Flammability of fuels

Flammability Limits There are both upper (or rich) and lower (or lean) limits of flammability of fuel-air or fuel-oxygen mixtures. Outside these hmits, a self-sustaining flame cannot form. Flammability limits for common fuels are listea in Table 27-18. 

Substance Autoignition temperature, °C Flammable (explosive) limits, percent by volume of fuel (25°C, 760 mm) ... 

Fuel Quantity of fuel per Gf" Flammability limit ia air, vol % gas Lower Higher Maximum flame speed, cm/s Spontaneous ignition temperature, °C Ignition d -jC energy, m ... 

In assessing the hazard of a UVCE or in investigating a UVCE it is often necessary to (1) estimate the maximum distance to the lower flammable hmit (LFL) and (2) determine the amount of gas in a vapor cloud above the LFL. Figure 26-31 shows the maximum distance to the lower flammable limit, i.e., in the centerline of the cloud, based on the previous method from Bodurtha (1980) for wind speeds of 1 iti/s (2.2 mi/h) and 5 m/s (11 mi/h). Maximum concentrations probably occur near 1 m/s. The volume of fuel from the LFL up to 100 percent may be estimated by... 

Install fire safe bottom valves Install fire safe valves on major solvent lines Install remote shut off of fuel sources Eliminate points of leakage (flanges, hoses). Replace with fixed/welded pipes Move flammable material storage away from vessel (e.g., pallets, etc.)... 

Ignition of a flammable dust-air mixture is more difficult than with flammable vapour-air mixtures. A larger source of heat is required, and a larger volume of fuel must be heated to the ignition point. The same range of potential ignition sources is applicable as for air-vapour mixtures. 

Flammable Limits The minimum and maximum concentration of fuel vapor or gas in a fuel vapor or gas/gaseous oxidant mixture (usually expressed in percent hy volume) defining the concentration range (flammable or explosive range) over which propagation of flame will occur on contact with an ignition source. See also Lower Flammable Limit and Upper Flammable Limit. 

The long list of vapor cloud explosion incidents indicates that the presence of a quantity of fuel constitutes a potential explosion hazard. If a quantity of flammable material is released, it will mix with air, and a flammable vapor cloud may result. If... 

The portion of fuel that should be included in the calculation The total amount released the amount flashed the amount flashed times an atomization factor or the flammable portion of the cloud after accounting for dispersion over time. 

In the first approach, a vapor cloud s potential explosive power is proportionally related to the total quantity of fuel present in the cloud, whether or not it is within flammable limits. This approach is the basis of conventional TNT-equivalency methods, in which the explosive power of a vapor cloud is expressed as an energetically equivalent charge of TNT located in the cloud s center. The value of the proportionality factor, that is, TNT equivalency, is deduced from damage patterns observed in a large number of vapor cloud explosion incidents. Consequently, vapor cloud explosion-blast hazard assessment on the basis of TNT equivalency may have limited utility. 

The two approaches lead to completely different procedures for vapor cloud explosion hazard assessment. If conventional TNT-equivalency methods are applied, explosive potential is primarily determined by the amount of fuel present in a cloud, whether or not within flammability limits. The cloud center is the potential blast center and is determined by cloud drift. 

Estimate the volumes of fuel-air mixture present in the individual areas identified as blast sources. This can be done on the basis of the overall dimensions of the areas and jets. Note that the flammable mixture may not fill an entire... 

Fireball A burning fuel-air cloud whose energy is emitted primarily in the form of radiant heat. The inner core of the cloud consists almost completely of fuel, whereas the outer layer (where ignition first occurs) consists of a flammable fuel-air mixture. As the buoyancy forces of hot gases increase, the burning cloud tends to rise, expand, and assume a spherical shape. 

Some 200 tonnes per annum of B2H6 is produced commercially, worldwide. Care should be taken in all these reactions because B2H6 is spontaneously flammable its heat of combustion (—A//°) is higher per unit weight of fuel than for any other substance except... 

Liquid Pool Flames. Liquid fuel or flammable spills often lead to fires involving a flame at the surface of the liquid. This type of diffusion flame moves across the surface of the liquid driven by evaporation of the fuel through heat transfer ahead of the flame. If the liquid pool or spill is formed at ambient conditions sufficient to vaporize enough fuel to form a flammable air/fuel mixture, then a flame can propagate through the mixture above the spill as a premixed flame. 

Vei y small solid fuel particles such as sawdust, agricultural grains, or coal dust can sustain flames when they are suspended in air. In fact, very serious fires have occurred in grain storage towers and coal mines because of the flammability of suspended dusts. The combustion of the individual particles follows the usual pattern of solid particle burning— devolatization and char burning. The combustion of the whole cloud of particles is similar to spray combustion and its characteristics depend on the nature of the fuel, size of the particles, and the number of particles in a given volume. 

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel. 

Thermosets and thermoplastics behave differently from each other in fires. Thermosets do not melt when heated but may well undergo further crosslinking. The presence of such additional crosslinks hinders movement of any volatile degradation products through the polymer matrix. Hence the combustion zone tends to be starved of fuel and for this reason thermosets tend to be relatively non-flammable. 

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