Last month’s article reviewed the operation of a centrifugal pump with a variable-speed drive (August, page 12). Logic suggested that straightforward application of the Affinity Laws at 50 percent flow would result in pump operation at 50 percent speed and yield 87.5 percent energy savings. This is NOT correct!
|Read Part I of this series:
“Part I: Variable-Speed Drive Energy Savings”
The overwhelming majority of centrifugal pumps need to overcome the static pressure in the piping system. Assume the liquid is being pumped to an atmospheric tank located six meters above the pump. This means that a pump speed of approximately 77 percent (square root of 0.6 per the second Affinity Law) is required to overcome the static head in the discharge pipe and initiate flow. There will be no flow at the 50 percent pump speed. Pump speed requirements will be even higher if the tank is pressurized and/or if flow with pipe friction is considered.
The proper methodology is to calculate the required pump discharge pressure under flowing conditions, then calculate the required speed, and then calculate the power requirements for that speed. These calculations are developed in my variable-speed drive book that is available at isa.org.
This oversight has been made by a number of people who took the time to write papers about justifying the application of variable-speed technology—papers that I had to reject out of hand. I suggest that it is fair to say that a large number of users make this same oversight. Hopefully this article will ensure that you are not one of them—and with some luck, it will help reduce the number of otherwise useful technical papers that I have to reject.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.
Mr. Spitzer can be reached at 845 623-1830 or www.spitzerandboyes.com. Click on the "Products" tab to find his "Consumer Guides" to various flow and level measurement technologies.