“Complacency is the enemy of both security and progress”
One of the most difficult aspects of dust explosion prevention and protection is a general lack of knowledge within industries handling combustible dusts.
In the case of combustible metal dusts, the hazards are increased. In addition to highly reactive materials, dust wetting can lead to release of highly flammable gas, dust mixing to toxic or self-heating substances, and traditional extinguishing methods such as spray hoses may cause severe and rapid explosions.
This post gives a review of the explosibility of combustible metal dusts with a focus on example scenarios, typical causes, and prevention techniques.
Dangerous and Deadly
Recent history has many examples demonstrating the danger of combustible metal dusts:
China (2014) 146 workers died in an aluminum dust explosion. Overall another 114 others were injured. Including family and friends, it is impossible to measure the number of lives affected by this incident. The facility was polishing aluminum rims at the time of the explosion. (Link)
California (2014) 11 workers burned, 2 of which were critically injured after an aluminum dust explosion. The company was polishing aluminum parts. (Link)
West Virginia (2010) Three people were killed and one person injured in a combustible metal dust explosion at a facility processing titanium and zirconium. (Link)
Potentially more concerning is the vast amount of combustible metal dust fires and explosions that go unreported. Some examples that typically occur in metal processing industries include:
On a Friday afternoon, a worker used an electric wet shop vac to vacuum aluminum dust off the floor. Monday morning rolls around and a worker gets the same wet vac for another clean up, flips the ‘on’ switch and the shop vac explodes. The cause of the explosion was water in the shop vac chemically reacting with the aluminum dust creating hydrogen gas. Monday morning when the motor was turned on, a spark was generated which in turn ignited the hydrogen gas and caused the shop vac to explode.
Other examples include cross-contamination of metal dusts from different materials:
A call came in one morning from the general manager of a manufacturing facility. A spark ignited the aluminum and steel dust mixture inside a wide belt sander resulting in an explosion. One employee was burned over 40% of his body. It was difficult for the general manager to wrap his head around the real possibility of an employee dying as a result of the explosion and if the employee did survive he would be disfigured, possibly disabled, and probably suffer health challenges for the rest of his life. My customer also expressed frustration because this particular employee had been warned multiple times not to cross contaminate metals between two wide belt sanders. This story is a tragic reminder of the hazards of mixing combustible metal dust
Knowledge is Power
There are at least three things to keep in mind when working with combustible metal dusts.
Metal dusts involve highly reactive materials
Dr. Ashok Dastidar with Fauske & Associates, LLC was kind enough to provide a comparison without using Kst and Pmax values so that a non-scientist/chemist can understand. For the “purposes of illustration, only”, Dr. Dastidar compared aluminum to TNT stating that 1 gram of aluminum dust has about the same explosive energy as 0.7 grams of TNT.
Be careful what you mix with
Certain metals when mixed create a chemical reaction that generates heat causing self-combustion. Furthermore, Aluminum dust (not completely submerged) mixed with water will generate hydrogen gas.
Do not spray with water
Metal dust fires require Class D fire extinguishers and agents. Water used as an extinguishing agent can actually intensify a metal dust fire. NFPA 484 mentions having Class D metal dust fire extinguishers ‘within easy reach’ of the processing equipment.
Combustible Metal Dust Standards
NFPA 484 is the “Standard for Combustible Metals” and provides a list of combustible metals and guidelines for handling dust generated from various processes such as sanding, sawing, grinding, cutting, polishing and abrasive blasting. Each metal has its own chapter addressing that particular metal’s characteristics.
There are chapters dedicated to dust collection and housekeeping. Although NFPA 484 is not a law, it does provide useful information and guidelines to help reduce the possibility of metal dust fires and explosions. Insurance adjusters and inspectors regularly reference NFPA 484. If you sell metal processing equipment and machinery, NFPA 484 is a useful resource for you and your customers who are processing combustible metals.
When in Doubt
Test your dust!
Testing your dust for explosibility provides a baseline you can use to determine the proper dust collection equipment for your specific application. Test results also help determine what housekeeping procedures to implement. The lab will provide assistance to ensure you take representative samples of your dust.
If you are still in doubt about equipment options, compliance, and housekeeping procedures, work with a professional combustible metal dust consultant.
Educate. Educate. Educate.
Facilities processing aluminum, titanium, magnesium or zirconium (refer to NFPA 484 for the complete list) should include combustible metal dust as part of their safety training and discussion. Safety seminars and workshops addressing combustible metal dust will usually prove to be time well spent.
Consider bringing in a professional safety consultant who specializes in combustible metal dust to assist with your safety plan and procedures. Have your dust tested and utilize information available from the US Chemical Safety Board, OSHA and NFPA 484, regarding combustible metal dust.
Lastly, there are knowledgeable people in the field of combustible metal dust, whether they are safety consultants, equipment manufacturers, test lab scientists, or engineers, they are available and able to provide assistance and guidance.