Maintenance of Gas Turbine 5465656

Maintenance of gas turbine :Maintenance, defined as the “upkeep of propertyupkeep of property,” is one of the most important operations in a plant. The manufacture and maintenance of turbomachinery aretotally different. The first involves the shaping and assembly of various parts to required tolerances, while the second, maintenance, involves restoration of thesetolerances through a series of intelligent compromises. Maintenance costs and availability are two of the most important concerns to a gas turbine equipment owner. Maintenance procedures are always controversial, since the definition of “upkeepupkeep” varies with the individual interpretation of each maintenance supervisor. Modern day turbomachinery is built to last between 30–40 years. Thus, the keeping of basic maintenance records and critical data is imperative for a good maintenance program. owners must develop a general understanding of the relationship between their operating plans and priorities for the plant, the skill level of operating and maintenance personnel, and the manufacturer's recommendations regarding the number and types of inspections.

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Economic justification is always the controlling factor forany program, and maintenance practices are not different.Maintenance costs can be minimized by, and are directly related to, good operation; likewise, better operating results can be obtained when the equipment is under the control of a planned maintenance program. Improper operation of mechanical equipment can can be as muchor more of the cause of its deterioration and failure as is actual, normal mechanical wear. Thus, operation and maintenance go together. So, the best maintenance program is the one that optimizes the owner’s costs and maximizes equipment availability. Combining the practice of preventive maintenance and total quality control and total employee involvement results in an innovative system for equipment maintenance that optimizes effectiveness, eliminates breakdowns, and promotes autonomous operator maintenance through day-to-day activities.

This concept known as Total Productive Maintenance (TPMTPM)

A new maintenance system is introduced based on the

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selection of all equipment “Life Cycle Cost.Life Cycle Cost.” This new system especially for major power plants is based on the combination of total condition monitoring, and the maintenance principles of total productive maintenance, and is called the:“Performance Based Total Productive MaintenancePerformance Based Total Productive Maintenance System.System.”The general maintenance system is classified into concepts:1). Panic maintenance based on breakdowns.2). Preventive maintenance.3). Performance based maintenance.4). Performance productive maintenance.5). Performance-based total productive maintenance (PTPMPTPM).

Key factors affecting maintenance planning

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The basic design and recommended maintenance of heavy-duty gas turbines are oriented toward:-Maximum periods of operation between inspection and overhauls.-In-place, on-site inspection and maintenance.-Use of local trade skills to disassemble, inspect and re-assemble.

The effect of maintenance factors for fuel, starts, environment and load duty cycles are cumulative.

As the maintenance factor increases, the time between inspections and component repairs decreases and it is possible that component replacement frequency will icrease .

Major factors influencing maintenance and equipment life :There are many factors that can influence equipment life and these must be understood and accounted for in the owner's maintenance planning.

Cyclic Effects Firing Temperature Fuel Steam/Water Injection

Continuous duty application: Rupture

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Creep deflection High-cycle fatiqueCorrosionOxidation ErrosionRubs/wearForeign object damagerupture

Cyclic duty application :Thermal mechanical fatique High-cycle-fatiqueRubs/wearForeign object damage

Inspection

The gas turbine is designed to withstand severe duty and to be maintained onsite.

Off-site repair required only on certain combustion components.

Major companies have some features designed to facilitate on-site maintenance like:

All casings, shells and frames are split on machine horizontal centerline.

Upper halves may be liftedindividually for access to internal parts.

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With upper-half compressor casings removed, all stator vanes can be slid circumferentially out of the casings for inspection or replacement without rotor removal.

All bearing housings and liners are split on the horizontal centerline so that may be inspected and replaced,when necessary.

The lower half of the Bearing liner can be removed with out removing the rotor.

All seals and shaft picking's are separate from the main bearing housings and casing structures and may be readily removed and replaced.

Maintenance inspection types may be classified asstandby,running and disassemply inspection.

The standby inspection: is performed when the unit isnot operating and includes routine servicing ofaccessory systems and devicecalibration.

The running inspection: is performed by observing key operating parameters while the turbine is running.

The disassembly inspection: requires opening the turbine for inspection of internal components and is Performed in varying degrees.

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Axial flow compressor after dismantling the

upper half casing:

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Hp rotor

Bearing No.2 on HProtor

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Labyrinth seal

Top cover of bearing no. 2 on HP rotor

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On most designs, fuel nozzles, can be removed for inspection,maintenance or replacement without lifting any casings.

Some damage will not be detectable through visual observation .

Accurate dimensional check and non-destructive examinations (NDE) techniques like fluorescent penetrant inspection may reveal additional discontinuities, such as stress cracking, that require more extensive repairs.

Typical gas turbine component damage that occurs during usage includes component cracking, foreign object damage (FOD), oxidation and corrosion.

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Borescope inspections: Aborescope is a device used to examine

components inside a cavity that cannot be

visually examined directly.

It bends in two or four directions to look

around a cavity.

The borescope contains its own light source

throughout the engine’s internal passages.

Allows rapid inspection of the following areas: Turbine Section Compressor section Combustion section

Removal of spark plugs and flame detectors from

combustion system allows access to these areas.

Borescope inspection is carried out because of the following benefits it can provide in the maintenance program:1. Perform internal on-site visual checks without disassembly.2. Detect abnormal conditions early to avoid failures.3. Determine degradation rates.4. Extend periods between scheduled inspections.5. Allow accurate planning and scheduling of maintenance actions.6. Monitor condition of internal components.7. Provide increased ability to predict required parts, special tools, and skilledmanpower.

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Gas Turbine Borescope Inspection Criteria: For Compressor Blades:

Foreign Object Damage (FOD) Dirt Build Up Corrosion Tip Erosion Trailing Edge Thinning Stator Blade Root Erosion Tip Clearance

For Combustion, (Liner and Transition Piece):

Carbon Build-Up Hot Spots Cracking Bulging Wear Missing Metal

For Turbine Nozzles: Foreign Object Damage (FOD) Corrosion Blocked Cooling Holes Cracks Trailing Edge Bowing Erosion Burning

For Turbine Buckets: Corrosion Blisters Cracks Tip Clearance Erosion Missing Metal

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This Figure shows the time savings we may obtain by the proper use of borescopic inspection for planned maintenance:

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Effect of planned maintenance with usage of borescope

Inspection for Degradation:

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Type of Damage Location Source of the Damage

Heavy oxide depositsPenetrationBlade tips missingDistortionCokingPenetrationCracksHeavy corrosionHeavy erosionHot streakingDentsHot spots

TBC coating flakingGougePieces missingSeal cracks

HPT vanes, blades HPT vanes, bladesHPT and compressorsCombustorFuel nozzle assemblyCombustorCombustor linersHPT vanes, bladesHPT vanes, bladesCombustorVanes, bladesHPT vanes, blades

HPT bladesHPT bladesBladesTransition piece

Environment, fuelCorrosion, erosionClearance, DOD/FODUneven combustionUneven combustionUneven combustionThermal stress, Wobbe number changeCoating defectLoss of film coolingFaulty fuel nozzleDOD/FODFaulty fuel spray pattern, liquidhydrocarbons in NG fuelTips rubbing, blocked cooling passagesDOD/FODDOD/FODUneven combustion

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