|Optimize Parallel Pumping Systems|
|Written by USDOE Office of Industrial Technologies|
|Thursday, 24 June 2010 08:02|
Optimize Parallel Pumping Systems
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
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, a 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.
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.
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 the figure). Each pump now operates at 80% efficiency while providing a capacity of 1,250 gpm. Although the fluid flow rate increases by only 25%, the electric power required by the pumping system increases by 62.2%:
P2 pumps = 0.746 kW/hp x (2,500 gpm x 159 ft) / (3,960 x 0.8 x 0.94)
= 99.6 kW
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 gallons/106 gallons)
= 512 kWh/million gallons
When both pumps are operating, the EI increases as follows:
EI2 = 99.6 kW / (2,500 gpm x 60 minutes/hour x million gallons/106 gallons)
= 665 kWh/million gallons
When both pumps are operating in parallel, approximately 30% more energy is required to pump the same volume of fluid. The electrical demand charge (kW draw) increases by more than 62%. If the current practice or baseline energy consumption is the result of operating both pumps in parallel, pumping energy use will decrease by 23% if process requirements allow the plant to use a single pump.
FOR ADDITIONAL INFORMATION, PLEASE CONTACT:
EERE Information Center
Industrial Technologies Program
Energy Efficiency and Renewable Energy
U.S. Department of Energy
Washington, DC 20585-0121