R. Todd Swinderman, P.E
Martin Engineering
Neponset, lllinois ABD
This paper discusses how many of the common practices in conveyor specification and design are out of date compared to the current expectations of users for safe, serviceable, reliable, and fugitive material-free conveyance of bulk solids. Ten of the most common mistakes made in the specification and design stages related to user expectations are reviewed along with the cost vs. price implications of those decisions. A general specification and design hierarchy that designers can use to improve conveyor design is included.
Introduction
Belt conveyor systems are often abused, through overuse and neglect. That is because suppliers and designers have done “too good” a job of providing very robust designs that continue to operate under all sorts of adverse conditions. This has reached the point that a conveyor is often considered a commodity that can be purchased by the kg, rather than an engineered system.
Many owners view a conveyor system as a simple system that merely transports bulk solids from point A to point B in at a prescribed rate. On the contrary, a conveyor is actually a complex and sophisticated system that interacts with all major processes. Short cuts taken in the specification and design of a conveyor will have dramatic and far reaching effects on safety, the environment and productivity.
Many specification and design decisions affect the future performance of a conveyor system. Leading trends in engineering routinely include designing for sustainability and life cycle costs
But with conveyors, the practice of buying on lowest price rather than lowest cost is the norm. But the game that is played by owners, which transfers funding for necessary design elements from capital expense to the operating budget, is seriously flawed, because the low bid design often can not be cost effectively remediated or the promised operating funds to correct shortcomings in the original equipment are never made available.
The symptoms of poor specifications, designing to meet price goals, and playing shell games with funding contribute to the numerous and expensive problems, like accidents, pollution and litigation that can persist over the life-time of the conveyor.
The following is a look at ten of the most common design choices that, if made with only price in mind, result in a less safe, less clean, and less productive conveyor system.
Ten Common Mistakes
1. Not Knowing Your Bulk Material
The science of bulk material handling has advanced to the point that the properties of the bulk solid should be determined for all but the smallest and least critical applications.
It has been common practice for decades to use only the bulk density and the angle of repose as the properties used to describe a bulk solid. As editor of CEMA’s handbook, Belt Conveyors for Bulk Materials I cannot tell you how many requests CEMA gets for bulk material properties that can be “just looked up in a table”. But there can be significant problems with this approach.
A very simple example of the dangers of the “look it up in a table” approach is to consider a very basic requirement – tonnage. The primary purpose of a belt conveyor is to deliver X tons per hour from point A to point B. If this is not accomplished all other requirements are insignificant; just ask anybody who has commissioned a system that can not deliver the specified tonnage.
CEMA Standard 550: Properties of Bulk Solids has eight different listings for coal. These bulk density ranges from ~ 600 to 980 kg/m3 (37 to 61 lb/ft3) From the average the variation in bulk density is ~790 ± 190 kg/m3 (49 ± 12 lb/ft3). Designing based on the average value of the bulk density could mean that the system through put could be over or under designed by ~ 25 %.
The angle of repose for these eight listing for coal varies from 27 to 45 degrees; that’s a variation of ± 9 degrees from the average. Designing the slope of hoppers or chutes based on the average could mean that the material does not flow at all. Or it might flow so freely it can not be controlled by chute geometry.
The cost of thoroughly testing a bulk solid depends on the number of samples (i.e. several points in the ore seam) and the range of conditions (i.e. moisture content) that are needed to define the envelope of bulk materials that may be carried on the belt. A typical set of tests to define a particular bulk solid cost about $30,000. The typical cost for system downtime is $1000 per minute. Over the lifetime of a conveyor system if just one plugged chute episode can be avoided the testing will have paid for itself. Similar arguments can be made for many other values that are critical to reducing future operating costs. For example lump size and the percentage of fines are often misrepresented in request for bid documents; the result is many disputes over contract performance.
Recommendation: Test samples of the actual bulk solid to be conveyed under the full range of expected moisture content and consolidating pressures and use this information to design the conveyor system.
2. Loading on the Transition
A common “trick of the trade” to meet price targets is to reduce the overall length of a conveyor by loading on the transition of the belt from flat to troughed. Another approach to shortening the overall length of the conveyor to meet price targets is a design technique known as half trough transition. When the practices of loading on the transition and half trough transition are used in combination the result can be increased belt wear, increased chute wear and increased spillage.
A typical cost for a conveyor is in the range of $10,000 per meter in the loading zone, and $5, 000 per meter elsewhere along its length. Reducing the distance at both the load and discharge zones a meter or more of conveyor length (and the resultant two meters of belting) can result in a savings of $15,000 to $20,000 per conveyor. Additional savings are found in the reduced size for the building that houses the conveyor.
But these cost-saving measures have a price too. Operating problems begin immediately with many designs that incorporate loading on the transition and/or using the half trough transition. The primary problem is fugitive material– that is, spillage and dust. In its transition from the flat tail pulley to the first full trough idler the belt is a hard-to-model complex 3D surface that varies based on belt tension (caused by variations in loading). It is virtually impossible to model this surface; consequently field-fitting of the chute to the belt line is required. This field work adds cost. A common rule of thumb is that it costs 10 times as much to do field fabrication than it does to do shop fabrication.
When the loading on the transition and/or using the half trough transition “tricks” are incorporated in a design, the result is a chute that starts out parallel to the belt in the transition and then must form a convex curve to follow the belt when fully troughed. The flexure point creates an entrapment point for fines that quickly wear the liner and the skirt seal and eventually groove the belt. The characteristic “half moon” wear area of the liner and skirt above idlers in the region where the loading is most turbulent leads to the escape of large quantities of fugitive materials that must be cleaned up – often by hand. The $15,000 to $20,000 saving quickly evaporates in cleanup costs, more frequent maintenance of the seal and liner, and the shortened belt life.
Numerous other design and maintenance issues result from this decision at the specification or design stage. Using a full trough transition design and waiting to load the belt until it is fully troughed can substantially eliminate all these negative effects.
Recommendation: Use the full trough transition distance recommended for the belt and belt width. Start loading after the first full trough idler.
3. Using Minimum Pulley Diameters
The diameters for the conveyor’s main pulleys are usually selected based on the minimum diameter recommended by the belt manufacturer for the life of the belt and splice based on belt tension. Generally no recognition is given to the concern that these pulley diameters may be too small to allow other components to function properly. When smaller drive pulleys are used it often necessitates use of snub pulleys to increase the wrap angle so there is sufficient friction to drive the conveyor. To increase the wrap the snub pulley must be close to the drive pulley. This limits the space available for cleaning the belt at the head pulley and often leads to severe buildup problems on the snub, which is the first rolling component to contact the dirty side of the belt.
When smaller main pulleys are used there is often inadequate space between the top and bottom runs of the belt for accessories that are critical to protecting the belt and maintaining good tracking. For example what sense does it make to install a tail protection plow when the stringer is directly in line with the plow discharge? Material ejected by the plow hits the stringer; a significant portion of that ejected material bounces back on to the belt negating the purpose of the plow. When pulleys are undersized, installation, inspection and maintenance of the plow are very difficult in the small space remaining between the bottom of the idler frame and the return run
Recommendation: Best practice is to select a pulley diameter that is at least 600 mm (24-inches) diameter or one size larger than the minimum recommended by the belt manufacturer.
4. Lack of Access
The examples of lack of proper access in many conveyor designs are so numerous that a paper could be written just on this topic alone. Conveyors are often placed in enclosures or tunnels where one side is so close to the wall that there is no room for a maintenance person to shuffle sideways along the conveyor. Access doors are located in odd places that allow a view of nothing and are so small that no maintenance can be done through them. Conveyors are so close to the floor that there is no room to clean under the conveyor. Substructures and accessories frequently protrude into walkways creating trip and bump hazards. The height of platforms around the head pulley are so low that it is impossible to reach components on the drive side for maintenance.
Recommendation: Follow CEMA recommendations for access and clearance, as detailed in Belt Conveyors for Bulk Materials 6th edition.
5. Covering Key Components with Piping and Conduit
It is a common omission not to control the location of conduit and piping runs on conveyor structure. The support structure of the conveyor makes a convenient rack system for mounting electrical conduit and the piping for plant air or water supply. The fact that this piping and conduit often impedes the installation and service of critical components such as belt wander switches, belt cleaners, plows, and return idlers is well-known. The conduit and piping rarely needs service or relocation while the components it incarcerates typically need frequent inspection and service. To add insult to injury, these plumbing runs are often on the side of the conveyor where there is a walkway supposedly installed to provide access.
Recommendation: Specify that conduit and piping runs not be allowed to block or impede access to critical components along the conveyor .At the head and tail pulley all conduit and piping should be installed with flexible conduit drops to connect components.
6. Insufficient Edge Sealing Distance
The free belt edge outside of the skirtboards in the loading zone of a conveyor is called the edge sealing distance. The CEMA standard is based on the distance between the inside dimensions of the skirtboards being equal to 2/3 the flat belt width, which does not account for the toughing angle. The European standard is based on a formula for free belt edge. Neither of these current standards provides adequate edge distance to accommodate the belt tracking and sealing systems required to meet today’s requirements for dust and spillage control. Rather than base the edge distance on a variable such as belt width, the free edge distance should be based on the distance needed to properly seal the belt. The allowance for belt tracking is based more on the structure and pulley face widths and does not vary significantly with belt width.
Recommendation: The free belt edge available for sealing the belt and allowing for belt mistracking should be at least 115 mm (4.5-inches), regardless of belt width.
7. Poor Chute Design
Chute design has improved in recent years through the use of Discrete Element Method (DEM) modeling programs but many chutes are still drafted vs. designed. The advantages of using DEM software to model the flow of the material in the chute and so design for consistent flow are well documented. However, if the properties of the bulk solid are not properly identified the results can be worse than using the old “rule of thumb” design methods. Even if the bulk material is well specified the approach to designing the structural support of the chute and the pulleys is based primarily on ease of fabrication and installation, rather than designing for the intended use, which requires proper access. Usually an A-frame type of head pulley support provides better access than a table frame design.
Recommendation: Test the bulk solid and use the properties that represent the worst-case flow to design the chute using DEM. Design the structure so that it does not impede access to critical components and allows adequate access for maintenance and future upgrades.
8. Inadequate Belt Cleaning
One can only conclude that as dust and spillage requirements tighten over time that more sophisticated belt cleaners in larger numbers will be required.
Belt cleaning is a critical function in many bulk material handling applications. Often an inadequate number of belt cleaners or cleaners with too low a duty rating are specified. In addition the space that is provided in the design does not allow for the proper installation and service of belt cleaners. Suppliers are pressured to meet price goals and end up supplying equipment that they know will not meet expectations. But the game is to make the specification vague enough (translation “or equal”) so you can pressure the supplier by presenting them with a choice; “if you don’t meet the price we will put in a simple design and let the customer deal with the problem”.
Recommendation: Include belt cleaning performance specifications in the conveyor requirements. Allow adequate space for scavenger conveyors if the head chute design is such that you cannot fit at least 3 cleaners in the design and the carryback can be captured in a dribble chute with near vertical walls.
9. Substituting Speed for Belt Width
Conveyors are routinely designed to travel at speeds of up to 7.5 to 11.5 m/s. (1400 to 2300 fpm) industries have established maximum transport speeds to limit degradation of the bulk solid and/or control dust. While these practices have their roots in practical experience they are often stretched to meet price goals. For example, suppose an 1800 mm (72-inch) conveyor traveling 3 m/s (600 fpm) can handle 4000 mtph (4400 tph). A 1200 mm (48-inch) conveyor could be designed to convey the same quantity of material if the speed was turned up to 7 m/s (~1375 fpm). This might save money on the initial purchase of steel and belting. But using this higher speed but narrower belt can create a number of problems, including increased wear in belt and chute, material degradation, loading problems, and chute plugging to name a few. At high speeds the material might never settle on the belt to its surcharge angle with the result of constant spillage.
Recommendation: Follow the suggested maximum conveying speeds listed in CEMA’s Belt Conveyors for Bulk Materials, 6th edition. Underrate or oversize the conveyor.
10. Failure to Allow for Upgrading
When the topic of upgrading a system is brought up the normal assumption is that the belt speed is being increased. Other than the drive and few other components the only thing that gets upgraded is the tons per hour output. Many designs leave no room for even the most modest upgrades/additions. With a minimal effort in the design phase and at little or no additional fabrication or installation cost some flexibility can be built into the system for performance-improving upgrades.
Not every conveyor needs every gadget attached to it and it is difficult to predict every problem. But experience has taught us that some modifications are GENERALLY needed at start up (or shortly thereafter) for the conveyor to meet customer expectations so that the contractor can turn the system over to production and get paid. Simple upgrades like replacing rollers in the impact zones with slider beds or adding a flow aid like a vibrator should be anticipated and space allowed in the original design. This makes the upgrade possible without incurring the cost of supplying the problem-solving component. It allows for cost-effective upgrading with minimal down time.
Recommendation: Use standard components to meet price targets but allow space in the design for problem solving upgrades to meet production/cost targets.
A New Hierarchy For Conveyor Design
Conveyor equipment manufacturers have begun to utilize a new hierarchy for design decisions that has the potential to revolutionize conveyor architecture.
The design is based on the following hierarchy to prioritize design decisions.
1. Capacity
2. Safety and Code Compliance
3. Control of Fugitive Materials
4. Service Friendliness
5. Cost Effectiveness
6. Upgradability
The changes from “traditional” conveyor design to a new conveyor architecture can result in improvements in environmental performance and production efficiency.
Conculusion
When faced with the pressures of getting a project based on the lowest bid, experienced material handling engineers should ask themselves would I be willing to be responsible for operating and maintaining this conveyor? If you can answer yes then you have probably avoided the professional ethical dilemma that owners and managers put you in by requiring designs based on purchase price rather than operating cost.
If we, as design professionals, cannot answer yes to this question then we need to start supporting each other by getting involved in changing the process. So how do we do that? In extreme cases it may mean resigning rather than approve a design that will not meet expectations. But a more productive answer would be to get creative in design and get involved in writing standards. If you step back and look at many of the design and fabrication practices they are simply done in a specific way because they have always been done that way. Many of the common mistakes mentioned in this paper can be resolved with the same amount of steel but arranging the components just a little differently and with only a few critical upgraded components required that add cost.
Recommendation: Use creative thinking to provide and promote conveyor designs that meet current needs and allow the systems to meet future requirements.
