Turbines

Gas turbine drivers generally consist of an axial compressor air section, and an axial flow expander for the rotating parts. The items discussed for axial compressors apply to the compressor section of the gas turbine and need not be repeated. The gas turbine is a complex machine, and because of this, would tend toward the unreliable. Because it is a production machine, as contrasted to the custom engineered concept used in many of the other machines, the designs are quite mature before being sold for general use. One of the more obvious areas of concern is the hot section parts. Fuel is burned in the turbine to generate the hot gas used to motivate the turbine. Here again is an area of compromise. For high efficiency, high firing temperatures are desirable. For parts life, the hotter the gas, the shorter the life. But as mentioned, because of the production nature of these units, the research to find the best compromise is not done in the installation but by off line research. Still, the limiting items for long run times are the hot section parts.

Steam turbines have more of the reliability factors in common with the centrifugal and axial compressor. Blading design considerations are quite similar to those discussed at the axial compressor. The rotor dynamics aspects are similar to the centrifugal compressor. Condensing steam turbines tend to have long bearing spans. These bearing spans are further extended if the design includes an extraction point and a condensing exhaust end, or worse, if double extraction is included with the condens ing exhaust end. Steam turbines tend to exhibit more flexibility in the bearing support system that must be included in the rotor dynamic analysis, adding to the complexity. Improvements in rotor dynamics analysis have contributed considerably to the solution of these problems.

Other items unique to steam turbines that must be considered are water carryover and fouling. Carryover can leave deposits on the blading, causing loss in performance and increased thrust loads. If carried to an extreme, thrust bearing failure could occur. If a large amount of water is carried over as a slug of liquid, catastrophic failure can occur.

To minimize the effects of deposit buildup on valve stems of trip or trip and throttle valves, a pull-to-close design is preferred. While somewhat more expensive, the added feature of hydraulic-assisted closure is desirable, particularly for the larger sizes.

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