Ideally, a centrifugal pump should be operated at or near its best efficiency point (BEP) flow rate in order to minimize the life cycle costs. However, all centrifugal pumps have sweet spots beyond the BEP that will yield acceptable efficiency and reliability.
Excerpt from the Sept. 2016 article from WaterWorld
There are limitations, though, on the minimum and maximum flow rates, beyond which the pumps should not be operated continuously (or for an extended period of time), in order to avoid premature failures.
A first step in avoiding these negative, low-efficiency and low-reliability conditions is to determine the pump BEP, preferred operating region (POR), and allowable operating region (AOR) flow rates. It is especially important to determine these flow regions because not all pump applications are static in nature or closely match the expected system demand. Because of this, pumps are often required to operate over a broad range of flow rates, which can adversely affect the pump efficiency and reliability.
A pump will always operate at the flow rate where the pump head-capacity curve intersects the system head-capacity curve. This means that it is also critical to accurately determine the true system H-Q curve, in order to establish the true operating flow rates.
Once these flow regions and the true system conditions are known, actions can be taken to maximize pump operation in the POR and avoid or minimize operation outside the AOR, thus optimizing pump life cycle costs.
BEP Flow Region
Pump performance and service life are optimized around a rate of flow designated as the BEP. At the BEP, the hydraulic efficiency is maximum, and the liquid enters the impeller vanes, casing tongue (discharge nozzle), and diffuser vanes in a shockless manner. At the BEP, flow through the impeller and diffuser vanes (if so equipped) is uniform, free of separation, and well-controlled.
Lower and higher flow rates cause mismatch between the flow and the impeller and casing vanes. This mismatch causes turbulence within the impeller and casing flow passages, which both block the flow passages and increases the local velocities. This increase in velocity increases vaporization (cavitation) within the liquid. The greater this resulting turbulence and cavitation, the lower the pump efficiency and reliability, and the more severe are the levels of vibration, noise and erosion.
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