Estimation of Available Safe Escape Time - Assignment Example

Name Course Date Title Available Safe Escape Time (ASET) ASET is the time that elapse from the time the fire ignites to the time that untenable conditions due to the presence of smoke, heat and toxic gases is reached. The occupants should be able to evacuate before untenable conditions is reached, as the conditions are intolerable. An exposure to such conditions can cause disorientation, incapacitation or death. Thus, ASET should be long enough to enable the occupants in a building to evacuate to a safe place before the end of ASET. ASET can determine through calculation or modeling technique (British Standards Institution, 2004). The building design is considered to be safe if ASET is greater RSET. The difference between the two is the margin of safety. The margin of safety takes care of the uncertainties that results from different phenomenon of fire situation. It can be increased by ensuring that the building is made of less flammable building materials and installation of fire control system to control fire and smoke. Apart from the type of materials used and the geometry of the building, ASET can be affected by factors such as the type of fire initiated such as fast, medium or ultrafast, the fire growth rate, the fire spread and the behavior of the occupants as they response to fire (British Standards Institution, 2004). Estimation of ASET Hand calculation ASET can be estimated through a calculation by defining the performance criteria such as the maximum upper layer temperature, lower layer height (LLW), the level of irritant or toxic and heat and the degree of visibility in the presence of smoke. The typical tenability limits are shown in the table below. T(Tosolini, Grimaz and Salzano, 2013) ASET can be calculated by using LLW as the performance criteria, as it requires minimum data input. The correlation proposed is shown below. (Tosolini et al., 2013, 223-228) Where H is the height of the enclosure (m), Af is the area of the floor (m2), pg is the density of the upper layer (kg.m3), pa is the air density temperature Ta (~293K), cp is the specific of air (1kJ/kg/K), g is the gravity force and αHRR is the factor for growth rate for t2 fires (kW/s2). Fire heat release rate, HRR (t) = αHRR.t2, where t is the time(s). Examples of scenarios adopted by Tosolini et al (2012) for the estimated ASET are shown below. (Tosolini, Grimaz & Salzano, 2013, 223-228) Zone models Zone models are one dimensional model that subdivide the building compartments into zones or control volumes with uniform physical quantities like temperature, gas concentration and smoke. The hot layer occupies the upper zone, while the cold layer occupies the lower zone. These models are used to predict fire behavior within an enclosure using differential equations for the conservation of momentum, mass and energy within each layer. The model assumes quiescent flow within the zone and neglect momentum equations such a Bernoulis’ law. The law is used to calculate the fluid flow between two compartments. Zone model estimate the temperature of the gas and the interface height between the layers due to buoyancy. The advantage of this type of modeling is that it is simple as it use differential equation and can be used easily in the estimation of the ASET in a compartment. The disadvantage is that it not easy to use this model in estimating value for complex geometry and strong wind as it is too simplified (Jones, 2001). Computational fluid dynamics (CFD) This model predict hot air and smoke flow due to fire, ventilation systems, and other factors in 3-dimension for time dependent and steady state applications using basic equations that govern fluid flow, also called Navier – Stokes formula (Jones, 2001). The equation for the conservation of mass used in CFD code a differential equation like as shown below. Where p is the density, uJ is velocity of the gas, xJ is the direction and t represents time. In CFD model the domain space is divided into a large number of computational cells or small control volumes. In the modeling differential equations which describe the conservation of momentum, mass concentration of substance and energy are computed for each cell. The assumption made is that the fire behaves just like the real fires derived from experimental observations, resulting in the use of the first principle of conservation of energy. The accuracy of this model physical models used in the CFD codes such as heat transfer, turbulence, combustion and buoyancy. The advantage of this model is that it is more detailed compared to zonal model (Jones, 2001; British Standards Institution, 2004). Tenability limit BS 7974 -6 provide the criteria used to define the tenability of in a fire scenario. A good building design ensures that the occupants are not affected heat or smoke during an escape. The factors that determine the safety of the occupants include visibility in the presence of smoke, the exposure to heat and toxicity of the inhaled gasses (British Standards Institution, 2004). These factors are discussed below. i) Temperature The growth of fire and smoke result in increase in temperature as the smoke causes heating. Very high temperature can result in incapacitation or death. The recommended temperature that can be tolerated by an individual should not exceed 1200C, and less than 600C for humid conditions due to the use of water sprinklers by fire fighters. Since the survival also depends on the length of exposure, the exposure of these elevated temperatures should not be long (British Standards Institution, 2004). ii) Visibility Smoke produced during combustion of materials cause visibility problem and hence the occupants will not be able to identify the exit routes during an escape. A visibility of at least 10m (d/m = 0.08) is recommended for large enclosures and long travel distances, though it can be less if the exit route is clear. A good building design can maintain the adequate visibility along the exit routes or the corridors. Smoke control systems such as ventilation systems can reduce the effect of smoke on visibility. The individuals with poor health or old are affected more by the smoke (British Standards Institution, 2004). iii) Smoke layer The layer of smoke should not be lower than 2m above the ground level. An exposure to excessive heat radiation emitted by smoke for a long time can lead to pain and skin burns. The maximum heat flux that an individual can be exposed to should be less than 2.5 kW/m2 for a short time (British Standards Institution, 2004). iv) Toxicity The smoke produced under fire condition usually contains poisonous toxic gases such as carbon monoxide and nitrogen oxide. These gases can lead to incapacitation or death depending on the duration of the exposure and the concentration of the gases. The occupants have been known to survive if the amount of carbon monoxide is less than 700pmm for one minute (British Standards Institution, 2003). Scenarios A fire is ignited through application of the source of heat, which causes the temperature at the surface of the fuel to rise. If the fuel is flammable liquid, it produces vapour, but solid materials decompose to from flammable volatiles. The combustion of fuel occurs at the surface in the gas phase. The reaction can propagate depending on the complex heat balance between radiative and convective heat gained by the fuel, heat flux, and heat loss to surrounding environment. From experiment it has been found that the critical radiant heat flux for ignition where there is flame is between 10 and 30 kWm-2. The critical heat flux for a spontaneous ignition is approximately 40kWm-2 (Hurley, 2016). The rate of heat release in the initial incubation period increases in probation to the square of time. Q = at2 Where a is a constant in kW/s, t is the time in seconds and Q is the heat release rate in kW. Fire can be slow, medium, fast or ultrafast and is dependent on the growth time characteristics. A fire inside an enclosure transfers heat by convection and radiation. In order for the combustion to occur, heat, fuel and oxygen must be present. The reaction terminate if one of them is removed. The reaction process involves thermal decomposition or pyrolysis of fuel. This process produces volatiles substances on the fuel surface, producing combustion products and releasing heat. The fire will continue to grow if there are no control measures. The fire growth can be predicted based on experiment and models (Kutz, 2005). References British Standards Institution, Application of fire safety engineering principles to the design of buildings: Part 6. London: BSI, 2003. British Standards Institution, 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, 2004. Hurley, Morgan J. Sfpe Handbook of Fire Protection Engineering. , 2016. Jones, Walter W. State of the Art in Zone Modeling of Fires. Gaithersburg, Md: National Institute of Standards and Technology, Building and Fire Research Laboratory, 2001. Tosolini, E, S Grimaz, and E Salzano. "A Sensitivity Analysis of Available Safe Egress Time Correlation." Chemical Engineering Transactions. 31 (2013): 223-228. Read More