Explosion limits, cause

Flammability Acrolein is very flammable its flash point is <0° C, but a toxic vapor cloud will develop before a flammable one. The flammable limits in air are 2.8% and 31.0% lower and upper explosive limits, respectively by volume. Acrolein is only partly soluble in water and will cause a floating fire, so alcohol type foam should be used in firefighting. The vapors are heavier than air and can travel along the ground and flash back from an ignition source. 

Tlie remainder of tliis cliapter provides information on relative physical properties of materials (flash points, upper and lower explosive limits, tlireshold limit values, etc.) and metliods to calculate tlie conditions tliat approach or are conducive to liazardous levels. Fire liazards in industrial plants are covered in Sections 7.2 and 7.3, and Sections 7.4 and 7.5 focus on accidental explosions. Sections 7.6 and 7.7 address toxic emissions and liazardous spills respectively. tliese latter types of accident frequently result in fires and explosions tliey can cause deatlis, serious injuries and financial losses. 

Explosive limits May develop sufficient internal pressure at ambient temperature to cause a container to rupture violently NFPA 1986... 

Hazard, i.e. the potential of the material to cause injury under certain conditions (flammability, explosion limits in air, ignition and autoignition temperatures, static electricity (explosions have occurred during drying due to static electricity), dust explosion, boiling point, fire protection (specification of extinguishers, compounds formed when firing), R S (nature of special risk and safety precautions). Table 5.2-5 lists hazards associated with typical chemical reactions. 

If a significant volume of gas (caused by a leak, for example) is exposed to an ignition source and this gas is mixed with air in proportions that are close to stoichiometric, the gas cloud can cause a lot of damage when it gives rise to a detonation. The accident at Flixborough is one example. The lower explosive limit of hydrocarbons is extremely low. If the carbon chain length exceeds 8, the autoinflammation temperature of a linear hydrocarbon is close to 200°C. All these parameters decrease with pressure. The table below shows to which extent pressure influences the AIT of ethylene ... 

Explosive limits No data Explosive with easily oxidizable substances, and lead nitrate-lead hypophosphite double salt 2-3 drops 90% peroxyformic acid causes violent explosion No data... 

Currently, pellistors are often used as guarding sensors in rooms where there is a risk of flammable gases leaking and causing explosion. Pellistors react to concentrations far below the explosion limits. As these pellistors have been specifically developed for this purpose, nearly all that are currently available work at an ambient temperature of below 50 °C. 

Dust presents a different type of hazard, because while it has a lower explosive limit, it does not have an upper explosive limit. This can result in a primary explosion, followed by secondary explosions as new air is provided. Secondly, dust does not diffuse away from its point of release, but settles out of the air and accumulates into layers. Unlike vapor, the dust explosion is caused by the radiant heat from one particle igniting the next. Because of this, the lower explosive limits for dusts are greatly higher than for vapors. Also, the size and shape of the dust particles are important factors in effecting its lower explosive limit. 

Figure 3.2 depicts the explosion limits of a stoichiometric mixture of hydrogen and oxygen. Explosion limits can be found for many different mixture ratios. The point X on Fig. 3.2 marks the conditions (773 K latm) described at the very beginning of this chapter in Fig. 3.1. It now becomes obvious that either increasing or decreasing the pressure at constant temperature can cause an explosion.

There is, of course, a chemical effect in carbon monoxide flames. This point was mentioned in the discussion of carbon monoxide explosion limits. Studies have shown that CO flame velocities increase appreciably when small amounts of hydrogen, hydrogen-containing fuels, or water are added. For 45% CO in air, the flame velocity passes through a maximum after approximately 5% by volume of water has been added. At this point, the flame velocity is 2.1 times the value with 0.7% H20 added. After the 5% maximum is attained a dilution effect begins to cause a decrease in flame speed. The effect and the maximum arise because a sufficient steady-state concentration of OH radicals must be established for the most effective explosive condition. 

Toxicology. Liquefied petroleum gas (LPG) is practically nontoxic below the explosive limits but may cause asphyxia by oxygen displacement at extremely high concentrations. ... 

Soils with high moisture or clay contents may reduce the efficiency of the LTTD system. Hot spots in the influent soils may cause fluctuations in the dryer temperature. The vaporized hydrocarbon concentration in the kiln must be kept below the lower explosive limit (LEL). 

The decomposition of the catalyst beads can cause a secondary air pollution emission consisting of the particulate dust generated by abrasion of the surface of the catalyst. Operating cost for catalyst replacement varies directly with catalyst attrition rate. The system can process waste streams with VOC concentrations of up to 25% of the lower explosive limit (LEL). The proprietary catalyst contains up to 10% chromium, including 4% hexavalent chromium. This could lead to the emission of hexavalent chromium in some applications of the technology. 

The H + O2 competition is responsible for several important aspects of combustion phenomena. For example, the second explosion limit for hydrogen-oxygen mixtures is explained by the competition between H + O2 branching and termination (Section 13.2.6). The observed reduction in hydrocarbon-air flame speeds with increased pressure between 1 and 10 atm is caused by the branching-termination competition. For a given temperature, as the pressure increases, the concentration of [M] increases, which favors the termination reaction. Thus the chain branching competes less favorably for a greater portion of the flame, which diminishes the flame speed [427]. 

Kach method suffers from one or more inherent sources of error. Method 1 is not readily adaptable to the determination of second explosion limits. If temperature equilibrium is reached very quickly by the gas flowing into the vessel, as the continued flow causes the pressure to increase, the system must first intersect the lower explosion limit. Method 2 can lead to large errors if explosion is preceded by an induction period. In the carbon monoxide-oxygen reaction, for example, it was found that the heating rate could considerably affect the results owing to the existence of a zone of slow reaction adjacent to the second limit and inhibition of the reaction by the product, carbon dioxide... 

Following fires in which endotracheal tubes became ignited by surgical lasers or electrocautery in atmospheres enriched by oxygen and/or nitrous oxide, the flammability of PVC, silicone rubber and red rubber tubes in enriched atmospheres was studied [1], Ozonised oxygen was reacted with hydrogen at low pressure to generate hydroxyl radicals. Pressure in the apparatus was maintained by a vacuum pump protected from ozone by a tube of heated silver foil. On two occasions there was an explosion in the plastic vent pipe from the vacuum pump. The vent gas should have been outside explosive limits and the exact cause is not clear the editor suspects peroxide formation. 

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