An industrial facility can reduce the energy costs associated with its pumping systems, and save both energy and money, in many ways. They include reducing the pumping system flow rate, lowering the operating pressure, operating the system for a shorter period of time each day, and, perhaps most important, improving the system’s overall efficiency.
Often, a pumping system runs inefficiently because its requirements differ from the original design conditions. The original design might have been too conservative, or oversized pumps might have been installed to accommodate future increases in plant capacity. The result is an imbalance that causes the system to be inefficient and thus more expensive to operate.
Correct Imbalanced Pumping Systems
If the imbalance between the system’s requirements and the actual (measured) discharge head and flow rate exceeds 20%, conduct a detailed review of your plant’s pumping system. Calculate the imbalance as follows:
Imbalance (%) = [(Qmeas x Hmeas)/(Qreq x Hreq) – 1] x 100% ,
Qmeas = measured flow rate, in gallons per minute (gpm)
Hmeas = measured discharge head, in feet
Qreq = required flow rate, in gpm
Hreq = required discharge head, in feet.
A pump may be incorrectly sized for current needs if it operates under throttled conditions, has a high bypass flow rate, or has a flow rate that varies more than 30% from its best efficiency point (BEP) flow rate. Such pumps can be prioritized for further analysis, according to the degree of imbalance or mismatch between actual and required conditions.
Energy-efficient solutions include using multiple pumps, adding smaller auxiliary (pony) pumps, trimming impellers, or adding a variable-speed drive. In some cases, it may be practical to replace an electric motor with a slower, synchronous-speed motor—e.g., using a motor that runs at 1,200 revolutions per minute (rpm) rather than one that runs at 1,800 rpm.
Conduct quick reviews like this periodically. Especially for multi pump systems, this can be a convenient way to identify opportunities to optimize a system at little or no cost.
This example shows the energy savings that can be obtained by not using an oversized pump. Assume that a process requires 1,500 tons of refrigeration during the three summer months, but only 425 tons for the remaining nine months. The process uses two chilled water pumps operating at 3,500 gpm and requiring 200 brake horsepower (bhp) each. Both are used in summer, but two-thirds of the flow rate is bypassed during the remaining months.
One 3,500-gpm pump is therefore replaced with a new 1,250-gpm pump designed to have the same discharge head as the original unit. Although the new pump requires only 50 bhp, it meets the plant’s chilled water requirements most of the year (in all but the summer months). The older pump now operates only in the summer.
Assuming continuous operation with an efficiency (ηm) of 93% for both motors, we can calculate the energy savings from operating the smaller pump as follows:
Savings = (200 hp – 50 hp)/ηm x 0.746 kW/hp x (9 months/12 months) x 8,760 hours/year
= 790,520 kWh/year
At an average energy cost of 5 cents per kWh, annual savings would be about $39,525.
- Survey your facility’s pumps.
- Identify flow rates that vary 30%or more from the BEP and systems imbalances greater than 20%.
- Identify misapplied, oversized, or throttled pumps and those with bypass lines.
- Assess opportunities to improve system efficiency.
- Consult with suppliers on the cost of trimming or replacing impellers and replacing pumps.
- Determine the cost-effectiveness of each improvement.
About Jonathon Bell
Jonathon Bell is an entrepreneur, focused on building his family's legacy in the industrial pump market. Currently, he is focused in Latin America, building Dynapro Pumps Mexico from the ground up while contributing in Canada & the United States with Sales & Marketing efforts.
His commitment is developing teams through individual and partnered coaching, to bring out the best in each team member and giving them the tools to help them reach their goals. Guiding and teaching the core values of passion, evolving, and team communication, his teams and members become top performers in their respective fields.
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DOE/GO-102005-2160 October 2005 Pumping Systems Tip Sheet #6
Variable Speed Pumping: A Guide to Successful Applications, Hydraulic Institute and Europump (www.pumps.org), 2004.
Conduct an In-Plant Pump Survey, DOE Pumping Systems Tip Sheet, 2005. Trim or Replace Impellers on Oversized Pumps, DOE Pumping Systems Tip Sheet, 2005
Optimize Parallel Pumping Systems, DOE Pumping Systems Tip Sheet, 2005. Adjustable Speed Pumping Applications, DOE Pumping Systems Tip Sheet, 2005.
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