A green product means different things to different people. For some, a green product is alternatively powered, such as some of the recently introduced solar-powered well pumps. For others, green refers to products made from sustainable materials. In this article, green means a pump or pump system that uses less energy or reduces material usage in its design.
Pumps are often considered the second most common machine in the world after the electric motor. Billions are running in the world, bringing drinking water, disposing of waste and working in cars, buses, planes, power plants, process facilities, cooling systems, etc. With all of these pumps running in the world, even a relatively small increase in efficiency could result in a huge reduction in energy consumption.
Energy reduction in pump and pump systems can come from several areas. Most pump professionals would agree that the biggest potential energy reduction opportunities lie in the proper application and control of pumps with respect to the system demand. Simply, this means properly selecting and controlling the correct pump for the application. Many end users probably believe that their pumps run close to their best efficiency. In a relatively famous study, "Expert Systems for Diagnosis and Performance of centrifugal pumps," the Finnish Technical Research Center (www.vtt.fi) found that the average pump operates at less than 40 percent efficiency in the field, and 10 percent of pumps operate at less than 10 percent efficiency. The solution to many of these efficiency problems is not selecting a new higher efficiency pump or motor, but analyzing the pump system to ensure that the pump is properly sized for the application.
CFD of impeller and volute. Image courtesy of Concepts NREC.
Several free online system calculation tools allow even a novice user to input the components of his system with pipe sizes and generate a system curve showing the expected losses from the pump system. One such tool, Pump-Flo (www.pumpflo.com) from Engineered Software, allows the user to develop a system curve and then select from 85 different manufacturers to find a pump that matches the requirements.
What about pump manufacturers? If the majority of the potential energy savings lies with the pump's proper application, is room left to improve the products from the manufacturer's point of view? I routinely find clients with many underperforming pumps in their portfolios. These pumps are far below the efficiency of the competitive pumps in the marketplace. Modern hydraulic design tools can help the design engineers create and analyze pumps that can compete in the global market with higher efficiencies, lower Net Positive Suction Head Required (NPSHr) and better reliability.
Modern mechanical design tools allow the calculation of stress levels in complicated shapes and allow the designer to remove excess metal in lower stress regions while maintaining the needed structural strength in others. This leads to less material usage, lighter components and less-expensive component costs. By combining these two initiatives, many pump companies can use these new tools to lower costs while providing a greener, more competitive product.
Modeling the Impeller
The impeller is the only element in the pump that adds energy to the liquid. Maximizing the amount of energy transferred into the liquid is the most crucial step in the improvement of a pump's performance and its efficiency. Traditionally, designers would manipulate the inlet and outlet widths and radii to achieve reasonable velocities and vane angles at the inlets and outlets of each pump component. Once the inlet and outlet angles were determined, the shape of the blading in-between was often determined more by art than science. Some designers used a linear angle change from inlet to outlet; others shaped the vane based on a logarithmic spiral. While these techniques produced many good pumps with excellent performance and high efficiencies, in too many others the lab results often fell short of expectations. This design technique relies heavily on the designer's feel and experience. If the designer was too aggressive with the change in angle, the impeller would end up with short vanes that were often incapable of guiding the fluid or imparting the required amount of energy. A less aggressive designer might change the vane angle at a lower rate, resulting in a long vane passage that increased the friction and reduced performance and efficiency.
Meanline pump analysis. Image courtesy of Concepts NREC.
Streamline curvature analysis of pump impeller. Image courtesy of Concepts NREC.