Understanding Pump Curves
Best Efficiency Point
(BEP)
The graph below shows the Head Capacity Characteristic including the Efficiency for a pump. The flow rate at which this pump converts input energy into hydraulic output energy most efficiently is 250 gpm, known as the Best Efficiency Point, or BEP. The BEP for this pump is located at a flow rate of about 72% of maximum flow for this pump. Most single stage centrifugal pumps have a BEP located at about 70% - 85% of maximum flow.
BEP for this pump is 250 gpm against a resistance of 176 Feet TDH, with 65% Efficiency.
How to read the graph above:
Follow the head flow curve (black curve line), from left to right starting at 0 gpm.
As you move from left to right, (towards higher flow rates), the curve intersects the first green efficiency line at ~120 gpm, showing that the efficiency of the pump at ~ 120 gpm is 50%. This means that the pump converts 50% of the input energy to hydraulic energy, the rest of the energy is lost and not useable.
As you continue to move to the right, the head flow curve crosses over higher and higher efficiency lines indicating that the pump operates at increasingly better efficiencies.
As flow increases the pump eventually achieves maximum efficiency when 250 gpm is flowing through the pump. At 250 gpm the pump is able to convert input energy to output energy with 65% efficiency. This is the BEP.
As you continue to follow the head capacity curve beyond BEP the efficiency drops. At 300 gpm efficiency is 62%.
Learn More
For a single volute pump, hydraulic forces surrounding the impeller, just below and near the BEP flow rate, are closer to being balanced than at any other flow rate, resulting in higher fluid flow efficiency, less radial shaft deflection, and therefore less vibration. At flow rates well above or well below BEP, hydraulic forces surrounding the impeller are less balanced, resulting in less fluid flow efficiency, and higher levels of shaft deflection and therefore vibration.
The further away from BEP a single volute pump operates, the lower the hydraulic efficiency and the more vibration occurs. Eventually as the flow rate moves further and further away from BEP, vibration may become harmful, contributing to the definition of the upper and lower limit of the pump's Operating Range or Window. the following types of pumps are much less vulnerable to flow rate radial reaction: Double Volute, Modified Concentric, and Concentric Flow (Multistage Turbines).
Low specific speed pumps react less to Off-BEP flow rates than high specific speed pumps. Where the type of pump desired exhibits large reactions to off BEP flow rates, good design requires proper accounting for the forces involved. Impellers may be suspended between bearings (as opposed to overhung), shafts can be made thicker and/or shorter (L3D4 ratio).
For Further Study Follow the Link Below
What is BEP Good for?
When designing a system the pump specifier attempts to match the best pump to the system by establishing priorities. Many systems, perhaps even most systems, do not have a steady continuous flow rate allowing a pump to operate only at its BEP. Therefore the system designer may chose a pump which has a BEP at the flow rate which the pump will operate at for the longest periods of time, while also operating at flows Off BEP with the least amount of radial thrust and vibration (Widest Operating Window or Range).
Related Subjects on this Website:
Glossary - Best Efficiency Point (BEP)
Glossary - Pump Characteristic
Glossary - Power Characteristic
How Fluids Move Through Impellers
Written By:
Richard Neff
President
Irrigation Craft, Inc.
Bibliography
The Pump Handbook
Third Edition, 2001
McGraw Hill
Karassik, Messina, Cooper, Heald
Centrifugal and Axial Flow Pumps
Second Edition, 1957, 1993 Reprint
A. J. Stepanoff, Ph.D.
The McNally Institute - William McNally
Clearwater, Florida
Follow the links below to pages focusing on other types of information provided in the Pump Characteristic and more, including graphs and explanations.
