Explosion efficiency

The problem of explosion of a vapor cloud is not only that it is potentially very destructive but also that it may occur some distance from the point of vapor release and may thus threaten a considerable area. If the explosion occurs in an unconfined vapor cloud, the energy in the blast wave is generally only a small fraction of the energy theoretically available from the combustion of all the material that constitutes the cloud. The ratio of the actual energy released to that theoretically available from the heat of combustion is referred to as the explosion efficiency. Explosion efficiencies are typically in the range of 1 to 10 percent. A value of 3 percent is often assumed. 

If the explosion occurs in an unconfined vapor cloud, the energy in the blast wave is only a small fraction of the energy calculated as the product of the cloud mass and the heat of combustion of the cloud material. On this basis, explosion efficiencies are typically in the range of 1-10%. 

In some cases, however, only the part of the cloud which is within the flammable range is considered to burn. This may be a factor of 10 less than the total cloud. For further discussion of explosion efficiency see CCPS (1989) or Feeds (1986),... 

If 10 tons of propane exploded with an explosive efficiency 0.05, 1,000 ft from you, what would be the peak positive overpressure Referring to equation 9.1-24, W = 0.0.5 ZE4 5E4/4,7E3 = 1E4 lb, where the heats ot combustion are from Table 9.1-4, and 10 tons is 2E4 lb. The scaled range is = 1000/IE4 = 45.6 from which a positive overpressure of 2... 

The explosion efficiency is one of the major problems in the equivalency method. The explosion efficiency is used to adjust the estimate for a number of factors, including incomplete mixing with air of the combustible material and incomplete conversion of the thermal energy to mechanical energy. The explosion efficiency is empirical, with most flammable cloud estimates varying between 1 % and 10%, as reported by a number of sources. Others have reported 5%, 10%, and 15% for flammable clouds of propane, diethyl ether, and acetylene, respectively. Explosion efficiencies can also be defined for solid materials, such as ammonium nitrate. 

Estimate the explosion efficiency, and calculate the equivalent mass of TNT using Equation 6-24. 

One thousand kilograms of methane escapes from a storage vessel, mixes with air, and explodes. Determine (a) the equivalent amount of TNT and (b) the side-on peak overpressure at a distance of 50 m from the blast. Assume an explosion efficiency of 2%. 

Qualitative studies26 have shown that (1) the ignition probability increases as the size of the vapor cloud increases, (2) vapor cloud fires are more common than explosions, (3) the explosion efficiency is usually small (approximately 2% of the combustion energy is converted into a blast wave), and (4) turbulent mixing of vapor and air and ignition of the cloud at a point remote from the release increases the impact of the explosion.27... 

Bugaut, F., Bernard, S., and Chirat, R. (1989) Theoretical prediction of high explosives efficiency application to NTO. Proc. 9th Symp. (Inti.) on Detonation, Office of the Chief of Naval Research, Arlington, VA, pp. 489 497. 

It was found that silver nitrate, silver bromide, and lead chloride had no appreciable sensitizing effect on lead azide when the impact was performed with a 240-g striker falling 29 cm. However, borax and chalcocite gave 100% explosion efficiency. Bismuthinite and galena had only small sensitizing effects in spite of their high melting points, but both of these materials were soft compared to tire lead azide. 

Thus, an approximate value for the TNT equivalent of a detonation-type explosion can be obtained from chemical structure (or, in many cases, from [29, 31]) for assessing blast effects. Similarly, the TNT equivalent for explosion of a mixture of a combustible material in air (vapor-cloud explosion) can be obtained from the heat of combustion (when multiplied by an explosion-efficiency factor, which may be of the order of 10%). 

It is important to apply conservative values to the proportionality constants used for the TNT method. An explosion efficiency of 0.06 to 0.10 should be used even in areas that are not tightly confined. Scaling factors should be averaged among several literature sources and used to calculate overpressure profiles. These data are often material-specific and, if not averaged, could introduce additional errors. 

The explosion efficiency depends on the method for determining the contributing mass of fuel. Models based on the total quantity released have lower efficiencies. Models based on the dispersed cloud mass have a higher efficiency. The original reference must be consulted for the details. 

The following methods for estimating the explosion efficiency are summarized by AIChE (1994) ... 

The application of an explosion efficiency represents one of the major problems with the TNT equivalency method. 

A logic diagram for the application of the TNT equivalency method is given in Figure 3.7. The main inputs are the mass and dimensions of the flammable cloud and an estimate of explosion efficiency. The main outputs are the peak side-on overpressure or damage levels with distance. 

The TNT equivalent model requires the specification of the explosion efficiency. The TNO multi-energy method requires the specification of the degree of confinement and the specification of a relative blast strength. 

All of the methods (except the TNT equivalency) require an estimate of the vapor concentration— this can be difficult to determine in a congested process area. The TNT equivalency model is easy to use. In the TNT approach a mass of fuel and a corresponding explosion efficiency must be selected. A weakness is the substantial physical difference between TNT detonations and VCE deflagrations. The TNO and Baker-Strehlow methods arc based on interpretations of actual VCE incidents—these models require additional data on the plant geometry to determine the confinement volume. The TNO method requires an estimate of the blast strength while the Baker-Strehlow method requires an estimate of the flame speed. 

The largest potential error with the TNT equivalency model is the choice of an explosion efficiency. One needs to ensute that the yield corresponds with the correct mass of fuel. An efficiency range of 1-10% affects predicted distances to... 

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