Fugitive material causes a number of problems in industrial settings, from liability risks to profit loss, and everything in between. Since material handling equipment such as belt conveyors can be common culprits in generating fugitive material, plant managers and operators are frequently looking for ways to minimize the potential for material to escape from such systems.
The following covers some of the most helpful tools that operators and plant managers can employ to prevent and manage fugitive material in bulk solids handling systems.
Why Fugitive Material is a Problem
Fugitive material, or material that escapes the handling line and infiltrates the surrounding environment, has a number of undesirable ramifications. Most notably, fugitive material is often a safety risk to workers, whether that’s because the material is caustic, creates slip and trip hazards, or presents other risks to employees or the surrounding environment. With safety the first priority in any operation, fugitive material that exposes workers to any sort of risk is simply not acceptable.
In addition to potential liabilities, fugitive material also incurs several direct and indirect costs that can significantly impact the bottom line. Costs typically come in the form of material or product loss, higher labor requirements for housekeeping and maintenance, reduced process efficiency and equipment performance, and even a shortened life for key production equipment.
In an age of increasing safety and environmental awareness, as well as maximized production efficiency, it’s no wonder why the prevention of fugitive material has become such a focal point in modern industry.
Preventing Fugitive Material
While fugitive material can be caused by a wide range of operational- and material-specific factors, there are several steps plant managers and operators can take to reduce the potential for their handling system to generate fugitive material, no matter what type of operation they’re working with.
Ensure Conveyor Belts are Properly Trained and Tensioned
Proper conveyor belt training is crucial to preventing material from unintentionally exiting the conveyor. Also referred to as tracking or belt alignment, belt training refers to how the belt rides on idlers and over pulleys, with the belt running centrally being ideal.
A mistracking belt will ride unevenly over pulleys and idlers, creating the opportunity for material to fall off the belt’s edge (not to mention other issues).
Since mistracking often occurs through wear and tear, conveyor belts should routinely be realigned as part of a preventative maintenance program. Off-center loading, as well as buildup of material, can also cause a conveyor belt to mistrack and should be kept in check.
Proper belt tension also plays an important role in minimizing fugitive material. Belts that have been tensioned too tightly cannot form the trough that contains material, instead allowing it to fall off the sides. Belts not tensioned enough are also problematic, as they allow for the belt to sag between idlers, again causing material to fall off the belt.
Belt conveyors, bucket elevators, and other types of handling equipment are designed around a specific handling capacity. When that capacity is increased beyond the original design, whether intentionally or unintentionally, it often leads to fugitive material in the surrounding area. This is commonly seen in older facilities that are pushing the limits of their existing capacity, as well as in operations where bottlenecks create surges in material feed.
The Conveyor Equipment Manufacturer’s Association (CEMA), which sets industry standards, recommends belt loading not exceed 75%. Accommodating additional capacity may mean investing in another conveyor system to handle the extra throughput, or using a variable frequency drive (VFD) to adjust to fluctuations in the process.
Enlist the Help of Belt Cleaners
Belt cleaners are an essential tool in minimizing fugitive material in the form of carryback. Carryback occurs when material does not discharge from the belt conveyor, and instead sticks to the return side. This often results in the material sloughing off along the length of the conveyor, causing the material to fall on whatever lies below. Carryback also encourages buildup on idlers and pulleys, which can be detrimental to the longevity of the conveyor.
Belt cleaners can be incorporated into either the head or tail pulley (or both) to remove any material that may have stuck to the belt, preventing it from being carried any further. Belt cleaners are fixed in position to ride against the conveyor belt, displacing any material that did not leave the belt and ideally, directing it to the discharge chute. If space does not allow, a separate collection chute can be incorporated to capture and direct the flow of material as needed. Adequate belt cleaner tension against the belt affects belt cleaner performance and should be regularly monitored.
Operations working with especially sticky or high-moisture materials can take a more aggressive approach and implement dual belt cleaners, which utilize a primary and secondary cleaner to remove adhered material.
When working with extremely flow-challenged materials, such as copper concentrates, an additional option is to incorporate a return belt cleanup under the discharge head section. This consists of a drag chain conveyor, which captures any carryback material and feeds it back into the material flow stream.
Belt turnovers are also an option in such settings; the turnover flips the belt over the return pulleys and re-flips it at the tail pulley in order to prevent any stuck-on material from falling off on idlers.
Invest in a Skirtboard Assembly
The addition of a skirtboard assembly to a belt conveyor is also useful in preventing material from escaping the system. Skirtboard assemblies typically consist of fabricated steel side plates and skirtboard rubber, which acts as a rubber sealing system.
Skirtboards are positioned at load points with the skirtboard rubber riding against the belting to prevent material from falling off the conveyor edge. While skirtboards are most often installed at the loading area, they can also be installed along the full length of a conveyor if desirable.
Numerous customizations exist for improving skirtboard performance in each setting, including different designs for the side plates and skirtboard rubber, various rubber compound options, and even hard liners to keep material away from skirtboard rubber.
Since skirtboard rubber can wear and become less effective over time, operations employing them should routinely inspect and adjust or replace the rubber to continue maintaining material containment; skirtboard rubber requires routine adjustment to ensure continued proper contact with the belt as the rubber wears.
Employ Dust Pick-Off Points Where Applicable
When working with a dusty material, managing fugitive material becomes even more challenging. One way to prevent dust from escaping the system is to use integrated dust pick-off points.
Pick-off points are exhaust ports that can be incorporated into a discharge hood or skirtboard assembly to remove dust as it is generated during loading. In the case of bucket elevators, which are already enclosed, dust pick-off points can be added to further prevent the potential for fugitive dust to escape.
Modify the Loading Area
The loading area is the most common place from which fugitive material originates. In addition to skirtboards and pick-off points, another way to prevent material from escaping at the loading zone is to modify the loading area in some way, either by adjusting idler spacing, selecting different idlers, or utilizing a slider bed or impact cradle.
When idlers are spaced too far apart at the loading zone, the impact of loading can cause the belt to sag between idlers. Even with skirtboards in place, the sag may be enough to allow material to escape the belt below skirtboards. By decreasing idler spacing, operators can prevent the belt from sagging.
Steel idlers can also be replaced with impact idlers at the loading zone to reduce material spillage. Impact idlers, which can be troughed or angled, are made up of rubber rolls, which better absorb the impact of loading.
Slider Beds and Impact Cradles
Both slider beds and impact cradles can be used in place of loading area idlers. These types of systems provide a solid surface on which the belt rides. Unlike with idlers, there are no gaps in the loading surface, so the material is fully supported, preventing the potential for it to spill off the conveyor. Slider beds provide an excellent option for containing material in lighter-duty applications, while impact cradles are best for settings with high-impact loading.
Regularly Inspect for Wear or Anomalies
A well-designed belt conveyor or bucket elevator generally does not allow for fugitive material to occur. As equipment ages and wears, however, fugitive material can become more common; degraded components and other age-related factors can increase the number of places where it’s possible for material to get through.
To prevent this from happening, it’s important to keep up on maintenance and regularly inspect equipment for any signs of wear, auditory or visual abnormalities, and the like. Identifying and resolving these issues early will prevent more extensive shutdowns, repairs, and cleanup in the long run. It will also help to keep the system running at peak performance.
Operations working with corrosive or abrasive materials must be especially vigilant about routinely inspecting equipment, particularly at high-wear areas, as these types of materials accelerate wear.
Keep Chutes Clear and in Proper Alignment
Fugitive material is often caused by some type of malfunction or blockage in the feeder, chute, or hopper. Chutes that see a high production capacity or uneven loading conditions may require routine realignment to ensure they align centrally with belts and won’t inadvertently direct material off the side of conveyors.
Operations working with materials that exhibit challenging flow characteristics may require a more watchful eye or a plugged-chute switch to ensure that hoppers and chutes do not become blocked or that material flow does not become obstructed or start ratholing.
Worn or damaged chutes and hoppers also create a high probability of material escaping the system and should therefore be repaired and replaced, as necessary.
Consider Material Behavior
As with chutes and hoppers, materials that are not inherently flowable tend to behave in a way that can encourage the potential for fugitive material; material building up at points in the system, failing to discharge from conveyors, or otherwise, can wreak havoc on a system not prepared for it.
Engineers must consider the material’s behavior during the design of the system. Sometimes, however, feed conditions or operational parameters change, causing a once-seamless operation to be troubled by constant shutdowns and cleanouts.
In such cases, modifying transfer points (or eliminating them, where possible) can help to resolve issues and bring the system back to homeostasis.
Alternative Ways to Manage Fugitive Material
In dire cases, some fugitive material may be unavoidable for one reason or another. In such settings, plant managers should work to minimize the effects of fugitive material. This might mean regularly monitoring and cleaning up to prevent fugitive material buildup from becoming excessive, as well as implementing aids to help contain any falling material, such as drip pans or deck plates.
Drip pans “catch” carryback material from the return belt and are often used over roadways and buildings, while deck plates are mounted onto the top of the conveyor frame, blowing carrying idlers to prevent any spilled material from falling onto the top of the return belt.
When fugitive material is occurring because of a dusty material, and dust management is especially critical, it may be worth considering a dust conditioning or agglomeration (pelletizing) system.
The inclusion of a dust conditioning or pelletizing system works by mixing the dusty material with a liquid binder or water to de-dust or condition the material, or in the case of a pelletizing system, granulate it. This approach still requires a well-designed handling system but is typically a highly effective technique in mitigating dust issues, as it prevents them from the start.
Fugitive material is a costly and often risky problem in plants handling bulk solids, whether losses come in the form of lost product or liability issues. Preventing fugitive material is therefore a critical aspect of ensuring an operation remains safe and efficient.
In plants struggling with fugitive material originating from the handling system, there are a number of ways to aid in preventing material from escaping the system. This starts with ensuring belts are properly trained and tensioned, and includes the use of helpful components such as belt cleaners and skirtboards. It also means regularly inspecting and repairing system components as necessary and may go as far as implementing a conditioning or agglomeration system to prevent dust from the start.
Plant managers struggling with a fugitive material problem in their handling system may also consider enlisting the help of a qualified expert to audit their process and equipment. Audits are a valuable way to quickly identify the source of an issue and expedite its resolution.
Dan Baxter is a material handling sales engineer at FEECO International. Baxter has a decade of experience in performing engineering analysis and design of bulk material handling equipment and systems. For more information, visit feeco.com.