Dust explosions are a real and present risk when conveying materials that give off combustible dust when handled or moved. While enclosed conveyors are more susceptible to dust explosions, uncontained dust can also ignite under the right circumstances. Thus, facilities which use open conveyors to move materials must be as equally cognizant of the dust explosion risk as those which are using enclosed conveyors.
In this blog, we focus on reducing the risk of dust explosions when elevating materials with enclosed conveyors. These conveyor types include bucket elevators, aeromechanical conveyors, flexible screw conveyors, drag conveyors, and enclosed belt conveyors.
Reducing the risk of dust explosions when elevating materials: regulatory pressures
Regulatory pressures for effective control of dust explosion risks continue to mount. In October 2015, the National Fire Protection Association (NFPA) in the USA published the NFPA 652 document: Standard on the Fundamentals of Combustible Dust (2016 edition). In publishing the standard, the NFPA sought to address the lack of appreciation and understanding of combustible dust hazards in industry. The NFPA 652 standard introduced a new term, Dust Hazard Analysis (DHA). DHA is aimed at facilities whose processes generate combustible dusts collected by simple dust collectors.
The NFPA 652 standard rolls up the guidelines and standards that NFPA previously issued for various industries and applications, including NFPA 61 (Agricultural and Food Processing), NFPA 654 (Manufacturing, Processing and Handling of Combustible Particulate Solids), NFPA 664 (Wood Processing and Woodworking Facilities), etc. By aggregating these various guidelines and standards, NFPA 652 now becomes the primary US industry standard for combustible dust.
Reducing the risk of dust explosions when elevating materials with enclosed conveyors
A dust explosion occurs in an enclosed conveyor when combustible dust accumulates in sufficient concentration within the interior space of the conveyor and is exposed to an ignition source. Since the ignition source is the trigger which sets off the explosion, controlling the possible sources of ignition through good equipment design can reduce the risk of an explosion ever occurring.
There are multiple possible sources of ignition for a dust explosion to occur within an enclosed conveyor. These sources are outlined in a guidance document for EU ATEX Directives that cover equipment used in potentially explosive atmospheres, published by the UK Solids Handling and Processing Association (SHAPA). Some of the ignition sources identified by SHAPA can be present in enclosed conveyor systems. They include:
- Hot material particles resulting from local chemical reactions or friction as the material is conveyed.
- Mechanically generated sparks from friction, impact and abrasion processes.
- Ingress of foreign materials between moving contact surfaces of the conveyor.
- Thermite or other exothermic chemical reactions. For example, impact between rust present on the conveyor and the conveying of light metals (i.e., aluminum and magnesium, and their alloys). In the conveying of light metals such as titanium and zirconium, friction of these materials against a hard surface may also create sparks.
- Electric sparks created from electric circuits used on machine controls.
- Discharge of accumulated static electric charge within the conveyor. Static charges may arise from friction as material is moved through the conveyor, most likely occurring as material is loaded into and discharged from non-metallic buckets.
How can the sources of ignition within an enclosed conveyor be addressed and controlled? Intelligent design and construction of an enclosed conveyor can address many of the ignition sources identified above and significantly reduce the risk of a dust explosion occurring.
For example, this blog details the numerous design features present in TipTrak™ Monocoque and PEC bucket elevators from UniTrak which suppress potential ignition sources and reduce the risk of a dust explosion occurring.
Manufacturers of other enclosed conveyor equipment types may incorporate similar design features or regulatory body approvals into their equipment to control dust explosion risk. Buyers of these equipment types should check with the manufacturer for the specific equipment features and approvals that can reduce the risk of a dust explosion occurring.
If you are concerned about reducing the risk of dust explosions when elevating materials, UniTrak can help. To find out more about the explosion-safe features in our conveying equipment lineup, please contact our sales team directly.