POWER April 2013 - 64

PLANT O&M
pads-so that free movement of the bearing
surfaces is possible, particularly while
undergoing startup and load changes. The
extent of the longitudinal and lateral thermal
expansion bore centers of the cylinders
and pad travel should be recorded for
future comparisons. This process should
be part of routine maintenance equipment
inspections.
Another factor concerns the difficulty
in assembling the HP turbine front bearing.
While the shaft is rotating in its journal
bearing, the shaft pushes oil from the
bottom of the bearing, causing the oil film
thickness to change. When this happens,
the centerline of the shaft moves up and to
one side. To account for this shaft movement,
the segmented bearing should automatically
adjust and the contact surface of
the journal bearing will remain in a good
position. If there is too much contact surface,
friction will increase on the bearing
surface, causing increased rubbing and
corrosion of the bearing surface and increasing
vibration and rotor eccentricity.
The result will be bearing oil leakage and
rubbing in the sealing glands. On the other
hand, if the bearing contact area decreases,
the oil film will cause uneven movement
of the rotor within the segmented bearing,
and the oil film will not form, also resulting
in increased vibration (Figure 2).
Don't overlook the concentricity of the
rotor with the bores and couplings. Correct
rotor alignment is lost when the axis
of one rotor is not continuous with the rotor
in the following casing, in multi-casing
steam turbines. The individually connected
drivelines must operate as one long continuous,
yet flexible driveline. After major
steam turbine maintenance, it is important
to confirm the alignment of the rotor to
the couplings as well as any other factors
that may cause change to the primary positions
of individual casings, bearing, and
rotors. During maintenance, if rubbing is
observed at the end or intermediate sealing
of the rotor or eccentricity of couplings,
it is necessary to realign the driveline to
avoid high turbine vibration, contact and
rubbing of glands or labyrinth seals, and
so on (Figure 3).
Remember that the axis from the center
of the gland's bore should coincide with
the axis of the turbine rotor during normal
temperatures experienced during turbine
operation. Furthermore, it is essential to
quickly identify any loss in sealing clearances
during steam turbine warm-up.
Changes in clearance may occur during
warming that could cause bending of the
cylinders due to differential temperatures
between the upper and lower sections of
the cylinder (Figure 4).
Finally, pay close attention to the possibility
of contact rubbing of blade tips
on the cylinder walls in reactive stages.
Rubbing can cause increased vibration to
blade bending, usually at the blade root
(Figure 5).
Rotor Imbalance Increases Vibration
Shaft curvature also shifts the rotation axis of
the shaft by moving the mass center of the rotor,
creating vibration. This vibration affects
blades in three significant ways.
First, the vibration causes blade structural
problems. The centrifugal forces encountered
during operation are significant, causing an
increase in the tensile forces in the blade
cross-section and, if the center of mass is not
on the radial line, bending stresses also occur.
Additionally, bending stresses are created in
blade joints under the pressure effects of the
2. Shaft must stay centered. Relocation of shaft center in a segmented bearing while
rotating can cause vibration. Courtesy: Ahwaz Power Generation Management Co.
Bearing
centerline
Shaft C
Drop in shaft
centerline from
pad rotation
Load on pivot (LOP)
64
Load between pivot (LBP)
www.powermag.com
POWER | April 2013
Shaft C
5. Avoid tenon extrusion rubs. This
is an example of tenon extrusion in high-pressure
rotor blades. Courtesy: Ahwaz Power
Generation Management Co.
4. Precise alignment required. This
photo shows the expansion key of the upper
half of a low-pressure cylinder. Courtesy: Ahwaz
Power Generation Management Co.
HP steam flowing axially through the turbine
cylinder. The magnitude of these stresses
is dependent on the flow rate of steam, the
temperature drop across the blade stage, the
rotational speed of the blades, and the blade
weight. The temperature of the steam, superheated
in the first stage and saturated in
the final stages, will have an effect on the
mechanical properties and corrosion of the
blade materials (Figure 6).
3. Properly align the diaphragms.
The diaphragms must be carefully aligned to
the shaft centerline within the casing of this
315-MW turbine to avoid contact with the rotor
blades. Courtesy: Ahwaz Power Generation
Management Co.
http://www.powermag.com
POWER April 2013

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