Some Effects of Operating Pumps Away from Best Efficiency Point
Vibration can be caused by many factors and can create bending moments in the shaft, resulting in poor pump performance and risk of shaft failure. Excessive vibration can occur when pumps operate too far to the right of BEP, due in part to cavitation which causes hydraulic imbalances within the impeller as voids are formed by the vaporization of the liquid. Excessive vibration can also occur due to higher bearing loads associated with pump operation closer to run-out or shut-off conditions.
Have you become so focused on the efficiency of the motors around your plant that you’re losing sight of the equipment those motors are driving? In many applications, pumps included, the answer to the efficiency question is best addressed when the complete system is studied.
Best Efficiency Point (BEP) Definition
Pumps should be selected to operate as close to BEP as possible. When a pump operates at BEP it is at it’s most efficient, meaning it is using input power as efficiently as possible and reducing energy costs. When a pump operates at BEP it also produces the lowest vibration readings, meaning that pump longevity is increased and maintenance costs are decreased.
Usually a pump curve will have flow rate on the horizontal axis (usually in l-min or m³-h) and the pressure on the vertical axis (usually bar or height in metres). The curve represents the flow rate the pump will produce at certain levels of pressure when the pump is operating properly, or oppositely, the pressure the pump will work at when it is providing a certain flow rate.
Utilize the system curve and the pump performance curve to select the proper pump for a particular application
They apply only to this pump running at 2900 RPM with the impeller diameters shown at the right end of each curve. At different speeds or with different impellers different curves would result. As an example of how to use the curves, assume 120 cubic meters per hour of water had to be pumped up to a tank on a 50-meter high building.
While some obvious reasons may cause a pump to run off of its factory-issued performance curve, they are often overlooked upon initial investigation.
Sounds like a reasonable request. However, other questions need to be answered, such as: Where is the pump running on its curve? If it is not running on its curve, why? Most pump problems can be attributed to system pipe and system friction changes.
Effective and efficient pump selection demands a thorough understanding of the system curve and how it reflects what happens in the actual system. This is especially true in today’s variable speed systems. It’s so important that even though we’ve touched on it many times before, we are going to “hit pause” in this series to focus specifically on the movement of the system curve in a variable speed pumping system.
Best Efficiency Point is Best Operating Point
As the vapor bubbles, formed during the onset of cavitation, migrate to the higher pressure regions of the impeller, they implode with enough force to send shock waves to the surrounding area which in turn breaks molecules from the parent metal, leaving behind the telltale signs of cavitation – pitting and erosion.