Scientific Principles of Fire Professionals - Assignment Example

Scientific Principles of Fire Professionals By: Institution: Course: Instructor: Date of submission: The fundamental physiochemical processes during the thermal degradation and combustion of polymeric material Polymers entail large molecules comprising of similar intra-molecular and inter-molecular forces as compounds of low molecular weight. They usually have an infinite boiling point because their molecular weight is usually high. On applying heat, solid polymeric materials experience both chemical and physical changes (Office of Aviation Research 2015, p. 1). Polymers are a large as well as fraction of load of fire in commercial environments, homes, and transportation. The most flammable polymers are the plastics. Flammability entails the capacity of a substance to have an easy ignition as well as burn at a rapid rate with a frame. It is one of the fire hazard indicator. A brief exposure of polymers to fire make then to experience a continuous burning. For the generation of fuel species to occur, the polymer’s intra-molecular and intermolecular chemical bonds need to be broken down. To do this, thermal energy must be supplied for ignition as well as to sustain burning. Flaming combustion can be categorized into chemical and physical processes which occur in three phases namely gas, mesophase, as well as condensed (solid/liquid phase) (Office of Aviation Research 2015, p. 6). The following is a fire cycle diagram. Figure 1: physical and chemical processes in framing combustion of polymeric materials. Source: office of Aviation Research Washington. The gas phase a) The Kinetics The condensed phase (liquids and solids) of ignitable compounds will only burn if it is possible to allow them to generate a volatile fuel as well as air mixture. This phenomena result in ignition as well as heat release in the gaseous state. Although there exist numerous chemical reactions in the flame which convert fuel and oxygen into stable products of combustion, experimental data and kinetic modelling have revealed that the sensitivity of the burning velocity is high in the following reactions which involve active radicals. Initiation: RH →k1 R• + •H Branching: •H + O2 →k2 •OH + •O Primary heat producing reaction (propagation): •OH + CO →k3 CO2 + •H Propagation: •OH + H2 →k4 H2O + •H Termination: •H + O2 →k5 •HO2 Inhibition: •H + HX →k6 H2 + •X Inhibition: •OH + HX →k7 H2O + •X Active radicals are denoted by •, R is fuel, H represent hydrogen, OH is hydroxyl, X is for phosphorous or halogen, while O is for oxygen. Thermochemistry If the combustion reactions in the kinetic progress to completion at a rate that bears flaming combustion, the chemical reaction involving atmospheric oxygen and generic fuel results in mineral acid (HX), water (H2O), carbon dioxide (CO2), and nitrogen (N2). Mesophase The mesophase entails the boundary between the condensed and gas phase during burning. Mesophase is composed of products that are thermally degradable in various phases inclusive of solid, liquid, as well as gases. At a typical rates of burning, the mesophase (polymer surface) subside at about 10-6 m/s Velocity. Generation of fuel is the rate-limiting phase in the framing combustion of the fuel, and it is directed primarily by mass and heat transportation rate to as well as from polymer respectively (Office of Aviation Research 2015, p. 7). Mesophase is where all the chemistry of thermal degradation that lead to the generation of volatile fuel, take place. The distribution of the temperature at the surface of burning is mainly as a result of prevailing balance of energy but the mesophase’ density, physical dimension, and viscosity are determined by thermal degradation chemistry and temperature field. For polymers with the capacity to thermally degrade, leading to solid char formation, the mesophase viscosity is higher compared to molten as well as solid polymer (Drysdale 2014 p. 1). Figure 2: the profile of mesophase’s viscocity, density, and temperature during flaming combustion The condensed phase The rate of transporting as well as storage of heat in the condensed phase is fundamentally essential since these processes are the determinants of ignition as well as burning rate of polymers. Among the thermal properties affecting polymer’s response to the external heat source exposure is the surface absorptivity. It is generally presumed that polymers’ behavior resemble that of gray services. Neglecting changes in the magnitude or slope of thermal properties that take place at phase change, for instance melting, glass transition, as well as polymers’ decomposition temperature, there can be the approximation of temperature reliance of k, p, as well as c for unstructured polymer (Office of Aviation Research 2015, p. 15). Such properties can be plotted as shown below The fire hazards associated with combustible materials Flammable and combustible liquids Combustible material presence is a representative of burning condition. The physical as well as the chemical components of the materials determine the burning process and phenomenon. When the ignitable source is present, flammable and combustible liquids are potential risk sources. First, the open as well as the closed vapor space above these materials provides an explosive and fire hazard. Combustion and especially explosion is likely to occur if the material occur in vapor-air mixture in a concentration which is suitable (Drysdale 2014 p. 1). Gases In relation to explosions and fire hazards, two main groups of gases do exist and these are non-combustible and combustible gases. Two hazardous situation do exist when it comes to gases and these are when gases are placed in the containers and when removed from the container. Extreme overpressure of the container holding the gas may lead to the explosion of the gases. Melting and glass transition When a thermoplastic material is heat, the primary change is usually from solid or glass state to fluid state. If such a transformation take place at a temperature which is below the temperature of decomposition, the likelihood that the material will flow is usually high. Flowing material can either further the growth of fire. In other cases, the material flowing may head towards the source of fire worsening the fire situation (Drysdale 2014 p. 1). Medium and large-scale prescriptive flammability tests The Steiner Tunnel Test The Steiner Tunnel Test primarily aims at calculating the wind-aided spread proclivity of the material tested. This method is highly recommended by the model building codes of the U.S. The description of Steiner Tunnel Test is in NFPA 255, ASTM E 84, “Standard Method of Test of Service Burning Characteristics of Building Materials”, and “Standard Test Method of Test for Surface Burning Characteristics of Building Materials”. the test is conducted in an aperture comprising of long as well as an enclosure resembling a tunnel that measures 0.31 by 0.45 by 8.7 m. the specimen of test is 0.51 m wide and 7.6 m tall and its mounting is in ceiling position. A force draft thru the tunnel is present from the end of the burner and its average initial velocity is 1.2m/s. a smoke photometer is fixed on the exhaust duct. On one side of the photometer is the white light source and a photocell is available on duct’s opposite side (Office of Aviation Research 2015, p. 43). Fig 2: Steiner Tunnel Apparatus The duration of the test is approximately 10 minutes. The basis of calculating flame spread index is the area under the location of flame tip curve versus time. The Radiant Flooring Panel Test Radiant Flooring Panel Test was developed in 1970s by NBS (national bureau of standards) while investigating floor coverings’ fire hazards. Its test apparatus comprises of air-gas-fueled radiant panel whose inclination is 30 degree. The initiation of the test is through open-frame ignition mainly from a pilot burner. The LOI Test This is among the methods utilized in material production control, assemblies, as well as in new products development. The test does not have good correlation with other flammability as well as fire tests, and at the same time, it does not offer a reliable material performance indication. It is an ideal tool for quality assurance especially for materials that are fire retarded. Its apparatus comprises of glass tube with a diameter of 75 to 100 mm and with a height of 450 to 500 mm. a mixture of nitrogen as well as oxygen gases are supplied at the tube’s bottom (Office of Aviation Research 2015, p. 45). The Pyrolysis Combustion Flow Calorimeter (PCFC) The method is commonly utilized in research to develop polymers that have improved fire performance. In PCFC, the gas phase and the condensed phase framing combustion processes are separately reproduced and this is mainly via pyrolyzate combustion in excess oxygen and milligram sample controlled pyrolysis in an inert gas. Fire propagation Apparatus FM Global Research developed this apparatus and its aim is to measure the flammability of the polymer material. The Room/Corner Test It is utilized to evaluate the characteristics of fire growth of ceiling and wall linings. The test material is lined with the ceiling and walls of approximately 2.4 by 3.6 by 2.4 m room. In one of the room’s rear corners, a gas burner is available. The combustion products are collected in a hood (Office of Aviation Research 2015, p. 47). Bibliographies Drysdale, D. 2014. The chemistry and physics of fire. International Labor Office. Office of Aviation Research. 2015. Polymer flammability. National Technical Information Service. Washington: Springfield. Read More