Mass unbalance is one of the most common faults found in steam turbine shafting. It was reported that about 70% of the total turbo-generator units newly put into commission needs high-speed dynamic balance in site. Because of longer shafting and relatively lower support stiffness, the vibration of multi-rotor bearing system, is much more sensitive to mass-unbalance. In most cases, trial masses and runs are required for the calculation of correction masses before balancing a turbo-generator rotor and such a procedure is time-consuming and expensive. Our experience shows that one balance for a turbo-generator rotor in China, by using traditional balance method, will take at least 3 to 5 runs, even 6 to 10 runs. That means 100∼500t oil will be consumed each time for balancing a turbo-generator unit with capacity of 200MW to 600MW. A balancing method without trial mass was proposed at the end of 1980’s. As it needs no trial runs if the magnitude and orientation of the rotor unbalance could be determined by calculating the amplitude-frequency and phase-frequency characteristics of various rotor sections, it has been adopted by highly skilled engineers. But the principle disadvantage of this method is that effective application requires a high degree of operator insight or knowledge of the support characteristics (i.e., requires data taken at previously balanced procedure, or from other units to determine the magnitude and location of the unbalance). This paper deduced empirical formula for dynamic balancing without trail mass at first. Then, based on the data of lag phase from the experience over 100 units, a balance method without trial mass was developed. Implementation of this method on a 1000MW turbo-generator rotor shows that it is an effective and economical procedure and the balancing risk is reduced.

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