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There is disagreement among many experts and suppliers. Some suppliers will high-speed balance flexible-shaft rotors while other suppliers making similar rotors do not.
API-684 states that compressor and turbine rotors generally do not require high-speed balancing. ISO-1940-1 says that "Flexible rotors normally require multi-plane balancing at high speed. Nevertheless, under certain conditions, a flexible rotor can also be balanced at low speed."
With new machinery, it is generally advisable to follow the supplier standards, since they are responsible for meeting vibration limits in the shop and the field.
The more difficult decision is what to do when a rotor is repaired. The writer suggests that the decision to require high-speed balancing be based on experience with rotors of a similar design, a review of the repaired rotor's vibration history, a review of the rotor-dynamic analysis and, in particular, a review of the proximity of the operating speed to the critical speeds and a review of the predicted-vibration response at the critical speeds, and an assessment of the risk of high vibration after the rotor is installed in the field.
There also are a couple of additional issues to consider with high-speed balancing. First, high-speed balancing does not replace the need for low-speed balancing. Low-speed balancing is required to ensure that the rotor and high-speed balance machine are not damaged due to excessive vibrations during balancing. Secondly, the high-speed balance machine support system stiffness might be considerably different than the field installation. This could have an effect on the observed critical speeds in the balance machine. Lastly, high-speed balancing requires the use of fluid film bearings. If possible, it is best to use the job bearings when balancing.
Conclusion
Although rotor balancing is a mature technology, the application and requirements need to be understood by the end user when purchasing new machines or repairing existing machines. The purpose of this article was to present many of the issues that need to be considered when balancing rotors. The references listed below contain additional information.
References
1. API-684, "Tutorial on the API Standard Paragraphs Covering Rotor Dynamics and Balancing: An Introduction to Lateral Critical and Train Torsional Analysis and Rotor Balancing," First Edition, API, Washington, D.C.
2. ISO 11342, "Mechanical Vibration - Methods and Criteria for the Mechanical Balancing of Flexible Rotors," ISO, Second Edition, Geneva, Switzerland.
3. ISO 1940/1, "Mechanical Vibration - Balance Quality Requirements of Rigid Rotors - Part 1: Determination of Residual Unbalance," ISO, First Edition, Geneva, Switzerland.
4. Halfen, Earl, "Shop Balancing Tolerances: A Practical Guide," Reliability Magazine, November/December, 1994.
5. "Balance Quality Requirements of Rigid Rotors, The Practical Application oif ISO 1940/1," IRD Balancing Technical Paper 1.
6. Brown, Royce, "Compressors Selection & Sizing," Houston, Texas, Gulf Publishing Company, 1986.
7. Randall L. Fox, "A Practical Guide To In-Place Balancing," Proceedings of the Tenth Turbomachinery Symposium, pp 113 - 129.
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