Operators in a powdered metals production facility heard a hissing noise near one of the plant furnaces and determined that it was a gas leak in the trench below the furnaces. The trench carried hydrogen, nitrogen, and cooling water runoff pipes as well as a vent pipe for the furnaces.
Maintenance personnel presumed that the leak was nonflammable nitrogen because there had recently been a nitrogen piping leak elsewhere in the plant. Using the plant's overhead crane, they removed some of the heavy trench covers. They determined that the leak was in an area that the crane could not reach, so they brought in a forklift with a chain to remove the trench covers in that area.
Eyewitnesses stated that as the first trench cover was wrenched from its position by the forklift, friction created sparks followed immediately by a powerful explosion. Several days after the explosion, Chemical Safety Board (CSB) investigators observed a large hole (~3x7 inches) in a corroded section of hydrogen vent piping inside the trench.
As the hydrogen-air mixture in the partially open trench exploded, the resulting overpressure dispersed large quantities of iron dust from the rafters and other surfaces in the plant, and some of this dust subsequently ignited. Eyewitnesses reported that embers were raining down and igniting iron dust flash fires in the area. Visibility was so poor due to dust and smoke that even with a flashlight, it was impossible to see more than 3 or 4 feet. Three plant employees eventually died from burn injuries despite wearing supposedly flash-fire-resistant garments. Two others suffered smoke-inhalation injuries. Due to the extensive nature of the injuries, and the abundance of both hydrogen and combustible dust present at the time of the incident, it is difficult to specifically determine which fuel, if not both, caused the fatal injuries to the victims.
In addition to the probable causes listed above, the lack of a standard operating procedure for hydrogen leak detection was one of the probable causes of this incident. Additional contributing factors included the following:
- Severe pipe corrosion due to the presence of hot water in the pipe trench
- Hydrogen piping located in a concealed space
- Limitations of the flash-fire-resistant garments worn by plant employees.
Key findings noted in the CSB report included:
- Significant accumulations of combustible iron powder fueled fatal flash fires when lofted near an ignition source.
- Facility management were aware of the combustibility hazard two years earlier but did not mitigate the hazard with engineering controls or housekeeping.
- The plant did not institute combustible gas monitoring or employee training to help avoid flammable gas fires and explosions.
- OSHA did not include iron and steel mills in its Combustible Dust National Emphasis Program.
- The 2006 International Fire Code (IFC) does not require enforcement of the more comprehensive and rigorous NFPA standards for the prevention of dust fires and explosions.
- The state and city did not enforce recommended IFC practices.
- The local fire department inspected the facility just months before the fatal hydrogen explosion and dust flash fire but did not cite or address the combustible dust hazards that were present.
- The flame-resistant clothing provided to the plant employees did not adequately protect them from the hydrogen explosion and dust flash fires.
- There was no corporate oversight regarding the management of combustible dusts even though there had been a succession of serious accidents at the facility in the past.
The CSB made recommendations regarding combustible dust hazards to OSHA, the International Code Council, the state, the company, the Metal Powder Producers Association, the city, and the local fire department. Recommendations to the company covered both combustible dust and flammable gases as shown below.
- Conduct periodic inspection audits of the facility for compliance with the relevant NFPA standards (484, 499, 497, 2, and 2113), using knowledgeable experts, and implement all recommended corrective actions.
- Develop a training program for combustible dust hazards for all employees and contractors.
- Implement a preventive maintenance program and leak detection/mitigation procedures for all flammable gas piping and gas processing equipment.
- Implement a near-miss reporting and investigation policy.