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Best Efficiency Point(BEP) and its significance

By Esteban M. Araza

The term Best Efficiency Point, or BEP, refers to the pump flow rate at a specified impeller diameter where the efficiency is highest on its performance curve. Most reference to BEP, such as per by API 610, is based on rated impeller diameter, whereas others are based on maximum diameter such as per examples below.

If the curve were at a cut diameter the flow rate (with the highest efficiency) should be extrapolated to the maximum diameter to calculate its BEP using Affinity Laws. 

BEP is the flow rate that is optimal for a given set of pump hydraulics; it is the point on the pump curve where the hydraulic losses are minimum. It is independent of the pump mechanical losses.

In a BEP evaluation the flow rate at BEP (e.g. 1000 GPM) is of primary interest rather than the efficiency itself (e.g. 85%) unless the BEP is being used to find the specific speed (Ns) for a hydraulic pump selection that will yield the highest possible rated efficiency. 

The location of BEP in a pump curve is the result of the hydraulic matching of the impeller and volute (or diffuser, in a diffuser pump); it may not correspond to the design flow of the impeller. An impeller will yield different values of BEPs if used in casings with different volutes. Similarly, a volute will yield different BEPs if used with different impellers.

A pump curve whose BEP coincides with the impeller design flow means that the impeller is hydraulically matched to the volute – the volute throat area is neither undersized, nor oversized for the flow. An undersized area will pull the BEP to the left, whereas an oversized area will pull the BEP to the right, of the pump curve. There are theoretical and empirical methods to estimate the shift in the location of BEP on the pump curve based on changes in the volute throat areas.As a Professional Engineer, I offer a wide range of services in the fields of civil, mechanical, and electrical engineering. With years of experience and a commitment to excellence, I am dedicated to delivering high-quality solutions for your engineering needs. Contact me today to learn more.