Estimating Lower Flammability Limits

The following discussion provides a plant caculation method for lower explosion limit concentrations for flare stacks or leaking valves that could ignite. 

Explosivity limits tor various pure components are given in Table 9. The limits of flammability (a concentration, C) for a mixture of gases can be computed from the following expression  

Application of this procedure to inadvertently ignited safety valve discharges can involve a special problem. Certain combinations of pressure ratio and length of safety valve riser can result in choked flow, with a pressure discontinuity at the exit. The pressure of the jet then adjusts to atmospheric pressure in a system of shock waves or expansion waves over a distance of a few pipe diameters. These waves can affect die local mixing of the jet with the crosswind. Since die calculation procedure incorporates correlations for subsonic jets, it cannot be expected to be entirely accurate in this case. Nevertheless, since the wave system occupies a very small portion of the flow field influenced by the jet, the procedure can still be counted on to provide a useful approximation of the gross flame length and flame shape when the actual discharge velocity and diameter are used in the calculation. 

Credit for additional height of the flame center for multiple valve installations may be taken by clustering the safety valve discharge pipes to the atmosphere. The following procedure should be used for determining equivalent diameter and exit velocity to be used in the flame center calculation. Diameter and velocity are based on the total actual area of the clustered vents. 

V jequ = actual velocity of any one vent. For Unequal Diameter Vents  

---

Pressure Safety Design Practices Estimating Lower Flammable Limits... 

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.)... 

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). 

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. 

Estimate the flash-point of acetone and compare it with the experimental value given in the literature. Hint Start with the basic principle that the fugacity in the vapor phase must equal that in the liquid phase. The lower flammable limit for acetone is 2.55 percent by volume. 

The flash-point of a liquid mixture can be estimated by determining the temperature at which the equilibrium concentration of the flammable vapors in the air reach a concentration such that ( i/LFLi) = 1.0 where yi is the vapor phase mole percent of component i and LFL( is the lower flammability limit concentration of component... 

The EPA and OSHA regulations [A-1, A-2, and A-3] apply to facilities from which a release of hazardous material could occur, at or above the quantities specified, as shown in bold in Table 17.A.1. Also shown are the Immediately Dangerous to Life and Health [IDLH] concentrations (for 30-minute exposure) [A-4] the Lower Flammable Limits and Heats of Combustion for combustible materials and the EPA toxic, thermal-radiation, and overpressure endpoints for public-exposure evaluations. N/L indicates that the chemical is not listed in the pertinent document. Estimated values are shown with a superscript ( ). Where no inhalation-toxicity data were available, the oral dose that caused 50% fatalities is shown only to indicate qualitatively the systemic toxicity (for example, an oral LD50 of 1000 mg/kg would be considered relatively nontoxic). The data are for pure chemicals, except where otherwise indicated, that is, without added diluents. Additional hazardous-properties information can be obtained via the Sax No. [A-6]. 

There are a few general rules which help in the estimation of flammability limits. In the case of hydrocarbons, the lower limit can be estimated from simple formula 6/number of carbon atoms in molecule for benzene and its derivatives the formula changes to 8/num-ber of carbon atoms. To calculate the upper limits, the number of hydrogen and carbon atoms is used in calculation. 

The lower flammability limit of a mixture can be estimated from Le Chatelier s Law ... 

When handling a solvent mixture, data are often available for the individual components but not for the mixture itself. Application of Le Chatelier s Law enables estimation of the limits for the mixture provided the limits of the individual components are known. If is the volume percentage of the combustible component n, with a lower flammable limit of then the lower limit, L ix, of the mixture is ... 

It should be noted that in cases where VCEs may be possible, the footprint of the flash fire zone (the zone within the lower flammability limit [LFL] of the material) should also be estimated and used in the overall risk estimation with its corresponding frequency. 

Combustible vapor-arr mixtures are flammable over a limited range of concentrations. The minimum volume % of vapor that gives a combustible mixture is called the lower flammable limit (LFL). Generally, the LFL is about half the stoichiometric mixture, the concentration required for complete combustion of the vapor in air. (a) If oxygen is 20.9 vol % of air, estimate the LFL for n-hexane, CgHj4. (b) What volume (in mL) of n-hexane (d = 0.660 g/cm ) is required to produce a flammable mixture of hexane in 1.000 m of air at STP ... 

Piqueras, C., Garcia-Serna, J. and Cocero, M. (2011). Estimation of lower flammability limits in high-pressure systems. Application to the direct synthesis of hydrogen peroxide using snpercritical and near-critical CO2 and air as diluents, J. Supercrit. Fluid, 56, pp. 33 40. 

When a combustible substance is mixed with air, the mixture will explode only when it is neither too rich nor too lean. The lower explosion limit (LEL) is the minimum volume percent of the substance in air with flammability, which is separated from the upper explosion limit (UEL) by the explosive concentration range. The tabulations in handbooks are based on experimental data, and sometimes derived from estimation methods based on the elemental composition of the fuel as CmEtxOy. Figure 6.11 shows the LEL for the series of normal paraffins and of 1-alcohols versus the number of carbon atoms. There are two ways to plot the results, which show that, for paraffins, the volume percent shows a steeply declining trend, but the weight percent shows a mildly increasing trend. One may conclude that a smaller volume percent of higher paraffin... 

---