Machine with abradable ridges and method   

Abstract: A machine having a fixed part including a portion with a smooth surface is provided. The machine also includes a rotating part configured to rotate relative to the fixed part, the rotating part directly facing the portion of the fixed part, and plural ridges formed on the portion of the fixed part directly facing the rotating part, the plural ridges comprising an abradable material, wherein the abradable material is configured to be inoperable at temperatures above about 1000° C.

This is a national stage application under 35 U.S.C. §371(c) of prior-filed, co-pending PCT patent application serial number PCT/US2010/052232, filed on Oct. 12, 2010, which claims priority to Italian Patent Application Serial No. CO2009A000045, filed on Oct. 30, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for producing abradable ridges in a machine having a rotating part and a fixed part.

2. Description of the Prior Art

Rotating machines, for example, gas turbines, used today in various technical fields (power systems, petrochemical plants, etc.) have at least a rotating part (rotor with blades) that rotates with respect to a fixed part (shroud). A fluid is typically injected at an input of the rotating machine to be accelerated/pressurized and the fluid is then ejected at an outlet of the rotating machine. Thus, a fluid flow is generated by the rotating blades. For a good efficiency of the rotating machine, a seal between the rotating part and the fixed part is desired to be achieved so that most of the fluid flow is engaged by the blades of the rotating part and does not leak over the tips of the blades, which is unwanted leakage.

One way to provide the seal between the rotating part and the fixed part of the rotating machine is to deposit an abradable material on the fixed part so that the tips of the blades together with the abradable material form a seal. If the abradable material includes a ceramic, then an abrasive material may be provided on tips of the blades of the rotating part to protect the tips when contacting the abradable material to form the seal. Such a method is described in U.S. Pat. No. 6,457,939, the entire content of which is incorporated here by reference. U.S. Pat. No. 6,251,526, the entire content of which is incorporated here by reference, describes profiled abradable ceramic coating systems, in which a porous ceramic coating is deposited onto a substrate with a profiled surface, e.g., a metal grid brazed onto the substrate surface (casing of the gas turbine) to form an abradable profiled surface. Because the blades of the rotor of the turbine may increase their size due to thermal expansion during the normal operation of the turbine and/or due to centrifugal effects produced by the high rotational speeds of the rotating part of the turbine during operation, the blades may move towards the casing and may remove part of the abradable material to achieve a smaller clearance. The differential expansion rate between the rotating part and the inner surface of the fixed part results in tips of the blades contacting the abradable material to carve grooves in the coating without contacting the casing itself. Thus, a custom-fitted seal with minimal leakage is formed in the turbine. However, a problem of such techniques is the grid brazed onto the substrate (casing) of the turbine, which may result in damage to the shroud upon profiling.

U.S. Pat. No. 6,887,528 and U.S. Patent Application Publication No. 2005/0003172, both of which are assigned to the assignee of the present patent application and the entire contents of which are incorporated here by reference, disclose a method for producing a profiled abradable coating on a casing of a gas turbine without providing a grid on the casing of the turbine. The abradable material includes a porous ceramic material that is able to withstand temperatures as high as 1500° C. The abradable layer is formed on the casing by using direct-write technology or plasma sprayed onto the substrate through a mask or a plasma gun. However, this method uses expensive materials for the plural ridges in order to withstand the high temperatures inside the gas turbines.

For a better understanding of the background art, the following example is discussed with regard to FIGS. 1 and 2. As shown in FIG. 1, traditional methods for improving a clearance between the tips of the blades and the fixed part of the turbine is to machine in the casing 10 of the turbine a grid 11 by removing part of the original material of the casing 10. Then, a thermal barrier coating (TBC) layer 12 (i.e., a high temperature resistant layer for protecting the casing from heat damage) is formed to not be in direct contact with a surface 14 of the casing 10. An abradable layer 16 is deposited on layer 12. A blade 18 of the rotating part faces the abradable layer 16 and may scrape this layer 16. As shown in FIG. 2, the abradable layer 16 and the TBC layer 12 may be shaped as a ridge 20 having a straight-line shape or ridge 22 having a zigzag shape. However, these traditional methods for providing a high temperature resistant seal in the turbines may be disadvantageous if used in other machines that do not experience a high temperature because casing 10 may be damaged when machining the grid 11 and/or may be expensive as the ceramic abradable material requires exotic components, as for example, yttria-stabilized zirconia.

Accordingly, it would be desirable to provide systems and methods for providing an abradable material on machines that do not operate in a high temperature environment.

SUMMARY

According to one exemplary embodiment, there is a machine that includes a fixed part having a portion with a smooth surface; a rotating part configured to rotate relative to the fixed part, the rotating part directly facing the portion of the fixed part; and plural ridges formed on the portion of the fixed part directly facing the rotating part, the plural ridges being made of an abradable material that is configured to be inoperable at temperatures above about 1000° C. At least one ridge of the plural ridges is curved.

According to another exemplary embodiment, there is a diaphragm of a compressor that includes a fixed part configured to accommodate at least an impeller of the compressor and having a portion with a smooth surface; and an abradable layer formed on the portion with the smooth surface of the fixed part. The abradable layer is machined to form plural ridges directly facing the impeller, the plural ridges\'being made of an abradable material that is configured to be inoperable at temperatures above about 1000° C., and at least one ridge of the plural ridges is continuously curved.

According to still another exemplary embodiment, there is a method of depositing an abradable material on a diaphragm of a machine. The method includes identifying in the diaphragm a portion with a smooth surface that directly faces a rotating part of the machine; depositing an abradable layer on the portion directly facing the rotating part, the abradable layer including an abradable material that is configured to be inoperable at temperatures above about 1000° C.; and machining plural ridges in the abradable layer such that at least one ridge of the plural ridges is curved.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

FIG. 1 is a schematic diagram of a portion of a conventional gas turbine with an abradable material deposited on a grid formed in the casing of the gas turbine;

FIG. 2 is a schematic diagram of a conventional pattern of the abradable material of FIG. 1;

FIG. 3 is a schematic diagram of a compressor;

FIG. 4 is a schematic diagram of an impeller of the compressor of FIG. 3;

FIG. 5 is a schematic diagram of a portion of a diaphragm of a compressor according to an exemplary embodiment of the present invention;

FIG. 6 is a schematic diagram of an abradable material deposited on a diaphragm of a compressor according to an exemplary embodiment of the present invention;

FIG. 7 is a schematic diagram of a pattern of plural ridges formed in an abradable material according to an exemplary embodiment of the present invention;

FIG. 8 is a schematic diagram of various ridge shapes that can be formed according to an exemplary embodiment of the present invention;

FIG. 9 is a schematic diagram of an interaction between ridges and an impeller of a compressor according to an exemplary embodiment of the present invention;

FIG. 10 is a schematic diagram of various layers that may be formed on a diaphragm of a compressor according to an exemplary embodiment of the present invention;

FIG. 11 is a graph showing advantages of curved patterns for the ridges formed on a diaphragm according to an exemplary embodiment of the present invention; and

FIG. 12 is a flow chart illustrating steps for forming the plural ridges on the diaphragm of a machine according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the present invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a compressor. However, the embodiments to be discussed next are not limited to compressors, but may be applied to other systems that require a seal between a rotating part and a fixed part.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

FIG. 3 illustrates an open impeller centrifugal compressor 30. The open impeller centrifugal compressor 30 has an impeller 32 connected to a shaft 34. Shaft 34 may be supported by bearings 36 and 38. The impeller 32 has a hub portion 40 and a blade portion 42. A fluid enters the centrifugal compressor 30 at an inlet 44, along an incoming direction A. The fluid reaches the impeller 32, where it is accelerated based on the centrifugal force while changing the fluid direction prior to being discharged at outlet 46 along direction B. A diaphragm 48, which faces the impeller 32, is part of the fixed part of the compressor 30. The diaphragm may be attached to a casing 49 of the compressor 30.

A detailed view of the impeller 32 is shown in FIG. 4. Other structures for the impeller 32 may be used. The specific shape of impeller 32 shown in FIG. 4 corresponds to an open impeller (no element is covering blade portion 42). A centrifugal compressor having this impeller is called an open impeller centrifugal compressor. The blade portion 42 may have multiple blades 50 having various contours, depending on the application/operation of the compressor. These multiple blades 50 rotate inside the diaphragm 48 such that tips 52 of the blades 50 may move closer or even touch the diaphragm 48 due to an elongation of the blades 50 because of thermal transients, and/or the high rotational speed of the blades 50 relative to the diaphragm 48, and/or critical vibrations.

To prevent damages to the tips 52 of the blades 50 and also to achieve a desirable seal between blade tips 52 and diaphragm 48, as has been discussed in the Background of the Invention section, an abrasive material may be coated on tips 52. However, no such abrasive material is used in this exemplary embodiment. Thus, tips 52 of the blades 50 are vulnerable to damage if they contact the strong material that the diaphragm 48 is made. For this reason, a continuous layer of abradable material is deposited on a portion of the diaphragm 48 that directly faces blades 50. This portion is shown in FIG. 5 as element 60. According to another exemplary embodiment, portion 60 may be smaller than shown in FIG. 5, i.e., may not extend the entire axial span of the blade portion 42. According to this exemplary embodiment, portion 60 may be one third of the axial span of the blade portion 42. In other words, considering axis X as being parallel to the rotation axis of impeller 32, the axial span of the blade portion 42 is between C and F. The axial span of portion 60, which has the abradable material thereon, may be between C and F or smaller, with the smallest axial span being between E and F.

Another feature of the novel exemplary embodiments is that the diaphragm 48, and more specifically, a surface 62 (see FIG. 5) of the diaphragm 48 that receives the abradable material is smooth, i.e., has no ridges, grids, or other formations intentionally formed in the metal of the diaphragm 48. In one application, the surface 62 of the portion 60 of the diaphragm 48, if represented in a XY plane, with a longitudinal axis of the diaphragm 48 along axis X, has a same sign of a partial derivative of a Y position with respect to X along the longitudinal axis of the diaphragm 48 ignoring normal tolerances accepted in the industry for making such large pieces of equipment. Further, even if small unevennesses are present in the surface 62 of portion 60, if these are not intentionally made, it is considered that the surface 62 is smooth. This is different from some gas turbine shrouds that have ridges or grids 11 intentionally formed in the casing 10 of the gas turbine prior to depositing the abradable material 16, as shown in FIG. 1.

Another difference between the traditional gas turbines and the novel embodiments is the temperature range. More specifically, the gas turbines are known to operate at high temperatures, e.g., higher than about 1000° C., while a compressor operates at lower temperatures, in the range from about 100 to about 400° C., and about 200° C. for a centrifugal compressor diaphragm. This large difference in the operation temperature of a gas turbine and a compressor makes the ceramic based abradable coatings of the traditional turbines not suitable/unnecessary for compressors. Thus, other materials, as will be discussed later, are used for coating the diaphragm of the compressors.

According to an exemplary embodiment illustrated in FIG. 6, the surface 62 of the diaphragm 48 may be directly covered with a smooth layer 70 of an abradable material. The layer 70 of abradable material may be directly deposited on the surface 62 of the diaphragm 48, which is different from the gas turbine case in which the TBC layer is formed on the casing prior to depositing the abradable material. The direct formation of the abradable material 70 on the surface 62 of diaphragm 48 is possible because of the lower temperature environment in which compressors operate.

Abradable materials to be used for compressors may be divided into metallic-based abradable materials and plastic-based abradable materials. These materials have a common property that they are not designed to withstand high temperatures, as those materials used in a gas turbine. In other words, the abradable materials to be used in the compressors may become inoperable (melt, peel, etc.) if used in the turbine of a gas turbine. In this regard, the abradable materials to be used, for example, in centrifugal compressors, are selected to operate at temperatures up to about 200° C. In another embodiment, depending on the type of compressor, the abradable materials may operate at temperatures up to about 400° C. Metallic abradable materials may include one or more of AlSi, AlSi and Polyester, NiCrFeBNAl, etc. Plastic abradable materials may include one or more of polytetrafluoroethylene (PTFE), Polyester, polyimide, etc.

It is noted that the metallic and/or plastic abradable material may be formed directly on the surface of the diaphragm 48, without any protection layers (for example, TBC layers) as is customary in the gas turbines. In this regard, a known ceramic abradable material is not directly deposited on the substrate but rather on a thermally resistant coating (layer 12 in FIG. 1), for protecting the substrate (the casing) from the high temperatures generated during the operation of the gas turbines. In another exemplary embodiment, such thermally protective coatings may be deposited on the diaphragm 48 prior to depositing the abradable material 70.

After the abradable material 70 has been deposited on the surface 62 of the diaphragm 48, the abradable material 70 may be machined to form ridges 72 having peaks 74 and valleys 76 as shown in FIG. 7. The shape of the ridges 72 may be diamond shape, straight lines, constantly curved, continuously curved, etc. A cross sectional view of ridges 72 is shown in FIG. 8. A shape of ridge 72, as shown in the cross sectional view in FIG. 8, may have a smooth shape as indicated by 80, or may have a triangular cross section as indicated by 82, or may have a rectangular cross section as indicated by 84, or other shapes. According to an exemplary embodiment, the diaphragm 48 may be provided with a combination of one or more of the above discussed shapes 80, 82, and 84. For exemplary purposes, a dimension “d” of the ridges 72 may be between about 0.0025 and about 0.102 mm for the rectangular shape and between about 0 and about 0.102 mm for the triangular shape, and a height “h” of the ridges 72 may be between about 0.1 and about 0.5 mm.

Once blades 50 are rotating with shaft 34 inside diaphragm 48, due to centrifugal effects and/or rotor unbalance and/or thermal transients, the blades may move radially or axially towards the diaphragm 48 to contact ridges 72. Depending on the degree of expansion of the blades 50, tips 52 of the blades 50 may touch and even break (remove) top parts of ridges 72 to form groove regions 90 as shown in FIG. 9. This close contact between ridges 72 and blades 50 may achieve the desired sealing between the rotating part and the fixed part of the compressor. In addition, the close contact of the tips 52 of the blades 50 with ridges 72, which are abradable and also have a soft structure due to their small physical dimensions, prevents the tips 52 of the blades 50 to suffer damages, given the fact that tips 52 have no protective abrasive materials. In addition, if a thickness of the ridge 72 is small, the material used to form the ridge may be dense.

According to another exemplary embodiment, the entire diaphragm 48 may be made of the abradable material so that the ridges 72 may be formed by machining the diaphragm 48 and not by depositing abradable material.

A more detailed view of the layers deposited on the surface 62 of the diaphragm 48 according to an exemplary embodiment is shown in FIG. 10. A bond coat layer 100 (for example, the bond coat can be NiAl or NiCrAlY) having a height h1 of around 0.125 mm optionally may be deposited on the diaphragm 48 by, for example, plasma spray process. Optionally, a layer 102 of DVC-TBC (Dense Vertically Cracked Thermal Barrier Coating) having a height h2 of about 1.00 mm may be deposited over layer 100. The abradable layer 70 is formed over layer 102 or directly on layer 100 or directly on diaphragm 48 and may have a height h3 of about 1.3 mm. Deviations from these exemplary numbers in the range of 5% to 50% are also possible.

Advantages of the novel abradable patterns discussed above are now discussed with regard to FIG. 11. FIG. 11 shows the variation of a total clearance reduction as a function of hot running rubbed clearance for various abradable ridge shapes having the same height. The hot running rubbed clearance is the actual clearance between the impeller and the diaphragm when the impeller rotates and the total clearance is the effective clearance due to the shape of the ridges and other parameters. FIG. 11 illustrates the relative effect of (i) abradable ridges with a curved pattern (curve 124), (ii) abradable ridges with 45 degrees straight line pattern (curve 122), and (iii) a smooth abradable layer with no ridges and no pattern (curve 120). For a given hot clearance (e.g., 102 mils, location 126 in FIG. 11), the abradable ridges with curved pattern provide an advantage of approximately 18 mils clearance reduction over the plural ridges with the straight line pattern. The curved pattern may provide approximately 40 mils clearance reduction over a compressor with no abradable layer versus approximately 27 mils clearance for the straight line pattern over the no abradable layer compressor. Curve 122 corresponds to plural ridges having a straight pattern inclined at 45 degrees relative to the axial direction of the compressor (see for example FIG. 2, ridges 22) and curve 124 corresponds to plural ridges having curved patterns (see for example FIG. 7, ridges 72). It is noted that the curved patterns curve 124 provides a higher clearance reduction (approximately 40 mils or 1 mm) than the straight pattern curve 122 (clearance reduction approximately 27 mils or 0.68 mm) and the smooth abradable layer curve 120 (approximately 23 mils or 0.58 mm) for the same hot running rubbed clearance 126. The total clearance reduction shown on the Y axis of FIG. 11 indicates that for a same height of the ridges 72 of the three curves 120, 122, and 124, the amount of fluid leaked between the moving part and the fixed part of the compressor is smaller for ridges 72 of curve 124 than for ridges 72 of curve 122. The shape of the ridges (straight versus curved) generate this effect of reduced clearance.

According to an exemplary embodiment, which is illustrated in FIG. 12, there is a method for depositing an abradable material on a diaphragm of a machine. The method includes a step 130 of identifying in the diaphragm a portion with a smooth surface that directly faces a rotating part of the machine; a step 132 of depositing an abradable layer on the portion directly facing the rotating part, the abradable layer including an abradable material that is configured to be inoperable at temperatures above about 1000° C.; and a step 134 of machining plural ridges in the abradable layer such that at least one ridge of the plural ridges is curved.

The disclosed exemplary embodiments provide a system and a method for depositing an abradable material on a fixed part of a machine having a moving part. However, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other example are intended to be within the scope of the claims.