Gas Turbine Handbook : Principles and Practices

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34 Gas Turbine Handbook: Principles and Practices Improvement in the properties of creep and rupture strength was steady from the late 1940s to the early 1970s, with the largest improvement of 390°F (200°C) in allowable operating temperature achieved in 1950 (Figure 3-9). The largest jump resulted from agehardening or precipitation strengthening (a technique utilizing aluminum and titanium in the nickel matrix to increase strength). Since 1960, there has been increased dependence on sophisticated cooling techniques for turbine blades and nozzles. Since 1970, turbine inlet temperatures have increased as much as 500°F (260°C), some as high as 2,640°F (1,450°C). The increase in turbine inlet temperature was made possible by new air cooling schemes and the incorporation of complex ceramic core bodies used in production of hollow, cooled cast parts as shown in Figure 3-10. 2 Turbine blades and nozzles are formed by investment casting. This is not new technology. For centuries bronze statues have been cast using this process. The critical part is the solidification of the liquid metal alloy after it is poured into the mold. It is during solidification that the alloy acquires its crystalline structure, which is a major determinant of the properties of the finished part. Many substances in nature have an atomic structure that is referred to as “crystalline,” which metallurgists refer to as “grains.” Undesirable grain sizes, shapes, and transition areas were responsible for premature cracking of turbine parts. This led to the development of the equiaxed casting process. The equiaxed process assures uniformity of the grain structure along all their axes. At elevated temperatures, component failure begins within and progresses through grain boundaries. Therefore, if failures occur at grain boundaries rather than within the grains, the full strength of the crystal itself is not being utilized. Further study led to the conclusion that strength could be improved if grain boundaries were aligned in the direction normal to the applied force (most of the stress in the blade is in the direction of centrifugal force-along the length of the blade). This elongated or columinar grain formation in a preferred direction, called directional solidification or DS, was introduced by Pratt & Whitney Aircraft in 1965. 2 Because grain boundaries remain the weak link in turbine blades, numerous techniques have been used to strengthen them. Even better than strengthening grain boundaries is eliminating them by producing parts consisting of a single crystal. Furthermore, the elimination of the grain boundary, also eliminates the need for additional boundary-strengthening elements. The reduc-
Hardware 35 Figure 3-9. Courtesy of Rolls-Royce Industrial & Marine Gas Turbines Limited. This chart represents a pictorial history of the development of turbine materials from early wrought alloys to the latest single crystal cast alloy blades. Improvements in material strength and durability is the single most contributing factor to the advances made in gas turbines. tion in the amount of boundary-strengthened elements (at least in some alloys) has raised the incipient melting point by 120°F (50°C). The evolution of equiaxed structure to directionally solidified structure to single crystal casting is shown for the same turbine blade in all three forms in Figure 3-11. Each advancement in this technology has increased the high temperature strength by approximately 85°F (30°C). 2 COMBUSTORS Combustor design is a complex task, often referred to as a “black art.” The combustor design took two distinct configurations fairly early in the evolution of the gas turbine. These are the can-annular
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Page 6 and 7: Dedication This book is dedicated t
Page 8 and 9: Contents PREFACE ..................
Page 10 and 11: Chapter 13—BOROSCOPE INSPECTION .
Page 12 and 13: Preface to the Third Edition The ne
Page 14 and 15: Acknowledgments I wish to thank the
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Page 28 and 29: Figure 2-3. Courtesy of United Tech
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Page 90 and 91: Figure 5-4. Courtesy of United Tech
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Chart 8-1 Gas Turbine Environments
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Appendix A-2 304 Gas Turbine Handbo
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5. Weather Hood Material __________
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Appendix C-6 Air/Oil Cooler Specifi
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9. Drive Requirements A. Hydraulic
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22. Packaging For Shipment Per Spec
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Parameters Units NH 3 CO H 2 CH 4 C
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.055 Parameters Units 1/0 .88/.12 .
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Koppers Foster Blue Coal Gas Winkle
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Gas Turbine Handbook : Principles and Practices

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