Keep your bucket elevators safe against combustible dust explosions, cautions Filip Ďurica
Bucket elevators and other vertical conveyors are part of the agriculture and grain industries and many other industries. They can be installed outside, inside or both, thanks to their design. Because of that, different types of explosion protection must be considered. Unfortunately, one type of explosion protection doesn’t fit all bucket elevators.
Vertical conveyors are a simple and effective technology frequently used in process industries. Their primary purpose is to quickly transport material of various sizes from bigger chunks to smaller dust-size particles. It is important to know that handling can generate dust particles below 500 µm (microns), which is recognised as presenting a dust explosion hazard.
Elevators can be installed inside or outside the facility and at the beginning of the production as well as at the end during the final steps of the process. All of these parameters and information must be considered when assessing explosion protection. The risk of combustible dust explosions is high because if explosion protection is chosen poorly and in the event of an explosion, a pressure wave can cause turbulence and stir up dust once again. This is followed by a flame front, which can act as an ignition source and quickly spread to connection equipment; in the worst-case scenario, secondary explosions may spread downstream or upstream.
Problem Areas And Explosion Prevention
In most cases, vertical conveyors transport organic materials such as wheat, wood chips, soya beans and other grain material. These are usually combustible, therefore there is a chance of an explosion. So, what type of prevention measurements can be done? The best way to prevent an explosion is to limit ignition sources to a minimum. The head and boot of an elevator are most prone to ignite dust explosions. The risk of an explosion can appear during the daily operation itself. If the bucket elevator or other vertical conveyor is being emptied or filled, the occurrence of a dust cloud is also possible; thus risk of combustible dust explosion is also present. Dust can also quickly adhere to the bucket elevator inside the legs casing, where it can form dust layers of several millimetres. Imagine a situation due to a malfunction of the bucket elevator; the belt can start vibrating, dust on the elevator walls can swirl and become a dust cloud. In some cases, a layer of dust 0.1mm thick is sufficient to create a potentially explosive atmosphere. Regular maintenance and sufficient cleaning are also necessary for limiting fire and explosion hazards.
Explosion Protection According To What Standard?
Bucket elevators and their protection are covered in National Fire Protection Association Standard NFPA 61, NFPA 68, and NFPA 69. Also, European Technical Report CEN/TR 16829 covers fire and explosion prevention and protection for bucket elevators. In general, you can protect your elevator with explosion venting panels, flameless venting devices or explosion suppression systems. Explosion isolation is a must, so the explosion is stopped from transferring to connecting parts of the technology.
Which Type Of Protection Is Best?
That depends. Explosion venting is used often as it is easy to install, requires minimum maintenance and explosion venting panels come in various shapes and sizes. Vent panels are effective and cost-efficient. Explosion venting panels must be located outside the facility due to the safety zones necessary for explosion deflagration. Flameless venting is suitable for indoor or partially indoor bucket elevators and other vertical conveyors. Protective mesh and explosion vent panel prevent flame front and pressure wave from propagating to open space.
Explosion suppression can be used as a protective measure to mitigate dust explosion. Explosion suppression systems start with extinguishing in the early stages of the explosion before there is an increase in pressure. Optical or pressure detectors can detect an explosion in a matter of milliseconds. The system then opens the valves of the explosion suppression container unit, and the pressure of the fire extinguisher pulls out special telescopic nozzles, which ensure efficient dispersion of the fire extinguisher into the entire protected area.
It is necessary to use explosion isolation in combination with either explosion (flameless) venting or explosion suppression. Mechanical or chemical barriers must be installed at the feeding and discharge point of a bucket elevator or to any other dust extraction pipeline. Isolation prevents propagating the explosion to another part of technology. Thus it protects the facility against significant damages that could heavily damage the production process.
RSBP has created a certified explosion protection system called Elevex. It is an effective solution for the explosion protection of vertical and horizontal conveyors. The system reduces the explosion pressure to an extremely low level. Although the use of the Elevex system does not prevent the occurrence of an explosion in an elevator or other similar transport system, the consequences can be reduced to a minimum. The system can be used in a variety of applications and can be used both for conveyors inside buildings and for those that are located outside. It can also be installed on existing technologies without additional modifications.
Filip Ďurica is with RSBP