Explosives overpressure

Capable of withstanding an explosion overpressure reduced by explosion suppression or explosion venting... 

At such low stresses, there would be no permanent deformation of an ASME code vessel subjected to an explosion overpressure. 

Containment (Explosion-Pressure-Resistant Design for Maximum Explosion Overpressure) An explosion-resistant construction is understood to mean the possibihty of designing vessels and equipment for the full maximum explosion ove (pressure, which is generally of the order P = 9 bar. The explosion-resistant vessel can then be designed as explosion pressure resistant or explosion pressure shock resistant. This protective measure is generally employed when small vessel volumes need to be protected, such as small filter units, fluidized-bed dryers, cyclones, rotaiy valves, or mill housings. 

One has to consider that all connected devices must also withstand the maximum explosion overpressure. 

Bv this method, in general, the expecl ed inherent maximum explosion overpressure of the order P = 7 to 10 bar will be reduced to a value of Pred.max < 2 bai. In this case, the static activation overpressure of the venting device is < 0.1 bar. The resulting P,ed,max i i y not exceed the design pressure of the equipment. The explosion as such is not prevented only the dangerous consequences are limited. However, subsequent fires must be expecl ed. 

The inertia, the opening behavior of the movable cover of the explosion door, and its arrangement (horizontal, vertical) can affect the venting efficiency EF. This results in a higher maximum explosion overpressure Fred,max Iri he protected vessel (see Fig. 26-39). 

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). 

Depending upon the design criteria of the installed suppression system, an unsuppressed explosion overpressure of around 7 to 10 bar is reduced to a suppressed reduced explosion overpressure which lies in the range of Fred,max = 0-2 to 1 bar. Thus, vessels need to be explosion resistant for an overpressure of maximum 1 bar (ISO Standard 6184/4, Explosion Protection Systems Paii 4 Determination of Efficacy of Explosion Suppression Systems, Geneva, 1985). 

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). 

Containment of explosion overpressure, i.e. by designing plant eapable of withstanding in exeess of the maximum explosion overpressure, or safe venting of forees, e.g. via blow-off panels, doors, membranes. 

A rupture disc may also be used in some cases for proteetion against internal explosion overpressure. However, this is a matter of special design and the appropriate safety group should be consulted. 

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 ... 

Thermal radiation 5kW/m (severe bums to bare skin within 30 seconds) Explosion overpressure 0.1 batg (minor structural damage, injuiy from falling masonry, glass etc.)... 

Facilities can be ranked based on the sum of the maximum hazard distances for each release. Only one hazard distance should be used for each release, even if there is the potential for more than one hazard (thermal radiation, explosion overpressure, toxic cloud and flammable vapor cloud). The highest-ranked facility will be the one whose potential releases would reach the greatest total distance. 

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

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

Containment of explosion overpressure or safe venting of forces, e.g. via blow-off panels, doors. 

Each scenario was modeled to determine the explosion overpressure at the three buildings under review. The resultant overpressures, and corresponding vulnerability estimates, for the most severe scenario in each process area are tabulated below. 

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. 

Maximum reduced explosion overpressure PreThe maximum pressure generated by an explosion of a dust-air mixture in a vented or suppressed vessel under systematically varied dust concentrations. 

Containment Containment is understood to mean the possibility of designing vessels and equipment for the full maximum explosion overpressure, which is generally from = 7 to 10 bar. The explosion-resistant vessel can then be designed as explosion-pressure-resistant or... 

One has to consider that all connected devices must also withstand the maximum explosion overpressure. The NFPA 69 Standard, Explosion Prevention System, 1997 European Standard prEN 14460, Explosion Resistant Equipment, 2005 and Kirby and Siwek, Preventing Failures of Equipment Subject to Explosions, Chemical Engineering, June 23, 1986, provide excellent guidance on the practice of containment. 

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