Gene Vogel
EASA Pump & Vibration Specialist

As with most other machines commonly repaired in EASA service centers, dynamic balancing on pump impellers is an important concern. Excessive imbalance imparts forces on bearings, reducing their lives and subjecting machine mountings to vibratory energy that deteriorates foundations.

Pump rotors are quite different than more familiar electric motor rotors from a dynamic balance perspective. The mass of an electric motor rotor is between the bearings, and the rotors are longer than their diameters. Many pump impellers are mounted in an overhung configuration, and the impellers will likely be narrower than their diameters. Narrow components may require special rules for allocating allowable residual imbalance (per ISO 21940-11), and special balancing techniques may be needed for efficient balancing in the balancing machine.

When a pump’s condition is evaluated during the repair process, a decision is made regarding whether the impeller balance should be checked/corrected in the balancing machine. Generally, it’s not practical to test run a pump to check vibration levels before repair. A decision to always perform a dynamic balance check on every pump repair would unnecessarily inflate the cost of some repair estimates. If there is no wear or damage to an impeller, and the need for repair is unrelated to machine vibration, the cost of checking the impeller balance may not be justified. Note that some pump shaft failures and impeller retaining bolt failures have been attributed to impeller imbalance forces. So, the decision to check impeller balance should be made with a thorough knowledge of the impact of impeller imbalance.

It is common practice to balance pump impellers on the pump shaft as an assembly. However, since the pump shaft itself could hardly be out of balance, an impeller can be balanced on a mandrel. This is the common practice for multi-impeller pumps. When calculating allowable residual imbalance tolerances, the entire rotor weight should be considered. Calculating tolerance amounts based on the impeller weight alone may result in much lower tolerances than necessary. For many pumps, balancing the impeller on the pump shaft will result in an overhung balancing configuration. The overhung configuration presents some challenges to the balancing process.

Single Plane v. Two Plane Balance
Next, the decision must be made to use a single plane or two plane balance. There used to be a guideline that if the length-to-diameter ratio of a rotor was less than half, and the rotating speed was less than 1000 rpm, single plane balancing was acceptable. That guideline is not reflected in the current ISO standard (ISO 21940-11). Most pumps operate above 1000 rpm, but many pump manufacturers only single plane balance impellers. That's fine if they control their manufacturing process to minimize "couple imbalance," but there have been numerous instances where that is not the case.

Incidentally, if you are unfamiliar with the term “couple imbalance,” you will find a tutorial on static-couple balancing very helpful. This is fundamental to balancing pump impellers (see the EASA technical article, "Most Common Methods of Balancing Overhung Rotors"). The recommendation is to two plane balance. Once the impeller is mounted in the balancing machine, there is little advantage to single plane balancing (Figure 1). 

Balance corrections on a pump impeller are made by removing weight, usually by grinding on the impeller shrouds. Areas ground should be smooth. Drilling holes is discouraged since the holes generate localized turbulence in the pumpage. Since it is difficult to “ungrind” a correction weight, the initial correction should be made by attaching putty to the impeller. When acceptable balance is achieved, grind the correction weights 180° from the putty location. If the results of initial balancing with putty indicate that a large amount of correction weight is required in a couple configuration, evaluate whether it will be practical to two plane balance the impeller. Large couple imbalance usually results from casting and machining anomalies in the manufacturing process. Again, manufacturers try to control these anomalies and only single plane balance impellers. When it is impractical to correct a couple imbalance on an impeller, it often is the “bad” impeller that should be replaced.

EASA recommends balancing pump impellers to G2.5 per the ISO 21940-11 standard. Special balancing procedures apply to high energy pumps as defined by the Hydraulic Institute standard (ANSI-HI Pump Standard). High Energy pumps are generally not encountered in EASA service centers. 

VTPs + Split Case Pumps
Two types of multistage pumps are common in EASA service centers: vertical turbine pumps (VTPs) and split case pumps. When balancing is required for multistage pumps, each impeller should be two plane balanced individually on a mandrel. Then the impeller should be assembled on the pump shaft, and the assembly should be checked to ensure it is within the G2.5 tolerance. Beyond this, the procedures for VTPs and split case pumps differ.

For VTPs, the impeller and any mounting collets or hardware should be match marked, and the rotor should be disassembled for proper assembly in the pump bowls. Also, when checking the balance of the assembled rotor, if it is out of spec, check the shaft runout and impeller mountings. Some slight trim balancing can be done on the assembled rotor, but excessive imbalance indicates a problem that needs to be corrected (Figure 2).

For multistage split case pumps, the rotor is assembled for final assembly in the casing except that the shaft nuts that hold the impellers in place are just snug, not tight. Any stage bushings or other stationary components that must be mounted between impellers can be installed. When the rotor is spun in the balancing machine, the stationary components can be tied with string to the balancing machine base. This will prevent them from rotating (with a little machine oil between them and the shaft). After the assembled rotor balance is confirmed within tolerance, tighten the shaft nuts and recheck the balance. Any significant change in the rotor balance with tight shaft nuts is likely due to runout of the shaft sleeve or impeller axial faces. Such runout will cause the shaft to bow when the shaft nuts are tightened.

A good understanding of static-couple balancing and the procedures for multistage pump rotors is essential for completing a quality pump repair. The EASA technical article mentioned above and the Fundamentals of Pump Repair Seminar and associated manual are helpful resources to reference.

Categories: Technical topics, Balancing, Pumps, Impellers, Vibration

Tags: Balancing Impeller

Rate this article:

1.7

Print