Processes where combustible or conductive dusts are being handled require dust collection equipment. Inside the dust collector or filter receiver, there is enough material to reach lower explosive limits (LEL) or necessary concentration of dust to cause an explosion.
For an explosion to occur, the following is required:
- Fuel/air mixture above LEL
- Ignition source
Both fuel/air mixture and confinement exist in any collector/receiver which makes an accidental ignition very difficult to control. Static electricity, an ember generated by the process of even a metal staple heating the wall of the collector receiver — any of these could be the proverbial match in the powder keg. Instances such as these are by no means rare. Dust-collector/filter receiver explosions represent 15 percent of all explosions taking place in industrial settings.
The best way to manage an explosion in a collector/receiver is to safely vent it to the outside. This is achieved through a properly selected explosion-relief venting system.
The mechanism of explosion venting is as follows: When deflagation pressure inside the dust collector reaches a certain value, the explosion relief panel ruptures at predetermined pressure called P-stat. The pressurized gases are discharged into the atmosphere either directly or via a vent duct. If the collector is located inside, a vent duct is required to connect the collector/receiver to the outdoors. The vent duct directs the flame and pressure outside the building.
While the collector/receiver is ventilating the blast to the outdoors, a sudden reduced deflagation pressure builds inside the unit. This pressure is called P-red. It’s very important to ensure that strength of collector/receiver is greater than P-red. Otherwise, even with the dust collector ventilating, the P-red pressure will destroy the collector and you run the risk of a secondary explosion.
Due to the back-pressure created by the duct, the P-red is significantly increased. As you can see in the figure above, a duct of up to 3m will increase the P-red from 0.15 bar (min. required) to 0.5 bar; a duct longer than 3m will increase the P-red to 1.5 bar or 10 times the value of a 3m duct. Since the P-red is directly linked to the strength of dust collector or receiver, additional reinforcement of the vessel is required.
This process is not very well understood in the industry. Quite often, collectors that are designed for only 20 inches of water or 0.05 bar are used to vent exploding dust. Obviously, in case of an explosion, such a collector/receiver will explode even with the explosion vent panel opening up. The ensuing blast may cause very serious damage to the building and do harm to the people working within it. Quite often, the initial explosion is followed up by a secondary blast and a fire.
Simon Fridlyand, P.Eng., is the president of S.A.F.E. Engineering, a company specializing in Pre-Start Health and Safety Reviews and audits for fire code compliance. You can reach him at (416) 447-9757.