The second-order critical speed of the generator rotor and the first-order critical speed of the exciter rotor are very close, which makes it a three-support system. This configuration causes mutual influence between imbalances, and the vibration mode characteristics at operating speed are not distinct, making it challenging to identify and analyze the source of imbalance. When the journal of the No. 7 bearing bush of the exciter is large, or when there is a significant second-order imbalance in the low-pressure rotor or an imbalance in the low-pressure wheel, the rotor's own imbalance is affected. This impacts the unit's speed-increasing characteristics and working performance, especially under lower-speed conditions, which are more complex. As a result, a comprehensive analysis is required to determine the exact location of the imbalance, increasing the difficulty of the balancing process.
The vibration treatment method for this type of issue involves analyzing the phase relationship between vibration amplitudes at operating speed, as well as the second-order critical speed of the generator and the first-order critical speed of the exciter. This allows for accurate identification of second-order and third-order imbalances in the generator rotor, as well as out-of-phase imbalances and first-order imbalances in the exciter rotor. Typically, the No. 5 vibration is first checked for potential issues with the low-pressure wheel or the low-pressure rotor, followed by balancing the generator rotor on the No. 5 side. If the No. 6 or No. 7 vibrations are large, the exciter rotor is balanced first, focusing on components such as the exciter wheel, rectifier ring, and fan end. After that, the generator rotor on the No. 6 side is addressed.
In our province, we have encountered issues with loosened short-axis bolts connecting the low-pressure wheel. Two instances of this problem occurred after maintenance during startup. In one case, manual tightening was used during overhaul, relying on experience rather than precise torque values. Due to variations in tightening force from person to person, the main symptom was abnormal vibration in No. 4 shaft. The amplitude and speed of the abnormal vibration were directly related to the bolt tightness. There was a noticeable difference between the shaft vibration and the bridge vibration, with the main frequency being the power frequency. The vibration was primarily speed-related, while the vibrations in No. 3 and No. 5 shafts remained relatively small. To resolve this fault, it is essential to ensure proper tightening of the short-axis bolts on the low-pressure wheel. Because this type of issue can be difficult to diagnose, sometimes unnecessary steps like low-pressure rotor dynamic balancing or low-speed wheel alignment are attempted, leading to delays and increased effort.
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