Approaches to Fire Growth - Article Example

FIRE GROWTH Insert Name PART ONE Introduction Fire growth covers the period from ignition to flashover, a process that is fuelled by flammable material that succumbs to the fire. For purposes of definition, fire growth is a comprehensive process comprised of ignition, flame spread and burning rate that are dependent on the fire environment. Fire disasters arise from a combustible source that holds flammable material that is ignited hence increasing the fire intensity. Notably, various elements such as the flame spread, heat release, smoke formation and flow of fire gases depend on the conditions of the fire environment. On the other hand, the compartment elements have an impact on the fire spread within a building. These elements include compartment size, ventilations, and the nature of the flammable materials within the compartment, furnishings and internal finishes that influence the fire growth (CFR 2010, p4). This paper seeks to discuss fire growth within compartments in two major parts. The first part aims at describing the characteristics and development stages in enclosure fires. The second part, the writer will discuss the factors influencing the development of fires within an enclosure by assessing in detail the stages of enclosure fire development. This approach enhances the reader’s understanding of fire growth from its initial stage to the last stage as well as identifying appropriate measures to prevent progressive fires within the compartments. However, the scope of this paper is limited to describing the fire growth process to facilitate planning through enhancing understanding on the nature and spread of fire. This will enable the reader focus on the development of fires within compartments and provide them with sufficient information to develop precautionary measures geared towards preventing progressive fire spread. Without identifying potential fire sources and ignitions, the development of fire policies is almost impossible as they facilitate fire hazards and risk assessment. Stages in Enclosure Fire Development Fire development within an enclosed space can be categorized into three major stages that are namely; pre-heat, burning and burnout (Drysdale 1999, p16). These stages are further broken down into sub-categories that signify the characteristics of the fire at that stage. The pre-heat stage comprises of ignition and growth sub-stages, the burning stage comprises of the flashover and fully developed fire while the burnout stage comprises of the decay stage. To enhance the understanding of the fire processes and stages, these sub-stages will be discussed individually where the subsequent fire characteristics and elements are to be identified. Ignition Stage The ignition stage is the first stage towards the development of a fire. For the ignition of a fire to occur, the presence of combustible material as well as the fire elements is necessary. Firstly, oxygen supports combustion and therefore this fire element has to be present for a fire to occur. Further to the presence of this element, the availability of combustible material at the source of the fire presents the danger of a potential fire. Combustible material includes objects made of polymers and paper among other materials that have low heat toleration levels (Mouritz & Gibson 2006, p23). However, with the presence of these two elements, a fire cannot occur unless it is ignited by a heat source. The ignition provides the heat energy required to start a fire to consume the flammable material. Therefore, the combination of these three elements characterizes the ignition stage that gives rise to a fire. Fire ignition may be as a result of the accidental interaction of the various elements such as electrical wires that provide a source of energy. These sources of ignition provide sources of heat energy that have the potential of starting a fire. However, as noted earlier, the source of energy has to be combined with the flammable material for a fire to develop. On the other hand, fire ignitions can be artificially instigated with the intention of malicious damage. Individuals can willingly ignite the flammable material that results into the development of the fire into the next stages. Quintere (2006, p22) states that flammable material present at the source of ignition determines to a great extent the intensity and spread of the fire within an enclosure. This especially arises from the ability of the material to resist the heat energy as well as transfer the energy. Through a hazard assessment, potential ignition sources are identified and adequate measures are structured to avoid the potential fire risk. Growth Stage With the three fire elements having interacted in the ignition stage resulting into a fire, the fire growth sets pace as the fire begins to increase in intensity and spread. The growth stage is characterized by the increased heat energy as well as consumption of the flammable material (Karlsson & Quintere 2000, p17). Heat energy released by the burning material mixes with the compartment’s cooler air hence increasing the room temperature. As the temperature increases, so does the heat energy as it continues to fill the compartment upwards towards the ceiling. Once this heat energy reaches the ceiling, the hot air begins to spread horizontally across the compartment and although the oxygen levels have reduced, the fire will continue to increase in intensity and hence increased pressures. This increases the temperature of the compartment’s wall and ceiling linings that propels the fire’s heat release rate. As a result, the fire flames continue to grow through the spread of fire or additional ignition within the compartment. However, since most interior linings within buildings are not able to handle high heat intensities, the high heat pressures push through the compartment layers and exit through the openings. At this stage, heat transfer is mainly through radiation that increases the heat flux at the lower level of the compartment (Rasbash, Ramachandran & Kandola 2004, p51). Notably, the size, fire load and ventilation profile of the compartment determines to a great extent the development of the fire. According to the building standards, compartments ought to be constructed to contain internal fires and minimize their spread to adjacent compartments (Stationery Office 2007, p67). Fire spread occurs at the growth stage as the fire increases the entire room temperature. Indicators of fires at this stage include brownish tainted windows, increased heat in adjacent rooms as well as increased room temperature within the compartment. The nature of the ventilation profile determines the spread of the fire to the next compartments as they facilitate the exchange of the high and cool temperatures into the compartment (Sanderson, Rubini & Moss 1999, p2). It should be noted that as the fire intensity increases, the velocity of the smoke discharge and air intake at the ventilations will increase. However, this is dependent on the external weather conditions and their impact on the air flow. Flashover Stage This is a rapid escalation of the heat intensity stage that transforms the fire’s classification from the growth stage and places it in the developed stage. Characteristically of this stage is the explosive heat intensity that increases the combustion of the flammable materials as well as the fire spread. At this stage, there is a large scale interaction of the entire combustible material lying within the compartment. Generally, for a fire to be classified under this stage, the temperature of the compartment’s upper part must range between 932o-1112oF and a heat transfer rate of 15-20kW/m2 (Furness & Muckett 2007, p69). During the flashover, burning gases and smoke will forcefully eject through openings and ventilations within the compartment as high velocity. This phenomenon is as a result of increased heat flux and intensity at both the upper and lower levels of the compartment. Similar to the growth stage, the compartment’s fire load, thermal properties and airflow influence the development of the fire. Fire loads provide fire fighters with information on the rate at which the fire is most likely to develop into the fully-developed stage. On the other hand, smoke exiting from the ventilations is an indicator of a fire at the flashover stage whereby the smoke is dark. However, the color of the smoke does not necessarily indicate a fire at the flashover stage. Increased smoke discharge is an indication of increasing room temperatures whereas its color indicates increased combustion rates within the compartment (Drysdale 1999, p61). Additionally, high temperatures at the top of the opening of the fire compartment can be collected via a thermal imaging camera. Such an observation is a strong indicator of deteriorating fire conditions and hence the possibility of a potential flashover. Another indication of a fire at the flashover stage is the presence of ghosting or rollover where isolated flames travel along the hot gas layer or across the ceiling (Furness & Muckett 2007, p77). A rollover moving along the ceiling into the adjacent compartment is a late indication of the flashover stage. Fires at this stage require sustainable oxygen levels for them to progress to the fully developed stage. Fully Developed Stage At this stage, the fire is uncontainable and it has the capacity to spread to adjacent compartments at high velocity. Although energy release is at its greatest, the fire is often limited by the available ventilation within the compartment. Notably, for fires to progress to the post-flashover stage, sufficient fuel and oxygen have to be present. Additionally, the initially ignited material(s) have to contain sufficient heat energy and heat release rates for the sustenance of the fire at the fully developed stage (CFR 2010, p16). Similarly, if the fire consumes the available oxygen, the heat release rate will reduce and the fire will slowly burn down. Characteristically, fires at the fully developed stage are externally visible as the flames emerge from the compartment’s openings. This is because of the unconsumed gases that accumulate at the ceiling and burn continuously as they exit the compartment through the available air spaces. Room temperatures in the burning compartment range between 1292o-2192oF (Rasbash, Ramachandran & Kandola 2004, p116). These temperatures depend upon the construction elements of the compartment whereby fires in these compartments can reach the fully developed level whereas the adjacent compartments are yet to be involved in the fire. The possibility of fire spread at this stage is high as hot gases from the compartment transfer heat to other combustible materials. Fires at the fully developed stage in one compartment poses a high risk to the adjacent compartments and its spread should be controlled. The building’s structure influences the fire development whereby the fire effects on the internal and external structure can transform the existing ventilation profile. At this stage, the compartment is completely dark hence impeding visibility and the volume of the smoke increases. In poorly ventilated compartments, the hot gas layer drops close to the floor level as the fire progresses (Sanderson, Rubini & Moss 1999, p3). Where single ventilation is present such as the compartment door, smoke will exit from the top as the air moves at the bottom. Air tracks at this level are clearly defined as they handle high velocities of smoke and air circulation. Substantial heat released from the compartment result into the alteration in the color of openings such as windows. Decay Stage Continued burning of combustible material depletes the oxygen levels within the compartment hence reduced heat intensity generated from the fire. On the other hand, fires may reach the decay stage due to the consumption of the available fuel that was ignited at the ignition stage and accelerated through the subsequent stages. Apparently, compartment fires may reach this stage without actually going through the flashlight and fully developed stages due to the low levels of heat energy and heat release rates (Karlsson & Quintere 2000, p23). On the other hand, the movement of fires into the decay stage depends upon the compartment’s ventilation. Where there is reduced oxygen levels within the compartment, the heat release rate decreases although the temperature may continue to increase periodically. However, the danger associated with this includes the presence of highly explosive materials within the compartment that will re-ignite the fire. In the event that the fire fighters were unable to control the fire at the ignition and growth stages, they can prevent additional damage at the decay stage. This is attributed to the reduced heat energy and release rates associated with the fire. However, such a move will be late to salvage any material that was present within the compartment during the fire. All in all, the decay stage is the final fire development stage as all combustible material is consumed and oxygen within the compartment is depleted. Conclusion The fire development stages analyses the cycle that fires undergo from ignition to decay. Knowledge on this cycle enable the fire fighters develop measures to handle the fire as well as project the development of the fire and develop the necessary measures required to contain the spread of the fire. Towards containing the fire, the compartment’s linings and structural elements should be considered as they have a direct influence on the development of the fire. Notably, for fires to develop, the oxygen, combustible material and a source of energy have to be present. Depletion or the absence of any of the three elements results into a failure in the fire and hence its abrupt decay. PART TWO Factors Influencing Fire Development in an Enclosure The fire protection system’s design phase is based on the knowledge of the enclosure’s integrity. Fire development within enclosures can be influenced by a number of variables that are present within the fire environment. These factors have the capability to accelerate or reduce the heat intensity and transfer rate generated by the initial ignition from the energy source. However, this depends upon the positive and negative elements within the enclosure that contribute positively or negatively to the fire. Examples of these factors include fire load, size of the enclosure, ignition source, nature and quantity of the fuel packages, location of the compartment ventilations and the structural materials that demarcate the compartment’s boundaries (CFR 2010, p4). These factors have a direct impact on the heat release rates and intensity that determine the spread of the fire within the compartment as well as the capability of the fire to progress across the multiple stages. This section of the paper seeks to discuss the various elements that influence the development of a fire within an enclosure. Notably, increased heat release rates are required to ensure the development of the fire to the next stage. For this fire development to occur, the compartment’s structural elements have to contribute positively to the ignited fire. On the other hand, the fire can die out sooner if there is the absence of favorable fire elements within the fire environment. Constant supply of the necessary fire elements results into the increased fire intensity and spread of the fire within the compartment as well as the adjacent compartments (Stationery Office 2007, p63). The analysis of these factors will enable the risk managers to develop an appropriate plan aimed at mitigating the impacts of a potential fire disaster to the compartment as well as within the building. The size and location of the ignition source The ignition source provides the initial energy required to ignite a fire at the point where the source combines with the combustible material. Heat energy released from the ignition source is transferred to the combustible material that is then transferred to other flammable objects within the compartment. Heat energy is known to move from high temperature regions to low temperature regions (Drysdale 1999, p7). This movement facilitates the spread of fire within the compartment that spreads to other adjacent compartments upon the acceleration of temperature and heat. Ignition sources can either be electrical or mechanical that has the capacity to generate heat energy capable of igniting combustible material near these sources. Electrical and mechanical energy can occur as a result of resistance, overload, sparking, compression or friction (Rasbash, Ramachandran & Kandola 2004, p157). The development of a fire is likely to be higher in compartment section where the ignition energy is highest. High ignition energies are emitted from large ignition sources as they have the capability to continuously supply constant energy to the combustible material within close range. Eventually, the combustible material is ignited and transfers heat through convection to other flammable objects within the compartment (CFR 2010, p18). However, the ignition source does not necessarily have to ignite another object for fires to occur. As identified earlier, ignition sources can either be electrical or mechanical that is manufactured of combustible materials. Large ignition sources are capable of generating high volumes of ignition energies and hence setting the source on fire. Subsequent combustion of the ignition source spreads fire through the building and hence the commencement of the fire development cycle. On the other hand, the proximity of the ignition source to the combustible material influences fire development. Flammable material close to the energy source is ignited through convection whereas material far from the source is ignited through radiation. Fires ignited through convection are able to develop much faster than fires ignited through radiation. Characteristics of the fuel package Fires ignited as a result of mechanical or electrical defaults are bound to subside eventually as the energy sources fail. However, such fires develop as a result of the ignition of combustible material within the proximity of the ignition source. Quintere (2006, p67) states that fuels are defined as the materials being oxidized during the combustion process that requires the interaction of the fuel, oxidizer and heat energy. The fuel packages can be distinctively classified into two major categories namely organic and inorganic fuels. In most cases, the organic materials are often consumed in fires due to their combustive elements. Organic materials can be further classified into hydrocarbons or cellulose depending on their subsequent elements (Quintere 2006, p69). The physical states of the fuels and their orientations have a direct impact on the fire development process. Solid, liquid and gaseous fuels have different ignition rates and hence the different rates in accelerating the development of fires after ignition. Additionally, the amount of the fuel packages within the compartment determines the development of the fire especially in the growth stage. Huge amounts of fuels within the compartment accelerate the development of fires as they are continuously ignited as well as the high heat transfer rates (Karlsson & Quintere 2000, p103). The proximity of the fuel packages to the ignition source directly influences the development of enclosure fires as they can easily be ignited as well as continuously absorb the heat energy from the ignition source. Fuel positioning within the compartment can fuel the development of the fire especially when they are ignited one after the other. This will increase the available heat energy present within the compartment and hence hasten the fire development process. Lastly, the surface area characteristic of the fuel packages has an impact on the fire development process. The surface area material determines to a great extent the transfer of the heat energy to the material contained therein. Fires develop slowly where the surface material of the package is able to withstand high energy levels and prevent the material from being ignited (Mouritz & Gibson 2006, p47). Compartment Geometry Compartment geometry refers to the size of the compartment both vertically and horizontally and its internal angles. The size of the compartment influences the fire development process in terms of the time taken to pressurize the high room temperatures and hence the fire spread. Large compartments are able to withstand high temperatures for a long period of time compared to the small compartments (CFR 2010, p6). Due to the large size, the subsequent flow of heat through radiation within the room takes longer. As the heat transfer is slower compared to smaller rooms, the room temperatures remain low and rise gradually. The heat transfer process within large rooms is often slow due to the high amounts of air within the compartment. However, the fire development process within large compartment can be accelerated by the availability of high amounts of fuel packages within the compartment. The compartment angles on the other hand influence the fire development process by reflecting the heat waves generated by the fire (Stationery Office 2007, p74). Equal distribution of the waves impedes the development of the fire as the room temperatures at a level ought to remain even before they proceed to the next level. This slows down the fire development process as the heat transfer remains stable over a long period of time. Additionally, the heat transfer rates increase evenly within the compartment depending on the similarity of the fuel packages available within the compartment. Compartment Ventilations The compartment ventilations are put in place to ensure the regular flow of air within the compartment. During the development of a fire, the compartment’s ventilations play an extended role in influencing the fire development process. Firstly, smoke within the compartment escapes through the available ventilations such as door and window spaces. The escape of the smoke through these openings reduces the compartment’s pressure that increase from the heat transfer process (Sanderson, Rubini & Moss 1999, p1). Since the pressure increases the heat velocity that in turn enhances the fire spread, the ventilation openings within the compartment have the capacity to accelerate or drag the fire development process. This eventuality is dependent on size of the ventilation openings and their capacity to allow the inflow of air and the outflow of smoke. Large ventilation openings are able to allow both air and smoke flows and hence stabilize the room temperatures. As a result, the fire takes longer to develop and hence the capability to contain its spread. Secondly, the development of fires is pegged to the availability of oxidants within the compartment that are provided by the compartment’s ventilation profile. During fires, pressurized airs occupy the upper level of the compartment whereas cool air occupies the lower layer. As a result, the inflow of air from the compartment’s exterior through the ventilation openings. However, a neutral plane exists where these two hot and cool layers meet where the hot gas exits and the cool air flows into the compartment (Sanderson, Rubini & Moss 1999, p3). This air exchange drags the fire development process as it depressurizes the air within the compartment. The inadequacy of ventilation openings hastens the fire development process within the compartment as the hot gas layer drops to the floor. Additionally, the compartment’s airflow depends on the weather conditions on the compartment’s exterior environment. Compartment Linings During fires, room temperatures increase due to increased combustion that eventually results into high heat transfer rates. At high room temperatures, the heat transfer rates impact on the compartment’s material properties of the enclosure boundaries. The interior compartment linings should be able to resist the heat transfers to ensure the containment of the fire spread. Internal linings ensure that the fires do not spread to adjacent compartments through the compartment walls (Stationery Office 2007, p63). Flammable material should not be used as linings for the compartment’s interior as they accelerate the combustion process. On the other hand, un-flammable material are used to repel the fire from the enclosure boundaries to ensure that they do not collapse or transfer heat to adjacent compartments. Apparently, the inability of the fire to spread through the compartment walls ensures that the fire is contained within its source. Material properties of the compartment linings ensure even heat levels within the compartment and hence directly impact on the fire development process (Stationery Office 2007, p65). Additionally, the enclosure linings should be slippery to avoid the sticking of fuel packages that probably explode from the internal fire. This avoids the presence of multiple fire sources within the compartment that accelerate the heat transfer and hence the room temperature. Room temperature will increase at an increasing rate compared to the flow of air within the compartment. Conclusion Internal compartment elements such as ventilation profile, size of ignition sources and the size of the compartment have a direct influence on the fire development process. The nature of these factors has the potential to accelerate or drag the fire development process depending on their appropriateness. The consideration of such elements provides the fire fighters with a clue on the ability of the fire to reach the decay stage as well as spread to the adjacent compartments. With this information, the fire fighters are able to develop an approach that will enable them to contain the fire spread. Notably these factors are evident in the first four stages of the fire development process and greatly determine the outcome of the compartment fire. Compartment occupants ought to focus on the identified factors to limit their contribution to the fire development process. This will enable the occupants to identify potential sources of fire hazards and formulate adequate measures towards averting a fire disaster. References CFR 2010. Compartment Fire Dev. & Flashover, Accessed on March 13,2010 from < http://www.cfbt-us.com/pdfs/01_cifr_fire_development_flashover.pdf >. Drysdale, D 1999, An Intro. To Fire Dynamics 2nd Ed, Michigan: Wiley. Furness, A & Muckett, M 2007, Intro. To Fire Safety Mgmt, Michigan: Butterworth-Heinemann. Karlsson, B & Quintere, JG 2000, Enclosure Fire Dynamics, Denver: CRC Prss. Mouritz, AP & Gibson, AG 2006, Fire Properties of Polymer Composite Materials, New Jersey: Springer. Quintere, JG 2006. Fund. of Fire Phenomena, California: John Wiley & Sns. Rasbash, D, Ramachandran, G & Kandola, B 2004, Evaluation of Fire Safety, California: John Wiley & Sns. Sanderson, V, Rubini, PA & Moss JB 1999. The Effect of Vent Size on a Compartment Fire, Proceedings of the 8th International Conference, pp.1189-1194. Accessed from < http://www.hull.ac.uk/php/331346/publications/interflam99_1.pdf > on March 13, 2010 Stationery Office 2007. Building Reg. Approved Doc. B: Fire Safety: Buildings Other Than Dwelling Houses, Manchester: The Stationery Office. Wilkie, CA & Morgan, AB 2009, Fire Retardancy of Polymeric Materials 2nd Ed, Denver: CRC Prss. Read More