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Turbine rotating blades cannot be field-repaired if they are cracked. If oneor two blades are damaged mechanically， the manufacturer may recommendfield repair or replacement of the damaged blades. However， if several bladesare fatigue cracked， it is recommended that the entire set be replaced， since the remaining blades have been exposed to the same operating conditionsand， therefore， have little fatigue life left.
Both top and bottom halves of the journal bearings should be inspectedfor misalignment wear as well as excessive ""in-line""wear， which can occur in turbines with frequent starts. An indication of the condition of the thrust-bearings can be made by removing a small section of the turbineshaft， usually on the governorend， and axially moving or bumping the shaft. The amount of axial shaft movement will indicate the thrust clear-anceand， if it is found to be0.012-0.015inches， it can be considered normal.
If the turbine is not out of alignment， or the shaft bowed as determined by the vertical and horizontal clearance checks or the appearance of the bearingsurfaces， it is not recommended that the rotor be removed. Some turbinedesigns， however， may require removal of the rotor to facilitate the removal of some bottom sections of the diaphragms or inlet vanes. If the rotor is removed， special care must be taken when separating the couplings. Coup-ling flanges must be marked and run-out checks made for alignment so thatthey can be properly reassembled. However， the work should always be done under the manufacturer"s supervision.
Work should be made to progress strictly in accordance with the plannedflow charts， which must be constantly kept updated. Extra work and delayswill probably be encountered; however， a well-planned program will includecertain allowances， and little change should be required. If any significantchange is encountered， the program should be revised to show the extra outage time and possible extra personnel required.
.ompressor Maintenance
Besides checking the hot section， the compressor blades in axial compres-sors should also be inspected. The compressor inspection should be con-ducted to determine the mechanical and aerodynamic condition of the compressor. Most axial-flow compressors have stacked rotors with boltsextending through all the discs. The bolts should be inspectedand， if anyareloose， the stretch on the bolts should be determined.
Axial compressor performance is sensitive to the condition of the rotorblades. During a major inspection， all blades should be cleaned and checkedfor cracks with a penetrant test. If cracks are found in anyblade， that bladeshould be replaced. Occasionally， small cracks can be blendedout， but this procedure should be approved by the manufacturer.
The amount of wear on an axial-flow compressor blade is usually a function of foreign particle ingestion. Dust is the most common foreign particle. The maximum and minimum chord lengths should be recordedand reported to the manufacturer， who in turn should be able to report the performance loss occasioned by wear and the decrease in structural strength.
If the air inlet is subjected to saltwater contamination， the rotor and stator blades should be checked for pitting. Severe pitting near the blade roots may lead to structural failures. The manufacturer should be informed of severe pitting.
Stator blades are as important as rotor blades. All the same cleaning，inspection， and nondestructive test procedures should apply. It should be noted that the wear pattern is somewhat different on the stator blades.Again， the manufacturer should be informed of the wear conditions and should in turn make recommendations concerning continuous operation or replacement.
On completion of required repair and replacement， the gas turbine should be reassembled. This reassembly should be done under careful and experi-enced supervision to ensure all work meets established criteria. Blade clear-ances， bearing clearances， and spacing should be checked and recorded during assembly. Special care should be taken to ensure that the machinist uses the proper torque when tightening bolts and nuts. There is a very strong tendency for machinists to apply a torque that ""feels"" right rather than using a torque wrench. Torque is a very important aspect of assembly. Impropertorqueing can cause component warpage and distortion， especially in those components subject to high temperatures during operation.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(88)
