Deflagration pressure

While the deflagration pressure ratio for ethylene oxide vapor is about 11 or less, Hquid mist decomposition can give much greater pressures and very fast rates of pressure rise (190). 

Deflagration Pressure The increase in pressure in a vessel from a deflagration results from an increase in temperature the ac tual maximum flame temperature for propane, for example, is I925°C (3497°F). No significant increase in moles of gas to cause pressure buildup results from combustion of propane in air. 

Peak deflagration pressure in closed equipment is approximately eight times the initial absolute pressure, whetner atmospheric, subatmo-spheric, or elevated. This maximum pressure occurs at a concentration just slightly richer in fuel than the stoichiometric concentration for combustion in air icA as shown in Table 26-14 for propane and methane ... 

Several environmental factors affec t maximum deflagration pressure and pressure rise, as highlighted in Table 26-16. 

TABLE 26-14 Optimum Concentrations for Maximum Deflagration Pressure... 

TABLE 26-16 Effect of Environmental Factors on Deflagration Pressure and Pressure Rise ... 

Maximum deflagration pressure -1- - Minor Minor Minor... 

Vessel volume has a large effect on the maximum rate of deflagration pressure rise the cubic law states, all else being equal... 

Deflagration pressure can be reduced substantially by suppression. Figure 26-30 shows the pressures measured in an ethylene-air explosion and a sodium bicarbonate-suppressed ethylene-air explosion. Fike Corporation, Blue Springs Missouri, and Fenwal Safety Systems, Marlborough, Mass., supply explosion suppression systems. 

Design vessel to accommodate maximum expected deflagration pressure... 

Provide deflagration pressure relief device/system... 

Design system to accommodate maximum expected deflagration pressure Provide prescrubbers/condensers to reduce load in duct... 

Design system for deflagration pressure containment where practical... 

Deflagration venting (see NFPA 68) is highly applicable to powder operations. This is because most powder operations are at atmospheric pressure, the rate of deflagration pressure rise is usually small enough for vent relief panels to be of a practical size, and the subsequent fire is limited because most powders cannot bum in bulk. 

Deflagration pressure containment is an approach for selecting the design pressure of a vessel so that it is capable of withstanding the maximum pressure resulting from an internal deflagration. Vessels or process equipment can be designed to either... 

NFPA 69 (1997) provides equations for calculating the required design pressures for both types of containment design. It also discusses the limitations of deflagration pressure containment design. 

It is common practice in the chemical process industries to provide isolation devices for stopping flame fronts, deflagration pressures, pressure piling, and flame-jet ignition between process equipment interconnected by pipes or ducts. There are several devices for providing this isolation as follows ... 

Optimum Mixture A specific mixture of fuel and oxidant that yields the most rapid comhustion at a specific measured quantity or that yields the lowest value of the minimum ignition energy or that produces the maximum deflagration pressure. The optimum mixture is not always the same for each comhustion property that is measured. 

Pj = the maximum initial pressure at which the combustible atmosphere exists, psig R = the ratio of the maximum deflagration pressure to the maximum initial pressure, as described in Code Par 5-3.3.1... 

The dimensionless ratio R is the ratio of the maximum deflagration pressure, in absolute pressure units, to the maximum initial pressure, in consistent absolute pressure units. For gas/air mixtures, R shall be taken as 9.0 for dust/air mixtures, R shall be taken as 10.0. 

Figure 7-65G. Alternate venting nomograph for dusts of ciass St.-I whose maximum deflagration pressure does not exceed 9 bar ga. Reprinted with permission, NFPA 68-1988, Deflagration Venting, (1988) National Fire Protection Association, Quincy, MA 02269. See note Figure 7-63A.

Figure 7-66. Increase in the reduced deflagration pressure for gases due to the effect of a vent duct. By permission, American Institute of Chemical Engineers, Meeting Mar. 6, 1988 by I. Swift (deceased) [56], with data from Ref. [54].

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