What are the consequences of choosing a pump that is too large or too small?

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Preface Engineers often worry that they are too conservative in system design. There are many uncertainties in the design process, including variations in actual operating conditions, changes in fluid properties, equipment aging over time, and pipe scaling. Engineers use design factors to explain these items in order to prevent the selection/purchase of sizes that are too small. They also explain the impact of aging on the system. However, some engineers fail to consider that applying excessive safety margins in the design actually increases the wear of components subject to overcurrent and shortens the system’s lifespan, especially when determining the size of the pump. When selecting pumps and piping systems, the main goal is usually to achieve the pressure (head) and flow rate required by the application, whether in a cooling water system, fuel delivery pipelines, chemical manufacturing plants, or many other applications. The flow rate required by the system is used to determine the total dynamic head of the pump, which is then used to compare the performance curves of all available pump types. Pumps that are too large or too small can have a serious impact on the system; this is why it is crucial to use an accurate operating point during this process. Excessive or insufficient impact: If the pump size selected is inadequate, the flow rate in the system will be below the required level. This will require an additional pump or adjustments to the system, such as opening the discharge valve (to make the pump operate under higher load conditions). Pumps of oversized size will provide more flow than the system requires. Depending on the application, it may be necessary to use a throttle valve or adjust the impeller to reduce the flow rate. If the pump’s operating point can be corrected by simply adjusting the valves in the system, it seems to have almost no impact on the system. However, when considering pump efficiency, the effects of oversized or undersized pumps become more apparent. The Best Efficiency Point (BEP) is the ideal operating point of a pump, at which the highest percentage of the energy used to operate the pump is transferred to the fluid. When the pump’s operating point deviates from the BEP, several things happen. The most obvious effect is a reduction in efficiency, as the pump requires more driving power. This energy that is not transferred into the fluid must be discharged in other forms, such as heat or vibration. Therefore, as the pump’s efficiency decreases, the vibration and heat generated by the pump increase. In a few cases, the impact on the pump is minimal, but the further the pump is from the BEP, the greater this impact becomes. Centrifugal pump standards (such as the Hydraulic Institute standard HI 9.6.3) generally recommend operating the pump within the range of approximately 80% to 110% of its BEP in order to avoid these effects. If it exceeds 110% of the BEP, the operator may face the risk of cavitation due to low net positive suction head (NPSH) margin, as well as damage caused by vibration and heat. And 80% of the negative effects below BEP, such as pump jamming, low-flow cavitation, internal recirculation, and high temperatures, may occur. Over time, the operation of such pumps leads to high maintenance costs, high energy costs, and a shorter service life for the pumps. So the question is, how can engineers effectively use design factors to avoid pump sizes that are too small or too large in the system? When carefully determining the overall design coefficient, one thing to consider is how much design coefficient the pump requires, and when this safety margin should be applied. Typically, the following parties may consider adding design elements to the design: system design engineers determine the system size, project managers review the design, and pump manufacturers recommend pumps. Care should be taken when understanding the assumptions and boundary conditions used to determine the system size, to prevent the inadvertent definition of unreasonable design coefficients. Consider operational limits: Do the size calculations and selected design coefficients take into account extreme operating conditions of the system? The system may have been effectively designed for its peak traffic volume, but does this change the original operating point? It is important to choose a pump that possesses operating characteristics that allow it to approach BEP under all conditions, and not merely for extreme operating conditions. If the requirements for the system change frequently, it may be necessary to consider options such as adding variable frequency drives (VFDs) to the pumps in order to keep their operation within the desired range. Choosing the right tools and determining the size of the system is a complex process, especially for large systems. Use tools that help simplify processes to reduce the chances of errors. When communicating with others and reporting information, using auto-adjustment tools can make the modeling approach clearer. An effective automated dimensioning tool allows engineers to quickly input manufacturer information into the design, and to ensure, through design verification, that the equipment will function as intended. This redundancy can prevent potential, costly errors. Furthermore, an effective automated size calculation tool will allow users to quickly compare different operating conditions and even take multiple related design scenarios into account while performing size calculations. In summary, it is crucial to design a system that meets operational requirements. However, when it comes to designing a system, it’s clearly not the case that bigger is better. An effective design not only helps to reduce material and installation costs during system construction, but also prevents additional wear on the components within the system (such as pumps).