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There is no doubt that the demands on publicly funded municipalities, privately owned water companies and private operators have changed over the past decade. The focus on energy use with regard to cost and carbon dioxide (CO2) footprint is increasing, along with the ever-present demand to reduce pump clogs and the associated risk of overflows and cost of service call-outs.
Reconciling these two requirements may seem problematic because traditional thinking concludes that energy consumption reductions are best achieved by installing higher-efficiency equipment—while at the same time higher efficiencies with solids-handling pumps are traditionally associated with an increased risk of clogging.
Analyzing data across developed countries shows that for municipal wastewater pumping stations, just under 60% of all call-outs to sites are a direct result of pump-related problems. It is clear that clogging still represents a major challenge to wastewater collection operators. This encourages users to choose impeller designs that have lower efficiencies but higher resistance to clogging. The end result, however, is higher energy consumption.
When discussing pumping equipment efficiency, (hydraulic, electrical, sustained, etc.) one has to understand the different efficiency components. In terms of the cost of power to operate, there is only one efficiency value that matters most and that is the total (water to wire) efficiency. This is what determines the P1 absorbed kW, which represents the true amount of electricity consumed at a specific duty point. With solids pumping, one also needs to understand the components of total efficiency and how they impact the ability to run clog-free.
With conventional pumping equipment, the user has more flexibility when choosing the motor and pump efficiency. Normally two separate components are purchased, of which the motors fall into various efficiency categories ranging from the lowest-level IE1/NEMA efficiencies up to the IE3/NEMA premium-efficiency ranges.
When trying to achieve the best balance between the risk of clogging and energy consumption, the starting point should always be to select the best commercially viable motor efficiency available. The reason for this is that motor efficiency is a win-win situation giving better total efficiency without any impact on the risk of clogging.
So how does this work when it comes to submersible wastewater pumps? Unfortunately, most submersible pumps do not incorporate advanced motor technology and efficiency standards, and still feature old technology that results in some of the lowest efficiency levels, such as IE1 (EFF 3). The good news is that the next generation of wastewater pumps is now available with premium-efficiency IE3 motors that allow optimized motor efficiency as a first step in improving energy consumption without increasing the risk of clogging.
The benefits of working with motor efficiency include:
A further consideration that should not be overlooked when working with motor efficiency is the added benefit of a cooler motor, resulting in a reduced need for cooling and significant increases in reliability as a result of the optimized conditions for the motor, bearings and mechanical seals.
Having selected a submersible pump with IE3 premium-efficiency motors, what is the next step to get the best total efficiency? Not only is the best motor efficiency required, but for the best total efficiency, the best hydraulic efficiency is required as well. And not just the best hydraulic efficiency, but also the highest “sustained” efficiency (i.e., proper hydraulic design so the leading edge sheds stringy materials and maintains the “clean water” efficiency even in solids-laden media).
Regrettably, because all pump curves show clean water efficiency, it is not possible to simply use selection software to choose the pump with the highest overall/sustainable efficiency. Without considering the solids-handling aspects of the impeller design, higher efficiencies may result on paper, but higher energy consumption and an increased risk of clogging may result in practice. One first must understand the application and the pumping station: What is the size of the station? How close is it to operator support in the event of a clog? What is the implication of a clog? Is frequent regular maintenance/adjustment acceptable? Is it a high-stringy solids or high-wear station, etc.?
If the above sounds too complicated, there are easy-to-use wastewater pumping station assessment tools that can help with this process, or you can contact your application support department for further advice.
Whether discussing reductions in water consumption or changes in personal hygiene habits, it is clear that the solids content in wastewater is increasing. More cases of soft clogging/fouling (rags and stringy material trapped in the impeller) are occurring, and the number of pumping stations considered to be at high risk is increasing as a result.
Impeller selection should focus on two areas, the first being the efficiency of the impeller and the second its resistance to clogging and fouling. In terms of assessing the intended choice, it is relatively easy when it comes to efficiency and hard clogging; both can be measured using agreed-upon methods in a test facility or by following impeller free solids passage requirements as given by industry standards (e.g., the “10 States Standards,” which specify minimum passage dimensions).
Based on a significant body of evidence, less than a 3-in. free solids passage increases the risk of hard clogging unless the station is fitted with screens at the inlet. All too often, free solids passage is the first sacrifice in achieving high efficiencies with some submersible pump manufacturers.
High-efficiency wastewater impellers that comply with 3-in. free solids passage requirements are indeed available and should always be the first choice.
All manufacturers claim to produce impeller designs that can handle stringy materials and rags. Unfortunately, there is not one agreed method of testing that can confirm these statements; basically it comes down to personal experience and subjective selection.
Companies now have invested considerable time in developing a standardized test method to enable benchmarking of different impeller designs and presenting a quantitative measure of the clog resistance of each impeller design. This can then be used in conjunction with the free solids passage and hydraulic efficiency to make sure that the impeller selected is optimized for a particular application.
This extensive testing also clearly demonstrates that there is a very large variance in clog resistance when comparing different existing impeller designs available for wastewater applications.
This method of quantitative testing and advanced CFD modeling techniques, have been used to develop next-generation wastewater impellers making use of optimized vane profiles and impeller designs to achieve the best balance between efficiency and clog resistance.
Impeller selection is not just a matter of efficiency when the equipment is new. Submersible sewage pumps are installed in a very aggressive, high-wear environment. Special attention must be given here, as new efficiency will not be maintained for long. One must decide how much manual intervention/maintenance is acceptable to regain the as-new efficiency over time.
The golden rule is not to select a high-efficiency impeller that requires regular maintenance if there is no plan to carry it out. If the golden rule is ignored, the result will be even higher energy consumption and more incidents of clogs than if one had selected a lower-efficiency design with better clog resistance from the onset.
For smaller pumps (<= 6 in.), also be aware of impeller designs that rely on a cutting action. The use of a cutting action demands a sharp edge that must be maintained over time. If this is ignored, increased clogging will result from larger pieces of rag being unable to pass through the impeller.
The design of next-generation, smaller wastewater impellers is focused on passing rags through the impeller without the need for a cutting action. This is achieved through optimizing the impeller and vane profiles to give a best-of-class clog resistance while maintaining high hydraulic efficiency levels.
When trying to achieve the best balance between energy optimization and pump clogging one must follow four important steps: