Explosion calculations Overpressure

Explosion calculations, 499-504 Estimating destruction, 501 Overpressure, 502 Pressure piling, 501, 504 Relief sizing, 505 Scaled distance, 502, 503 Schock from velocity, 503 TNT equivalent, 499-504 Explosion characteristics of dusts, 515 Explosion suppression, 518 Explosion venting, gases/vapors, 504 Bleves, 504 Explosions, 482 Blast pressure. 496 Combustion, 482 Confined, 482 Damage, 498-501 Deflagration, 482 Detonation, 483... 

Planas-Cuchi, E., Salla, J., and Casal, J. (2004) Calculating overpressure from BLEVE explosions. Journal of Loss Prevention in the Process Industries 17, 431-436. 

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. 

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. 

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. 

Blast overpressures calculated by the TNT-equivalency method are in reasonable agreement with the overpressures deduced from observed damage (Sadee et al. 1976/1977). This is to be expected, because the Flixborough incident is one of the major vapor cloud explosion incidents on which the TNT-equivalency value of... 

In this section, three examples of blast calculations of BLEVEs and pressure vessel bursts will be given. The first example is designed to illustrate the use of all three methods described in Section 6.3.2. The second is a continuation of sample problem 9.1.5, the BLEVE of a tank truck. A variation in the calculation method is presented instead of determination of the blast parameters at a given distance from the explosion, the distance is calculated at which a given overpressure is reached. The third example is a case study of a BLEVE in San Juan Ixhuatepec (Mexico City). 

Based on the pressure and impulse of the incident blast wave, the maximum velocity can be calculated of a human body during transportation by the explosion wind. Figure C-4 shows the impact velocity for the lethality criterion for whole body impact as a function of side-on overpressure and impulse... 

Different materials pose different hazards, including thermal radiation, explosion overpressure, and toxic and flammable vapor clouds. Some materials pose only one hazard, while others may pose all four. For the purposes of ranking facilities you will need to estimate the laigest area affected by the potential hazards. You can arrive at such an estimate by calculating the greatest downwind distance to a particular level of hazatd. The following thresholds are commonly applied ... 

Calculate the total burning time of tlie octane pool in Illustrative Exatiiple 3. Calculate tlie peak overpressure of a 50-pound TNT explosion at a distance 200 feet from the ignition point, if tlie peak oi erpressure at 1000 feel is 0.10 psi when 150 pounds of TNT is detonated. 

Rupture disks are used for the same purpose as safety valves and, in addition, serve to relieve internal explosions in many applications. If the pressure rise can be anticipated, then the volume change corresponding to this change can be calculated by simple gas laws, and the capacity of the disk at the relieving pressure is knowm. The system must be examined and the possible causes of overpressure and their respective relief capacities identified before a reliable size can be determined. See Figure 7-14. 

An overpressure after an explosion is noted as 0.5 psi. The calculated scaled distance Z is 75 ft/ (Ib) . Thus for a one pound charge, windows are broken at a distance of 75 feet. How far tvill windows be broken for a 500 lb. charge ... 

From the calculated building damage versus response relationship and the empirical probability of serious injury or fatality versus damage relationship discussed above, the relationship between explosion overpressure (or other effects) and probability of serious injury or fatality may be constructed in a manner that accounts for the detailed structural characteristics of plant buildings. The steps involved are similar to risk screening (Chapter 4), with the addition of detailed quantitative structural evaluation of plant buildings and detailed quantitative frequency assessment as described in the next section. 

The use of vent ducts results in an increase in Pred,max- The maximum reduced explosion overpressure P/red,max caused by the downstream vent duct can be calculated with Eqs. (23-13) and (23-14). 

Independent of the location of the vent duct, the maximum reduced explosion overpressure P) j, lla.( caused by the downstream vent duct can be calculated for vessels having an LID ratio of 1 with Eq. (23-19). 

It could be argued that vapor cloud explosions for hydrocarbon facilities need only be calculated for those facilities that contain large volumes of volatile hydrocarbon gases that can be accidentally released and where some degree of confinement or congestion exist. The most probable amount for an incident to occur is taken as 4,536 kgs (10,000 lbs ), however incidents have been recorded where only 907 kgs (2,000 lbs.) has been released. Additionally, an actual calculation of worst case releases to produce 0.2 bar (3 psio) at say 46 meters (150 ft.), indicates a minimum of 907 kgs (2,000 lbs.) of material is needed to cause that amount of overpressure. A limit of 907 kgs (2,000 lbs.) release of hydrocarbon vapor is considered a prudent and conservative approach. 

The objective in calculating explosion overpressure levels is to determine if a facility has the potential to experience the hazardous effects of an explosion and, if so, to mitigate the results of these explosions. The calculations can also serve to demonstrate where mitigating measures are not needed due to the lack of a potential to produce damaging overpressures either because low explosion effects or distance from the explosion for the facility under evaluation. 

Calculation of blast overpressure parameters There are three major methods in use today. One is the TNT Equivalency Methbd which gives inaccurate results for vapor cloud explosions. The other two methods are the Strehlow Curves from Baker 1983 and the Multi-Energy Method from TNO 1985. Both provide a family of curves based on flame speed or explosion strength. These curves are used to select dimensionless parameters which are then unsealed to determine the actual overpressures. 

Vapor cloud explosions (VCE) defined 3-2 overpressure calculation 3-... 

Historical data from industrial explosions are hard to accurately quantify as these can only be approximated by back calculating from observed deformations of structures. Blast overpressures from vapor cloud explosions are especially difficult to quantify because they tend to be directional, come from multiple sources, and vary with site conditions. Additionally, there is less information available than for high explosives. In one company s review of five recent vapor cloud explosion incidents, as measured at a range of 200 to 1,000 feet (60 to 300 meters), peak reflected pressures in the range from 2 psi (14 kPa) with a 35 ms duration to 12 psi (83 kPa) with a 33 ms duration have occurred. These pressures correspond to side-on overpressures ranging from 1 psi (7 kPa) to 5.5 psi (38 kPa). An extensive list of this type of explosion data is included in Lenoir 1993. 

PHAST (Process Hazard Analysis Software Tool)—This is a conglomerate package for gas dispersion and fire modeling. PHAST is capable of calculating the formation of a cloud or pool to final dispersion calculating concentrations, fire radiation, toxicity, and explosion overpressure endpoints. 

The explosion had all the characteristics of a detonation. A plot of overpressure destruction vs. distance statics fits exactly on known TNT detonation curves. Based on the stoichiometric combustion, my calculations indicate that the explosion had the force of 10 to 12 tons of TNT. 

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