Bulk material handlers depend on properly engineered conveyor transfer points for steady and efficient production. Without them, as tons of material drop with intense impact from receiving chutes onto conveyor belts, (fugitive) material can collect on floors and equipment, creating numerous operational problems.
Fugitive material often poses health and safety hazards in addition to requiring personnel to halt operations for costly housekeeping and maintenance. Achieving material containment at conveyor transfer points thus becomes mission-critical for plant operators.
Well-designed conveyor transfer points have become increasingly important at facilities that generate silica dust from crystalline silica, a common mineral in materials such as quartz, sand, clay, gravel, cement, and asphalt.
Grinding, crushing, and transfer of materials containing crystalline silica releases airborne silica dust. In bulk material handling, if an airborne dust particle is larger than 60 μm, it will be visible and settle swiftly. At 30 μm and smaller, it will not be visible unless reflecting light and will take more time to drift down.
Silica dust particles are at least 100 times smaller than typical beach sand. When less than 10 μm, they will remain airborne for as long as several hours until gravity and electrostatic forces help them settle onto surfaces. At these sizes, they also can easily enter the lungs, where they are even more toxic than coal dust.
The silica particles are so noxious that if 5.6 grams (the weight of a quarter) of respirable silica dust were distributed through the air in a large football stadium, a person breathing the air would be considered overexposed.
Prolonged exposure to silica dust is linked to major health problems such as silicosis (a debilitating lung disease), lung cancer, kidney disease, and chronic obstructive pulmonary disease. Airborne silica dust also can result in potential combustion, reduced visibility at the plant, and slime that causes slips and falls.
For all of these reasons, OSHA and MSHA have developed regulations pertaining to silica dust safety. A lack of compliance can lead to silica-related OSHA and MSHA fines.
At a bulk material handling facility, conveyor transfer point issues such as damaged idlers, improperly sealed transfer chutes, and uneven conveyor belt loading can produce loose material and fugitive dust that remain constant liabilities.
Conveyor Transfer Point Design Considerations
Proficient conveyor transfer point engineers will consider every item that influences system performance and the optimal containment of material and dust, including silica dust. Conveyor transfer solutions that adapt to a plant’s specific requirements will include central factors such as the following.
Load zone location
The load zone should be set up past the tail pulley, where the belt is fully troughed for central belt loading of the bulk material. Ideally, the load zone should be on a horizontal plane. Materials loaded vertically onto an incline conveyor are subject to negative control features, such as material rollback and disturbances created when loading against the direction of belt travel.
Chute and skirtboard design, construction, and placement
The chute is probably the most important feature at the transfer point. It should consistently transfer bulk material with maximum flow with the delivery and minimize pluggage and material degradation, particularly for friable material.
The drop height of the chute is vital as well. If the chute is too high, the material’s impact on the conveyor belt will be more damaging. If the chute is too low, it can block material from reaching the conveyor. Good design will calculate proper minimum working clearances and geometry for the materials being handled.
Because chutes place the bulk material on the conveyor belt, chutes and rubber skirting must be securely fastened to the conveyor structure to help prevent spillage and dust for the application.
The skirting assembly will have a low profile with only a few inches of clearance, allowing convenient installation and maintenance in space-restricted areas.
Skirtboards typically have an inside dimension equal to 2/3 of the conveyor belt width. They should be as short as possible but long enough to settle the material on the belt. Their design also should relieve any gaps. The skirtboard bottom should be adjustable for wear and have a flexible sealing device for preventing belt damage.
Loading of material in the direction of travel
The system design should not have bulk material being loaded from the side, which can push the belt into the conveyor frame, causing cover abrasion and edge wear. Soft loading and center loading in the direction of belt travel are critical to optimal performance, minimal wear, and the reduction of dust and spillage.
Matching load and belt speed
The load speed should be close to the belt speed as material loads from one belt onto another. This lessens cover wear, requires less horsepower for the belt, and decreases spillage/dust formation.
Even with transfer chutes that help reduce material impact on the conveyor belt, many high-volume facilities will still need one or more heavy-duty impact cradles to absorb the force of the falling stream. The cradles can be equipped with rubber or urethane impact bars to minimize belt friction as well.
Support beams in the cradle center will be set slightly below the receiving belt’s line of travel. This helps the belt avoid prolonged friction when running while empty. At the same time, it can absorb hard impacts during loading while maintaining a tight belt seal.
Within the settling zone, which follows the impact cradle in the conveyor chute box, slider cradles can then create a troughed belt to center the material and quickly reduce disruption, which helps dust to settle.
The simplest discharge from a conveyor belt will let the bulk material pass over a terminal pulley. In most installations, the troughed belt goes through a transition section onto a flat pulley. To prevent the material in the trough from spilling over a flat belt’s edges, the time to pass through should not exceed one second.
The constant stream of material crashing on the receiving belt’s impact point will often make the transfer point turbulent. Slowing the airflow in the skirted area allows dust to settle on the material path. Proper chute design can be very effective in minimizing the problem at the source by controlling induced air, entrained air, and displaced air.
To contain the mixture of air and agitated material, the conveyor belt must be stable and well-supported for properly sealing components. Without a stable belt line, the belt will sag between idlers and sealing components will not prevent air and fines from escaping through gaps, resulting in extra spillage and dust.
Conveyor Transfer Point Solutions: MaxZone® & MaxZone® Plus
Safety- and production-minded bulk material handlers will apply an integrated approach to conveyor transfer points with specific technologies for all dust challenges, including the distinctive ones posed by silica dust.
Conveyor transfer solutions such as the MaxZone Modular Skirtboard and Belt Support System address the vital facets of conveyor transfer points and material containment. With the dust control items for increasing plant compliance and safety and reducing housekeeping time and material loss, the system includes key components such as:
- XN Externally Adjusted Internal Wearliner that seals the conveyor system and extends the life and production of transfer chutes. All maintenance also can be achieved from the load zone exterior – no confined-space access is required.
- XN Liner Sealing System with clamps and seals on the sides of the load zone for containing respirable dust and eliminating spillage
- Warrior Impact Bed with a stiff, rigid frame and soft rubber bars to cushion and absorb impact on the conveyor belt during loading
- Warrior Roll & Guide Support Bed with low-friction slider bars on the wing section and rollers in the center to create a seamless seal against load zone spillage and dust
- Simple Slide Idler to allow roller frames to slide into place close together for greater belt support without having to remove adjacent idlers or adjust the belt. These are also provided with impact rolls for the load zones, where structural interferences make serviceability difficult or impossible using other equipment such as impact beds.
- Training Idler for instant correction of belt misalignment without special structural modifications
- Unique Peaked-Hood Design for greater containment of escaped material and less settling of dust on flat surfaces
- Tailseal with skirting and strip rubber for a tight seal at the rear of the chute work to prevent spillage or dust created by material rollback
- Dust-Tight Inspection Doors with easy access to key areas for simple maintenance or inspection by one person
- Primary and Secondary Belt Cleaners to conclusively resolve carryback issues
The system reinforces employee safety as well by eliminating exposure to rolling components that is otherwise required for housekeeping and maintenance.
MaxZone components require no welding. Bolted connections also add to structural rigidity and ease in modification should production rates change, material characteristics change, or airflow conditions require modification.
Transfer point solutions will further include engineered transfer chutes for consistently centered belt loading that reduces pluggage, spillage, and fugitive dust while preventing belt mistracking and enhancing material flow. Benetech’s patented InteliFlo® load chute with the J-Glide® discharge applies a round chute design and adjustable horizontal loading for proper center loading.
In cases where time or budget constraints preclude a chute replacement, the patented MaxZone Plus System acts as a modified load zone chute for center loading that keeps material flowing and reduces transfer point spillage and dust.
The system includes a deflector and side kicker plates that can both be adjusted to correctly center-load material for a smoother transition onto the moving belt. Removable 6” side panels further allow height adjustment according to load zone requirements. The system also can be adapted to existing XN Wearliners and retrofitted to other material containment systems.
Because the MaxZone Plus System often eliminates the need for an engineered transfer point chute, bulk material handlers can reduce capital expenses by up to $200,000 or more. The system’s design can save a facility tens of thousands in clean-up costs as well.
Benetech: Your Ally in Bulk Material Handling
Benetech is a team of seasoned bulk material handling engineers with an up-close understanding of what can interfere with your daily goals. We support operators with conveyor transfer points for safer, more productive and compliant facilities, including those that need to control silica dust. To discover more about our conveyor transfer solutions for your operation, contact us at (630) 844-1300 to speak with a specialist.