Second Pump Installed in Parallel to Improve Energy Efficiency of Water Circulation SystemÂ
A pump used in an open system for pumping water from circulation tank into two wire tanks was worn down and had decreased efficiency. Analysis was done to compare the cost benefit of installing two modifications to the existing system. With an increased demand required only during startup, the conclusion was installing a new smaller pump parallel to the old pump to provide the base load duty. The old pump would only be used during the start up period.
Category: Blogs, Case Studies September 20, 2025
Intended Audience: Water sector, General Pump System Selection
Objective: Improve the efficiency of an existing circulation tank pump system.
Description of System: Water was being pumped from the paper machine circulation water tank into two wire tanks, which were maintained in an overflow state to guarantee constant pressure. The rate of flow was controlled by throttle valves to maintain the surface level. By monitoring the duty cycle, it was found that the basic rate of flow demand of the system was 540 to 720 m3/h (2370 to 3160 USgpm) and the maximum demand of 1080 m3/h (4740 USgpm) was only required during start up.
The installed pump was also worn and had decreased efficiency.
Description of Intervention: Two alternatives were considered:
A) A new smaller pump would be installed parallel to the old pump to provide the base load duty. The old pump would be used only during the brief start up period. (New valve installations are required.)
B) A frequency converter would be installed to control the existing pump. This also would require the motor to be changed or rewound.
See Table 1 below:
Table 1: Summary of ResultsÂ
Summary of Results: With basically two distinct demand rates and constant head requirements, the two pumps offer a better pay-back and LCC than the variable speed alternative. For full cost analysis refer to Table 1.
Conclusion: Those with distinct duty conditions, rather than infinitely variable conditions, and with a high percentage of operating time, will benefit from installing a second pump rather than operating a single, larger pump at off-design duty. In addition, they will often give a better payback than using a variable frequency drive.
Written by:
Members of the Committee, 1st EditionÂ
Published In: Pump Life Cycle Cost: A Guide to LCC Analysis for Pumping Systems, 2nd EditionÂ
Year of Publication: 2021Â
Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping Systems – 2nd Edition – Pumps.org
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