Many systems are designed utilizing centrifugal pumps to move fluid through the system. Currently there is an effort to minimize operational costs by upgrading constant speed pumps with Variable Frequency Drives (VFDs) or providing an initial design using VFDs. Process Engineers and Control Engineers need to understand the proper application, operational requirements and constraints associated with both the constant speed and variable speed designs.

In order to understand the pump operation in the process system, the Process and Control Engineers need to understand the information the pump manufacturer provides and the pump’s relationship to the system into which it will be installed.

This whitepaper will review typical pump curves provided by the manufacturer, the relationship of these curves to the system in which the pump is installed and the differences between the constant speed and variable speed operation.

Older pump operating and system resistance curves in the plant files are typically based on constant speed operation of the pump. By using the pump “Affinity Laws” the control and process engineers can generate variable speed curves for that pump. Using these updated curves the engineers can then evaluate and recommend approaches to the system control, pump protection and develop associated costs and benefits…

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