A few weeks ago, we discussed an explosion that occurred in a wood pellet plant located in British Columbia in October 2014. The fire and subsequent explosion was said to have started inside equipment used to dry wood fiber during a routine maintenance shutdown and was extinguished by the equipment’s built-in fire suppression system. One of the questions that came to mind for us was if the plant was on shutdown, meaning normal production would have been halted, what was the source of the heat and the fuel for the fire?

After reviewing the public fire investigation records, it appears that the source of the heat contributing to ignition was the hot dryer and hot residue of combustible material inside the dryer. The dryer had been shut down and was cooling down in order for the workers to get inside to scrape and clean off the residue build-up on the interior surfaces. At that point, we assume that the dryer had not cooled down enough for the workers to get inside. This can be assumed because the three workers were still outside the dryer on the work platform and had not yet made entry into the dryer. They positioned an electric fan to blow cool air into the dryer to make sure they had sufficient clear cool air to breath once they were able to work inside the dryer. Unfortunately, as soon as they turned on the fan and started blowing fresh air into the dryer interior, the interior exploded, knocking all three workers off of the platform. All three workers suffered burns from the explosion. The automatic deluge sprinkler system inside the dryer is credited with extinguishing the fire and the fire never spread outside of the dryer interior.

After studying the fire investigation reports, some additional questions come to mind regarding possible root cause elements:

  1. Was fuel present inside the dryer in the form of unburned combustible vapors that was hot enough to self-ignite with the introduction of additional oxygen by the ventilation fan?
  2. Was fuel present inside the dryer in the form of accumulated combustible residue that was thick enough to promote self-heating and, eventually, flaming combustion?
  3. Was the dryer gas-fired, and if so, could gas have leaked into the dryer interior after it was shut down? For this to happen, there would have to have been a malfunction of the burner safety management system, such as a leaking safety shut off valve (SSOV).

We may never learn the answers to these questions. Based upon our experience, however, we know that the risk of similar incidents can be greatly minimized by accomplishing the following:

  • Regularly clean the interior surfaces of dryers and exhaust ductwork to prevent the accumulation of combustible residue to thicknesses that can self-ignite and promote fire spread.
  • Regularly conduct inspections, testing and maintenance on burner safety management systems in strict accordance with the manufacturer’s requirements and recommendations.
  •  Regularly conduct inspections, testing and maintenance on all dryer equipment safety sensors and control systems in strict accordance with the manufacturers’ requirements and recommendations.