In past “Pumping Prescriptions” columns this year, I have discussed the procedure of piping size selection when given the process flow requirement and how this affects the pump’s power consumption. In this column, two computer calculation tools will be detailed. The tools are the Energy Consumption Calculator and the Pump Performance Estimator. Both can be found at pump-magazine.com/pump_magazine/pump_magazine.htm. These tools provide a quick and basic evaluation of piping’s effect on pump energy and the effect’s associated cost.
As always, the best way to understand a concept is with an example. In our example, a process must move water at a rate of 8,000 gallons per minute. The water must be transported 5 miles across flat terrain with no significant elevation. In this type of application, most of the hydraulic losses result from pipe friction. The process flow is a non-stop 24 hours per day and seven days per week.
Energy Consumption Calculator
The Energy Consumption Calculator should be used first. This tool will estimate the friction losses associated with pumping the water through a selected pipe diameter (18 inches) and length (5 miles or 26,250 feet). A straight run of pipe is assumed with no additional losses for bends, turns or other restrictions along the way. More detailed programs are available to handle more complex piping. However, for this type of calculation, a simplified straight piping run will suffice.
Clean (or relatively clean) water is assumed to flow through reasonably new pipe in this calculation. For old, rusty or partially clogged piping, additional adjustments must be made. For example, entering a smaller pipe diameter will simulate a partial blockage. Table 1 shows the first iteration of the Energy Consumption Calculator’s results.
The 5 miles of pipe resulted in 836.3 feet of hydraulic losses, which needs to be overcome by the pump. The pump efficiency assumed in the initial calculation is 80 percent. As a result, the required pump power selected was 2,112 horsepower. At $0.10 per kilowatt-hour (kW-hr), this pump would haves a $1,380,028 per year energy cost.
Pump Performance Estimator
Once the pump head (836.3 feet) has been determined, the Pump Performance Estimator can be used to define a more accurate pump efficiency value. The pump efficiency adjustment is calculated by the Pump Performance Estimator (see Table 2). Notice that the more accurate pump efficiency is 83.9 percent, which reduces the required power to 2,013 horsepower.
With the new horsepower and efficiency, the new yearly energy cost is $1,315,879 (see Table 3).
Alternate Pipe Diameter
What happens if the pipe diameter is larger? We will enter the same flow, distance and efficiency information but change the pipe diameter to 20 inches (see Table 4).
The head losses decreased by almost half, down to 493.8 feet. The power required was also reduced to 1,189 horsepower. With this new pipe diameter, a smaller and likely less expensive pump can be installed to produce the required process flow.
Piping size can be a major contributor to energy savings. Use the Energy Consumption Calculator and the Pump Performance Estimator for a quick estimate of the piping’s effect on the system. By entering the pipe length, diameter, required flow and the percentage of approximate operating time during the year, end users can obtain a reasonable estimation of the impact of any changes to the system.
Using the Energy Consumption Calculator, evaluate how much additional energy is wasted if the pipe in the example is clogged at half the initial area. The answers will be posted in future articles, and the detailed solution will be presented at the next scheduled Pump School training during the PumpTec Conference 2014 in September.