For effective mechanical conveying of bulk solids in the chemical process industries (CPI), equipment should be tailored to the specific needs of the solids-handling application
The sheer diversity of bulk-solid materials encountered across the chemical process industries (CPI), including agricultural chemicals, catalysts, adsorbents and many others, requires an equally diverse approach to solids-handling systems. To meet these requirements, equipment manufacturers offer an extensive array of optional components and equipment customizations that allow chemical producers to design bulk-solids systems that are not only safe, reliable and efficient, but also uniquely tailored to the needs of the particular process (Figure 1). This article discusses the essential considerations for mechanical conveying of bulk solids and how those considerations translate into equipment modifications.
FIGURE 1. This operation involves conveyors with gallery enclosures and ventilation/dust-control ducting
Whether handling fine chemicals or intermediate materials, a mechanical conveying system in the CPI must be designed to prioritize safety, efficiency and product integrity. Here are some considerations that are essential during the design stage.
Safety considerations
Safety should always be the number one priority in designing bulk-handling equipment for any industry, but it is perhaps even more important when it comes to mechanical conveying of solid chemicals, which can bring additional challenges.
In addition to creating potential slip and trip hazards if not properly contained, many chemical products present additional risks, such as combustion or explosion potential, air-quality hazards, skin irritants and more. As such, safety considerations for mechanical conveyors must go beyond the standard guarding that protects employees from pinch points and moving components. Fortunately, many options are available to tailor systems to the specific risks associated with the plant, making each system as safe as possible.
Explosion protection. Depending on the potential for danger, facilities working with chemical reactions or combustible dust will often need to employ explosion-protection measures. Dust management, discussed later in this article, is also vital in this effort. Explosion-protection measures are less of a concern for belt conveyors and other material-handling equipment, which are open to the atmosphere. However, fire-suppression piping and sprinklers may be required. In the case of bucket elevators, which are enclosed, special design considerations may need to be considered for safe operation regarding explosion protection.
Safety cables and shut-offs. Pull cords and emergency stops are always required for mechanical conveyors, and serve as critical tools in minimizing injury and further risks in the event of an incident.
Dust containment
While the level of dust prevention and containment required at any given facility varies significantly, all chemical producers typically want to minimize dust as much as possible. In addition to being a safety hazard, dust can represent significant profit loss in the form of unusable or lost product, particularly for producers working with valuable materials. Dust can also cut into profits by degrading production equipment, resulting in increased maintenance costs, unnecessary downtime and the need to replace failed equipment more frequently than necessary.
With a mechanical-conveying and handling system, dust can be largely mitigated by incorporating a few small, but effective measures, including the following:
Dust pick-offs. The incorporation of dust pick-off points at feed and discharge areas creates a vacuum (negative pressure), capturing fugitive dust and funneling it to a baghouse. Once in the baghouse, dust is then filtered out of the air and can be recycled.
Skirtboards. Skirtboards, which incorporate rubber strips that seal along the conveying belt, are a vital integration, especially at points where solids are being fed onto a conveyor (Figure 2). These rubber strips contain dust resulting from solid material dropping from a hopper or other feeder onto a conveyor. For extra protection against fugitive dust or material spillage, apron seals are a special type of skirtboard rubber seal. Apron seals contain an extra rubber lip, creating a two-layer rubber seal. Positioned along the edges of the conveyor at the loading area, these components seal off the area, preventing the escape of fugitive material, whether it is dust or potential spillage. Skirtboards can also be integrated along the entire length of the conveyor for added dust control.
FIGURE 2. This belt feeder includes a skirtboard
Taconite seals. Taconite seals can also be employed to prevent dust from infiltrating bearings. This is an extra seal that can be filled with grease and purged to prevent dust from contaminating the lubricant and damaging the bearing.
Motorized head pulley. A motorized head pulley is an upgrade on the traditional motor and reducer setup that can be helpful in mitigating risk from dust. With a motorized head pulley, the motor and reducer are internally contained within the head pulley of the conveyor, preventing dust infiltration.
Minimizing spillage
As with dust, fugitive material in the form of spillage must also be prevented to avoid product loss and potential safety hazards. Spillage can occur for a number of reasons both in design and operation, making a blanket solution impossible. In addition to skirtboards and apron seals at loading areas, the following can help to minimize the potential for spillage.
Belt cleaners. Belt cleaners are recommended on all conveyors to prevent material from sticking on the belt after discharge. If allowed to remain on the belt, material can build up around idlers and pulleys, degrading equipment and creating a potential safety hazard (Figure 3).
FIGURE 3. Primary belt cleaners prevent material from sticking on the belt’s surface
Proper belt tensioning. Proper belt tensioning is an often-overlooked culprit in contributing to spillage. A belt tension that is too high will prevent proper troughing, encouraging material to spill off the belt. A belt tension that is too low could allow the belt to sag between idlers, again, creating the opportunity for material to fall off the belt.
Slider bed. A slider bed can be used in lieu of idlers at the loading area. This eliminates sag between idlers, which can contribute to spillage. Reduced idler spacing, as well as an increase in trough depth (idler angle), can also be employed in this effort (Figure 4).
FIGURE 4. Slider beds can be used to eliminate sag between idlers on a belt conveyor
Feed design. The design of the feed area for a conveying system must prioritize even and precise loading/feeding of solid material. This might require the use of materials that prohibit sticking of the material, as well as considering the geometry of the feed hopper. Belt speed and feed rate must also be optimized to minimize spillage.
Deck plates and drip pans. Deck plates and drip pans do not prevent material spillage, but in cases where spillage is inevitable, they can help to keep the material contained, avoiding housekeeping issues. Deck plates are positioned on top of the conveyor framing, just under the carrying idlers, while drip pans are positioned below the return idlers at the very bottom of the conveyor.
Material compatibility
The compatibility between the materials of construction of the mechanical conveyor and the material being handled is also a critical consideration. Since many chemicals exhibit some level of corrosive quality, materials of construction must be carefully selected to suit the anticipated level of corrosion. This might mean incorporating surface treatments, such as three-coat paint systems, galvanized steel or using special materials of construction, such as 316L stainless steel. In cases of extreme corrosion, duplex stainless steel may be desirable.
It is also important to minimize areas within the conveying system where material could catch and build up or become trapped, as this could accelerate corrosive wear. In bucket elevators, for example, casings can be internally seal-welded to prevent material from becoming trapped between the vertical angles and sheet metal.
The belt material and cover compounds must also be carefully selected. This might include the incorporation of flame retardance, oil resistance, chemical resistance, heat resistance or other considerations.
Operating environment
Facilities with conveyors that operate outdoors require protection from the elements, which could otherwise not only contaminate or disperse the material, but could also cause material to stick on the belt, creating a larger issue. In this effort, weather covers or full gallery enclosures (depending on the need for access to the conveyor) are recommended.
Automation
As mechanical conveying technology continues to advance, automation and control systems are becoming increasingly valuable to chemical facilities. With the ability to track and trend data and intervene on potential issues, these systems give operators the opportunity to streamline production for a highly efficient operation.
Maintenance considerations
The potential for chemical products to degrade equipment means plant managers will need to prioritize preventive maintenance programs. As such, systems should be designed with accessibility and ease of maintenance in mind. This often translates to the incorporation of the following:
- Service platforms and walkways
- Ladders and safety cages (Figure 5)
- Access and inspection doors (for elevators only)
FIGURE 5. The photo shows a bucket elevator with service platforms, ladders and safety cages. Explosion venting (open panels) can also be seen
Concluding remarks
The diverse requirements of the chemical industry make a one-size-fits-all approach to bulk handling inefficient and fraught with potential problems. Further, the plethora of options in equipment modifications and customizations available gives producers the opportunity to tailor equipment to their exact needs, arriving at a system that is safe, reliable and efficient. For these reasons, producers should keep several key considerations in mind during the design of a handling system, including safety, dust containment, maintenance considerations and more. Those unsure of where to start should work with a qualified original equipment manufacturer (OEM) backed by proven experience in the industry.
Edited by Scott Jenkins
Acknowledgement
All photos appear courtesy of FEECO International.
Author
Dan Baxter is a material handling sales engineer at FEECO International (3913 Algoma Road, Green Bay, WI 54311; Phone: 1-920-468-1000; Email: sales@feeco.com; Website: www.feeco.com). Baxter has more than a decade of experience in performing engineering analysis and design of bulk-material handling equipment and systems. He holds a B.S. degree in mechanical engineering from the University of Wisconsin – Platteville.