Quiz Corner: Pump flow at reduced speed

Water is pumped up to a tank located 80 feet above a pump rated for 100 gallons per minute at 100 feet of head at 3,600 rpm. What is the flow rate when the pump operates at 80% speed?

A.    0 gallons per minute

B.    30 gallons per minute

C.    60 gallons per minute

D.    80 gallons per minute

The first Affinity Law implies that the flow generated by the pump is proportional to pump speed so one might be tempted to select Answer D.

However, the pump must generate sufficient pressure to enable the fluid to flow. The pump discharge pressure varies as the square of the pump speed so the pump discharge pressure at 80% speed is approximately 64 feet (0.8 x 0.8 x 100). However, the tank is located 80 feet above the pump so not enough pressure is generated for the water to flow. Answer A is correct.

Additional complicating factors

The flow at higher pump speeds can be determined using system curves in conjunction with the pump curve showing the relationship between pump speed and resultant head generated by the pump.

David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.