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Introduction to Fire

Introduction to Fire

The self-sustaining nature of fire makes it extremely dangerous if uncontrolled. Fire has been described as a “living entity consuming both oxygen and matter in order to survive”. Even with its numerous uses, uncontrolled it can be very disastrous. Every year about 5,000 people are killed by fires in Europe and more than 4,000 people in USA. Direct property loss through fire is roughly 0.2% of the gross domestic product and the total cost of fires is around 1% of the gross domestic product (Beyer, 2005). This is the driving force for scientists and safety bodies to develop new materials to tackle this problem. Once a fire starts in a room containing flammable materials; it will generate heat, which can heat up and ignite additional combustible materials. As a consequence the rate at which the fire progresses speeds up because more and more heat is released and a progressive increase in the room temperature is observed. The radiant heat and temperature can rise to such an extent that all materials within the room are ignited very easily, resulting in an extremely high rate of fire spread. This point in time is called flashover and leads to a fully developed fire. Flashover normally occurs at around 500 °C and an incident heat flux at floor level of 20 kW/m2. Escape from the room will then be virtually impossible and the spread of the fire to other rooms is highly likely. When a fire reaches flashover, every polymer will release roughly 20% of its weight as carbon monoxide, resulting in excess toxic smoke. Consequently, most people die in big fires and 90% of fire deaths are the result of fires becoming “too big”, resulting in too much toxic smoke.

 Fire test Methods

The 1988 edition of the compilation of fire tests by the American Society for Testing and Materials (ASTM) alone lists some 77 tests. ASTM is only one of many US and international organizations publishing fire test standards; the actual number of fire tests in use is at least in the hundreds. It is customary to divide the actual fire test standards into two broad categories: (i) reaction to fire, or flammability (pre-flashover), and (ii) fire endurance, or fire resistance (post-flashover). Reaction to fire is how a material or product responds to heating or to a fire. This includes ignitability, flame spread, heat release rate, and the production of various toxic, obstructing, corrosive etc., products of combustion. Reaction to fire largely concerns the emission of undesired components, e.g. how much heat is emitted, how much smoke, or how fast does the first emission start (ignitability). A reaction to fire test is typically performed on combustibles. Fire endurance, by contrast, asks the questions: how well does a product prevent the spread of fire beyond the confines of the room? Such a test is performed on barriers to fire and load bearing elements, such as walls, floors, ceilings, doors, windows and related items. Manufacturers of resins, flame retardants, and plastic products are accustomed to describing reaction to fire performance according to two tests: the UL 94 vertical Bunsen burner test and limiting oxygen index (LOI) test. The LOI test determines how low of an oxygen fraction the test samples can continue burning in a candle-like configuration. It has never been correlated to any aspect of full scale fires. The UL 94 test was developed to determine the resistance to ignition of small plastic parts, such as may be found inside electrical switches. For this purpose, it is an accurate simulation of a real fire source.

 The Necessity for Heat Release Rate Tests

Heat release rate (HRR) is the driving force of a fire. This happens as a means of positive feedback in that heat produces more heat. This occurrence does not happen for all variables, for instance, with carbon monoxide. Carbon monoxide does not produce more carbon monoxide. Most other variables in the fire are correlated to HRR. The generation of most other undesirable fire products tends to increase with increasing HRR. Smoke, toxic gases, room temperature and other fire hazard variables generally progress to increase with HRR, as HRR intensifies. Furthermore a high HRR indicates a high threat to life. Some fire hazard variables do not relate to threats to life. An example could be, a product shows easy ignitability or flame spread rates, however this does not necessarily mean that the fire conditions are expected to be dangerous. Such behavior may merely suggest a predisposition to nuisance fires. However high HRR is intrinsically dangerous and should be avoided. This is because high HRR causes high temperatures and high heat flux environments, which may prove lethal to occupants

The Cone Calorimeter

The cone calorimeter is the most significant dynamic bench scale instrument in the field of fire testing. Heat release rate is the key measurement required to assess the fire severity and development for materials and products. The cone calorimeter was first announced in 1982 by workers at the National Institute of Standards and Technology (NIST) formerly NBS, in the USA, with input predominantly from Vytenis Babrauskas (1982). However ISO 5660-1 standard was only published in its final form in 1993 and the smoke evolution measurement was supplemented later in 2001 ISO 5660 part 2 (2002). The American Society for Testing and Materials has also recognized the cone calorimeter as a certified reaction to fire apparatus in ASTM 1354 (2004). The two cone calorimeter standards are identical, except for the fact that the ISO standard does not include the smoke measurement. As a result, heat release rate research using the cone calorimeter started from that point on. It was decided to produce an improved bench scale heat release rate test which would overcome the deficiencies of existing small scale heat release rate tests which relied upon the measurement of the outflow enthalpy of enclosed systems. Oxygen consumption calorimetry was identified as the best measurement method. In 1917, Thornton showed that for a large number of organic liquids and gases, a relatively constant net amount of heat is released per unit mass of oxygen consumed for complete combustion. Huggett (1980) found that this was also true for organic solids and obtained an average value for this constant of 13 MJ/kg of O2. This value may be used for practical applications and is accurate with very few exceptions to within ± 5%. Thornton’s rule implies that it is sufficient to measure the oxygen consumed in a combustion system in order to determine the net heat release. Therefore, for example, in compartment fires, the oxygen consumption technique is much more accurate and easier to implement than methods based on measuring all the terms in a heat balance of the compartment. The cone calorimeter is designed to investigate “reaction to fire” of materials and products intended for industrial or commercial market use. This test allows an estimation of parameters such as shown below:

 

Parameter                                 Unit

  1. Heat Release Rate (HRR): kW/m2
  2. Average Heat Release Rate: kW/m2
  3. Total Heat Released (THR): MJ/m²
  4. Effective Heat of Combustion (EHC): MJ/kg
  5. Specific Extinction Area (SEA): m²/kg
  6. Exhaust Flow Rate m3/s
  7. Mass Loss Rate (MLR): g/s
  8. Final Sample Mass: g
  9. Time to Sustained Ignition: s
  10. CO/CO2 Production (optional): g/s

 

Reaction to fire tests ascertain whether the material takes part in the fire, their contribution to flame spread and their tendency to propagate and expand the fire by altering the thermal environment (preheating). The main importance in looking at the reaction to fire is to study the behavior of the material before flashover; this is the phenomenon in which the furniture and other materials in the room ignite virtually simultaneously. The intention is to study the shouldering to combustion performance of the material before full scale fire transpires, as this is the interim before the fire can be controlled and resultantly extinguished. According to the ISO concept, the phenomenon accompanying the fire such as ignitability, flame spread and heat release rate can be grouped into the primary effects of fire. The secondary fire effects, smoke and toxic fire gases, occur alongside these phenomena, particularly as the rate of flame spread increases. Together with the radiant heat release and lack of oxygen they represent the greatest danger to people. The principal result of using the cone calorimeter is a heat release rate curve over the duration of the test. The HRR due to combustion is determined using oxygen consumption methodology. The overwhelming importance of the role of HRR in fires must be made apparent. Heat release rate is not just one of many variables used to describe a fire. It is, in fact, the single most important variable in describing fire hazards. The information gathered from the cone calorimetry tests can be used in computer modeling to evaluate what would happen in a large scale fire.