The U.S. Chemical Safety Board (CSB) said its ongoing investigation has revealed that the explosion at the Hoeganaes facility in Gallatin, Tennessee was caused by hydrogen gas leaking into a trench from a large hole in a vent pipe -- but that combustible dust played a part as well.

The CSB released an update on the inquiry late last week. The blast and ensuing fire at the the plant – which killed two workers and critically injured a third – occurred on May 27th.

Witnesses interviewed during the course of the investigation said the the explosion occurred when a forklift operator attempted to lift a cover from a trench containing a number of process pipes carrying hydrogen, nitrogen and cooling water. The cover was being lifted because two annealing operators heard a hissing sound in the trench and suspected a gas leak.

“As the cover was pried opened, an explosion occurred,” according to CSB Team Lead Johnnie Banks. “Some witnesses saw a flash of light; some heard a muffled boom and felt the building shaking from the explosion. The building filled with dust and the lights went out. Witnesses saw burning dust raining down from above.

“The initial explosion, we now know, involved hydrogen gas that had been leaking into the trench from a large hole in the vent pipe. However, the witness statements as well as the physical evidence leave no doubt that combustible iron dust was also involved in the aftermath of the explosion.” Banks said that after the incident, CSB investigators observed splatterings of burned iron dust.

A hydrogen fire, described as three to four feet high, continued until an operator in the area closed a valve on the hydrogen piping.

The three victims were flown to Vanderbilt Hospital’s Burn Unit. Two succumbed to their injuries, while a third remains hospitalized with extensive burns.

Banks said his team has located the 3”x7” hole in the hydrogen pipe through which flammable hydrogen leaked. “The trench that held the pipe showed signs of dust intrusion, pipes inside in the trench showed signs of corrosion. The CSB now plans further efforts to understand why the piping failure occurred.”

Additionally, he said the CSB would examine the flame retardant work uniforms worn by the victims, which he said was heavily damaged by fire – “indicating an intense thermal event.”

The Hoeganaes facility was the result of flash fires on January 31st, which fatally burned two workers, and on March 29th – which injured one worker.

During the current visit and two previous trips to the plant, Banks said his team observed “alarming quantities” of metal dust near the incident locations. “This was of particular concern as metal dust flash fires present a greater burn injury threat than flammable gas or vapor flash fires. Metal dust fires have the potential to radiate more heat and some metals burn at extremely high temperatures in comparison to other combustible materials. In addition to visible dust particles in the air, 2 to 3-inch layers of dust were observed on flat surfaces, rafters, and railings throughout the facility.”

“The CSB’s investigation will continue to move forward with a comprehensive examination of existing codes, standards, and inspection procedures applicable to this facility. We will also be testing additional dust collected from the most recent accident site. “

The CSB is an independent federal agency charged with investigating chemical accidents and reporting on their root causes.