Flammable mixtures Calculations

The violence of the confined explosions (flammable v apor, gas, and dust) can be calculaled by the e.xplosion pressme mid energy eqmitions introduced earlier. The behavior of flammability (or e.xplosivity) limits of flammable mixtures and dusts is also covered in preceding sections. 

Le Chatelier s Rule allows the calculation of the lower flammability (explosibility) limits for flammable mixtures ... 

Some organic compounds can be in solution with water and the mixture may still be a flammable mixture. The vapors above these mixtures such as ethanol, methanol, or acetone can form flammable mixtures with air. Bodurtha [39] and Albaugh and Pratt [47] discuss the use of Raoult s law (activity coefficients) in evaluating the effects. Figures 7-52A and B illustrate the vapor-liquid data for ethyl alcohol and the flash point of various concentrations, the shaded area of flammability limits, and the UEL. Note that some of the plots are calculated and bear experimental data verification. 

Centrifugal pumps, 181 Discharge systems, 187 Example calculation, 186 Flow friction losses, 185. 186 Friction losses, pipe, see Chapter 2 Friction, 188 Pressure head, 184—186 Static head, 184-186 Suction head, 184, 185 Suction lift, 184, 185 Suction systems, 186 Hvdroclones, 265—267 Application system, 267 Ignition, flammable mixtures, 493 Impellers, centrifugal, reducing diameter, 203 Impellers,... 

Any oxygen contained in a mixture may be considered as though it were a part of the air required for die combustion, and the analysis of the flammable mixture should be converted to an air-free basis before the flammable limits are calculated. 

A mixture of methane and air is capable of being ignited only if the mole percent of methane is between 5% and 15%. A mixture containing 9.0 mo e% methane in air flowing at a rate of 700. kg/h is to be diluted with pure air to reduce the methane concentration to the lower flammability limit. Calculate the required flow rate of air in mol/h and the percent by mass of oxygen in the product gas. Note Air may be taken to consist of 21 mole% O2 and 79% N2 and to have an average molecular weight of 29.0.)... 

The maximum pressure rise [Equation (4.6)] following ignition of a flammable mixture in a closed vessel is proportional to the initial pressure and to the ratios rijn and TJT. At atmospheric pressure, the ratio njrii is approximately 1.0 for a stoichiometric methane-air mixture and, from the calculations of Smith, Edwards, and Brinkley, TJT, is ... 

We then compare the results of the calculations in order to determine the dimensions of the flammable gas. We also comment on the influence of obstacles on the formation of the flammable mixture and on the possible occurrence of spontaneous ignition at the start of the leakage. 

The use of the stationary jet model JET 1 makes it possible to calculate, to within 20% the distance along the axis at which the conentration is equal to the LFL amd also the volume of the flammable mixture formed. 

Schofield (1984) reports that experiments on the behavior of flammable mixtures in large volumes (30 m or 1000 ft ) indicate that venting calculations developed from small scale experiments may oversize the vents. Evaluation of container strengths can be a main source of error. Vessels are often stronger than safety factors assume and this factor may be conservative in terms of the frequency or probability of vessel rupture, but conversely, not conservative in terms of calculating the consequences of rupture. 

A calculation of the flammability limits of complex gas mixtures is carried out by the application of the mixture rule. Stated simply, the mixture rule is that if two limit mixUires of different gases are added together, the resulting mixmre also will be a limit mixture. The mathematical statement of this law is as follows ... 

In these examples, the inerts CO2 and Nj are combined with the combustibles H2 and CO and the small amount of CH4 is taken alone. Next, the ratio of inert to combustible is obtained for each group, as shown in Table 5, and the flammable limits for each such mixture are obtained from Figure 14. The mixture rule formula is now applied, using the data as just obtained, and the limits are calculated as shown in Table 5. 

The summary at the bottom of Table 5 indicates the relative agreement between the calculated data and that experimentally determined for this particular producer gas. It is suggested that the difference between calculated and determined data in this case may be due more to inaccuracies in the analysis of the produeer gas (particularly for methane) than to the fault of the mixture rule formula. This points up the fact that reliable gas analyses also are a necessary part of the calculated flammability limit data. 

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