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.
Additional Complicating Factors
Presuming that additional process and installation information is given, this problem would be further complicated if the vessel is operated under pressure. This is because different vessel pressures will require different pump pressures and, hence, different pump speeds.
David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
