The Impact of Oversized or Undersized Pumps

Engineers often worry about being too conservative in their system designs. Many uncertainties exist during the design process, including variations in actual operating conditions, changes in fluid characteristics, equipment aging over time, and pipe scaling. Engineers use design factors to account for these factors and prevent undersized selection/purchase. They also account for the effects of aging on the system.

However, some engineers fail to consider that applying excessive safety margins in the design can actually increase wear on flow-through components and shorten system life, especially when determining pump size.

When selecting pumps and piping systems, the primary objective is usually to achieve the required pressure (head) and flow rate for the application, whether it's a cooling water system, fuel delivery pipeline, chemical production plant, or one of many other applications. The required system flow rate is used to determine the pump's total dynamic head, which is then used to compare the performance curves of all available pump types. Oversized or undersized pumps can have serious consequences for the system, which is why using an accurate operating point is crucial in this process.

The Impact of Oversized or Undersized Pumps

If the selected pump is undersized, the flow rate in the system will be lower than required. This will require an additional pump or system adjustments, such as opening a drain valve (allowing the pump to operate at a higher capacity). An oversized pump will provide more flow than the system needs. Depending on the application, this may require using a throttle valve or adjusting the impeller to reduce the flow. If the pump's operating point can be corrected simply by adjusting valves in the system, it may seem to have little impact on the system. However, the effects of an oversized or undersized pump become more pronounced when considering pump efficiency.

The Optimal Efficiency Point (BEP) is the ideal operating point for a pump, where the maximum percentage of energy used to operate the pump is transferred to the fluid. When a pump's operating point deviates from the BEP, several things happen. The most obvious effect is reduced efficiency, and the pump requires more drive power. This energy not transferred to the fluid must be output in other forms, such as heat or vibration. Therefore, as pump efficiency decreases, the vibration and heat generated by the pump increase. In a few cases, this has a small impact on the pump, but the further the pump is from the BEP, the greater the effect.

Centrifugal pump standards (such as Hydraulic Institute Standard HI 9.6.3) typically recommend operating pumps within approximately 80% to 110% of the Base Equivalent (BEP) to avoid these effects. Exceeding 110% of the BEP may expose operators to the risk of cavitation due to low Net Positive Suction Head (NPSH) margin, as well as damage from vibration and heat. Below 80% of the BEP, negative effects such as pump seizure, low-flow cavitation, internal backflow, and high temperatures may occur. Over time, operating such pumps leads to high maintenance costs, high energy costs, and a shorter pump lifespan. So, the question arises: how can engineers effectively utilize design factors to avoid undersized or oversized pumps in the system?

Carefully Determining Overall Design Factors

One thing to consider is how many design factors the pump requires and when to apply that safety margin. Typically, the following parties may consider adding design factors to the design: the system design engineer determines the system dimensions, the project manager reviews the design, and the pump manufacturer recommends the pump. Care should be taken when understanding the assumptions and boundary conditions used to determine system size to avoid unknowingly defining unreasonable design factors.

Consider Operating Limits

Do the sizing calculations and selected design factors take into account extreme operating conditions? While an effective design may have been made for the system's peak flow rate, does this alter the original operating point? It is important to select pumps with characteristics that allow them to approach their BEP (Best Before Expectation) under all conditions, not just extreme operating requirements. If system requirements change frequently, options such as adding a variable frequency drive (VFD) to the pump may need to be considered to keep pump operation within the desired range.

Choose the Right Tools

Determining system size, especially for large systems, is a complex process. Using tools that help streamline the process reduces the chance of errors. Using automated adjustment tools can make modeling methodologies clearer when communicating and reporting information to others. An effective automated sizing tool allows engineers to quickly input manufacturer information into the design and verify that the proposed equipment will operate as expected. This redundancy prevents potentially costly errors. Furthermore, an effective automated dimensional calculation tool will allow users to quickly compare different operating conditions and even consider multiple related design scenarios when performing dimensional calculations.

Summary

Designing a system that meets operational requirements is crucial. However, when it comes to designing a system, bigger is clearly not always better. Effective design not only helps reduce material and installation costs when building the system but also prevents additional wear on components within the system, such as pumps.