Dust or Gas Explosion: Case Study of Dryer Explosion and Design Venting

A recent explosion occurred in a single burner, recirculating solids ring dryer. No one was reported injured as a result of the explosion, however the explosion caused significant damage to the dryer and minor damage to sections of the facility. Despite the dryer having been designed with seven explosion doors, the explosion caused a section of the dryer recycle duct to rupture and various doors on the dryer equipment to fail. As a result of the blast, two of the explosion doors on the external ring duct were separated from their hinges, with one landing on the upper roof section, while the other fell back through the roof of the facility in the area of workers. A fire in the various sections of ductwork of the dryer ensued just after the explosion which caused significant damage to the equipment. Fire suppression efforts using water and foam lasted approximately 90 minutes when the fire was ultimately extinguished.

Just after the incident the facility owners thought the accident was caused by a dust explosion, as they recently converted to a new process for treating the solid particulates prior to entering the dryer. Moreover, process data also showed that the explosion occurred seconds after a burner trip when air was being added to the dryer to purge the system. The combustion management system utilized an air-to-fuel ratio controller, along with mass flow meters for both combustion air and the natural gas for the burner. This paper describes the investigation into the cause of the explosion and design blast venting used on the dryer. The CFD tool FLACS was used to help evaluate the consequences associated with a dust or gas explosion during the incident. This included analysis of the process data to determine the gas mixture within the dryer at the time of the explosion as well as dust explosion testing of the solid particles in the dryer in order to evaluate its potential contribution. Finally, deficiencies were identified in the blast vent design employed on the dryer for deflagrations associated with credible gas or dust explosions.