Explosion hazards overpressure

Commonly used criteria includes SG-22, and CIA 1992. Both documents specify at least two blast overpressures for buildings spaced 100 feet (30 meters) from a vapor cloud explosion hazard as follows ... 

Note The high pressures referred to are of the order of 2000-6000 psi. Since we are working with liquids instead of gases, high pressures do not pose an explosion hazard. Leaks occur on overpressurizing the worse problems to be expected are drips, streams, and puddles. 

Although a vapor box or containment may be effective for concentration reduction, it may increase the explosion hazards. Higher explosion peak overpressure may be realized in the near field because of cloud geometry and partial blockage (Melhem and Croce, 1994). 

Inert system design The dryer atmosphere is commonly inerted with nitrogen, but superheated steam or self-inertized systems are also possible. Self-inertized systems are not feasible for flammable solvent systems. These systems must be operated with a small overpressure to ensure no oxygen ingress. And continuous on-line oxygen concentration monitoring is required to ensure that oxygen levels remain well below the explosion hazard limit. 

L = P for explosion hazards (P is the overpressure resulting from the explosion)... 

Some cylinders have contents that change their chemical characteristic over time (decompose, polymerize, etc.). Examples include 1,3-butadiene, hydrogen bromide and diborane. These changes may eventually result in overpressurization that exceeds the specification of the cylinder. This situation renders the cylinder unsafe for transport. Contents of cylinders that have become contaminated with foreign material may also result in a chemical reaction causing overpressurization or an explosive hazard. These cylinders should be handled with extreme care and only by specially trained and qualified personnel. 

The hazard posed can be limited by maintaining a zone free of people and property around a storage area of explosive material. The minimum radius of the zone depends on the type and quantity of explosive, the extent and type of barrica ding, and the magnitude of loss that would be encountered if an explosive incident occurred. The maximum distance to which hazardous explosive effects propagate depends on the blast overpressure created, which as a first approximation is a function of the cube root of the explosive weight, W. This is termed the quantity distance and is defined as... 

Polymerization Exothermic reaction which, unless carefully controlled, can run-away and create a thermal explosion or vessel overpressurization Refer to Table 7.20 for common monomers Certain processes require polymerization of feedstock at high pressure, with associated hazards Many vinyl monomers (e.g. vinyl chloride, acrylonitrile) pose a chronic toxicity hazard Refer to Table 7.19 for basic precautions... 

Overpressure can lead directly to all three hazards. It can lead directly and immediately to injury, to fire or explosion if there is an ignition source, and to pollution if there is not enough containment. Therefore, we must have a very high level of assurance that overpressure is going to have a very low frequency of occurrence. 

Leaks cannot lead directly to personal injury. They can lead to fire or explosion if there is an ignition source and to oil pollution if there is inadequate containment. Both the immediacy of the hazard developing and the magnitude of the hazard will be smaller with leaks than with overpressure. Thus, although it is necessary to protect against leaks, thi.s protection will not require the same level of safety that is required to protect against 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 Effects of Explosions in the Process Industries." First paper of the Major Hazards Assessment Panel—Overpressure Working Party. 

On April 8,1998, at 20 18, an explosion and fire occurred during the production of Automate Yellow 96 Dye at Morton International, Inc. Yellow 96 dye was produced by mixing and reacting two chemicals, ortho-nitrochlorobenzene (o-NCB) and 2-ethylhexylamine (2-EHA). The explosion and fire were the consequence of a runaway reaction, which overpressurized a 2000-gal capacity chemical reactor vessel and released flammable material that ignited. Nine employees were injured, including two seriously, and potentially hazardous materials were released into... 

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. 

The objective in calculating explosion overpressure levels is to determine if a facility has the potential to experience the hazardous effects of an explosion and, if so, to mitigate the results of these explosions. The calculations can also serve to demonstrate where mitigating measures are not needed due to the lack of a potential to produce damaging overpressures either because low explosion effects or distance from the explosion for the facility under evaluation. 

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