Page 200 - Steam Turbines--Design, Applications, and Rerating by Heinz-Bloch, Murari-Singh

Rotor Dynamics Technology 179

were open.This created a four-pad load on the pad configuration with its
different bearing characteristics, which changed the shaft orbit and
increased the rotor response. This phenomenon is illustrated in Fig.
10.9, which is a force vector plot for one of the bearings.
The vertical force is due to rotor weight. The sketch shows a 360°
nozzle box with the valve-opening sequence and the direction of rota-
tion, as indicated. The directions of the resultant forces within each
nozzle arc are shown.
The first valve supplies 90° of nozzle arc in the lower half. The steam
flow is limited to about a quarter of full flow, but a large proportion of
the total turbine power is developed in the first-stage partial arc. As
the second valve opens, the resultant force vector rotates toward the
pivot point angle of the adjacent tilting-pad. When it has rotated 45°,it
is in line with the pad, and the bearing characteristics are those of a
four-pad load on the pad configuration.
When the rotor force vector is directed between the pads of a four-pad
bearing, the oil film stiffness and damping coefficients are nearly sym-
metrical. The shaft orbits are reasonably circular and well damped.
When the force vector is directed toward a pad, the coefficients in line
with the pad are vastly different from those that are perpendicular to
the force vector. This produces a narrow, elliptical (line) orbit of much
greater amplitude. In addition, the critical speed responses change and
become more complicated.This condition will not occur during a no-load
factory test, but only during operation with load.

10.10 Design Procedure
To avoid large changes in bearing characteristics due to force vector
changes, a design procedure has been established to evaluate the
first-stage, control valve, partial arc forces due to the valve-opening
sequence. Extraction valve forces are analyzed in the same way. Worst-
case combinations of the forces from both control stages are then iden-
tified. The effect of these forces relative to the rotor weight, the bearing
loads, and the bearing type is then determined.
Another aspect of the same problem occurs in conjunction with tur-
bines that have partial arc diaphragms in the steam path, which are
required for efficiency reasons. The design practice is to circumferen-
tially index the partial arcs of the succeeding stages in the direction of
rotation. This diaphragm partial arc effect exists whether or not the
first stage is a partial arc. Even in some cases of longer, heavier rotors,
this effect results in significant changes in the bearing loads and can
rotate the resultant rotor force vectors around to the adjacent pad
pivot points.
In modern steam turbine design, the combined effects of the gravity
forces and the partial arc forces (both control stages and the