Common Misconceptions

The Truth About Variable Speed Pumps

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Variable Speed Pumps Not Efficient in Lift Stations

Variable Speed Pumps have been introduced into the waste water lift station design with hopes of improved efficiency operation. However, Variable Speed Pumps:

  • Cost more to purchase than single speed pumps
  • Have been proven to be less energy efficient than single speed pumps in waste water lift stations by actual documented tests

The following is an excerpt out of:
Assessing variable speed pump efficiency in water distribution systems
By Thomas Walski

“Use of variable speed pumping with storage: I have conducted numerous analyses comparing variable speed pumping to constant speed pump in systems for the relatively flat system head curves in water distribution systems (Walski, 2001, 2005, 2111; Walski, Bowdler and Wu, 2005). In each case, when a pump is selected to correctly match the system, the constant speed pump has a lower energy cost than the variable speed pump whether the storage is on the discharge side as in elevated water storage or on the suction side as in a wet well at a sewage pumping station. I have done these comparisons so many times that I jokingly refer to it as Walski's Law, "The most efficient speed to run a variable speed pump is OFF." As long as there is storage to enable the pump to be turned off, it is best to run it at an efficient point and then turn it off. Running a variable speed pump at a low speed is inefficient."

(Dr. Thomas Walski, Vice President of Engineering for Haestad Methods, was recently elected to the membership rank of "Fellow" by the American Society of Civil Engineers (ASCE). Dr. Walski is one of the most published and recognized water-resources modeling experts and educators in the world. He is a three-time winner of the best paper award in Distribution and Plant Operation for the Journal of the American Water Works Association and is the former editor of the Journal of Environmental Engineering.)

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