Danger in the ductwork
April 10, 2003
Under controlled conditions, aluminum and steel powders can be mixed to create a chemical reaction to produce heat for welding. If you mix these materials under uncontrolled conditions, you will want to call the fire department.
Why, you may ask, would anyone want to start an uncontrolled chemical reaction that is really hard to stop? Well, our shop did not want to, but we did anyway, and I hope you learn from my lesson. Actually, they are lessons (plural) because we repeated the disaster twice, although with some variation on the theme.
The first time we had a fire was about 20 years ago. We had been using wide-belt graining machines to finish our sheet metal parts. These machines were hooked up to vacuum systems to collect the dust that results from the sanding process.
The vacuum system was made from a rack of canvas tubes that hung inside a big metal box. This box occupied a fair amount of space in the sanding department. The blower motor and dust box were outside the building.
One day someone noticed a smoldering fume emitting from the vacuum system. Apparently, a spark from the steel grainer had ignited one of the canvas tubes. It did not seem like an emergency, but we evacuated the building anyway and called for help.
While waiting for the fire department to arrive, I decided it might be a good idea to keep the dust box cool, so I went outside and grabbed a garden hose. When the water first hit the dust box, it really sizzled and steamed! I could hear the approaching sirens as I directed the water up to the metal ductwork that led to the blower cage.
By the time the first firefighter walked up, I had made a pretty good-sized puddle of water. I explained to him that we had smoking metal dust in the vacuum system.
Suddenly the smoldering became a billowing. The wisps of steam were replaced with blinding clouds of superheated stench. The firefighters scrambled and grabbed hoses and yelled at me to get away.
As I walked backward, I was amazed to see the firefighter become engulfed in a cloud of flame, sparks, and smoke as he chopped through the dust box with an ax. As he staggered back, a high-pressure fire hose covered him in a life-saving fog.
With a ferocious whoosh, the remnants of the dust collector melted and collapsed. Fortunately, the building was constructed of cinderblock and the dust box had only a relatively small amount of fuel. Because of these, the firefighters were able to keep the heat under control with water, even though they could not put out the fire until all of the metal powder burned away. Another group of firefighters deployed inside the building miraculously kept the eruption from burning through the roof.
Later the fire investigation officer advised me to "always think of your sanding equipment as if it is about to explode."
We learned from this disaster. For sanding aluminum, we replaced the dry dust collector with a wet vacuum system that pulls the metal dust through a vat of water instead of canvas socks. Today, as part of preventive maintenance, we make sure to clean the sanding machines and their ducting when switching from steel to aluminum.
We are careful to keep the aluminum separated from the steel. This is important also because the steel dust makes black pits in clear anodized finishes on aluminum. After that first disaster, more than 10 years passed without incident.
Then one day, about five years ago, wet aluminum mud got mixed with rusty steel mud. Some organic material may have been mixed in too. It started to steam and smolder. We called the fire department and evacuated the building.
One of our brave employees risked his life by spraying a dry chemical fire extinguisher in an attempt to keep the flare-up from blowing up. Fortunately, he did not get hurt. By the time the fire department arrived, it had spread to the ductwork in the vacuum system. They determined that we had an uncontrolled chemical reaction in process and called the HazMat unit.
When it was over, I was terribly thankful no one was killed. It was a bonus that the building was still standing. I blessed and cursed my heroic employee. We replaced nearly all of the machinery in the sanding department-again. This time we did some roof work too. The tar shingles had gotten hot enough to boil and bubble.
Since then we have been even more careful to segregate the aluminum dust from the steel dust. We make sure the fire department knows what materials are being processed and how we store them. Every Friday we carefully clean the sanding machines to make sure no residual piles of metal dust are trapped in corners or in the ductwork.
Having literally faced death and destruction, I am constantly vigilant to make sure that someone new or inexperienced does not carelessly mix aluminum powder with steel powder. A spark or bio-generated heat can easily ignite this dust. Putting it out requires heroic effort, expert training, fast reaction, and great expense.