Available and Required Safe Egress Time - Assignment Example

Name Course Date Part B: Available Safe Escape Time (ASET) ASET is the time between the start of a fire ignition to the time the untenable conditions are reached. The tenability is determined by factors such as visibility, toxicity of gases and the amount of heat radiated by the fire. The objective of ASET is to evacuate all the occupants to a safe place before the harmful condition is attained (CIBSE, 2010). The rate of fire spread depends on the type and the amount of the materials used in the construction, the height of the ceiling, the building geometry, the physical barriers along the escape routes and smoke ventilation systems. Margin of safety is obtained from the difference between RSET and ASET. Margin of safety covers the uncertainties associated with various potential fire scenarios. In an ideal building design that takes care of the safety of the occupants, the occupants must be able to move to a safe place without being subjected to any physical harm (Hurley, 2016). Tenability limit The criteria for determining the tenability of a fire situation has been defined in the British codes such as BS 7974. The objective is to make sure that the occupants move to a safe place in or outside the building without being hurt. The safety of individuals depends of factors such as visibility through smoke, radiative heat flux and the presence of toxic gases (British Standards Institution, 2001). The parameters used in determination of tenability criteria are discussed below. Temperature -As the fire grows and the smoke spread, the temperature increases which can cause disorientation or death to an individual. The maximum temperature that an individual can be exposed to for a short time without being incapacitated is 1200C, and is less than 600C in a scenario where the humidity is high due to the presence water from fire fighters. The maximum amount of heat flux that an individual can be exposed to for a short time should be less than 2.5 kW/m2 (British Standards Institution, 2004). Visibility -Visibility is very significant for the escape of the occupants as it enables them to identify the exit routes. A smoke is produced as the materials burn produce smoke. The visibility design for long travel in large enclosures should be more than 10m (0.1m-1). Adequate visibility should be maintained through the exit routes including the corridors (British Standards Institution, 2001). Smoke layer - PD 7974-2 covers the spread of smoke and toxic gases and the criteria used to control them. Hot gases accumulate near the ceiling before moving downwards to increase the temperature of the room. The hot smoke layer can radiate heat that can cause pain or skin burn, and thus should not move below the 2.1 m from the ground level. It has been found that the maximum heat flux exposed to an individual should not be higher than 2.5 kW/m2 in a short duration (British Standards Institution, 2001). Toxicity - Apart from the heat radiated by the smoke, the smoke consists of toxic and poisonous gases such as nitrogen oxide and carbon monoxide. An exposure to a high concentration of these gases for a long duration can lead to incapacitation or death. For example, an exposure to carbon monoxide for a long time especially more than 1000pmm for more than one minute has been known to cause death (British Standards Institution, 2001). Estimation of ASET (a) Calculation LLW criteria do not require many details such as heat in every point of the enclosure or the concentration of the toxic species. In these criteria it is assumed that the conditions in an enclosure are untenable for safe evacuation if the LLH drops to a given value. The criteria enable easy and fast definition of ASET as it adopts a two layer fire model as shown below. (Tosolini et al., 2013, 224) The equation used to calculate ASET utilizes the equation for the conservation energy and mass. ASET can be calculated by using lower layer height as the performance criteria, as it requires minimum data input. The correlation proposed is shown below. The assumption is that the room has been adequately ventilated to reduce the buildup of pressure due to gases expansion. (Tosolini et al., 2013, 223-228) Where H is the height of the enclosure (m), Af is the area of the floor (m2), ρg is the density of the upper layer (kg.m3), ρa 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) (Tosolini et al., 2012). (b) Simulation CFD and Zone models are the main computer models used in many fire scenarios especially in designing fire safety in a building and control of smoke. The models predict a fire conditions using numerical equation as a function of time. The two methods are discussed below. (i) Zone models These types of model represent an enclosure under fire as two layers with homogenous characteristics called zones or control volumes. The layer in the upper zone consists of hot smoke gases, and the lower zone consists of cold gases. The model is dependent on the first principles and the assumptions that the control volumes follow the conservation equations for energy and mass. The properties such as the density, temperature and the concentration of species are uniform for each zone (Bong, Wen Jiann, 2012). Zone models can be confirmed using an experimental data of small volumes as an example of an occupied compartment. If a fire occurs in an enclosure, a fire plume is induced; a smoke moves up to the ceiling before turning towards, creating a stratified smoke. As the zone model does not obey conservation equation, the layer is considered to be created instantly. A small compartment is considered to be representing such as scenario. The advantage of this technique is that the calculated run times are fast in a computer (Bong, Wen Jiann, 2012). (ii) Computational Fluid Dynamics (CFD) An enclosure in CFD model is defined in three dimensions geometry and is divided into a large number of small cells. The governing equations are computed numerically depending on basic laws the conservation of momentum, energy and mass for the each cell in the domain space. CFD model is evidently more complex compared to zone model, as it provides more details of fluid flow and transfer of heat in a specific point to be studied in an enclosure as a function of time. Though, the number of run times in the computation is considered to be longer owing to the number of calculations to be done (Bong, Wen Jiann, 2012, 10342). It is assumed that the fire behaves like in real fires similar to the experimental values. The accuracy of this model depends on the assumptions used in the model codes. The assumption covers the buoyancy, heat transfer, combustion and turbulence. This model has less assumptions compared to zonal model, thus it has less errors (Hurley, 2016). Scenarios The time for escape for the occupants start from the time the fire is detected to the time untenable conditions are reached. If the flash over is reached, all the combustible materials in the room would be involved in fire, and the occupants still in the room may not possibly survive and the fire spread easily to other parts of the building. Fire suppressors such as sprinklers increase the ASET significantly (British Standards Institution, 2001). The fuel combustion occurs at the surface of the material, and the reaction can spread based on combination of the radiation and convection of heat gained by fuel, heat flux, and heat loss to surrounding. The critical radiant heat flux for ignition is approximately 30 kW/m2 and spontaneous ignition is occurs at 40kW/m2. The fire will continue to grow if there are no control measures and the fire growth can be predicted based on experiment and models. The spread of fire is limited by passive or active fire safety provisions. The fire retardants in a room minimize the spread of smoke and fire. Fire suppressors such as water sprinklers control fire spread significantly (Hurley, 2016). References Bong, Wen Jiann. Limitations of Zone Models and CFD Models for Natural Smoke Filling in Large Spaces, University of Canterbury. Department of Civil and Natural Resources Engineering, 2012. . British Standards Institution. Application of Fire Safety Engineering Principles to the Design of Buildings: Code of Practice. London: BSI, 2001. CIBSE Guide E, Fire Safety Engineering, Chartered Institution of Building Services Engineers, 2010. http://app.knovel.com/hotlink/toc/id:kpCIBSEGEK/cibse-guide-e Hurley, M. J. (2016). SFPE handbook of fire protection engineering. Tosolini, E., Grimaz, S., & Salzano, E. (May 29, 2013). A sensitivity analysis of available safe egress time correlation. Chemical Engineering Transactions, 31, 223-228. Read More