Cloud Explosion Overpressures

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 

The criteria selected for an overpressure hazard is normally taken as 0.2 bar (3.0 psio). Although fatalities due to direct effects of an explosion may require up to 2.0 bar (29.0 psio) or higher, significantly lower levels result in damages to structures and buildings that would likely cause a fatality to occur. An overpressure of 0.2 to 0.28 bar (3.0 to 4.0 psio) would destroy a frameless steel panel building, 0.35 bar (5.0 psio) would snap wooden utility poles and severely damage facility structures, and 0.35 to 0.5 bar (5.0 to 7.0 psio) would cause complete destruction of houses. 

A natural gas and air mixture is only likely to explode if all of the following conditions are met  

It could be argued that vapor cloud explosions for hydrocarbon facilities need only be calculated for those facilities that contain large volumes of volatile hydrocarbon gases that can be accidentally released and where some degree of confinement or congestion exist. The most probable amount for an incident to occur is taken as 4,536 kgs (10,000 lbs ), however incidents have been recorded where only 907 kgs (2,000 lbs.) has been released. Additionally, an actual calculation of worst case releases to produce 0.2 bar (3 psio) at say 46 meters (150 ft.), indicates a minimum of 907 kgs (2,000 lbs.) of material is needed to cause that amount of overpressure. A limit of 907 kgs (2,000 lbs.) release of hydrocarbon vapor is considered a prudent and conservative approach. 

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As an aid in determining the severity of vapor cloud explosions, overpressure radius circles are normally plotted on a plot plan from the source of leakage or ignition. Computer applications are available that can easily calculate and plot these on electronic plant design and drafting applications. These overpressure circles can be determined at the levels at which destructive damage may occur to the facility from the worst case credible event (WCCE). 

Volume of vessel (free volume V) Shape of vessel (area and aspect ratio) Type of dust cloud distribution (ISO method/pneumatic-loading method) Dust explosihility characteristics Maximum explosion overpressure P ax Maximum explosion constant K ax Minimum ignition temperature MIT Type of explosion suppressant and its suppression efficiency Type of HRD suppressors number and free volume of HRD suppressors and the outlet diameter and valve opening time Suppressant charge and propelling agent pressure Fittings elbow and/or stub pipe and type of nozzle Type of explosion detector(s) dynamic or threshold pressure, UV or IR radiation, effective system activation overpressure Hardware deployment location of HRD suppressor(s) on vessel... 

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. 

Vapor cloud explosions can cause damaging overpressures (CCPS, 1994b). 

A vapor cloud explosion may be simply defined as an explosion occurring outdoors, producing a damaging overpressure (Factory Mutual Research Corporation, 1990). 

It begins with the release of a large quantity of flammable vaporizing liquid or gas from a storage tank, process or transport vessel, or pipeline. Generally speaking, several features need to be present for a vapor cloud explosion with damaging overpressure to occur. 

A deflagration can best be described as a combustion mode in which the propagation rate is dominated by both molecular and turbulent transport processes. In the absence of turbulence (i.e., under laminar or near-laminar conditions), flame speeds for normal hydrocarbons are in the order of 5 to 30 meters per second. Such speeds are too low to produce any significant blast overpressure. Thus, under near-laminar-flow conditions, the vapor cloud will merely bum, and the event would simply be described as a large fiash fire. Therefore, turbulence is always present in vapor cloud explosions. Research tests have shown that turbulence will significantly enhance the combustion rate in defiagrations. 

Making a detailed estimate of the full loading of an object by a blast wave is only possible by use of multidimensional gas-dynamic codes such as BLAST (Van den Berg 1990). However, if the problem is sufficiently simplified, analytic methods may do as well. For such methods, it is sufficient to describe the blast wave somewhere in the field in terms of the side-on peak overpressure and the positive-phase duration. Blast models used for vapor cloud explosion blast modeling (Section 4.3) give the distribution of these blast parameters in the explosion s vicinity. 

Figure 4.5. Flame velocity, peak overpressure, and overpressure duration in gas cloud explosions following vessels bursts (Giesbrecht et al. 1981).

Figure 4.7. Maximum overpressure in vapor cloud explosions after critical-flow propane jet release dependent on orifice diameter (a) undisturbed jet (b) jet into obstacles and confinement.

The very first stage of flame propagation upon ignition, during which the flame has a spherical shape, mainly determines the blast peak overpressure produced by the entire vapor cloud explosion. 

On the basis of an extended experimental program described in Section 4.1.3, Harris and Wickens (1989) concluded that overpressure effects produced by vapor cloud explosions are largely determined by the combustion which develops only in the congested/obstructed areas in the cloud. For natural gas, these conclusions were used to develop an improved TNT-equivalency method for the prediction of vapor cloud explosion blast. This approach is no longer based on the entire mass of flammable material released, but on the mass of material that can be contained in stoichiometric proportions in any severely congested region of the cloud. 

Figure 4.20. Dimensionless blast side-on overpressure for vapor cloud explosions (Strehlow etal. 1979).

Explosively Dispersed Vapor Cloud Explosions (Giesbrecht et al. 1981). The Giesbrecht et al. (1981) model is based on a series of small-scale experiments in which vessels of various sizes (0.226-10001) containing propylene were ruptured. (See Section 4.1.2, especially Figure 4.5.) Flame speed, maximum overpressure, and positive-phase duration observed in explosively dispersed clouds are represented as a function of fuel mass. 

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. 

TNT-equi valency methods express explosive potential of a vapor cloud in terms of a charge of TNT. TNT-blast characteristics are well known fiom empirical data both in the form of blast parameters (side-on peak overpressure and positive-phase duration) and of corresponding damage potential. Because the value of TNT-equiva-lency used for blast modeling is directly related to damage patterns observed in major vapor cloud explosion incidents, the TNT-blast model is attractive if overall damage potential of a vapor cloud is the only concern. 

TABLE 7.6. Side-On Peak Overpressure for Several Distances from Charge Expressing Explosive Power of the Flixborough Vapor Cloud Explosion... 

Blast overpressures calculated by the TNT-equivalency method are in reasonable agreement with the overpressures deduced from observed damage (Sadee et al. 1976/1977). This is to be expected, because the Flixborough incident is one of the major vapor cloud explosion incidents on which the TNT-equivalency value of... 

In this section, the blast from the BLEVE will be investigated but not the blast which may be caused by a vapor cloud explosion. A variation in the calculation method will be presented. Instead of determination of blast parameters at a given distance from the explosion, the distance at which a given overpressure is reached will be calculated. The distance to which fragments may be thrown will be calculated in Section 9.3. 

Vapor cloud explosion The explosion resulting from the ignition of a cloud of flammable vapor, gas, or mist in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure. 

Different materials pose different hazards, including thermal radiation, explosion overpressure, and toxic and flammable vapor clouds. Some materials pose only one hazard, while others may pose all four. For the purposes of ranking facilities you will need to estimate the laigest area affected by the potential hazards. You can arrive at such an estimate by calculating the greatest downwind distance to a particular level of hazatd. The following thresholds are commonly applied ... 

Facilities can be ranked based on the sum of the maximum hazard distances for each release. Only one hazard distance should be used for each release, even if there is the potential for more than one hazard (thermal radiation, explosion overpressure, toxic cloud and flammable vapor cloud). The highest-ranked facility will be the one whose potential releases would reach the greatest total distance. 

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. 

Figure 6-24 Sachs-scaled overpressure and Sachs-scaled positive-phase duration for the TNO multi-energy blast model. Source Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEs (New York American Institute of Chemical Engineers, 1994) used by permission.

Consider the explosion of a propane-air vapor cloud confined beneath a storage tank. The tank is supported 1 m off the ground by concrete piles. The concentration of vapor in the cloud is assumed to be at stoichiometric concentrations. Assume a cloud volume of 2094 m3, confined below the tank, representing the volume underneath the tank. Determine the overpressure from this vapor cloud explosion at a distance of 100 m from the blast using the TNO multi-energy method. 

Unconfined vapor cloud explosion explosive oxidation of a flammable vapor cloud in a nonconfined space (e.g., not in vessels or buildings) the flame speed may accelerate to high velocities and can produce significant blast overpressures, particularly in densely packed plant areas. 

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. 

F. Blast effects from a nearby explosion (unconfined vapor cloud explosion, bursting vessel, etc.), such as blast overpressure, projectiles, structural damage... 

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