R. Todd Swinderman, Martin Engineering, outlines the factors that operators must consider when upgrading their conveyor systems in order to ensure their modifications actually improve performance.
When greater production is needed to meet rising demand or when lower quality raw materials require greater amounts to be processed per unit of output in order to retain the same level of production, many operators simply speed up the conveyor. Rather than increasing capacity as intended, speeding up the conveyor often results in reduced capacity, because changes in the trajectory of the discharged material can cause buildup and the clogging of hoppers or chutes, leading to unscheduled downtime.
More tonnage means more carryback, dust, and spillage, degrading workplace safety and increasing labour costs for cleanup. A greater volume and weight could also require a more powerful drive, which may weigh more, requiring structural changes and potentially additional space, limiting access for maintenance.
As plant engineers, operators, and maintenance mechanics make undocumented or unproven changes, over time, the conveyor operation and physical characteristics can morph the system. In some cases, the proper answer to the question, ‘Can we increase capacity on the existing conveyor?’ should be ‘No, we need to start over.’
Original design
Prior to the modification of a conveyor, it is recommended to verify that the current system is operating in an environment and on an application for which it was originally designed. The existing conveyor may have been repurposed over the years by modifying chutes, adding feed points, or changing the slope to accommodate process changes. In situations where the conveyors are many decades old, the original design specifications and drawings could be incomplete or lost.
Conveyor design is an iterative process. Purchasing a conveyor at the lowest capital cost is generally accompanied by significant design compromises. Even if it matches previous conveyor structures, the design is likely to use the maximum loading capacity on the narrowest belt, travelling at the maximum speed for the raw material, while only meeting the minimum safety standards and codes.
When sold on lowest price, the supplier’s goal is to win the low bid and make it through the warranty period without costly rectifications being required. If the goal was to design a conveyor with the lowest cost of ownership over its intended life, it was likely designed with less than maximum loading, a slightly wider belt, and the capacity to run at a reasonable speed, while exceeding minimum safety standards and code requirements. The best practice is to re-establish the original design intent and compare it to the existing conveyor.
Conveyor technology changes over time, particularly in belting and calculation methods. Until the 1980s, without the aid of computers and design software, conveyors were designed using hand calculations and experience. It is amazing how many conveyor designers still use the 1977 5th Edition (or earlier) of the Conveyor Equipment Manufacturers Association (CEMA) design guide ‘Belt Conveyors for Bulk Solids’, which relies on research from the 1940s. The 6th Edition indicated that the hand calculation method was an inaccurate predictor of the actual power needed for proper conveying. The most recent 7th Edition requires predicting power within – 0% to + 10% of actual. Much research and development has been conducted in regard to conveyor power requirements, which has resulted in several low-cost design software options.
Upgrade design
First, define the problem the conveyor upgrade plan is trying to solve. It may seem obvious, but a lack of understanding of the primary reason(s) for an upgrade could cause suppliers to address symptoms rather than root causes. The new design might not address the primary need for a performance upgrade.
For example, if the chutes are plugging or there is a spillage, then it might not be a conveyor issue, but instead an operator or maintenance issue. If the problem is belt damage, mistracking, or tripping the breakers, it may be due to a misalignment of the structure and idlers. Surge loading the conveyor in an attempt to catch up for lost time spent cleaning could result in more spillages.
The bulk material
Another critical early step in an upgrade project is understanding the physical properties of the material being handled. Knowledge of properties such as solid density, bulk density, and particle distribution are crucial to a well-designed conveyor. Original test results for the material are likely out of date due to changes in the sources and variations in the extracted raw cargo over time.
Discrete element modelling (DEM) software helps model the flow of bulk solids through chutes and onto conveyors. Laboratories can perform the tests, or operators can conduct their own basic tests using the information in the CEMA publication, ANSI/CEMA Standard 550 – Classification and Definitions of Bulk Materials.
