Straightening a rotor

One of the problems that might have to be faced when operating steam turbines is shaft inflection, or rotor bowing, a condition that can be either temporary or permanent. Temporary bowing is usually solved with the cooling down of the turbine, so that it does not need further actions. Permanent bowing, on the other hand, requires understanding of the underlying root cause and subsequent correction.

There are several conditions that can result in the bowing of a rotor. A common one is a long standstill in hot condition. Another is an uneven steam flow with uneven temperatures in one or more admission branches. But in many cases, the most serious causes of permanent bowing are attributable to rubbing, which may cause a plastic deformation of a small portion of the rotor; and the presence of water, usually entering the hot turbine from the heat exchangers (known as quenching of a rotor portion).

If a rotor has a Total Indicated Run Out of above 0.2 mm, this deflection is difficult to recover by rotor balancing. As a consequence of this amount of bowing, it is not possible to run the rotor above critical speed, rendering the turbine inoperable. At this point, there are two possible choices: replace the rotor, or straighten and rebalance it.

There are ways of carrying out this second option in situ such as cold mechanical straightening using jacks — thermomechanical straightening. Both options have been applied successfully for small rotors with only light bowing. Depending on the inflection line and the amount of inflection, there is also the possibility of machining the rotor shaft (offsetting the shaft centerline). Peening, or hammering, is also limited to small rotors with light bowing. A more effective approach can be “Hot Spotting,” which involves the controlled application of a thermal source to a well-defined point followed by a controlled cool down.

This entails several steps. First, is an in-depth analysis of the rotor.Tests must be carried out to assess the material structure and whether the metallurgical structure has suffered any modification.

 

You can read the rest of this article in the March April Edition of Turbomachinery International