Flammable limits, defined

An alternate method for flash point prediction is the method of Gmehling and Rasmussen and depends on the lower flammabihty limit (discussed later). Vapor pressure as a function of temperature is also required. The method is generally not as accurate as the preceding method as flammability limit errors are propagated. The authors have also extended the method to defined mixtures of organics. 

Flammability Range - Defined as the difference between the UEL and LEL. This difference is an indication of how wide the flammability limits of a chemical are. The wider this range, the more hazardous the chemical may be considered from a fire standpoint. 

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. 

Flanumbility limits (or explosion limits) for a flammable gas define tlie concentration range of a gas-air ini. ture witliin wliich an ignition source can start a self-propagating reaction. Tlie minimmn and maximmn fuel concentrations in air tliat will produce a self-sustaining reaction mider given conditions are called tlie lower Jlammability limit (LFL) and tlie upper Jlammability limit (UFL). (The abbreviations LEL and UEL, for lower and upper explosivity limits, are sometimes used.) The flanunability limits are functions of... 

IDLH data are currently available for 380 materials. Because IDLH values were developed to protect healthy worker populations, they must be adjusted for sensitive populations, such as older, disabled, or ill populations. For flammable vapors the IDLH concentration is defined as one-tenth of the lower flammability limit (LFL) concentration. Also note that IDLH levels have not been peer-reviewed and that no substantive documentation for the values exists. 

The flash point and flammability limits are not fundamental properties but are defined only by the specific experimental apparatus and procedure used. 

Figure 6-5 Maximum pressure for methane combustion in a 20-L sphere. The flammability limits are defined at 1 psig maximum pressure. Data from C. V. Mashuga and D. A. Crowl, Process Safety Progress (1998), 17(3) 176-183 and J. M. Kuchta, Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries A Manual, US Bureau of Mines Report 680 (Washington, DC US Bureau of Mines, 1985).

The experiment is repeated over a range of concentrations. A plot of the maximum pressure versus fuel concentration is used to determine the flammability limits, as shown in Fig. 23-9. A pressure increase of 7 percent over initial ambient pressure is used to define... 

In considering the ignition of vapors, two important concepts are flammability limit and flammability range. Values of the vapor/air ratio below which ignition cannot occur because of insufficient fuel define the lower flammability limit. Similarly, values of the vapor/air ratio above which ignition cannot occur because of insufficient air define the upper flammability limit. The difference between upper and lower flammability limits at a specified temperature is the flammability range. 

Physical and Chemical Properties. The physical and chemical properties of heptachlor and heptachlor epoxide are sufficiently well defined to allow assessments of the environmental fate of the compounds to be made (ACGIH 1986 Chapman 1989 HSDB 1990a MacKay 1982 OHM/TADS 1985a, 1985b). Some physical and chemical properties of heptachlor epoxide that are not relevant to environmental fate are lacking. Knowledge of these properties, such as odor, flashpoint, and flammability limits, would be useful for workers involved in the manufacture, use, or clean-up of heptachlor and heptachlor epoxide. 

No single parameter defines flammability. Commonly used terms are flash point, flammability limits, autoignition temperature, minimum ignition energy and burning velocity. 

Flammable vapor burns in air only over a limited range of fuel-to-air concentrations. The flammable range is defined by two parameters the Lower flammable limit (LFL) and the upper flammable limit (UFL). These two terms are also called the lower explosive limit (LEL) and the upper explosive limit (UEL). 

Most liquids respond to a temperature rise through a thermodynamic phase change to gas. For ignition to occur, the fuel concentration in air must be in a range that defines a flammable mixture. These bounding limits are commonly referred to as the lower flammability limit (LFL) and upper flammable limit (UFL). These are the lowest and highest fuel concentrations in air (by volume) that will support flame propagation. Fuel concentrations below the LFL or above the UFL are too lean or rich, respectively, and will not support combustion. 

The criteria defined by the CHETAH program (see COMPUTATION OF HAZARDS) were used to predict lower and upper flammable limits for various organic (C,H,0,N) compounds, results generally being in good agreement with experimental values [8]. 

Dust concentrations in major equipment may be designed to be below the lower flammable limit, but this often cannot be depended on in actual operation. Dust concentrations cannot be safely designed to be above an upper flammable limit because such a limit is ill-defined.2... 

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... 

Tlie flash point of a flanunable liquid is defined as the temperature at which tlie vapor pressure of the liquid is tlie same as the vapor pressure corresponding to tlie lower flammability limit concentration. Tlie tliree major methods of measuring Uie flash point are... 

How many grams of tlie ether must evaporate to reach Uie nia. inimn percent of ether vapors tliat would still be flanuiiable This niaximum percentage of apor is was earlier defined as tlie upper flammable limit (UFL) which for dietliyl ether is 35% v/v. 

FIGURE 8.3. Illustration of the dependence of the adiabatic flame temperature and of the Arrhenius factor on the equivalence ratio, exhibiting extinction condition for defining flammability limits. 

There exist two values of the mole percent of fuel in a fuel-air mixture—the lower or lean flammability limit and the upper or rich flammability limit—that define a range within which self-sustaining combustion can occur. A fuel-air mixture whose composition falls outside these limits is incapable of igniting or exploding, even if exposed to a spark or flame. The composition range between the two flammability limits is called the explosive range of the mixture. 

Certain other rather stringent limitations are attached to flammable solids and liquids as to the precise conditions under which they will burn or explode. The explosive or flammable limits of a material refers to the ratio between the amount of potentially flammable vapor in mixture with ambient air or oxygen. The lower limits are defined as the minimum concentration of vapor in air or oxygen below which the propagation of flame does not occur on contact with a source of ignition. Likewise, there is a maximum concentration which ignition does not occur. As may be expected, these limits have wide variation dependent upon the particular material involved. 

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