Vapor air mixture

Flammability. Since almost all ethers bum in air, an assessment of their potential ha2ards depends on flash points and ignition temperatures. The flash point of a Hquid is the lowest temperature at which vapors are given off in sufficient quantities for the vapor—air mixture above the surface of the... 

Electrostatic Discharges An electrostatic discharge takes place when a gas or vapor-air mixture is stressed, electrically, to its breakdown value. Depending upon the specific circumstances, the breakdown appears as one of four types of discharges, which vary greatly in origin, appearance, duration, and incendivity. 

Self-Generated Discharges Vapor-air mixtures can be ignited by sparks from highly charged liquids. It is said that such liquids cany their own match. Typical causes of such charging for poorly conductive (<50 pS/m) liquids include ... 

To determine the level of electrification on an insulating surface, an elec tric field meter should always be used. There is a direct relationship between the charge density on the surface of an insulator and the elec tric field intensity at the surface. Measurements should be made at locations where the insulating surface is several inches away from other insulating or conduc tive surfaces. The area of the measured surface should be large, compared to the field of view of the meter. In locations where a flammable vapor-air mixture has an MIE greater than 0. 2 mj, field intensities of 500 kV/m or more should be considered unsafe. 

Among the principal reasons for providing inerting on reactors and vessels is the desirability of eliminating flammable vapor-air mixtures that can be caused by ... 

Roofs of tanks have to be exchanged for floating types to minimize the dangerous atmosphere of hydrocarbon vapor-air mixture which may explode by sparking due to the static electricity. 

The equilibrium vapor pressure above a confined liquid depends only on temperature. The fraction of the total pressure exerted by vapor pressure determines the composition of the vapor-air mixture. Thus when the total pressure is reduced, for example at high elevations or in vacuum tmcks, the vapor concentration in air increases. Since flash points are reported at a... 

Flare systems are subject to potential flashback and internal explosion since flammable vapor/air mixtures may be formed in the stack or inlet piping by the entry of air, and the pilot constitutes a continuous ignition source. Flares are therefore always provided with flashback protection, which prevents a flame front from travelling back to the upstream piping and equipment. Design details are described later. 

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. 

The CEN standard rises the Enropean classificadon system for flammability which has more groups (based on the MESG of die vapor-air mixture) than the US classificadon system. 

Data on dispersion and combustion of aerosol-air clouds are scarce, although Burgoyne (1963) showed that the lower flannmability limits on a weight basis of hydrocarbon aerosol-air mixtures are in the same range as those of gas- or vapor-air mixtures, namely, about 50 g/m. ... 

A BLEVE can cause damage from its blast wave and from container fragments such fragments can be propelled for hundreds of meters. If the vapor-air mixture is flammable, the BLEVE can form a fireball with intense heat radiation. Each effect is discussed in the following sections. 

Rapid and extensive thermal decomposition and oxidation reactions in vapor-air mixture at this temperature. Lower limit determined at 302°F... 

Section 5-3.3.2 where flammable vapor-air mixtures may exist under normal operation. 

Outdoor equipment of the type covered in Flammable and Combustible Liquids Code, NFPA, 30-1993 (ANSI), Section 5-3.3.2, where flammable vapor-air mixtures may exist under normal operation... 

Water-cooling in towers operates on the evaporative principles, which are a combination of several heat/mass transfer processes. The most important of these is the transfer of liquid into a vapor/air mixture, as, for example, the surface area of a droplet of water. Convective transfer occurs as a result of the difference in temperature between the water and the surrounding air. Both these processes take place at the interface of the water surface and the air. Thus it is considered to behave as a film of saturated air at the same temperature as the bulk of the water droplet. 

Flash point The temperature at which the vapor-air mixture above a liquid is capable of sustaining combustion after ignition from an external energy source. 

Flammability limits Vapor-air mixtures will ignite and burn only over a well-specified range of compositions. The mixture will not burn when the composition is lower than the lower flammable limit (LFL) the mixture is too lean for combustion. The mixture is also not combustible when the composition is too rich that is, when it is above the upper flammable limit (UFL). A mixture is flammable only when the composition is between the LFL and the UFL. Commonly used units are volume percent fuel (percentage of fuel plus air). 

Flammability limits for vapors are determined experimentally in a specially designed closed vessel apparatus (see Figure 6-14 on page 255). Vapor-air mixtures of known concentration are added and then ignited. The maximum explosion pressure is measured. This test is repeated with different concentrations to establish the range of flammability for the specific gas. Figure 6-5 shows the results for methane. 

Calculate the range of temperatures within which the vapor-air mixture above the liquid surface in a can of n-hexane at atmospheric pressure will be flammable. Data are found in Table 4.5. Calculate the range of ambient pressures within which the vapor/air mixture above the liquid surface in a can of n-decane (n-C10H22) will be flammable at 25 °C. 

Calculate the temperature at which the vapor pressure of n-decane corresponds to a stoichiometric vapor-air mixture. Compare your result with the value quoted for the firepoint of n-decane in Table 6.1. 

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