In a gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. In aircraft applications, the hot exhaust gases flow from the back of the engine, driving the aircraft forwardly.
The turbine blades are mounted on a turbine disk, which rotates on a shaft inside a generally cylindrical tunnel defined by a hollow stationary shroud structure. The stationary shroud structure is formed of a series of stationary shrouds that extend around the circumference of the tunnel in an end-to-end fashion. The stationary shroud structure has such a segmented arrangement to accommodate the thermal expansion experienced during each engine cycle as the stationary shroud structure is cycled between room temperature and a maximum service temperature of over 2000 degrees F. Each of the stationary shrouds has an internal gas path surface that is a segment of a cylinder, and a support structure that backs the gas path surface and provides for attachment to the adjacent structure. Additionally, in power generation applications, the gas path surface of the shrouds includes shroud block seal teeth that protrude from the shroud path surface. During turbine operation, the shroud block seal teeth act as a seal to minimize the escape of gas between the turbine blade and the shroud gas path surface.
During service, the shroud and the shroud seal teeth may be damaged by fatigue, erosion, and other mechanisms. One form of the damage is the wearing away of material from the shrouds, at locations such as the end faces, the forward and aft edges, shroud teeth, and elsewhere. As material is worn away and during multiple repair cycles when material is removed by machining operations, the shroud gradually becomes undersize in at least one dimension of the support structure and can provide a potential leak path for gas. When the shroud has become too small in at least one dimension of the support structure to continue to be functional, it is discarded.
There is a need for an improved approach to responding to damage to gas turbine engine shrouds, and particularly to protruding structures on the gas path face such as seal teeth. The shrouds are made of expensive nickel-base or cobalt-base superalloys, and the discarding of a shroud represents a substantial cost. The present invention fulfills this need, and further provides related advantages.