(43d) The Design of Venting of Gas Explosions in the Presence of a Discharge Duct

Venting devices are the main solution for the mitigation of accidental explosion. However, relief ducts are mandatory for the discharge of combustion products to safe location. On the other hand, the presence of a duct is likely to increase the severity of the explosion with respect to simply vented vessels. To this regard, several mechanisms have been proposed to account for the enhanced violence of the explosion in such configuration: secondary explosion in the duct (burn-up), frictional drag and inertia of the gas column in the duct, acoustic and Helmholtz oscillations. The relative effects of the cited phenomena and their weight on maximum pressure reached during ducted vented explosion is uncertain. As a consequence, appropriate design of duct-venting configuration is still a matter of debate and it's not surprising that the available guidelines for the design of ducted vents for gas explosions as those proposed by Bartknecht (1981), also reported in NFPA 68 (2002), can lead to gross errors [Russo & Di Benedetto, 2006]. The authors have recently proposed experimental and complex numerical analyses (Computational Fluid Dynamics) [Ferrara et al., 2006] for the analysis of duct-vented phenomena on the peak pressure measured in either small, medium or large scale equipment. Results of these studies highlighted that the two main phenomena responsible for the increased peak pressure were: i) the explosion occurring in the initial sections of the duct leading to a backflow from the duct towards the main vessel, thus restricting the effective vent section; ii) the turbulisation of flame within the combustion chamber, due to the backflow, which enhances the combustion rate and the rate of pressure rise. On the basis of previous results, in this paper we proposed engineering correlations based on semi-empirical engineering methodologies which are able to reproduce the behaviour of explosion in combustion chamber taking into account the presence of a duct fitted on the vent panel. The engineering correlations are also intended to develop empirical design guidelines conservative with respect to those given by NFPA 68 (2002).

References

Bartknecht W., Explosions, Course Prevention Protection, Springer-Verlag, Berlin, 1981

Ferrara G., Russo G., Di Benedetto A., Salzano E.,2nd International Conference on Safety & Environment in Process Industry, CISAP2, Napoli, 2006.

NFPA 68 (2002), Guide for venting of deflagration, National Fire Protection Association, Quincy, 2002.

Ponizy B., Leyer J.C., Comb. Flame, 116, 1999, 259.

Russo P., Di Benedetto A., Process Safety and Environmental Protection, 84(B6), 2006, 1.