BLEVE and Fireball

It is very difficult to predict the number of projectiles and where they will be propelled. These methods are more suited for accident investigations, where the number, size and location of the fragments is known. 

In general, vessels of pressurized gas do not have sufficient stored energy to represent a threat from shock wave beyond the plant boundaries. These techniques find greater application involving in-plant risks. 

These types of incidents can result in domino effects particularly fi om the effects of the projectiles produced. Very few CPQRA studies have ever incorporated projectile effects on a quantitative basis. 

A process engineer should be able to perform each type of calculation in a few 

Several integrated analysis packages contain explosion fragment capability. These include  

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Flammability Hazards—several endpoint criteria are used. For flammable liquids that form large pool fires, a steady heat load or thermal radiation criterion often expressed in BTU/hr-ft or kW/m is used. For determining hazards from short-duration BLEVEs and fireballs, an integrated dose criterion, which represents the area of a time-dependent heat flux, is used. For determining flash fire hazards, a flammable vapor has to be diluted to within its flammable range, with the concentration often usually expressed in ppm, mg/m, or as a volume percent. 

The vessels all failed catastrophieally in less than five minutes and resulted in boiling liquid expanding vapor explosions (BLEVEs) and fireballs. 

Boiling liquid expanding vapor explosions (BLEVEs) are one of the most severe accidents that can occur in the process industry or in the transportation of hazardous materials. Strictly speaking, these explosions do not necessarily imply thermal effects. However, in most cases the substance involved is a fuel that causes a severe fireball after the explosion. Usually BLEVE refers to the combination of these two phenomena, BLEVE and fireball, i.e., to an accident simultaneously involving mechanical and thermal effects. 

In this section, the main features of BLEVEs and fireballs are discussed and a practical methodology for estimating their effects is described. 

Later on, the turbulence of the fire entrains air into the fireball. Simultaneously, the thermal radiation vaporizes the liquid droplets and heats the mixture. As a result of these processes, the whole mass turbulently increases in volume, evolving towards an approximately spherical shape that rises, leaving a wake of variable diameter. Such fireballs can be very large, causing a very strong thermal radiation. The combined action of BLEVE and fireball can be summarized therefore in the following effects ... 

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). 

This section covers radiation due to BLEVEs with accompanying fireballs. First, a brief description is given of experimental investigations of BLEVEs and their fireballs. Next, some fireball models, primarily for predicting fireball diameter and combustion duration, are presented. Most of these models evolved from experimental results. Finally, some radiation models, based on experiments and theory, are given. 

Hasegawa and Sato (1977) showed that, when the calculated amount of flash vaporization equals 36% or more, all released fuel contributes to the BLEVE and eventually to the fireball. For lower flash-vaporization ratios, part of the fuel forms the BLEVE, and the remainder forms a pool. It is assumed that, if flash vaporization is below 36%, three times the calculated quantity of the flash vaporization contributes to the BLEVE. 

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. 

Figure 4 Radiation intensity from a BLEVE and resulting fireball as a function of distance for propylene storage tanks in three different sizes.

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 heat flux, E, from BLEVEs is in the range 200 to 350 kW/m is much higher than in pool fires because the flame is not smoky. Roberts (1981) and Hymes (1983) estimate the surface heat flux as the radiative fraction of the total heat of combustion according to equation 9.1-32, where E is the surface emitted flux (kW/m ), M is the mass of LPG in the BLEVE (kg) h, is the heat of combustion (kJ/kg), is the maximum fireball diameter (m) f is the radiation fraction, (typically 0.25-0.4). t is the fireball duration (s). The view factor is approximated by equation 9.1-34. where D is the fireball diameter (m), and x is the distance from the sphere center to the target (m). At this point the radiation flux may be calculated (equation 9.1-30). 

BLEVEs are more commonly associated with releases of flammable liquids from vessels as a consequence of external fires. Such BLEVEs produce, in addition to blast and fragmentation ejects, buoyant fireballs whose radiant energy can bum exposed skin and ignite nearby combustible materials. A vessel may rupture for a... 

A BLEVE involving a container of flammable liquid will be accompanied by a fireball if the BLEVE is fire-induced. The rapid vaporization and expansion following loss of containment results in a cloud of almost pure vapor and mist. After ignition, this cloud starts to bum at its surface, where mixing with air is possible. In the buoyancy stage, combustion propagates to the center of the cloud causing a massive fireball. 

At 5 45 A.M., a flash fire resulted. The vapor cloud is assumed to have penetrated houses, which were subsequently destroyed by internal explosions. A violent explosion, probably involving the BLEVE of several storage tanks, occurred 1 minute after the flash fire. It resulted in a fireball and the propulsion of one or two cylindrical tanks. Heat and fragments resulted in additional BLEVEs. 

A fireball then developed. Several ensuing explosions, fireballs, and BLEVEs destroyed the refinery almost completely, causing the deaths of seven people and injuries of ten. 

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