TNT equivalence

M. M. Swisdak, Jr., "Maximum TNT Equivalence of Naval Propellants," V2, in 21 st Explosives Safety Symposium, U.S. Dept, of Defense Explosive Safety Board, Alexandria, Va., 1984. 

An explosion model is used to predict the overpressure resulting from the explosion of a given mass of material. The overpressure is the pressure wave emanating from a explosion. The pressure wave creates most of the damage. The overpressure is calculated using a TNT equivalency technique. The result is dependent on the mass of material and the distance away from the explosion. Suitable correlations are available (2). A detailed discussion of source and consequence models may be found in References 2, 8, and 9. 

Example The combustion process in large vapor clouds is not known completely and studies are in progress to improve understanding of this important subject. Special study is usually needed to assess the hazard of a large vapor release or to investigate a UVCE. The TNT equivalent method is used in this example other methods have been proposed. Whatever the method used for dispersion and pressure development, a check should be made to determine if any govern-mentaf unit requires a specific type of analysis. 

Tests conducted by the Eastman Kodak Company have shown that tert-butyl azidoformate [Formic acid, azido, -butyl ester], also known as iert-butoxy carbonyl azide and 1-BOC azide, is a thermally unstable, shock-sensitive compound (TNT equivalence 45%). 

It is convenient to calculate a TNT equivalent of a physical explosion to use the military results of Figures 9.1-4 and 5. Baker et al. (1983) give a recipe for the rupture of a gas filled container assuming expansion occurs isothermally and the perfect gas laws apply (equation 9.1-25), where W is... 

The TNT equivalence of the blast was estimated to be 20-60 tons (Davenport, 1983). The area of total destruction was 430,000 ft (40,000 m ) and the area of total destruction plus severe damage was 3,200,000 ft (300,000 m ) (Figures 2.7-2.9). The main cause of the explosion was the turbulence generated by the release itself. The release did, however, occur in a very congested area. 

Probably no more than 500 kg of liquid methane was involved. This would have formed a cloud 1 m deep (3 ft) and 40 m (130 ft) in radius (assuming a stoichiometric mixture). TNT equivalency was estimated to be 1000-2000 kg, which implies that the yield was 18-36%. 

The literature is inconsistent on definitions. TNT equivalency is also called equivalency factor, yield factor, efficiency, or efficiency factor. 

In a numerical exercise described in section 4.2.2, it was shown that, for a stoichiometric, hydrocarbon-air detonation, the theoretical maximum efficiency of conversion of heat of combustion into blast is equal to approximately 40%. If the blast energy of TNT is equal to the energy brought into the air as blast by a TNT detonation, a TNT equivalency of approximately 40% would be the theoretical upper limit for a gas explosion process under atmospheric conditions. However, the initial stages in the process of shock propagation in the immediate vicinity of... 

Furthermore, accidental vapor cloud explosions are anything but detonations of the full amount of available fuel. Therefore, practical values for TNT equivalencies of vapor cloud explosions are much lower than the theoretical upper limit. Reported values for TNT equivalency, deduced from the damage observed in many vapor cloud explosion incidents, range from a fraction of one percent up to some tens of percent (Gugan 1978 and Pritchard 1989). For most major vapor cloud explosion incidents, however, TNT equivalencies have been deduced to range from 1% to 10%, based on the heat of combustion of the full quantity of fuel released. Apparently, only a small part of the total available combustion energy is generally involved in actual explosive combustion. 

Methods for vapor cloud explosion blast prediction based on TNT equivalency are widely used. Over the years, many authors, companies, and authorities have developed their own procedures and recommendations with respect to issues surrounding such predictions. Some of the differences in these procedures include the following ... 

The value of TNT equivalency A value based on an average deduced from observations in major incidents or a safe and conservative value (whether or not dependent on the presence of partial confinement/obstruction and nature of the fuel). 

Brasie and Simpson (1968) use the Kingery and Pannill (1964) TNT blast data to represent blast parameter distributions, and the US Atomic Energy Commission s recommendations (Glasstone 1962) for the attendant structural damage. Brasie and Simpson (1968) base their recommendation for the TNT equivalency of vapor clouds on the damage observed in three chemical-plant explosion incidents. Analyzing the... 

Although values for TNT equivalency ranging from 0.3% to 4% have been observed, Brasie and Simpson recommend, for predictive purposes, conservative values for TNT equivalency as follows 2% for near-field, and 5% for far-field effects (based on energy), applied to the full quantity of fuel released. 

TNT equivalency should be applied to the quantity of fuel calculated with the above equations. For planning purposes, Brasie (1976) recommends the use of TNT equivalencies of 2%, 5%, and 10% (based on energy) in calculations to determine the sensitivity of geometry to the yield. 

In addition, Eichler and Napadensky derived TNT equivalencies from the damage observed in some major vapor cloud explosion incidents of the 1970s ... 

The Flixborough explosion was analyzed on the basis of damage figures presented by Munday and Cave (1975). Assuming a 60,000 kg cyclohexane release, they found a TNT equivalency of 7.8% on the basis of energy, which corresponds with a mass equivalency of 81.7%. These equivalences were calculated on the basis of the full quantity of material released. 

For the Port Hudson vapor cloud explosion, they found TNT equivalencies of 8.7% and 96%, based on energy and mass basis, respectively. These equivalencies were calculated from damage data presented by Burgess and Zabetakis (1973), and are based on the full quantity of fuel (31,750 gallons, 70,000 kg) of propane released. 

Although the blast effects of the East St. Louis tank-car accident (NTSB 1973) were found to be highly asymmetric, average TNT equivalencies of 10% on an energy basis and 109% on a mass basis were found. These equivalencies were calculated based on the assumption of a full tank-car inventory (55,000 kg) of a mixture of propylene and propane. 

Another tank car was punctured at Decatur (NTSB report 1975). TNT equivalencies of 4.3-10.2% and 47-111% were calculated on energy and mass bases, respectively. These equivalencies were calculated based upon a full tank car inventory (152,375 lb, 68,000 kg) of isobutane. 

Taking into account the possibility of highly directional blast effects, Eichler and Napadensky (1977) recommend the use of a safe and conservative value for TNT equivalency, namely, between 20% and 40%, for the determination of safe standoff distances between transportation routes and nuclear power plants. This value is based on energy it should be applied to the total amount of hydrocarbon in the largest single, pressurized storage tank being transported. 

Although it recognized that much higher values have been occasionally observed in vapor cloud explosion incidents, the U.K. Health Safety Executive (HSE) states that surveys by Brasie and Simpson (1968), Davenport (1977, 1983), and Kletz (1977) show that most major vapor cloud explosions have developed between 1% and 3% of available energy. It therefore recommends that a value of 3% of TNT equivalency be used for predictive purposes, calculated from the theoretical combustion energy present in the cloud. 

Exxon recognizes that blast effects by vapor cloud explosions are influenced by the presence of partial conflnement and/or obstruction in the cloud. Therefore, in order to determine an equivalent TNT yield for vapor clouds, Exxon recommends use of the following values for TNT equivalency on an energy basis ... 

Industrial Risk Insurers (1990) states that the TNT equivalency of actual chemical plant vapor cloud explosions is in the range of 1% to 5%. A value of 2% based on... 

The equivalent charge weight of TNT is calculated on the basis of the entire cloud content. FMRC recommends that a material-dependent yield factor be applied. Three types of material are distinguished Class I (relatively nonreactive materials such as propane, butane, and ordinary flammable liquids) Class II (moderately reactive materials such as ethylene, diethyl ether, and acrolein) and Class III (highly reactive materials such as acetylene). These classes were developed based on the work of Lewis (1980). Energy-based TNT equivalencies assigned to these classes are as follows ... 

One of the complicating factors in the use of a TNT-blast model for vapor cloud explosion blast modeling is the effect of distance on the TNT equivalency observed in actual incidents. Properly speaking, TNT blast characteristics do not correspond with gas explosion blast. That is, far-field gas explosion blast effects must be represented by much heavier TNT charges than intermediate distances. 

To express the maximum potential explosive power of a fuel, a safe and conservative value for TNT equivalencies of vapor cloud explosions was estimated from literature data on major incidents, after correction for virtual distance. Prugh (1987) concluded that the maximum energy-based TNT equivalency is highly depen-... 

These TNT equivalencies should be used in combination with high-explosive blast data by Baker (1973). Instead of graphical representation, Prugh (1987) recommends the use of simple equations which relate basic blast parameters to distance from the explosion center. These expressions can be readily implemented in a spreadsheet on a personal computer. 

On the basis of an extended experimental program described in Section 4.1.3, Harris and Wickens (1989) concluded that overpressure effects produced by vapor cloud explosions are largely determined by the combustion which develops only in the congested/obstructed areas in the cloud. For natural gas, these conclusions were used to develop an improved TNT-equivalency method for the prediction of vapor cloud explosion blast. This approach is no longer based on the entire mass of flammable material released, but on the mass of material that can be contained in stoichiometric proportions in any severely congested region of the cloud. 

Vapor cloud explosion blast models presented so far have not addressed a major feature of gas explosions, namely, variability in blast strength. Furthermore, TNT blast characteristics do not correspond well to those of gas-explosion blasts, as evidenced by the influence of distance on TNT equivalency observed in vapor cloud explosion blasts. 

A comprehensive collection of estimates of TNT equivalencies was deduced from damage patterns observed in major accidental vapor cloud explosions (Gugan 1978). From these estimates, it can be concluded that there is little, if any, correlation between the quantity of combustion energy involved in a vapor cloud explosion... 

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