In many large powerplants，refineries， and petrochemicalcomplexes， about one-third of the failures are due to machinery failure; it is therefore necessary to redesign parts of a machine to improve reliability.
The maintenance practice of one large refinery is to replace gas turbine control systems with state-of-the-art electronics and ""plug-in"" concepts for ease of maintenance. These installations have been highly successful in thatmaintenance has been minimal， and can usually be accomplished on-stream. Another replaces all journal bearings with tilting pad bearings.
Inaddition， the new control systems increase turbineperformance， while speed control and flexibility are greatly improved. The original designhas been supplemented to include a self-contained alarmsystem， a semi-automatic sequential startsystem， and a complete trip and protectionsystem， as well as the electronic controls. The cost of this system is substan-tially less than the cost of a similar device offered by the OEM on new machines.
The gas turbines major limitations on the life are the combustorcans， first stage turbine nozzles and first stage turbine blades as seen in Figure 21-6. The effect of dry Low NOx combustors have been very negative on the availability of Combined CyclePower Plants， especially those with dual fuel capability. Flash back problems are a very major problem as they tendto create burning in the pre-mix section of the combustor， and cause failure of the pre-mix tubes. These pre-mix tubes are also very susceptible to resonance vibrations.
Bearing failures are one of the major causes of failures in turbomachinery. The changing of various types of radial bearings from cylindrical and.or
30 25 20 15 10 5 0
Compressor Combustor First Stage First Stage Controls Bearings Seals Couplings Generator Blades Cans Nozzles Blades
Figure 21-.. Contributions of various ma.or components to gas turbine do.n time.
pressure dam babbitted sleeve bearings to tilting pad journal bearings isbecoming common in the industry. In mostcases， this gives better stability，eliminates oil whirl， and under misalignment condition， is more forgiving.
Thrust bearingchanges， from the simple， tapered land thrust bearings totilting pad thrust bearings with leveling links (.ingsbury type)， is another area of common change. These types of bearings absorb sudden load surges and liquid slugs. Many users have changed out the inactive thrust bearing to carry the same load as the active thrust bearings. This has been the case in older gas turbines where traditionally the load carrying capacity of the inactive thrust bearing was 1.3of the active thrust bearing. As gas turbines got older the leakages increased and the thrust forces were altered greatly leading to failures in the inactive thrust bearings.
A major plant replaces the entire large journal and thrust bearings in their main machinery to tilting pad bearings in their plant as a matter of practice.
Material changes of the babbit are sometimes undertaken. Changing fromthe more common steel backed babbitted bearings to the copper alloys， withthis babbitted pads， conducts surface heat away at a fasterrate， thus increas-ing the load carrying capacity. In some instances， a 50-100% load carrying capacity improvement can be achieved. Some equipment manufacturers are offering bearing-upgrading kits for their machine in service.
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本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 3(80)
