Balancing is an important technical procedure both when assembling a new turbine and during the operation of turbine equipment. As the rotor rotates, wear of the rotor shaft supporting surfaces occurs. This leads to a gradual displacement of the center of mass – unbalance. Ideally, it should be located on the axis of the rotor rotation. The unbalance and accompanying vibrations are results of ineffective lubrication and bearing defects, as well as disturbances in operational conditions and failures in the automation and control system. Cavitation and surging effects also accelerate turbine unbalance.
As a result, the capacity of the entire unit drops. This is also followed by an increase in noise levels and a gradual increase in vibrations with far-reaching negative prospects in case the rotor is not balanced in time.
Rotor balancing process
The balancing process involves balancing the distributed masses of the rotor that is rotating on test fixture or bearings. Balancing can be static and dynamic. At the stage of static balancing of the rotor and other rotating parts of the turbine, the center of mass is aligned in the direction of the axis of rotation in order to minimize run-out during the rotor rotation and eliminate vibrations of the entire turbine. Static balancing is performed using special stands.
The general balancing procedure involves identification of the existing center of mass displacement vector, which will lead to a point on the outer circumference of the rotor. A rotor with a static unbalance, mounted in bearings on fixtures, will turn its "heavy point" downward due to gravity.
The balancing and displacement of the center of mass in the direction of the axis of rotation is performed after installing additional weights on the side of the rotor opposite to the point. For a comprehensive balancing, it may be necessary to install several weights in different places. Their mass is selected empirically. At the same time, a minimum deviation of the center of mass is achieved, meeting the tolerances according to the standards..
Moving, adding, or removing correction weights can be done by surfacing, built-up welding, screwing or unscrewing special screws. Other options are spot drilling or milling of excess weight around the rotor circumference at separate points that are indicated by the center of mass displacement vector.
Upon completion of the static rotor balancing, it will be able to maintain a stable position in any orientation during 360 degrees rotation.
Unbalance, that appears after the installation of a pre-balanced (static) rotor and assembly of the turbine, is a common problem for such large-sized equipment as a turbine. Dynamic balancing involves measuring the rotor vibration resistance parameters in the operating mode. Dynamic unbalance on a rotating rotor is manifested due to centers of mass being displaced along the axis of rotation. This can be the result of inhomogeneity of the rotor material, uneven wear of the blades, adhesion of foreign bodies during operation or other reasons.
Dynamic balancing of the assembled mechanisms, rotating on their own fixtures (bearings), is performed in order to identify and eliminate possible problems in the operating mode. Measurements are performed at standard rotational speeds using specialized measuring instruments and rotor-mounted sensors.
Based on the measurement results, the vibration level is determined, the mass and the angle of installation for the correction weights are calculated. Upon the installation of the weights, the vibration is re-measured and, if necessary, the weights balance is adjusted.
Dynamic balancing is performed after assembling the new equipment or repairing a turbine, and as a preventive measure for equipment in operation.
Balancing of the turbine rotor on a timely basis allows to:
- reduce the vibration level and extend the service life of individual components and parts of the turbine;
- reduce energy losses and increase turbine capacity;
- prevent possible subsequent breakdowns and accidents.
Timely rotor balancing guarantees a long and efficient operation of the turbine.