WO2002023069A2

WO2002023069A2 - Floating brush seal optionally disposed in labyrinth seal

Info

Publication number: WO2002023069A2
Application number: PCT/US2001/028537
Authority: WO
Inventors: William J. Catlow; Eric Sulda
Original assignee: Turbocare, Inc.
Priority date: 2000-09-14
Filing date: 2001-09-13
Publication date: 2002-03-21
Also published as: AU2001290828A1; WO2002023069A3

Abstract

A floating brush seal (101) is provided for a gas or steam turbine or a compressor that typically includes labyrinth seals (307). A separate holder is provided, or a groove or recess is provided in a labyrinth seal, and the brush seal is allowed to float therein because it is supported on a spring (311) and is not affixed to the holder or the labyrinth seal. The spring or other resilient member urges the brush seal towards the shaft or rotor (111), preferably only so much as it weighs so that the seal effectively floats because the spring supports only the weight of the brush seal. Use of a floating brush seal allows for the use of bristles having a desired stiffness (more or less stiff) and avoids interference transients that would degrade the bristles and decrease the useful life of the brush seal.

Description

FLOATING BRUSH SEAL OPTIONALLY DISPOSED IN

LABYRINTH SEAL

BACKGROUND OF THE INVENTION.

1. Field of the Invention

This invention is related to seals for turbines (gas and steam) and compressors, and more particularly to a floating brash seal disposed therein, optionally being disposed in a labyrinth seal or any suitable carrier, and to methods for making, using, and installing the same.

2. The State of the Art.

As relates to this invention, turbines generally include two parts: a stationary casing and a moving (rotating) shaft disposed therein. Fluid flowing between the casing and the shaft, flowing from a higher pressure zone to a lower pressure zone across blades attached to the shaft, causes the shaft to rotate. This rotating member, in turn, can be comiected to a generator, whereby electricity is generated. For electricity generators, the working fluid is typically steam. In summary, a working fluid flows between the casing and the shaft and is present along the length of the turbine device at various stages of pressure, depending upon the blades and other devices through which the compressible fluid must flow.

To assure that the compressible working fluid does not leak from an area of one pressure to a lower pressure, other than through the desired flow path impinging the blades attached to the shaft, a number of seals are typically used in the turbine. One of the most common types of seals is a toothed labyrinth seal, which generally consists of a number of arcuate sections disposed in a circle around the shaft and located between the shaft and the casing. On the face of the labyrinth seal abutting the shaft are a number of fins or teeth which do not contact, but come close to contacting, the shaft. In cross section, a labyrinth seal assumes a geometry similar to a fat I-beam, with the bottom section wider than the top. The advantage of using this I-beam geometry is that the casing can be made with a large groove into which the top portion of the I-beam labyrinth seal can be mounted. The shaft may also have a number of platforms or lands, essentially raised sections (which, in cross section, would provide a shaft of a larger diameter); in such cases, the labyrinth seal has fins of different lengths to accommodate the platforms on the shaft.

In addition to labyrinth seals, another type of seal used in turbines is a brash (or finger) seal. As the name suggests, a conventional brush seal is made of a number of bristles, typically comprised of metal, closely bound together, and thus effecting a seal across the brash. In typical operations, a brush seal is used as an alternative to a labyrinth seal. Alternatively, such as shown in US 5,599,026 and GB 2,301,635 (the disclosures of which are incorporated herein by reference), one or more brush seals can be incorporated into a labyrinth seal. In such a configuration, the brash seal takes the place of one or more of the fins on the labyrinth seal.

In U.S. Patent Nos. 4,436,311, 5,395,124, 5,810,365, and 5,934,684 to Brandon (the disclosures of which are each incorporated herein by reference), the problem of rubbing in packing ring design has been addressed by providing a retractable segmented packing ring structure between each rotor and turbine shaft. The manner in which the quality of the labyrinth seal is improved with this design is described as follows. During startup operation, when low frequency rotor vibration is predominant, the diaphragm-mounted packing ring segments are spring-biased in a radial direction away from the turbine shaft, reducing the risk of fin-tip portion rubbing and packing ring damage. As the rotor increases its angular speed, low frequency vibration is naturally reduced, and the packing ring segments are forced to move closer to the turbine shaft by steam pressure, improving the quality of the labyrinth seal among the rows of fins and surface projections in the packing ring structure, thereby improving the efficiency of the turbine. An alternative solution to the problem of rubbing in packing ring designs is disclosed in UK Patent Application Publication No. GB 2 301 635 A. In this UK Patent Publication, a brash-type element is installed between a pair of fins extending from the packing ring segments mounted on the diaphragm. The function of the brash seal is to improve the quality of the labyrinth seal during all phases of operation. A major shortcoming with this design, however, is that during startup operations it does not provide a way of protecting the tip portions of the fin seals without designing a high degree of clearance into the design. Consequently, by virtue of such increased clearance requirements, the quality of the labyrinth seal provided by this prior art packing seal design is necessarily compromised. In addition, the brash seal design disclosed in this prior art publication is very expensive to manufacture, decreasing the price-to- performance ratio of this packing ring structure.

While there are some advantages to the combination of labyrinth and a brash seal, there are also a number of disadvantages. During installation, because the brush seal is rigidly fixed in the labyrinth seal (or a brash seal alone) in a manner which does not allow movement radially, the bristles are liable to be permanently deformed during installation. Similarly during operation, because the brash seal does not have any freedom of movement radially away from the shaft, interference with the shaft can cause bending and or wear of the bristles, thus decreasing the useful brush life and/or diininishing the sealing properties.

SUMMARY AND OBJECTS OF THE INVENTION.

In light of the foregoing, various objects of this invention include providing a floating brash seal, providing an improved combination labyrinth and brash seal, providing a brush seal that is adapted for radial movement, providing an improved method for installing a brush seal that avoids damage to the bristles, to provide a brash seal having bristles of a desired stiffness and yet compliant with transients, and other objects as will become apparent. In smnmary, in one embodiment this invention provides a brash seal supported around a rotatable shaft effective to allow the brash seal to move with the shaft during transients.

In another embodiment this invention provides a combination labyrinth seal and brash seal, wherein the brush seal is disposed in a recess in the labyrinth seal on a base that is adapted for radial movement.

In another embodiment, this invention provides a brash seal that is affixed to a base adapted for radial movement.

This invention thus also provides an improved turbine including the unproved floating brush seal and/or the labyrinth and brush seal combination.

In yet another embodiment, this invention provides a method for installing a combination labyrinth seal and brash seal in a turbine, wherein the brush seal is disposed in a recess in the labyrinth seal on a base that is adapted for radial movement, by fixing the brash seal in the labyrinth seal with at least one sacrificial shim prior to installation, and wherein the shim rapidly degrades once the turbine has reached operating speed.

This invention also provides a method for installing a floating brush seal by providing a carrier for the brash seal, providing a support for the brush seal, and suspending the brush seal concentrically with the shaft.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 depicts an idealized exploded perspective view of a brash seal in cross section.

Fig. 2 depicts an idealized plan view showing a brash seal assembled from two halves.

Fig. 3 depicts an idealized cross section of a turbine casing showing the labyrinth and brash seals.

Fig. 4a shows an idealized cross sectional view of the installation of the brush seal secured in location in a labyrinth seal by sacrificial shims and supported by a spring. Fig. 4b shows an idealized cross sectional view of the installed brash seal element under load. Fig. 4C shows an idealized cross sectional view of a brush seal element under load and located in a labyrinth seal. Figs. 4D and 4E are analogous to Fig. 4C but with a brash seal having a greater height and disposed in a recess having a greater length, for use with retractable packing.

Fig. 5 is an exploded perspective view of a turbine end gland housing having a floating brash seal installed therein.

Fig. 6 is a partial cross-sectional view of a turbine end gland housing having a floating brush seal installed therein.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

As mentioned above, this invention is applicable to seals used in steam and gas turbines and in compressors. The particular seals with which this invention are useful are likely to be shaft seals and end seals.

As best shown in the cross sectional, perspective, exploded view of Figure 1, the brash seal element 101 includes a packet of bristles 103 disposed between the interior surfaces of front plate 105 and a back plate 107. The front and back plates are welded together at the outer surface 109 to form a unitary brush element; by "outer" is meant the circumferentially outer surface. The back plate is mounted on the lower pressure side of the seal and is preferably longer than the front plate because it must support the brash packet against the pressure of the working fluid.

The brush seal element 101 is mounted around the area 111 where the turbine shaft would be located, as best shown in Figure 2. The brash seal element is preferably divided into arcuate segments; for example, as shown in the figure, semicircular arcuate segments 113a and 113b. The segmented construction allows the brush seal to be installed around the shaft. The arcuate segments are connected togetlier by screws 115a and 115b that fit into screw threaded recesses 117a and 117b so that the free end of the packet of bristles 103 is adjacent to the shaft 111. The top portion of the brash seal element will inherently have a close tolerance with the shaft due to gravity while the lower portion will be pulled away from the shaft. As described in more detail below, the floating brash seal of this invention utilizes a spring to center the brash seal on the shaft while allowing the seal to move with transients induced in the shaft.

Figure 3 depicts an idealized cross section of a turbine casing and rotor with non-retractable packing. The casing is typically constructed similar to a split mold, so that the top half of the casing can be removed to service the equipment. As shown in Fig. 3, the top part of the casing 301 (also called a diaphragm for each pressure section of the turbine) is shown exploded apart from the bottom part 303. The shaft or rotor is located centrally in area 111. In the embodiment shown in Fig. 3, the brash seal is installed in combination with a packing ring, namely as shown a labyrinth seal; the teeth (or fins) of the labyrinth seal 309 are disposed radially outwardly of the brash seal 101. In the preferred embodiment, the brash seal is formed from two semi-circular pieces, a lower and an upper portion; the labyrinth seal likewise may be formed from two semicircular pieces, but is typically comprised of multiple arcuate portions that form a circle around the shaft; the packing ring as illustrated includes six segments with three on the top (307a-c) and three on the bottom (307d-f). The labyrinth seal can be divided into more or fewer segments but four or six are commonly used. A leaf spring 311 is shown supporting the labyrinth seal packing.

At the bottom portion of the brush seal shown as mounted in Fig. 3 is a wave spring 305 (a wave-shaped leaf-type spring) designed to center the brush seal with respect to the center or axis of the shaft. The spring is designed to counteract the weight of gravity so that, effectively, the spring does not provide any resistance against movement of the brash seal downward (other than the effect of gravity). Thus, the spring constant, which may need to be adjusted in practice, is chosen based on the weight of the brush seal. While a leaf spring is shown in Fig. 3, which could be either flat as shown or curved (e.g., in an "S" shape), the spring could be a coiled or helical spring instead. Further, while a compression spring is shown in Fig. 3, a tension spring could be used instead of or in combination with a compression spring; the tension sprmg would be located at the top portion of the brush seal. The spring can be consfructed of any suitable material, especially metals such as steels, preferably stainless steel and high sti'ength, corrosion resistant alloys such as ferrous alloys containing nickel and/or chromium (e.g., INCONEL x718 and INCONEL 750).

The bristles of the brash seal are preferably directed essentially normal to the shaft with respect to the axial direction. That is, the bristles should be oriented parallel with a plane that is perpendicular to the axis of the turbine shaft. Also, the bristles are preferably angled in the direction of rotation of the shaft; thus, the bristles point slightly tangentially with respect to the axis of the shaft.

The bristles themselves are preferably made from metal wire, such as stainless steel, although in various applications it is possible that plastics, such as high temperature polymers, and especially filled polymers (e.g., filled with particles, fibers, platlets, and combinations of these morphologies of ceramics and/or metals) would also be suitable. Preferred materials for the bristles are metals such as stainless steel, cobalt- and/or chromium-based superalloys (e.g., Haynes 25 and Haynes 214) are preferred; ceramics and plastics, especially high temperature and filled plastics. The wires typically have a diameter chosen between 1 mil (0.001 inch) and 10 mils, although larger and smaller diameter wires can be suitable depending on the application. The density of the bristles is typically described in terms of the number of bristles in an area defined by a circumferential inch by a given depth (axially); the depth usually ranges from 20 mils to 100 mils. For example, a typical brush seal installation has a brash made with 2.8 mil wire that is 50 mils wide; accordingly, there are about 357 bristles circumferentially by about 18 bristles axially, for a density of about 6375 bristles.

The brash seal can but need not contact the shaft depending on the particular application and installation for which it is designed. Operation of the brash seal with interference (contact) with the shaft results in bristle wear, and so the effectiveness of the seal may diminish over time. By seating the brush seal on a resilient member such as the aforementioned spring, the brash seal can move radially to accommodate movement of the shaft rather than being damaged by interference with the shaft. This "floating" brash seal design provides an improved seal by eliminating damage to the seal due to interference between the brash seal and the shaft when the brash is designed not to contact the shaft. However, in some applications it may be desirable to design the brash seal so that the bristles actually contact the shaft around its circumference and wear away during use. Every shaft used in a turbine (or compressor) is unique, and each has its own dynamics, vibrations, and other idiosyncracies. Those who operate and service a particular turbine will become aware of these characteristics, and so when the turbine is rebuilt, a brash seal may be inserted alone or in combination with a labyrinth seal (as discussed below). The design of the brash seal will depend on the particular characteristics of the shaft and the turbine, and so it may be desirable to design the brash with a certain tolerance, or with no tolerance and have the bristles wear down to the design tolerance during use. For example, if a shaft typically has a 5 mil interference during startup (that is, the shaft is apt to move 0.005 inches in some radial direction due to transients during startup), the brash seal may be designed with a 5 mil tolerance so that the bristles are not damaged during startup, and the sealing characteristics take into account the 5 mil design gap between the shaft and the brash. The excursions of the shaft (the transient movements) are not necessarily isotropic and can be unidirectional; in these cases, it may be more desirable to design a brash seal having an inner diameter smaller than the outer diameter of the shaft so that the bristles wear away during use until the shaft and seal reach an equilibrium after continued operation. Of course, for a newly built turbine, the startup and operating characteristics of the shaft may likely not be known until the turbine is shut down, the casing opened, and the seals serviced. At that time, it may be decided to change the seal design. Additionally, the bristles will be chosen of a material having a desired stiffness that depends on whether the brush is intended to have a tolerance (stiffer bristles) or is designed to contact the shaft and wear during use (less stiff bristles).

The portion of the brush seal that abuts the spring preferably includes a lubricant or a low friction coating to facilitate the brash seal tracking the movement of the shaft, independent of shaft vibration and stationary part distortion. Suitable low friction coatings are among those commercially available, such as polyfluoroethanes (e.g., TEFLON brand lubricants, available from DuPont deNemours, Wilmington, DE), molybdenum disulfide (e.g., MOLYKOTE brand lubricants, from Dow Corning Corp., Midland, MI), and other high temperature lubricants (e.g., HI-T-LUBE brand lubricant, from General Magnaplate, Linden, NJ; PS304, from NASA's Tribology Branch, a chrome-oxide-based plasma spray coating that contains silver and a barium fluoride/calcium fluoride eutectic). The operating conditions under which the ring floats and tracks the shaft movement are a function of the bristle stiffness, the coefficient of friction between the brash seal and the shoulder recess (e.g., at 425 in Fig. 4B), and the pressure drop across the seal. The lubricant should be provided at areas such as the abutment between the brash seal and the shoulder recess 425. Preferably the seal is designed to float during transient periods (e.g., start-up and shut-down/trip) when the potential seal interference is most severe and the pressure drop across the seal is low.

Thus, the floating brush seal can be disposed in a toothed labyrinth seal that forms part of a packing ring segment. (Although a brash seal is technically a labyrinth seal because the working fluid must take a tortious path to pass through the seal, the toothed seal is conventionally referred to as a labyrinth seal.) Referring then to Fig. 4A, the combined seal of this invention comprises a labyrinth seal as part of a packing ring segment, and there are typically four or six segments that form a complete packing ring; the most preferred embodiment are those segments that are retractable, as described in the aforementioned Brandon patents. Each segment 307 has an outer ring 401 connected by an neck portion 403 to an inner ring 405 on which are disposed a series of fins or teeth 407 directed to the shaft 409 and which are separated from the shaft by a clearance 411 (typically about 25 mils in the closed position). The segment shown is on the bottom portion of the casing (e.g., 307d-f). Some of the fins engage platforms (or lands) 413 on the surface of the shaft. As mentioned above, the labyrinth seal preferably has the geometry of an I-beam. The shoulders of the diaphragm engage the neck portion of the segment and corresponding lips in the casing (not shown in this figure) so that the labyrinth seal has limited movement radially with respect to the shaft. Disposed in the body of the labyrinth seal is a brash seal 101. The body of the labyrinth seal includes an opening or recess 417 adapted to receive the brash seal assembly. This assembly is disposed throughout the circumference of the labyrinth seal much as the labyrinth seal is disposed throughout the circumference of the casing. The base (closed end) of the recess preferably comprises the resilient spring member 111 capable of urging the brash seal away from the recess in the labyrinth seal and towards the shaft, preferably only so much as to support the brush seal assembly. Fig. 4A depicts the structure during assembly of the seal segments, brash seal, and the like. During assembly, the seal segments are generally separated from the shoulders of the diaphragm by a gap 419. Also during assembly, it may be desirable to fix the brash seal in place in the seal segment using a pair of shims 421. Preferably these shims are made of plastic (e.g., polystyrene, poly(methyl methacrylate), etc.) or another material than cannot withstand the conditions in the local environment during normal operation, and so are sacrificial. The shims maintain the brash seal in the desired position during installation. After the turbine is started and has reached normal operating conditions, the shims degrade and effectively disappear. Brush seals rigidly held, such as in the labyrinth seal as in the aforementioned patents, can have an interference of as much as 0.010-0.015 inch because the brush seal holder itself is also rigidly fixed to a stationary piece of the turbine, typically by a hook. If the turbine has been serviced previously, and/or experienced one or more significant transients, the holder can be distorted, causing interference between the brash seal and the shaft. Additionally, during servicing, inaccuracies can be introduced into the positioning of the holder, again causing interference between the brush seal and the shaft. Any deviation from true concentricity between the brush seal and the shaft will result in interference. The present floating brash seal design eliminates this interference by allowing the brash seal to move radially by floating on the resilient spring member. Although floating toothed seal (or rubbing strip) designs exist (e.g., the aformentioned patents to Brandon), because the tooth is non-resilient, contact between the tooth and the shaft will cause wear on the tooth, and so reduce its effectiveness as a seal. As mentioned above, the present design allows an initial, brief interference between the brush seal and the shaft if there is any contact, after which the brash seal simply tracks the shaft and moves radially so that the bristles do not operate with interference. The combination of a floating seal with compliant bristles allows the present floating brush seal to tolerate short time, transient operation without damage. Because the floating brash seal can tolerate and compensate for transient interference with the shaft, such as during start-up, the bristles can be made stiffer, thus effecting a better seal.

As shown in Fig. 4B, after start up, the pressure of the working fluid forces the seal segments towards the downsfream (lower pressure) portion of the turbine, forwards the right in the figure. This force causes the neck portion of the segment to abut the shoulder, as shown at point 423. Similarly, the pressure differential across the seal causes the brash seal to move towards the downstream side and abut the side of the recess, as shown at point 425. Conversely, in the upper portion of the seal segment (i.e., 307a-c), as shown in Fig. 4C, there is no need for a resilient member in the recess 417. The weld 427 at the top of the brash seal holding the front plate, back plate, and bristles is also shown.

In the situation where a retractable packing is used, such as in the aforementioned Brandon patents, as shown in Fig. 4D, the recess 417 is provided with a greater height H_R (or depth) and the brash seal is provided with a greater height H_s, so that the brash seal effectively acts as a guide for the retractable packing. While clearance is usually designed with respect to the distance between the fins and the shaft (e.g., 411), for ease of discussion the packing can be considered to have a seal clearance C_s in the closed position away from the shaft. When the packing is retracted, as shown in Fig. 4E, it moves away from the shaft 111 to a retracted clearance position C_R away from the shaft that is larger than C_s, although the clearance between the brash seal and the shaft does not change. The deeper recess in the packing and the extended length H_s of the brash seal allow the packing to move out and back while keeping the brush seal in the same relative position with respect to the packing. Knowing the difference between C_s and C_R allows one to determine suitable values for the recess depth H_R and the brush seal height H_s to enable the brash seal to maintain a portion of itself 431 within the recess even when the seal is fully retracted, so that when the seal segments close the brash seal will remain disposed in the recess.

Fig. 5 shows an exploded perspective view of a turbine end gland housing having a floating brash seal installed therein; that is, it is the portion of the casing or housing at the end of the turbine through which the shaft extends and exits. Similar to the main casing, the end casing is comprised of two halves 501 and 503 which are joined together by a plurality of bolts, one large hex bolt 505 being shown extending through a bore in the one half to engage a correspondingly threaded female bore in the other half of the casing. The two casing halves are aligned preferably by a pair of casing dowels, one 507 of which is shown, that extend through aligned, corresponding bores in both halves. The casing shown in Fig. 5 is actually a configuration for a single stage turbine in which, in each of the grooves 509a-g, would have been a carbon seal ring. For the present invention, in each of three grooves are disposed floating brush seals. Each brush seal has a lower 511a and an upper 511b portion as described previously. The lower portion preferably has a notch 513 that is engaged by a key 515 to prevent the brush seal from rotating; the key engages a flat 517 in the groove in the casing. (Although two notches are shown, preferably only one needs to be used). Disposed directly under the brush seal (z^'.e., bottom dead center) are a pair of coil springs 519 on which the brash seal floats. These springs as shown cooperate merely with the surface of the groove in the housing and do not require any other apparatus to prevent then movement circumferentially during operation. It would be more preferably to use one or more leaf springs in place of the coil springs shown. This end casing is bolted to the mam turbine casing, and leakage between the two is prevented by a seal disposed in a groove 521 in the end casing.

Another view of a similar embodiment is shown in Fig. 6, which depicts a partial cross-sectional view through the end casing of a turbine. Shown is half of the end casing 601 and its cooperation with the turbine shaft 603. Towards the main turbine area are a pair of steam chambers 605 and between the end casing and the shaft are a plurality of interleaved labyrinth teeth 607 created by caulked- in seal strips. Towards the end of the casing are a pair of floating brash seals 609a and 609b. While the permanent interleaved labyrinth teeth created by the caulked-in seal strips will likely break and/or wear away during transients, especially significant transients such as during start-up and shut-down, the floating brash seals will not degrade in the same manner and should retain their sealing function through the transients.

The foregoing description is meant to be illustrative and not limiting. Various changes, modifications, and additions may become apparent to the skilled artisan upon a perusal of this specification, and such are meant to be within the scope and spirit of the invention as defined by the claims.

Claims

What is claimed is:

1. A turbine comprising a shaft in a casing and brash seal disposed cfrcumferentially around the shaft, the brash seal supported by a spring having a force constant approximately equal to the weight of the brash seal.

2. The turbine of claim 1, wherein the spring is a compression spring, a tension spring, or a combination thereof.

3. The turbine of claim 1 , wherein the spring is a leaf spring, a coil spring, or a combination thereof.

4. The combined labyrinth seal and brash seal of claim 1 , wherem a resilient member is disposed in the recess and is adapted to contact the brash seal and urge the brash seal away from the labyrinth seal.

5. The combined labyrinth seal and brash seal of claim 2, wherein the resilient member is a spring.

6. A combined labyrinth seal and brash seal, wherein the labyrinth seal comprises: a recess; the brush seal is disposed in said recess, not affixed thereto, and moveable therein; and a sacrificial shim disposed between the brash seal and the recess effective to maintain the brash seal in a desired position in the recess.

7. A gas or steam turbine or a compressor having a labyrinth seal, wherein the improvement comprises substituting said labyrinth with a combined labyrinth seal and brash seal, wherein the labyrinth seal comprises a recess and the brush seal is disposed therein, not affixed thereto, and moveable therein.

8. The improved gas or steam turbine or compressor of claim 5, wherein a resilient member is disposed in the recess and is adapted to contact the brash seal and urge the brash seal away from the labyrinth seal.

9. The improved gas or steam turbine or a compressor of claim 6, wherein the resilient member is a spring.

10. The improved gas or steam turbine or a compressor of claim 6, wherein the recess in the labyrinth seal comprises a plurality of resilient members.

11. The improved gas or steam turbine or a compressor of claim 8, wherein the recess in the labyrinth seal comprising a plurality of resilient members is provided in only a portion of the labyrinth seal.

12. The unproved gas or steam turbine or a compressor of claim 9, wherein the recess in the labyrinth seal comprising a plurality of resilient members is provided in a lower portion of the labyrinth seal.

13. A method for installing a seal into device which is a steam or gas turbine or a compressor, wherein the improvement comprises installing the combined labyrinth and brush seal defined by claim 4 and operating the device to sacrifice the shims.