Vapor Cloud Explosions VCE

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. 

Davenport (1977, 1983) and Lenoir and Davenport (1992) have summarized numerous VCE incidents. All (with one possible exception) were deflagrations rather than detonations. They found that VCEs accounted for 37% of the number of property losses in excess of 50 million (corrected to 1991 dollars) and accounted for 50% of the overall dollars paid. Pietersen and Huerta (1985) has stimmarized some key features of 80 flash fires. 

AIChE/CCPS (1994) provides an excellent summary of vapor cloud behavior. They describe four features which must be present in order for a VCE to occur. First, the release material must be flammable. Second, a cloud of sufficient size must form prior to ignition, with ignition delays of from 1 to 5 min considered the most probable for generating vapor cloud explosions. Lenoir and Davenport (1992) analyzed historical data on ignition delays, and found delay times firom 6 s to as long as 60 min. Third, a sufficient amount of the cloud must be within the flammable range. Fourth, sufficient confinement or turbulent mixing of a portion of the vapor cloud must be present. 

AIChE/CCPS (1994) also provides the following summary  

In the experiments described, no explosive blast-generating combustion was observed if initially quiescent and fully unconfined fiicl-air mixtures were ignited by low-energy ignition sources. Experimental data also indicate that turbulence is the governing factor in blast generation and that it may intensify combustion to the level that will result in an explosion. 

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Vapor Cloud Explosion (VCE) Explosive oxidation of a vapor cloud in a non-confined space (not in vessels, buildings, etc.). The flame speed may accelerate to high velocities and produce significant blast overpressure. Vapor cloud explosions in plant areas with dense equipment layouts may show acceleration in flame speed and intensification of blast. 

The properties of the process materials and their conditions of use pose the potential for a vapor cloud explosion (VCE) and vessel rupture in the following process areas ... 

Potential explosion phenomena include vapor cloud explosions (VCEs), confined explosions, condensed-phase explosions, exothermic chemical reactions, boiling liquid expanding vapor explosions (BLEVEs), and pressure-volume (PV) ruptures. Potential fire phenomena include flash fires, pool fires, jet fires, and fireballs. Guidelines for evaluating the characteristics of VCEs, BLEVEs, and flash fires are provided in another CCPS publication (Ref. 5). The basic principles from Reference 5 for evaluating characteristics of these phenomena are briefly summarized in this appendix. In addition, the basic principles for evaluating characteristics of the other explosion and fire phenomena listed above are briefly summarized, and references for detailed evaluation of characteristics are provided. 

The TNT equivalency method also uses an overpressure curve that applies to point source detonations of TNT. Vapor cloud explosions (VCEs) are explosions that occur because of the release of flammable vapor over a large volume and are most commonly deflagrations. In addition, the method is unable to consider the effects of flame speed acceleration resulting from confinement. As a result, the overpressure curve for TNT tends to overpredict the overpressure near the VCE and to underpredict at distances away from the VCE. 

The most dangerous and destructive explosions in the chemical process industries are vapor cloud explosions (VCEs). These explosions occur in a sequence of steps ... 

Vapor Cloud Explosions A vapor cloud explosion (VCE) occurs when a large quantity of flammable material is released, is mixed with enough air to form a flammable mixture, and is ignited. Damage from a VCE is due mostly to the overpressure, but significant damage to equipment and personnel may occur due to thermal radiation from the resulting fireball. 

Previous studies of Vapor Cloud Explosions (VCE) have used a correlation between the mass of a gas in the cloud and equivalent mass of TNT to predict explosion overpressures. This was always thought to give conservative results, but recent research evidence indicates that this approach is not accurate to natural gas and air mixtures. The TNT models do not correlate well in the areas near to the point of ignition, and generally over estimate the level of overpressures in the near field. Experiments on methane explosions in "unconfined" areas have indicated a maximum overpressure of 0.2 bar (2.9 psio). This overpressure then decays with distance Therefore newer computer models have been generated to better simulate the effects... 

Vapor Cloud Explosion (VCE) - Is an explosion resulting from the ignition of a cloud of flammable vapor, gas or mist in which the flame speed accelerates to produce an overpressure. 

Vapor cloud explosions (VCE) defined 3-2 overpressure calculation 3-... 

Incident The loss of containment of material or energy (e.g., a prmcture or fittings leak of ammonia on a railcar). Not all events propagate into incidents. Incident Outcome The physical manifestation of the incident for toxic materials, the incident outcome is a toxic release, while for flammable materials, the incident outcome could be a boihng liqtrid expanding vapor explosion (BLEVE), flash fire, vapor cloud explosion (VCE), etc. (e.g., for a leak of chlorine from a railcar, the incident outcome is a toxic release). Likelihood A measure of the expected probability or frequency of occurrence of an event (e.g., events/year). 

Overpressure for explosions (for a confined explosion, boiling liquid expanding vapor explosion [BLEVE], or vapor cloud explosion [VCE])... 

Vapor cloud explosion (VCE) Two-phase discharge, heavy gas dispersion and/or VCE... 

Many types of outcomes are possible for a release. This includes vapor cloud explosions (VCE) (Section 3.1), flash fires (Section 3.2), physical explosions (Section 3.3), boiling liquid expanding vapor explosions (BLEVE) and fireballs (Section 3.4), confined explosions (Section 3.5), and pool fires and jet fires (Section 3.6). Figure 3.1 provides a basis for logically describing accidental explosion and fire scenarios. The output of the bottom of this diagram are various incident outcomes with particular eflfects (e.g., vapor cloud explosion resulting in a shock wave). 

When the contents of the vessel are released both a shock wave and projectiles result. The effects are more similar to a detonation than a vapor cloud explosion (VCE). The extent of a shock wave depends on die phase of the vessel contents originally present. Table 3.6 describes the various scenarios. 

The physical models described in Chapter 2 generate a variety of incident outcomes that arc caused by release of hazardous material or energy. Dispersion models (Section 2.3) estimate concentrations and/or doses of dispersed vapor vapor cloud explosions (VCE) (Section 3.1), physical c q)losion models (Section 3.3), fireball models (Section 3.4), and confined explosion models (Section 3.5) estimate shock wave overpressures and fragment velocities. Pool fire models (Section 3.6), jet fire models (Section 3.7), BLEVE models (Section 3.4) and flash fire models (Section 3.2) predict radiant flux. These models rely on the general principle that severity of outcome is a function of distance from the source of release. 

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