Patent Application
20080056889
Clearance control devices such as abradable seals have been proposed in the past to accommodate rotor to casing clearance changes. See for example U.S. Pat. Nos. 6,340,286, 6,457,552; and Published Application Nos. 2005-0003172, US 2005-0164027 and US 2005-0111967, the disclosure of each of which is incorporated herein by this reference. Such clearance control devices allow the designer to decrease the cold built clearance of the turbine or engine, which decreases unwanted leakage, thus improving the performance and/or efficiency of the turbine or engine.
The invention relates generally to an abradable seal material provided at the interface between a stationary seal component and a rotating portion of the turbine. More particularly, the invention relates to an abradable seal material provided either on a seal gap facing surface of a flange projecting axially from a radially inner end portion of a stationary turbine blade or nozzle assembly, or on the opposed seal gap facing surface of a seal plate projecting axially from a shank portion of a rotating bucket. An example embodiment of the invention is described herein below as incorporated in a gas turbine.
Reference numerals 14, 16, 18 denote seal plates, so-called angel wings, which extend axially from the upstream and downstream surfaces of the shank portion 20 of the moving bucket and respectively terminate in radially outwardly extending tip(s), teeth or fins 22, 24, 26. Sealing structures or flanges 28, 30, 32, typically referred to as discourager seals, project axially from respective upstream and downstream stationary nozzle assemblies 34, 36 for defining a seal with the angel wings of the moving blade shank 20. These seal assemblies 22/28, 24/30, 26/32 are intended to prevent more than the necessary amount of cooling air from leaking into the high temperature combustion gas passage and being wasted. Conventionally, the gap between angel wing tip 22 and the discourager seal 28 at the radially outer portion of the shank is about 140 mils (3.56 mm) whereas the gap between the radially inner angel wing tip 24 and discourager seal 30 is about 125 mils (3.17 mm). Thus, conventionally, the sealing performance is not always good. Consequently, more than a desired amount of the cooling/sealing air tends to leak into the high temperature combustion gas passage so that the amount of cooling air is increased, thereby inviting deterioration in the performance of the gas turbine.
Referring to
The abradable seal material provided according to example embodiments of the invention may be metallic or ceramic as deemed appropriate. The abradable seal material is applied directly on the seal surface, the radially inner surface of the discourager seal(s) in the illustrated embodiment. In this regard, the abradable seal material may take the form of an abradable coating, e.g., sprayed on, the seal surface. Examples of abradable coatings which may be applied according to example embodiments of the invention may be found in U.S. Patent Publication Nos. 2005-0164027 and 2005-0003172, the disclosures of each of which are incorporated herein by this reference. The depth of the abradable coating can range from about 10 to 150 mils (about 0.25 to 3.81 mm).
In the illustrated example embodiment, the discourager seals 28,30,32 are designed as replaceable inserts selectively insertable within the stationary blade/nozzle assembly and the abradable material is applied to the radially inner surface thereof. In the alternative, the abradable seal material may be applied to an integrally formed seal flange and/or, in the absence of a seal flange, to the radially inner surface of the nozzle inner shroud, suitably disposed for defining a seal gap with an angel wing tip of the moving bucket. Although, as described hereinabove, the abradable material may be applied to the radially inner surface of one or more of the discourager seals or other seal structure of the nozzle, it is to be understood that, as an alternative, the abradable seal material may be applied to the tip(s) of one or more of the angel wings themselves, although this ultimately results in a lesser wear area.
In an example embodiment, the depth of the abradable seal material is defined as a 50 mil (1.27 mm) coating applied to the stationary discourager seal. As will be appreciated, applying a 50 mil coating to the radially inner surface of the radially outer discourager seal 28 effectively tightens up the clearance between discourager seal 28 and angel wing tip 22 from 140 mils to less than 100 mils. Thus, a 50 mil abradable seal member or coating applied to the stationary discourager seal tightens up the angel wing clearance by over one third. An analysis of flow with the abradable seal material present demonstrates that providing the abradable seal results in about 15-20% reduction in purge flow due to the tightening up of the clearance as above mentioned.
Thus, abradable seals provided according to example embodiments of the invention improve turbine performance by physically reducing the clearance between the bucket angel wing tooth and discourager seal. The reduction in clearance is possible due to the abradable seal's ability to be rubbed without damaging the bucket tooth tips. In this regard, it is expected that the rubbing of the abradable seals on the discouragers is not circumferential but rather the result of pinch point effects. Thus, clearance reduction at the angel wings could provide additional turbine performance gains.
The provision of an abradable seal as described hereinabove also mitigates angel wing tooth wear by providing for abradable contact without metal to metal hard rub, i.e., contact of the angel wing tip and the underlying hard surface of the discourager seal. Thus, the angel wing abradable seals give good clearance reduction and offers additional performance gains in reducing the required purge flow and minimizing bucket angel wing tooth wear and discourager seal damage, thereby increasing their application lives.
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.
