POWER April 2017 - 30

OPERATIONS & MAINTENANCE
3. Rubs. Contact between rotating and stationary components can cause vibration and damage.
These spill strips have been rubbed and as a result " machined out. " Courtesy: EPRI.
lead to forced outages to repair the damage
(Figure 3). Rubs can be due to a variety of
issues, including but not limited to improper
maintenance practices, inadequate insulation
installation, or poor startup processes.
Generators
Issues with generators on the cycling steam
turbines may be the direct result of cycling or
exacerbated by the cyclic nature of operations.
Dusting and Greasing. One of the most
4. Greasy windings. Cycling can cause
additional vibration in the generator, which can
manifest as a greasy substance caused by the
presence of oil inside the generator combining
with dust created by windings rubbing
against each other. Courtesy: EPRI
of the cyclic operations.
Airfoil Damage Due to Water or Wet
Steam Ingress. Due to the push to get units
online quicker, or the desire to try to run at
extremely low loads, today's CCGT facilities
have experienced issues with damage to
airfoils, both rotating and stationary, resulting
in cutting or thinning (Figure 2). This
damage is often repairable but can be very
expensive and time consuming.
Last-Stage Blade Resonance. Surprislevels,
in contrast to fossil boilers, which
have only one. As a result, most modern
CCGT facilities are outfitted with steam turbines
that have been specifically designed for
combined-cycle operations rather than steam
turbines that were originally intended for
process duty or fossil-based service.
With the many different facilities responding
in the study, researchers received a wide
diversity of responses. Some operators stated
that they have seen no negative consequences
from high cycling of their steam turbines,
while others stated that they have seen fairly
drastic consequences. One of the project
managers interviewed at a non-OEM service
provider stated, " There is a lot more being
asked of these steam turbines with regards to
cyclic duty, and problems are showing up a
lot sooner than compared to their older utility-grade
counterparts. "
Issues identified in the study were separated
into four areas: steam turbines proper,
generators, plant startup and shutdown, and
control valves.
Steam Turbines
Steam turbines proper have been experiencing
a number of problems as the direct result
30
ingly, a number of issues were noted with a
vibrational response in the last-stage blades,
resulting from a natural resonance, which has
led to the failure of many last-stage blades.
This issue has been identified in multiple
manufacturers' units and multiple frame
sizes.
Casing Cracks and Deformation. Due
to the impact of thermal fatigue and potential
water or wet steam ingress, several examples
have been documented of casing cracks as
well as component deformation.
Rotor Bow. At least one plant, which was
the subject of a site visit, experienced a rotor
bow to the extent that it required remedial action
and will require further repairs during an
upcoming maintenance interval.
Managing Differential Expansion.
Several sites have noted issues with managing
differential expansion, and at least one
site had issues with the proper setup and
calibration of the differential expansion instrumentation.
Having proper knowledge of
the proper setup of this instrumentation and
proper understanding of how to manage the
relative growth between rotor and casing
during cycling are critical to preventing potentially
significant damage to the critical
clearances.
Rubs. Rubs between rotating and stationary
components, whether radial or axial, can
at best cause momentary vibration and marginal
opening of tight clearances, and at worst
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prevalent issues with cycling generators is
loose end windings, which has led to multiple
sites experiencing dusting, caused by the
rubbing together of adjacent windings. If this
happens in the presence of oil that has migrated
into the generator, the oil and the dust
caused by the friction will create a grease-like
substance (Figure 4). The presence of dusting
or greasing is not the problem; rather, it is a
symptom of a problem that if left uncorrected
may eventually lead to shorted turns. Some
sites have installed end winding vibration
probes to aid in the detection of vibration.
Oil Ingress. Another common issue with
most generators is oil getting in through the
oil seals. In hydrogen-cooled generators, this
ingress can occur if problems exist with the
seal oil pressure regulators. On air-cooled
units, it can happen if the inlet air filters become
dirty and the differential pressure increases
to the point that air is pulled through
the bearings. Either way, oil, once it is inside
the generator, can atomize with the cooling
media, either air or hydrogen, flowing in and
around the windings of both the stator and
the rotor. This oil can act as a lubricant on
windings that are already prone to vibration,
as noted above, and can cause them to vibrate
much more readily, leading to the greasing
described previously.
Thermal Sensitivity. While not specifically
a cyclic operational issue, thermal sensitivity
in generators has been known to be
aggravated by cycling operations.
Rotor Pole Cracking. At least one site
reported rotor pole cracking, which was a
direct result of the additional stresses placed
on this unit as a result of the cycling operations.
If such cracking has occurred at this
one plant, and it is attributable to cycling, the
likelihood that it may occur at other similarly
situated facilities is increased.
Control Valves
Due to extreme cycling, a number of issues
have been identified with STG control valves.
Thermal Fatigue Leading to Cracking.
Naturally, these valves are experiencing
multiple heating and cooling cycles, as well
as occasional water ingestion or wet steam,
which will lead to localized quenching of the
components. This quenching, in turn, will
POWER | April 2017

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POWER April 2017

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