Unconfined blasts

Unconfined Blasts. See in Vol 2, B180-R to B182-R under Blast Effects in Air ... 

An explosion on board an aircraft is clearly a confined explosion, with the effect that the energy and the pressure involved are considerably higher than those for an unconfined blast. Hence, the explosion characteristics cannot be determined using simplified empirical forms such as those given in Eqns (13.2) and (13.3). Various explosion scenarios (including analysis of, for example, different charge locations... 

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. 

The insensitivity of nitromethane to detonation by shock under normal conditions of handling has been demonstrated by a number of fljH-scale tests. Sensitivity to shock increases with temperature at 60° C, nitromethane can be detonated by a No. 8 blasting cap. Nitroethane can be initiated only when heated near its boiling point under heavy confinement neither it or the nitropropanes can be detonated in unconfined conditions. 

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%. 

The blast resulting from the remaining unconfined and unobstructed parts of a cloud can be modeled by assuming a low initial strength. For extended and quiescent parts, assume minimum strength of 1. For more nonquiescent parts, which are in low-intensity turbulent motion, for instance, beeause of the momentum of a fuel release, assume a strength of 3. 

Munday, G., and L. Cave. 1975. Evaluation of blast wave damage from very large unconfined vapor cloud explosions. International Atomic Energy Agency, Vienna. 

Pickles, J. H., and S. H. Bittleston. 1983. Unconfined vapor cloud explosions—The asymmetrical blast from an elongated explosion. Combustion and Flame. 51 45-53. 

Zeeuwen, J. P., C. J. M. Van Wingerden, andR. M. Dauwe. 1983. Experimental investigation into the blast effect produced by unconfined vapor cloud explosions. 4ih Int. Symp. Loss Prevention and Safety Promotion in the Process Industries. Harrogate. UK, IChemE Symp. Series 80 D20-D29. 

Experimental data (Section 4.1) may be used to estimate a blast s initial strength. These data indicate that deflagrative gas explosions may develop overpressures ranging from a few millibars under completely unconfined or unobstructed conditions to greater than 10 bars under severely confined and obstructed conditions. 

The multienergy method applies only if detonation of unconfined parts of a vapor cloud can be ruled out. If so, the explosive potential of a vapor cloud is determined primarily by the blast-generative properties of the environment in which the vapor is released and disperses. Consequently, a vapor cloud explosion can be regarded as a number of subexplosions. Therefore, the first step in applying the multienergy method in vapor cloud explosion hazard assessment is... 

Consequently, if none of these conditions is present, no blast effects are to be expected. That is, under fully unconfined and unobstructed conditions, the cloud bums as a flash fire, and the major hazard encountered is heat effect from thermal radiation. 

Equally severe in practice is the requirement of the break 2 test. To pass this test the explosive must have a low power when fired in an unconfined condition. The actual power has not been quantitatively measured, but is probably in the region of 15% blasting gelatine. To achieve such a result, it is necessary deliberately to design the explosive in such a way that only partial reaction occurs in the unstemmed condition. Such partial reaction can be achieved by either of two ways. 

An explosion occurring in a confined vessel or structure can rupture the vessel or structure, resulting in the projection of debris over a wide area. This debris, or missiles, can cause appreciable injury to people and damage to structures and process equipment. Unconfined explosions also create missiles by blast wave impact and subsequent translation of structures. 

Unconfined vapor cloud explosion explosive oxidation of a flammable vapor cloud in a nonconfined space (e.g., not in vessels or buildings) the flame speed may accelerate to high velocities and can produce significant blast overpressures, particularly in densely packed plant areas. 

F. Blast effects from a nearby explosion (unconfined vapor cloud explosion, bursting vessel, etc.), such as blast overpressure, projectiles, structural damage... 

Blast, Spherical. When a spherical explosive charge is detonated in air, underground, or underwater it produces waves which are spherical in shape. Such waves can also emerge from the end of an unconfined cylindrical charge, as described by Cook(1958), pp 100-01 (See also Fig on p B183 of Vol 2)... 

Cook (Ref) reported that only the S-wave and the induced ground wave were apparent in seismic disturbances from surface explns at Tooele, Utah and for unconfined or dirt-covered shots the induced ground wave had an index 5-30 times greater than the S-wave. Synchronous ground-wave induction apparently requires simply matching of the frequency of the air blast with the natural frequency of the ground in the region of the blast. 

Blasting agents are powerful explosive agents that cannot be detonated by means of a blasting cap when unconfined and are, therefore, very safe to handle. A powerful booster is needed to start detonation. Blasting agents are usually ammonium nitrate (melting point 169.6°C, density 1.725) mixtures sensitized with nonexplosive fuels such as oil or wax. 

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