Page 112 - Steam Turbines Design, Applications, and Rerating

Rotors for Impulse Turbines 93

Rotor response plots, along with those of bearing pedestals, can be
generated for the rotor with initial low-speed balance corrections, and
for final high-speed corrections. Typical final results give less than 0.5
mils rotor vibration and pedestal velocity equivalent to less than 10
percent of rotor static load at maximum continuous speed.

Some advantages of at-speed balancing

1. Compensation of unbalance distribution of flexible rotors and effect
of oil film on spin axis results in lower vibration levels, permitting
more margin for changes due to service conditions. This apples both
to at-speed and while traversing through the first critical speed.
2. Residual stresses from assembly and temporary rotor bends due to
internal moments can be alleviated by running rotor to overspeed
limit, resulting in final conditions prior to final balancing.
3. Less metal removal is required since the process is more effective.
4. Sensitivity and balancing accuracy are actually increased due to
pedestal flexibility and mass, as compared to more rigid and mas-
sive machinery supports.
5. Shipments of new equipment and start-up of repaired or rerated
rotors are expedited.
6. Unbalance response calculations can be verified in the at-speed bal-
ance machine. Differences in the support stiffness of the balance
machine and the actual turbine casing have to be taken into consid-
eration for this comparison.

Some disadvantages of at-speed balancing
1. With pedestal stiffness and oscillating mass used to obtain a force-
measuring system, rotor critical speeds are reduced to below field
values. Sensitivity is increased even higher when second critical
speed is reduced to below trip speed, and added care is required in
rare cases whenever it is reduced to below maximum operating
speed. This sometimes requires increasing allowable limits.
2. Due to drive coupling limitations, heavy rotors must be lifted for
start-up using hydrostatic lift bearings. Lift grooves in bearings
affect oil film stiffness and damping, which may further reduce crit-
ical speeds.
3. After at-speed balancing, the low-speed balance readings can be
above standard tolerances in ounces per inch, as they are measured
on an oil film and are only equivalent resultants measured in a rigid
state. In this case, documented values should be used if there is a