Quiz Corner: Estimating pump discharge pressure and energy savings at reduced speed

An existing centrifugal pump and control valve operated continuously to provide between 95 and 105 gallons per minute of water to a process approximately 60 feet above the pump. The pump operated at a discharge pressure of approximately 100 feet of water column. The pumping system was subsequently modified to incorporate a variable speed drive and eliminate the control valve. What are the approximate energy savings and approximate discharge pressure if the variable speed drive operates at 90 percent speed to provide the same water flow after the modification? 

A. 10 percent energy savings and 80 feet of water column

B. 10 percent energy savings and 90 feet of water column

C. 20 percent energy savings and 80 feet of water column

D. 20 percent energy savings and 90 feet of water column

E. 30 percent energy savings and 80 feet of water column

F.  30 percent energy savings and 90 feet of water column

This pumping system and modifications are the same as presented previously with the added twist of energy savings. 

Water flow is approximately 100 gallons per minute at a discharge pressure of approximately 100 feet of water column at full speed. The same 100 gallons per minute flows at 90 percent speed where the pump discharge pressure is approximately 81 feet of water column (0.9 x 0.9 x 100) per the Affinity Law relating speed and discharge pressure. Therefore, Answer B, Answer D and Answer F are not correct. 

Energy consumption will be approximately 73 percent (0.9 x 0.9 x 0.9 x 100) of full speed per the Affinity Law relating speed and brake horsepower. This results in an energy savings of approximately 27 percent. Answer E is the best answer. 

Additional complicating factors

Often there is only limited information available. However, it is similarly common to receive a glut of information. In this case, the problem was intentionally verbose and rich with information — none of which is necessary because only one answer approximates the correct relationship between energy savings and discharge pressure.