Optimizing Pumping Systems to Minimize First or Life-cycle Cost
Judy Hodgson, DuPont Company; Trey Walters, P.E., Applied Flow Technology - Presented at the 19th International Pump Users Symposium, February 2002
The potential cost and energy savings from pumping systems is great. Recent studies have found that pumping systems account for about 20 percent of world energy usage (Frenning, et al., 2001). Efforts that minimize wasted energy in these systems would not only have substantial economic savings, but an equally important environmental impact, as well. Although savings can be made by optimizing existing systems, the greatest opportunities are in systems yet to be built. The reason being that in new designs the piping can be included as one of the variables that the engineer can modify to optimize the system. In large existing systems, it would be cost prohibitive to make a piping change.
Unfortunately, pumping system design engineers work in an environment where budget and schedule constraints limit their ability to optimize their designs using traditional methods. The number of variables in complex pumping systems makes such optimization impractical, even with modern hydraulic analysis software. Most of the design engineer’s effort is focused on ensuring the system will merely function properly. With the abundant opportunity for cost and energy reduction in new pumping systems, the need exists for technologies that will allow engineers to optimize pumping system designs to minimize cost and energy usage. The commercial software, AFT Mercury (now the Automated Network Sizing Module), addresses this need.
Numerical optimization methods offer a powerful new technology for pump users when combined with pumping system analysis software. Whether the design goal is to reduce first costs or life-cycle costs, this technology promises to significantly reduce pumping system costs and energy usage.
Optimization methods work by automatically selecting pipe and pump sizes to minimize cost. Design engineers define the constraints for the system, such as flowrate, NPSH margin, or fluid velocity. The optimization software then finds the combination of pipe and pump sizes to minimize the cost while satisfying the constraints.
A new design concept is introduced called the optimal pumping system operating point (OPSOP). In simple terms, the OPSOP uses cost data to identify the optimum tradeoff in pipe, pump, and (optionally) energy costs for a system that may have one or more duty points. Using this information, a new and improved method of pump sizing is described.
To establish benchmark comparisons for typical petrochemical pumping systems, these optimization methods were applied to four previously designed systems. With a modest amount of effort, first cost reductions were as much as 17 percent, and life-cycle cost reductions were as much as 72 percent (based on 10 years), with savings of over $100,000 in several cases.