Explosions confined vapor cloud

In particular, great care must be take when evaluating tradeoffs for a containment building for a flammable and toxic material such as hydrogen cyanide. A leak or fire inside the building could cause a confined vapor cloud explosion which destroys the building. The total risk may actually increase. 

Especially, for gaseous mixtures exploding in the open the term unconfined vapor cloud explosion (UVCE) is used, whereas when it explodes in confined spaces the term confined vapor cloud explosion (CVCE) is used. In a very poor or very rich fuel mixture, but still within flammability limits, the flame front travels in the cloud at low velocity and insignificant pressure increase—a phenomenon known as flash fire. 

Failure of a 3/8-in compression fitting on a 1000-2500-psi ethylene line in a pipe trench resulted in a spill of 200-500 lb of ethylene. A cloud was formed and ignited, giving an explosion equivalent to 0.12-0.30 ton of TNT. This accident took place in a courtyard, giving a partially confined vapor cloud explosion. Two people were killed and 17 were injured property loss was 6.5 million. 

A relatively small amount of flammable material, a few kilograms, can lead to an explosion when released into the confined space of a building. This is known as a confined vapor cloud explosion. 

A second hazard associated with nitrogen is the possibility that it may be inadvertently used as the source of combustion air to a fired heater. The reduced oxygen in the air supply could cause the heater s flame to go out, thus creating the potential for a confined vapor cloud explosion. 

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. 

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.

A more deterministic estimate of a vapor cloud s blast-damage potential is possible only if the actual conditions within the cloud are considered. This is the starting point in the multienergy concept for vapor cloud explosion blast modeling (Van den Berg 1985). Harris and Wickens (1989) make use of this concept by suggesting that blast effects be modeled by applying a 20% TNT equivalency only to that portion of the vapor cloud which is partially confined and/or obstructed. 

The consequence of the second approach is that, if detonation of unconfined parts of a vapor cloud can be ruled out, the cloud s explosive potential is not primarily determined by the fuel-air mixture in itself, but instead by the nature of the fuel-release environment. The multienergy model is based on the concept that explosive combustion can develop only in an intensely turbulent mixture or in obstructed and/or partially confined areas of the cloud. Hence, a vapor cloud explosion is modeled as a number of subexplosions corresponding to the number of areas within the cloud which bum under intensely turbulent conditions. 

By modeling site-specific conditions Inventories of material having explosion or fire potential that can be released Plant volumes and degree of confinement or obstruction (if Multienergy or similar method is used) if vapor cloud explosion is a concern... 

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. 

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. 

Based on the accident investigation, the explosive energy was equivalent to 0.3 ton TNT. Therefore the fraction of energy manifested in the explosion is 0.3/2.69 = 11.2%. This 11.2% is considerably higher than the 2% normally observed (see section 6-13) for unconfined vapor cloud explosions. The higher energy conversion is a result of the explosion occurring in a partially confined area. 

The amount of explosion overpressure is determined by the flame speed of the explosion. Flame speed is a function of the turbulence created within the vapor cloud that is released and the level of fuel mixture within the combustible limits. Maximum flame velocities in test conditions are usually obtained in mixtures that contain slightly more fuel than is required for stoichiometric combustion. Turbulence is created by the confinement and congestion within the particular area. Modem open air explosion theories suggest that all onshore hydrocarbon process plants have enough congestion and confinement to produce vapor cloud explosions. Certainly confinement and congestion are available on most offshore production platforms to some degree. 

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. 

Finally, there must be a flame acceleration mechanism, such as congested areas, within the flammable portion of the vapor cloud. The overpressures produced by a vapor cloud explosion are determined by the speed of flame propagation through the cloud. Objects in the flame pathway (such as congested areas of piping, process equipment, etc.) enhance vapor and flame turbulence. This turbulence results in a much faster flame speed which, in turn, can produce significant overpressures. Confinement that limits flame expansion, such as solid decks in tnulti-lcvc process structures, also increases flame speed. Without flame acceleration, a large fireball or flash fire can result, but not an explosion. 

Thus, the center of a VCE is not necessarily where the flammable material is released, the point of ignition, or the center of the vapor cloud. Slather, the center of a vapor cloud explosion is usually an area of congesiion/confinemenl within the vapor cloud. If there are multiple areas of congestion or confinement within the flammable portion of a vapor cloud, multiple explosions can occur as the flame front propagates through each congestcd/confined area. 

Sprayed water acts as an air mover, drawing in air that dilutes the cloud and also helps to drive the vapors from under structures and away from equipment. If vapors were to remain trapped, the cloud would be partly confined, increasing the chance of a vapor cloud explosion. 

REDIFEM—This fire model has applications including steady state releases of compressible gas/vapor, incompressible liquid and transient release from a gas vessel, Gaussian Plume models, continuous free momentum, BLEVE, and confined and unconfined vapor cloud explosions. REDIEEM is reported to have internal validation with ISO 9001 and checked against PHAST and ERED. 

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