Introduction to Fires and Dynamic Thermal Stress Analysis

This manuscript explains the fire phenomena and introduces the different types of industrial fires that should be identified and characterized during the development of a risk-based quantitative assessment; i.e., flash fires, pool fires, jet fires and fireballs. It addresses specific criteria for the following primary fire types with potential for domino effect; i.e., pool and jet fires. An advanced and time efficient quantitative approach is proposed for accurately estimating the Time to Failure (TTF) of process equipment or any other type of structure of interest being impacted by fires. The approach is suitable for ensuring which are the most appropriate risk reduction measures (active and passive) to be considered during the decision-making process and to predict if there is enough time to either prevent or mitigate the fire outcomes with the aim to prevent escalation; i.e., Dynamic Thermal Stress Analysis (DTSA).

Fire is the rapid exothermic oxidation of an ignited fuel. The fuel can be in solid, liquid or vapor form. Vapor and liquid fuels are generally easier to ignite. The combustion always occurs in the vapor phase; liquids are volatized and solids are decomposed into vapor before combustion. When fuel, oxidizer and an ignition source are present at the necessary levels, burning will occur [1]. The essential elements for combustion are fuel, an oxidizer and an ignition source; i.e., the fire triangle (see Figure 01).

This means a fire will not occur if:

  1. Fuel is not present or is not present in sufficient quantities.
  2. An oxidizer is not present or is not present in sufficient quantities.
  3. The ignition source is not energetic enough to initiate the fire.

Some of the commonly used definitions associated with fires and explosions are given by reference [1]:

  • Combustion or fire: combustion or fire is a chemical reaction in which a substance combines with an oxidant and releases energy. Part of the energy released is used to sustain the reaction.
  • Ignition: ignition of a flammable mixture may be caused by a flammable mixture encountering a source of ignition with sufficient energy or the gas reaching a temperature high enough to cause the gas to autoignite.
  • Autoignition temperature: a fixed temperature above which adequate energy is available in the environment to provide an ignition source.
  • Flash point: lowest temperature at which it gives off enough vapor to form an ignitable mixture with air. At the flash point the vapor will burn but only briefly; inadequate vapor is produced to maintain combustion. The flash point generally increases with increasing pressure. There are several different experimental methods used to determine flash points. Each method produces a somewhat different value. The two most commonly used methods are open cup and closed cup, depending on the physical configuration of the experimental equipment.
  • Fire point: lowest temperature at which a vapor above a liquid will continue to burn once ignited. The fire point temperature is higher than the flash point.
  • Flammability limits: vapor-air mixtures will ignite and burn only over a well-specified range of compositions. The mixture will not burn when the composition is lower than the Lower Flammable Limit (LFL); the mixture is too lean for combustion. The mixture is also not combustible when the composition is too rich; that is, when it is above the Upper Flammable Limit (UFL). A mixture is flammable only when the composition is between the LFL and the UFL. Commonly used units are volume percent fuel (percentage of fuel plus air).

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