In today’s industrial landscape, efficiency is often a priority. Companies are becoming more sustainable, driven both by market trends and an increasingly regulated business landscape.
There is always a tradeoff between peak efficiency and peak output — typically, pushing equipment to its extremes will lead to reduced reliability and efficiency as it strains to provide a higher workload. High levels of heat, vibration, and force are the result of maximizing a pump’s output. In addition to reduced lifetime, the pump will run at a lower efficiency, causing an increase in energy consumption and operating costs.
While it is important to maximize a pump’s reliability and efficiency, there are some scenarios in which it may be worth operating at peak output at the expense of reliability or efficiency.
Where Should a Pump Operate on Its Curve
Every pump has a point on its curve called the best efficiency point (BEP). This is the optimal operating point where efficiency is maximized as a result of how the geometry of the impeller and volute allow flow to move through the pump. Operating at BEP is where the pump not only maximizes efficiency (reducing
energy consumption), but it also maximizes reliability.
Ideally every pump selection should run at BEP, but this is unrealistic as there are a limited number of hydraulic curves for an infinite amount of potential pumping applications. It’s analogous to clothing sizes; there is an infinite number of body types, and it makes little sense for clothing manufacturers to tailor each piece of clothing to every customer. Therefore, they provide a set size that properly fits a range of customers. For centrifugal pumps, a couple of ranges are provided in ANSI/HI 9.6.3 Guideline for Operating Regions.
This guideline’s operating ranges are called the Preferred Operating Region (POR) and the Acceptable Operating Region (AOR). The POR is a region on the pump curve that ranges from 70% of the BEP flow to 120% of BEP flow. This region is analogous to a well fitted shirt, where pump operation is optimal. The AOR is a region determined by the pump manufacturer by testing that indicates the pump will generally be okay operating in. This region includes the POR within it and is often represented by the full published curve. While the manufacturer deems operation in the AOR to be acceptable, it is important to note efficiency and reliability decrease the further you move away from the POR. Operating on the far left of the AOR is like wearing a tight shirt showing some belly and operating on the far-right side of the curve is like wearing a baggy shirt. This ill-fitting shirt, while providing utility, will be uncomfortable and may not be a long-term solution.
When to Prioritize Output over Efficiency
When selecting a pump, it is a good rule of thumb to ensure that it’s operating in the POR and has high efficiency. However, just as exceptions to rules can be made in a variety of pumping applications, this one is no different.
One case in which efficiency may be sacrificed for output is in the design of heavy-duty pumps. These pumps are designed with thicker and heavier rotor components to prolong use in high wear applications. Bulky and dull impeller vanes may create more drag and reduce efficiency, but will not erode quickly and will remain in service for longer. The trade-off that must be considered here is the additional energy cost to move a heavier rotor against the reduction maintenance and parts costs.
Another instance where it may make sense to prioritize operation over efficiency is in the use of special solids handling pump styles such as cutters, choppers, grinders and vortex pumps. Cutters, choppers and grinders are all similar in that they use a cutting device that sits at the suction port. This cutting action is effective at reducing solids to a size that prevents clogs but comes at the expense of pump efficiency. Vortex style pumps have impellers that sit recessed in the volute and do not have a close fit to the suction plate. This additional space is ideal for passing abrasive solids with minimal contact to impeller. However, a drawback to this design is that it is less efficient. The tradeoff in the use of solids handling pump styles is additional energy cost to using a less efficient pump against the reduction in maintenance and parts costs.
There are also instances in which maximizing output supersedes efficiency in cases of short term or intermittent operation. Applications such as jobsite dewatering, disaster remediation, and bypass operate in locations temporarily until the job is done. Due to the short-term nature of these jobs, the end user may value the pump’s ability to provide high flow or pump at high head as opposed to saving money on energy or maintenance costs. Additionally, end users who require pumping at multiple sites will take the efficiency / reliability tradeoff and use a pump that can run satisfactory (in the AOR) as opposed to purchasing / carrying around multiple pumps that are perfect fits for each short term application. The tradeoff in short term operation applications is flexibility or urgency at the risk of increased energy and maintenance costs.
Conclusion
An industry professional recently stated “a pump in for repair has an efficiency of zero” which captures both ends of the pump efficiency versus max operation topic. Maximizing efficiency is a key factor when selecting a pump, but it is also important to remember the primary goal of pump selection is to put the right pump into the right application. A pump with thicker material or with a solids handling design will remain in operation longer than alternatives with higher efficiency. Alternatively, a contractor may find more utility and convenience in a single pump than having a fleet of options sized for every application collecting dust on their shelf. These nuances in pump selection highlight the importance of having a discussion with the end user to get a better understanding of their needs. Pump selections with limited information could be leaving money on the table.