- 12 July 2005-

Variable speed pumping: myths and legends

Variable speed drives offer many advantages in the operation of centrifugal pumps but, in spite of this, many barriers remain to implementing the technology to its full potential. In this article from the USA, Mike Pemberton of ITT Goulds Pumps seeks to dispel the myths and legends surrounding variable frequency technology and its application, arguing that it has now developed to the point that these barriers are, for the most part, perceived rather than actual.

In common language, a myth is a fiction – something that is untrue. A legend is a story from the past about a subject that was, or is believed to have been, historical. So, myths and legends encompass beliefs that may or may not be true. And legends are often so embellished that it is hard to discern how much of the story is factually true.

When considering the application of a variable frequency drive (VFD) for centrifugal pump control, one often encounters myths and legends. How much is fiction and how much is fact? Today, it is increasingly important to sort through the misinformation in order to realize the significant benefits through application of this tech-nology.

Traditionally, a fixed-speed pump and control valve have been used to regulate process flow. Even though VFD technology has grown rapidly in acceptance, the technology and benefits are often poorly understood. As such, there is uncertainty surrounding its use for pump control. There are any number of reasons for this confusion, including:

• Lack of knowledge about the difference in hydraulic performance between fixed and variable speed pump control

• Lack of knowledge concerning control and failure modes, especially for mission-critical applications

• Perception that a VFD is always more expensive than a control valve

• Concerns about the reliability of the electronics platform.

“Therefore, even the lover of myth is in a sense a philosopher; for myth is composed of wonders” – Aristotle

Many of these concerns stem from bad experiences before VFD technology matured. These legacy issues can be put into perspective by relating them to the evolution of PC technology. The cost of low-voltage VF drives continues to drop while reliability and functionality increase. As a result, VFD technology has become a highly reliable, cost-effective alternative to using fixed-speed pumping systems in low static head applications. There are compound benefits through implementation. These include energy and maintenance savings, pump and process reliability improvements, better process control and less fugitive emissions. Also, on new projects, VFD application can reduce overall capital cost by eliminating valves and starters plus the associated wiring and pneumatic lines. In many cases, smaller pumps with lower horsepower motors can be used. In terms of piping, smaller diameters often suffice and by-pass lines can be eliminated.

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