VFDs can reduce energy consumption and water loss in variable pumping systems.

As the cost of energy and water and wastewater services continues to rise, operators are looking for ways to reduce the costs of these basic, yet crucial, utilities. Proposed energy efficiency standards from the U.S. Department of Energy (DOE) and other federal measures could help.

In the water sector, the DOE has a mandate to develop and enforce national minimum energy efficiency standards for any product if significant savings can be realized in a cost-justified and technically feasible manner. During the past year, the DOE and stakeholders have been negotiating a regulation for pump efficiency. That rule, now in the Notice of Proposed Rulemaking (NOPR) phase, would apply only to clean water pumps of these types:

  • End suction close-coupled
  • End suction frame-mounted/own bearings
  • In-line
  • Radially split, multi-stage, vertical, in-line, diffuser casing
  • Vertical turbine submersible

The energy efficiency of existing water distribution systems is greatly impacted by pumps and motors. Utilities are incentivizing facilities to install variable frequency drives (VFDs) to reduce electrical surges caused by starting and stopping equipment. These controls can reduce water hammer and lower maintenance costs.

By operating a pump/motor system at constant pressure or flow, a VFD can help reduce energy consumption. This energy consumption is the result of the three affinity laws, which describe the relationship between speed of the pump (n), flow (Q), pressure head (H) and electrical power consumption (P) (see Equations 1-3).

Equation 1-3

These calculations show that the power to run a pump is relative to the cube of the speed. If a pump is slowed by 20 percent, it will experience a nearly 50 percent reduction in power or energy consumption. Even small decreases in speed can result in energy savings. VFDs can also regulate the pressure of water distribution systems. Before VFDs were commonly used, water distribution systems typically incorporated pressure switches to control pumps. When the system hit a low pressure point, the pump turned on; when it hit a higher pressure point, the pump shut off. A facility may have used two pumps: a smaller one for slow night hours and a larger unit for during the day. While this type of control often causes pressure fluctuations, it is common practice.

A VFD can maintain pressure at a constant set point by increasing pump/motor speed. By maintaining constant system pressure, a VFD can help reduce water leakage. For example, a ¼-inch hole in a distribution pipe can leak as much as 4 million gallons per year at 50 pounds per square inch (psi) or more than 5 million gallons per year at 80 psi. By keeping the pressure more constant, VFDs can help reduce non-revenue water losses (see Figure 1).

Figure 1. Water losses at different operational pressures (Courtesy of Danfoss)Figure 1. Water losses at different operational pressures (Courtesy of Danfoss)

While VFDs can help reduce energy consumption, which could lead to lower operating costs, some additional savings may not be initially realized:

  • Protection of pumps and assets
  • Reduction of maintenance costs
  • Lower risk of bacteria/contamination of tap water
  • Lower risk of road breaks
  • Reduction in pipe repair cost
  • Extended service life of network
  • Postponed investment in system upgrades
  • Improved control performance
  • Increased redundancy
  • Reduced load on supervisory control and data acquisition (SCADA) system because of dedicated VFD software features

The wastewater side of the system requires more processes and a greater number of motors and pumps. But because wastewater pumps operate in different conditions than clean water pumps, they have not been included in the DOE rule. While the DOE could develop a rule for wastewater pumps in the future, the schedule for efficiency rulemaking under the current administration appears full, and a federal initiative in the near term seems unlikely.

The U.S. Environmental Protection Agency (EPA) through its Clean Air Act (section 111d), on the other hand, will require states to develop compliance plans, which could include state energy efficiency programs. Wastewater treatment plants could be included in these regulations. While states are unlikely to require a minimum efficiency specifically for wastewater pumps, the EPA could incentivize plants to reduce energy use.

Wastewater operations can begin to increase efficiency by adding VFDs to motors and pumps in the same way many clean water applications have. Utility rebates can help with acquisition and installation costs, and the resulting lower energy bill can reduce operating costs.

Additional VFD Features

For smaller facilities, using a larger SCADA system may be out of the question. But today's VFDs typically include integral controllers that can be used to maintain level, pressure, flow, dissolved oxygen (DO), turbidity or other monitored processes. An analog signal sent from the monitoring equipment to the VFD allows the VFD to maintain a constant process output. Internal communication modules transmit data to other areas, allowing end users to monitor the current status.

Integral cascade controllers can operate multiple pumps simultaneously, allowing for better overall control. When used in a variable pumping system, where pumps of different flow capabilities are installed and staggered, VFDs can increase system efficiency. This type of control can offer the most efficient operation of a pumping system and the highest system efficiency.

In almost any size system, the use of VFDs on pump motors means greater flexibility and better process control. By reducing the surges that a plant may see during an event or normal loading, VFDs allow a plant to more closely handle waste influx by regulating the flows into the plant. VFDs can assist the existing SCADA or overseeing system.

Drives can also offer features—such as a deragging function—that help maintain pump efficiency. This feature will rotate the impeller backward (on a pump that can be operated backward) to dislodge solids, stringy materials or other debris, which can help the impeller pump at optimum performance.

In addition to deragging capabilities, drives designed specifically for the water/wastewater market tend to offer additional features that facilitate motor/pump operation, including initial ramp and/or check valve ramping, flow counting, flow confirmation, and no-flow or low-flow/dry pump protection. Additionally, analog and digital output cards, communication cards and specific application cards are available to assist in control functions.

As the water/wastewater industry continues to modernize and expand—and issues such as reducing energy consumption and water loss become more important and regulated—VFDs will become more commonplace in many applications.