Page 199 - Steam Turbines Design, Applications, and Rerating

178 Chapter Ten

10.9 Partial Arc Forces
For many years, it has been the practice of turbine designers to arrange
the steam control valve-opening sequence so that the first valve to
open will result in a downward steam force on the rotor. The reason for
this was to avoid unloading the bearings during partial arc operation,
which might cause undesirable rotor vibration.
More recently, in the case of a small, high-power turbine in service, it
was discovered that the entire valve-opening sequence must be consid-
ered, because the partial arc steam forces were large relative to the rotor
weight. This could push the rotor into a sector of the bearing where the
dynamic characteristics of the bearing would be different than what was
expected. The vertical gravity load was directed between the pads of a
four-pad bearing. But during partial arc operation, the combined gravity
and steam force vector rotated as much as 60° so that the resultant load
could be directly in line with the adjacent pad when two control valves

Figure 10.9 Force vector plot shows load orientation on
bearing changing with steam admission sequence. (Gen-
eral Electric Company, Fitchburg, Mass.)