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 a parallel pumping conﬁguration (see the ﬁgure). A greater increase in ﬂow rate will be seen when adding a parallel pump 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, 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.
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.
A split-case centrifugal pump operates close to its BEP while providing a ﬂow rate of 2,000 gallons per minute (gpm) at a total head of 138 feet (ft). The static head is 100 ft. The pump operates at an efﬁciency of 90% while pumping ﬂuid with a speciﬁc gravity of 1. With a drive motor efﬁciency of 94%, 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 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.
- 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
About Jonathon Bell
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DOE/GO-102006-2227 October 2006 Pumping Systems Tip Sheet #8
Control Strategies for Centrifugal Pumps with Variable Flow Rate Requirements, U.S. Department of Energy Pumping Systems Tip Sheet #12, 2006.
U.S. Department of Energy Washington, DC 20585-0121 www.eere.energy.gov/industry
“Control Valve Replacement Savings,” U.S. Department of Energy Performance Optimization Tip, Energy Matters, July 1998; available online at: http://www.nrel.gov/docs/legosti/fy98/23382.pdf