What are the causes of imbalance in steam turbine rotors?

Reasons for imbalance: shaft bending, blade fouling, uneven wear during operation, component failure, etc. The consequence is increased vibration during operation.

There is liquid accumulation in the cylinder or foreign objects causing water hammer; the pump shaft bends due to lack of regular rotation after shutdown; the speed is increased too quickly when starting up; the impeller is rusted or fouled, resulting in imbalance; improper control of clearance during maintenance leads to friction; the couplings are not aligned

In my opinion, the imbalance of the turbine rotor is mainly caused by salt accumulation on the impeller. Secondly, it’s possible that the gaps during assembly weren’t adjusted properly; if it’s due to other issues, then the problem is more serious. It’s generally unlikely.

Unbalance is a fault condition resulting from the misalignment of the mass and the geometric center line (the center of mass being not on the axis of rotation). The consequence of unbalance is an increase in additional loads, and it is one of the most common causes of damage to equipment and its components. Rotor imbalance is a fault caused by the eccentric mass of rotor components or defects in those components. There are many specific reasons for rotor imbalance, which can be classified into primary imbalance, progressive imbalance, and sudden imbalance, depending on the process by which the imbalance occurs. The original imbalance is caused by factors such as rotor manufacturing errors, assembly errors, and uneven material properties; if the dynamic balance does not meet the required accuracy standards at the time of leaving the factory, significant vibration will occur right from the start of operation. Gradual imbalance is caused by factors such as uneven scaling on the rotor, uneven deposition of dust in the medium, uneven wear of the blades and impellers by particles in the medium, and erosion of the rotor by the working medium. It is manifested as the amplitude gradually increasing as operating time prolongs. Sudden imbalance is caused by the detachment of components on the rotor or the accumulation of foreign objects in the impeller flow channels, leading to a sudden and significant increase in the vibration level of the unit, which then stabilizes at a certain level. 　　Unbalanced classification: Unbalance can be further divided into static unbalance, couple unbalance, and dynamic unbalance based on its mechanism. 　　Static imbalance: There is an unbalanced mass within the central plane of the rotor, causing the axis’s center of mass to deviate from the center line of rotation, although these two lines remain parallel. 　　 　　Even imbalance: There are unbalanced masses of equal mass at the two ends of the rotor, with a 180-degree phase difference between them; this causes the axis’s center of mass to intersect the axis of rotation at the center of gravity. 　　Dynamic imbalance: a random combination of static imbalance and torque imbalance, in which the mass center line of the shaft is neither parallel nor intersecting with the axis of rotation. 　　Causes of imbalance in rotating components: 1. Asymmetric shape of the rotating component itself; 2. Tolerances resulting from manufacturing processes; 3. Improper assembly; 4. Deformation of the rotating component during operation; 5. Damage and wear of the rotating component; 6. Presence of foreign objects attached to the rotating component. Correction of imbalance: Imbalance can be corrected on-site using dynamic balancing systems or dynamic balancers for online dynamic balancing. 　　Advantages of on-site online dynamic balancing: 1. There is no need to remove the rotating components, which reduces downtime and eliminates waste of labor and resources. 2. Correction can be carried out at the actual operating speed; usually, when components are removed and balanced on a balancing machine, it is not possible to do so at high speeds. 3. Many factors affect balance, including the balance of all rotating components (impellers, shafts, couplings, keyways, etc.), as well as installation tolerances and alignment angles of the rotating parts. Therefore, on-site online balancing yields better balance results; however, this approach is less effective for large-scale equipment. 4. Vibration problems in rotating equipment are sometimes not related to balance issues, and an online dynamic balancer can immediately detect whether there is a problem with balance, thus avoiding unnecessary balancing corrections

In summary, the reasons for rotor imbalance are: 1. Material defects, such as uneven material density during rotor manufacturing； 2. The arrangement of keys, pins, holes, etc. in the rotor structure design is asymmetric ； 3. Some components in the rotor are poorly manufactured, resulting in installation errors during assembly ； 4. Rotor bending, such as bending caused by uneven heating of the rotor, etc ； 5. Uneven scaling or corrosion of the rotor during operation, or the breakage of blades, struts, shrouds, etc., on the rotor can all cause rotor imbalance.