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Design of turbine blades to obtain higher efficiency and damping has beendone. In somecases， this has improved efficiency by 8-10%， and stopped failures in these blades. Steam injection has been utilized in gas turbines to improve efficiency and to increase the power output. Redesign of various bleed-off ports has reduced tip stalls and their accompanying blade failures.
Today"s machinery， which is pushing the state-of-the-art in design， needs more than ""simple fixes."" This is one major reason why so much redesign takes place in the field. Maintenance engineers are no longer just required torepair， they are required in many cases to make revisions. Continual improvements and updating of the machinery is required to obtain the long runs and high efficiencies desirable in today"s turbomachinery.
Maintenance Scheduling
The scheduling of maintenance inspections and overhauls is an essential part of the total maintenance philosophy. As we move from ""Breakdown"" or ""Panic"" maintenance towards a performance based total productive maintenance system， total condition monitoring and diagnostics becomes an integral part of both operation and maintenance. Total condition monitoring and diagnostic examines both the mechanical and performance of the machinery and then carries out diagnostics. Condition monitoringsystems， which are only mechanical systems without performance inputs give less than half of the picture and can be very unreliable. .nscheduled maintenance is very costly and should be avoided. To properly scheduleoverhauls， both mechanical and performance data must be gatheredand evaluated. As indicatedearlier， we want to consider repairs during a planned ""turnaround"" not ""random""repairs， which are frequently done on an ""emergency"" basis andwhere， due to timerestraints， techniques are sometimesused， which are questionable and should only be used in emergencies.
To plan for a""turnaround，"" one must be guided by the operatinghistory of the given plant and， if it is the first ""turnaround，"" by conditions found in other plants utilizing the same or closely similar process and machinery. This is how the time between subsequent ""turnarounds"" has been extended to three years or more in many instances. By utilizing the operating history and inspection at previous ""turnarounds"" at this orsimilar installations， one can get a fair idea of what parts are most likelyto be found deteriorated and， therefore， must be replaced and.or repaired， and what other work should be done to the unit while it is down. It shouldbe pointed outthat， with modern turbomachinery， items such as bearings，seals， filters andcertain instrumentation， which areprecision made， areseldom， ifever， repaired except in an emergency; such items are replaced with new parts.
This means that parts must be ordered in advance for the ""turnaround"" and other work must be planned so that the whole operation may proceed smoothly and without holdups that could have been foreseen. This usually means close collaboration with the manufacturer or consultant and the OEM (or specialty service shop) sothat handling facilities，servicemen，parts， cleaning facilities， inspection facilities， chrome plating and.or metaliz-ing facilities， balancing facilities， and some cases even heat treatment facil-ities， are available and will be open for production at the proper timerequired. This is the planning， which must be done in detail before the shutdown with sufficient lead-time available in order to have replacement parts available at the job site.
The old maxim ""if it ain"t broke don"t fix it"" is very applicable in today"s machinery. A study conducted at a major nuclear power facility found that 35% of the failures occurred after a major turnaround. This is why total condition monitoring is necessary in any performance based total productive maintenance system and leads to overhauls being planned on proper data evaluation of the machinery rather than on a fixed interval.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(81)
