Instantaneous overpressure

The side-on overpressure at some distance x from an explosion center is shown in idealized form in Figure 40. The pressure rises quickly to a maximum value P and then decays for a period of time t, until the pressure equals the atmospheric pressure Pq subsequently, the pressure drops below atmospheric pressure and then rises again to Pq. Brode has expressed the instantaneous overpressure as... 

Guirao and Bach (1979) used the flux-corrected transport method (a finite-difference method) to calculate blast from fuel-air explosions (see also Chapter 4). Three of their calculations were of a volumetric explosion, that is, an explosion in which the unbumed fuel-air mixture is instantaneously transformed into combustion gases. By this route, they obtained spheres whose pressure ratios (identical with temperature ratios) were 8.3 to 17.2, and whose ratios of specific heats were 1.136 to 1.26. Their calculations of shock overpressure compare well with those of Baker et al. (1975). In addition, they calculated the work done by the expanding contact surface between combustion products and their surroundings. They found that only 27% to 37% of the combustion energy was translated into work. 

STEAM EXPLOSION Overpressure associated with the rapid expansion in volume on instantaneous conversion of water to steam. 

The blast load is modeled as a triangular-shaped overpressure time curve. The blast overpressure rises instantaneously to the peak overpressure, B, then decays linearly with a blast pressure duration, T. The pressure is uniformly distributed over the surface of the plate and is applied perpendicular to the pane. 

Internal Detonations or Explosions An internal detonation or explosion may occur due to several scenarios. Air leakage into the system may cause a combustible mixture to form, undesired chemical reactions may occur, and extremely rapid vapor expansion may occur. These almost instantaneous events have to be carefully protected against as many overpressure devices do not react quickly enough to prevent the vessel from rupturing. 

Another way of expressing QD values is to state them as the cube root of the expl wt because certain detonation phenomena scale according to a cube root law. One of these is the instantaneous peak overpressure with distance (Ref 11). Damage can be related to overpressure by the cube root law except with respect to damage within inhabited structures and with respect to flying debris, for both of which a square root law is more nearly correct. 

Figure 9 is plotted together with the instantaneous phase saturation pressures. Figure 11 shows that the evolution of saturation pressures of the instantaneous phase closely follows the reservoir pressure trend until, in the past 5 Ma, reservoir pressures increase sharply. Undersaturation of the highly compartmentalized Snorre reservoir can, in this case, be stated to be a relatively recent event attributable to the latest subsidence and overpressuring event. 

Most pressure vessels require rupture disks for which designs are specified according to ASME or other international standards codes. They may protect from either overpressure or vacuum conditions. It is composed of a membrane that will instantaneously fail (within milUseconds) at a specified differenfial pressure. Figure 4.18 shows a horizontal vessel that failed because it was poorly vented or due to a failure in the rupture disk. 

Typically applied where dust explosion potentials exist in enclosures from potential overpressures generated from dust or vapor. Spark detectors (i.e., infrared detectors) or pressure detectors are typically used that instantaneously initiate a suppression system to inhibit or extinguish the spark and prevent an explosion. See also Explosion Isolation. 

The mechanism of vessel failure appears to be a two-step process The formation of an initiating overpressure crack in the high-temperature, vapor-wetted walls of the vessel, followed by the final catastrophic unzipping of the containment and a nearly instantaneous release of its contents. The distribution and hashing of the lading causes a fireball if the contents are flammable. The failure of the vessel and the surface emissive power of the BLEVE fireball do not appear to be directly related to the superheat of the contents at failure and indeed may be most severe for conditions when the vessel fails while undergoing a pressure reduction at low superheat. 

When such reflection occurs, an object precisely at the surface will experience a pressure increase, since the reflected wave is formed instantaneously the value of the overpressure thus experienced at the surface is generally considered to be entirely a reflected pressure. In the region near ground zero, this total reflected overpressure will be more than twice the value of the peak overpressure of incident blast wave. 

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