CONVEYING environment . 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 lifecycle costs .”
To help mine managers avoid the pitfalls of buying only on the purchase price , Martin Engineering experts have compiled a list summarising ten of the most common design choices likely to result in a conveyor that is less safe , less clean and less productive over time . Here we include four of them – not knowing the bulk material , loading on the transition , using minimum pulley diameters , and lack of access – with recommendations for solutions on each .
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 . “ But there can be significant problems with this approach . 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 to 980 kg / m 3 ( 37-61 lb / ft 3 ). That represents a large potential variation from the average bulk density : ~ 790 + 190 kg / m3 ( 49 + 12 lb / ft 3 ). So , designing a system to accommodate the average value means that throughput could be over- or under-designed by + 25 %. 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 .”
Test samples of the actual material to be conveyed should be used under the full range of expected moisture content and consolidating pressures , then this information taken to design the conveyor system .
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 , ie ‘ loading on the transition .’ Another approach to shortening the overall length of the conveyor to meet price targets is a design technique known as ‘ halftrough transition .’ “ These practices can yield an upfront construction cost savings of $ 15,000 to $ 20,000 per conveyor . However , when the practices are used , the result can be a drastic increase in belt wear , chute wear and spillage .
“ But these cost-saving measures have a price . 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 to $ 20,000 savings quickly evaporates in cleanup costs , more frequent maintenance of the seal and liner , and reduced belt life .”
Martin Engineering recommends using the full trough transition distance recommended for the belt and belt width and starting loading after the first full trough idler .
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 . Swinderman : “ 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 .”
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 .
Finally , 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 . Conveyors may be so close to the floor that there is no room to clean under the conveyor . Further , the location of platforms and drive components around the head pulley are often so misplaced that it ’ s impossible to reach components for proper inspection or maintenance .”
Strategically-located access doors facilitate inspection and service . Martin Engineering suggests following CEMA recommendations for access and clearance , as detailed in Belt Conveyors for Bulk Materials , 7th edition . IM
FEBRUARY 2024 | International Mining 55