Burning Dependance on Oxygen - Lab Report Example

The presence of oxygen has to be there in a fuel to support combustion. This is called the limiting oxygen. The presence of higher limiting oxygen will reduce flammability. The presence of oxygen in the air has been found as 21% and limiting oxygen index of less than 21% has been found to help burning in open air situation. Oxygen index method has been found to describe the tendency of a material to catch flame. This has been used as the method to find the flammability of polymers. Limiting oxygen index has also been found to determine the numerical measure of flammability. The limiting oxygen index has also been used for testing the relative flammability of rubbers, textiles, paper, coating and other materials. The main attraction of using limiting oxygen index is that the equipment used is inexpensive and that requires only a small sample size. The relative flammability of fire retarded items has been investigated with these methods and that has helped to find the efficiency of fire retardancy mechanisms. The sample is burnt in an atmosphere that is controlled and the standard procedure is to ignite the top of the sample .The ignition is done using gas flame which has t o be withdrawn once the ignition has occurred and the aim of the experiment is to find the lowest oxygen concenteration.The mixture of nitrogen and oxygen will be flowing upwards and their role is to support the sustained burning in a vertical flame. The extinguishing of the flame means that the level of the oxygen has to be increased and the sample has to be reignited again. The term critical measure means that the sample should be burning for a certain amount of time or the sample must be burning till certain length is consumed. The effectiveness of the fire retarders are calculated by the change in the critical oxygen concentration. The burning behaviour and the tendency to spread flame within a limiting oxygen index of more than 21% will be reduced once the ignition source has been removed. It has been found that self sustaining combustion has not been possible if limiting oxygen index has been equal or greater than 100. Methodology of testing The testing specimens should be kept at an ideal temperature of at 23ºC ±2°C and and 50 ±5 % RH and the testing apparatus also should be maintained at ambient temperature. The temperature of the oxygen and nitrogen gas mixture is 23ºC ±2°C. The first step is to ensure that the valves of nitrogen and oxygen are kept at off position .The oxygen and nitrogen gas bottles are opened at line pressure of 29 psi or 2.0 bar if the instrument fitted is standard flow meter. If the instrument fitted is air-flow meter, then the line pressure should be 39.9 psi or 2.75 bar. The instrument has to be turned on by connecting to the supply and proper steps should be taken to ensure that temperature and oxygen concentration display are functioning. Then the extraction system has to be turned on after positioning the chimney. Once the unit has been warmed up and stabilized, the nitrogen valve is turned on. The nitrogen valve has to be fully opened and the Flostat should be adjusted to get the flow rate as equivalent to 40 mm/s through the chimeney. The flow reading has to be taken at the top of float .The thing that has to be kept in mind is that a glass chimney that has 75mm diameter ,10.61 min-1 will be the recording for 40 mm/s linear flow and in an internal diameter of 100 mm,this will be 18.81 min-i.The oxygen analyzer should be made to zero reading or 0.0%.After this the nitrogen valve should be completely turned off and then only oxygen valve should be opened. The two gases should be in the same pressure and steps should be taken for it. The oxygen valve should be turned only after it has met certain specifications. The span adjust on the front panel should read the oxygen concentration as 99.5% in case of commercial grade oxygen and 100% in case of high purity oxygen. Then the oxygen valve should be turned to off. The initial concentration of oxygen that has to be used has to be selected and this can be found by igniting a test specimen in air and understanding the burning behaviour. The specimen is found to be burning rapidly means that 18% of the oxygen is needed and if the specimen is not found to be burning gently, then 21% of the oxygen is needed. The higher difficulty in burning means that an oxygen concentrate of 25% is needed. The nitrogen valve has to be turned on slowly and then only oxygen valve has to be turned on. The oxygen valve needle should be fully opened and to require more than 50% of the oxygen. One of the needle valve should be fully opened to get the required concenteration.The opening of the valves should be a slow process .This has to be done to prevent an initial surge of flow and the nitrogen valve has to be fully opened before the oxygen. The neck valve on the igniter should be closed and the propane bottle should be on. The neck valve should be opened only a little and the gas at the end of tube should be ignited at the end. The tube should be placed vertically and the neck valve should be adjusted to obtain valve. The flame should be projected 16 ±4 mm vertically downwards. The propane igniters should be removed and the length of the flame in the air should be noted and after that the neck valve should be turned off and gas valves should be in off position. The chimney should be removed and the specimen should be made vertically and centrally in the appropriate holder and this should be placed in the stem. A test chimney should be placed vertically over the specimen and ensure that the specimen op should be at least 100 mm above the top of lower debris screen. The specimen can be repositioned if needed. The gases should be turned once again and this should be started with nitrogen first and then oxygen should be turned on.A time period of 30 seconds will be there before the gases fill the chimney and the required oxygen concentration can be got by turning the needle on. The igniters neck valve should be opened and the valve should be adjusted in such a way that the flame is lowered into the chimney and the specimen is tried to be ignited. The procedure A is used for specimens 1,2,3,4 and Procedure B is used for specimen 5. Details of Procedures The procedure A should be done in such a way that the flame is applied at the top of the specimen and this should be done in a sweeping motion. The whole surface should be covered and the care should be taken to ensure that the flame does not touch the edge of the specimen and the flame is removed every five seconds and that is done to ensure that the top of the flame is burning and if it is burning the igniters should be removed and the timer should be started after turning off. If the specimen is not burning, the igniters should be reapplied at the top of the specimen and this should removed after five seconds has passed off and this should be done for a maximum of 30 seconds. This should be done till the material is termed in ignitable and O response recorded. The procedure B has to be used in such a way that the igniters is used to apply flame at the end face of the specimen and it should be done at depth of approximately 6 mm on vertical faces. The flame is applied for 30 seconds and it should be removed every five seconds to observe the specimens. The first part of the analysis involves determining the preliminary oxygen concentration. Here oxygen concentration changes are of any convenient step size until two oxygen concentrations have been found that differ by ≤1 % and of which one gave an “O” response and the other an “X” response. Note that these need not be from consecutive specimens. The preliminary oxygen concentration is taken to be the concentration which gave the “O” response. The main part of the analysis is determining the oxygen index based on the preliminary oxygen concentration. Test another specimen at the preliminary oxygen concentration and record its response (this need not be the same response as the run previously performed at this concentration). This test is the first in the NL and NT series of tests. Once testing has been completed turn both gas bottles off and allow the lines connecting to the apparatus to bleed of gas before turning the nitrogen and oxygen valves to the off position. Press the power button to turn the unit off and disconnect the mains cable. Ensure that the propane bottle is turned off and flare off the excess propane in the line (by opening the neck valve and igniting the gas The oxygen index is given below in the table: Oxygen Index Sample Radiant Heat Output Weight prioir to test Weight after test Observations Carpet 2 35.19 31.01 15seconds ignition Carpet 4 41.04 40.34 9 seconds Flashing, 11 seconds ignition Carpet 5 37.28 36.56 5 seconds ignition Underlay 2 35.28 32.67 9 seconds ignition Underlay 4 37.99 37.07 2 seconds ignition Underlay 5 33.97 31.44 2 seconds ignition Carpet 1 39.51 38.95 2mins 30 seconds - Dancing angels, 2 minutes 46 seconds ignition Carpet 2 41.57 41.36 19 seconds gasification 47 seconds flashing 61 seconds ignition Carpet 2 38.47 37.79 12 seconds gasification 45 seconds flashing 54 seconds ignition Carpet 3 42.9 42.48 6 seconds gasification 25 seconds ignition Lino 1 59.75 58.5 46 seconds Lino 2 57.7 57.02 13 seconds gasification 15 seconds blistering 20 seconds ignition Read More