Every pump has a Best Efficiency Point (BEP), the flow at which the pump operates at the maximum hydraulic performance. BEP is the optimal operation point, and to achieve the maximum efficiency and lowest running cost, pumps should work as close as possible to the BEP. At BEP, the risk of pump failure is minimal, and the pump has the longest life expectancy.
Matching the requested duty point with the pump BEP is not usually possible. But if there are no specific reasons for doing otherwise, a pump should be selected so that the operation point lies close to the BEP. Ideally, a centrifugal pump shall operate in a flow range of ±10% of BEP; the acceptable working flow range changes based on the pump's hydraulic design and manufacturer recommendations. The further away the operation point is from the pump BEP, the lower the pump efficiency will be, and the higher the vibration and noise levels will be.
API 610 (11th ed.) guidelines for pump selection define various operating regions for centrifugal pumps:
- Allowable operating region: set by the manufacturer, and typically the flow rates between the minimum flow rate of the pump (or shutoff head) and the run-out point (or maximum flow rate that the pump driver can support). The allowable operating region specifies how far, on either side of BEP, the pump can be operated without being subjected to some short-term damage.
- Rated region: the flow rates between 80% and 110% of BEP.
- Preferred operating region: the flow rates between 70% and 120% of the BEP (operating under normal conditions).
There are many consequences of running a pump too far to the left or right of its BEP for an extended period. Off-BEP operation can lead to early pump failure. Pumps are subjected to minimum unbalanced hydraulic forces at the best efficiency point, promoting a smooth operation.
The hydraulic pressure is unbalanced inside of a centrifugal pump. In a volute casing pump, usually, the impeller is not sitting in the center of the casing to increase the pressure toward the pump discharge. This unbalanced pressure acts on the impeller and applies radial loads to the impeller and shaft. The formed radial load is a function of impeller size and pressure differential inside the pump casing. In general, the impeller radial load is directly related to impeller diameter and width, increasing both above and below BEP flow rates significantly.
The pressure difference between the front and back of impeller shrouds creates an axial load, usually towards the pump inlet, and resulting axial loading on the shaft. The unbalanced radial and axial loads on the impeller cause shaft deflection, which decreases mechanical seals, bearings, shaft, and shaft sleeve working life and reduces the critical speed of the shaft. For most centrifugal pumps, by increasing flow after the best efficiency point, the NPSHr also increases. When the NPSHr exceeds the NPSHa, air bubbles form by vaporizing the liquid, and cavitation occurs.
In order to reduce the lifecycle costs of centrifugal pumps and achieve optimum performance and service life, it is essential to operate the pump close to its best efficiency point (BEP). At BEP, vibration and undesired hydraulic forces are minimal, and the pump is subjected to a lower risk of flow separation. The selection of a pump that works in the preferred operating region ensures a good shaft, seal, and bearing life.