Fire Engineering and Engineering Desing - Essay Example

FIRE ENGINEERING Fire Engineering What is combustion? Combustion is defined as a complex sequence of exothermic reactions between an oxidant and a fuel. The reaction is accompanied by release of heat, gas fumes and light. The chemical reaction resulting to the production of fire is as shown below; Where is a chemical that supports combustion and O2 is the oxygen available from the atmosphere. The products of combustion are carbon dioxide and water vapor 2. Fire safety in building Fire safety is concerned mainly with (a) Developing means of suppressing the fire, these methods are also refereed to as active fire suppression systems (b) Developing means of preventing fire outbreak. These include educating people about the different methods of preventing fire outbreaks as well as means of escaping when a fire erupts (c) Lastly, it deals with the different methods of protecting buildings which are refereed to passive fire suppression systems and involve fitting buildings with fire proofing materials. 3. Functional requirement of approved document B This document covers fire safety related matters, it includes (a) means of warning and escape This rule stipulates that building should be designed and constructed with appropriate provisions for warning people about an impending fire, it also dictate the provisions for escape when a fire arises. (b) internal fire spread (linings) The building should be designed so that they can limit fire spread, to do this the building should (i) Reduce the spread of flame over the walls and floors (ii) When ignited, the rate of heat release should be reasonable This means that building should be have walls, ceilings and other internal surfaces coated with lining materials (c) Internal fire spread. (building structure ) (i)The building should be designed so that in case of fire, its stability should be maintained for a reasonably long period. (ii)The walls between two buildings should be designed so that they can resist fire spread between those buildings. (d) External fire spread (i)The external building structures shall be designed and build to prevent the fire spread over the walls of buildings and from one building to another. (ii)The building roof should be designed so as to resist the spread of fire over the roof and from one building to another. (e) Access to fire facilities and fire services (i)The building shall be designed and constructed so as to assist the fire suppression teams in combating fire with ease. (ii)The building shall be designed so as to get enable the fire appliances to gain access to the building 4. Means of escape from fire The word “means of escape from fire” is a terminology used to describe the different structural means of providing a safe route for people to escape in case of fire outbreaks. Safe escape route are provided in all points of a building and directs people to a safe place without any assistance from the fire fighting team or from the people outside. (5) the most important information when designing a means of escape from fire is The number of exits required The exit physical dimensions Number of occupants in a given building The minimum travel distance The time taken for people to be evacuated from a Burning building (6)The importance of performing fire tests The most important reason for performing fire test is to determine whether or not the fire suppression system meets the minimum requirement as set by different building codes and legislation. Tests are done by companies holding international accreditation and certification (7) Why is it important to carry more than one experiment? It is important to perform more than one experiment to actually proof that the installed product will not fail in times of fire. Conducting of one experiment may give conflicting results that may indicate that the product is ok but another test may reveal the fire fighting product will not work. (8) Trench effect This is the process by which the fire rapidly spreads or climbs up an inclined surface. The trench effects are caused by the flash over concept and the Coanda effect. The flash over effects is a condition that occurs when a fire spreads on a surface rapidly due to the surface emitting flammable gases hot enough to ignite themselves. The Coanda effect is the tendency of fast moving stream of air to deflect towards the surface nearby it. This is because the fast moving streams of air tend to experience a decrease in static pressure that creates a pressure difference between the areas far from the wall as well as the wall itself (9) Flammability limit The flammability limit gives the proportion of combustible gases in a mixture between which the mixture is flammable. The flammability limit is also referred to as the explosive limit. There are two extremes defined by the lower flammable limit and the upper flammable limit. The lower flammable limit: this limit describes the leanest mixture that is still combustible/ flammable The highest flammable limit: this limit describes the richest flammable mixture. Adjustment is done to the flammable gases mixture to either increase the lowest flammable mixture or reduce the highest flammable mixture. Inert gas are used for flammability adjustments (10) The five main factors that affect the development of fire in a compartment The size and geometry of the compartment The compartment ventilation conditions The thermal properties of the compartment boundary The density, distribution and fire load type The combustion behaviour of the fire (11) Stages of fire development There are five main stages involved in the development of a fire these include; 1) Incipient stage For ignition to occur heat, fuel and oxygen have to be present. The first stage of fire development occurs once the fire ignites. At this stage there is no visible smoke, flame and there is little heat. The development of the fire at this stage depends upon the fuel. Air compartment also contribute to its growth as it provide oxygen to continue developing the fire. At this stage, the radiant heat warms up the fuel and continues with the pyrolsis process. Transfer of energy occurs as a plume of hot air rises while the cold air occupies the lower part of the compartment. When the plume of hot air reaches the ceiling, it starts spreading horizontally. As the process continues, the air in the compartment becomes hot leading to the next stage. No clear transition can be drawn to the next stage of fire development. 2) Growth stage This occurs if there is enough oxygen in the room to allow for more fuel to be involved in releasing the heat. The gas layer comprise of two layers with one hot layer extending from the ceiling while the collect layer is at the bottom. The temperature of the compartment and other objects in the room rises due to the hot plume and the hot particulates in the smoke. As the temperature rises, the hot air in the compartment increases in pressure and pushes down trying to come out from any opening while the cooler air resulting in the inward movement of air. The layer where the hot air and the cold air meet is called the neutral plane. As the growth stage continue the method of heat transfer changes from convection to radiation. This increases the heat flux at the floor level. 3) Flashover The flash over process is the transition of fire from the growth stage to a fully developed fire. The transition of to a fully developed fire results from all combustible materials in a compartment being involved in the fire. The flashover occurs at 5000C -6000C. it also occurs if the heat flux which is a measure of the heat transfer to the floor reaches 15-20 KW/m2. When the flashover occurs, the entire compartment openings are pushed out with a big velocity. Sometimes flash over does not occur especially at times when there is insufficient energy to raise the compartment temperature. 4) Fully developed stage The energy release is greatest at this stage. The combustion process is rigorous but is limited by the unavailability of oxygen in the compartment. Unburned gases accumulate at the ceiling and burn as they leave the compartment. The fire temperature at this stage is about 700-12000C 5) Decay stage As the available fuel is consumed and the oxygen in the compartment reduces significantly, the heat release reduces but the temperature may continue to rise. This also presents a challenge as the room may contain hot pyrolized fuel and other flammable gaseous products. 12) Categories of fire There are five major classes of fuels causing fires, these include; 1. Fires caused by wood, trash and paper. These fires are refereed to as class A fire. 2. Fires caused by flammable liquid such as paint, propane butane, grease and gasoline. The are classified as class B 3. Fires energized by electrical equipment malfunction. They are refereed to as class C 4. Fires caused by combustion of metal such as sodium, aluminum and magnesium. they are classified as class D 5. Fires caused by greases, cooking oils and animal fats. Classified as class K 13 parameters determined using cone calorimeter Parameters determined by the cone calorimeter test include; 1. The time of ignition 2. Rate of mass loss 3. Effective heat of combustion 4. Smoke extinction area 14) Ignitability This term refers to the ease with which a material can be ignited. 15) Positive pressure ventilation The term positive pressure ventilation consists of strategies of using a fan to create pressure in a compartment or room in order to expel the heat and smoke and hence suppress the fire. This helps to secure the fire fighters as well as aiding fire rescue operations. 16 difference between temperature and heat Temperature is a number that is related to average kinetic energy of molecules in a substance. When the temperature is measured in Kelvin, the number is directly proportional to the kinetic energy possessed by the molecules. Heat on the other hand represents the total energy possessed by a substance. This total energy is the summation of the kinetic energy and the total potential. 17. Importance of investigating the causes of fire It is important to investigate the causes of fire as this helps in accurate formulation of fire prevention techniques. The information obtained is also used to educate people about the prevention methods they can adopt in-order to prevent fire outbreak. 18. Difference between diffusion flame and premixed frame A diffusion flame is the one in which the oxidizer combines with the fuel through diffusion process. The speed of the flame is limited by the rate of diffusion. Due to this fact, the flame burns with more soot and burns slower. A premixed flame: This type of flame occurs where the oxidizer and the flame mix before reaching the front of the flame. The resulting flame is thin as all the reactants are readily available. The flow of the fuel- oxidizer mixture is laminar. For a flow rate below the flame speed the flame moves upstream until it is fully consumed or it encounters a flame holder. If the flow rate corresponds to the speed, a stationary flat flame is obtained. If the flow rate is above the flame speed the flame assumes a conical shape. 19 flash point of a fuel This is the lowest temperature in which a flammable liquid can form an ignitable mixture with air. Flash point is used to describe a characteristic of liquid fuel. 20: importance of vapor in the combustion process Vapors are important in the combustion process in that they convey heat by convection and radiation from the frame to other parts. The nature of vapor produced, type and density of the vapor greatly affects how fire spreads in a building. 21: oxygen limiting test This is a widely used test and quality control tool for the determination of relative flammability of polymeric materials. The limiting oxygen index is determined by evaluating the minimum concentration of oxygen in oxygen –nitrogen mixture required to support downward burning of a vertically mounted test specimen. The accuracy for reproducing the experiment is +0.5%. The test was originally designed for testing plastics but it use has been extended to cover paper, coatings, textiles and other materials. The LOI is conducted in accordance to the BS 2782(part1, Method 141), ISO 4589-2 and ASTM D2863. Thus limiting oxygen index is a measure of the percentage of oxygen that is present to support combustion. Products with higher LOI value have lower flammability. 22. Disadvantages of the limiting oxygen index test The test method indicates the basic property of the plastic but does not describe how the plastic will behave or react during combusting in the open atmosphere. The test results obtained from these experiments vary considerably. This makes the method more unreliable. 23 piloted ignition and auto ignition Ignition is the process by which rapid exothermic reaction is initiated and then propagates rapidly causing the materials involved in the combustion to undergo change. The process produces high temperatures Piloted ignition: involves a small spark plug which gives initial energy to start a fire. Auto ignition: this involves the starting of a fire when a substance reaches the auto ignition temperature or the temperature that can start a fire. It results from a flame developing spontaneously in a flammable mixture of gases 24. Experimental analysis Test Heat release rate (h) Deviations from the mean (h-) (Deviation from the mean)2 () 1 44.80 1.255 1.575025 2 42.15 -1.395 1.946025 3 42.97 -0.575 0.330625 4 43.60 0.055 0.003025 5 43.88 0.335 0.112225 6 44.80 1.255 1.575025 7 42.79 -0.755 0.570025 8 45.10 1.555 2.418025 9 41.62 -1.925 3.705625 10 43.74 0.195 0.038025 = 435.45 =12.27365 Mean Mean = = The mean for the data above is 43.545 Variance The variance is given by Hence the variance is given by; Hence, the variance = = 1.227 Standard deviation The standard deviation is given by square root of the variance; Then, Uncertainty From the experiment, taking the uncertainty to be 0.5 it means that the mean should be between 43.545 +/- 0.5 The uncertainty in the standard deviation is given by 25 bang box test The data from the experiment is as shown below Test Number of drops Height attained by lid 1 2 0 2 6 20 3 10 40 4 12 80 5 14 120 6 16 150 7 18 110 8 20 90 On plotting the graph of the number of drops against the height attained by the lid, the resulting graph is as shown below Diagram showing the graphical representation of data collected from the bang box experiment. From the experiment it can be seen that as the number of drops increased, the height increased proportionally up to a certain extent when it stated to reduce drastically with increase in the number of drops. References Britton, L. G. 2007. Using Material Data in Static Hazard Assessment. NFPA 77 – 2007. American Chemistry Council. 2008. Working With Modern Hydrocarbon And Oxygenated Solvents: A Guide To Flammability. Solvents Industry Group. [Online]. Available at http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 Accessed 21 August 2009. Moodie, K. 1992. The Kings Cross Fire: Damage Assessment and Overview of the Technical Investigation. Fire Safety Journal. Simcox, S; Wilkes, S & Jones, I.P. 1992. Computer Simulation of the Flows of Hot Gases from the Fire at Kings Cross Underground Station Fire Safety Journal. IStructE. 2009. Guide to the Advanced Fire Safety Engineering Of Structures. [Online]. Available at http://www.istructe.org/publications/files/Advanced_fire_safety-Sample_pages.pdf . Accessed 19 August 2009. Hartin, E. (2008) Reading the fire: Heat and flame. Retrieved January 19, 2008 from www.firehouse.com. Read More