preface

Engineers often worry that they are too conservative in system design. There are many uncertainties in the design process, including differences in actual operating conditions, changes in fluid characteristics, equipment aging over time, and pipeline scaling. Engineers use design factors to explain these projects to prevent selection/purchase of undersized sizes. They also explain the impact of aging on the system.

However, some engineers did not consider that applying too much safety margin in the design would actually increase the wear of flow components and shorten the system life, especially when determining the size of the pump.

When selecting pumps and piping systems, the main goal is usually to achieve the required pressure (head) and flow rate for the application, whether it is cooling water systems, fuel delivery pipelines, chemical production plants, or one of many other applications. The required flow rate of the system is used to determine the total dynamic head of the pump, and then used to compare the performance curves of all optional pump types. A pump that is too large or too small may have a serious impact on the system, which is why using accurate operating points in this process is crucial.

Excessive or minimal impact

If the selected pump size is insufficient, the flow rate in the system will be lower than the requirement. This will require additional pumps or adjustments to the system, such as opening the discharge valve (allowing the pump to operate under excessive operating conditions). The oversized pump 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 flow. If the working point of the pump 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 impact of pump size being too large or too small becomes more apparent.

The Best Efficiency Point (BEP) is the ideal operating point for the pump, where the maximum percentage of energy used to run the pump is transferred to the fluid. When the working point of the pump deviates from BEP, several things happen. The most obvious impact is a decrease in efficiency, as the pump requires more driving power. This energy that is not transmitted to the fluid must be output in other forms, such as heat or vibration. Therefore, as the efficiency of the pump decreases, the vibration and heat generated by the pump increase. In a small number of cases, the impact on the pump is minimal, but the farther the pump is from BEP, the greater the impact.

Centrifugal pump standards (such as the Hydraulic Institute standard HI 9.6.3) typically recommend operating the pump within the range of approximately 80% to 110% of BEP to avoid these effects. If it exceeds 110% of BEP, operators may face the risk of cavitation due to low net positive suction head (NPSH) margin, as well as damage caused by vibration and heat. The negative effects below 80% of BEP, such as pump jamming, low flow cavitation, internal reflux, and high temperatures, may occur. Over time, the operation of such pumps will result in high maintenance costs, high energy costs, and shorter pump lifespan. So the question arises, how can engineers effectively utilize design factors to avoid pump sizes being too small or too large in the system?

Carefully determine the overall design coefficient

One thing to consider is how many design factors are required for the pump and when to apply this safety margin. Usually, the following parties may consider adding design elements to the design: the system design engineer determines the system size, the project manager reviews the design, and the pump manufacturer recommends the pump. When understanding the assumptions and boundary conditions used to determine the size of the system, caution should be exercised to prevent unreasonable design coefficients from being unknowingly defined.

Consider operating limits

Do the size calculations and selected design factors take into account the extreme operating conditions of the system? Perhaps an effective design has been made for the peak traffic of the system, but has this changed the original working point? It is important to choose pumps with operating characteristics that enable them to approach BEP in all situations, rather than just for extreme operating conditions. If the demand for the system changes frequently, options such as adding variable frequency drives (VFDs) to the pump may need to be considered to maintain the pump's operation within the expected range.

Choose the right tool

Determining the size of a system, especially for large systems, is a complex process. Use tools that help streamline processes to reduce the chance of errors. When communicating and reporting information to others, using automatic adjustment tools can make modeling methods clearer. 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 based on the design. This redundancy can prevent potential costly errors. In addition, an effective automated size calculation tool will allow users to quickly compare different operating scenarios and even consider multiple related design situations when performing size calculations.

summary

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

（Source | WeChat official account: new vision of water treatment）