Page 111 - Steam Turbines Design, Applications, and Rerating

92 Chapter Four

assumed to be in balance. The wheels are assembled to the shaft using
keys and a shrink fit. When cool, the shaft is checked for straightness
and placed in a balance machine for dynamic balancing. The correction
required at this step is usually quite small since each wheel has
already undergone static balancing. The remaining parts such as the
thrust disk and overspeed cup assembly are then put on the shaft and
a final correction made to the edge of the thrust disk if required.
The integral, or solid rotor, requires a somewhat different approach.
Since the wheels are part of the shaft, they cannot be balanced indi-
vidually. The shaft is therefore dynamically balanced after machining,
but before any blades or buckets are installed. Since it is symmetrical,
very little correction is usually required. Wheels are then bladed in
small groups, or one at a time, with a balance performed between each
group. In this way, if an unbalance is introduced by the blading proce-
dure, it is corrected at the same general plane of imbalance rather than
elsewhere on the rotor. As with the built-up rotor, the final balance is
done after the smaller parts, thrust disk, etc., are installed.

4.7.1 At-speed rotor balancing
A preferred balance procedure is to perform balance corrections utiliz-
ing an at-speed facility. The rotor is to be assembled, given a prelimi-
nary low-speed balance, and then a final at-speed balance using either
job bearings or those of the same type to be supplied in the unit.
Low-speed balancing of multimass rotors is an attempt to improve
the mass distribution to more nearly coincide with the axis of rotation.
This process only measures the sum of unbalance at two journal loca-
tions with the rotor in a rigid state. Unbalance forces can still exist at
service speed, especially for flexible rotors where bending of the rotor
can exaggerate a specific unbalance distribution. Incremental low-
speed balancing can help alleviate the offset of unbalance distribution,
as only two major mass components are summed in each balancing
operation.
However, the most effective results can be obtained by measuring
dynamic forces at the bearings at maximum service speed using bear-
ings that closely simulate actual operating rotor/bearing dynamics. For
flexible rotors, where bending deflections at speed modify the resulting
forces from residual unbalance, balancing corrections can be made in
any of several planes. Trial weights can determine the most effective
permanent corrections, minimizing deflections and resultant bearing
forces. With an at-speed balancing process, incremental rotor assembly
and balance are thus not required. Only a preliminary three-plane,
low-speed balance is needed for flexible rotors to limit initial rotor
response, prior to making corrections using at-speed measurements.