Ignition vapor cloud explosions

Make sure that piping and equipment inspections and preventive maintenance tasks are completed as required to ensure mechanical integrify of process equipment. Containment of fuel is the best way to prevent vapor cloud explosions. Ignition sources forflammable vapor clouds for example, heaters, vehicles, unclassified electrical areas, hot work, static discharge — are difficult to control. 

LFnconfined vapor cloud explosion Ignited spills of combustible fuels... 

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. 

On July 28,1948, a rail car containing liquefied dimethyl ether ruptured and released its entire contents. The rupture was due to the generation of excessive pressures created by long solar exposure following initial overfilling. The gas was ignited after 10 to 25 seconds. The ensuing vapor cloud explosion caused the death of 207 people and injured 3818. 

On March 26, 1980, a power shovel was relocating a tank containing 1500 I (750 kg 1650 lb) liquid propane. During maneuvering, the tank fell from the shovel a portion of its contents was releas as a result. After a delay of 30 seconds, the ensuing vapor cloud was ignited. The explosion caused substantial blast and fire damage. There were no casualties. 

The tank truck left the road near the campsite and crashed at 4 29 p.m. (Figure 2.29). Propylene seems to have been released. The resulting vapor cloud was ignited, possibly by camp cooking hies. One or two explosions then occurred. (Some witnesses heard two explosions.)... 

Generally, at any moment of time the concentration of components within a vapor cloud is highly nonhomogeneous and fluctuates considerably. The degree of homogeneity of a fuel-air mixture largely determines whether the fuel-air mixture is able to maintain a detonative combustion process. This factor is a primary determinant of possible blast effects produced by a vapor cloud explosion upon ignition. It is, therefore, important to understand the basic mechanism of turbulent dispersion. 

Accidental vapor cloud explosions do not occur under controlled conditions. Various experimental programs have been carried out simulating real accidents. Quantities of fuel were spilled, dispersed by natural mechanisms, and ignited. Full-scale experiments on flame propagation in fuel-air clouds are extremely laborious and expensive, so only a few such experiments have been conducted. 

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. 

Harrison, A. J., and J. A. Eyre. 1986. Vapor cloud explosions—The effect of obstacles and jet ignition on the combustion of gas clouds, 5th Int. Symp. Proc. Loss Prevention and Safety Promotion in the Process Industries. Cannes, France. 38 1, 38 13. 

Taylor, P. H. 1985. Vapor cloud explosions—The directional blast wave from an elongated cloud with edge ignition. Comb. Sci. Tech. 44 207-219. 

A flash fire is the nonexplosive combustion of a vapor cloud resulting from a release of flammable material into the open air, which, after mixing with air, ignites. In Section 4.1, experiments on vapor cloud explosions were reviewed. They showed that combustion in a vapor cloud develops an explosive intensity and attendant blast effects only in areas where intensely turbulent combustion develops and only if certain conditions are met. Where these conditions are not present, no blast should occur. The cloud then bums as a flash fire, and its major hazard is from the effect of heat from thermal radiation. 

Although the experiments reported by Maurer et al. (1977) were performed for a completely different reason, namely, to study effects of vapor cloud explosions (see Section 6.4), fireballs were nevertheless generated. These experiments involved vessles of various sizes (0.226-1000 1) and containing propylene at 40 to 60 bar gauge pressure. The vessels were ruptured, and the released propylene was ignited after a preselected time lag. One of these tests, involving 452 kg of propylene, produced a fireball 45 m in diameter. 

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. 

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. 

IIA. vapor cloud explosion, caused by ignition of tlie gas released, centered at tlie release point, and unaffected by weather conditions... 

If the release forms a vapor cloud that premixes with air before ignition occurs, and turbulence is developed (for example, by the flame front propagating through a process structure), the flame speed can accelerate sufficiently to cause a blast. This event is referred to as a vapor cloud explosion. In addition to blast effects, radiant heat and flame contact effects may also occur. Flashback to the source may cause a pool and/or jet fire. 

Flammability Vessel/enclosuie rupture following ignition of contained vapors + air Vapor cloud explosion Flash fire Pool 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. 

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... 

Water curtains can also cool or eliminate available ignition source to a released vapor cloud. In this fashion they can also be a mitigating feature to prevent vapor cloud explosions. Hot surfaces, sparking devices and open flames in the immediate area of a vapor release can all be eliminated as a result of a directed water curtain where these sources exist. 

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. 

On June 22, 1997, an explosion and fire destroyed an olefin plant at the Shell Deer Park, Texas chemical complex. Thirty workers received minor injuries and extensive damage resulted to both the plant and adjacent community. The incident was caused by the sudden failure of a pneumatic check valve in a 300-psig gas compressor discharge line that led to a large release of propylene vapor. A few minutes after the release, the gas ignited causing a vapor cloud explosion and flash fire. 

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