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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 action. Permanent bowing, on the other hand, requires understanding of the underlying root cause and subsequent correction (Figure 1.)
Figure 1: Analysis showing the amount of bowing in a steam turbine[/caption]
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 man y 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 re-balance 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 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.
Before hot spotting begins, the rotor needs to be stress relieved using heat treatment. As for hot spotting itself, it can be summarized as follows: a rapid warm up of the selected areas while keeping adjacent areas cool (Figure 2).
Figure 2: Hot spotting involves the rapid warm up of specific areas while keeping adjacent areas cool[/caption]
The warm material be gins to expand elastically, but is axially constrained by the adjacent cold material. Thus, the hot material plastically expands outside its original shape. When cooling down, the material tries to shrink back to its original bounds, but the shape has now changed. In this new shape, the material that is plastically expanded beyond its original borders cannot occupy the same space when cooled. After hot spotting, the stress originated during the process needs to be relieved through a post-heat treatment (which may recover the metallurgical characteristics decayed during spotting). A final lathing to finish the rotor shaft may also be applied.
Tests are then repeated to guarantee that the material can safely operate.
Some special tooling should be prepared in advance, such as rotor-holding tools during heat treatment, special torches, and water-cooled shields.
Other equipment can typically to be found on site, e.g., cranes, slings, inspection equipment, as well as a lathe that is able to carry the rotor and the balancing machine, plus the heat treatment equipment.
Although it is not possible to guarantee the success of rotor straightening, in our experience this approach has obtained a 100% success rate. All rotors were straightened within one month of commencement (Figure 3).
Figure 3: “Hot spotting” of bowed rotors has been successfully applied on many sites[/caption]