Flammability limit

These tests cover the methods of determination of the minimum temperature at which vapors in equilibrium with hquid solvent are sufficiently concentrated to form flammable mixtures with air at atmospheric pressure and concentration limits of chemicals. Flammable (explosive) limits are the percent levels, volume by volume, of a flammable vapor or gas mixed in air between which the propagation of a flame or an explosion will occur upon the 

The concentration limits of flammability are determined using another method. The method is limited to atmospheric pressure and temperature of 150 C. Equipment is similar to that used in the previous method. A uniform mixture of vapor and air is ignited and flame propagation from ignition source is noted. The concentration of flammable components is varied until a composition is found which is capable to propagate flame. 

A variety of apparatus such as the small scale closed tester, the tag open cup, the tag closed tester, the Cleveland open cup, the Pensky-Martens closed cup, and the equilibrium method are used to determine the flash point. The selection of method is based suggestions included in separate standard.  

The small scale clos tester is a metal cup with a thermometer fitted below the bottom of the internal chamber with a hinge mounted cover having filling orifice. The sample is introduced to the cup and the cup is maintained at a constant temperature by means of temperature controller. After a specific time, a test flame is applied for 2.5 s and an observation is made whether or not flash has occurred. If flash did not occur the cup is cleaned, a new sample is introduced and the temperature is increased by 5 C. The measurements are repeated until the flash point is determined with accuracy of TC. 

The tag open cup is a larger unit equipped with water-glycol bath for temperature control or a solid carbon dioxide-acetone bath for lower flash points. A much larger sample is used with this equipment and the temperature is gradually increased at a rate of l C/min. A taper flame is passed for 1 s in 1 min intervals until the flash point is detected. The tag closed tester can use either a manual or an automated procedure. A sample volume of 50 ml is used. Either a gas flame or an electric ignitor is used. In the automated mode, the equipment is programmed to perform the standard procedure. 

It is important to specify the direction of flame propagation. Since it may be assumed as an approximation that a flame cannot propagate downward in a mixture contained within a vertical tube if the convection current it produces is faster than the speed of the flame, the limits for upward propagation are usually slightly wider than those for downward propagation or those for which the containing tube is in a horizontal position. 

In view of the accelerating effect of temperature on chemical reactions, it is reasonable to expect that limits of flammability should be broadened if the temperature is increased. This trend is confirmed experimentally. The increase is slight and it appears to give a linear variation for hydrocarbons. 

As noted from the data in Appendix E, the upper limit for many fuels is about 3 times stoichiometric and the lower limit is about 50% of stoichiometric. 

TABLE 4.3 Flammability Limits of Some Fuels in Air3 

As increasing amounts of an incombustible gas or vapor are added to the atmosphere, the flammability limits of a gaseous fuel in the atmosphere approach one another and finally meet. Inert diluents such as C02, N2, or Ar merely replace part of the 02 in the mixture, but these inert gases do not have 

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). 

The lower flammability limit is the minimum proportion of fuel in air that will support combustion. The upper flammable limit is the maximum concentration of fuel in air that can support combustion. In popular terms, a mixture below the LFL/LEL is too lean to burn or explode and a mixture above the UFL/UEL is too rich to burn or explode. 

For example, the lower flammability limit of methane in air at sea level is a concentration (by volume or partial pressure) of about 5%. The upper flammability limit is about 15% by volume or partial pressure. Heavier hydrocarbons tend to have lower LFLs. The LFL and UFL of some common hydrocarbons are given in Table B-2. 

Flammability limits are not absolute, but are dependant on temperature, pressure, and other variables. Care must be exercised in using flammability limit data when conditions are different from ambient. For example, in reactors and thermal oxidizers. 

in general, there is a range of fuel concentration in air within which a flame can be propagated. From the analysis above it will be apparent that this range will widen (CfL will decrease and Cnj will increase) as the initial 

At the lower limit of flammability there is more oxygen available than is required for stoichiometric combustion of the fuel. For example, the lower flammability limit for propane in air at 20 C is 2.2% by volume. 

In a fuel-air mixture with 2.2% propane the ratio of air to propane is  

For example, for propane, since under stoichiometric conditions five volumes of oxygen are required per volume of fuel propane, 

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Flammability limits. A flammable gas will bum in air only over a limited range of composition. Below a certain concentration of the flammable gas, the lower flammability limit, the mixture is too lean to burn, i.e., lacks fuel. Above a certain concentration, the upper flammability limit, it is too rich to burn, i.e., lacks oxygen. Concentrations between these limits constitute the flammable range. 

Flammability limits are also affected by temperature. An increase in temperature usually widens the flammable range. 

The flammability limits of a hydrocarbon depend on its chemical nature and its molecular weight. Table 4.14 gives values for some common hydrocarbons. 

Targets and spirals have been observed in the CIMA/CDIMA system [13] and also in dilute flames (i.e. flames close to their lean flammability limits) in situations of enlianced heat loss [33]. In such systems, substantial fiiel is left unbumt. Spiral waves have also been implicated in the onset of cardiac arrhytlnnia [32] the nomial contractive events occurring across the atria in the mannnalian heart are, in some sense, equivalent to a wave pulse initiated from the sino-atrial node, which acts as a pacemaker. If this pulse becomes fragmented, perhaps by passing over a region of heart muscle tissue of lower excitability, then spiral structures (in 3D, these are scroll waves) or re-entrant waves may develop. These have the incorrect... 

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. 

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. 

Inert Gas Dilution. Inert gas dilution involves the use of additives that produce large volumes of noncombustible gases when the polymer is decomposed. These gases dilute the oxygen supply to the flame or dilute the fuel concentration below the flammability limit. Metal hydroxides, metal carbonates, and some nitrogen-producing compounds function in this way as flame retardants (see Flame retardants, antimony and other inorganic compounds). 

In the absence of air, TEE disproportionates violently to give carbon and carbon tetrafluoride the same amount of energy is generated as in black powder explosions. This type of decomposition is initiated thermally and equipment hot spots must be avoided. The flammability limits of TEE are 14—43% it bums when mixed with air and forms explosive mixtures with air and oxygen. It can be stored in steel cylinders under controlled conditions inhibited with a suitable stabilizer. The oxygen content of the vapor phase should not exceed 10 ppm. Although TEE is nontoxic, it may be contaminated by highly toxic fluorocarbon compounds. 

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 ... 

Air is compressed to modest pressures, typically 100 to 200 kPa ( 15-30 psig) with either a centrifugal or radial compressor, and mixed with superheated vaporized butane. Static mixers are normally employed to ensure good mixing. Butane concentrations are often limited to less than 1.7 mol 1 to stay below the lower flammable limit of butane (144). Operation of the reactor at butane concentrations below the flammable limit does not eliminate the requirement for combustion venting, and consequendy most processes use mpture disks on both the inlet and exit reactor heads. A dow diagram of the Huntsman fixed-bed maleic anhydride process is shown in Figure 1. 

Compound Flash point, °C Flammability limit, vol % Lower Upper Autoignition temperature, °C... 

Safety is a critical aspect in the design of phenol plants. Oxidation of cumene to CHP occurs at conditions close to the flammable limits. Furthermore, the CHP is a potentially unstable material which can violendy decompose under certain conditions. Thus, phenol plants must be carefully designed and provided with weU-designed control and safety systems. 

Simple asphyxiant value shown is 10% of the lower flammable limit (LFL). 

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. 

Halothane. Halothane or Fluothane, 2-bromo-2-chloro-l,l,l-trifluoroethane [151 -67-7] is a colorless Hquid with a pleasant odor. Its lower flammability limit, 4.8% in 70% N2O/30% O2, renders it essentially nonflammable. It has a vapor pressure of 32.5 kPa (244 mm Hg) at 20 °C and is stable to soda lime. However, it is photochemicaHy reactive. 

Lower flammability limits (LFL) are expressed as vol % in dry ambient air. No entry means ASHRAE Safety Classification (2) (see Fig. 3). NR = not rated. 

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