A silo explosion that killed a firefighter at Robbins Lumber Mill in Searsmont on Friday was apparently a rare occurrence in Maine, but industrial silo explosions happen once a month on average in the U.S. and Europe, according to experts.
The Searsmont fire began in an area where wood shavings are packed into plastic bags, a mill official told the Bangor Daily News last week, and spread to a nearby silo filled with wood shavings that exploded.
At least a dozen people were injured and one firefighter, 27-year-old Andrew Cross of Morrill, was killed by the blast.
Silo fires have happened in Maine before, but explosions are unusual. BDN archives record only one explosion, in 2017 at an empty sulfur silo in Detroit owned by an agricultural supply company.
But silo fires and explosions are frequent elsewhere, according to Jeff Nichols, who worked for decades in silo fire and explosion prevention for industries that deal with “combustible dusts.” Silo fires happen daily, he said.
Dust is so combustible because of its high surface area relative to its mass. When small particles are suspended in the air, each one is surrounded by the oxygen it needs to burn instantaneously if it meets a spark or flame.
Lumber mills are just one type of facility that can accumulate potentially explosive dust from manufacturing processes. Americans are injured or killed in combustible dust accidents every year, according to the Occupational Safety and Health Administration.
Grain silo explosions have long been an issue for farmers storing crops. Chemicals, metals and plastics also create combustible dust.
“It could be sugar, it could be spices, it could be flour, it could be anything,” Nichols said.
A 2008 dust explosion at a Georgia sugar refinery killed 14 people and injured three dozen more, bringing more attention to combustible dust and tighter regulations in the U.S. In British Columbia, two sawmill explosions in 2012 led to new regulations.
Combustible dust or gases build up between stored materials and the top of a silo during a fire, which can cause an explosion while firefighters work, according to an expert from the Canadian province’s safety program.
Fires and explosions in wood pellet storage silos happened often enough there that the regulators launched a safety initiative around them.
Typically, sawmills bring in wet wood that’s dried in a kiln, according to Nichols. Further processing like cutting and planing creates combustible dust, which is pulled away from machinery by extraction systems connected to duct work.
A separator then routes fine dust and shavings apart, Nichols said. Shavings are often processed further into sawdust or fuel; the lighter dust is very fine and very explosive, and can be sold or used as fuel.
There are numerous fire and explosion prevention measures such mills can use, both technical and administrative, according to Nichols.
The Robbins mill’s fire or explosion prevention measures aren’t publicly known.
Joel Davis, chief investigator for the state fire marshal’s office, on Tuesday declined to answer questions about the firefighting response or fire response training for mill employees, saying that’s beyond the scope of the agency’s role.
The family said Monday they are no longer commenting until more information is available.
We’re fundraising to support the newsroom this spring. But on this page let’s take a break. Here are some efforts raising funds to support victims of the Searsmont fire. We hope you’ll consider a gift to support the families.
Typical silo fires start with a spark from inside the mill, where cutting and sawing generates friction and heat, that travels through the dust collection system to the silo, according to Nichols.
It can sit and smolder inside until the material is emptied and the spark is exposed to combustible dust.
In an enclosed space like a silo, a flash fire — which happens when a cloud of dust gets ignited — can lead to an explosion as contained pressure builds. Waves of pressure and fireballs push against the sides of the silo. A stronger secondary explosion can follow from dust dislodged by the first.
Silos with an explosion prevention system can vent out that pressure and fireball, or suppress flames with foam or carbon dioxide. Duct work can also have spark-detection and extinguishing systems installed, among other possible prevention measures, Nichols said.
Some firefighting research recommends fighting silo fires by injecting non-combustible gas that will suppress flames, instead of opening the silo. Adding oxygen to the combustible gas built up inside can cause an explosion.
Volunteer fire departments in rural areas often don’t have access to expensive injecting equipment, according to Nichols. In a past appearance on the Dust Safety Science podcast, he also said volunteer departments may not have adequate training in dealing with combustible dust.
Ken Desmond, president of the Maine State Federation of Firefighters, said he hadn’t heard of such equipment being available to departments in the state.
In Nichols’ experience in the South, mills are also usually located in rural areas staffed by volunteer fire departments, and he urges companies to coordinate with them.
“These local communities have it tough,” he said Monday.
Without gas injection technology, spraying water can keep dust clouds down to prevent an explosion, but it typically tunnels around the woodpile and comes out the bottom without reaching the burning inside.
Companies can also add controls on the administrative side, managing factors like how materials are moved, how fast they’re cut and how often bearings or blades are replaced, Nichols said. Even having an employee walk around the plant with a heat gun helps catch hotspots.
To Nichols, preventing fires is a key focus before an explosion happens.
“A lot of bad things happen when you get to that point,” he said.
Federal and state investigators are looking into the cause of the fire and explosion, and may be onsite throughout the week.
