Patent Application
20130139386
The present invention relates to turbines, and more particularly, to a method of fabricating an abradable honeycomb as an integral part of a nozzle sealing flange which is engaged by an angel wing of a rotating bucket in a gas turbine to limit cooling air from leaking into the gas turbine's high temperature combustion gas passage.
This invention was made with Government support under contract number DE-FC26-05NT42643 awarded by the Department Of Energy. The Government has certain rights in this invention.
A gas turbine is a rotary engine that extracts energy from the flow of combustion gas through the turbine. It has an upstream air intake or inlet, a compressor coupled to a combustor, a downstream turbine that receives combustion gas from the combustor, and a gas outlet or exhaust nozzle.
The compressor and turbine sections include at least one circumferential row of rotating rotor blades or buckets. The free ends or tips of the buckets are surrounded by a stator casing. The base portions of the rotor blades are flanked by the inner shrouds of stator blades or nozzles located upstream and downstream of the rotating buckets.
Seal assemblies are typically used to prevent or limit cooling air from leaking into a gas turbine's high temperature combustion gas passage from between moving buckets and stationary nozzles. These seal assemblies typically include seal plates, referred to as “angel wings”, which extend axially from the upstream and downstream surfaces of the shank portions of the moving buckets, and which terminate in radially outwardly extending tips or teeth. The seal assemblies also include sealing structures or flanges projecting axially from upstream and downstream from stationary nozzle assemblies to define seals with the angel wings of the moving bucket shanks. However, the sealing performance of these seal assemblies is not always good, such that more than a desired amount of the cooling air tends to leak into the high temperature combustion gas passage so that the amount of cooling air is increased, causing deterioration in the performance of the gas turbine.
The efficiency of the turbine depends, in part, on the radial clearance or gap between the angel wings and the adjacent sealing structures. If the clearance is too small, the angel wings will strike the adjacent sealing structures during certain turbine operating conditions. If the clearance is too large, excessive valuable cooling air will leak from the rotor wheelspace into the hot gas path, decreasing the turbine's efficiency.
An abradable seal can be used to improve turbine performance by physically reducing the clearance between the sealing flange of a nozzle and an opposed angel wing seal plate of a bucket. An abradable seal material is sometimes located on the radially inner surface of the seal of the stationary nozzle, so as to be located within the annular gap between the inner surface of the nozzle seal and the end tips of the angel wing of the rotating bucket.
One type of sealing material that is used in sealing assemblies is an abradable honeycomb. While an abradable honeycomb has the ability to significantly reduce the leakage of cooling air into the high temperature combustion gas passage, the secure attachment of traditionally made abradable honeycomb pieces at a bucket/nozzle interface is difficult to ensure.
Angel wing abradable seals using honeycomb on the nozzle have been shown to provide significant sealing improvement. However, attaching a honeycomb piece reliably to an angel wing is difficult, since brazing thin pieces of metal ribbon means a very small attachment region. Currently, the honeycomb is sometimes brazed directly to a part with the hope that it will stay on. One alternative to this is to braze the honeycomb to a plate and then braze or weld the plate onto the angel wing.
The present invention uses sinker electrical discharge machining (“EDM”) to plunge/burn a honeycomb directly into a turbine nozzle sealing flange so it becomes an integral part of the nozzle sealing flange so that when a turbine bucket angel wing in the turbine's sealing assembly rubs the nozzle sealing flange, there is improved sealing between the nozzle sealing flange and the bucket angel wing with less risk of honeycomb failure. Optionally, the honeycomb resulting from the electrode with the sinker electrical discharge machining device being plunged into the block of material can be coated with a coating suitable for corrosion resistance and/or improved abradability. In addition, the block of material in which the honeycomb is formed can be either an integral part of the turbine nozzle or separate from the turbine nozzle. In the latter case, the block of material is welded or brazed to the turbine nozzle after the honeycomb has been formed in such block.
In an exemplary embodiment of the invention, a method of making a turbine nozzle sealing flange with a plurality of abradable cavities formed in the sealing flange comprises the steps of providing a block of material suitable for serving as a nozzle sealing flange, providing a sinker electrical discharge machining device for machining features into the block of material, providing an electrode for use with the sinker electrical discharge machining device to perform the machining of the features in the block of material, the electrode being shaped to form the plurality of cavities in the block of material, and using the electrode with the sinker electrical discharge machining device to plunge the electrode directly into the block of material, to thereby form the nozzle sealing flange with the plurality of abradable cavities.
In another exemplary embodiment of the invention, a method of limiting cooling air from leaking into a gas turbine's high temperature combustion gas passage comprises the steps of providing one or more blocks of material suitable for forming one or more corresponding turbine nozzle sealing flanges, providing a sinker electrical discharge machining device for machining features into the block(s) of material, and providing an electrode for use with the sinker electrical discharge machining device to perform the machining of the features into the block(s) of material, the electrode having a series of crisscrossing negative grooves which form a positive honeycomb pattern that can be formed directly into the block(s) of material by the positive electrode burning the honeycomb pattern into each of the one or more blocks of material when the electrode is pressed into the block of material.
In a further exemplary embodiment of the invention, a method of making a part with a predetermined integral abradable shape comprises the steps of providing a block of material suitable for making the part, providing a sinker electrical discharge machining device for machining features into the block of material, providing an electrode for use with the sinker electrical discharge machining device to perform the machining of the features in the block of material, the electrode being shaped to form the predetermined shape in the block of material, using the electrode with the sinker electrical discharge machining device to burn the predetermined integral abradable shape directly into the block of material, and optionally coating the resulting predetermined integral abradable shape burned into the block of material with a coating suitable for corrosion resistance and/or improved abradability.
Seal plates, typically referred to as “angel wings” 14, extend axially from the upstream and downstream surfaces of the shank portions of the rotating buckets 15. Each of the angel wings 14 ends in a radially outwardly extending tip or cutter tooth 17. Sealing structures or flanges 16 project axially from upstream and downstream stationary nozzle assemblies 11 for defining seals with the angel wings 14 of the rotating buckets 15. The seal assemblies 12 and 14 are intended to limit the amount of cooling air from leaking into the high temperature combustion gas passage.
After the honeycomb 18 has been formed, it can optionally be coated with a coating suitable for oxidation and corrosion resistance and/or improved abradability. An example of such a coating would be an aluminide intermetallic coating.
It should be noted that although
As can be seen from
Thus,
Sinker EDM (Electrical Discharge Machining) is a process that lends itself to precision machining of features when conventional machining is inappropriate. For instance, complex features, small precise features and tight tolerance features are all examples of operations well suited for sinker EDM, in which a “positive” shaped electrode can be machined and then “sunk” (burned) into a desired part. With EDM, no pressure is applied to the material being machined, as the features are burned to their shape, rather than abraded. A more complicated electrode shape than the diamond shaped honeycomb of the electrode 42 shown in
In one alternative embodiment, a thicker sidewall along the leading edge 19 and/or the trailing edges 21 (where no rub occurs) could be retained to improve strength. Additional embodiments could include the formation of varied shapes and sizes. The EDM technology enables having certain walls thicker in one axis or location than another, which could allow improved sealing, while not impacting abradability (thicker in the flow direction, than in the rub direction, for example). The EDM technology can also enable angling of the honeycomb in a preferred direction, again to improve sealing. The thickness of honeycomb could be varied in one direction versus another preferential direction. The sunken shape, such as a honeycomb, could also have another non-normal orientation that could not easily be made any other way.
The use sinker electrical discharge machining (“EDM”) to directly plunge the honeycomb 18 into the nozzle sealing flange 16 itself, so that the honeycomb becomes an integral part of the nozzle sealing flange, enables the abradable angel wing seal by providing a reliable method for putting honeycomb on a nozzle sealing flange. It also results in a sealing flange that provides improved sealing between the angel wing and the sealing flange.
The use of the sinker electrical discharge machining enables a new sealing technology may have applications in other turbine locations, as well in other areas of application.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
