Fire Safety Science and Engineering - Literature review Example

Note: Please just send a message if you need anything. Thanks! Literature Review The Nature of Fire Fire can only occur if oxygen, fuel, and ignition source are present – the fire triangle. According to Hughes & Hughes (2012), oxygen is present in the air while soft furnishings, wood, paper, flammable liquids, and others can serve as fuel for the fire. An ignition source on the other hand can be small fire itself, a hot surface, and failing electrical equipment (p.103). In other words, the absence of one of these ingredients reduced the risk of fire, as fire will never occur. Moreover, the presence of oxygen does not necessarily mean that the fire triangle is complete and capable of starting a fire because there is a certain amount of oxygen required for combustion, which is dependent on the chemical composition of the solid fuel. For instance, if there is too little oxygen, the solid fuel will not burn. In contrast, too much oxygen will make solid fuel hotter and burn faster (National Fire Protection Association, 2004, p.128). Extinguishing a fire requires breaking the chain of the fire triangle by removing one of the three elements. Failure to break the chain will let the fire consume all fuel in its path while heat from the fire transfer from surface to surface through conduction, convection, and radiation (Pepper, 2010, p.105). Flames are manifestations of the reaction between fuel and oxygen. According to Stollard & Abrahams (1999), solid or liquid fuel vaporised as it is heated and these flammable vapours mix with oxygen produces diffusion flames where the rate of burning is determined by the rate of mixing (p.5). In other words, it is not the fuel that burns but the mixed fuel vapours and oxygen controlled by the amount of ventilation, fuel, and room configuration. The relationship of oxygen and solid fuel is important in understanding the behaviour of fire. According to Quintiere (1998), although the fire triangle is useful in explaining the nature of fire, it is also important to understand how fuel reacts with oxygen. The burning rate of solid or liquid fuel for instance is highly dependent on the amount of oxygen (p.102). For this reason, a fire under hyperbolic condition (i.e. tunnel) is often intense because there is large amount of oxygen supporting fuel combustion. Moreover, a rise from 21 to 28% for oxygen will double the burning rate (Stellman, 1998, p.6). In contrast, oxygen in wildland and ground fires is not an important variable in ignition or growth of fire as an open-air fire is considered free burning with unlimited oxygen. Wind blowing in this type of fire brings more oxygen to the fire thus increasing the rate of combustion and direction of fire spread (International Association of Fire Chiefs, 2008, p.603). According to Corbett (2009), an open-air free burning fire is one that is able to utilize the correct amount of oxygen, heat, and fuel. If this free burning fire is transferred into a confined room in a building, this ideal ratio will change as accumulating hot gases and smoke will reduce the amount of oxygen necessary for combustion. Consequently, as the ration of heat, fuel, and oxygen becomes unbalanced, the level of oxygen will drop until it can no longer support free burning. The fire will then begin to smoulder and generate more smoke and fire gases (p. 325). However, small quantity of oxygen and rising heated smoke and gases in a confined space create significant pressure that can result to explosion when oxygen is suddenly introduced through a vent such as a door or a window (Mittendorf, 1998, p.279). Fire performance is dependent on different factors such surface spread of flame, fire penetration, ease of ignition, fuel contribution, and oxygen content (Marsh, 2010, p. 128). Fire spread is influenced by both material and environmental factors such as the material distribution, height and density, shape, and design of the affected room. Other factors include physical and technological properties such as heat transfer, orientation, surface roughness, porosity, melting behaviour, size and arrangement of the ignition source, ventilation and oxygen availability (Prager & Rosteck, 2006, p.9). Ignitability of a material is an important factor in fire performance as it determines the likelihood of fire starting. Similarly, the spread of flame over a material is essential because it concerns transmission of heat and its release rate, which is the driving force of a fire. Since fire is an exothermal process, it will grow rapidly particularly when materials, product, and the environment support higher release rate (Rapra Technology, 1993, p.55). Types of Compartment Ventilation and Fire Behaviour Compartment fires can be either fuel-controlled or ventilation controlled. A fuel-controlled fire usually occurs when there is sufficient oxygen available inside the compartment to support combustion and growth of fire. In contrast, a ventilation controlled fire is one that fuel-rich but under-ventilated or with limited oxygen (Carvel & Beard, 2005, p.234). Fire within a compartment varies depending on building design and ventilation conditions. For instance, compartment geometry and properties can influence fire development because of heat build-up and radiation from its boundaries to the fuel. Similarly, fire development can also be affected by ventilations such as window and door openings – natural ventilation or mechanical ventilations – HVAC ducts, exhaust fans, etc. - as they supply fresh air into the compartment and affect the outflow toxic gases (Yung, 2008, p.87). The size, height, area, and location of natural ventilation (doors, windows, cracks in wall and other openings), often dictate the maximum heat release rate of fire in a compartment (Carvel & Beard, 2005, p.234). Gravity vents is one example of natural ventilation that is commonly used in large warehouse. However, they are not intended to supply fresh air but facilitate smoke extraction during a fire. Gravity vents relies on the buoyancy of the hot products of combustion that usually rise towards the ceiling. Some warehouses used exhaust fans that are automatically activated in the event of fire but they require special protection to withstand heat (International Association of Fire Chiefs, 2011, p.236). Smoke and heat are escaping and continue to rise as the fire grows thus vents near the ceiling help eliminate toxic gases and heat from the room. In contrast, vents located at the lower level fill the voids by supplying fresh air and supporting further combustion. This is called air entrainment or the process by which new air is being introduced in the compartment (Chandler, 2009, p.153). Natural ventilations use pressure difference caused by wind and temperature in and out of the building. However, due to wind suction forces, positioning of ventilation openings must be correct and able to support natural extraction of hot air from the building. Sensor-controlled roof ventilations with opening actuators can be designed as a natural smoke and heat venting system (Wurm, 2007, p.96). In some circumstances, airflow in a compartment fire is provided through mechanical means (forced ventilation) such as an HVAC and other air handling system. Compartment ventilation is not only intended for fresh air supply but smoke management during a fire. For instance, some HVAC systems are link to the fire alarm and all relevant air-handling units are shut down and closed all supply and return air dampers when smoke is detected. Some use the HVAC system to depressurized the smoke zone by activating air-handling units or dedicated smoke management exhaust fans. (International Association of Fire Chiefs, 2011, p.234). Advantages of mechanical ventilation over natural ventilation can be summarized as the ability to deliver the required flow rate, can easily integrate into air-condition system, can be directed and controlled to service critical areas, and can be placed anywhere with available electricity. However, mechanical ventilation cost more and subject to equipment failure, utility service interruption, poor design, poor maintenance, and poor management. In contrast, properly installed and maintained natural ventilation can provide economical but with high ventilation rate, using natural forces (Atkinson, 2009, p.10). Compartment Ventilation and Fire Safety Compartment ventilation as mentioned earlier serve two purpose – supply fresh air and extract toxic gases. In terms of fire safety, these ventilations must be designed with fire development in mind. For instance, doors must have self-closers to ensure that they are closed when a fire occurs. Similarly, ventilations intended for air exchange must be capable of eliminating smoke and heat during a fire (Yung, 2008, p.87). In the United Kingdom, compartment ventilation is considered essential to fire safety. For instance, the Building Regulations Approved Document limits the openings or ventilations in compartment floor or walls and recommend used of fire resisting doors and shutters, and sealing of concealed cavities, ventilation ducts, pipes passages, and other services to prevent fire and smoke from spreading (Furness & Muckett, 2007, p.184). In a study conducted by Wang (2002) about fire behaviour in a large compartment, the fire started to die down due to lack of oxygen. However, when the stand-by fire brigade was asked to break a pane of the double glazing window, the fire started to grow again due to the influx of fresh air (p.74). This study demonstrates the importance of ventilation to fire safety as fire growth can be controlled by minimizing supply of oxygen. According to Harmathy (1985), the combustion rate inside a compartment is equal to the available oxygen supplied by the incoming air (p.148) thus limiting air supply also limits combustion – ventilation-controlled fire. Fire safety requires sufficient knowledge of ventilation-controlled fire as the presence of hot gases at the ceiling level radiate heat to combustible materials that can result to deadly flashover. According to the International Association of Fire Chiefs (2008), an averaged sized room with combustible furnishings and an open door will be involved in a flashover within 5 to 10 minutes after ignition (p.138). In contrast, introducing fresh air in a compartment fire with a door and other ventilation closed by opening a door can result to backdraught, a hazardous phenomenon where the entire compartment erupt into flames due to air entrainment and mixing of oxygen with unburned fuel volatiles (Carvel & Beard, 2005, p.234). According to Cote (2011), instant burning occurs as soon as an opening is made in the closed compartment because the vaporized fuel is already hot and only need oxygen to burn. Similarly, rapid expansion of the gases inside the compartment pushes them out through the opening resulting to a rolling fireball or flashover (p.128). Fire and associated ventilation-related phenomenon can harm both building occupants and fire fighter who may be exposed to fire, smoke, and toxic gases. Exposure can lead to dangerous conditions such as thermal injury, toxicity, asphyxiation, and obscuration (Cote, 2011, p.129). For this reason, a number of ventilation-related fire fighting technique is introduced such as horizontal and vertical ventilation for different types of buildings. Fire fighters performing horizontal ventilation open windows, doors, and other openings to allow natural ventilation. Vertical ventilation on the other hand is taking advantage of roof natural features such as skylights, scuttle covers, and others. This technique will allow rising hot gases to escape. In a single story building, ventilation efforts are often focus on creating an opening close to the fire in order to prevent fire spread. In contrast, ventilation efforts in multi-story buildings are focused on vertical passageways such stairwells, hallways and on the roof to delay flashover by drawing out hot gases (Corbett, 2009, p.381). Approved Document B of UK Building Regulations aims to ensure the safety of the occupants and others who may be affected by a fire in closed environment (Davies, 2010, p.82). It should be noted that fire safety recommendations such as provisions for suitable means of escape, fire spread prevention, performance of mechanical ventilation, and others are ventilation related. For instance, means of escape requires routes free of smoke or toxic gases, fire spread prevention demand self-closing and fire resistant openings such as doors while performance of mechanical ventilation demands effective smoke extraction. Analysis of Case Studies Fire in Stardust Club, Dublin – 1981 This accident claim the lives of 48 people as the fire occurred in the dance hall where about 800 people were dancing. Investigation after the fire suggests that the fire started in the West Alcove, an area measuring about 17 x 10 metres that was partially cut-off from the dance hall by a roller blind. This area contains rows of PVC covered polyurethane foam seats (where the fire started) and carpet tiles. The fire spread rapidly as the highly combustible burning seats and carpet tiles produced a burning rate of about 800kW/m, sufficient to generate high levels of radiation and hot gases under the ceiling resulting to its collapsed and shattering of the roof (Rasbash et al, 2004, p.40). The noteworthy point about this fire incident in terms of ventilation and fire safety is the collapsed of the ceiling that led to the shattering of the roof and natural venting of heat and smoke. Note that if no venting had occurred, the fire would kill many more of the 800 people present in the dance hall. By analysis, the natural ventilation created through the roof reduced the pressure and heat transfer inside the building and prevent the occurrence of flashover. Although more fresh air was introduced that support further combustion, the large natural vent removed toxic and dangerous combustion products including flammable gases that are likely to explode inside the hall. Bradford Football Stadium Fire – 1985 The fire in Bradford Football Stadium occurred during a football match and it started in the main stand occupied by around 5000 people. Investigation of the incident suggests that the bleacher seats and flooring were made of high combustible light wood materials. The fire started in the accumulated trash under the wooden bleachers and spread upwards into the aisle and then to the rear concourse where the main exit is located (Cote, 2003, p.1). By analysis, the stadium is an open sport complex rich in fresh air and oxygen supply. In fact, the movement of the fire was greatly influenced by the light breeze coming from the playing field (Cote, 2003, p.1). In addition, the main grandstand has a roof where rising gases can accumulate and ignite instantly when mix with oxygen. Clearly, this fire was well ventilated and spread quickly due to tremendous amount of oxygen supporting combustion of large amount of available fuel. King Cross underground Fire – 1987 The fire at King’s Cross left 31 people dead and the result of the investigation suggest that fire who claimed their lives started in 40 m wooden escalator. A lighted match falling through the gap into residues of lubricating grease ignited the fire. This small fire grew and spread up the escalator and burned the combustible material in the booking hall above. Further investigation of the incident reveals that the flames within the trench of the escalator did not move upwards but confined, convective, and generated about 150kW/m2 of heat transfer rate – the mechanism of rapid fire spread (Rasbash et al, 2004, p.41). By analysis, rapid fire spread is actually possible as residues of lubricating grease and wood are highly combustible particularly in the presence of large ventilation with abundant oxygen supply coming from above the escalator. Since this case can be considered a tunnel fire, it is possible that smoke, heat and hot gases accumulated in this closed environment and resulted to flashover. In fact, witnesses saw a sudden eruption of black smoke and flames into the ticket hall (Fennell, 1988, p.100) Woolworth’s Retail Store Fire – 1979 This fire accident left 10 people dead due to smoke inhalation. The fire started in stacked polyurethane foam mattresses and chairs near the furniture display area. Hot gases (polyurethane) rapidly rise and spread to the ceiling after ignition. Calculated heat output reached 23MW in 2.5 minutes. Eyewitness reported very rapid growth of fire and large amount of thick choking smoke. Victims were found near but not in the protected escape routes while others were trapped behind barred windows (Stollard & Abrahams, 1999, p.66). Analysis of the accident report suggests that victims died from toxic gases coming from the burning polyurethane products. The presence of large amount of thick choking smoke also suggests that there was not enough ventilation for both heat and toxic gases to escape. There was no report of a flashover but the way victims were found indicate that they were instantly rendered unconscious by toxic gases before they were burned. References Atkinson J, (2009), Natural Ventilation for Infection Control in Health-Care Settings, Jones & Bartlett Publishers, UK Davies A, (2010), Workplace Law Handbook 2011- Health and Safety, Workplace Law Group, UK Carvel R. & Beard A, (2005), The Handbook of Tunnel Fire Safety, Thomas Telford, UK Chandler R, (2009), Fire and Arson Investigation, Cengage Learning, US Corbett G, (2009), Fire Engineering’s Handbook for Firefighters I and II + Skill Drills, Fire Engineering Books, UK Cote A, (2003), Organizing for Fire and Rescue Services, Jones & Bartlett Learning, UK Fennell D, (1988), Investigation into the King’s Cross Underground Fire, Stationery Office, UK Hartmathy T, (1985), Fire Safety Science and Engineering, ASTM International, US Hughes P. & Hughes L, (2012), Easy Guide to Health and Safety, Routledge, UK International Association of Fire Chiefs, (2008), Fundamental of Fire Fighter Skills, Jones & Bartlett Learning, UK International Association of Fire Chiefs, (2011), Fire Inspector: Principles and Practice, Jones & Bartlett Publishers, UK Marsh E, (2000), Composite in Infrastructure- Building New Markets, Elsevier, UK Mittendorf J, (1998), Truck Company Operations, Fire Engineering Books, UK National Fire Protection Association, (2004), Fundamentals of Fire Fighter Skills, Jones & Bartlett Learning, UK Pepper I, (2010), Crime Scene Investigation: Methods and Procedures, McGraw-Hill International, UK Prager F. & Rosteck H, (2006), Polyurethane and Fire, John Wiley & Sons, US Quintiere J, (1998), Principles of Fire Behaviour, Cengage Learning, US Rapra Technology, (1993), Enhancing Polymers Using Additives and Modifiers, iSmithers Rapra Publishing, UK Rashbash D, Ramachandran G, Kandola B, & Law M, (2004), Evaluation of Fire Safety, John Wiley & Sons, US Stellman J, (1998), Encyclopaedia of Occupational Health and Safety, International Labour Organization, UK Stollard P. & Abrahams J, (1999), Fire from First Principles: A Design Guide to Building Fire Safety, Taylor & Francis, UK Wang Y, (2002), Steel and Composite Structures: Analysis and Design for Fire Safety, Taylor & Francis, UK Wurm J, (2007), Glass Structures, Springer, UK Yung D, (2008), Principles of Fire Risk Assessment in Buildings, John Wiley & Sons, UK Read More