When multiple pumps operate continuously as part of a parallel pumping system, there can be opportunities for significant energy savings. For example, lead and spare (or lag) pumps are frequently operated together when a single pump could meet process flow rate requirements. This can result from a common misconception – that operating two identical pumps in parallel doubles the flow rate. Although parallel operation does increase the flow rate, it also causes greater fluid friction losses, results in a higher discharge pressure, reduces the flow rate provided by each pump, and alters the efficiency of each pump. In addition, more energy is required to transfer a given fluid volume.
Parallel Pumping Basics
Designers can expand the operating range of a pumping system by specifying parallel pumping configuration (see Figure 1). A greater increase in flow rate will be seen when adding parallel pumps to a static head-dominated system. Parallel pumps can be staged and controlled to operate the number of pumps needed to meet variable flow rate requirements efficiently.
The total system flow rate is equal to the sum of the flow rates or contributions from each pump at the system head or discharge pressure. Parallel pumps provide balanced or equal flow rates when the same models are used and their impeller diameters and rotational speeds are identical. When possible, recommended design practice is to have parallel pumps moved from beyond Best Efficiency Point (BEP) at low system flow rates (fewer pumps operating) to the left of BEP at the highest flow rate. An ideal scenario will allow the pumps to have the highest possible average operating efficiency for the overall flow rate vs. time profile.
Dissimilar pumps may be installed in parallel, as well, as long as the pumps have similar shutoff head characteristics and/or are not operated together continuously unless provisions are made to prevent dead-heading.
Applications
In general, parallel pumps provide good operating flexibility in static head-dominated systems, but are not nearly as effective in friction-dominated systems. It is advisable to avoid operating two pumps in parallel whenever a single pump can meet system requirements. One exception is certain storage applications with time-of-day energy rates or high “peak period” demand charges. Also, be sure to take into consideration the amount of energy consumed by multiple pumps in contrast to the amount consumed by a single pump with adjustable speed drive control. Multiple pumps should be selected with head-versus-capacity performance curves that rise at a constant rate when these pumps approach no-flow or shutoff head.
Some efficient high-head/low-capacity centrifugal pumps used in process industries have “drooping” pump performance curves. These pumps supply peak pressure at a certain flow rate, and the pumping head decreases in approaching no-flow conditions. Identical pumps with drooping head-versus-capacity curves should not operate in parallel at variable flow rates under conditions in which capacity requirements can approach zero.
Example
A split-case centrifugal pump operates close to its BEP while providing a flow rate of 2,000-gpm at a total head of 138-ft. The static head is 100-ft. The pump operates at an efficiency of 90 percent while pumping fluid with a specific gravity of 1. With a drive motor efficiency of 94 percent, the pumping plant requires 61.4-kW of input power.
When an identical parallel pump is switched on, the operating point of the composite system shifts to 2,500-gpm at 159-ft of head (see Figure 1). Each pump now operates at 80 percent efficiency while providing a capacity of 1,250-gpm. Although the fluid flow rate increases by only 25 percent, the electric power required by the pumping system increases by 62.2 percent:
P2pumps = 0.746 kW/hp x (2,500 gpm x 159 ft) ÷ 3,960 x 0.8 x 0.94 = 99.6kW
For fluid transfer applications, it is helpful to examine the energy required per million gallons of fluid pumped. When a single pump is operating, the energy intensity (EI) is as follows:
EI1 = 61.4 kW ÷ 2,000 gpm x 60 minutes/hour x million gal/106 = 512 kWh/million gallons
When both pumps are operating, the EI increases as follows:
EI2 = 99.6 kW ÷ 2,500 gpm X 60 min/h x million gal/106 gal = 664 kWh/million gallons
When both pumps are operating in parallel, approximately 30 percent more energy is required to pump the same volume of fluid. The electrical demand charge (kW draw) increases by more than 62 percent. If the current practice or baseline energy consumption is the result of operating both pumps in parallel, pumping energy use will decrease by 23 percent if process requirements allow the plant to use a single pump.
References
Control Strategies for Centrifugal Pumps with Variable Flow Rate Requirements, U.S. Department of Energy Pumping Systems Tip Sheet #12, 2006.
Pump Systems Matter, www.PumpSystemsMatter.org
Suggested Actions
- Consider operating the minimum number of pumps that the system requires at any given time; one exception might involve off-peak pumping to storage tanks.
- Evaluate and compare multiple-pump scenarios to single-pump systems with adjustable speed controls.
Pumps & Systems, December 2006
Hydraulic Institute (HI)
Hydraulic Institute, the largest association of pump producers in North America, serves member companies and pump users worldwide by developing comprehensive industry standards, expanding knowledge by providing education and training, and serving as a forum for the exchange of industry information. In addition to the ANSI/HI pump standards, HI has a variety of resources for pump users and specifiers, including Pump LCC and VSP guidebooks, “7 Ways To Save Energy” training program and more. To download FREE executive summaries of HI’s “Pump Life Cycle Costs”, “Variable Speed Pumping”, and an index to ANSI/HI Standards, visit www.Pumps.org and www.PumpLearning.org.
Pump Systems Matter™ (PSM).
Developed by the Hydraulic Institute, PSM is an educational initiative created to help North American pump users gain a more competitive business advantage through strategic, broad-based energy management and pump system performance optimization. PSM’s mission is to provide end-users, engineering consultants and pump suppliers with tools and collaborative opportunities to integrate pump system performance optimization and efficient energy management practices into normal business operations.
PSM is seeking the active support and involvement of energy efficiency organizations, utilities, pump users, consulting engineering firms, government agencies, and other associations. For more information on PSM, to become a sponsor, or to download PSM’s FREE Pump System Improvement Modeling Tool™ (PSIM), an educational tool designed to show pump systems engineers how modeling tools can reduce cost and conserve energy, visit www.PumpSystemsMatter.org.
U.S. Department of Energy (DOE)
DOE’s Industrial Technologies Program (ITP), through partnerships with industry, government, and non-governmental organizations, develops and delivers advanced energy efficiency, renewable energy, and pollution prevention technologies for industrial applications. ITP has launched the Save Energy Now initiative to help the nation’s manufacturing facilities continue to thrive during a time of diminished energy supplies and rising costs. As a part of this initiative, ITP is sending DOE Energy Experts to the nation’s most energy-intensive manufacturing facilities to conduct 200 Energy Savings Assessments. See www.eere.energy.gov/industry for additional information on DOE’s energy efficiency activities.
BestPractices emphasizes opportunities for savings in plant systems such as motor, steam, compressed air, and process heating systems. BestPractices is a part of the Industrial Technologies Program, and offers a variety of resources addressing ways to reduce energy and maintenance costs in industrial process systems. This includes training workshops, software tools, a series of sourcebooks, case studies, tip sheets, and other materials, including several which focus on opportunities in pumping systems. For example, the Pumping System Assessment Tool (PSAT) aids in the assessment of pumping system efficiency and estimating energy and cost savings.
For more information, please contact: EERE Information Center; 1-877-EERE-INF (1-877-337-3463);
www.eere.energy.gov/industry/bestpractices.