Engine & Turbine Technology
concentrated in just a single detail of the entire component. Since most life-limiting processes in gas turbine components are controlled by processes that are based on diffusion kinetics, which, in turn, are highly material temperature controlled, small changes in metal temperature can lead to dramatic extensions of component lifetime. Tus, modifications can usually be small changes with big consequences.
were reluctant to introduce them on airfoil surfaces. When increasing power and efficiency requirements led to higher firing temperatures and internal cooling technology could not keep pace, and when the requirement to achieve acceptable lifetimes from components grew to a nearly unachievable level, TBCs were introduced on airfoils. In many cases, lifetime was doubled or tripled, and, surprisingly, few or no effects were found in internal efficiency or power output. Te leading edge of a gas turbine blade is exposed to
the severest risk of overheating because of the high rate of heat exchange that is caused by the impingement of hot gases. Leading edges, thus, are provided with the majority of the cooling air and are frequently designed as a relatively thin-walled pipe that is cooled by internal airflow. Despite the large amount of cooling air, leading
edges are still vulnerable to overheating and thermal cracking, as is demonstrated by many sets of damaged components. It frequently proves that cracking in a set of components is more pronounced in components with thin leading edges than in the ones with thick leading edges.
A thick leading edge will conduct more heat to
Fig. 3. Hot corrsion damage in first stage gas turbine blades.
A good impression of the stresses and temperatures of gas turbine components can frequently be obtained with a straightforward analysis. Metal temperature distribution can be assessed with fair accuracy by determining the ageing of the alloy in various areas. A simple and straightforward analysis can yield a reliable identification of safe zones for repair or modification, critical danger mzones, and transition zones. It is obvious that this analysis must be carried out and interpreted conservatively. Torough knowledge of superalloy metallurgy,
properties, processing, coating, etc, is a prerequisite to undertaking any steps in modifying gas turbine components. Despite that fact, most modifications are a result of straightforward sound thinking and acting. Solutions can address a very specific case or a wide array of problems. Termal-barrier coatings (TBC) in industrial gas
turbines are plasma-sprayed ceramic coatings that act as an insulator between hot gases and cooled components. A TBC not only reduces average metal temperature, but also reduces steep thermal transients. TBCs were introduced for combustion components many decades ago. Because of their surface roughness, gas turbine original equipment manufacturers (OEM) and users
cooler parts of the airfoil than a thin one. In reverse engineering, component modification to thicker leading edges is more easily incorporated than in repair of existing components although reliable processes are available for the latter as well. Cooling air is a scarce commodity in gas turbines. Components that lack cooling will deteriorate within an unacceptably short period of time. When lifetime is determined by the corrosion rate of the base material, one option is to simply increase wall thickness of that component. In the case described here, the modification was
more complex than usual. Border conditions were: no changes in total cooling airflow for this component and no changes in the contour of the airfoil were allowed. Te original component was produced from uncoated Inconel 939 and had thin walls at the trailing-edge cooling-air exit slot. Te modifications were as follows: internal cavity and the cooling-air exit slot modified to double minimal wall thickness; cooling-air impingement insert modified for cooling-air distribution to the hottest sections; change of material from IN939 to IN738LC with better resistance to internal oxidation and nitridation; application of an oxidation-resistant aluminum diffusion coating. In these components, lifetime was tripled by this combination of actions. l
A version of this article first appeared in Sulzer Technical Review, Sulzer Management Ltd, Winterthur, Switzerland.
Sjef Mattheij is with Sulzer Turbo Services Venlo BV, Lomm, The Netherlands.
www.sulzerts.com. 30
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