By R. Todd Swinderman, P.E., Barbara A. Wheatall and Andrew D. Marti, Martin Engineering
Until recently, conveyor system design philosophies have been comfortably lodged in the style of the 1950s. Armed with information on the required capacity, space available and budget constraints, the engineer has used a state-of-the-art CAD system to design essentially the same conveyor that his or her father (or grandfather) might have designed. Little time has been spent on refinements that could make a conveyor system safer to operate and maintain. And little engineering time has been devoted to modifications that could make a conveyor system cleaner and more productive.
Rather than rely on these old-school philosophies any longer, conveyor designers are now focusing on an approach that examines all aspects of the conveyor system and challenges some of the traditional practices. This is evidenced by three recent developments in the architecture of coal handling systems that address safety concerns and combustible dust issues, improve coal flow through transfer chutes and facilitate maintenance.
Engineered-flow transfer chutes and air-supported belt conveyors were introduced into bulk material handling systems in the 1990s. Modern conveyor architecture with its dust accumulation-resistant structure is currently gaining acceptance. And the benefits of combining these cutting-edge technologies are now beginning to be fully appreciated.
Engineered-flow Transfer Chutes
In a conveyor system, the purpose of a transfer chute is to connect two pieces of equipment: a conveyor to a conveyor, a crusher to a conveyor or a conveyor to a hopper. Transfer chutes have typically been an afterthought in conveyor-system design, with the primary focus being on cost control. Although this budget-conscious approach may have been beneficial at the time of the initial capital expenditure, the lifetime cost has been excessive in systems where chutes have plugged repeatedly or allowed large amounts of airborne dust to be created and released.
Designed from material testing and flow studies, “engineered-flow” transfer chutes provide better material control, continuous flow at higher capacities and reductions in spillage and dust. By regulating the movement of the coal stream, engineered chutes improve load placement on the belt, eliminate chute blockages, reduce safety hazards and minimize maintenance costs.
Engineered-flow chutes include a “hood” discharge chute and a “spoon” receiving chute along with a flow-engineered enclosure that contains the stream of material as it leaves the head pulley until it is deposited onto the receiving belt at as close to belt speed as practicable. This reduces the impact that degrades the material and wears the belt at the same time it minimizes air expulsion that drives dust into the air (see illustration above).
Because the hood and the spoon are designed to intercept the material trajectory at a low angle of incidence, they guide the consolidated coal stream through a custom-designed drop chute until it is center-loaded on the receiving belt. Since the hood and spoon are designed with both the material specifications and the flow requirements as criteria, the chute can operate at the required rate without risk of plugs or chute blockages that will choke operations.
The second noteworthy development in conveyor system technology evolution is the air-supported conveyor. This uses a film of air rising from a troughed pan to support the belt rather than traditional rollers. These enclosed conveyors offer a number of benefits, including improved tracking, improved control of dust and spillage and reduced friction and power consumption. Enclosed conveyors are gaining in popularity as new designs make the technology cost-competitive.
Air-supported conveyors can span longer distances with less structure because of the structural strength of the plenum and pans.
An air-supported conveyor supports the belt and its cargo with a thin film of air rather than the troughing rolls used by conventional belt conveyor systems. Air is supplied by a low-pressure centrifugal fan and released through a trough-shaped pan below the conveyor belt. A series of holes drilled in the center of the pan along the length of the conveyorbetween the air-carrying chamber (plenum) and beltenables the air, supplied by the blower through the holes in the pan, to lift and support the loaded belt.
The air film eliminates the need for most idlers on the carrying side of the conveyor; conventional return idlers may be used for the return run of the belt. With no troughing idlers, budgets typically designated for replacing rolling components and maintenance labor needed to accomplish that replacement are reduced.
Compared to conventional stringer or truss conveyors, air-supported conveyors can span longer distances with less structure because of the structural strength of the plenum and pans. This reduces the capital investment required for the conveyor system.
As engineered-flow transfer chutes and air-supported conveyors were installed in power plants during the 1990s, designers began to consider other aspects of conveyor construction and to question the logic behind traditional concepts. The result has been the emergence of a new hierarchy for conveyor system design.
A New Design Hierarchy
Modern design techniquessuch as discrete element modeling (DEM) for chute design and 3D modeling for fabricationhave been used to improve conveyor system reliability, productivity and safety while reducing the total cost of ownership. To achieve cleaner, safer and more productive equipment, however, conveyor equipment manufacturers are beginning to consider a new hierarchy for design decisions.
Capacity The primary purpose of any conveyor system is to reliably deliver the necessary quantity of material to the next piece of process equipment. When a conveyor system has been shut down (whether for routine maintenance or emergency repairs) it cannot be productive. Conveyor systems that are safe, service-friendly and easy to clean spend more time transporting cargo than conveyor systems that chronically require maintenance attention.
Safety and Code Compliance Safety issues normally correspond to unsafe operating conditions, which are also detrimental to conveyor equipment. Airborne dust not only finds its way into lungs, but also into bearings. Fugitive material accumulating on walkways not only creates tripping hazards, but also causes premature wear on rollers and other moving components. A safe, service-friendly belt conveyor system experiences fewer unscheduled maintenance outages and is consequently more productive.
Dust accumulation-resistant structure sheds fugitive material.
Belt conveyor designs should meet or exceed all local codes and regulations for structural, safety and environmental requirements. In addition to minimizing respirable dust, a conveyor designed to ensure worker safety should include the use of barrier guards that protect workers from moving equipment while providing visibility to monitor performance. Specialty guarding now available includes return roller baskets and v-belt guards. The implementation of such new designs will improve the ease of cleaning around and changing out equipment without jeopardizing the workers responsible for these tasks.
Control of Fugitive Materials Following the March 2008 update to the OSHA National Emphasis Program (NEP) regarding combustible dust, housekeeping procedures and prevention of dust accumulation have generated increased attention. OSHA has announced it will issue a citation for the presence of combustible dust if dust accumulations exceed 1/32-inch deep (the thickness of a paper clip) and cover at least 5 percent of a room’s total area or 1,000 square feet. Accumulations on overhead beams, joists, ducts and the tops of equipment are included when determining the dust coverage area.
The dust accumulation-resistant conveyor structure addresses this issue with its sloped surfaces that prevent the accumulation of coal dust on cross braces, decking and door frames. By orienting structural members at 45 degrees to horizontal (see photograph) structural components shed, rather than accumulate, fugitive materials. Dust accumulation-resistant surfaces make it unlikely that cleanup crew members will have to reach under the belt with tools to remove buildup. Yet it is important to note that they are as structurally sound as traditional components, such as C-channel and angle iron. Structural members that cannot be oriented to reduce dust buildup should be fit with angular dust plates or caps to reduce material buildup in hard-to-clean areas.
Service Friendliness Many maintenance procedures critical to conveyor system operation may be accomplished safely while a belt is in operation if the equipment is properly designed and if the maintenance staff is properly trained. Belt cleaners can be designed to be safely serviced while the belt is running.
Other tasks that can only be done while the belt is shut down can be made easier and faster if maintenance is considered a design priority. Wear liner repositioned so it is placed on the outside of the skirtboardwhere it can be easily inspected, accurately installed and easily replacedis a simple modification potentially saving thousands of maintenance hours.
Another critical detail of service-friendly design is allowing sufficient access for inspection and maintenance. Adequate access includes providing space such as walkways and platforms and eliminating obstructions that interfere with conveyor maintenance. Whenever possible, utility piping or conduit should be run overhead and flexible conduit that can be moved out of the way to accommodate maintenance should be used.
Cost Effectiveness In addition to the amount paid to acquire the conveyor, the conveyor’s lifetime cost includes all of the other costs relating to operation over its lifetime. These extra expenses, which usually cover maintaining the conveyor system, are often greater than the purchase price. Over time, the lowest-bid process that considers only the initial purchase price has slowed the evolution of clean, safe and productive conveyor designs. While the initial purchase price may be lower for a system with no adjustment capabilities and no consideration for replacing wear components, the higher costs required for properly installing and maintaining components and cleaning up fugitive materials may exceed the price of a system that addresses these factors during the design phase.
By using modern, computer-based design and fabrication techniques, the conveyor structure can be designed to be dust accumulation resistant without increasing the cost per kilogram of fabrication. With foresight during the design process, a basic conveyor system with upgrade flexibility may be purchased. Then as budgets allow, it can be retrofit with standard and some specialized components at minimal cost to make the conveyor cleaner and safer, yet still cost-competitive to operate. Designing to accommodate future upgrades requires a new mindset; the actual cost of providing upgrade flexibility is minimal.
Upgradability Designing a conveyor system for upgradability and making components track-mounted and service-friendly can reduce downtime and control fugitive materials. Designers routinely consider capacity upgrades, but they rarely include provisions for component upgrades. A track-mount system provides flexibility for quickly installing different problem-solving components, such as an additional belt cleaner or training idler. The use of a pre-engineered mounting hole pattern in the structure around the conveyor transfer point allows for the quick and easy installation of new or improved components, such as replacing idlers with a belt support cradle.
When the preceding hierarchy of design elements is incorporated into the engineering of a belt conveyor system, the result is a concept known as modern conveyor architecture.
Modern Conveyor Architecture
Safety and maintenance-friendly features are the dominant characteristics of the new architecture for modern conveying. Wear liners are installed outside of skirtboards to provide for safer installation and maintenance. The external wear liners effectively protect the skirtboards and the sealing system, yet may be easily inspected without implementing confined-space procedures. Impact cradles, support cradles and idlers incorporate track-mounted sub-assemblies (see photograph) slide out on one side for maintenance, minimizing downtime and enhancing safety. Belt cleaners may be designed to permit service by a certified conveyor technician while the belt is still operating, which reduces carryback and improves the consistency of the belt cleaning.
Impact cradles, support cradles and idlers incorporate track-mounted sub-assemblies that slide out on one side for maintenance.
Preventing the escape of fugitive material is another important aspect of modern conveyor architecture. Modular chute wall, which simplifies the design and construction of transfer point skirtboards and stilling zones, helps to manage air flow and control dust. Skirtboard covers with a “peaked roof” prevent escape of airborne dust from the conveyor loading zone while keeping workers away from moving cargo and rolling components.
The return on investment in any of these conveyor system designs is significant in terms of increased safety and productivity and reduced fugitive material and maintenance. These benefits are enhanced by combining the individual systems, as when an engineered-flow chute is fed by an air-supported conveyor, and all components are engineered and designed using the principles of modern conveyor architecture.
For example, a Wyoming power generation facility has taken advantage of this synergistic approach to conveyor system design. The plant’s fuel is friable Powder River Basin (PRB) coal that yields significant amounts of coal dust and related dust problems. Company management emphasizes cleanliness inside all of their facilities and are conscious of maintaining environmentally friendly coal-handling systems. To improve the coal-handling systems and maintain an environmentally conscious operation, management elected to replace some transfer chutes and belt conveyors that were not working to their design intentions.
The coal-handling system upgrades at this plant were completed in three stages. First, the new engineered-load chutes were installed on an existing air-supported conveyor in July 2007. That autumn, an air-supported conveyor was commissioned for the bunker room and new load chutes for one of the pant leg chutes below the primary crusher. Finally, in July 2008, the second load chute beneath the primary crusher was upgraded with the installation of an engineered-flow chute.
This power generation facility has benefited from the installation of these leading edge conveyor technologies with:
- Reduced risk of fire and explosions inherent in PRB coal
- Reduced risk of employee health issues due to exposure to dust
- Improved cleanliness in the coal handling system with minimal maintenance requirements
- Reduced impact, spillage and dusting to extend component life
- Increased safety associated with the reduction in maintenance efforts
- Center loading of the air-supported conveyor, reducing mistracking, spillage and other problems.
An updated conveyor design philosophy that incorporates engineered-flow chutes, air-supported conveyor and modern conveyor architecture yields economic, environmental and safety benefits for the conveyor system owner as well as for the workers who operate and maintain it. With a state-of-the-art strategy for controlling fugitive material and an aggressive approach to employee safety, power plants will be better prepared to face the challenges of 21st century coal handling.
Authors: R. Todd Swinderman, P.E., is chief technology officer and a member of the board of directors for Martin Engineering. He is also president of the Conveyor Equipment Manufacturers Association (CEMA).
Barbara Wheatall is projects coordinator for Martin Engineering.
Andrew Marti is global marketing communications administrator for Martin Engineering.