Quiz corner: Estimating pump discharge pressure 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 is the approximate pump discharge pressure if the variable speed drive operates at 90 percent speed to provide the same water flow after the modification? 

A. 100 feet of water column

B. 90 feet of water column

C. 80 feet of water column

D. 70 feet of water column

Water flow is approximately 100 gallons per minute at a discharge pressure of 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, the best answer is Answer C. 

Additional complicating factors

This calculation is a straightforward application of the Affinity Law describing the relationship between discharge pressure and speed. Note that the flow remains the same despite being driven by a significantly lower discharge pressure. This can occur because the pressure drop across the control valve has been eliminated due to the removal of the control valve.  

It may seem counterintuitive to have the same flow at a lower pump discharge pressure. However, it is wasteful to dissipate hydraulic energy (pressure) across a control valve when a variable speed drive can control the pump in a more elegant and energy-efficient manner.