Vapor explosivity limits

Precaution Combustible exposed to heat or flame can react with oxidizers explosive as vapor explosive limits 0.8% (212 F) to 5% (302 F) may be explosive on prolonged close contact with air Hazardous Decomp. Prods. Heated to decomp., emits acrid smoke and... 

Precaution Combustible explosive as vapor explosive limits 0.9-9.5% reacts with water producing CO2 violent reactions possible with compds. contg. act. hydrogen violent polymerizations possible with... 

The autoignition temperature is the minimum temperature required for self-sustained combustion in the absence of an external ignition source. The value depends on specified test conditions. Tht flammable (explosive) limits specify the range of concentration of the vapor in air (in percent by volume) for which a flame can propagate. Below the lower flammable limit, the gas mixture is too lean to burn above the flammable limit, the mixture is too rich. Additional compounds can be found in National Fire Protection Association, National Fire Protection Handbook, 14th ed., 1991. 

Flash points and autoignition temperatures are given in Table 11. The vapor can travel along the ground to an ignition source. In the event of fire, foam, carbon dioxide, and dry chemical are preferred extinguishers. The lower and upper explosion limits are 1% and 7%. 

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. 

The explosive limits of hydrazine in air are 4.7—100 vol %, the upper limit (100 vol %) indicating that hydrazine vapor is self-explosive. Decomposition can be touched off by catalytic surfaces. The presence of inert gases significantly raises the lower explosive limit (10) (Table 2). 

Properties of the principal hydrocarbons found in commercial hexane are shown in Table 9. The flash point of / -hexane is —21.7 °C and the autoignition temperature is 225°C. The explosive limits of hexane vapor in air are 1.1—7.5%. Above 2°C the equiUbrium mixture of hexane and air above the Hquid is too rich to fall within these limits (42). 

Undesirable combustible gases and vapors can be destroyed by heating to the autoignition temperature in the presence of sufficient oxygen to ensure complete oxidation to CO2 and H2O. Gas incinerators are appHed to streams that are high energy, eg, pentane, or are too dilute to support combustion by themselves. The gas composition is limited typicaUy to 25% or less of the lower explosive limit. Gases that are sufficiendy concentrated to support... 

Diketene is a flammable Hquid with a flash point of 33°C and an autoignition temperature of 275°C. It decomposes rapidly above 98°C with slow decomposition occurring even at RT. The vapors are denser than air (relative density 2.9, air air = 1). The explosive limits in air are 2—11.7 vol % (135). In case of fire, water mist, light and stabilized foam, as well as powder of the potassium or ammonium sulfate-type should be used. Do not use basic extinguisher powders and do not add water to a closed container. 

The lower flammable limit (LEL) or lower explosive limit (LEL) is the minimum concentration of vapor in air below which a flame is not propagated when an ignition source is present (61—64). Below this concentration, the mixture is considered too lean to bum. The lower flammable limit and the flash point of a flammable Hquid are closely related by the Hquid s vapor pressure characteristics. 

Fig. 6. Approximate explosive limits of styrene monomer vapor in equiUbrium with Hquid styrene in air, where represents the explosive region. To...

Processes involving oxygen and nitrogen oxides as catalysts have been operated commercially using either vapor- or Hquid-phase reactors. The vapor-phase reactors require particularly close control because of the wide explosive limit of dimethyl sulfide in oxygen (1—83.5 vol %) plants in operation use Hquid-phase reactions. Figure 2 is a schematic diagram for the Hquid-phase process. The product stream from the reactor is neutralized with aqueous caustic and is vacuum-evaporated, and the DMSO is dried in a distillation column to obtain the product. 

Many polymer films, eg, polyethylene and polyacrylonitrile, are permeable to carbon tetrachloride vapor (1). Carbon tetrachloride vapor affects the explosion limits of several gaseous mixtures, eg, air-hydrogen and air-methane. The extinctive effect that carbon tetrachloride has on a flame, mainly because of its cooling action, is derived from its high thermal capacity (2). 

Some vent streams, such as light hydrocarbons, can be discharged directly to the atmosphere even though they are flammable and explosive. This can be done because the high-velocity discharge entrains sufficient air to lower the hydrocarbon concentration below the lower explosive limit (API RP 521, 1997). Toxic vapors must be sent to a flare or scrubber to render them harmless. Multiphase streams, such as those discharged as a result of a runaway reaction, for example, must first be routed to separation or containment equipment before final discharge to a flare or scrubber. 

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. 

The lower explosive limit (LEL) is the minimum concentration of a vapor in air that will support a flame when ignited. The flash point is the lowest temperature of a liquid that produces sufficient vapor for an open flame to ignite in air. 

Flammable or Explosive Limits — the upper and lower vapor eoneentrations at whieh a mixture will bum or explode. The lower explosive limit of p-xylene is 1.1 pereent by volume in air, whereas the upper explosive limit is 7.0 percent in air. A mixture of p-xylene vapor and air having a coneentration of <1.1 pereent in air is too lean in p-xylene vapor to bum. Conversely, a mixture containing more than 7.0 percent is too rieh in p-xylene to bum. By subtraetion (7.0 - 1.1), p-.xylene is said to have a flammable range of 5.9. Materials having low explosive limits and wide flammable ranges are extremely dangerous. 

Explosive limits are expressed in percent by volume of vapor in air. LELs and UELs have been determined in fire and safety, and health laboratories for all substances likely to be found in industry. Typical values for some solvents and gases are given in Table 3. 

The volatile solvents recoverable by the activated carbon system or any other system are nearly all organic, and many of them form flammable or explosive mixtures with air. Such mixtures may lie between upper and lower explosive limits. The activated carbon system can avoid the explosive range by staying well below the lowest percentage of vapor which is still explosive it functions well at very low concentrations. The system also recovers solvents efficiently even in the presence of water the recovery efficiency is high (98 percent and 99 percent are not unusual) it may be fully automatic. The annual maintenance charge rarely exceeds 5 percent of the cost of equipment. The recovery expense may be as low as 0.2 cent per pound in some installations it rarely exceeds 1 cent per pound. 

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. 

Among common areas where explosion can occur are coal mines, petrochemical plants, chemical plants, paint shops, grain handling industry, etc. Explosive limits for gases and vapors are expressed as percentages (% ), and may be defined as minimum and maximum concentrations of a flammable gas or vapor between which ignition occur. Concentrations below the lower explosive limit (LEL) are too lean to burn, while those above the upper explosive limit (UEL) are too rich. Table 7.8 lists explosive limits for some common gases. 

A liquid not considered flammable may still have an explosive potential. An example is dichloromethane or methylene chloride, often used in paint strippers, which evaporates very quickly. It is not flammable, but its vapors may be explosive (explosive limits 12% to 22%). 

In catalytic incineration, there are limitations concerning the effluent streams to be treated. Waste gases with organic compound contents higher than 20% of LET (lower explosion limit) are not suitable, as the heat content released in the oxidation process increases the catalyst bed temperature above 650 °C. This is normally the maximum permissible temperature to which a catalyst bed can be continuously exposed. The problem is solved by dilution-, this method increases the furnace volume and hence the investment and operation costs. Concentrations between 2% and 20% of LET are optimal, The catalytic incinerator is not recommended without prefiltration for waste gases containing particulate matter or liquids which cannot be vaporized. The waste gas must not contain catalyst poisons, such as phosphorus, arsenic, antimony, lead, zinc, mercury, tin, sulfur, or iron oxide.(see Table 1.3.111... 

Explosive limits The maximum and minimum concentrations of a mixture of gas, dust, or vapor in air or another gas which will explode if ignited. 

As vent collection systems normally contain vapor/air mixtures, they are inherently unsafe. They normally operate outside the flammable range, and precautions are taken to prevent them from entering it, but it is difficult to think of everything that might go wrong. For example, an explosion occurred in a system that collected flarmnable vapor and air from the vents on a number of tanks and fed the mixture into a furnace. The system was designed to run at 10% of the lower explosion limit, but when the system was isolated in error, the vapor concentration rose. When the flow was restored, a plug of rich gas was fed into the furnace, where it mixed with air and exploded [17]. Reference 34 describes ten other incidents. 

The vapor-air mixture in the drum is close to the optimum for an explosion. This usually occurs about midway between the lower and upper explosive limits. 

NamA Flash Point °F Explosive Limits in air % by Volume Aufoignition Temperature F. Vapor Density... 

Explosive limits in air, % by vol. Auto- ignition temperature. °F Specific gravity (water = 1.0) Vapor density (air = 1.0) Melting point. °r Boiling point or range. °F Water solu- bility Suitable extin- guishing agents Hazard... 

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