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ID269
Improved Unit Reliability & Availability
Through Optimised Predictive
Maintenance
Detlef Schoeler
Dipl.-Ing. Peter O. Miranda
Siemens AG, Power Generation, Germany
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Siemens Power Generation is focused on providing our customers with solutions that will
help reduce the overall maintenance, repair and replacement parts cost while improving
overall unit reliability and availability. One simple solution -- predictive maintenance -- is
crucial to maintaining your assets and managing your operational costs.
The following presentations outline a variety of Siemens proven predictive maintenance
tools that are benefiting gas and steam customers around the world -- comprehensive pre-
outage planning, audits, life assessments, and unique tooling that will move you from a
reactive to a predictive mode for long-term cost savings.
1. Introduction
The primary objective of every power plant utility is to optimise the efficiency as much as
possible. That means the reliability and availability of a power plant has to be maintained on
a high level.
Long term experience has shown that a well prepared overhaul makes you sleep comfortable
(see slide 1). Preventive maintenance and thorough outage preparation allows for a short
outage duration, enhanced availability and high cost efficiency. As a key factor of
prevention, minor inspections enable an assessment of the gas turbines status to detect wear
and tear and allow to discuss the repair or replacement of components during the upcoming
major overhaul. In addition minor inspections also ensure the detection of upcoming failures
to prevent major damages to the machine.
Certain components with a long manufacturing lead time or a time consuming refurbishment
process make an early planning essential to ensure the availability of these parts and
overcome unacceptable outage times and additional expediting fees.
A malprepared major overhaul on a Siemens frame recently caused an unscheduled
downtime of over 3 months due to unavailability of long lead time components which could
have easily been avoided with a thorough preventive maintenance and subsequent outage
planning.
The advantages of preventive maintenance inspections are illustrated with examples.
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2. Goals of Preventive Maintenance
The goal of a well prepared maintenance is to avoid unforeseen findings (see slide 2) that
could have been identified and taken care of during earlier minor inspections. This approach
prevents damages and leads to optimised planning and enhances availability.
An exceptionally well planned major overhaul on a Siemens V94.3A in South America
enabled the hand-over of the unit to the customer 3 days ahead of the already tight schedule.
Everybody operating a gas turbine power plant can easily calculate the additional costs or
connected savings related to preventive maintenance.
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3. Examples for Siemens V94.2 and V64.3 Gas Turbines
Many unsuccessful major overhauls are solely the result of a lack of communication
between the operator and the OEM. Especially units that are in operation for a long time
and where the contact to the OEM is not that close, are more likely to face problems.
The next slides show how important a pre planning is and how a repairable finding can lead
to cost intensive outage delays.
In the first example (see slide 3 and 4) we see an inner casing of a V64.3 Unit. Roughly
8000 hours before the planned outage the heat shields are showing scaling of different
degrees. The wall thickness can be measured and the scaling propagation be evaluated by
the OEM based on his wide fleet experience. The blue coloured areas in slide 7 mark the
recommended spare parts for the major outage. We as manufacturer are able to manufacture
new heat shields by using the stored data from the factory for laser cutting . That means the
spare parts can be at side before the outage starts. The time advantage here is 1 week.
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The second example (see slide 5 and 6) shows normal wear and tear at the transition
between flame tube and mixing casing of an V94.2 Unit. A repair segment is required if the
wear exceeds the tolerable limits. Such segments can be ordered and manufactured months
ahead of the planned outage. If such a finding is discovered while the major outage is
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ongoing, the manufacturing time for a specially suited segment would cause additional
outage time of 2 weeks, not including raw material sourcing.
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The last example shows compressor findings. Under harsh running conditions and long
operation time (above 2nd major outage) wear may occur at the inner ring of a compressor
diaphragm . Therefore are videoscopic examination (see slide 7 and 8) will be executed if
the running behaviour of a gas turbine was suspicious.
In the worst case it might be necessary to perform a repair or to exchange the diaphragm.
With a pre outage inspection 8000 hour before a planned outage or in other words 1 Year
ahead it will be no problem to manufacture a spare diaphragm and have it available for the
upcoming outage. On the other side if the order for a new diaphragm would be given during
the outage, this might cause an outage extension up to 7 months in case the diaphragm is
damaged beyond repair and a new one needs to be manufactured from scratch.
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4. Pre Outage Planning Schedule
The first planning meeting for a major overhaul should be at least two years ahead (see slide
9). At this meeting the general scope of work and parts is defined and RFQ (request for
quotation) is initiated including long lead time components, modification and upgrades or
life time extension as well as first refurbishment planning based on scrap rates from the
OEM experience.
Around one year ahead a last minor inspection will rectify the planning forecast (as
demonstrated under item 3) and assist to finalise the scope of work following a discussion of
special findings required repair- and refurbishment-activities. At this time also the personnel
and tool availability needs to be clarified.
In a final meeting 24 weeks ahead, a common review of time schedules and ordered parts
should be held.
After such a well prepared major overhaul a common evaluation of the outage should be
commenced establishing the lessons learnt for further optimisation.
It is of course also possible to place an order for a turn-key major or hot gas path outage
and make you sleep comfortable.
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5. Summary
As final conclusion it can be stated that minor inspections are the key factor of prevention .
They enable an assessment of the gas turbines status to detect wear and tear and allow to
discuss the repair or replacement of components during the upcoming major overhaul (see
slide 10). In addition minor inspections also ensure the detection of upcoming failures to
prevent major damages to the machine. Certain components with a long manufacturing lead
time or a time consuming refurbishment process make an early planning essential to ensure
the availability of these parts and overcome unacceptable outage times and additional
expediting fees.
As shown in three examples an early detection of spare parts and necessary repair methods
leads to a significant outage time reduction. Where the time reduction can lye between a
few days up to several months.
Following the pre outage planning schedule will be a great benefit for each power plant
operator to come to a minimized outage time, and subsequently to high reliability figures
and considerable cost reductions.
Additional to the predictive maintenance there is further option to enhance reliability and
availability by implementing upgrades in order to enlarge outage intervals.
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INTRODUCTION
A manageable reduction in maintenance costs might make the difference between a stranded
asset and a competitive generating unit. However, consideration must be given to the
potential consequences with each resource application. A short-term decision not to replace
a part immediately but to postpone it till the next scheduled outage may save immediate
maintenance dollars. This decision however could have a detrimental impact on future
consequences, such as increasing the risk of a forced outage.
The decision to not replace or repair a part may result in decreased thermal efficiency. The
increased fuel cost during the next run cycle may result in a larger net loss than the cost of
replacing the part. As the steam path ages, its thermal performance decreases due to
degradation in parts. Although the worn parts may still be mechanically sound, their
thermal performance has decreased. In most cases it is not obvious whether the part should
be repaired or replaced. Repairing the part instead of replacing it may result I less initial
cost however, the heat rate differential may be sufficient enough to warrant replacing the
part instead. At other times however, very little thermal performance or reliability
improvement may be realized from complete replacement.
During all major inspections, a steam path audit is performed to determine the overall
condition of the rotor’s blading and sealing. The audit typically includes the HP-IP, LP and
boiler feed pump turbines. The audit identifies deviations and classifies them as either being
reliability or efficiency related. Secondly, a material sample is removed from the hot section
of older HP rotors and mechanically tested. The test results are then analysed with rotor
stresses to determine the remaining life of the rotor. Generators are inspected and
electrically tested to verify their integrity. Using the FAST GenSM inspection, removal of the
rotor is not required.
Upon start-up, enthalpy drop testing is performed to establish a baseline for future thermal
performance monitoring. Additional future testing is performed to monitor the turbine’s
steam path condition. Future outages and repairs are based on the turbine’s thermal
performance verses the standard outage approach of performing maintenance based on
operational hours.
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STEAM PATH AUDITS
Steam path audits, to some extent, are performed during every major overhaul. The audit
may consist of either a brief visual inspection to a very detailed inspection and analysis of
the entire steam path.
The visual inspection provides information regarding the obvious condition of the turbine.
The complete steam path audit consists of:
• Visual inspection of all steam path areas
• Sealing loss analysis
- Spring back seals
- Seal strips/spills
• Blading loss analysis
- Blade damage
- Evaluate airfoil profile
• Surface roughness on stationary and rotating components
• Changes in area due to erosion
Thermal testing provides a complete profile of the turbine steam path. Enthalpy drop test is
the best indication on how the unit is currently performing thermally. Once the current
condition is determine, an evaluation is made against original design conditions to establish
a base and determine the degradation in efficiency. Analysis calculations are then
performed to determine the financial pay back for performing repairs to the turbine. This
allows the owner to manage limited repair dollars based on the most benefit.
The steam path audit output is a recommendation list that prioritises repairs based on the
following criteria:
1. Reliability short term - must perform in order to continually operate the unit
2. Reliability long term - will reduce future maintenance/repair costs while
decreasing the potential for a forced outage
3. Efficiency improvement - will improve turbine heat rate
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Usually, short-term reliability repairs are performed to make the unit operational. This
classification consists of items such as replacing broken bolting, replacing or repairing
cracked blades, applying thermal sprays coating to minimize erosion condition and
performing rotor and stationary component repairs. Other areas for improve reliability and
operational efficiency is replacement of all turbine seals and addressing valve repairs. Long-
term reliability repairs correct issues that are not an immediate threat but could have a
negative effect on the long-term operation of the unit if left un-repaired. Examples would be
items such as: addressing creep related issues on rotating and stationary components or
design conditions with inherent failure features. Efficiency improvements may consist of
either minor or major improvements. Decreasing seal clearances or modifying the seal
design are examples of minor improvements. On impulse design type turbines for example,
the use of retractable packing has shown to improve unit reliability and efficiency. Major
improvements could consist of replacing blades with high efficiency airfoil sections or
changing the entire steam path.
LIFE ASSESSMENTS
Typically, the HP or IP rotor sees the most severe operating conditions. These rotors are
highly stressed and operate in high temperature environments. Their lives are further
reduced by load cycling or performing start/stop cycles.
The rotor’s remaining life is one critical factor that should be considered prior to performing
extensive repairs or upgrades. Rotor life analysis is performed to determine the remaining
useful life of the rotor with the current operational conditions. Rotor bore examinations
should be performed, especially on older high pressure turbine rotors which are susceptible
to creep induced rotor bowing. Analysis of the rotor’s internal condition will determine it’s
long term integrity and remaining useful life. Fracture mechanic techniques are used to
calculate the remaining life of the rotor under the same operational conditions.
Results of the rotor life analysis provide two very important pieces of information for
managing the cost of the turbine generator system. The first is “calculated remaining life”,
this value provides guidance on the pay back period for repairs performed on the rotor or
turbine-generator system. The second deals with operational considerations. Additional
analysis can be performed to either increase or decrease the cycling capability of the unit,
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allowing the operator to optimise the remaining life. An informed decision can be made to
cycle the unit faster to meet system demands since the detrimental effect to the rotor is
understood.
FAST GenSM IV Generator Inspection
The Solution The Bottom Line
In the effort to optimise the O&M budget and maximize unit availability, electric utilities are
moving from a reactive to a predictive approach in turbine-generator inspection and
maintenance.
This initiative requires selection and application of state-of-the-art inspection techniques and
skilled evaluation of results. Inspection must deliver accurate and repeatable data in
minimum time and with minimum disassembly. Evaluation capabilities must include (1)
upfront selection of the appropriate inspection techniques and (2) analysis that yields
conclusions and service recommendations based on:
• Specific inspection results
• Original design data
• Data bases from similar units
• Unit-specific trends, formulated and further refined with each periodic inspection
At the same time, maintaining high availability demands inspection and evaluation services
to diagnose suspected problems in operation. In response, Siemens has developed FAST
GENSM IV, utilizing state-of-the-art technology, electronics and miniaturized robotics.
Siemens introduces a powerful new FAST GenSM IV generator inspection tool. It offers
many breakthrough inspection features – providing a faster, more reliable, comprehensive,
and effective generator inspections.
The FAST GenSM IV inspection can be performed without rotor disassembly, making this
robotic tool a safe, thorough and indispensable means of condition-based, preventive
maintenance and outage planning.
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FAST GenSM IV technology, pioneered by Westinghouse Electric Corporation’s Power
Generation business in the 1980s, has been providing power plant operators with cost
effective on-site generator inspection services for over two decades.
Fig. 1: A FAST GenSM IV generator specialist conducts stator, rotor and wedge inspections
conveniently on site, featuring narrated video taping plus high resolution photos.
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Fig. 2: The SCAMP (Stator Core Assessment Magnetic Probe) Carriage performs
Electromagnetic Core Imperfection Detection (EL CID) and high definition visual
inspection of the stator surface, rotor body and vents.
Customer Benefits
A FAST GenSM IV inspection can be performed in a short scheduled outage, saving you
approximately one-half of the traditional “crawl through” generator inspection time. It may
also considerably reduce the overall time and costs of your outage.
Benefits include:
• Proactive planning of generator maintenance using inspection data for assessments,
trending, predictions, and recommendations for future maintenance and unit
reliability
• Reduced inspection time of up to 50% due to minimal disassembly
• Increased accuracy and thoroughness of the inspection due to powerful digital
visual imaging and enhanced electronic capabilities
• Narrated video by experienced generator specialist
• Easy-to-analyse graphical results in a computerized report format
• In-situ technique avoids removing and reinstalling the rotor just to perform an
inspection – and the potential associated handling risks.
Features
The FAST GenSM IV generator inspection service offers expanded capabilities
and new flexibility:
• Computer operated, precision robotic system with digital data format
• Carriage design allows manual extraction if required
• Flexible, visual system that includes two high definition miniature colour CCD
cameras for video and still images, and brighter, shock proof, solid state illumination
• Completely redesigned Magna-CatTM drive system (patent pending) fits air gaps as
small as 22 mm
• Faster drive speed – four times the torque of the FAST GENSM III inspection carriage
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• Immediate diagnostic results available on-site for interpretation with off-site service
engineering assessment.
This leading edge technology can significantly reduce forced outage time and the predictive
maintenance approach can impact future major outages. Inspections can be performed on
Siemens, Westinghouse and Parsons generators, and on non-OEM generators on a case-by-
case basis.
Fig. 4: The Wedge Tightness Carriage inspects tightness of the stator wedges with the rotor
in-situ or removed, digitally mapping them for non-subjective analysis.
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Fig. 3: A sample Wedge Tightness Inspection printout shows real-time digital
mapping of individual wedges with the green-red scale demonstrating the degree of
tightness. This inspection data allows you to proactively plan maintenance on your
generator and enables you to better manage your plant assets.
Fig. 5: This 3D image highlights the articulation of the robotic carriage. This aids in
accessibility for machines with tight axial clearances.
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FAST GenSM IV: Corrective and Predictive Application Modes
When used in a single-point, problem-solving mode, FAST Gen IV evaluation involves data
analysis that considers:
• Inspection results
• Design data
• Operation and maintenance history, of the subject unit
The goal is identification of the root cause of current conditions, with recommendations for
interim repair or immediate return to service, with maintenance planned for the next
scheduled outage.
FAST GenSM IV: Summary
When used in a program of periodic inspection, FAST GenSM IV evaluation will include
trend analysis of unit-specific conditions over time. This trend analysis combined with
astute utility attention can be used to predict unit conditions and refine plans for preventive
maintenance. Working in this predictive mode, Utilities will achieve control in making
more informed decisions for cost-effective maintenance, in time for budget and outage
planning.
Conclusion
Performing rotor life assessments, steam path audits, thermal testing and generator electrical
testing provide essential information regarding the turbine-generator information allows the
owner to manage the unit’s repair cost while optimising the dollars spent.
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Formal evaluation of this information allows economic analysis to determine which repairs
or part replacements should be performed in order to maintain turbine-generator reliability.
Units with good remaining life are given priority for repair dollars to maintain or improve
their condition, units with little remaining life receive only essential repair dollars.
References
1) Zink, John C. PhD, “Competition Brings Powerful Changes”, Power Engineering,
December 97, pg 18.
2) Jagmohan, Takhar S., Collins, Robert V., Schaefer, Jack E., “Run/Retire Decision on a
26-Year-Old LP Turbine Rotor Based on Boresonic and Material Test Results and Fracture
Mechanics Analysis”, American Power Conference, April 1979.
3) Cotton, Ken C., Evaluating and Improving Steam Turbine Performance Cotton Fact Inc.,
Rexford, New York, 1993.
4) Sanders, William P., Turbine Steam Path Engineering for Operations and Maintenance
Staff, Aurora, Ontario, Canada, 1988.
5) Mühle E.-E PhD, Neuhaus E., Dadek A., Siegel M., “Assessment of Steam Turbine
Components Subject to Creep for Extended Operation Time – 20 Years of Experience”,
Siemens AG, Bereich Energieerzeugung (KWU), 1994.