Pumps and Systems, May 2009
Balancing is essential for minimizing vibration, increasing bearing life and minimizing downtime and repair costs. Follow the guidelines from these standards to balance centrifugal pumps properly.
The Importance of Balancing
Unbalance, according to DIN/ISO 1925, is "that condition which exists in a rotor when vibratory force or motion is imparted to its bearings as a result of centrifugal forces." An unbalanced rotor will cause vibration and stress in the rotor and its supporting structure. The bearings and support systems of a pump experience higher loading, and the mechanical seals can become damaged. Balancing the rotating element of a pump will help to minimize vibration, structural stresses, premature wear and power loss. If the pump is not properly balanced, its service life is significantly reduced, and it can prematurely fail. Unbalance of rotating components is the most common cause of equipment vibration, and dynamic balancing is required to increase the pump system's overall reliability.
A pump can be unbalanced from many possible factors, including manufacturing and assembly errors and field conditions. Common causes for unbalance in pump impellers include, but are not limited to:
- Mounting errors during stacking of components
- Machining and fabrication tolerances
- Eccentric mounting of components and the rotating assembly
- Nonsymmetrical rotors
- Bent shaft due to mechanical conditions or thermal effects
- Non-homogenous component material
- Distortion of rotating elements due to stresses or temperature changes
- Gradual accumulation of product within or around the component
- Product flowing through an impeller during normal operating conditions
- Corrosion and erosion of rotating components in contact with the product that the pump displaces
Balancing describes the procedure by which the mass distribution of a rotating component or assembly is checked to see whether it is within acceptable limits corresponding to the service speed. If necessary, the mass distribution is improved so that minimal vibration and unbalance forces are imparted to the bearing system. To ensure quality of product, maintain low overall system vibrations and maximize the machinery's availability, it is advantageous to follow the successful balancing techniques from industry organizations.
Rotating pump components and impellers are balanced at pump assembly and during production, and should be balanced again when the pump is rebuilt or repaired.
Industry Standard Guidelines
Industry standards such as API (American Petroleum Institute), ISO (International Organization for Standardization) and ANSI (American National Standards Institute) provide repair shops and OEMs with important information and requirements for pumps, turbines and compressors, including balancing guidelines. A pump service and overhaul shop responsible for the rebuild and repair of pump systems should rely on these standards for balancing procedures, and the OEM will use these guidelines to establish balance tolerances and correction methods for manufacturing the pump.
In addition to requirements and guidance on basic design, function and analysis for all types of rotating components found in centrifugal pumps, these industry standards provide explanations for determining balancing tolerance, balancing acceptance and balancing machine performance testing.
According to API Standard 610, residual unbalance refers to the amount of unbalance remaining in a rotor after balancing. Balance tolerances are established as maximum allowable residual unbalance for each correction plane of the rotor. Balance tolerances can be determined by different methods. The API Standard states that the balance tolerance, or maximum allowable residual unbalance per correction plane, equals:
U = 4W/N
Where:
U = Balance tolerance, in units of oz-in (for each correction plane)
W = Bearing journal static weight at each end of the rotor,
N = Maximum service speed of the rotor.
The 4W/N API Standard balance tolerance equates approximately to an ISO Grade of 0.7 (ISO 1940 tolerances are a highly accepted standard for balance tolerance requirements). With a modern balancing machine, along with an experienced operator and any proper tooling, these limits may be achievable but not repeatable.
To achieve an ISO 1940-1 Quality Grade of G1, the rotor must be mounted in its own service bearings (and not removed prior to installing into service) on a suitable belt driven balancing machine. To achieve a quality grade better than G1, the rotor must be mounted in its own service bearings, completely assembled as a unit in its own housing, run under service conditions (preload of bearings and temperature) and self-driven. Quality grades better than G1 are reserved for small, high speed, self-driven rotors such as gyroscopes, optical scanners, dental turbines and special motors.
API 610, 9th edition, states that main rotating components of a pump, such as an impeller, shall be dynamically balanced to ISO 1940-1 grade 2.5. It then states that these same components shall be dynamically balanced, if specified, to ISO 1940-1 grade G1, which is equivalent to 4W/N (oz-in). In actuality, the API 4W/N equates approximately to an ISO grade of 0.7.
ISO also recommends a margin of safety for changes in the residual unbalance between the time when the rotor was balanced originally and when an inspector checks it again. Apparent changes in the residual unbalance can be attributed to calibration differences between balancing and inspection machines, bearing or journal changes, environmental fluctuations (heat/humidity), tooling inaccuracies, drive errors, stress relieving of components or damage from rotor handling.
The recommendation is to allow the balancing machine operator a margin set below the recommended tolerance and the inspector a margin set above the recommended tolerance to account for these potential changes. As an example, for ISO Quality Grades from G16 to G2.5 a margin of -10 percent is recommended for balancing and a +15 percent margin is recommended for inspection. For a G1 quality grade, a -20 percent margin is recommended for balancing and a +25 percent is recommended for inspection.
In some cases, the manufacturer or repair shop must reconsider whether to continue to balance components to quality grades of G1 or better. The additional time and energy spent trying to achieve a tolerance that is not repeatable once it is installed is simply not cost effective.
The balancing procedures recommended by API, ISO or ANSI for general balancing applications and specific applications such as overhung (outboard) impellers, impellers between bearings and vertically suspended impellers should be closely reviewed and evaluated. The pump manufacturer or repair shop may specify a residual unbalance test be performed at the completion of the final balancing of the rotor before removing the rotor from the balancing machine. The API 610 and API 617 documents outline a procedure for performing a residual unbalance test to determine if the residual unbalance found in the rotor after balancing is indeed within the tolerance specified. The test is used to confirm that the balancing machine readout and calibration is correct, and there was no operator error.
The residual unbalance test is used to help rule out possible errors in machine calibration, operator error, machine installation errors, set up inaccuracies or faulty machine components (bad pickup or cable). The rotor's residual unbalance is tested by using a known amount of unbalance, or trial weight, and adding it to the rotor in six equally spaced radial positions around the rotor. This is repeated for each balancing machine correction plane. It is important to ensure that the trial weight is placed at the correct radius for each radial position on the rotor. The angular location of the test weight displayed on the balancing machine should closely approximate the location of the trial weight on the rotor. The readings are recorded and plotted per the API specification.
Figure 1. Vector display of the six trial weights traversed.
In the readings, the distance between the centers of the circle from the origin of the polar plot corresponds to the actual residual unbalance present in the rotor for that correction plane. The API documents recommend that if the residual unbalance amount determined from the test is greater than the tolerance specified for that rotor, then the rotor is to be balanced more accurately by making the necessary corrections and repeating the test.
Figure 2. Vector display showing the resultant residual unbalance. Note the tolerance band given by the shaded area.
The residual unbalance test is another aspect of the balancing process that must be assessed. The additional time and energy used to perform this test should be considered when accounting for cycle time, throughput or repair turnaround time. The time and cost associated with performing the test should be factored into deciding if it will be specified to perform the test on every rotor going on a balancing machine, or only for each new rotor type set up or operator shift change, etc.
For commonality throughout the pump industry and streamlined balancing operations, balancer operators should become familiar with these industry standard guidelines to balance and check pump components and assemblies. These guidelines are developed from common industry experience to give an operator, engineering department or designer recommended balancing techniques. The decision must ultimately be made as to how closely an organization will follow these guidelines and what approach will safely and cost effectively produce the best results for a particular application.