Thermal radiation effects

This accident has the potential to seriously injure 50 people because of blast overpressure and thermal radiation effects. 

In order to compute the thermal radiation effects produced by a burning vapor cloud, it is necessary to know the flame s temperature, size, and dynamics during its propagation through the cloud. Thermal radiation intercepted by an object in the vicinity is determined by the emissive power of the flame (determined by the flame temperature), the flame s emissivity, the view factor, and an atmospheric-attenuation factor. The fundamentals of heat-radiation modeling are described in Section 3.5. 

Zeeuwen et al. (1983) observed the atmospheric dispersion and combustion of large spills of propane (1000-4000 kg) in open and level terrain on the Musselbanks, located on the south bank of the Westerscheldt estuary in The Netherlands. Thermal radiation effects were not measured because the main objective of this experimental program was to investigate blast effects from vapor cloud explosions. 

Thermal radiation effect on pressurized hydrogen vessel. (From Rigas, F. and Sklavounos, S., hit. ]. Hydrogen Energ., 30,1501,2005. With permission from International Association of Hydrogen Energy.)... 

Since 1978, large-scale LNG spill tests have been conducted by a joint team from Lawrence Livermore National Laboratory (LLNL) and the Naval Weapons Center (NWC) (Koopman et al., 1981). The test site was located at NWC, China Lake, California. The program, sponsored primarily by the Department of Energy, had as its principal objective the acquisition of data to aid in modeling both vapor dispersion and thermal radiation effects (from LNG vapor cloud fires). 

The effect of including the thermal radiation effects is to reduce the value of the critical radius. 

Thermal Radiation Effects. Thermal radiation effects arise from flash fires, pool fires, jet fires, or fireballs. These involve the combustion of flammable mixtures. The intensity of thermal radiation (measured in terms of thermal radiation flux or energy per unit area and time) at a receptor outside a fire depends on its distance from the Are, the flame height, flame emissive power, and atmospheric transmissivity. 

The major difficulty presented to anyone involved in CPQRA is in selecting the proper outcomes based on the available information and determining the consequences. The consequences of concern in CPQRA studies for explosions in general arc blast overpressure effects and projectile effects for fires and fireballs the consequences of concern arc thermal radiation effects. Each of these types of explosions and fires can be modeled to produce blast, projectile and/or thermal radiation effects appropriate for use in CPQRA studies and these techniques are described in the designated sections. 

A process engineer with some understanding of thermal radiation effects could use BLEVE models quite easily. A half-day calculation period should be allowed unless the procedure is computerized in which case much more rapid calculation and exploration of sensitivities is possible. Spreadsheet can be readily applied. 

Hymes, 1., 1983, The Physiological and Pathological Effects of Thermal Radiation, UKAEA Safety and Reliability Directorate, Report SRD R275, Culcheth U.K. 

The air Temperature senaor shall be offeccivcly protected from -my effects of thermal radiation cxmiing from hot or C(>ld walls. 

Investigations of the effects of BLEVEs (Chapter 6) are usually limited to the aspect of thermal radiation. Blast and fragmentation have been of less interest, and hence, not studied in detail. Furthermore, most experiments in thermal radiation have been performed on a small scale. 

Accident scenarios leading to vapor cloud explosions, flash fires, and BLEVEs were described in the previous chapter. Blast effects are a characteristic feature of both vapor cloud explosions and BLEVEs. Fireballs and flash fires cause damage primarily from heat effects caused by thermal radiation. This chapter describes the basic concepts underlying these phenomena. 

A flash fire is the nonexplosive combustion of a vapor cloud resulting from a release of flammable material into the open air, which, after mixing with air, ignites. In Section 4.1, experiments on vapor cloud explosions were reviewed. They showed that combustion in a vapor cloud develops an explosive intensity and attendant blast effects only in areas where intensely turbulent combustion develops and only if certain conditions are met. Where these conditions are not present, no blast should occur. The cloud then bums as a flash fire, and its major hazard is from the effect of heat from thermal radiation. 

The literature provides little information on the effects of thermal radiation from flash fires, probably because thermal radiation hazards from burning vapor clouds are considered less significant than possible blast effects. Furthermore, flash combustion of a vapor cloud normally lasts no more than a few tens of seconds. Therefore, the total intercepted radiation by an object near a flash fire is substantially lower than in case of a pool fire. 

Thermal effects depend on radiation intensity and duration of radiation exposure. American Petroleum Institute s Recommended Practice 521 (1982) reviews the effects of thermal radiation on people. In Table 6.5, data on time to reach pain threshold are given. As a point of comparison, the solar radiation intensity on a clear, hot summer day is about 1 kW/m (317 Btu/hr/ft ). Criteria for thermal damage are shown in Table 6.6 (CCPS, 1989) and Figure 6.10 (Hymes 1983). 

Hymes, J. 1983. The physiological and pathological effects of thermal radiation. SRD R 275. U.K. Atomic Energy Authority. 

Consequently, if none of these conditions is present, no blast effects are to be expected. That is, under fully unconfined and unobstructed conditions, the cloud bums as a flash fire, and the major hazard encountered is heat effect from thermal radiation. 

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. 

Ozone forms a layer around the Earth that insulates against thermal radiation. This layer is being destroyed by pollutants (principally fluorocarbons). The effect of the depletion of the ozone layer is to warm the Earth (and hence exacerbate the greenhouse effect) and may also lead to an increase in the incidence of skin cancers. 

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