Stock material or miscellaneous articles – All metal or with adjacent metals – Porous
Reexamination Certificate
2001-05-24
2003-12-09
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Porous
C428S652000, C428S678000, C428S680000, C428S681000, C427S455000
Reexamination Certificate
active
06660405
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to coatings applied to metal components of gas turbine engines, radial inflow compressors and radial turbines, including micro-turbines and turbo-chargers, that are exposed to high temperature environments and, in particular, to a new type of abradable coating applied to turbine shrouds used in gas turbine engines in order to improve the performance and efficiency of the turbine blades (also known as “buckets”). Although the present invention has been found particularly useful in stage 1 turbine shrouds, the same coating developments can be used in other stages of gas turbine engines, as well as on hot gas path metal components of other rotating equipment exposed to high temperature environments. The present invention can also be used to repair and/or replace the coatings on metal components already in service, such as coated turbine shrouds.
Gas turbine engines are used in a wide variety of different applications, most notably electrical power generation. Such engines typically include a turbocompressor that compresses air to a high pressure by means of a multi-stage axial flow compressor. The compressed air passes through a combustor which accepts air and fuel from a fuel supply and provides continuous combustion, thus raising the temperature and pressure of the working gases to a high level. The combustor delivers the high temperature gases to the turbine, which in turn extracts work from the high pressure gas working fluid as it expands from the high pressure developed by the compressor down to atmospheric pressure.
As the gases leave the combustor, the temperature can easily exceed the acceptable temperature limitations for the materials of construction in the nozzles and buckets in the turbine. Although the hot gases cool as they expand, the temperature of the exhaust gases normally remains well above ambient. Thus, extensive cooling of the early stages of the turbine is essential to ensure that the components have adequate life. The high temperature in early stages of the turbine creates a variety of problems relating to the integrity, metallurgy and life expectancy of components coming in contact with the hot gas, such as the rotating buckets and turbine shroud. Although high combustion temperatures normally are desirable for a more efficient engine, the high gas temperatures may require that air be taken away from the compressor to cool the turbine parts, which tends to reduce overall engine efficiency. One aim of the present invention is to enable the stationary shroud to cope with the high gas temperatures without having to increase cooling air.
In order to achieve maximum engine efficiency (and corresponding maximum electrical power generation), it is also important that the buckets rotate within the turbine housing or “shroud” without interference and with the highest possible efficiency relative to the amount of energy available from the expanding working fluid.
During operation, the turbine housing (shroud) and a portion of the hub remain fixed relative to the rotating buckets. Typically, the highest efficiencies can be achieved by maintaining a minimum threshold clearance between the shroud and the bucket tips to thereby prevent unwanted “leakage” of gas over or around the tip of the buckets. Increased clearances will lead to leakage problem can cause significant decreases in overall efficiency of the gas turbine engine. Only a minimum amount of “leakage” of the hot gases at the outer periphery of the buckets, i.e., the small annular space between the bucket tips and turbine housing, can be tolerated without sacrificing engine efficiency.
The need to maintain adequate clearance without significant loss of efficiency is made more difficult by the fact that as the turbine rotates, centrifugal forces acting on the turbine components can cause the buckets to expand radially in the direction of the shroud, particularly when influenced by the high operating temperatures. Thus, it is important to establish the lowest effective running clearances between the shroud and bucket tips at the maximum anticipated operating temperatures.
A significant loss of gas turbine efficiency can also result from wear of the bucket tips if, for example, the shroud is distorted or the bucket tips rub against the shroud creating metal-to-metal contact. Again, any such deterioration of the buckets at the interface with the shroud when the turbine rotates will eventually cause significant reductions in overall engine performance and efficiency.
In the past, abradable type coatings have been applied to the turbine shroud to help establish a minimum, i.e., optimum, running clearance between the shroud and bucket tips under steady-state temperature conditions. In particular, coatings have been applied to the surface of the shroud opposite the buckets using a material that can be readily abraded by the tips of the buckets as they turn inside the housing at high speed with little or no damage to the bucket tips. Initially, a small clearance exists between the bucket tips and the coating when the gas turbine is stopped and the components are at ambient temperature. Later, during normal operation, the centrifugal forces and increased heat generated by the system inevitably results in at least some radial extension of the bucket tips, causing them to contact the coating on the shroud and wear away a part of the coating to establish the minimum running clearance. As detailed below, the relationship between the type of material used to form the abradable coating and the temperature of the turbine shroud can play a critical role in the overall efficiency and reliability of the entire engine. Without abradable coatings, the cold clearances between the bucket tips and shroud must be large enough to prevent contact between the rotating bucket tips and the shroud during later high temperature operation. With abradable coatings, on the other hand, the cold clearances can be reduced with the assurance that if contact occurs, the sacrificial part will be the abradable coating and not the bucket tip.
As noted in prior art patents describing abradable coatings for use in turbocompressors and gas turbines (see e.g., U.S. Pat. No. 5,472,315), a number of design factors must be considered in selecting an appropriate material for use as an abradable coating on the shroud, depending upon the coating composition, the specific end use, and the operating conditions of the turbine, particularly the highest anticipated working fluid temperature. Ideally, the cutting mechanism (e.g., the bucket blade tips) can be made sufficiently strong and the coating on the shroud will be brittle enough at high temperatures to be abraded without causing damage to the bucket tips themselves. That is, at the maximum operating temperature, the shroud coating should be preferentially abraded in lieu of any loss of metal on the bucket tips.
Thus, the need exists for an abradable coating system that will allow for the use of bucket tips at elevated temperatures without requiring any tip reinforcement (such as the application of aluminum oxide and/or abrasive grits such as cubic boron nitride). A need also exists for an improved abradable coating system that can be used if necessary in conjunction with reinforced bucket tips in order to provide even longer term reliability and improved operating efficiency.
In addition, any coating material that is removed (abraded) from the shroud should not affect downstream engine components. The abradable material must also be securely bonded to the turbine shroud and remain bonded while portions of the coating are removed by the bucket blades during startup, shut-down or a hot-restart. Preferably, the abradable coating material remains bonded to the shroud for the entire operational life of the gas turbine and does not significantly degrade over time. Ideally, the coating should also remain secured to the shroud during a large number of operational cycles, that is, despite repeated thermal cycling of the gas turbine engine during startup and shu
Ghasripoor Farshad
Gray Dennis M.
Lau Yuk-Chiu
Ng Chek Beng
General Electric Co.
Nixon & Vanderhye P.C.
Savage Jason L
Zimmerman John J.
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