Pumping systems are found in nearly every type of infrastructure: municipal water and wastewater; commercial buildings; industries such as oil, gas and mining; irrigation; and even residential applications. The sheer number of pumping applications accounts for nearly 20% of the world’s electrical energy demand. Moreover, pumping accounts for 25% to 50% of energy use in certain industrial operations.
The energy consumed by a pumping system depends on the design of the pump, the design of the system and the way the system is sized and operated.
Pumping, especially centrifugal pump applications, will produce the largest potential for energy savings, even more than energy-efficient motors. Most people are concerned about initial costs, but when looking at overall life cycle costs, energy accounts for 40%, more than any other single element.
The way the pump system is sized and operated is key. In many cases the daily and weekly pump operating cycle varies greatly, resulting in less than optimum operating conditions. In other applications, pump systems may be oversized to allow for future expansion, which reduces system efficiency and increases energy consumption.
As part of a growing trend, manufacturers are revamping designs to improve efficiency, utilizing improved hydraulic designs. The result is improved operational economy, which will reduce the total life-cycle cost. Pump systems are typically composed of the pump, driver, piping, valves and end-use equipment. Any evaluation of the pumping system should consider complete system dynamics versus the pump itself.
One key element of pump system design is the driver, which may include a variable frequency drive (VFD). Some people might believe that drives are the solution for everything. But if you have a properly sized pump that runs at constant pressure and flow 24/7, you probably do not need a drive; a traditional motor starter, or soft start (for inrush and mechanical benefits), is probably all that is required.
However, VFDs do offer many benefits in certain applications:
- If the pump is variable pressure or flow, then a VFD can provide more enhanced energy savings than mechanical means;
- If the pump is a constant speed pump that is oversized, reducing the pump speed can correct for oversizing. The reduction in speed will also save energy; and
- Most applications will be variable loading and use an oversized motor. For this, the VFD can solve both challenges simultaneously.
Benefits of VFDs in Pumping
The secret to the success of VFDs in energy savings is something called the affinity laws. Given a set of operating conditions, these relationship laws demonstrate what will happen to your system if you vary your operation from these conditions. An interesting aspect of these laws is that power usage possesses a cubed relationship to speed. If you decrease a pump to 50% of full speed, you will decrease power consumption by about 88%. This surpasses the energy savings from mechanical means such as inlet valve throttling. Once the ramifications of the affinity laws in pumping are understood, the benefit of VFDs is revealed.
The next step for system optimization is intelligent pumping, which is defined by the ARC Advisory Group as the combination of a pump and a VFD with digital control capability.
Intelligent pumping takes the system to the next level of optimization. It goes beyond simply regulating the pump speed to meet demand and looks at items like measuring and running the most efficient pump during peak energy rates and flagging the least efficient pump for maintenance. Other items include cavitation and pipe burst protection and anti-jam and pipe-fill functions.
According to the U.S. Department of Energy, the potential energy savings with intelligent pumping systems can be as much as 20%.
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