The influence if we select a centrifugal pump too large or too small
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, pipeline scaling, etc. Engineers use design factors to interpret these items to prevent sizing/purchasing too small. They also explain the effects of aging on the system.
However, some engineers did not consider that the application of excessive safety margin in the design would actually increase the wear of overcurrent components and shorten the life of the system, especially when determining the size of the centrifugal pump.
When selecting pump and piping systems, the main objective is usually to achieve the pressure (head) and flow required for the application, whether it is a cooling water system, a fuel transfer pipeline, a chemical plant, or one of many other applications. The flow required by the system is used to determine the total dynamic head of the pump and then used to compare the performance curves of all available pump models. Too large or too small a pump can have a serious impact on the system, which is why it is important to use accurate operating points in this process.
If the selected pump size is insufficient, the flow in the system will be lower than required. This will require additional pumps or adjustments to the system, such as opening the drain valve (allowing the pump to operate under a larger operating condition). Oversized pumps will provide more flow than the system requires. Depending on the application, it may be necessary to reduce the flow by using a throttle valve or by adjusting the impeller. If the working point of the pump can be corrected by simply adjusting the valve in the system, it seems to have little effect on the system. However, when considering the pump efficiency, the influence of too large or too small pump size becomes more obvious.
The best efficiency point (BEP) is the ideal working point of the centrifugal pump, where the maximum percentage of energy used to run the centrifugal pump is transferred to the fluid. When the operating point of the pump deviates from the BEP, several things happen. The most obvious effect is that the efficiency is reduced, and the centrifugal pump needs more driving power. This energy not transferred to the fluid must be output in other forms, such as heat or vibration. Therefore, as the pump efficiency decreases, the vibration and heat generated by the centrifugal pump increase. In a few cases, this has little impact on the centrifugal pump, but the farther the centrifugal pump is from BEP, the greater the impact.
Centrifugal pump standards (such as Hydraulic Institute standard HI 9.6.3) generally recommend operating the pump in the range of about 80% to 110% of BEP to avoid these effects. If it exceeds 110% of BEP, the operator may face the risk of cavitation due to low NPSH margin, as well as damage caused by vibration and heat. However, 80% of the negative effects below BEP, such as pump seizure, low flow cavitation, internal backflow and high temperature may occur. Over time, the operation of such pumps will lead to high maintenance costs, high energy costs and short pump life. So the problem is, how can engineers effectively use design factors to avoid too small or too large pump size in the system?
