DSS255: Typical hazards in grain milling with Matt Williamson

In today’s episode of the Dust Safety Science podcast, we’re talking about typical hazards in grain milling, sifting, and sorting applications. We’re doing that with Matt Williamson, Director of Engineering at ADF Engineering, based just outside of Dayton, Ohio.

We received a question from a man working at a rice milling site in Malaysia, asking about the risks of combustible dust hazards specific to rice milling. This is different from other grains like soy or wheat. From our location in Canada, it’s challenging to directly assess the hazards in Malaysia. However, we provided him with information and connected him with local experts in Malaysia who could conduct a hazard assessment on site.

This interaction led to the idea of sharing information about such hazards and the assessment process more broadly. We decided to discuss this topic on our podcast, planning to split it into two episodes. The first episode will focus on the common hazards in grain milling, sifting, and sorting. We’ll talk about issues like smoldering, material heating at the site, and necessary precautions. The second episode will delve into the hazard analysis and assessment process for dust hazards in more detail, as it’s a subject with a lot to cover. This way, we aim to provide practical and helpful information on this important safety topic.

What Does a Grain Milling Application Look Like?

Matt explained that grain mills, regardless of the grain type (rice, soybeans, wheat, rye), mainly come in two varieties: dry mills and wet mills. Wet mills are more complex as they separate grains into different components. Dry mills are simpler but have a higher risk of dust hazards.

In a typical mill, one of the most hazardous areas is the grain unloading section. Grains are unloaded from trucks or rail cars into a pit. From this pit, conveyors (such as drag or screw conveyors) transport the grains to an elevator. This elevator moves the grains to further conveyors that direct them to silos or dryers. Grain dryers are a significant fire risk area, often being direct-fired with propane, posing an ignition risk.

Unloaded grains are usually wet. After drying, they pass through a cleaner to remove debris like leaves or rodents. Then, they are transported back to an elevator and into silos for storage. These silos, typically large metal bins or concrete structures, present another risk area. In concrete silos, particularly, the interconnected structure (honeycomb pattern) makes it hard to isolate one silo from another in case of fire or smoldering materials. Shared walls and vents lead to a risk of chain reactions if an explosion occurs.

After storage, the grains are moved, usually via a bucket elevator, to a dehulling operation. This entire process, from unloading to storage, involves various stages where safety and hazard management are essential due to the risks of fire, explosion, and dust hazards.

In grain milling, including rice and soybeans, the process often involves removing large fibrous components like pods and hulls. These are typically transported by a pneumatic system to a separate grinder, which grinds them for use in animal feed. Ground hulls and pods are prone to smoldering and fires, particularly when wet.

After dehulling, the process varies depending on the product. For oil extraction, the grains may go through a pre-conditioner and then into an expelling press or a chemical extractor to remove the oil. The remaining pulp, or the material not used for oil, is then processed in a hammer mill. This milling reduces the grain to a finer size, after which it is screened. Larger particles are recycled back, while finer particles move forward in the process.

The resulting material, often fine and dust-like, is stored again, posing risks of smoldering and fires. The next step is screening for packaging, which may include passing through a magnet or metal detector. The final stage, loadout, is a major source of dust. Here, the ground material is dumped into bulk trucks or sometimes railcars. Without proper dust collection measures, such as a hood over the truck, this stage can be particularly dusty and challenging.

Throughout the entire grain milling process, from unloading to loadout, dust collection systems are critical. These systems capture dust from various points like elevators, conveyors, and mills. Large dust collectors, especially at the loadout stage, are essential. They may be connected to hoods over truck loadouts or around spouts. These dust collectors are crucial for managing combustible dust hazards and must be properly designed from the start. Retrofitting older facilities to comply with combustible dust safety standards can be challenging.

Are the Layouts Different?

Matt has observed mills in various, often unusual locations. He has seen facilities where multiple grains are processed in a single large room filled with mills. This setup is notably hazardous due to the increased risk of dust accumulation and potential explosions. Another notable location he has encountered is mills situated directly under the silos in the skirt space. This confined area beneath a grain silo is particularly prone to combustible dust explosions, making it a risky placement for a mill. Additionally, Matt has seen mills both outside and inside buildings, indicating the wide range of environments where milling operations can take place.

What is the Process With Wet Mills?

In a wet mill, typically used for corn, the process involves separating the corn into multiple components. The outer fiber casing of the corn is directed to one part of the process, while the germ, rich in oil, goes to another area for oil extraction. Additionally, the glutinous material and the endosperm, which contains starch, are separated. The starch may be converted into ethanol or used to make products like high fructose corn syrup. This process allows for the creation of various products from corn.

The term ‘wet mill’ indicates that the material is processed while wet, reducing dust production during the milling stage. However, at the end of the process, the various materials obtained, such as germ, starch, gluten (like corn gluten meal), and fiber or fiber pellets, are dry. Each of these materials has different characteristics regarding combustibility.

Therefore, while wet milling initially mitigates dust hazards due to the moisture, it introduces complexity later in the process. As the final products are dry, they present various combustibility risks that need to be managed effectively.

What About Smouldering?

When discussing smouldering in grain storage, Matt explained that the presence of even a small amount of moisture can be problematic. Grains are organic materials that can degrade over time. This degradation, especially in slightly moist and finely ground grains, can lead to heat generation, often occurring deep within a stored pile.

In cases where grains are stored in flat storage areas on concrete, it’s easier to detect smouldering as smoke and signs of decomposition are more visible. However, in silos, particularly concrete silos, identifying smouldering becomes more challenging. The smouldering could be happening deep within the pile, making it difficult to see smoke or detect hot spots visually. This hidden nature of the heat generation in silos poses a significant risk, as it can go unnoticed until it becomes a more serious problem.

What Other Fire Issues Come Up at Grain Mills?

In grain milling operations, issues often arise during or immediately after unloading grains into silos. These challenges typically occur with materials that are still wet or have not been dried properly. Additionally, these materials might contain a high amount of fine particles, which haven’t been sifted out yet. These fine particles can have a very low ignition energy, making them highly susceptible to ignition from something as minor as a brush static discharge.

Such static buildup during the loading of silos can lead to fires igniting within the grain pile. As more material is unloaded from a truck and added to the pile, the risk of an explosion increases. This scenario has been observed in multiple instances.

Another issue arises when materials are stored in silos not designed for them. For example, when a company tests a new formulation or stores a different type of material in a silo, unexpected problems can occur. These might include the buildup of static discharge leading to a fire within the silo. Once a fire or smolder starts, or in the event of an explosion that blows out the explosion panels at the top of the bin, it becomes extremely difficult to remove the material from the silo.

In such situations, it is common for professionals to be called in after the incident. They often find that the only solution is to physically cut open the bottom of the bin or create large openings to remove the material and stop the smoldering or fire. This highlights the importance of proper material handling and storage in grain milling operations to prevent such hazardous situations.

How Does Plugging Occur?

In grain milling operations, bucket elevators often experience plugging. This problem often involves the discharge area of these elevators or the conveyor systems that follow them. Sometimes, these conveyors can become jammed or blocked, leading to a buildup of material at their ends. Over time, this buildup can grow significant enough to back up and reach the elevator, causing a blockage in the entire system. This scenario is a common occurrence with bucket elevators and highlights the need for regular monitoring and maintenance to prevent such blockages and ensure smooth operation.

What Are Other Common Issues?

In the design of grain milling facilities, particularly concerning silo access, there are often significant oversights. Many of these facilities do not adequately consider the need for easy access to the tops of silos, especially for emergency response vehicles. This lack of accessibility can create dangerous situations in emergencies, such as fires.

Concrete silos, for instance, are often designed with limited or no access around them. In case of a fire within these tall silos, the only feasible approach for firefighting efforts is often from above. This positioning can be extremely hazardous, as there is a risk of explosion in such scenarios.

The ideal solution is to incorporate proper access points in the original design of these facilities. This means including built-in ladders or other safe access methods that allow emergency responders to reach the top of the equipment without the need to stand on potentially dangerous areas. Proper upfront design can significantly enhance safety and efficiency in responding to emergencies at these facilities.

The 1977 Westwego, Louisiana incident serves as a stark example of the dangers associated with grain storage facilities. In this case, there was an unexpected explosion in a group of concrete silos, even though the facility was not operational that day. One of the silos collapsed, destroying an occupied building and resulting in over 30 fatalities. As a consequence of this tragic event, current regulations under NFPA 61 prohibit having occupied buildings within 50 feet of a concrete silo. However, there’s an ongoing debate about whether this distance is sufficient for safety.

One common issue is the placement of dust collectors in proximity to silos. It’s not unusual to find dust collectors either adjacent to or, even more problematically, mounted on top of silos. In the event of an explosion or fire in the dust collector, especially when it’s on top of a tall silo, the situation becomes extremely hazardous. An explosion can cause the dust collector to release a fireball over the silos, where grain residues from maintenance and cleaning activities often accumulate, creating a highly flammable environment.

This placement of dust collectors poses a significant risk because it essentially directs intense heat and flames towards abundant fuel sources. Additionally, such a location makes it almost impossible for emergency responders to safely access and address the fire. This highlights the need for more careful consideration of equipment placement and safety protocols in the design and operation of grain storage facilities.

Conclusion

Remember, the key to safety in grain milling is not just in understanding the risks but in taking proactive steps to manage and mitigate these risks effectively. Stay tuned for more informative discussions and insights in the next episode, where Matt goes into more detail on the subject.

If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Matt Williamson directly:

Email: [email protected]
LinkedIn: https://www.linkedin.com/in/matthew-williamson-760111215/
Website: https://www.adfengineering.com/

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.