Page 113 - Steam Turbines Design, Applications, and Rerating

94 Chapter Four

check balance in another low-speed machine prior to operation, to
retain the low vibration during testing.

4.8 Balance Tolerance
The degree of balance that must be obtained with a rotor is governed
by the weight of the rotor and the speed at which it runs. The faster
and lighter a rotor is, the more precise is the required balance. The
usual tolerance is given by the formula adopted by API in 1987. It
states that the maximum allowable residual imbalance per plane (jour-
nal) is to be calculated as follows:

4W
U =
N
In SI units, this translates to:

6350W
U =
N
where U = residual unbalance, oz-in (g-mm)
W = journal static weight load, lb (kg)
N = maximum continuous speed, r/min

Finally, during the shop test of the machine, assembled with the bal-
anced rotor, operating at its maximum continuous speed or at any
other speed within the specific operating speed range, the peak-to-peak
amplitude of unfiltered vibration in any plane, measured on the shaft
adjacent and relative to each radial bearing, may not exceed the fol-
lowing value or 2.0 mils (50 μm), whichever is less:

12,000
A =
N
In SI units, this translates to:

12,000
A = 25.4
N
where A = amplitude of unfiltered vibration, mils (μm) peak to peak
N = maximum continuous speed, r/min