The BEP, or best efficiency point, is the point at which the pump operates at peak efficiency. At the BEP, flow enters and leaves the pump with a minimum amount of flow separation, turbulence, and other losses. The closer a pump operates to its BEP, the less wear the pump will experience, which increases the reliability of the pump. In the below image, the BEP is located at the apex of the pump efficiency curve (green line). If a vertical line is drawn from the BEP down to the pump curve (black line), the head and flow rate at the BEP can be determined by looking at where this line is in relation to horizontal and vertical axes.
A closely related topic to the BEP is the preferred operating region, or POR. The POR is a range of rates of flow to either side of predicted BEP within which the hydraulic efficiency and the operational reliability of the pump are not substantially degraded. Within this region, the design service life of the pump will not be affected by the internal hydraulic loads or flow-induced vibration. Operating a pump within the POR ensures higher reliability and lower energy consumption. A typical range for the POR is shown on in Image 1. Note that the POR is defined in ANSI/HI 9.6.3 Rotodynamic Pumps Guideline for operating regions, and the POR changes depending if the pump radial (centrifugal), mixed or axial flow type.
A wider range of flows, outside the POR, over which the service life of a pump is acceptable, is called the allowable operating region (AOR). The limits to AOR are determined by requirements other than energy consumption and should defined with the help of the pump manufacturer. Some factors that determine the AOR are:
1.hydraulic loads (bearing life, fatigue, mechanical contact, shaft deflection, thrust, etc.)
2.liquid temperature rise
3.bearing housing or shaft vibration
6.net positive suction head
Image 1. BEP illustrated on head capacity curve with corresponding POR and AOR (Images courtesy of the Hydraulic Institute)
There are a multitude of negative consequences that arise when a pump is operated significantly above or below its BEP that can result in accelerated pump wear and premature failures. For example, operating at excessively low flow rates could cause higher radial loading on the impeller causing excessive shaft deflection leading to premature pump seal failures. At excessively high flow rates the pump’s required suction head may not be met and the pump could cavitate.
Shaanxi DaoLin Mechanical & Electrical Co.,Ltd
Room1107, Building B, Sail Times Plaza, Hou Wei Zhai, Wei Yang
District, 710085, Xi’an City, Shaanxi Province, China
Tel.:+86(029)84523807-802 Fax: +86(029)84523807-808