Discovering accessible Secure Release Time - Assignment Example

Determining Available Safe Escape Time (ASET) Name Date Available Safe Escape Time (ASET) Buildings are designed in a way that in case of fire outbreak, there will be enough time and exits for escape. Available Safe Escape Time (ASET) is a time that starts from the fire ignition to the time in which all the occupants will have escaped from the building. Fire safety engineering has a principle that in every building ASET should be greater than the required safe escape tine (RSET) by a certain margin. An ideal building is the one that ensures that in case of fire breakout, people inside the building should be able to reach safety without being hart or coming into contact with fire (British Standards Institution, 2004). In any accident like fire in a building, there are some inevitable circumstances in which the occupants may be injured or get exposed to fire especially if they were in an area where fire started at. The effects of such scenarios should be accessed as part of risk assessment. In fire breakouts, there certain factors that affect the safety of the people inside the building, this include the point in which the fire went on, how the occupants behave, and how fast the fire spreads (British Standards Institution, 2004). Another factor is the materials used in building and finishing the building like ceiling boards. For safety of the occupants, the buildings should have fire protection like sprinklers and enough evacuation exits. ASET is calculated using the formula below (British Standards Institution, 2004) Where is the detection time? Is the time for the onset of hazardous environmental conditions? Is the notification time. Apart from calculation, it can also be determined by using fire modelling techniques. This modelling techniques begins with finding heat release history of the combustible materials inside the building as well as the combustion products like carbon dioxide and soot. The information will be used in the fire model calculation to find the time available from the time of fire ignition to the time of untenable condition (Hurley, 2015). Other criteria’s used to find ASET include temperatures beyond 60 degrees centigrade, building visibility, and concentration of carbon monoxide. Tenability limits There is a criteria for defining tenability of the fire in a fire scenario. Tenability of the building is determined by factors like room visibility, intensity of heat, and inhalation of toxic gases (Hurley, 2015). When the occupants are exposed to excess of this factors, untenable conditions is reached. This factors affect a person depending of other things like age, health status, and discipline. These tenability criteria are discussed below. Heat When there is a fire breakout in a building, heat produced in many occasions become excessive, meaning that it is in a state in which it can burn a person. A person can withstand certain temperatures but when it reaches a level that cannot withstand it will result in death (Hurley, 2015). Prolong exposure to hot environment for more than 15 minutes can cause stroke. For a short time with low humidity, a person can withstand temperatures of around 100 degrees centigrade, but for high humidity and for short time, a person can withstand temperatures of about 45 degrees centigrade. Individual tolerance to high temperatures depend on the ability of the body to cool through sweating. High humidity reduce evaporation hence lowering tolerance time. Visibility Fire produce smoke but the amount of smoke produced depend on the materials being burned. Oil produce little smoke while solid materials such as wood and plastics produce a lot of smoke. Smoke reduce the distance in which occupants can see, this will prevent them from getting the right exit on time and it can also cause stampede (Hurley, 2015). Visibility in escape routes should always be clear and a distance of twelve meters is recommended. Toxic gases When fire gets on toxic materials, it will produce toxic gases like carbon monoxide. This gases if a person inhale it will cause incapacitation or death depending on the amount inhale and the time a person is exposed to. There are many other toxic gases produced in fires and their quantity varies depending on the fuel and environmental conditions, toxicity is generally dominated by gases such as CO and HCN. A person will lose consciousness if the environment has oxygen of less than 12 percent or as a result of carbon dioxide narcotic effects, this occur where the amount of oxygen in the environment is above 6 percent. Smoke layer The layer of the smoke should be above 2 meters from the ground. There is a heat radiation that accompany the smoke and exposure to that radiation for a long time will burn the skin. The minimum amount of hat that a person should be exposed to for a short time should be below 2.5 kW/m2. Determining ASET The equation for conservation of mass and energy is used to determine ASET. In using the equation, it is assumed that the building has openings in its surrounding to reduce building pressure since the gases will be expanding as it burns. Other assumptions that are made include assuming that the density of upper layer is the same, there is no heat loss at the boundaries or is minimal, and two layers are formed, in which the upper layer is smoke while the lower layer has no smoke (Hurley, 2015). The equation used is: where and H- Room height S- Area of the floor α- growth rate factor ρg- upper layer density ρa- air density g- Gravitational force Cp- specific heat Scenarios Before untenable conditions are reached, there are certain factors that affect the time, this include the room ventilation, building materials used to construct the building, compartment geometry, and the layout (Bergman & Incropera, 2011). Engineers when making fire assessment in a building, they should consider development scenario. A fire scenario is the development and spread of fire until it consumes a building or stopped from spreading. Afire scenario for any building is developed by looking at other scenarios of other similar buildings. For fire to ignite, oxygen and energy must be present, ignition source provide the first energy, this source can be faulty electric wires, arsonist, or lighting a cigarette (Bergman & Incropera, 2011). The growth of the fire depend of the amount of fire ignited and the surrounding materials, if there is fuel around it will grow fast. As it develops, the room gets hot and materials that are vulnerable to fire can get ignited. If fire safety materials like sprinklers or carbon dioxide fire extinguishers are present in the room, the fire can be easily put off before spreading to other places. In an explosion scenario, hot gases produced will spread through the room at a high speed. Since the gas produced is hot, the smoke that accompanies it will rise because of buoyancy and fill the upper part of the room (Bergman & Incropera, 2011). The smoke affect egress process depending on the room size, ventilation, and the rate of smoke release, this is done through poor visibility and reducing untenable conditions. Fire modelling Fire modelling is a significant tool used in fire scenarios. It is used by engineers to characterize hazards resulting from accidental fires and to develop measures that will ensure that in case of fire breakout no life will be lost as a result of it (Bergman & Incropera, 2011). There are different models and all this models are used to predict fire behaviour and risk. The models that are commonly used are Zone and CFD models (Bergman & Incropera, 2011). ASET can be determined by using both models, CFD model calculate the evolving distribution of smoke, fire gases, and temperatures in the building when there is a fire breakout while Zone model solve equations based on zone assumptions. Zone models In this models, the building is divided into different zones whose physical quantities are the same. The models are designed in a way that it can predict fire breakout using differential equations for conservation of momentum, mass, and energy (Bergman & Incropera, 2011). Its advantages is that it needs low memory, it can be used to represent large structures, and it uses differential equations that work out easily. Its disadvantages is that when there is a strong ventilation or fire source in the building, it will make errors because its condition is very simplified. It will only provide the whole details with no fine details. Another thing is that the system is somehow complex and it requires and expert to feed the data and perform data analysis (Bergman & Incropera, 2011). To find ASET using Zone model, the model divides the room into hot upper layer and lower cool layer. The model calculation gives estimates of key conditions of each layer as a function of time. Computational fluid dynamics (CFD) model CFD is a kind of model that is divided into several computational cells. To solve quantities in every cell finite technique approached is applied. It is a good model because it is cable of solving complex solutions for conservation of energy and momentum using first principle method (Bergman & Incropera, 2011). The model fall in the same category with fire dynamics simulator. To find ASET using CFD model the model calculates the evolving distribution of smoke, fire gases, and temperatures in the building. It uses basic equations that govern fluid flow, also called Navier – Stokes formula. The equation for the conservation of mass used in CFD code differential equation is shown below. Its assumptions This model has certain assumptions, one is that it is assumed that fire behaviour is the same as physical or real fires, an example is that the movement of smoke is derived from experiments or real fires hence the first principle is used to solve the problem of conservation of energy and momentum (Xie, Lu, Kong, & Wang, 2012). References Bergman, T. L., & Incropera, F. P. (2011). Fundamentals of heat and mass transfer. Hoboken, NJ: Wiley. British Standards Institution. (2004). The application of fire safety engineering principles to fire safety design of buildings - Part 6: Human factors: Life safety strategies - Occupant evacuation, behaviour and condition (Sub-system 6) PD 7974-6:2004. London: BSI. Xie, Q., Lu, S., Kong, D., & Wang, J. (2012). The effect of uncertain parameters on evacuation time in commercial buildings. Journal of Fire Sciences, 30, 1, 55-67. Hurley, M. J. (2015). SFPE handbook of fire protection engineering. . Read More