Side-on overpressures

Venting an explosion ahead of a flame arrester can reduce the thermal flux and the impulse to which the arrester is subjected. Test results indicate that peak side-on overpressure is halved, specific impulse is reduced by a factor of three, and the temperature is substantially reduced. However, overpressure and flame speed at the flame arrester do not appear to be changed significantly. 

For describing structural loading functions needed for design analysis, the use of overdriven detonation data representing the net overpressure (run-up side less protected side overpressure) on the arrester element and supporting structure is preferable to data representing only the run-up side, side-on overpressure. However, the run-up side transient history of side-on overpressure for overdriven detonations should provide a conservative estimate for design purposes (see Chapter 6). 

In Figure 3.8a, a plane shock wave is moving toward a rigid structure. As the incident wave encounters the front wall, the portion striking the wall is reflected and builds up a local, reflected overpressure. For weak waves, the reflected overpressure is slightly greater than twice the incident (side-on) overpressure. As the incident (side-on) overpressure increases, the reflected pressure multiplier increases. See Appendix C, Eq. (C-1.4). 

In summary, an object s blast loading has two components. The first is a transient pressure distribution induced by the overpressure of the blast wave. This component of blast loading is determined primarily by reflection and lateral rarefaction of the reflected overpressure. The height and duration of reflected overpressure are determined by the peak side-on overpressure of the blast wave and the lateral dimensions of the object, respectively. The Blast loading of objects with substantial... 

Figure 4.18. Peak side-on overpressure due to a surface TNT explosion according to Marshall (1976). (TNT in kilograms.)...

Figure 4.19. Diameters of side-on overpressure circles for various explosive yields (1 ton 2000 lb) (based on free-air bursts).

Figure 4.20. Dimensionless blast side-on overpressure for vapor cloud explosions (Strehlow etal. 1979).

A safe and most conservative estimate of the strength of the sources of strong blast can be made if a maximum strength of 10 is assumed. However, a source strength of 7 seems to more accurately represent actual experience. Furthermore, for side-on overpressures below about 0.5 bar, no differences appear for source strengths ranging from 7 to 10. 

Once the energy quantities E and the initial blast strengths of the individual equivalent fuel-air charges are estimated, the Sachs-scaled blast side-on overpressure and positive-phase duration at some distance R from a blast source can be read from the blast charts in Figure 4.24 after calculation of the Sachs-scaled distance ... 

The real blast side-on overpressure and positive-phase duration can be calculated from the Sachs-scaled quantities ... 

TABLE 6.10. Conditions of Failure of Side-on Overpressure-Sensitive Elements (Glasstone, 1957)... 

Structural Element Failure Approx. Side-on Overpressure (bar) (psi) ... 

To determine the nondimensional side-on overpressure P, read from Figure 6.21 or 6.22 for the appropriate R. Use the curve labelled high explosive if Figure 6.21 is used. 

To determine the nondimensional side-on overpressure P, read P from Figure 6.21 for the appropriate R (calculated in Step 3 of the basic method). Use the curve which goes through the starting point, or else draw a curve through the starting point parallel to the nearest curve. Continue with Step 6 of the basic method in Section 6.3.3.2. 

Once the energy quantities E and the initial blast strengths of the individual equivalent fuel-air charges are estimated, the Sachs-scaled blast side-on overpressure and... 

The BLEVE of the spheres probably shifted a number of cylindrical vessels from their foundations. Furthermore, it probably produced damage in the built-up area. However, because destruction by intense fire in that area was complete, this cannot be confirmed. Beyond a range of 300 m, no glass damage due to blast was observed. This indicates that the side-on overpressure at that range was well below 3 kPa (0.03 bar), which is a nondimensional pressure /, of 0.04. 

Figure B-1. Pressure impulse diagrams for damage to brick houses. Line 1 Threshold for light damage. Line 2 Threshold or moderate damage partial collapse of roof some bearing wall failures. Line 3 Threshold for severe damage 50 to 75 percent of bearing wall destruction. P, side-on overpressure. /, side-on impulse (Baker et al. 1983).

The ear is a very sensitive and complex organ that responds to very small variations in pressure. It was argued in Hirsch (1968) that ear drum rupture is decisive as to ear damage from blast waves. Figure C-3 shows the percentage of eardrum ruptures as a function of side-on overpressure F,. 

Figure C-4. Impact velocity and injury criteria as a function of side-on overpressure and impulse (Bowen et al. 1968).

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... 

The gauge records eimbient pressure Pq. At arrival time ta, the pressure rises quite abruptly (discontinuously, in an ideal wave) to a peak value Pj + Pq. The pressure then decays to ambient in total time tg + T+, drops to a partial vacuum of amplitude Pj, and eventually returns to Po in total time tg + T+ + T. The quantity P is usually termed the peak side-on overpressure, or merely the peak overpressure. The portion of the time history above initial ambient pressure is called the positive phase, of duration T+. That portion below Po, of amplitude Ps and duration T, is called the negative phase. 

A control building is located 125 ft (38 m) from a facility that handles highly reactive materials having the potential for explosion. The building can withstand a 12 psi (0.83 bar) side-on overpressure, 20 ms blast load. 

The company standards classify this facility as "high explosion potential," requiring control buildings that are located at this distance [125 ft (38 m)] from a potential explosion be designed to a minimum load of 10 psi (0.69 bar) side-on overpressure and 20 ms blast load. It was determined that this... 

Peak Side-on Overpressure, psi (bar) Consequences to Buildings Possible Building Occupant Injury Consequences... 

In the course of evaluating the risk to a nearby control room, blast parameters were calculated using the Multienergy method. At 300 ft (90 m), the peak side-on overpressure was determined to be 1.5 psi (0.10 bar). By the estimates shown in Table 3.5 and Table 4.8, at 1.5 psi (0.10 bar) sheet metal can be ripped off and internal walls can be damaged. It was felt that, at this level, the building could sustain sufficient damage to cause serious injury to the occupants, and further study evaluation should be performed. 

A small engineering building is located 350 ft (107 m) from the process unit discussed in Example 8. It has an occupancy load of 500 person-hours, which exceeds the company s occupancy criteria. The building is constructed of unreinforced concrete and contains several windows. Earlier calculations estimated the incident side-on overpressure to be 0.5 psi at 350 ft (0.069 bar at 105 m). 

By the estimates shown in Table 4.8, the calculated incident side-on overpressure of 0.5 psi (0.069 bar) should not cause significant damage to the building. However, glass breakage could occur. It was determined that the windows should be eliminated or strengthened. 

Peak side-on overpressure during the positive phase of the blast wave... 

Side-on overpressure The pressure experienced by an object as a blast wave passes by it. 

The total inventory of flammable material that could be released was determined, and the TNT equivalence method (from Reference 5) was applied. Using this information, an incident side-on overpressure of 3 psi at 150 ft (0.21 bar at 45 m) was calculated. On this basis, it was determined that the building could sustain the maximum anticipated blast overpressure, and no further evaluation was needed. 

Chapter 3 offers guidance on evaluating the characteristics (e.g., peak side-on overpressure, duration) of potential explosion or fire phenomena. Additional information is briefly summarized in Appendix A of this document. Information on performing more detailed evaluations of the consequences of these phenomena on buildings, structures, contents, and occupants is presented below. 

Process Unit Building No. Side-on Overpressure (psi) Incident Frequency Building Type Occupant Vulner- ability Fractional Time of Attendance Individual 1 Risk I... 

Figure 6-23 Correlation between scaled distance and explosion peak side-on overpressure for a TNT explosion occurring on a flat surface. Source G. F. Kinney and K. J. Graham, Explosive Shocks in Air (Berlin Springer-Verlag, 1985).

From Figure 6-23 the scaled overpressure is 0.055. Thus, if the ambient pressure is 1 atm, then the resulting side-on overpressure is estimated at (0.055)(101.3 kPa) = 5.6 kPa (0.81 psi). From Table 6-9 this overpressure will cause minor damage to house structures. 

---