An important and commonly-used parameter in evaluating the characteristic and performance of a centrifugal pump is its Best Efficiency Point (BEP). The term BEP refers to the flow rate in the performance curve at its maximum impeller diameter where the efficiency is at its highest.
The BEP is an important parameter because:
Other pump parameters such as specific speed (Ns), suction specific speed (Nss), hydrodynamic size (Z), viscosity correction factors, etc., are calculated from data - such as flow, head, and NPSHR - taken from BEP.
It is common practice to determine a pump's recommended minimum continuous stable flow (MCSF), its allowable operating flow range (AOR), and its referred operating flow range (POR), as a percentage of its BEP.
Many users prefer their pumps to operate within 80% to 110% of BEP for optimum performance, to minimize their hydraulic radial and axial thrust loads, and to avoid vibration problem induced by low flow recirculation.
Typically, the pump's hydraulic thrust loads, and shaft deflection, are lowest when the pump operates at, or very close to, its BEP.
The flow at BEP is not necessarily the same as the design flow, or optimum efficiency of its impeller. The BEP is controlled by both the impeller and the volute. A particular impeller will yield different performances with varying BEP if it is installed in different sizes of volutes.
A volute with a smaller throat area will pull the BEP to a lower flow rate, and will lower the pump NS. A volute with larger throat area will pull the BEP to a higher flow rate, and will increase the pump NS. An impeller which is matched to the correct volute throat area will yield the optimum pump performance.
A pump BEP can be altered easily, either reduced or increased in value, thereby relying on BEP in determining other parameter rather misleading or risky, such as the pump's MCSF, AOR, or POR.
Is it a good practice to base the minimum stable flow, the preferred operating flow range, and the allowable operating flow range on a percentage of BEP? Is there an alternate method to this practice?
[ * ] the term 'volute' is used for simplicity but it can also refer to a diffuser. [ ** ] Some information are excluded. Read more.