Part II: Throttling the pump

Making the case for variable-speed drives

Spitzer Headshot
Spitzer Headshot
David W. Spitzer

Last month, I was irked that the extractor feed pump operated at full speed with its flow being throttled by a control valve located in plain sight approximately one meter above the pump discharge. It seemed straightforward to me that it would be much more efficient to reduce the pump speed so that the pump output could match its load, thereby eliminating not only the dissipation of hydraulic energy across the control valve, but also eliminating the control valve entirely.

The concept of using variable-speed drives to throttle pumps and save energy was received with some apprehension. My peers had no problems with the equipment (there were a handful of variable-speed drives in the plant on specialized equipment) and with the concept. However, they wanted assurances that capital expenditures for variable-speed drives would result in predictable energy savings. That said, as a relatively new engineer at the plant with limited experience and no published literature/software available to calculate energy savings, I was first asked to demonstrate that I could accurately predict the energy savings.

We all agreed to test my calculations by temporarily connecting an unused variable-speed drive to a feed pump for a distillation column. The predicted energy savings using the variable-speed drive were calculated to be approximately 45 percent as compared to full-speed operation. The energy savings based upon measurements taken during actual full-speed and actual reduced-speed operation were between 40 and 50 percent.

Now that the ability to accurately calculate energy savings had been validated, variable-speed drive projects could be credibly presented for funding. While bothersome, the process of gaining credibility forced me to formalize my calculations and enabled me to write Variable Speed Drives: Principles and Applications for Energy Cost Savings so that others can have the calculations readily available for use.

Next month I will address the extractor feed pump.

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.

NOTE: David W. Spitzer’s Variable Speed Drives: Principles and Applications for Energy Cost Savings is available for purchase through ISA at

More in Variable Frequency Drives