Decreased vibration and increased seal life are among the benefits.
by Steven Boren
August 10, 2018

Another major benefit of VFDs is in mitigating stress on an aging pump system infrastructure. In many municipalities where the piping system is old and decaying, a full voltage start of the pumps across the line could cause damage to pipes, pumps and motors, and increase expenditures required to maintain the system. The soft start and stop function that the VFDs provide prevents the violent start and rapid acceleration of inertia that occurs with a full voltage start while eliminating the electrical inrush to the pump motor, which can decrease a municipality’s total electrical demand, thus minimizing monthly electrical expenses. This minimizes the mechanical stress on the motor winding, pump bearings, shaft and impeller keyway. It also eliminates water hammer in the pump system, which is detrimental to an aging piping infrastructure.

Achieving a Higher Efficiency

In some cases higher pump efficiencies can be achieved with the use of VFDs if the operating point of the system is to the right of the BEP.

When slowing the pump down in systems that have a fair amount of static head, the pump can be operated at a speed that will operate at the pump BEP. Adding a VFD to a pumping system will not guarantee energy savings on every application. When evaluating the use of VFDs on the basis of energy savings alone, one must take into account specific energy. In short, specific energy can be defined as the cost to pump a given amount of fluid annually. This evaluation should take into consideration total system efficiency, annual gallons pumped by the system, cost of energy, pumping system characteristics, pump specific concerns and other criteria.

Additional VFD Benefits

  • Decreased vibration in the pump and pumping system when pump speeds are decreased to control flow versus using control valves
  • Decreased noise as pump speed is decreased
  • Increased seal life within the pump, as long as the pump operates in the preferred operating range (POR)
  • Decreased net positive suction head required (NPSHR) as pump speed decreases with a VFD to allow sumps to be pumped to a lower level without causing cavitation within the pump
  • Reduced potential of cavitation
    • Fluid pressure drops as the fluid is split into separate streams as it enters different vanes of the impeller.
    • If fluid pressure coming into the pump is at insufficient levels above the vapor pressure in the impeller, bubbles will result.
    • As these bubbles travel up the impeller, pressure increases causing the bubbles to collapse. The pressure wave resulting from bursting bubbles is cavitation.
    • The cavitation bubbles are small explosions that also release enough energy to create small pits in the impeller surface. Over time, these pits become larger causing excessive wear on the impeller, and ultimately failure.

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