walls of any damage. 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 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 n 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 not. In most cases, fire always moves in 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.