Building Service Chapter 2

PSMZA Course Note (Chapter 2) 2.0 FIRE SAFETY AND PREVENTION SYSTEM Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition. The flame is the visible portion of the fire. If hot enough, the gases may become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different. Fire in its most common foam can result in conflagration, which has the potential to cause physical damage through burning. Fire is an important process that affects ecological systems across the globe. The positive effects of fire include stimulating growth and maintaining various ecological systems. Fire has been used by humans for cooking, generating heat, signaling, and propulsion purposes. The negative effects of fire include water contamination, soil erosion, atmospheric pollution and hazard to life and property. 2.1 Source of Fire The fire triangle or combustion triangle is a simple model for understanding the necessary ingredients for most fires. The triangle illustrates the three elements a fire needs to ignite: heat, fuel, and an oxidizing agent (usually oxygen). A fire naturally occurs when the elements are present and combined in the right mixture, and a fire can be prevented or extinguished by removing any one of the elements in the fire triangle. For example, covering a fire with a fire blanket removes the "oxygen" part of the triangle and can extinguish a fire. Figure 2.1: Fire triangle Figure 2.2: Fire tetrahedron Fires start when a flammable a combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas or another oxygen-rich compound, is exposed to a source of heat or ambient temperature above the flash point for the fuel mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. This is commonly called the fire tetrahedron. Fire cannot exist without all of these elements in place and in the right proportions. For example, a flammable liquid will start burning only if the fuel and oxygen are in the right proportions. Some fuel-oxygen mixes may require a catalyst, a substance that is not directly involved in any chemical reaction during combustion, but which enables the reactants to combust more readily. Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of an oxidizer and fuel. If the oxidizer is oxygen from the surrounding air, the presence of a force of gravity, or of some similar force caused by acceleration, is necessary to produce convection, which removes combustion products and brings a supply of oxygen to the fire. Without gravity, a fire rapidly surrounds itself with its own combustion products and non-oxidizing gases from the air, which exclude oxygen and extinguish it. Because of this, the risk of fire in a spacecraft is Ver. 1 (MSH-Jun213): CC608 Building Services 1 PSMZA Course Note (Chapter 2) small when it is coasting in inertial flight. Of course, this does not apply if oxygen is supplied to the fire by some process other than thermal convection. Fire can be extinguished by removing any one of the elements of the fire tetrahedron. Consider a natural gas flame, such as from a stovetop burner. The fire can be extinguished by any of the following: i. Turning off the gas supply, which removes the fuel source. ii. Covering the flame completely, which smothers the flame as the combustion both uses the available oxidizer (the oxygen in the air) and displaces it from the area around the flame with CO2. iii. Application of water, which removes heat from the fire faster than the fire can produce it. iv. Application of a retardant chemical such as Halon to the flame, which retards the chemical reaction itself until the rate of combustion is too slow to maintain the chain reaction. 2.1.1 Stages of Fire Development i. Pre-flashover Stage Fire remains limited in size initially, and can be easily extinguished using a portable fire extinguisher at first. Detection may not occur until flames become visible or when heat is produced. ii. Flashover Stage Heat becomes intense and high enough to ignite common combustible materials within the room, leading to a fully developed fire. This can happen within minutes of the pre-flashover stage when the proper conditions are in place. iii. Post-flashover Stage Fully developed phase of a fire, whereby all exposed combustibles in the room are involved. This may result in total loss of collections within the room; the entire building is threatened. Flames may spread to other rooms through hallways and ceiling voids. Fire will eventually burn out when all combustibles are consumed. Because fire can grow and spread rapidly, it is important to detect and extinguish it at the earliest stage possible in order to reduce the risk of serious damage, injury or loss. Figure 2.3: Fire development stages Ver. 1 (MSH-Jun213): CC608 Building Services 2 PSMZA Course Note (Chapter 2) 2.1.2 Source of Fire Fuel Anything that burns is fuel for a fire. To look for the things that will burn reasonably easily and are in enough quantity to provide fuel for a fire or cause it to spread to another fuel source. Some of the most common 'fuels' found in factories and warehouses are: i. Flammable liquid-based products, such as paints and varnishes ii. Flammable chemicals, such as certain cleaning products and photocopier chemicals iii. Flammable gases such as liquefied petroleum gas (LPG) and flammable refrigerants iv. Stored goods and high piled or racked storage v. Foodstuffs containing sugar and oils, such as sugar-coated cereal and butter vi. Plastics and rubber, such as video tapes, polyurethane foam-filled furniture vii. Paper products, such as stationery, advertising material and decorations; viii. Packaging materials ix. Plastic and timber storage aids both in use and idle, such as pallets and palletizers x. Combustible insulation, such as panels constructed with combustible cores; xi. Textiles and soft furnishings, such as hanging curtains and clothing displays xii. Waste products, particularly finely divided items such as shredded paper and wood shavings, offcuts, dust and litter/rubbish. 2.1.3 Source of Fire Ignition/Heat Some typical sources of ignition include: i. Exterior and natural sources such as lightning ii. Electrical sources such as faulty or overloaded wiring, electrical panels, electrical equipment and appliances, and HVAC (heating/ventilation/air conditioning) systems iii. Proximity of combustible materials to a heat source such as portable heaters iv. Open flames such as candles and food warmers used during catered events v. "interpretive fires" such as fireplaces, cook stoves, candles, blacksmith shops vi. Construction and renovation activities such as hot work example welding and paint removal vii. Improper use, storage, and/or disposal of flammable liquids such as paint thinners viii. Smoking materials ix. Gas leaks 2.2 The Spreading of The Fire Most fires start in the contents of a building. But if the flames are not quickly extinguished while in the content phase; they will extend to, and throughout the structure. It spreads throughout concealed spaces, poke through walls, common roof or attic spaces. Sometimes even along the outside of the building. Its cause of heat transfer. Heat transfer is a major factor in the ignition, growth, spread, decay and extinction of a fire. It is important to note that heat is always transferred from the hotter object to the cooler object - heat energy transferred to and object increases the object's temperature, and heat energy transferred Figure 2.4: Fire spreading from and object decreases the object's temperature Ver. 1 (MSH-Jun213): CC608 Building Services 3 PSMZA Course Note (Chapter 2) Fires can spread by four method: i. Direct ignition The ignition system for the fire combustion creates such as a lightning spark that ignites the fuel. Example of direct ignition was open flames, lightning, lighted cigarette butts and hot ashes. Figure 2.5: The fire ignition ii. Radiation Radiation is heat transfer by electromagnetic waves. It is the type of heat one feels when sitting in front of a fireplace or around a campfire. It travels in straight lines at the speed of light. This is the reason that when facing the fire, only the front is warmed. The backside is not warmed until the person turns around. The earth is heated by the sun through radiation. Sunburns are a “fact of life” when people are exposed to the sun very long. Most of the preheating of fuels ahead of a fire is by radiation of heat from the fire. As the fire front gets closer, the amount of radiant heat received is increased. Figure 2.6: The fire radiation iii. Convection Convection is heat transfer by the movement of liquids or gasses. Convection is the transfer of heat by the physical movement of hot masses of air. As air is heated, it expands (as do all objects). As it expands, it becomes lighter than the surrounding air and it rises. This is why the air near the ceiling of a heated room is warmer than that near the floor. The cooler air rushes in from the sides. It is heated in turn and it also rises. Soon a convection column is foamed above the fire which can be seen by the smoke that is carried aloft in it.

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