There are events associated with the operation of turbines that can induce bends into the rotor. Normally, if these are not too severe (in excess of about 0.006 0.009"), the rotors can often be balanced and can continue to operate. However, with this form of operation, the rotating blade rows at and opposite the position of the maximum bend are no longer concentric with the stationary row, and the lap will be consumed at both the inner and outer diametral positions (see chapter 2).

Note: If a rotor designed to operate at high temperatures is used with a known bend, but has been balanced to allow such operation, there is the possibility the bend will slowly correct itself. This will require continual field re-balancing to keep vibration at acceptable levels.

Operation with a bent rotor, even if adequately balanced, does not represent a satisfactory condition, and should be corrected as soon as possible. However, in order to return the rotor to service, it may be necessary to accept such operation for a period.

The bends may be of a nature that they can be corrected relatively easily. This condition is normally referred to as a "temporary bend," and the rotor metallurgical structure is essentially unchanged. Other situations are such that the rotor will have sustained a "permanent bend," with a change in the metallurgical structure, and will possibly have sustained mechanical damage or have formed surface cracks. When this condition occurs it is not...