Coal Age

JUN 2018

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June 2018 21 shovels & excavators continued Evaluating Performance: Hydraulic Excavators vs. Rope Shovels At Haulage & Loading 2017, University of Alberta Professor Tim Jo- seph presented the results of ongoing studies that compare the dig performance for a hydraulic excavator versus an electric rope shovel. They evaluated similar mining-class machines in terms of the ener- gy required to excavate a unit quantity of the same material from a mining face. One of the initial findings was that the energy per unit excavation quantity for rope shovels and hydraulic excavators working in the same muckpile are, in fact, identical. This proved the fact that the amount of energy to excavate a unit quantity, regardless of the excavating tool, is constant. The energy required is related to the ma- terial to be moved and not the machine. Knowing the efficiencies of the two types of machines and the operating conditions, the researchers measured the power draw for both types of machines on a per ton basis, Joseph explained. On av- erage, the hydraulic excavator was loading 76 metric tons (mt) per bucket and pulling 14.7 megaJoules to do it, while the rope shovel was loading 119 mt per dipper and pulling 23 megaJoules. "This average specific energy per ton moved was measured over several hundred cycles," Joseph said. "Both machines gave identical specific energy, 193 kiloJoules/mt, confirming the power and energy required to dig the blasted material was the same no matter the machine, and that diggability depends on fragmentation." But, these results do not indicate whether one primary excavator is better than another; nor does it represent the total picture driving the selection of one excavating tool over another. The total cost of ownership (TCO) including capital (plus sustaining capital), operating and maintenance (O&M) costs, time, availability and utilization (pro- ductivity and efficiency of use) must also include the qualitative deci- sions regarding mining method and the mode of application; much of which is driven by the geological, operating and environmental condi- tions that are highly site specific. Another parallel study of rope shovel and hydraulic excavator per- formance focused on availability, productivity and life cycle costs (with a sub-focus on maintenance costs) for up to 60,000 operating hours per machine, where data was available. Each machine's performance was assessed by operating hours, maintenance activity impacting availability, and productivity per cubic meter (m 3 ) of bucket capacity as a comparison normalizing factor, regardless of excavator size. Cost of ownership, including capital outlay, oper- ating and maintenance costs including but not limited to spare parts, fuels, lubricants, electricity and consumables, were considered. The researchers pulled together costs for both types of machines and projected that over the expected life of the machine, 25 years for a rope shovel and nine years for a hydraulic excavator. "If the [TCO] was com- pared on a production equivalent basis, the availability difference between the rope shov- el and hydraulic excavator amounted to 20% or $53 million over seven years," Joseph said. "The rub, however, is that everyone calculates availability differently." To establish an availability standard, they conducted a massive survey of surface mines and plotted availability looking at dis- tribution. Taking the mean, they found an 85% correlation for the avail- ability standard developed. "If we compare the data for the two types of machines, the availability of the rope shovel is roughly twice that of the hydraulic excavator," Joseph said. "Looking at mines sites with both types of machines working in similar mining conditions, it was close to double, confirming the results of the study. This would be a practical ref- erence tool for engineers to predict availability as a function of hours." The O&M costs for a hydraulic excavator are higher than a rope shovel on a production-equivalent basis. As the life of the hydrau- lic excavator deteriorates, the rope shovel holds constant, Joseph explained. "The rope shovels are production machines, while the hy- draulic excavators are moving about the mine," Joseph said. "In true productivity comparisons, they should be much closer, but they are not because of how they are used. "Production wise, the biggest difference is primarily found in the dig cycle time," Joseph said. "It seems as though operators take more time with hydraulic excavators in face loading applications to improve the bucket's fill factor. The difference between 48 seconds for a rope shovel compared to 60 seconds for a hydraulic excavator makes a huge difference over the life of the machine." Maintenance costs for a hydraulic excavator are higher. "The difference in maintenance costs between a rope shovel and a hydrau- lic excavator is 5.5 times," Joseph said. "This is important for the equipment selection process. With exception of the electrics and the electronic controls, rope shovels haven't changed much, Joseph explained. "On the other hand, the systems that have changed on the hydraulic excavators have changed a lot," Joseph said. "These machines are more complex, which requires more training and that slows the process. Over the course of 25 years, we see an availability of 67% for the rope shovel and 85%-87% for the hydraulic excavator, but the hydraulic shovel is not always loading." The results of the research also indicated higher production rates by excavator of similar capacity and age. In general, electric rope shovels, with higher initial purchase cost, exhibited lower service cost per m 3 of capacity, becoming overall more cost effective within 5 years (~30,000 hours) of operation over the hydraulic excavator coun- terpart of similar capacity.

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