Life Management System
for Gas Turbine Blades—An Industry Consortium Program
SwRI developed methods and software
to predict the life of gas turbine hot section blades
for GE frame gas turbines. Extensive testing of materials of new and service-exposed components were conducted to
develop pertinent material properties, which were used
in the life assessment algorithm and software. The
consortium participants were able to use these data in
their turbine fleet life management programs to realize
economic gains by prudently scheduling the inspection
and maintenance intervals while maintaining good
availability and reliability of their engines.
Root Cause Failure
Analysis of Aeroderivative Engine Blades and Vanes
Failure analysis of cracked hot
section blades, vanes, and shrouds were conducted
through several individual investigations to identify
the damage mechanisms in the engine components of most
engine manufacturers. Mechanisms of failures or
degradation include high-cycle fatigue (HCF), hot corrosion,
creep, high-temperature oxidation, and thermal fatigue.
Most probable cause of component failure was identified
and corrective actions recommended to the turbine users.
Further investigative programs were recommended as
needed to better understand unusual root cause of
failures.
Gas Turbine Hot Section
Coating Life Assessment Model
In gas turbines, the life and
integrity of the coatings applied to protect
the components from environmental attack and
degradation govern hot section life and maintenance
intervals. SwRI developed state-of-the-art coating life
prediction methods for several substrate/coating
systems. This model has been calibrated with service
experience and can be used to predict the life of
specific coatings on turbine blades in service.
Qualification of Turbine
Blade Material Substitution
A turbine owner sought to improve
reliability and reduce maintenance cost of its
large-frame gas turbine fleet by substituting materials
and coatings. Materials, coatings, and vibration data
were evaluated by SwRI to qualify an alternate material
and design for the first row of hot section blades.
Vendors were evaluated and qualified to manufacture the
blades and apply selected coatings. The turbine owner
realized significant increased blade life and reduced
maintenance costs as a result.
Blade Walk Research
An industry-wide compressor blade
failure problem on large-frame gas turbines was
investigated in a $1,000,000 EPRI-supported program. A
study of turbine overhaul procedures revealed several
practices that can damage blade retention devices. SwRI
monitored blade tip clearance, casing distortion, and
thermal-gradient sensors during a three-year period of
turbine operation. This computer-controlled monitoring
showed that blade tip rubbing was also a major
contributor to the blade walk problem. Tip rubbing was
caused by inadequate cool-down of the unit after
operation. Videos were developed to train utility
personnel about measures to prevent damage during both
operation and overhaul.
Balancing of
Aeroderivative Power Turbine for Oil Pipeline Service
A power turbine experienced
excessive vibration that was diagnosed by SwRI as
excessive imbalance. Correction balance weights were
calculated based on data recorded on site, and the
problem was resolved by attaching weights to the
coupling shaft. A support pedestal crack was also
discovered, and a weld was repaired during a subsequent
outage.
Bearing Failure of
Aeroderivative Gas Turbine
Failure of a main roller bearing was
attributed to misalignment of bearing supports caused by
uneven cooling and distortion of the turbine case.
Resulting malfunction of the squeeze-film damper caused
excessive dynamic loads and reduced bearing life.
Adjacent roller and ball bearings sustained slight
consequential damage. The failure cause was documented
to support revision of alignment procedures.
Audit of an Air Force
Turbine Blade Thermal Spray Coating Facility
SwRI conducted a detailed audit and
qualification of several coatings on various aircraft
components. Technical orders, work control documents,
process operation sheets, coating qualification
procedures, and coating specifications were reviewed.
Several anomalies were discovered in the documents, and
corrective actions were recommended. Results of a
round-robin metallurgical evaluation were also reviewed.
The review resulted in rejection of some coatings procedures
because of quality problems.
Vibration Diagnostic
System for Westinghouse 501 Gas Turbines
A data acquisition and diagnostic
system was developed through cooperative EPRI and utility
funding. Data acquired from an unattended system is
analyzed for turning gear time, critical speeds,
load-time integration, and other parameters. Peak levels
are trended, and an expert system diagnoses the root
cause of problems when excessive vibrations occurred. A
user-friendly balancing program is built into the
system. Several systems have been installed on units
with telephone modem access for engineering and
maintenance staff data access. At one site, diagnostic
systems were installed on four units that were then
interconnected with a LAN.
Vibration of
Aeroderivative Gas Turbine in Pipeline Compression
Service
A series of forced outages caused by
excessive vibration resulted in reduction in gas transmission
service and excessive cost for repair and maintenance.
An on-site investigation revealed severe vibration jumps
because of slippage at rotor joints, nonlinearity of a main
bearing support structure, and defective signal cables.
A correction plan was developed to improve cables, and a
closer tolerance balance criterion was established to
reduce rotor excitation levels.
Rotating Labyrinth Seal
Windage and Leakage Tests
A 15,000-rpm seal test facility was
fabricated for a major aircraft engine manufacturer, and
tests were conducted on various compressor discharge
seals. The test program provided critical data to verify
predictive algorithms for controlling seal parameters to
improve engine performance.
Damage Assessment of
Aeroderivative Turbine Combustor Section
This investigation combined
metallurgical evaluation of thermal-damaged surfaces and
thermo-mechanical analysis of the energy impinging on
affected diffuser parts. Analyses revealed melting of
critical components because of uncontrolled combustion in
inappropriate chambers. The
combustion was caused by incomplete
purging during startup.
Fretting Problems of
Aeroderivative Hot Section Blading
Visual inspection indicated that
blade vibration played a significant role in the blade
fretting damage. Fretting sites are consistent with the
location of disk fracture initiation that had caused
early rejection of expensive parts. Blade vibration
damper refurbishment was recommended.
Compressor Vane Failure
of Aeroderivative Gas Turbine
Examinations of failed parts
revealed severe fretting of the inner shroud segments of
compressor vanes and high-cycle-fatigue failure of one
airfoil. Diagnosis of turbine vibration revealed that
rotating stall develops at low power operation, which
was a preferred operating condition. It was deduced that
long periods at low-power operation leads to fretting
damage accumulation. The problem was resolved by
adjusting the operating schedule to avoid rotating
stall.
High-Cycle-Fatigue of
Aeroderivative Fourth-Stage Turbine Blade
A recurrent fourth-stage hot section
blade failure problem was traced to blade tip rubbing
due to alignment and tip clearance control issues
related to design and installation. Revision of the
design to increase tip clearances eliminated tip rubbing
and blade failure.
Aeroderivative Gas
Turbine First-Stage Nozzle Failure
Visual examination indicated several
nozzles had severe distortion due to thermal distress.
Flow testing revealed the distorted vanes were
significantly deficient in cooling flow capacity.
Thermal imaging of the nozzle revealed a blockage of
internal flow passages. A metallurgical examination of
the section vanes revealed flow passages blocked by an
accumulation of calcium and silicone deposits indicating
ingestion of fine dust particles. An improved engine air
filter system was recommended.
Aeroderivative Gas
Turbine Booster Compressor Blade Failure
A metallurgical examination revealed
corrosion pitting had lowered the effective endurance
limits of the blade material. Estimated vibratory
stresses were compared with various other stages, which
indicated the failed blade was likely to have acceptable
vibratory stresses if stress concentration from
corrosion pitting can be eliminated. A
corrosion-resistant coating was recommended.
Fire Damage on an
Industrial Gas Turbine
A fire in the compressor section
damaged variable vane bushings and prevented the rotor
from running after shutdown. SwRI investigation
identified the most likely cause of the fire as leakage
from fuel gas nozzles in the combustor section.
Aeroderivative Gas
Turbine Bearing Failures
A series of four thrust and roller
bearings from different turbines was examined. All
bearings showed evidence of overheating that was
attributed to excessive radial and/or thrust loads and
improper lubricant quantity. Some damage was a result of
improper material, heat treatment, or excessively
reconditioned bearings. Improved quality control was
recommended, and measures were taken to prevent lube oil
contamination.
Design Analysis for Lube
Oil System Modifications
The owners of an aeroderivative-driven
gas pipeline compressor wanted to reduce cost by
separation of the synthetic lube oil system for the gas
turbine from a mineral oil system for the compressor.
SwRI redesigned the lube oil system to provide proper
flow control to both components in the rotating train.
Hot Section Failure of
Aeroderivative Gas Turbine
Severe damage was found on the
first- and second-stage blades and vanes with diminished
damage on third- and fourth-stage airfoils. A single
first-stage turbine blade failed in a root extension
section, creating massive consequential damage that
included burst-through of the turbine case. The isolated
blade failure is attributed to blade material deficiency
and excessive excitation from first-stage nozzles.
Blade and Nozzle
Integrity in Aeroderivative Turbine for Power Generation
Service
Repeated blade and nozzle failures
were experienced in liquid fuel-fired engines. A root
cause failure analysis found that fuel or airborne
contamination caused hot corrosion failures of HP and PT
blades and nozzles. A coating rainbow test program
provided data to support alternate vane and blade
coating recommendations to mitigate the hot corrosion
problem. The new coatings improved hot section
reliability, which has significantly improved turbine
availability.
Failure of Aeroderivative
Gas Turbine Compressor
Failure was traced to fatigue of a
third-stage compressor vane airfoil that had initiated
in a corrosion pit at mid cord. The loose airfoil caused
subsequent secondary damage to the compressor and hot
sections. The extent of damage was documented, and
recommendations were made for repair and replacement.
Coating recommendations were provided to mitigate future
corrosion damage.
Flow Validation Testing
of Aeroderivative Gas Turbine Compressor Inlet Scroll
A scale model of an inlet scroll was
designed and fabricated; an induced draft fan was fitted
and flow velocities were profiled. Slight modifications
to the inlet, based on these data, provided a
substantial decrease in flow losses and improved turbine
performance.
Burner Fatigue of
Industrial Gas Turbine
Metallurgical examination provided
evidence that a combination of high-cycle fatigue and
hot spots were the primary cause of damage. An
investigation was conducted using dynamic pressure
transducers and accelerometers located in the combustion
section. The problem was identified as a combustor
rumble problem created by pulsating flow in the fuel
supply line. Pulsation suppression devices for the fuel
lines were recommended.
Combustor Dome Erosion
An aeroderivative turbine combustor
had experienced erosion at the dome due to injection of
water for NOx control. SwRI rig testing indicated that
both corrosion and erosion affect the cumulative damage
process. These findings redirected the OEM approach for
improved combustor redesign.
Coupling Failures of an
Industrial Gas Turbine Driven Compressor Train
Problems were identified as a result
of design analysis errors in calculation of torsional
critical speed. A strain gage telemetry system was
attached to the coupling, which showed that three
torsional critical speeds of various severities were
within the turbine operating speed range. An interim fix
was recommended to limit operating speed to avoid
excessive excitation of the most severe torsional
critical speeds until a redesigned coupling could be
provided.
Service Degradation and
Refurbishment Assessment of Hot Section Blades
Tests of hot section blades and
coatings were conducted to understand the degree of
degradation and to quantify the effects of service
exposure on the blades. Metallurgical and mechanical
tests were conducted by SwRI to determine the quality of
the refurbishment process used by the OEM to reverse the
blade damage. Recommendations were made to the turbine
user regarding the suitability of the components for
continued service.
High Vibrations on Large
Industrial Gas Turbine for Power Generation
Two turbine-generator trains
experienced high vibrations in operation and failure to
start problems due to excessive vibration on passing
through its critical speeds. A multi-speed, multi-plane
rotor balancing developed by SwRI was used to reduce
vibration to acceptable levels at critical speeds and at
running speed.
Heat Exchanger Shell
Vibration in Air Separation Plant
High noise, vibration, and pulsation
of the first-stage cooler was associated with low
inlet-guide vane settings. Tests involved resonant
frequency of cooler shell by impact testing, mapping of
acoustic mode shape throughout the cooler and
compressor, mapping of circumferential acoustic modes at
the inlet guide vane, and static pressure mapping across
the IGV and other compressor locations. Vortex shedding
excitation from flow separation at the IGV was
determined as the most likely cause. Several alternative
solutions were recommended, including throttling with an
alternative flow restriction, redesign of the IGV,
impeller or diffuser section, or detune dominant
acoustic and structural resonance away from preferred
excitation frequencies.
Diagnosis of
Turbo-Expander Seal and Blade Tip Damage
A turbo-expander in a process plant
experienced excessive vibration that caused damage to
seals and blade tip rubbing. Modal testing confirmed
that shaft resonant frequencies are within the operating
speed range. A diagnostic investigation revealed that
the rotor casing and supports were subjected to
excessive loading due to thermal expansion of the
connecting piping that were inadequately isolated by
expansion joints. A redesign of the expansion joints
relieved the problem.
Industrial Gas Turbine
Crossfire Tube Bellows Failures
A metallurgical examination revealed
crossfire tube bellows had failed. High-cycle fatigue,
diagnosis of vibration from sensors installed on site,
and subsequent testing at the SwRI Flow Facility
revealed vortex shedding excitation that corresponds
with bellows structural natural frequencies.
Gas Turbine Exhaust
Diffuser Vane Failure
An on-site diagnostic investigation
using high-temperature strain gages on the diffuser vane
showed high strain levels that corresponded to natural
frequencies identified by impact modal testing.
Pulsation data revealed a strong acoustic source that
corresponds to strain levels in the 200-Hz frequency
range. These pulsations were attributed to vortex
shedding mechanisms.
