Fire Spread Processes and Fire Protection - Assignment Example

Fire protection: questions Question1 A: compare the following fire equations a) dMwood/dt = 0.092A*sqrt (h) (Kg/s) b) dM/dt = 0.09(Qpw2)1/3h (Kg/s) Where A (m2) = area of opening W (m) = width of opening H (m) = height of opening QpkW = convective rate of heat release Solution The first equation is used to represent fire with a constant rate of heat release. The plot for such a case is shown below: Heat release rate Time This model is basically applied in the Kawagoe’s model of room fire. This is a room which has a single opening. The equation is used to approximate the burning rate of wood. The burning period (tb) for this case is given by: - tb = Mf/dMf/dT. In this case, Mf is the combustible mass Rate of heat release Q (MW) is given by: Q = dMf/dt ΔHc, where dMf/dt is constant hence Q becomes constant also. The second equation is used for calculation in calculation of the approximate figures for design of a mass flow. This equation is different from the first equation in that with additional knowledge of the mass flux, the equation can be used to determine the temperature for a fire. B: the fire occurs in a room of dimensions 6m by 4m by 3m and a window 1m by 2m. maximum rate of heat release is 1.053MW. Solution Averaged temperature of the room: - Specific heat of the air: cp = 1.03kJ/ (kg. K) Mass flux: dM/dt = 0.09(Qpw2)1/3h = 0.09{(Q/1.5)*w2}1/3h Temperature: T = Ta + [Qp/cp* dM/dt]= Ta + Q/[1.5 cp* dM/dt] = Ta + (Q/1.5)2/3/0.09w2/3hcp = 20 + (1053/1.5) 2/3 / 0.09*22/3*1*1.03 = 556.770C According to class t square fire model, the temperature development is: - To get the time instant, Q = at2 = 1053 For a medium fire, the model constant a = 0.117 Q = 1053 = 0.117 t12. t1 = sqrt (1053/0.0117) = 300 seconds The heat that is released in this time is calculated as below: E1 = at13/3 = (0.0117/3)*3003 = 105300kJ = 105.3MJ Curve Temperature Time Question 2 The building is made up of concrete and steel. The doors are wooden and glazed windows framed of uPVC plastic. In case of an accidental fire: - a) Possible fire spreads in the in the building and around it include: wood, the conditioning ducts and the uPVC material b) Fire spread processes The process of fire spread occurs through the heat transfer methods of conduction, convection and radiation. When the fire is ignited, the heat from the fire gets transferred to the nearby fuels which are unburnt and immediately any unburnt combustibles in the proximity. Solar radiation from the windows increases the amounts of the spread of the fire due to an increase in the heating effect. Due to the presence of air resulting from the air conditioning, increases the height of the flames which will make the flames to hang over the unburnt fuels. This increases the heat transfer through convection. When smoke is trapped in the building, there is additional heat transfer due to radiation of the energy. This increases the heat rate and the smoke causing the fire to increase and grow more rapidly. This is mainly due to the increased speed of burning the unburnt fuel and the combustibles. c) Calculation of minimum thickness of the wall separating the rooms to prevent fire spread through conduction Thermal conductivity of concrete: 0.76W/ (m.K) Fire room temperature: 5850C Neighboring room ignition point: 2500C Maximum heat flux through the walls: 1.25kW/m2 Change in temperature is: ΔT = TA - TB = (585+273) K – (250+273) K = 335K Q = -kA* ΔT/ Δx, Q/A = -k * ΔT/ Δx, -11250 = -1.7W (m.K) * 335/ Δx = 569.5/ Δx Δx = 569.5W/m / 11250W/m2 Δx = 0.506 meters Question 3 Building criteria for construction of external walls External walls have some basic functions related to prevention of the spread of fire which include; prevention of the fire spreads from the low storeys to the upper storeys, confining the fire within the burning building until it gets confined or diminishes and inhibition of fire spread across the boundaries of the building. The external wall is not supposed to provide a medium which will allow fire to spread if that is likely to be a safety risk or a health risk. In tall buildings, such kinds of risks may be presented by the use of thermal insulations that are combustible or as over-claddings or even in cavities which are ventilated. In buildings which are over 18 storeys above the ground, the insulation material which is in use in the ventilated cavities in the construction of the external walls should have a very low combustibility. This is however is not applicable to the construction of walls with “masonry cavity”. The roof covering reduces fire spread just like the external walls. They differ in that the external walls prevent transmission of fire through radiation whilst the roof covering prevents the spread of fire from one building to the rest. In the design of the roof coverings, there is no restriction to the usage of plastic roof lights, the roof covering needs to be a material with a low combustibility foe a distance of at least 3 meters. The roof lights should be a distance away from the compartments walls. Components of a standard fire test There are three main components which determine the end point criteria in a standard fire test. They include; Stability: the criterion for stability is the ability of a specimen for a particular element which bears a load to support the test load that it has in an appropriate manner. It has to do this without going in excess of a criterion which has been specified with respect to the extent and or rate of deformation. Integrity: the ability of a specimen of an element used for separation to contain a fire to a certain criteria which is specified for collapse, cracks and fissures, freedom from any holes and flaming sustained on exposed surface. Insulation: this is the ability of a specimen of an element used for separation to inhibit the increase in temperature of the face that is not exposed to certain levels which are specified. Fire resistance Fire resistance is the time for which a building construction element can withstand exposure to standard temperature time and also to some regime of pressure without the element losing its load bearing function or even the fire separation function or both of them. The fire resistance test includes the subjection of the building components to a heat environment that is standard for specific time duration. A fire resistance test is passed if the end point criteria do not get reached before the test period gets reached. The heating environment in which the specimen under test is exposed to is produced in some furnaces which are designed specifically with a fuel input that is controlled so as to allow reproduction of a standard relationship of time and temperature. The standard temperature time curve is given below Where T is the temperature of the fire, T0 is the initial temperature and t is the time in minutes. Termination criteria Failure in insulation occurs when, the temperature of the mean exposed face increases by over 1400C above its first value or 1210C above the ambient temperature. This is according to the BS476, part 20 and the ASTM E-119 respectively. The insulation failure also occurs when there is a recorded temperature of above 1800C over the initial mean temperature of the face that is not exposed. Insulation also occurs when there is integrity failure. The test specimen is considered to fail if it exceeds a deflection of L/20 or a rate of deflection of L2/9000d in an interval of one minute. L is the specimen’s clear span and d the distance the structural section top to the design tension zone bottom. Drawbacks of the standard fire test procedure Expense: if a full fire test is required, it is very expensive to conduct the test in terms of preparation of the specimen and also the actual test of the specimen. Data which is obtained for a particular test is applicable for the test only and thus a slight mistake will constitute to repetition of the test which is even more expensive. Limitations of the specimens: due to the size of the test furnaces, large specimen cannot be tested and instead representative specimens are used. It is thus quite hard to make realistic tests to some beams since the specimen cannot be scaled. Reduction of the sizes can lead to errors in the results. Effects of continuity and restraints: due to the limitations of the size of the specimens and also the loading arrangements in the furnaces, it becomes impossible to test specimens which have end conditions which are not idealized. Question five Case of an accidental fire in a room in building constructed with steel. On getting subjected to a fire, the structure reduces in its strength and gets weak Part A: reasons for the reduction in steel strength Steel starts losing its strength with an increase in the temperature. At 5500C, steel loses approximately 40% of the strength at room temperature and the same percentage of its modulus of elasticity. The equations below show the effect of temperature on the modulus of elasticity and also the yield strength of steel. Yield strength at 0° ≤ T ≤ 600°C Yield strength at 600° ≤ T ≤ 1000°C Modulus of Elasticity at 0° ≤ T ≤ 600°C Modulus of Elasticity at 600° ≤ T ≤ 1000°C Mechanical properties of steel at high temperatures Methods protection methods for steel Fire protection methods are various and dependent on the load of the fire, rating of the fire and the type of structural members that have been used. Some of the common fire protection methods are indicated below: Spray protection: the thickness of the spray protection is dependent on the rating of the fire that is required. This is a low cost system that can be applied very fast. Due to the fact it is usually applied in surfaces with undulating kinds of finish, it is applied in areas with hidden views. Board protection: this method is very effective though quite expensive. It is used in columns or in exposed beams. Steel does not need to be prepared while using this method. Intumescent coatings expand and form insulation in the event of a fire outbreak. The method is known for not increasing the overall member dimensions. Concrete encasement: this is a more traditional fire prevention method. Due to the composite action of steel and concrete, there is high load resistance as well as a high fire resistance. The methods increase the amount of deadweight as compared to steel frames which are protected. Read More