Drives with embedded pump functions can provide advantages.
by Dan Peters
June 29, 2018
  • line fill or pre-charge function to fill empty lines at a reduced speed to prevent water hammer when line filling is complete
  • automated regulation of vital variables such as constant pressure, level or flow
  • multiple sensor inputs to allow an automated response to system changes, such as suction control for constant pressure systems
  • multiplexing pumps to allow for wide variations in demand while maintaining efficiency
  • loss of pump prime detection
  • high, low feedback detection with action, fault or alarm
  • no flow detection, or sleep, with auto restart on demand
  • auxiliary control for lube pumps, tank level or low city pressure in booster applications
  • flow control or limiting and accumulations
  • multiple set points and many more functions that vary by manufacturer
three VFDsImage 3. Building booster station with three VFDs communicating serially for multiplexing with no PLC
two pump systemImage 4. Two-pump system with one on the VFD and one running across the line and controlled by the VFD

One of the most complex applications that had been the exclusive implementation of the PLC in past years is multiplexing multiple pumps and bringing them on and off as demand changes.

This is preferred to sizing the pump on expected maximum demand, which can result in running the pump to the left of the performance curve. This is a condition that can accelerate pump wear as well as result in a loss of wire to water efficiency. In some cases, end users have oversized a pump in anticipation of future demand, resulting in running the pump in an inefficient mode for extended periods of time. If that increase does not materialize, the result is a waste of the capital investment.

A more elegant option is to multiplex multiple pumps together that discharge into a common manifold.

Using a VFD capable of multiplexing allows the end user to purchase pumps that match current demand and add additional pumps at a later date if the demand requires additional flow. All that would be required is the common manifold to plumb in the additional pumps. Such a technique has been successfully applied to agricultural and building booster stations for more than a decade.

vertical turbinesImage 5. 150-hp vertical turbines multiplexed with one running on VFD and three running across the line
pumps multiplexedImage 6. Four pumps multiplexed on one VFD running one pump and the other three running across the line

These systems vary in size from multiple 10-horsepower (hp) pumps on building booster applications to multiple 500-hp lift stations supplying large pivot farms. Currently, there are hundreds of these systems operating in the United States.

Even if the maximum demand is known and if the actual need at any one point is variable, the maximum demand can be divided up with multiple pumps to better match their performance curves.

Any form of multiplexing gives the user multiple options, and losing one pump will not result in bringing the entire system down. If any pump fails, there are others still running, and if the VFD fails, the remaining pumps can run in bypass. Getting a replacement VFD quickly is a better prospect than getting a replacement PLC programmed on short notice.

VFDs may not replace PLCs in all applications. However, VFDs are now capable of replacing many to the advantage of the end user in the form of reduced procurement cost, enhanced control of setup, available technical support, training of staff and overall standardization. VFD manufacturers are constantly working to increase features so the future will belong to more VFDs with embedded pump specific firmware.

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