Bucket Elevators and Dust Explosion Safety

A male industrial worker stands on a grain silo with documents in his hands and makes a visual inspection.
Bucket elevators are extensively used in industrial manufacturing facilities such as agriculture, food processing, and biomass plants to vertically transport bulk materials, typically for storage or further processing.

Over the years, bucket elevators have been identified as a frequent source of combustible dust explosions. These explosions occur when a mixture of fuel and oxygen is dispersed within a confined space and meets an ignition source. The rapid expansion of the resulting fireball generates destructive pressure that can exceed the vessel’s structural strength, leading to deformation or rupture. Under the right conditions, secondary explosions and fires often accompany the initial explosion. All these elements are frequently present in a bucket elevator during regular operation. Without effective procedures and mitigation measures, these explosion risks can result in catastrophic outcomes. Research conducted by Purdue University’s Agricultural Particulates Research Group found that between 2006 and 2020, 78 dust explosions occurred in bucket elevator facilities in the United States, leading to 56 injuries and 10 fatalities.  This paper aims to review industry protocols for identification and mitigation of the risks associated with combustible dust in bucket elevators. 

Damaged bucket elevator and silo

Hazard Identification

To mitigate the risks of catastrophic losses, hazard identification is crucial. A bucket elevator operates using a series of buckets attached to a continuous loop of belt or chain. The system includes a head pulley or sprocket, which drives the belt or chain, and a tail pulley or sprocket, which maintains tension. As the belt or chain moves, the attached buckets scoop material from the elevator’s boot and carry it upward. Upon reaching the discharge point at the head of the elevator, the material is unloaded. The empty buckets then return to the boot, completing the cycle. While this cycle is in process, the components necessary for a dust explosion, including the introduction of ignition sources, can be present.  Friction-induced heat or sparks may arise if the drive components are improperly installed or misaligned. Additionally, overheated bearings and motors, faulty or inadequate wiring, and the discharge of static buildup can serve as ignition sources. These risks underscore the importance of consistent maintenance and diligent monitoring of equipment.

Dust Hazards Analysis (DHA)

Owners and operators will identify these hazards and associated risks through a process called a Dust Hazards Analysis (DHA). According to the National Fire Protection Association (NFPA), a DHA is “a systematic review to identify and evaluate the potential fire, flash fire, or explosion hazards associated with the presence of one or more combustible particulate solids in a process or facility.” Additionally, NFPA 660 —”Standard for Combustible Dusts and Particulate Solids”—offers comprehensive guidance on protection strategies and the design of mitigation solutions.

Preventive Measures

The ultimate goal is for an explosion not to occur in an industrial process. To reduce the risk of such an event, the first line of defense is to eliminate one of the required components for a combustible dust explosion. For a bucket elevator process, controlling ignition sources are one of the most common and practical approaches. A few implementations of ignition control include:

  • Overheat Detection: Monitoring the temperature of bearings and motors in the drive system can identify trends and prompt corrective actions before component failure.
  • Electrical Bonding and Grounding: Proper grounding dissipates stray electrical charges to the earth, reducing the risk of static discharge.
  • Misalignment Detection: Sensors that detect belt or chain misalignment enable preemptive maintenance, minimizing wear, damage, and failure points.
  • Zero Speed Switches: Identifies when the shaft has stopped rotating and signals an alarm to the process control.
  • Spark Detection Systems: These systems, combined with water sprays or inerting gas, can extinguish ignition sources conveyed into the bucket elevator from upstream equipment.

Proper housekeeping is another essential preventive measure. Regular cleanings, the use of properly designed dust collection systems, safe material handling practices, routine maintenance, and operator training will improve safety and reduce the risk of a fire or explosion propagating beyond the bucket elevator.

Protection Measures

Even with the most robust explosion prevention measures in place, a facility cannot eliminate the risk of an explosion. For this reason, NFPA 660 requires explosion protection on bucket elevators handling combustible dust. Previous standards, such as NFPA 61 (Agricultural and Food Processing), NFPA 652 (Combustible Dust Standard), and NFPA 654 (Dust Fire and Explosion Prevention) among other industry specific standards, existed independently. These standards have since been consolidated into NFPA 660, providing unified safety requirements for combustible dust hazards.

Explosion Venting

Explosion venting is a critical safety measure that protects process equipment by relieving overpressure during a dust explosion. The system employs specially designed pressure relief membranes (or rupture panels) that relieve at a predetermined pressure, preserving the structural integrity of the bucket elevator. To ensure compliance, venting systems must adhere to NFPA 68 (Explosion Venting Standard) or NFPA 660-Chapter 21, which outline methods for calculating explosion relief area, determining vent spacing, and identifying optimal placement. These guidelines account for factors such as elevator geometry, material properties, and process operating conditions.

Effective venting requires careful evaluation of vent locations to ensure the fireball is safely directed away from personnel and critical infrastructure. When conventional venting is impractical due to space constraints or facility layout, flameless venting solutions may be implemented. These devices combine a rupture vent, or valve plate, with a flame-arresting mechanism, such as a mesh filter, to retain particulate matter and prevent the release of flames. However, it is essential to consider the required safety barrier around the flameless vent as heat dissipation and toxic gas emissions from the flameless vents need to be considered to maintain personnel safety and regulatory compliance. In some cases, advanced modeling tools could be used to assist in optimizing vent design and placement, ensuring maximum effectiveness in mitigating explosion risks.

CFD mesh example of bucket elevator simulation (left). CFD results with combustion progress (right).

Explosion Suppression

For some applications, venting may not be feasible or economically practical due to the height of the elevator and number of vent location required to meet the prescriptive solution. In those cases, explosion suppression systems can be implemented as the explosion protection solution. Suppression systems are composed of advanced detectors, centralized control units, and rapid-acting suppressors that collectively identify and neutralize a growing fireball within milliseconds. Detectors utilize optical or pressure-based sensors to detect the initial stages of a combustion event, signaling the control unit to activate the suppressors. Suppressors then release a suppressant agent, such as a chemical powder, designed to quench the fireball and stop the pressure build-up before it exceeds the structural integrity of the equipment.

The placement and quantity of suppression units are determined based on the dimensions of the bucket elevator, the characteristics of the combustible material, and advanced modeling techniques to ensure effective coverage. For added safety, suppression systems are integrated with explosion isolation measures to prevent flame propagation through interconnections and mitigate the risk of secondary explosions.

Explosion Isolation

Explosion isolation systems serve a critical role in preventing the propagation of combustion through interconnected process equipment.  Explosion isolation significantly reduces the risk of secondary explosions and cascading damage throughout a facility. These systems employ various methods, such as chemical isolation barriers, fast-acting mechanical valves, and rotary airlocks, to create a physical or chemical block that halts flame from spreading. For gravity-fed chutes, explosion isolation often involves mechanical barriers which rapidly seal off the chute upon activation. In mechanical conveyors, isolation is typically achieved using chemical suppression systems that discharge suppressing agent to disrupt flame propagation. Pneumatic dust pickup points are safeguarded through specialized isolation valves or high-speed extinguishing barriers, which ensure that flames cannot travel back into the system or ignite upstream components. As is the case with venting or suppression systems, advanced computational fluid dynamics (CFD) modeling is often used during the design phase to simulate flame propagation scenarios and optimize the placement based on response times of explosion isolation components, ensuring maximum system effectiveness.

Conclusion

The risks associated with combustible dust explosions in bucket elevators have been recognized for over a century, with NFPA 61 first being introduced in 1923. These hazards require a thorough understanding of potential risks and the implementation of the proper protective measures. Over the years, the NFPA codes have grown to reflect best practices, but to a novice reader, can be cumbersome and overwhelming to fully comprehend its requirements. It is imperative, and recommended, to contact an industry expert to assist throughout the process from explaining the DHA and identified risks to selecting the most effective solutions to combat those risks. Trusted experts can further explain the how various preventative and protective system will help, but may also be limited, to ensure the best, most informed decisions are made.

Ultimately, the proper mitigation for combustible dust risks in bucket elevators will come down to the materials being conveyed and the operational conditions of the facility. When the resultant plan is properly implemented, it can significantly minimize the potential for catastrophic losses, ensuring the safety of equipment, operations, and most importantly, personnel.

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