Estimating Flammability Limits

For some situations it may be necessary to estimate the flammability limits without experimental data. Flammability limits are easily measured experimental determination is always recommended. 

Jones7 found that for many hydrocarbon vapors the LFL and the UFL are a function of the stoichiometric concentration (Cst) of fuel  

The stoichiometric concentration for most organic compounds is determined using the general combustion reaction 

Zabetakis, Fire and Explosion Hazards at Temperature and Pressure Extremes, AlCHE Inst. Chem. Engr. Symp., ser. 2, Chem. Engr. Extreme Cond. Proc. Symp. (1965), pp. 99-104. 

Inflammation Limits and Their Practical Application in Hazardous Industrial Operations, Chem. Rev. (1938), 22(1) 1-26. 

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Estimating Flammability Limits There are a number of very approximate methods available to estimate flammability limits. However, for critical safety values, experimental determination as close as possible to actual process conditions is always recommended. 

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... 

FIG. 26-31 Estimated maximum downwind distance to lower flammable limit L, percent by volume at ground level in centerline of vapor cloud, vs. continuous dense vapor release rate at ground level. E atmospheric stability. Level terrain. Momentary concentrations for L. Moles are gram moles u is wind speed. (From Bodmtha, 1980, p. 105, by permission.)... 

Pressure Safety Design Practices Estimating Lower Flammable Limits... 

IDLH). For a combustible release, the code gives an estimate of the mass of vapor within the flammable limits. Model Description References Model description and references I... 

Both LFL and UFL valnes for mixtnres can be estimated by nse of the Le Chatelier eqnation (Growl and Lonvar 1990). However, the methods have some limitations with respect to calcnlating the UFL for certain mixtnres. Britton (1996) determined that the eqnation does not apply to the UFL of mixtnres containing decomposable components snch as ethylene oxide or to mixtnres containing ethyl ether. Mashnga and Growl (2000) discnss the derivation of Le Chatelier s mixing rnle for flammable limits. 

Estimating the amount of material within flammable limits (usually by dispersion modeling) and multiplying this by the heat of combustion times an efficiency factor (usually higher than the one applied above, generally 5% to 20%). 

Blast scale was determined by use of dispersion calculations to estimate fuel quantity within flammability limits present in the cloud. Initial blast strength was determined by factors which have been found to be major factors affecting the process of turbulent, premixed combustion, for example, the fuel s nature and the existence within the cloud of partial confinement or obstacles. 

Conventional TNT-equivalency methods state a proportional relationship between the total quantity of flammable material released or present in the cloud (whether or not mixed within flammability limits) and an equivalent weight of TNT expressing the cloud s explosive power. The value of the proportionality factor—called TNT equivalency, yield factor, or efficiency factor—is directly deduced from damage patterns observed in a large number of major vapor cloud explosion incidents. Over the years, many authorities and companies have developed their own practices for estimating the quantity of flammable material in a cloud, as well as for prescribing values for equivalency, or yield factor. Hence, a survey of the literature reveals a variety of methods. 

Method 3 (Figure 6-12) Given the flammability limits in air, the procedure is as follows Use steps 1 and 3 from method 1. Estimate the LOC using Equation 6-16. This is only an estimate, and usually (but not always) provides a conservative LOC. 

The ability to predict Su is limited by the same factors used to predict the autoignition or flammability limits. However, an approximate analysis first considered by Mallard and Le Chatelier in 1883 [7] can be useful for quantitative estimates. 

It is further found that the adiabatic flame temperature is approximately 1300 °C for mixtures involving inert diluents at the lower flammable limit concentration. The accuracy of this approximation is illustrated in Figure 4.19 for propane in air. This approximate relationship allows us to estimate the lower limit under a variety of conditions. Consider the resultant temperature due to combustion of a given mixture. The adiabatic flame temperature (7f ad), given by Equation (2.22) for a mixture of fuel (Xp), oxygen (Xo2) and inert diluent (Xd) originally at 7U, where all of the fuel is consumed, is... 

It is reported that the adiabatic flame temperature for H2 at the lower flammability limit (LFL) in air is 700 °C. From this information, estimate the LFL, in % by volume, for the hydrogen-air mixture at 25 °C. Assume water is in its vapor phase within the products. 

Ducros, M. and H. Sannier, "Application du Programme CHETAH a l Etude de la Sensibilisation de Composes Oxygenes et a l Estimation des Limites Interfer-ieures dTnflammabilite" ("Application of the CHETAH Program for the Study of the Sensibility of Oxygen Containing Mixtures and the Estimation of the Lower Flammability Limits"), /. Haz. Mat., 19 (1988). 

Vapor Mixtures Frequently, flammability data are required for vapor mixtures. The flammability limits for the mixture are estimated by using LeChatelier s rule [LeChatelier, Estimation of Firedamp by Flammability Limits, Ann. Mines (1891), ser. 8, 19 388-395, with translation in Process Safety Progress, 23(3) 172]. 

Flammability limits can also be estimated by using calculated adiabatic flame temperatures and a chemical equilibrium program [Mashuga and Crowl, Flammability Zone Prediction Using Calculated Adiabatic Flame Temperatures, Process Safety Progress, 18 (3) (1999)]. 

Flash point is one of the most important fire safety characteristics and hence it is a very important consideration in solvent design. The flammability limit of a solvent is characterized by its flash point, which is the temperature at which the mixture of air and vapor above the liquid can be ignited (Mullin, 1961). It is the lowest point at which the vapor pressure of a liquid will produce a flammable mixture. The flash point of the solvent can be estimated using the following group contribution method (ICAS, 2003)... 

Use laminar premixed free-flame calculations with a detailed reaction mechanism for hydrocarbon oxidation (e.g., GRI-Mech (GRIM30. mec)) to estimate the lean flammability limit for this gas composition in air, assuming that the mixture is flammable if the predicted flame speed is equal to or above 5 cm/s. For comparison, the lean flammability limits for methane and ethane are fuel-air equivalence ratios of 0.46 and 0.50, respectively. 

Estimate the time required for the natural gas/air mixture in the room to reach the lean flammability limit, assuming that the gas in the room is well mixed and there is no ventilation. 

From the point of view of the potential for a fire, the closed cup flash point determination is usually the most important. In a perfect closed cup test, the vapor pressure is in equilibrium with the liquid at the temperature of the test. At the flash point, the vapor composition is at the lower flammable limit. In fact, the lower flammable limit can be estimated from vapor pressure data (for a pure compound). Open cup flash points are generally higher and, thus less conservative, than closed cup determinations. The value determined in an open cup test is subject to air movement at the open face of the cup and true vapor-liquid equilibrium probably does not occur. 

Flammable limits for mixtures of flammable gases and vapors. For mixtures of several flammable gases and vapors, the flammable limits can be estimated by application of Le Chatelier s equation, if the flammable limits of the components are known 2... 

The flammability limits of mixtures can be estimated from the data for individual fuels by using le Chatelier s principle... 

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