Thermal ignition sources

Thermal ignition sources A source that will cause the ignition of a flammable gas, vapor, or dust, such as an electric spark, flame, or hot surface. 

Fig. 1. Pressure required for propagation of decomposition flame through commercially pure acetylene free of solvent and water vapor in long horizontal pipes. Gas initially at room temperature ignition by thermal nonshock sources. Curve shows approximate least pressure for propagation (0), detonation,...

The predetonation distance (the distance the decomposition flame travels before it becomes a detonation) depends primarily on the pressure and pipe diameter when acetylene in a long pipe is ignited by a thermal, nonshock source. Figure 2 shows reported experimental data for quiescent, room temperature acetylene in closed, horizontal pipes substantially longer than the predetonation distance (44,46,52,56,58,64,66,67). The predetonation distance may be much less if the gas is in turbulent flow or if the ignition source is a high explosive charge. 

Fig. 2. Predetonation distances (in m) observed in acetylene at various pressures in horizontal pipes of various diameters. Gas quiescent, at room temperature, ignition by thermal nonshock sources (44,46,52,56,58,64,66,67). To convert kPa to atm, divide by 101.3.

Certain materials which are generally considered to be stable at ordinary temperatures can inflame even in tlie absence of normal ignition sources. Such spontaneous combustion results from exotliermic autoxidation when the heat liberated exceeds that dissipated by the system. Materials prone to self-heating are listed in Table 6.7. In most cases, such fires involve relatively large, enclosed or thermally-insulated masses, and spontaneous combustion usually occurs after prolonged storage. 

A massive amount of propane is instantaneously released in an open field. The cloud assumes a flat, circular shape as it spreads. When the internal fuel concentration in the cloud is about 10% by volume, the cloud s dimensions are approximately 1 m deep and 100 m in diameter. Then the cloud reaches an ignition source at its edge. Because turbulence-inducing effects are absent in this situation, blast effects are not anticipated. Therefore, thermal radiation and direct flame contact are the only hazardous effects encountered. Wind speed is 2 m/s. Relative humidity is 50%. Compute the incident heat flux as a function of time through a vertical surface at 100 m distance from the center of the cloud. 

The sustained decomposition of a substance without introduction of any other apparent ignition source besides thermal energy and without air or other oxidants present. Autodecomposition is the result of a thermal self-decomposition reaction for given initial conditions (temperature, pressure, volume) at which the rate of heat evolution exceeds the rate of heat loss from the reacting system, thus resulting in an increasing reaction temperature and reaction rate. 

Furniture calorimeters were developed in the 1980s in several laboratories to obtain this kind of data.70 71 The first furniture calorimeter test standard was published in 1987 in the Nordic countries as NT Fire 032. Furniture calorimeter test standards have been developed by ASTM for chairs, mattresses, and stacked chairs. The corresponding designations are ASTM E 1537, ASTM E 1590, and ASTM E 1822, respectively. The California Bureau of Home Furnishings and Thermal Insulation (CBHFTI) developed California Technical Bulletins (CAL TB) 133 and 603. These documents describe fire test procedures to qualify seating furniture and mattresses, respectively, for use in public occupancies in California. CAL TB 603 has been superseded by the Federal CPSC standard 16 CFR 1633. The primary difference between the various chair and mattress tests is the ignition source. 

Thermal radiation hazards result from liquid hydrocarbon pool fires, flash fires, turbulent jet fires, and fireballs (BLEVE). A release may be ignited immediately or some time later, and the ignition source may be at the point of release or at a distance downwind, as shown in Figure 2.2. Gas venting... 

The general problem has been to extend the usefulness of the induction parameter model proposed by Oran et al. (1). This induction parameter model (IPM) is proposed as a means to enable one to estimate, relatively easily, the energy necessary to achieve ignition when using a thermal heating source Much of the calibration of this model, for example the effect of deposition volume (quench volume), can be done with one-dimensional models, and shock tube experiments. There are phenomena, however, which must be studied in two or three dimensions. Examples are turbulence and buoyancy. This paper discusses the effect of buoyancy and possible extensions to the IPM. 

Energetic materials can be initiated using thermal, mechanical or electrostatic ignition sources and do not need atmospheric oxygen to maintain the exothermic reaction. 

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