There is one more dust explosion at Hoeganaes facility, Tennessee on May 27th in which two persons died and another is seriously injured. Earlier in the explosion on 31st January, two persons died and a person was injured during the flash fire on 29th March. My post on 14th May gives some details of iron dust explosion provided at CSB website. The latest incident appears to be initiated by hydrogen gas leak from 3x7 inch hole on a hydrogen pipe in a trench. The trench also contains nitrogen and water lines. After hearing hissing sound from the trench, nearby annealing furnace operators informed maintenance who unsuccessfully tried to remove the trench cover. Later tried to open with the metal chain hooked to a forklift and during cover opening, a flash and explosion occurred. The statement for news conference on June 3, 2011 by CSB gives status of their investigations and a short video (1:16 min) on iron dust flash fire experiment.
The shortfalls at the plant identified are poor containment of dust, improper dust collection, thick layers of dust, unsuitable electrical fittings and availability of various ignition sources nearby.These types of causes are well reported and NFPA Standard 484 and we fail to take measures to eliminate these types of causes.
Providing dust containment and collection, elimination of ignition sources, flameproof electrical fittings, providing and maintaining obstruction-free aisles, regular wet mopping and incineration of mops at safe distance, testing of electrical continuity between equipment and earth pits, maintaining 50-60% humidity, use of antistatic uniform and PPE are some of the measures that will surely help in avoiding such incidents.