Patent Application
20060249911
The present invention relates to turbines, and more particularly, to the use of an abradable coating and/or an abrasive coating with turbine brush seals to increase their pressure drop and temperature capabilities.
Turbine brush seals are contacting seals which typically include a highly flexible seal pack that consists of thousands of wire bristles that continuously adapt to the moving surface of a turbine rotor. Typically, the wire bristles are made from a collection of very fine metal bristles that form a curtain to produce a seal, yet are sufficiently compliant to withstand movement of the rotor. Typically, the bristles have low wear rates.
Currently, brush seals are applied to high pressure (HP), intermediate pressure (IP), low pressure (LP) and end packing regions of single-flow and opposed-flow combined rotors. However, the application of brush seals to such regions is limited by the backing-plate bending stress and bristle-bending stress capabilities of the brush seal assembly.
One of the limitations precluding the use of brush seals in applications having higher temperatures and higher pressure drops is the materials currently available for backing plates and bristles. Another is the fence height required when dealing with backing plates made from materials with limited bending stress. Height is the distance between a turbine rotor and the bottom edge of a backing plate. One difficulty in using a higher strength brush backing plate that withstands higher temperatures is the damage that would result to a rotor as a consequence of contact between the rotor and the higher strength backing plate. Nevertheless, because of the operating advantages provided by brush seals, it would be desirable to use brush seals where high temperature and high pressure drops are normally observed.
The present invention uses the application of an abradable coating and/or an abrasive coating to the surface of the rotor of a steam or gas turbine so as to increase the temperature and pressure capabilities of conventional brush seals. The combination of an abradable coating or an abrasive coating with a brush seal assembly results in a seal configuration that can withstand severe conditions in steam and gas turbines. The coatings of the present invention are suitable for high temperature environments, and serve to increase the pressure drop capability of conventional brush seals. The pressure capability of brush seals is closely related to fence height. In the present invention, the risk of rubbing a backing plate against a rotor so as to cause damage to the rotor is reduced by applying the abradable or abrasive coating. Application of the abradable or abrasive coating allows fence height to be reduced, and thus, the pressure drop and temperature capability of a brush seal increased commensurately. As the fence height is reduced, the bristle bending stress will also be reduced, giving the brush seal the capability to withstand higher pressure drops and temperatures. The other benefit is that higher strength materials can be used for the backing plate without being in risk of damaging the rotor. The abradable or abrasive coating also protects the rotor. The higher strength materials for the backing plate also assist the brush in withstanding higher pressures and temperatures.
Brush holder assembly 10 is urged against surface 13 of rotor 11 by a spring assembly (not shown) exerting a positive force against assembly 10 in the direction of rotor 11. The distance between surface 13 of rotor 11 and the bottom edge 21 of backing plate 15 is the “fence height” 22 of brush holder assembly 10. In time, fence height 22 will be diminished as a result of wire bristles 12 rubbing against surface 13 of rotor 11 and wearing down as rotor 11 rotates during operation. Eventually, bottom surface 21 of backing plate 15 will contact surface 13 of rotor 11, as a result of wire bristles 12 wearing away. At this point, bottom edge 21 of backing plate 15 has the potential to damage surface 13 of rotor 11 as it rubs against surface 13.
During operation of rotor 11, steam or air (depending on whether the turbine is a steam or gas turbine) moves, as shown in
To reduce the risk of damage to rotor 11 resulting from backing plate 15 rubbing against surface 13 of rotor 11, the present invention uses an abradable coating or an abrasive coating that is applied to the surface 13 of rotor 11. The application of either of these coatings to surface 13 serves to protect surface 13 to prevent damage to it in an instance where backing plate 15 rubs against surface 13. The application of the abrasive or abradable coating to surface 13 of rotor 11 allows fence height 22 to be reduced. In addition, a higher strength brush backing plate 15 that withstands higher temperatures can be used because the contact between plate 15 and surface 13 of rotor 11 results in no damage to the rotor, but rather a gradual reduction of backing plate 15 as it wears.
Reduction in fence height 22 effectively allows a greater percentage of wire bristles 12 to be supported by backing plate 15, which will then have a longer length as a consequence of the reduced distance between plate 15 and rotor 11. The higher strength material used for plate 15 provides further support for wire bristles 12. Preferably, the backing plate provides support for approximately seventy-five percent of the length of the wire bristles. The result is the ability of the wire brush assembly 10 to withstand an increase in pressure drop capability and a suitability for higher temperature environments because of the greater support provided to bristles 12 by a longer backing plate 15 made from a stronger material.
Materials for backing plates are well known to those skilled in the art. Exemplary materials consist of austenitic stainless steel, ferritic stainless steel, a nickel based super-alloy, a cobalt based super-alloy, a polymeric material and combinations thereof. One example of a preferred stainless steel material would be 409SS.
When an abrasive coating is applied to surface 13 of rotor 11, contact between backing plate 15 and surface 13 of rotor 11 results in backing plate 15 being worn, rather than surface 13 of rotor 11 being damaged. When an abradable coating is applied to surface 13 of rotor 11, contact between backing plate 15 and surface 13 of rotor 11 results in the abradable coating being worn, rather than surface 13 of rotor 11 being damaged.
The abradable coating can be plasma sprayed or flame sprayed onto surface 13 of rotor 11, depending on the type of material used. A heat treatment may also be appropriate with the application of the material, again, depending on the particular abradable material used. Materials for abradable coatings are well known to those skilled in the art, such as those described in U.S. Pat. No. 6,547,522 and U.S. Patent Application Publication No. US 2004/0126225, the contents of which are incorporated herein by reference. Such coatings are considered exemplary materials for the preferred embodiment.
The abrasive coating can be plasma sprayed or flame sprayed onto surface 13 of rotor 11, depending on the type of material used. A heat treatment may also be appropriate with the application of the material, again, depending on the particular abrasive coating used. Exemplary abrasive coatings 80 include alumina, chromium carbide and stellite.
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.
