What degree of impeller balancing should be used to limit pump vibration?
High levels of residual unbalance in rotating parts can generate high unbalance forces causing excessive loading on bearings and shaft and inducing high levels of vibration.
Pump impellers are typically balanced to International Organization for Standardization (ISO) 1940 balancing quality standard G6.3 or better. (ISO 1940 for values relating to other balancing grades). It is important to note that the G6.3 is a generality. Some equipment may operate at higher speeds and require better balance quality to meet vibration standards, and other equipment may be lower speed and ruggedly built for heavy duty.
of the Hydraulic Institute)
For example, the impellers of slurry pumps will have wear during operation which will alter the balance; therefore, slurry pumps are designed to operate with a large amount of imbalance in the impeller. For balance of mud pump type impellers, refer to the American National Standards Institute/Hydraulic Institute (ANSI/HI) latest edition 12.1-12.6.
Another important factor to consider is the balance between single-plane and dual-plane. Depending on the geometry of the component, it may be satisfactory to perform a single-plane spin equilibrium. Components are generally balanced in one plane if the diameter/width ratio (D/b) is 6.0 or greater (Image 1). Two-plane (or dynamic) balancing is usually done another way.
Imbalance is only one cause of vibration in rotating equipment, but it is arguably the most common. It is therefore logical to pay attention to the balance of the rotating elements to limit vibrations; however, a pump with the most accurate balance may still exhibit excessive vibration due to operation away from the design point, misalignment, resonance, or improper installation.
For more information on turbine balancing and pump vibration, see “ANSI/HI 9.6.4 Rotodynamic Pumps for Vibration Measurement and Allowable Values” at www.pompes.org.
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