Martin Engineering outlines the 10 most common design choices likely to compromise conveyor safety – and how to avoid them.
Conveyor systems are the veins and arteries of mining and processing.
Mined material is extracted with differing sizes, weights and traits which react in their own unique ways to being conveyed.
Some spread, some collect on one side, some stick and some emit excessive dust. These different qualities can require slight changes in conveyor design.
Modern systems are being tasked with moving greater volumes of cargo and at higher speeds than ever before.
Yet some common practices in conveyor specification and design could be considered out of date compared to the current safety requirements, serviceability innovations and greater control over fugitive material.
Many decisions affect the initial and future performance of a conveyor system, with leading trends that include designing for lower risk, greater sustainability and reduced life cycle costs.
To help mine managers avoid the pitfalls of buying only on the purchase price, experts have compiled a list summarising 10 of the most common design choices likely to result in a conveyor that is less safe, less clean and less productive over time.
Not knowing the bulk material
For decades, it has been common practice to use only the bulk density and angle of repose to describe a bulk solid.
The Conveyor Equipment Manufacturers Association (CEMA) receives an untold number of requests for material properties that can just be looked up in a table, as if every material variation can be effectively captured in a textbook.
A simple example of the dangers can be found by considering a very basic requirement: tonnage.
CEMA Standard 550: properties of bulk solids has eight different bulk density listings for coal, ranging from ~600–980kg/m3.
That represents a large potential variation from the average bulk density: ~790+190 kg/m3.
Designing a system to accommodate the average value means that throughput could be over- or under-designed by +25 per cent.
Further, the angle of repose for these eight coal listings varies from 27–45
°, a possible variation of +9° from the average.
Designing the slope of hoppers or chutes based on the average value could mean that the bulk material doesn’t flow at all, or it might flow so freely that it can’t be adequately controlled by the chute geometry.
Recommendation
Test samples of the actual material to be conveyed under the full range of expected moisture content and consolidating pressures, then use this information to design the conveyor system.
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 where the belt transitions from flat to troughed – “loading on the transition”.
Another approach to shortening the overall length of the conveyor to meet price targets is a design technique known as “half-trough transition”.
These practices can yield an upfront construction cost savings of $15,000–$20,000 per conveyor. However, when the practices are used, the result can be a drastic increase in belt wear, chute wear and spillage.
When loading on the transition, operating problems begin immediately with the primary issue being fugitive material (spillage and dust).
In its transition from the flat tail pulley to the first full trough idler, it is virtually impossible to accurately model the complex 3D belt surface.
A common rule of thumb is that it costs 10 times as much to do field fabrication as shop fabrication.
When loading on the transition and/or using the half-trough transition 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.
This flexure creates an entrapment point for fines that quickly wear the liner and skirt seal, eventually grooving the belt.
With this in mind, the $15,000–$20,000 savings quickly evaporates in cleanup costs, more frequent maintenance of the seal and liner, and reduced belt life.
Recommendation
Use the full trough transition distance recommended for the belt and belt width. Start loading after the first full trough idler.
Using minimum pulley diameters
The diameters for the conveyor’s main pulleys are usually selected based on the minimum 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 the 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, which limits the space available for cleaning the belt at the head pulley and often leads to severe buildup on the snub, which is the first rolling component to contact the dirty side of the belt.
Smaller main pulleys often leave inadequate space between the top and bottom runs of the belt for accessories that are critical to protecting the belt and maintaining good tracking.
Recommendation
Best practice is to select a pulley diameter that is at least 600mm diameter or one size larger than the minimum recommended by the belt manufacturer.
Lack of access
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 may be located in odd places that allow a minimal view and are so small that no inspection or maintenance can be done through them, and conveyors may be so close to the floor that there is no room to clean underneath.
Recommendation
Follow CEMA recommendations for access and clearance, as detailed in Belt Conveyors for Bulk Materials, 7th edition.
Covering key components with piping and conduit
It’s a common omission not to control the location of conduit and piping runs on a conveyor structure.
The fact that this piping and conduit often impede the installation and service of critical components such as belt wander switches, belt cleaners, plows and return idlers is well recognised.
The conduit and piping rarely need maintenance or relocation, while the components that surround it typically do need frequent inspection and service.
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.
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 two thirds the flat belt width, which does not account for the toughing angle.
The European standard is based on a formula for free belt edge.
Calculated to prevent spillage between carrying idlers, neither standard provides adequate edge distance to accommodate the belt tracking and sealing systems.
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 115mm, regardless of belt width.
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 rather than designed.
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, with one leg vertical, provides better access than a table frame design.
Recommendation
Test the bulk material 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, yet allows adequate access for maintenance as well as future upgrades.
Inadequate belt cleaning
Suppliers are pressured to meet price goals and end up providing equipment that they know will not meet expectations.
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 may not allow the proper installation and service of belt cleaners.
Recommendation
Include belt cleaning performance specifications in the conveyor requirements.
Allow adequate space for scavenger conveyors if the head chute design is such that at least three cleaners cannot fit in the available space and the carryback can be captured in a dribble chute with near-vertical walls.
Substituting speed for belt width
Conveyors are routinely designed to travel at speeds as high as 7.5–11.5 metres per second.
Some industries have established maximum transport speeds to limit the degradation of the cargo and/or control dust.
While these practices have their roots in practical experience, they are often stretched to meet price goals.
Dust and spillage are directly related to belt speed and tonnage, while wear is a function of the square of the material stream.
The trade-offs between width and speed should be considered carefully.
Recommendation
Follow the suggested maximum conveying speeds listed in CEMA’s Belt Conveyors for Bulk Materials, 7th edition. Underrate or oversize the conveyor.
Failure to allow for upgrading
Many designs leave no room for even modest upgrades or additions.
Upgrading by changing the speed alone often results in a throughput decrease rather than an increase, due to plugging problems created by the change in material trajectory or the existing chute cross-section creating a flow restriction.
With 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.
Recommendation
Use standard components to meet price targets, but allow space in the design for problem-solving upgrades to meet production/cost targets.
Deciding to ignore these problematic areas, and purchasing solely on price, usually results in less throughput than specified, higher operating and maintenance costs than budgeted, and reduced safety.
Each of the issues, if addressed in the specification and design stages, can easily be justified based on life cycle costing and cost avoidance.
This feature appeared in the May–June edition of Safe to Work.