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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.