State of the art automatic condition diagnostic, monitoring, and balancing software

4. Machine balancing in situ.

Question 4.3.

While balancing powerful turbines we often face a situation when the coordinates of the heavy point and hence the vibration of the turbine on the rotating frequency is very dependent on the load value. A typical explanation, viz. the uneven thermal expansion of the rotating units, does not explain the simplest experiments when a sudden change of the load leads to a sudden change of the vibration. Then, what are the real physical processes in such cases?
(The question was asked by the participants of the all-Russia conference "The Problems of Vibration and Vibrodiagnostics of Equipment on Power Plants ".)

The question is answered by Alexej V. Barkov:

The uneven thermal expansion of the rotors with the turbine-wheels is one of the main but not the only reason of the vibration change on the rotating frequency when the load changes. Presumably you have seen one of two other probable reasons – dis-similar aerodynamic properties of certain blades (a part of turbine-wheel) or a defect of the coupling that joins the turbine shaft with the generator (or any other machine).

• In the first case, because of the disparate aerodynamic resistance of the blades the center of the gas medium pressure applied to the turbine-wheel shifts against the geometric center of the wheel. Hence an additional rotating moment of forces appears (mainly couple unbalance of the turbine-wheel) that depends on the load and changes abruptly with the load changes.

• In the second case, when there is a defect of the coupling, the axes of the moments transfer of the turbine and generator do not coincide. It also results in the appearance of a rotating radial force applied to the coupling. The amplitude of the force is proportional to the moment transferred and the displacement of the axes of the moment transmission, and hence it changes stepwise when the load changes. Very often this effect appears when the fit of coupling on the shaft is out of tolerance, when there is a change in rigidity, or when the center of one of the pins of the coupling has shifted as a result of manufacturing fault or wear. But most often this defect appears in the tooth-type coupling as a result of wear.

How can you detect that one of these reasons exists in your turbine for the vibration increase at rotating frequency? The simplest way is to abruptly change the value of load applied to the turbine without changing the rotating frequency. If the vibration of the turbine changes immediately then the source of one of the rotation forces is present. To find out whether the source of the vibration increase is the aerodynamic features of a certain turbine-wheel, it is necessary to measure the envelope spectra of the high frequency random vibration of the turbine casing in the section of each turbine-wheel separating preliminary the harmonic (blade) and random components in the high frequency vibration signal. (See the answer on the question 1-8 and D. Carter’s article in the article section of our Web site.) How to balance the turbine in such a case? It is necessary to balance the turbine together with the generator optimizing the balancing weights for several typical modes of operation. For this case you can use a software for balancing the multimode operation machines. Such programs are designed by several companies. In particular our companies supply the software of different complexity for balancing such machines in field.