3. .umber o starts. Each start is equivalent to about 50 hours of operation.
4. .umber o ull load trips. This is very hard on the turbine and is nearly equivalent to about 400-500 hours of operation.
5. Type o material. The properties of the blade and nozzle vanes are avery important factor. The new blade materials， which are the singlecrystal structures， have done much to help the life of these blades inthe higher temperatures， which are used in these new turbines. It must be remembered that if more than about 8% of the air is used in cooling than the advantage of going to higher temperatures is lost.
The .arson-Miller parameters， which describe an alloy.s stress rup-ture characteristics over a wide range of temperature，life， and stress， is very useful in comparing the elevated temperature capabilities of many alloys.
6. Types o coatings. The use of coatings in both compressor and tur-bines has extended the life of most of the components. Coatings are also being used on combustor liners. The new overlay coatings are more corrosion-resistant as compared to the old diffusion coatings. The coatings of the compressor are now more prevalent especially since some of the new compressors are operating at very high-pressureratios， which translate into high exit temperatures from the compres-sor. Compressor coatings also tend to reduce the frictional losses and can have a very rapid payback.
Identification of Losses
The losses that are encountered in a plant can be divided into two groups，uncontrollable losses， and controllable losses. The uncontrollable losses areusually environmental conditions， such as temperature， pressure， humidity， and turbine aging. The controllable losses are those that the operator can have some degree of control over and can take corrective actions:
1. .ressure drop across the inlet ilter. This can be remedied by cleaning or replacing the filter.
2. Compressor ouling. .n-line water cleaning can restore part of the drop encountered.
3. .uel lower heating .alue. In many plants， on-line fuel analyzers have been introduced to not only monitor the turbine performance but toalso calculate the fuel payments， which are usually based on the energy content of the fuel.
4. Turbine back pressure. In thiscase， the operator is relatively limited since he cannot do anything about the downstream design. If there is some obstruction in the ducting to the H.SG that can be removed or if the duct has collapsed in an area the duct could be replaced.
Compressor .erothermal Characteristics and Compressor Surge
Figure 19-19 shows a typical performance map for a centrifugal compres-sor， showing efficiency islands and constant aerodynamic speed lines. The total pressure ratio can be seen to change with flow and speed. Usually compressors are operated on a working line separated by some safety margin from the surge line.
Compressor surge is essentially a situation of unstable operation and should，therefore， be avoided in both design and operation. Surge has been tradition-ally defined as the lower limit of stable operation of a compressor and involves the reversal of flow. This reversal of flow occurs because of some kind ofaerodynamic instability within the system. Usually， it is a part of the compres-sor that is the cause of the aerodynamic instability， though it is possible that the system arrangement could be capable of magnifying this instability. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(55)
