Constant speed booster replacement with Grundfos variable speed system

For years, constant speed systems have been used in designs for the plumbing industry. These designs include three main components: pumps, constant speed starters and pressure reducing valves that regulate the pumps. Pumps are sized to satisfy peak demands, which comprise only 15% of booster system operating times; however, constant speed booster systems, which use restricting devices (PRVs), waste energy by operating 85% of the time. Operation of a constant speed booster system could be compared to running a car with an internal combustion engine by pressing the gas pedal fully and regulating speed with a brake pedal.

*Constant speed pump performance.

In addition to wasted energy, there are additional disadvantages associated with running constant speed booster systems:

  • Pressure-reducing valve maintenance (PRV diaphragm is easily damaged)
  • Loud operation
  • Water hammering
  • Electrical spikes during full-voltage starts

*Factory assembled triplex constant speed system with pressure reducing valves.

There are multiple field assembled constant speed booster systems across the USA. Pumps, control panels, and valves for these systems are supplied separately and installed in the field. Standalone booster systems are used for small as well as large high-rise buildings and have energy consumption and performance disadvantages as well as additional challenges associated with the involvement of multiple part vendors and multiple points of responsibility.

*Field assembled duplex constant speed system with pressure reducing valves.Era of variable speed pumpsRegulating the speed of three phase motors was a significant challenge for electrical engineers until the widespread acceptance of variable speed drives (VFDs) in the industry. In the past, even large utility pumps ran at full speed to satisfy 15% peak loads, which wasted energy in the bypass lines. Electricians used different techniques in order to reduce starting currents and regulate the speed of AC motors (i.e., resistors, transformers and Y/D starters etc.). VFDs use complex technology to convert AC to DC using rectifiers and then to invert DC to variable frequency AC.

VFD evolution has been a long process with reliability issues in the first stages of development and implementation. Drives were expensive and only used on large industrial motors. VFD technology has improved and advanced tremendously over the last decade. Today, the cost and reliability of a VFD is comparable to the cost and reliability of a NEMA combination starter. Modern variable frequency drives provide a compact and competitive solution to the problem of regulating speed on AC motors. VFD-based booster systems allow for the reduction of power used to drive a pump during periods of reduced demand.

*Diagram shows 50% energy savings with 20% speed reduction.

*Grundfos® Multi-B variable speed duplex booster system.

In addition to energy savings, there are other benefits to running at lower speeds. Hydraulic forces on an impeller, created by the pressure profile inside the pump casing, reduce approximately with the square of speed. These forces are carried by pump bearings, so reducing speed increases bearing life. In addition, vibration and noise are reduced and seal life is increased, provided that the duty point remains within the allowable operating range.

*Grundfos® MPC variable speed duplex booster system.

Most existing booster systems are oversized, many by more than 20%, and thus provide substantial opportunities for system optimization; whereas, Grundfos® booster systems can adjust performance to a system demand. Also, Grundfos® provides VFDs integrated in their motors. All components that come in contact with water are constructed entirely with stainless steel and are NSF approved.


Variable Speed Pumping: A Guide to Successful Applications: Executive Summary. Washington, D.C.: U.S. Dept. of Energy, 2004. DOE/GO-102004-1913. U.S. Department of Energy. U.S. Department of Energy Industrial Technologies Program, Europump, and Hydraulic Institute, 1 May 2004. Web. 5 June 2015.

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