POWER April 2013 - 62

PLANT O&M
Steam Turbine Rotor Vibration
Failures: Causes and Solutions
Steam turbine rotors bend during operation, but the bearing and supports
are designed to keep the static and dynamic forces under control. However,
bending can cause impact between stationary and rotating parts-
often cascading impacts. An operator of many utility-scale steam turbines
shares its extensive field experience identifying the root cause of failures
as well as successful solutions.
By Farshad Shamoradi, Naser Mohammadi, and Mahdi Heidari Kahkesh, Ahwaz Power Generation Management Co.
Uneven cooling of the rotor, particularly
R
otor bending that results in premature
failure of steam turbine blades and
other internal components is one of the
most serious problems experienced in power
plant operations. The problems often reduce
plant availability by limiting generation and
increase plant operation and maintenance
cost. Extreme rotor bending problems often
involve interaction between the turbine's rotor
and stationary parts. Rotor bending may
be caused by a variety of static and dynamic
factors, many of which will be explored in
this article.
We begin with mechanical factors related
to the rotor, the largest rotating assembly in
the turbine. Working from the inside out, we
next look at rotor balance issues, followed by
rotor and casing misalignment problems, and
problems caused by the casing. The discussion
is based on the authors' experiences at
the six-unit, 1,890-MW Ramin Power Plant,
located in Ahwaz, Iran. The units were commissioned
from 1980 through 1985.
Avoid Rotor Rubs
It almost goes without saying that rubbing in
the labyrinths or diaphragms, caused by insufficient
clearances, disrupts the end sealing
of the rotor. This situation commonly occurs
when the high-mass rotor at operating speed
comes in contact with a stationary surface,
typically caused by a too-small clearance
between the labyrinth or diaphragm gland
seals and the rotor. Secondarily, there may be
a localized temperature increase at the point
of contact, causing increased metal temperatures
at the point of contact due to friction.
The forces produced by the impact of the
large rotating rotor mass with the poorly
functioning stationary seals often impress a
layer of metal on the surface of the rotor. The
rub can cause elastic deformation of the rotor
at the point of impact and temporary rotor
shaft bending. The shaft bending will usually
cause increased vibration levels (Figure 1).
62
after shutdown, also causes the rotor to contact
stationary parts. After a unit shuts down,
the relatively high-temperature rotor may
bend solely due to the mass of the rotor and
the distance between bearing supports, if left
in a stationary position to cool. This situation
can cause permanent shaft bending.
The effect of a permanent shaft bend
caused by uneven cooling will immediately
appear as high rotor vibration at the next
startup. The vibration is caused by insufficient
clearance between stationary and
rotating parts, as well as a shaft located offcenter
in its bearing. Even if the clearance
change is small, there may be significant
rubbing along the rotor to cause damage.
Again, the rubbing causes friction between
stationary and rotating parts, localized heating
of the rotor metal at the contact point,
and shaft bending.
Furthermore, uneven shaft warming
caused by rubbing between rotating and stationary
parts can cause further bending of
the shaft in the same direction of the existing
bow and cause additional contact with
stationary parts, increasing temperatures and
therefore causing more bending. The effect
cascades if allowed to continue. If the bending
is allowed to continue, it is possible that
the yield strength of the metal could be exceeded,
causing a permanent deformation of
the shaft. The allowed bending in 3,000-rpm
turbines is up to 0.02-0.03 mm in each section.
When on turning gearing, the limit is
0.05 mm.
To avoid rotor bending during cooling,
turbine vendors provide very specific instructions
on the allowable rate of cooling.
For example, the turbine should remain on
turning gear until the high-pressure (HP) cylinder
temperature is below 150C and the oil
temperature is below 75C. The turbine vendor
also defines the rotational speed of the
turning gear.
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POWER | April 2013
Prevent Rotor and Casing
Misalignment
Misalignment of the coupling between two
shafts or between a shaft and bearing may
cause bending in the system. Misalignment
between two shafts of an integrated
rotor can cause an eccentricity of the mass
center of the rotor, and this eccentricity at
high rotational speed will produce a centrifugal
force in the radial direction, causing
bending of the rotor. Misalignment
between the axis of rotation and the axis of
the shaft can also cause bending in the rotor.
There are six primary factors that can
cause misalignment.
A poor connection between the turbine
casing and the bearing pads on the foundation
frame is one cause. If a pad experiences
increased friction or stops sliding during
thermal expansion (usually during startup)
in the axial direction, the result is a tipping
torque on the casing. This torque can cause
a misalignment between the casing and bearing
surface, causing vibration in the forward
end of the turbine, foundation frame surface
support deformation, and bearing pad stall.
Also pay close attention to the foundation
frame-including bolts, keys, and
1. Blade rubs cause bending. Rubbing
in the sealing of a high-pressure rotor of
a 300-MW unit caused bending of this rotor
and blade tip rubs. Courtesy: Ahwaz Power
Generation Management Co.

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POWER April 2013

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