David W. Spitzer

A centrifugal pump is designed to operate at a water flow of 100 liters per minute and pressure of 100 meters of water column at full speed. What is the approximate flowrate if the pump is operated at 60 percent speed?

A. 77 liters per minute
B. 60 liters per minute
C. 36 liters per minute
D. 22 liters per minute
E. None of the above

Commentary
The Affinity Laws for centrifugal pumps are given as:
• Flow is proportional to the pump speed
• Pressure is proportional to the square of the pump speed
• Brake horsepower (energy input) is proportional to the cube of the pump speed

Noting that the first Affinity Law states that the flow is proportional to pump speed, it would seem logical that the flow would be 60 liters per minute at 60 percent speed. Therefore, Answer B would appear to be the correct answer.

Not so fast. The pump operates at 60 percent speed, so it will generate a pressure of approximately 36 meters of water column. If the application involves pumping water into a process vessel located 40 meters above the pump, the flowrate will be zero – not 60 liters per minute. In this application, no flow will occur until the pressure of the pump exceeds 40 meters of water column, which is at approximately 63 percent speed. In short, there is not enough information about the process and installation in the problem statement to determine the flow at 60 percent of full speed, so the correct answer is Answer E.