Overpressure explosion effects

A principal parameter characterizing an explosion is the overpressure. Explosion effect modeling generally is based on TNT explosions to calculate the overpressure as a function of distance. Although the effect of a TNT explosion differs from that of a physical or a chemical explosion (particularly in the near-field), the TNT model is the most popular because a large data base exists for... 

The hazard posed can be limited by maintaining a zone free of people and property around a storage area of explosive material. The minimum radius of the zone depends on the type and quantity of explosive, the extent and type of barrica ding, and the magnitude of loss that would be encountered if an explosive incident occurred. The maximum distance to which hazardous explosive effects propagate depends on the blast overpressure created, which as a first approximation is a function of the cube root of the explosive weight, W. This is termed the quantity distance and is defined as... 

Explosion consequences in terms of overpressure and other effects may be evaluated by appropriate methods such as those described in Reference 5 and Appendix A. In evaluating the consequences of potential explosions, all these methods account for the energy of the explosion, the location of the explosion source, and attenuation of explosion effects with distance from the explosion source. From such an evaluation, maximum blast parameters can be determined at all locations of interest. Evaluation results can be graphically expressed by plotting contours of equal blast overpressure on a site plan of the facility, as shown in Figure 4.4. 

Whenever the operation to be performed involves the potential to cause the initiation of the propellant, explosive or pyrotechnic (PEP) component(s) of a munition item, the APE is either operated by remote control, with the operator behind a protection wall or barrier, or it is enclosed in a protective barricade or operational shield. Barricades or operational shields are designed to protect personnel and assets from the effects of blast overpressures, thermal effects or fireball, and fragments result from the initiation of PEP components, such as the fuze, primer, propelling charge, burster, etc. 

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. 

The calculation of explosion effects is a complex topic involving many variables. Table 14.4 shows some overpressure values with typical effects. 

An important parameter for describing explosion effects is the peak side-on overpressure, ps. It can be represented by the following relationship for ps (in kPa), which stems from [57] and was scaled here to produce agreement with the Marshall curve (cf. [15])... 

Explosion Effects. Explosion overpressure effects that are of interest here result either from the rapid combustion of a fuel-air mixture (confined explosion or VCE), or a sudden release of pressure energy (BLEVE). 

Explosion effect models predict the impact of blast overpressure and projectiles on people and objects. 

The basis for explosion effect estimation is experimental data from TNT explosions. These data are for detonations and there may be differences with respect to longer duration deflagration overpressures. 

For explosion effects, some risk analysts assume that structures exposed to a 3 psi peak side-on overpressure, or higher, will suffer major damage, and assume 50% fatalities within this range (corresponding to a probit value of 5). 

Protection against explosions is typically provided by explosion-venting, using panels or membranes which vent an incipient explosion before it can develop dangerous pressures (11,60). Protection from explosions can be provided by isolation, either by distance or barricades. Because of the destmctive effects of explosions, improvement in explosion-prevention instmmentation, control systems, or overpressure protection should receive high priority. 

The Effects of Explosions in the Process Industries, Eoss Prevention bulletin 068, Overpressure Working Party, Major Ha2ards Assessment Panel, Institution of Chemical Engineers, Apr., 1986. 

Explosion-Pressure-Resistant Design for Reduced Maximum Explosion Overpressure with Explosion Suppression Explosion suppression systems provide one means to prevent the buildup of an inadmissibly high pressure, which is the consequence of explosions of combustible material in vessels. They operate by effectively extinguishing explosion flames in the initial stage of the explosion. An explosion of combustible material can generally be regarded as successfully suppressed when the maximum explosion overpressure can be lowered to a reduced explosion overpressure of not more than 1 bar (see Fig. 26-40). 

Dynamic explosion detectors use a piezoresistive pressure sensor installed behind the large-area, gas-tight, welded membrane. To ensure optimum pressure transference from the membrane to the active sensor element, the space between the membrane and the sensor is filled with a special, highly elastic oil. The construc tion is such that the dynamic explosion detec tor can withstand overpressures of 10 bar without any damage or effect on its setup characteristic. The operational range is adjustable between 0 and 5 bar abs. Dynamic explo-... 

Volume of vessel (free volume V) Shape of vessel (area and aspect ratio) Type of dust cloud distribution (ISO method/pneumatic-loading method) Dust explosihility characteristics Maximum explosion overpressure P ax Maximum explosion constant K ax Minimum ignition temperature MIT Type of explosion suppressant and its suppression efficiency Type of HRD suppressors number and free volume of HRD suppressors and the outlet diameter and valve opening time Suppressant charge and propelling agent pressure Fittings elbow and/or stub pipe and type of nozzle Type of explosion detector(s) dynamic or threshold pressure, UV or IR radiation, effective system activation overpressure Hardware deployment location of HRD suppressor(s) on vessel... 

For any proposed suppression system design, it is necessary to ascribe with confidence an effective worst-case suppressed maximum explosion overpressure Pred.max- Provided that the suppressed explosion overpressure is less than the process equipment pressure shoclc resistance and provided further that this projected suppression is achieved with a sufficient margin of safety, explosion protection security is assured. These two criteria are mutually independent, but both must be satisfied if a suppression system is to be deployed to provide industrial explosion protection. 

Explosion isolation can also be effected by rapid action barrier valves. At present, they can be arranged only in horizontal pipehnes and are suitable, in general, only for streams with a small amount of dust. Such valves are thus frequently used to protect ventilation lines. As a certain explosion overpressure is necessaiy to close such valves, a distinction is made between self-actuated and externally actuated barrier valves (Fig. 26-46). 

By avoiding knock-on effects for example, if storage tanks have weak seam roofs, an explosion or overpressuring may blow the roof off, but the contents will not be spilled (see Section 5.2). 

Detonation flame arresters and associated piping mnst be able to withstand the effects of explosion transients that inclnde the pressnre or pres-snre-related force, the specific impulse (integral of the dme versus overpressure) or net impulse (upstream minns downstream impulse) highspeed gas momentnm transfer and flnx, and temperatnre-, heat-, and dier-mal-flnx related loads (White and Oswald 1992). Wliite and Oswald con-... 

Smaller explosions occurred until about 6 00 a.m. There was no evidence of strong overpressure effects, although a television news broadcast showed broken windows at 3.5 km (2 miles) from the plant. 

Flame acceleration does not generate extremely high overpressures. That is, numerical simulation of an explosion process with a steady flame speed equal to the highest flame speed observed results in a conservative estimate of its blast effects. 

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. 

Table 6.10 presents some damage effects. It may give the impression that damage is related only to a blast wave s peak overpressure, but this is not the case. For certain types of structures, impulse and dynamic pressure (wind force), rather than overpressure, determine the extent of damage. Table 6.10 was prepared for blast waves of nuclear explosions, and generally provides conservative predictions for other types of explosions. More information on the damage caused by blast waves can be found in Appendix B. 

This appendix is a summary of the woiit published in the so-called Green Book (1989). Possible effects of explosions on humans include blast-wave overpressure effects, explosion-wind effects, impact from fragments and debris, collapse of buildings, and heat-radiation effects. Heat-radiation effects ate not treated here see Chapter 6, Figure 6.10 and Table 6.6. 

Fire and explosion models describe the magnitude and physical effects (heat radiation, explosion overpressure) resulting from a fire or e.xplosion. 

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