In today’s episode of the Dust Safety Science podcast, Michel Vandeweyer, Explosion Safety Consultant at ISMA, based out of Antwerp, Belgium, discusses recommended ways to protect bucket elevators from explosion risks. During the interview, he answers questions like the following:
- What is a bucket elevator?
- Why are they used in industry?
- What are different components?
- What are some of the hazards with regard to fire and explosions?
- What are some of the methods for prevention and protection?
“As an explosion safety consultant, I advise companies on how to manage their explosion risks,” Michel explained. “We don’t sell hardware. We only try to make the world a bit safer by giving specific recommendations and looking for the best solution for the customer. So by doing so, we try to stay on top of the different standards and all the different scientific work that is done in the field.”
About Bucket Elevators and Their Use in Industry
Bucket elevators are widely used in various industries, primarily for moving materials vertically. The process begins at the bottom part of the elevator, known as the boot, where materials are loaded. These materials are then scooped up by buckets attached to a belt or chain. As the belt rotates around pulleys, it carries the buckets filled with material to the top part, or the head, of the elevator. Here, the materials are discharged as the belt makes its turn.
The main driving mechanism, usually situated at the head, powers the belt, causing the buckets to ascend and descend. These buckets travel through channels, which can be round or square, referred to as the legs of the elevator. Elevators may have a single leg for both ascending and descending buckets or double legs, with one leg for each direction.
Bucket elevators are essential for moving large volumes of materials quickly. This makes them integral to industries handling significant quantities, such as the food and feed industry, grain handling, sugar production, and breweries for malt supply.
One of their primary benefits is their efficient use of space; they occupy minimal ground area since they move materials vertically. Unlike traditional conveyor belts that require more space and can only move materials at an angle, bucket elevators can reach heights up to 40 meters, making them a noticeable feature in industrial areas.
Despite their visibility, they are often placed inside production buildings, typically in central locations, to save space. However, this positioning presents challenges in implementing explosion protection measures, which are necessary due to the potential risks associated with their operation.
What Are the Risks Associated With Bucket Elevators?
There has been a lot of discussion and analysis regarding the safety risks associated with bucket elevators. Recognizing these concerns, there is a specific European technical report, TR16,829, introduced in 2016, alongside various NFPA standards -namely NFPA 61, 68, and 69- that address these safety issues. These documents outline the risks and provide guidelines for safely managing bucket elevators.
The core risk with bucket elevators involves the potential for dust explosions, which require two main elements: an explosive dust atmosphere and an ignition source. When these two factors converge, a dust explosion can occur.
How is an Explosive Dust Atmosphere Created?
The creation of an explosive dust atmosphere in bucket elevators happens through several mechanisms. As buckets scoop up material at the elevator’s base and transport it upward, some of the material spills, separating into heavy particles that fall quickly and fine particles that stay suspended in air. This separation creates a dust cloud within the elevator, particularly in the upward and then the downward legs.
Even when the amount of fine particles in the material is small, the phenomenon repeats in the downward leg as the material is discharged at the top but not all of it reaches its intended destination, causing some to fall back through the elevator. Here, the same separation occurs: heavier particles descend quickly, while dust remains airborne, reaching explosive concentrations.
The mechanical movement within bucket elevators also contributes to the formation of dust. For instance, sugar, which is typically not considered a risk for dust explosions in its coarse, crystallized form, can become hazardous when broken into dust by mechanical action. This can happen both due to the elevator’s mechanical components and when materials are fed into the elevator through devices like screw conveyors, which generate friction and thus dust.
What About Facilities With Lower Dust Levels?
In transport systems that handle materials with only small amounts of dust, the risk of dust building up to explosive levels is generally lower. The primary concern in such systems is the accumulation of dust layers inside the casing, which doesn’t apply as much to bucket elevators.
Interestingly, bucket elevators present a significant risk even when they transport products with minimal dust. Contrary to what might be expected, pure dust can sometimes be too dense to ignite or only cause a weak explosion because the concentration is above the upper explosion limit. This was observed in experiments in England with elevators carrying pure starch or flour, where ignitions were difficult to achieve or resulted in weak explosions. Follow-up studies in Germany confirmed these findings, showing that the most violent explosions occurred when elevators were started without any product, and the dust accumulated on the walls was dislodged by vibrations.
ATEX Zoning in Bucket Elevators
This leads to considerations for ATEX zoning, a European system for classifying areas where an explosive atmosphere may occur. Inside a bucket elevator, it’s often designated as zone 20, indicating a constant risk of explosive atmospheres, similar to the risk level inside a dust filter. However, if the elevator transports coarser products with very few fines or relatively wet products, a less dangerous zone 21 classification might apply.
For instance, in sugar refineries, the initial bucket elevator transporting freshly dried sugar, which still retains some moisture, might be considered zone 21. In contrast, elevators handling drier materials are typically classified as zone 20 due to the higher risk.
Given the persistent risk of explosive atmospheres inside bucket elevators, it’s crucial to eliminate ignition sources with a high degree of certainty. This highlights a key challenge with bucket elevators compared to other transport systems, where simply limiting the speed to one meter per second can significantly reduce the risk of ignition. The need to manage ignition sources carefully makes safety measures in bucket elevators particularly critical.
Considerations When Reducing Speed
Reducing the speed of a bucket elevator to one meter per second can prevent metal components from generating enough kinetic energy to create hotspots or glowing particles, effectively reducing the risk of ignition. However, setting the speed of a traditional bucket elevator below this threshold can interfere with its operation, as the product might not be ejected properly at the top and instead fall back down the elevator, rendering it ineffective.
Bucket elevators commonly operate at minimum speeds of 3 to 4 meters per second to ensure proper functioning, but these higher speeds increase the risk of generating ignition sources. Several factors can contribute to this risk, including misalignment, loose buckets, failed ball bearings, and belt slippage.
The dynamic nature of these components means they can quickly generate heat, posing a significant risk in an environment classified as zone 20, where the presence of an explosive atmosphere is considered constant. Under ATEX regulations, measures must be taken to prevent ignition, accounting for the possibility of multiple simultaneous failures without leading to ignition, which is a challenging standard to meet.
For example, to mitigate the risk of a bearing overheating, temperature monitoring can be implemented. However, this system must be fail-proof, ensuring that even if both the bearing and the temperature monitoring system fail, an ignition source is not produced. Beyond the mechanical aspects of the elevator, external ignition sources, such as smoldering particles from biomass and organic materials, must also be considered. These materials can easily ignite under certain conditions, and processes like drying and milling can produce hot particles capable of sparking fires.
Therefore, even with internal ignition sources controlled, the challenge remains in preventing external hot particles from entering the elevator. These particles can find ample fuel in the elevator’s boot and, propelled by the fast-moving buckets, create the perfect conditions for a fire to ignite and spread rapidly throughout the elevator, acting like a chimney. This combination of factors can lead to a dust explosion, an event that occurs with alarming frequency.
Other Methods of Prevention and Protection
Limiting the tip speed of a bucket elevator to one meter per second might seem like a viable safety measure, but it’s not practical for traditional elevators. This speed limit is primarily aimed at preventing metal-to-metal contact from generating excessive heat. However, when dealing with materials that have high friction coefficients, this limit becomes ineffective, as friction can cause temperatures to rise more rapidly. In bucket elevators, belts are usually made from anti-static, flame-retardant materials rather than metal. Yet, when these belts misalign at high speeds, they can quickly generate hot spots due to friction with the elevator casing.
Despite some companies claiming to have effective misalignment detection systems that are well-maintained and regularly tested, physical evidence often suggests otherwise. One clear sign of misalignment is the presence of circular markings on the exterior of the elevator at the head or the foot, indicating that the belt has rubbed against the casing, generating enough heat to discolor the paint. This is a significant indicator that the misalignment detection system is not functioning correctly.
For any organization operating bucket elevators, it’s advisable to regularly inspect the head and foot for such markings. Discovering these signs should prompt immediate action to check and adjust the misalignment detection system. After addressing the issue, repainting the affected area can help monitor for new misalignments. This makes it easier to identify problems early, rather than dismissing them as old, unchanged marks. This approach serves as a practical tip for ensuring the safety and proper maintenance of bucket elevators.
Installing temperature monitoring on bearings, especially in food and feed processing facilities, is also essential for safety. Bearings are often located on the exterior of machinery, where cleanliness may not always be optimal. Since bearings are prone to failure over time, they can quickly overheat, posing a risk if they’re covered in dust, which can shorten their lifespan. Keeping bearings clean while monitoring their temperature is critical for preventing overheating.
Slip detection is another important safety feature. Overloading or blockages can cause the belt to slip on the pulley, generating significant heat through friction. Additionally, installing misalignment detection helps prevent the belt from rubbing against the casing, which can lead to overheating and potential ignition sources. An anti-runback brake is also crucial, as the significant weight difference between loaded upward-moving buckets and empty downward-moving ones could cause the belt to rapidly reverse if a pulley fails, potentially leading to dangerous speeds and ignition risks.
The choice of belt material is vital. Given the high speeds at which bucket elevators operate, static electricity can become a concern. Using insulating materials, like simple rubber belts, could turn the elevator into a large static generator, which is hazardous in combustible atmospheres. Therefore, belts should be made from anti-static materials to minimize electrostatic discharges and also be flame retardant to prevent them from fueling a fire in case of severe friction.
These measures significantly enhance the safety of bucket elevators. However, even with diligent maintenance and these precautions, risks remain. Preventive measures cannot completely guard against external ignition sources entering the elevator. Malfunctions in upstream equipment, hot work activities, self-heating of biomass, and other factors can introduce smoldering materials into the elevator. Such materials can accumulate at the bottom, where fast-moving buckets can exacerbate a fire. Additionally, foreign objects like metal rods entering the elevator can create ignition sources, despite all implemented safety measures.
Thus, while preventive steps are crucial, they cannot entirely eliminate the risk of ignition sources. Protection against the consequences of a potential explosion is necessary, highlighting the complex challenge of ensuring bucket elevator safety.
What Protection Measures Are Typically Used for Bucket Elevators?
To safeguard bucket elevators in industrial settings, vent panels are installed at strategic points. These panels release flames and pressure from an explosion, maintaining pressure within safe limits. The guidance for the placement of these vents, based on the explosion potential of the material (KST value) and the maximum pressure the elevator can withstand, is detailed in a technical report that offers step-by-step instructions for designing elevator protection.
However, since most bucket elevators are located indoors, using vent panels within such environments poses significant risks. It’s common to find vent panels installed without any safety precautions, which is a concern, particularly in places like the food and feed industry where dust accumulation is frequent. Releasing a fireball indoors through venting can be extremely dangerous.
An alternative to traditional venting is the use of flameless vents, which should be approached with caution due to their limited efficiency in certain conditions. Even with flameless venting, maintaining safe distances for people and equipment is necessary because, although the flame may be extinguished by the vent’s metal mesh, a loud noise and hot gases can still be emitted.
Explosion isolation is crucial alongside venting. For dust filters, isolation might be achieved with rotary valves or non-return valves, but such solutions are impractical for bucket elevators due to the volume of material they handle. Therefore, explosion suppression systems, or high rate discharge (HRD) systems, become a viable alternative. These systems detect an explosion’s onset and release an extinguishing agent, like sodium bicarbonate, at high speed to suppress the explosion. Sodium bicarbonate is particularly favored for its food safety and effectiveness as a fire extinguishing agent.
These suppression systems also offer the advantage of serving as an isolation mechanism through the creation of a chemical barrier. This barrier prevents the flame from passing through the elevator’s inlet and outlet, effectively stopping the explosion. Installing these systems at strategic locations, such as the elevator head and boot, helps suppress and isolate explosions, preventing them from traveling through the elevator legs, where they could cause significant damage due to pressure build-up.
In some cases, chemical barriers are used alongside venting to isolate the elevator further, but this requires careful consideration to avoid the extinguishing powder being dispersed by the air movement caused by venting. Consulting with experts and potentially adding extra suppression units can ensure the effective isolation of connected systems and prevent flame propagation.
How Are Protection Options Evaluated?
Assuring the safety of a bucket elevator begins with assessing its structural strength. For instance, if the elevator is constructed using only 1.5mm thick sheet metal, its ability to withstand pressure is severely limited. In such cases, the pressure generated by an explosion could easily exceed the elevator’s structural capacity, rendering further protective measures ineffective. It’s important to calculate the maximum pressure the elevator can tolerate, but the foundational concern is whether the elevator itself is robust enough to support any explosion protection strategy.
The second key point is the elevator’s location—whether it’s situated indoors or outdoors. This determines the feasibility of using venting systems to release explosion pressure and flames safely. However, even for elevators located outside, releasing large fireballs into the surrounding area is undesirable due to the potential risk and damage. Therefore, regardless of the elevator’s location, explosion suppression systems are often recommended as the most appropriate safety measure. These systems mitigate the risk of damage by controlling and containing the effects of an explosion within the elevator, avoiding the challenges associated with venting, especially in locations where it’s not practical or safe to do so.
Conclusion
In the planning stage of a new facility or installation, consultants often advise against using standard bucket elevators due to safety concerns. Instead, safer alternatives like “Z elevators” are recommended. These operate at slower speeds with rotating buckets that tip over to discharge material, minimizing mechanical ignition sources and dust production. Another option is disc conveyors, which nearly eliminate the possibility of dust cloud formation due to their design with minimal free space.
However, bucket elevators may still be necessary in situations requiring the transport of large volumes of material or where they are already installed. In these cases, understanding and managing the associated risks is crucial. This includes implementing protection systems, like explosion suppression, and ensuring that personnel have the training and insights they need to do their jobs safely.
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Michel Vandeweyer directly:
Website: https://www.isma.be/
LinkedIn: https://www.linkedin.com/in/michel-vandeweyer-0200071a0/
Email: [email protected]
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Resources mentioned
The resources mentioned in this episode are listed below.
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Companies
ISMA
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DSS260: Protection From Explosion Risks in Bucket Elevators with Michel Vandeweyer
