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
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.