Dust Explosions: Case Study of Dryer Explosions and Poor Explosion Vent Design

Four recent explosions occurred in a single burner, recirculating solids ring dryer. The dryer was equipped with a recirculating fan, a burner section, a section to introduce the solids, a ring duct, a pre-separator and four cyclones. Seven explosion doors were also used to protect the integrity of the dryer in the event a dust explosion, which close the vent area after the explosion. During each of the four events, at least four of the explosion doors opened and plant personnel observed flames emanating from the dryer. While the actual overpressures caused by the four dust explosions were successfully vented via the explosion doors and prevented failure of the dryer, significant damage was caused on three occasions by an ensuing vacuum in the dryer. The magnitude of the negative pressure caused was so great that it collapsed and permanently deformed: (1) the return duct to the recirculating fan on one occasion; (2) one of the four cyclones on another occasion; and (3) the remaining three cyclones that had yet been affected by the previous events. Significant production delays and down time were observed while the equipment was replaced.

Examination of the process data and interviews with plant operators revealed that each of the explosions was caused by its own unique and different chain of events. For example one event was the result of a clogged cyclone, another event occurred during an extended shutdown where the recirculation fan was left running without the burner or solid feed in operation, and yet another occurred near the end of a controlled shut down of the dryer. The only common consequence amongst the various events was the observed overpressue due to resulting deflagration that opened 4-5 explosion doors, followed by a significant vacuum which collapsed sections of the dryer in three of the four events. This paper describes the investigation into the cause of the explosions and severe negative pressure events. The CFD tool DESC was used to help determine what conditions exacerbate the negative pressure events in the dryer. In addition, DESC was used to identify deficiencies in and provide necessary remediation measures to this dryer vent design. This included systematically evaluating the various parameters affecting the dryer pressures during explosion events and included: reactivity of the dust (K_St), size and location of the dust cloud, static opening pressure and weight of the explosion doors, design and layout of the explosion vents, and the inclusion of vacuum breakers.