Engine & Turbine Technology
Repair assessment and modification of gas turbine components
Gas turbine components have different life-limiting failure modes. Life- limiting deterioration is frequently created by a detail of a component. A thorough analysis and creative thinking can lead to modifications that result in substantial extensions of turbine lifetime. Sief Mattheij reports.
Los componentes de las turbinas de gas tienen distintos modos de fallo que limitan su vida. Este deterioro suele deberse a un detalle de algún componente. Un análisis exhaustivo y unas ideas creativas pueden conducir a modificaciones que permiten una importante ampliación de la vida útil de la turbina. Informa Sief Mattheij.
Gasturbinenkomponenten haben unterschiedliche Ursachen, welche die Lebensdauer beeinflussen. Der die Lebensdauer beendende Verschleiß wird häufig nur von einem einzigen Bauteil der Komponente verursacht. Eine sorgfältige Analyse und kreatives Decken kann zu Modifikationen führen, welche die Lebensdauer der Turbine erheblich verlängern. Bericht von Sief Mattheij.
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as turbine components are subjected to high temperatures, as well as high stress levels, and are exposed to aggressive gases at the same time. Gas turbines
have to be fired to highest possible temperatures to get the best efficiency and the highest output. Fighting degradation of components exposed to high temperature is a continuous challenge. Te steady-state temperature is the first
factor. It controls oxidation and corrosion rates, degradation of base material quality, and creep lifetime. Creep lifetime is very dependent on material temperature. Gas turbine hot-section components are made of nickel-base and cobalt- base superalloys. Excessive temperature can affect the material integrally—sometimes irreversibly. Termal cycling creates cyclic stress loads, which can be very severe. Most cracking of gas turbine components is a consequence of thermal cycling. Termal cycling cracks can occur in sound material as well as in aged material. Steady-state stress levels in materials that are subjected to high temperature will lead to creep, which is a slow continuous plastic deformation of the material. Cyclic stresses can lead to fatigue, which is the initiation and growth of cracks by cyclic stress. Cyclic temperature changes while starting and stopping can create very high cyclic stresses. As a result, cracks can initiate and grow during even a very low number of stress cycles. Fatigue by thermal cycling thus is known as
‘thermal fatigue’ and ‘lowcycle fatigue’. When a component is weakened by internal degradation of the base material, it becomes much more sensitive to thermal fatigue. Cyclic stresses can be created by external mechanical excitation, like tip rubbing or rattling of combustion components or by irregularities in the gas flow pattern that create cyclic pressure loads on components. Usually, the cyclic stress level is low and the vibration frequency is high. It is worth mentioning that the crack growth rate under
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identical stress and temperature conditions is very similar for most superalloys. Stronger alloys show a longer crack initiation time, but, after initiation, the crack growth rate is nearly the same as for weaker alloys. Many high-cycle fatigue failures are caused
by components that have developed cracks by excitation of a harmonic mode. At the same time, it is very important to understand that pure cyclic mechanical overstressing (by forced vibration) can make a component fail just as easily. In all cases, it is important to determine and address the root cause of the mechanical excitation rather than just focus on the determination and trimming of natural frequencies. Initiation of cracks, growth, and ultimate failure usually take place in the range of thousands to some millions of cycles. Because of the high frequency, this can happen rather quickly.
Fig. 1. First stage blade after coating removal and cleaning. It exhibits thermal cycling and cracking in the leading edge and one the air foil.
Te excitation of a natural frequency in gas turbine components can lead to severe amplification of the cyclic stress level and to failure in a very brief period of time (seconds to minutes). Especially ,long rotating blades can be sensitive to excitation. Superalloys are a blend of many elements, of which, many are very sensitive to oxidation. Like in stainless steel, the alloys are protected against
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