Blast propagation study
As a result of a vented gas explosion or an unconfined explosion, one or more external overpressure regions may be generated. Pressure waves are emitted in all possible directions from these regions. If a flame is strongly accelerated and decelerated several times (e.g. due to congested regions with sufficient spacing), there may be more than one peak of the blast. A vented explosion may also give more than one pressure peak in the blast wave, due to opening of relief panels, external explosion and strong venting from the vessel due to significant internal combustion after the external explosion. The presence of several vent openings (see the figure above) may also give rise to separate pressure waves that may interfere.
Blast waves propagate with the speed of sound (~340 m/s). The pressure decay will vary with confinement: in a tunnel-type (1-D) geometry there will be almost no pressure decay. In a more open (3-D) situation, the decay of pressure will be inversely proportional to the radius (1/R), in the near field it may be higher due to energy loss. Pressures at remote buildings will be higher than the free field blast pressure, due to reflection and focusing effects.
Popular methods for prediction of far field blast propagation include TNT-equivalence methods, the Multi-energy method, Baker-Strehlow and several more. These require an assumption of the energy available, as well as the explosion pressure, and based on this, overpressures in the far field are generated. In general poor estimates of internal explosion strength is available, and an idealized unconfined hemispherical source for the blast waves is assumed. Factors like ignition location and geometry are not considered. These methods yield a quick answer, however, the degree of conservatism - if any - depends on the assumptions made and is difficult to assess.
As an example, the Multi-energy method has been applied to large-scale explosion tests. Even when the total amount of gas used in the experiments was used as input to the calculations, the predicted overpressures were significantly lower than measured.
CFD-methods like FLACS are preferred due to their ability to generate a good representation of the initial explosion as well as accounting for spatial effects like shielding, focusing, reflection and pressure wave interference due to multiple openings. As an example, FLACS can be used to design protective shielding against explosion loads.
Contact
Lars Rogstadkjernet
Phone no: +47 55 57 43 22
Email: larsr(at)gexcon.com
