BLEVEs described

Useful formulas for BLEVE fireballs (CeSP, 1989) are given by equations 9.1-27 thru 9.1-30, where M = initial mass of flammable liquid (kg). The initial diameter describes the short duration initial ground level hemispherical flaming-volume before buoyancy lifts it to an equilibrium height. 

This text is intended to provide an overview of methods for estimating the characteristics of vapor cloud explosions, flash flies, and boiling-liquid-expanding-vapor explosions (BLEVEs) for practicing engineers. The volume summarizes and evaluates all the current information, identifies areas where information is lacking, and describes current and planned research in the field. 

This chapter describes the main features of vapor cloud explosions, flash fires, and BLEVEs. It identifies the similarities and differences among them. Effects described are supported by several case histories. Chapter 3 will present details of dispersion, deflagration, detonation, ignition, blast, and radiation. 

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. 

As previously described, full-scale BLEVE experiments by British Gas (KXX) and 2000 kg of butane and propane released at 0.75 and 1.5 MPa) give average... 

As described in Section 6.2.1., British Gas performed full-scale tests with LPG BLEVEs similar to those conducted by BASF. The experimenters measured very low overpressures firom the evaporating liquid, followed by a shock that was probably the so-called second shock, and by the pressure wave from the vapor cloud explosion (see Figure 6.6). The pressure wave firom the vapor cloud explosion probably resulted from experimental procedures involving ignition of the release. The liquid was below the superheat limit temperature at time of burst. 

Thus, the BLEVE theory predicts that, when the temperature of a superheated liquid is below T, liquid flashing cannot give rise to a blast wave. This theory is based on the solid foundations of kinetic gas theory and experimental observations of homogeneous nucleation boiling. It is also supported by the experiments of BASF and British Gas. However, because no systematic study has been conducted, there is no proof that the process described actually governs the type of flashing that causes strong blast waves. Furthermore, rapid vaporization of a superheated liquid below its superheat limit temperature can also produce a blast wave, albeit a weak... 

In this chapter, applications of the calculation methods used to predict the hazards of BLEVEs, as described in Chapter 6, are demonstrated in the solution of sample problems. Fire-induced BLEVEs are often accompanied by fireballs hence, problems include calculation of radiation effects. A BLEVE may also produce blast waves and propel vessel fragments for long distances. The problems include calculations for estimating these effects as well. Calculation methods for addressing each of these hazards will be demonstrated separately in the following order radiation, blast effects, and fragmentation effects. 

In this section, three examples of blast calculations of BLEVEs and pressure vessel bursts will be given. The first example is designed to illustrate the use of all three methods described in Section 6.3.2. The second is a continuation of sample problem 9.1.5, the BLEVE of a tank truck. A variation in the calculation method is presented instead of determination of the blast parameters at a given distance from the explosion, the distance is calculated at which a given overpressure is reached. The third example is a case study of a BLEVE in San Juan Ixhuatepec (Mexico City). 

Kirkw ood [30] describes Bleves referenced to flammable liquids as occurring when a confined liquid is heated above its atmospheric boiling point by an external source of heat or fire and is suddenly released by the rupture of the container due to overpressurization by the expanding liquid. A portion of the superheated liquid immediately... 

Ref. [40] points out that the effects of a Bleve depends on whether the liquid in the vessel is flammable. The initial explosion may generate a blast wave and fragments from the vessel. For a flammable material, the conditions described in Ref. [34] above may result, and even a vapor cloud explosion may result. 

As discussed in Section 3.2.1, other explosion events can occur that impact process plant buildings, including condensed-phase explosions, uncontrolled chemical reactions, PV ruptures, and BLEVEs. Appendix A and Reference 5 describe the information needed and the methods available for calculating blast parameters from these events. 

Explosion, or BLCBE, involving multiple site initiation during the explosive stage, is described [2], A study of Bleves in propane tanks is combined with a procedure for predicting whether a tank will BLEVE or merely produce a jet leak on overheating [3]. The relationship between BLEVE conditions and subsequent fireballs has been studied [4]... 

Roberts makes clear that P is intended to be the vapor pressure when the failure occurs. In a BLEVE, this might be the relief valve setting Pq, whereas in a fireball resulting from an impact failure, it will be the vapor pressure at ambient temperature, as is used in FLARE (described in Appendix C). For a fireball following a release of gas (as opposed to liquefied gas), P should be the storage pressure. 

BLEVE (Boiling Liquid Expanding Vapor Explosion) See Boilover the same phenomenon may occur in a pressurized container, resulting in an explosion or bursting of the tank or vessel in which a fire is occurring. The term is almost exclusively used to describe a disastrous effect from a crude oil fire. 

The consequences of exposure to a fireball resulting from a BLEVE can be determined as described in the previous section. 

Experimental and theoretical work described in [66] suggests that the blast wave caused by a BLEVE is due to the vapour energy whilst the process of flash depressurization after containment failure is too slow for causing a blast wave. 

Missile damage from BLEVEs is more difficult to model and of relatively little importance in risk assessments. A statistical account of the extent of missile damage from actual BLEVEs involving primarily LPG is described in Lees (1996). 

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

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 addresses a special case of a catastrophic rupture of a pressure vessel. A boiling liquid expanding vapor explosion (BLEVE) occurs when there is a sudden loss of containment of a pressure vessel containing a superheated liquid or liquified gas. This section describes the methods used to calculate the effects of the vessel rupture and the fireball that results if the released liquid is flammable and is ignited. 

The output of a BLEVE model is usually the radiant flux level and duration. Overpressure effects, if important, can also be obtained using a detailed procedure described elsewhere (AIChE, 1994). Fragment niunbcrs and ranges can be estimated, but a probabilistic approach is necessary to determine consequences. 

BLEVE dimensions and durations have been smdied by many authors and the empirical basis consists of several well-described incidents, as well as many smaller laboratory trials. The use of a surface emitted flux estimate is the greatest weakness, as this is not a fundamental property. Fragment correlations are subject to the same weaknesses discussed in Section 3.3.4. 

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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