Page 201 - Steam Turbines--Design, Applications, and Rerating by Heinz-Bloch, Murari-Singh

180 Chapter Ten

diaphragm stages) are evaluated, and bearings are selected so that the
rotation of the force vector does not cause the journal attitude angle to
approach the adjacent pad pivot point. In some cases, the partial arc
diaphragms are repositioned to avoid this condition.

10.11 Rotor Response

If it has been determined that the combination of partial arc forces and
rotor weight can produce a resultant force vector that rotates by a sig-
nificant amount, the bearing stiffness and damping coefficients are
calculated for this condition. With the new coefficients, rotor response
to unbalanced calculations are made to ensure that the vibration
response is acceptable with the partial arc conditions.

10.12 Instability Mechanisms
Over the years, many causes of unusually high, nonsynchronous vibra-
tion due to instability have been experienced. The most common cause
has probably been bearing oil film whirl, which has been associated
with high-speed, lightly loaded, fixed-arc bearings. The use of tilting-
pad bearings, which have extremely small oil film cross-coupling
effects, has been the solution to this type of problem.
Instability due to hysteretic damping is associated with rotors hav-
ing parts, such as wheels, that are shrunk-on to the shafts. This prob-
lem is avoided by designing rotors with integral wheels, thrust collars,
and coupling flanges.
Rotors that have bore holes or internal cavities may be subject to
instability due to trapped fluid. With modern forging practices, process
controls, and ultrasonic inspection results, mechanical drive turbine
rotors have not required bore holes for the removal of centerline indi-
cations or for bore sonic inspections.
Bore holes are usually required for larger rotors, in which case the
hole is sealed at each end to prevent the entrapment of fluid.
Other types of instability have been caused by close, nonuniform,
seal or blade tip clearances. Contact or near-contact friction has also
caused problems.
Stability criteria that have been developed for bearing oil film forces
and for steam forces are successful in providing assurance that the
classic violent vibration associated with instability will not occur.

10.13 Subsynchronous Vibration
Many of the known instability mechanisms cause vibration at frequen-
cies lower than those corresponding to running speed. Most often, the
vibration frequency coincides with the fundamental rotor natural fre-