Dusts lower explosion limit

OSHA and ACGIH have not estabhshed specific airborne exposure limits for PVB and PVF resias however, some products may contain sufficient fines to be considered nuisance dust and present dust explosion potential if sufficient quantities are dispersed ia air. Unformulated PVB and PVF resias have flash poiats above 370°C. The lower explosive limit (lei) for PVB dust ia air is about 20 g/m. ... 

For combustible dusts, the explosibility limits do not have the same meaning as with flammable gases and flammable vapors, owing to the interaction between dust layers and suspended dust. This protective measure can, for example, be used when dust deposits are avoided in operating areas or in the air stream of clean air lines after filter installations WTiere in normal operation the lower explosibility limit is not reached. However, dust deposits must be anticipated with time. When these dust deposits are whirled up in the air, an explosion hazard can arise. Such a hazard can be avoided by regular cleaning. The dust can be extracted directly at its point of origin by suitable ventilation measures. 

Hybrid mixture A suspension of dust in air/vapor. Such mixtures may be flammable below the lower explosive limit of the vapor and can be ignited by low energy sparks. 

Dust particles have a lower explosive limit expressed in mg/m and almost no upper limit. Examples of LEL for dusts are polystyrene, 0.02 mg/m corn starch, 0.04 mg/m and coal, 0.055 mg/m. ... 

The finely powdered silicide is a significant dust explosion hazard [1]. The lower explosion limit for a calcium-silicon dust cloud of mean particle size 9.7 pm was measured as 79 g/m3, in good agreement with a calculated value [2], Other dust cloud parameters are presented and related to predictions [3],... 

For a dust ignition to occur, the suspended solids concentration must lie between lower and upper limits which vary from material to material and are influenced by subtle factors such as particle shape and size distribution. From the standpoint of assessing safety hazards in commercial and industrial operations, the lower explosive limit is the more important one. The rationale of this statement is that, if the possibility of exceeding the lower limit in a powder-handling operation can not be completely ruled out, then a hazard must be recognized and appropriate measures taken. Over the... 

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. 

The lower explosive limit and minimum explosive concentrations of flax, wool, cotton, jute, hemp and sisal fibres are of the same order of magnitude as those of highly explosive dusts [15], The explosibility of pyrites dusts with sulfur contents above 20% was evaluated experimentally. Dusts of 30% sulfur content gave explosion pressures of 3 bar at pressure rise rates of 16 bar/sec. Mixtures of 60% pyrites and 40% powdered limestone still showed significant pressure effects, and the proportion of limestone actually needed to suppress explosions was considerably above the values currently accepted by mining industries [16], Effects of mixtures of particle sizes in combustible dusts upon minimum ignition temperature (T ") and upon presence or absence of explosion were studied. Presence of 30% of fines in a coarse dust lowers Tf significantly [17], Experimental explosions of polyethylene,... 

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. 

Aluminum powder forms explosive mixtures with air the lower explosive limit being ca. 35-50 g/cm3. Dust-free aluminum paste is therefore used in most applications. 

The finely powdered silicide is a significant dust explosion hazard [1]. The lower explosion limit for a calcium—silicon dust cloud of mean particle size 9.7 pm was... 

For a large number of flammable dusts, the lower explosion limit lies between 0.02 and 0.06 kg/m3. The upper explosion limit is in the range of 2-6 kg/m3, but this number is of limited importance. 

A fundamental solution to the dust explosion problem is to use a wet process so that dust suspensions do not occur at all. If a wet process can be used, it is one of the most satisfactory methods. However, the process must be wet enough to be effective. Some dusts with a high moisture content can still ignite. Dust concentrations in major equipment can be designed below the lower flammable limit, but this often cannot be counted on in operation. Dust concentrations cannot be safely designed to be above an upper flammable limit, because such a limit is ill-defined (Lees, 1980). For a large number of flammable dusts, the lower explosion limit lies between 20 and 60 g/m3. The... 

Control methods which focus on any of these three conditions will be effective to prevent explosions. For example, probably the most important method for controlling dust explosions is to limit the concentration of dust in the air to below the lower explosive limit (LEL). Besides controlling dust in the air, housekeeping is important as well so that there is no possibility of settled dust s becoming resuspended and producing a dust cloud above the LEL. 

SUCROSE or SUGAR (57-50-1) CijHjjO A reducing agent. Dust forms explosive mixture in air (lower explosive limit 0.045 g/1 Fire Rating 2). Violent reaction with... 

Dust-explosion control is more difficult to evaluate, as it depends on the particle size, hygro opicity, surface, and combustibility of the dust. The lower explosive limit of air-borne dust generally ranges from 0.01 to 0.5 oz/ft of air. More detailed information is available from the NFPA. 

Dust-air mixtures are flammable only within a certain range of concentrations just like gas-air mixtures. This range is marked by the lower explosion limit (LEL) and the upper explosion limit (UEL). 

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