Pumps & Systems, April 2013
In the February 2013 issue of Pumps & Systems, a simple system, shown in Figure 1, was examined. Given only the flow rate and the piping details, the head loss was calculated to find the power required to drive the pump. With this information, the operators could determine the motor size needed for the system. The 800 gallons per minute (gpm) was calculated against 143 feet of head, and the power was calculated using the formula below:
BHP = (Q x H x SG) / (EFF x 3,960)
Q = flow
H = head
EFF = pump efficiency
SG = specific gravity (typically about 0.9 for oil)
3,960 = the unit conversion coefficient
Figure 1. Initial system
Figure 2. Screen shot of the Efficiency Estimator Matrix
Since the pump efficiency was not known, it was estimated at 50 percent and resulted in the following:
BHP = (800 x 143) / (0.5 x 3,960) = 57.8 horsepower
In this application, a 60-horsepower motor was used. A calculator program, the Efficiency Estimator Matrix, will be used to calculate the power more accurately.
Go to www.mj-scope.com/pump_tools/pump_efficiency
.htm to use the program calculator.
The Efficiency Estimator Matrix indicates an 81.3 percent pump efficiency, which is better than the initial estimate of 50 percent (see Figure 2).
The program also indicates a 32-horsepower requirement, which means that a 40-horsepower motor should be selected, not a 60-horsepower motor, which was used as a rough first estimate. This will translate into monetary savings.
Note that the Efficiency Estimator Matrix was supplied with an 1,800-rpm pump speed. How would the answer change if the selected motor speed was 3,600 rpm?
What other factors should be considered when choosing the speed?
For homework, try a few alternatives—the Efficiency Estimator Matrix will make it easy. Note also how the estimated impeller diameter changes with speed. Send your calculations to me (firstname.lastname@example.org), and they may be published in an upcoming issue of Pumps & Systems. P&S
Editor’s Note: For the Efficiency Estimator Matrix to work, readers may need to download a Microsoft Web component from:
Readers should only need to download the program once.