Pumps & Systems, February 2009
The operating costs of pumps, in relation to the service life, are considerably higher than the initial investment. Realizing savings on existing pumps means looking at operational efficiency and differential head. Mechanical retrofits are common in the market, but do we know enough about power retrofits to improve operations and save money on pumps? First, it is important to know if the "correct" pump is in place or if the pump is still producing the desired output after years of use. Over time, it may have become unprofitable, technically outdated and in desperate need of maintenance.
The reasons for a loss in the desired output of the pump can be found in the following:
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The pump operating conditions might have been changed
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The market might be using new (better) techniques
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Conditions of the suction side and excessive wearing clearances have undergone a negative change, i.e., cavitation occurs; power and pump thrust can also be affected, requiring the existing pumps to be overhauled due to their accumulated operating hours
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The pumps are to be used or relocated for different purpose
Energy Costs
Figure 1 demonstrates how energy cost varies with power input and annual operating hours. The higher the annual energy cost, the greater the savings from improving the energy balance. Reducing energy demand by 13.8-kW leads to annual savings of $10,000 (considering a total of 8,000 annual operating hours and a specific energy cost of $0.09 per kWh).
The exact potential to optimize the energy consumption and operational performance on the installed pumps is often unknown. We might be focused on keeping the pumps running and not on potential performance improvements. For this reason, it is important to analyze the existing installation and look into its operating conditions to see if energy consumption and operational performance can be improved. The first step in this process is to identify the exact operating conditions needed in the operating process. These might have changed slightly since the pump was first selected. It is also important to make field performance tests to measure the actual flow, suction pressure, discharge pressure, power and pumping liquid characteristics. Comparing the test output with actual requirements gives a good insight into the needed changes.
Implementing the Changes
Ruhrpumpen developed the Synergetic Retrofit® method to adapt existing pump installations to new operating conditions, improve efficiency and save power by replacing old impellers and wear rings with optimally adjusted impellers and wear rings . All pump part dimensions and price determining parts such as casing, pump shaft, shaft seal and bearing are retained without any alteration. This also applies to the baseplate and foundation.
Partial flow and high flow impellers made according to this technology can extend the life cycle of pumps, optimize a pump's adaptation to plant conditions, obtain a stable performance curve and reduce energy costs (power), NPSHR, vibration, noise level and bearing load.
This particular form of retrofitting [af1] and overhauling can be a rewarding not only for pumps with high power but also for lower power pumps where many pumps of the same size are in an installation. . However, , even a small reduction in the power per pump may lead to considerable energy cost savings.
Case Studies
Improving the Efficiency of a Process Pump
Figure 2 shows the curves of a pump in the original condition as well as after retrofitting with a newly installed impeller. The surfaces of the spirals had also been improved. It was possible to improve the efficiency by 9 percent, thus reducing the energy consumption up to 7-kW. 25 pumps in this size are used in one installation, so substantial energy costs can be saved annually.
Increase in the Capacity of a Cooling Plant
In another example, the capacity of the cooling plant of a power station with four vertical wet pit cooling-water pumps was to be increased from 22,500-m3/h to 26,000-m3/h. At the same time, the required head was increased from H = 29.4-m to H = 37.4-m. Various options were available to achieve the goal. The first option was to operate the plant with four pumps instead of three since replacing the existing impellers with new impellers would provide a greater head. However, four pumps with enlarged impeller diameters would operate with a considerably reduced efficiency in partial-flow operation, and a standby pump would no longer be available.
Another possibility consisted of operating the existing pumps at a higher speed. However, this solution would require fitting the motors with a variable speed control system. Each individual pump would also operate in the unfavorable high flow range (with regard to cavitation), so that the submergence would have to be increased in the intake structure.
Because it appeared unfavorable to use the existing pumps for the new operating data, the use of a larger pump part was considered. However, in the case of the standard size suitable for this purpose, the main dimensions of the pump part were considerably larger than those of the existing size. The larger outside diameter of the diffuser bowl would have made it necessary to use a new foundation ring with an enlarged passage. Due to the connected modifications to the foundation, this option was also unfavorable.
The best solution proved to be designing a new pump part that consisted of an impeller, a diffuser bowl and a bellmouth and whose connection dimensions corresponded exactly to the dimensions of the pump part to be replaced. The scope of the modifications could be kept as small as possible and, compared to the four-pump operation option, 1.92-MWh of energy was saved every year.
Conclusion
If there is inefficiency in a pump installation, it is important to calculate possible savings with an upgrade or synergetic retrofit. It is crucial to be proactive and always optimize pump performance for operational performance, reduced energy consumption and cost savings.