Definition of Compartment Fire Situations - Term Paper Example

RUNNING HEAD: Literature Review Literature Review of Compartment Fires Name: Instructor: Course Unit: Date: 2.2 Compartment Fire: Fire is a fast, continual chemical change that liberates heat and light and is accompanied by flame; it results from the exothermic oxidation of a combustible material and will persist till the fuel concentration decreases to a minimum rate or unless extinguished. Compartment fire envelops the complete fundamental nature of fire growth. The word ‘compartment’ in here refers to any enclosed space that controls the critical air supply and thermal surroundings of the fire. These factors have power over the spread and growth of the fire, its maximum burning rate and its length. Fire in enclosed spaces can be differentiated into three stages. Stage one is fire development whereby fire raises in magnitude from a small ‘primary’ fire. If no act is applied to contain the fire, it in the long run cultivates into a maximum magnitude that is restricted by the concentration of fuel at hand, technically the fire is termed to be fuel controlled, or the quantity of air accessible through ventilation apertures, and in other words it is ventilation limited. When the entire fuel is utilized, the fire will diminish in magnitude or perish. These stages of fire development can be seen in Figure 1 below: Developed Flashover Fully developed Cooling phase Temperature Effect on structure Effect on people Effect on operation Time Figure 1: Stages of fire development The fully developed fire is influenced through the following factors; the dimension and nature of the enclosed space; the quantity, circulation and category of fuel in the enclosed space; the number, circulation and type of ventilation of the enclosed space; and the form and nature of construction materials consisting of the roof, ceiling, fortifications and flooring of the enclosed space. The consequence of every stage of a compartment fire relies on the fire safety system gadget under deliberation. The fully developed fire and its decay phase are considerable for the reliability of the structural basics. The factor of dimension and nature of the enclosed space is depicted in Figure 2 that demonstrates different situations; Figure 2: Compartment fire situations (a) Centers on usual building compartment fires representative of living or working areas in which the area height is technically approximately three meters; (b) Focuses on high, large spaces where the enclosed oxygen reservoir is essential; (c) Permeable enclosures where annihilation & oscillatory combustion have been pragmatic; (d) Enclosures in which the only vent is at the roof, for instance boats, and underground rooms. An air starved fire has low air inlet and low fire gases outlet as in Figure 2. (a), (b) and (d) above which illustrate that the air inlet and outlet of the gases all take place within the same vent, however the fire rig brought into play by Andrew, Ledger and Phylaktou, which is air starved fires, the air enters all the way through the bottom of the rig at the same time as the gases get away through the vent at roof of the enclosure at a controlled ventilation rate as in Figure 2. (c), above. Flashover is a phrase challenging further deliberation. It’s an observable fact that is more often than not understandable to the eyewitness of fire growth. In broad sense, flashover is the changeover involving the developing fire that is still reasonably gentle and the fully developed fire. It by and large distinguishes between the fuels controlled or well ventilated fire and the ventilation limited fire. Flashover can be set off by more than a few methods. On the condition that the enclosure does not get adequate air, the fire can bring into being large flames outside the enclosure. A ventilation-limited fire can have burning generally at the vents and significant toxicity concerns take place due to the unfinished combustion course of action. Remote ignition is one of the mechanisms of flashover, which is the abrupt ignition by auto-ignition due to flaming brands, as a consequence of radiant heating. The radiant heating is predominantly from the enclosure ceiling and hot upper gases due to their large extent. The threshold for the auto- ignition of many regular materials is approximately 20 kW/m2, this is according to Waterman principle. This value of flux, calculated at the floor, is universally grasped as an operational principle for flashover. It also corresponds to gas temperatures of about 500–600oC as observed by Hagglund et al. Of essence is that; these observations made by Waterman and Hagglund et al were not conducted under air-starved state of affairs. Furthermore, comparisons of the standard temperature-time curves with temperatures measured during full scale experimental compartment fires reveal that actual fires behave in a different way to these in the standard test as illustrated in Figure 3. Figure 3: Rrelationships between standard temperature-time curves with the temperatures measured during compartments fires Other flashover mechanisms include; rapid spread which upshots in the order of 1 m/s; Burning instability; Oxygen supply, which can encourage back-draft and entails a fire burning in a very ventilation-limited state; and Boil-over, which occurs when water is sprayed on to a less dense burning liquid with a boiling temperature higher than that of water. 2.2 Effect of Ventilation on Fires: Ventilation controlled fires have often been examined in test rigs that have open window or door situations where passive fire protection can be practical to limit the air supply to fire. This is not a very level-headed way of determining the toxic gases from a fire as the legislations require doors and windows to be closed. A closed door or minute leakage would lead to partial combustion hence decrease in energy release rate and increase in the concentration of unburnt toxic gases like carbon monoxide. Well-ventilated fires with high fire temperatures are the most suitable for fire resistance testing because they create the worst case fire scenarios in terms of loss of compartmentation. In the case of fire toxicity studies, they appear to be inappropriate or unsuitable as they do not create the worst case scenarios, apart from the carbon monoxide toxicity. This work labours to show that material fire testing for toxicity in terms of acidic gases and smoke should be undertaken under air-starved low temperature conditions. If high hydrocarbons are present in air-starved fires, acidic and irritant toxic gases have a tendency to be formed due to the partial oxidation products of hydrocarbons. Andrews et al. (14-16) introduced a ventilation parameter Kin that is the air in leakage equivalent open area, Av, divided by the cross-sectional area of a cubic room of equivalent volume (V) to the test room. This definition allows air in leakage to be assessed for any shape of room. This definition can be expressed as Kin = Av/V2/3 . Their work revealed that the yield of toxic products in fires is a function of the fire equivalence ratio. Ibid., discovered that the tabulations of fire product yields measured in a Cone Calorimeter which is freely ventilated equipment may not represent the actual yields in enclosed air starved fires. A lot of of these flow features are demonstrated in photographs taken by Quintiere et al., for ‘corridor’ flows, made noticeable by white smoke streaks as in Figure.4. Figure 4: (a) Smoke layers in a model corridor as a function of the exit door width (b) Smoke streak lines showing four directional flows (c) Smoke streak lines illustrating mixing of the inlet flow from the right at the vent with the corridor upper layer. 2.3 Toxic gases from enclosed fires: Toxic gases from compartment fires are hazardous to human beings especially the vulnerable ones (Young, sick and the old). These toxic gases are categorized into three groups; narcosis-producing toxicants, irritants and toxicants showing other or extraordinary effects. The narcosis-producing toxicants include anorexia and hypoxia conditions, carbon monoxide, carbon dioxide and hydrogen cyanide. The last two gases inhibit the uptake of oxygen by blood. The irritant group is further sub-divided into two, this is, sensory irritation for instance irritation of the eyes and upper respiratory tract, and pulmonary irritation which affects the lungs. The main toxic products are carbon monoxide, hydrogen cyanide and irritants and the quantity of each depends on the thermal decomposition of the fire load, which again depends on the temperature and oxygen supply. Analysis of toxic gases in enclosure fires in a fully furnished room shows that the CO level was up to 70% away from the fire room. The peak of HCN was1200 and 150ppm while that of temperature was 800oC, and zero oxygen levels. In the later stages of fire, the heat release rate becomes limited by the air entrainment and for steady state fires the air entrainment can equal the air consumption. This can lead to an under estimation of the air consumption by a fire in the early stages and an over estimation of the global fire equivalence ratio. The calculation of the Air/Fuel ratio by mass from the gas composition involves the CO/CO2 ratio which makes it similar to the CO/CO2 ratio though it’s a perfection since the unburned hydrocarbons are integrated. For A/F ratio by full carbon balance to be determined, the elemental composition of the fire load has to be known or assumed. The equation for A/F is adequate for pure hydrocarbon fires but a more general computation procedure is necessary for fuels with high oxygen content, such as methanol or wood as in the present work. There has been development in the relevant A/F calculation procedures for these fuels. This procedure enables different fire loads and fire materials to be compared under real fire conditions and hence is a better way of ranking the fire materials for their toxic gas release than current standard methods, such as the cone calorimeter, which do not simulate the practical air/fuel ratios of fires. The equation 1 was utilized by Andrews et al as the conversion equation, but it is satisfactorily accurate to take the fire product gases as air and error is less than two percent, otherwise the calculation becomes rather cumbersome. Gas Concentration kg/kg fuel = constant x volume concentration (%) [1+A/F] (1) Where; the constant is the ratio of the species molecular mass to the molecular mass of the fire product gases. The Air/Fuel ratio would be appropriate in this current exertion. 2.4 Toxicity Evaluation: The current exertion tags along the principle of the N-Gas model introduce by Levin and Babrauskas et al. which stands on the assumption that a small number (‘N’) of gases in the smoke accounts for a large percentage of the observed toxic potency. These gases are seven and include CO, CO2, HCN, NO2, low O2, HCl and HBr. The summation of the relative toxicity of all the 7 gases is what is called the ‘N’ gas value. It’s clear from the model that irritant or acidic gas are not considered, thus it is not perfect for the current work. In an attempt to determine the combined effects of irritants(58) Purser developed the fractional irritant concentration (FIC) which expresses the concentration of each irritant present as a fraction of the concentration taken to be more irritating. The FIC’s for each irritant is then added up to give a total FIC, if it’s equal to 1 then it is predicted that the smoke would be highly irritant and is enough to slow down escape attempts but if the FIC reaches 4 and above then it is likely that escape would not occur. More than a few measures for assessing gas toxicity are accessible for instance, Lethal concentration 50 (LC50), threshold limit value (TLV), short term exposure limit (STEL), immediately dangerous to health (IDLH) and control of substances hazardous to health (COSHH). Any analysis of relative toxicity in gas mixtures will always place much more emphasis on CO using LC50 data than COSHH 15 minute limits and on the other hand COSHH will always place much more emphasis on acrolein. Therefore any measurement of toxicity in fires depends on which toxicity limit data is being used. This work will compare the two toxicity assessment procedures, but the COSHH method is preferred because of its statutory validity in legal cases anywhere in Europe, also the present work is interested in the risk to people outside the room of fire origin and not the risk to people in the room of fire origin because toxic gases that leak from a room are diluted by air and the CO2 and O2 depletion levels are not really important. Regardless of the many devices for combustion atmosphere analysis for instance, gas chromatography/mass spectrometry (GC/MS), infrared (IR) spectroscopy, non-dispersive infrared (NDIR) spectroscopy, and gas chromatography (GC), this exertion applies the Fourier transform infra-red (FTIR) spectroscopy specifically to investigate the toxic gases from wood crib fires. Reference: Andrews, G.E., Boulter, S., Burell, G., Cox, M., Daham, B, LI, H. & Phylaktou, H.N. 2007, ‘Toxic Gas Measurements Using FTIR for Combustion of COH Materials in Air Starved Enclosed Fires’ Third European Combustion Meeting, ECM, Quintiere, J. G., McCaffrey, B. J. & Rinkinen, W. 1978, “Visualization of room fire induced smoke movement in a corridor”, Fire and Materials, vol.12, no.1, pp.18–24. Read More