DSS190: Safety Share: Wood Hogger Fires and Deflagrations at Sawmills

In today’s episode of the Dust Safety Science podcast is a safety share about wood hogger fires and deflagrations at sawmills. It’s based on a Manufacturing Safety Alert that’s put out by the BC Forest Safety Council and the Manufacturing Advisory Group (MAG)..

To summarize the case we’re discussing today, there was a smouldering fire in a hogger machine. Hogging machines create hog fuel, which is basically wood waste.

A smaller fire broke out in the machine and workers put it out. In the process, a piece of burning wood escaped and left the two conveyor outfeeds. Workers stopped the machine to deal with the burning wood. Then, 10 minutes later, another fire broke out in a pile of hog fuel at the outfeed of another conveyor because the material wasn’t all collected. Despite their best efforts, they were unable to capture all of the burning mass.

High winds and other flammable materials in the mill yard prevented workers from putting out this second fire. A second hog fuel conveyor then caught fire, spreading to several structures and wood waste products. Fortunately, no one was injured but the spreading fire that began in the hogging machine caused quite a lot of damage.

2016 Investigation and Recommendations

WorksafeBC reported this incident summary back in 2016. According to investigators, it was a friction fire caused by inadequate maintenance of the hog machine. WorksafeBC listed hog fuel accumulation under and on conveyors as the first contributing factor. Because this hog fuel and sawdust and wood were very dry in the summer, they were a strong fuel and WorksafeBC recommended regular cleanup of the hog fuel, sawdust, and wood material on and around the hogger.

Inadequate training and fire procedures were the second contributing factor. WorksafeBC listed a few things that could have been done:

An employee could have been posted to act as a firewatch during firefighting and afterwards in order to prevent or at least notice the fire spreading.
Provide workers with basic skills to fight fires, monitor fire spread, and know when to call the fire department to extinguish the fires.

Thirdly, they mentioned that there was no method for detecting smouldering fires. In this case, the fire was detected by a passerby. Consequently, they suggested that smoke detectors could have prevented the fire from spreading and growing.

Several months ago, the BC Forest Safety Council released another Manufacturing Safety Alert. The date and location are not given, but according to that alert, smoke was found inside a hogger. When the access panels were removed, it was determined that the blockage in the hogger was causing the grinding, friction, heat, build-up, and smouldering combustion. A large portion of the room was engulfed in flames as an employee tried to clear the hogger with a bar. Fortunately, no one was injured.

This Manufacturer Safety Alert features a very insightful picture showing five workers in the hogger room. The room’s height isn’t apparent but it’s not that big: basically, it just fits the hogger. You see a set of stairs going in, and these workers are around the hogger trying to get it cleared. The top picture shows a flame, which is probably the start of the deflagration. Afterward, they show a second picture where a fireball occupies around 15% or 20% of the room.

The machine is surrounded by fugitive dust. It’s mostly around the base of the hogger. They don’t go into a lot of details about what happened other than saying there was a deflagration. But you can see that the employees kicked out that smouldering mass in the vicinity of combustible dust while clearing the hogger with the bar. There was a large flash fire at a point where the combustible dust was dispersed in the air.

Again, fortunately, no one was injured. There wasn’t enough fugitive dust to cause a pressure rise in the room and destroy equipment, but there was a very dangerous near-miss in terms of this open-air deflagration directly near the workers. It serves as a reminder of the importance of working carefully with smouldering masses when combustible dust is nearby.

Conclusion

In the second alert, BC Forest Safety Council recommends that when clearing equipment during a potential fire situation, always inspect the area for combustibles before moving the equipment and ensure the hogger areas are regularly checked for combustible dust to minimize accumulations, and review the emergency procedures to ensure they have clear instructions on how to deal with equipment fires. These measures can control incidents when they do occur, minimize their damage, and save lives.

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