Abstract
International standards for dust explosion relief design (NFPA 68 2013, EN 14491 2012, VDI 3673) have largely been developed for conventional bulk material handling silos with volumes of less than 10,000 m3, with empirical calculations being used to determine the total suitable vent areas required to achieve explosionprotection in the event of an internal incipient explosion. However, these standards generally do not advise on the effects of the distribution of this total area across the vessel. Some standards state that the distribution of the relief vent area is unimportant as long as the total required vent area is achieved. For larger vessels, suchas large biomass storage silos with volumes up to 100,000 m3, Computational Fluid Dynamics (CFD) codesare often used to undertake explosion simulations in order to determine the effects of likely worst-case explosions and subsequently determine a suitable area of explosion relief venting. The effects of the distribution of these vent areas is again not often considered in the design of such vessels.
This paper presents the results of research undertaken by Gexcon using the FLACS-DustEx CFD code, showing the effects of various relief vent arrangements for a given vessel and given total relief vent area. The initial results suggest that flames emitted through relief vents during an explosion can interact with one another outside of the vessel, increasing turbulence and reducing the efficiency of the venting process. This subsequently results in an increase in the reduced explosion pressure (Pred) within the vessel during an explosion. The level of interaction, and hence the magnitude of the change in the reduced overpressure, has been found to vary with the manner in which the total vent area is distributed across the vessel. The results of this initial research are discussed, along with preliminary conclusions and recommendations. Other aspects are also discussed, such as statements in the standards that may be misleading and the effects of structures located in-between large areas of relief vents on a vessel.
