Pit & Quarry, August 2013

vice pump Determine the horsepower necessary for each of these components to find the total horsepower requirements for the dredge Dredge owners need to understand how the diesel engine horsepower requirements relate to the electric motor kilowatt requirements There is a standard equation used to calculate equivalent kilowatt hours to horsepower Electric motors actually require less energy than their diesel counterparts This is because electric motors utilize the energy in electricity at approximately 95 percent efficiency while diesel engines utilize the energy in diesel at approximately 41 percent efficiency Approximately 5 percent of the total energy consumed by a diesel engine is lost through heat expelled by the radiator DSC used its model SH 7560 30D dredge in the conversion project See Table 1 for specifications of the diesel and electric models With a diesel engine our dredge required 80218 hp For the electric motor we needed 85424 hp Johnson says We calculated the total horsepower requirements for both the diesel engine and electric motor then used the standard conversion formula 85424 hp An electric motors per hour cost is about half that of a diesel engine x 07456999 to convert horsepower to kilowatts 63701 In its cost analysis DSC obtained regional electricity prices from the U S Energy Information Administration www eia gov The company also calculated off road diesel fuel costs and used that figure 339 in its study Their research revealed that an electric motors per hour cost is about half that of a diesel engine The average hourly horsepower demand with our diesel engine was 80218 hp Johnson says With diesel prices at 339 per gallon the cost of running the diesel engine for one hour was 13024 The kWh demand for the dredges electric motor was 63701 at a cost of 0103 kW which comes to 6561 per hour about 50 percent less than the TABLE 1 Dredge Horsepower Demand Diesel Electric Dredge Pump 64456 Dredge Pump 64974 Engine Cooling 3000 Engine Cooling 0 Cutter 7508 Cutter 7508 Swing Winch 3754 Swing Winch 3754 Service Water Pump 1500 Service Water Pump 1500 Total Avg HP 80218 Total Avg HP 77736 per hour cost of the diesel engine Using those per hour cost figures Johnsons team calculated that use of a diesel engine incurred an energy cost of 182 cents per ton The annual per ton cost for energy penciled out at 267130 When an electric motor was used on the dredge the energy cost per ton was at 86 cents a total of 137391 per year Additional benefits Thats a huge savings Johnson says But the benefits of the electric motor go even further Theres far less maintenance because monthly oil changes are eliminated That enhances uptime by about 2 percent Electric motors dont need a radiator to cool the power unit and there are far fewer moving parts in an electric motor than a diesel engine When maintenance requirements decrease overall efficiency increases because you have less downtime As part of the conversion a dredge extension section was added to make room for the electric transformer The dredges increased length increased the swing distance of the arc which meant the dredge had to move less often The result was added productivity Tons per hour went from 714 to 764 Between the increased uptime and savings in energy costs youre at about 4856 percent of original operating costs Johnson says Add the 50 tons x 91 Drive Efficiency 85424 x 07457 kW 63701 kW per hour increased productivity and its clear that the electric motor has a significant economic and production advantage Calculating the cost of conversion is the next step in completing a feasibility analysis In its project DSC included the cost of equipment necessary to install the electric motor cost of the electric motors shore mounted electrical substation related transportation costs and special equipment and field service related costs see Table 2 The DSC conversion project required a dredge extension section 1000 hp dredge pump and hydraulic electric motor 1000 hp variable frequency drive 20 hp SW pump motor and switch dredge mounted 5kV x former and switch shore side substation 15kV 5kV 15kV cables from pole to substation and 1000 ft of floating power cable with floats Equipment costs totaled 842550 Adding the numbers Our total conversion cost was 1029810 Johnson says We didnt cut corners or reduce the number of required laborers or do anything special that would impact the final timeline or costs to cast the conversion process in a more favorable light We wanted our conversion figures to reflect actual costs and conditions DSCs conversion took about four weeks to complete One supervisor and three workers provided the labor work DREDGING 20 PIT QUARRY August 2013 www pitandquarry com