GB2138892A

GB2138892A - Ribbed cover piece for steam turbine rotor bucket assembly

Info

Publication number: GB2138892A
Application number: GB08410399A
Authority: GB
Inventors: Kenneth Elmer Robbins; William James Sumner
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1983-04-25
Filing date: 1984-04-24
Publication date: 1984-10-31
Also published as: JPH068601B2; FR2544792B1; IT8420591A1; IT1176082B; DE3413628C2; GB8410399D0; IT8420591A0; JPS601303A; GB2138892B; FR2544792A1; CH666325A5; DE3413628A1

Abstract

The cover piece includes a single integral sealing rib 17 extending radially from its outer surface towards the surrounding housing. Each cover piece interconnects the outer tips 19 of adjacent rotor buckets 13, the adjacent ribs 17 being tangentially aligned with respect to each other to form, in one embodiment, a substantially continuous, radially extending, ring 21 about the turbine stage. <IMAGE>

Description

SPECIFICATION Ribbed cover piece for turbine rotor and bucket assembly Background of the Invention This invention relates generally to cover pieces for steam turbine buckets as part of an axial flow steam turbine rotor and bucket assembly, and more particularly to ribbed covers which provide sealing against steam flow around the radially extensive tip portions of the buckets.

Basically, energy is imparted to the rotor and bucket assembly of the steam turbine by a working fluid, commonly steam. The steam is vented into a generally cylindrical chamber defined by the inner shell of the turbine housing. A plurality of rotor buckets are circumferentially aligned on a shaft and affixed at an axial position thereon. The shaft is coaxially and rotatably mounted within the chamber. Large steam turbines usually include several sets of rotor buckets affixed to the shaft and each set is axially spaced apart on the rotor shaft. Each set or stage of the steam turbine extracts a part of the steam's energy by changing that energy into mechanical energy, as evidenced by the rotation of rotor buckets and shaft. The steam is admitted into the chamber at a desired axial location and flows at least in one axial direction through a working passage.The working passage is generally defined by the axially displaced stages of the turbine as well as the circumferential working area encompassed by the turbine blades in each stage.

When the steam is confined to that axial working passage, the turbine operates more efficiently than if the steam is not so confined to that working passage. A cover or cover piece connects the outer tip portions of a pair of buckets together. A plurality of covers, which correspond to the plurality of rotor buckets in the turbine stage, form a circumferential band around the radially extensive tip portions of the buckets. This circumferential band of covers prevents steam from escaping out of the axial working passage by limiting the radial flow of steam past the outer tip portions of the buckets. The rotor and bucket assembly must be relatively free to rotate within the turbine shell, therefore a radial clearance gap exists between the radially extensive tips of the rotor buckets or the outer surface of the covers, and the inner surface of the shell.

As stated above, the steam flows in at least one axial direction through several stages of rotor buckets. The buckets have increasingly large radii from a point near the admission of steam to the last stages of the turbine rotor which are proximate the exhaust conduit or hood of the turbine. In those last stages, the pressure drop across the rotor buckets is the greatest.

In the last stages of a low pressure steam turbine, the steam is saturated, therefore water droplets are apt to form in that section of the turbine. Generally, the water droplets are forced against the inner surface of the shell surrounding the turbine stage by centrifugal force and/or the pressure of the steam flow. Although the water droplets generally have a low absolute velocity, the relative velocity with respect to the radially outer tip portions of the blades is great. The relative velocity of the water droplets near the bucket tips in a low pressure turbine which has a last stage, active bucket length of 33 1/2 inches is approximately two thousand feet per second. The force at which a water droplet impacts a bucket blade is related to the size or mass of that particular droplet and the relative velocity of the droplet with respect to the bucket.Since the speed of the turbine is essentially established by other parameters, the potential problems caused by water droplets, such as erosion, lower torque, and loss of efficiency, can be minimized by providing a turbine rotor and bucket assembly which effectively limits the amount and size of water droplets in the axial working passage of the turbine. In addition, it has been noted that the steam which condenses in the last few stages of the turbine forms a water film which flows axially along the inner surface of the shell.

As stated earlier, the pressure drop across the last few stages of the turbine is greatest as compared with the other upstream stages.

Also, the pressure differential is higher near the radially outer portion of the rotating blades as compared with the root or radially inner portion of the blades. Therefore, the greater the radial clearance gap between the bucket tip covers and the inner-surface of the shell, the greater the loss of steam which flows axially around the outer radial tips of the buckets, and hence, the lower the efficiency of that particular stage of the turbine.

To minimize the loss of steam flow around the tip portions of the buckets, sealing strips have been placed on the inner surface of the turbine shell radially opposite the tip portions and covers in prior art apparatus. Generally, the sealing strips form a ring around the turbine stage and extend radially inward towards the bucket tip portions to narrow the radial clearance gap therebetween. The crosssectional geometry of the sealing strips, the number of strips utilized per stage and the axial placement of the strips on the inner surface of the shell is based upon the study of fluid mechanics in a steam turbine. The sealing strip(s) should be axially located such that the strip(s) are approximately opposite the steady state centerline of the rotating buckets.

The steady state centerline is the centerline of the buckets when the turbine is in normal operation at rated speed. However, since the rotor shaft, upon which the buckets are mounted, expands due to thermal reaction to the steam, the optimum axial placement of the sealing strip(s), i.e., at the steady state centerline, is not easily ascertained. Also, the axial position of the rotating blades changes during the operation of the turbine, especially when the turbine experiences transient changes in the mechanical load attached to it or changes in the steam supplied to the turbine. This movement presents a problem relating to the placement of the strips which should be radially opposite the rotating bucket tips.

Since the sealing strips project radially inward, the axially flowing film of water, which flows along the surface of the inner shell, is deflected towards the rotating tip portions of the rotor buckets and covers associated therewith. The water film leaves the inner surface of the shell in the form of water droplets.

These water droplets, in combination with the speed of the bucket tip portions, are detrimental to the apparatus as stated earlier. To limit the introduction of water droplets into the working passage of the turbine, a moisture removal slot is located immediately upstream of the sealing strips in prior art devices. The moisture removal slot allows a portion of the steam flow to escape as is required for the removal of the water film. Hence, although steam flow around the outer tip portions of the rotor buckets is minimized by the sealing strips, some steam flow is lost through the radial moisture removal slot.

One prior art device, disclosed in a U.S.

patent issued to Gross, No. 3,575,523, utilizes an airfoil radially extending from the cover's surface in combination with two inwardly extending flanges, one flange immediately upstream of the turbine stage and the other immediately downstream. The airfoil is an askewed rib radially extending above the cover's outer surface. The airfoil, in combination with the flanges, is effective to pump the steam flow trapped therebetween back into the working passage of the turbine. A radial moisture removal slot, immediately upstream of the first flange, is required if the water flowing axially along the inner surface of the shell is to be prevented from entering the working passage. Therefore, some steam flow is sacrificed.Notwithstanding the presence of the flanges, the pumping action developed by the askewed airfoils would disturb the smooth stream lines of the main flow of steam through the axial working passage such that the efficiency of the turbine stage would decline. This loss of efficiency is a result of the reversed flow of steam caused by the pumping action of the airfoil. Also, the pumping action may effect the smooth flow of water along the inner surface such that the water may leave the surface and impact the turbine buckets.

Other prior art devices have utilized common labyrinth seals in the radial clearance gap. The covers, in these latter devices, include several radially extending ribs which cooperate with inwardly projecting flanges on the inner surface of the shell and provide a labyrinth seal for the turbine stage. As stated earlier, the inwardly projecting flanges would require a radial moisture removal slot immediately upstream of the flanges.

Objects of the Invention It is an object of this invention to provide a more efficient means for circumferentially sealing the radial clearance gap between the portions of the rotor buckets outer tip or the covers associated therewith and the inner surface of the turbine shell.

It is a further object of the invention to provide for a more efficient operation of the stages of the turbine by eliminating the moisture removal slot and therefore conserving the steam flow through those stages.

It is an additional object of the present invention to provide a ribbed cover or cover piece with a single rib which extends radially outward from the tip portions of the bucket assembly thereby eliminating the sealing strip and the moisture removal slot and minimizing potential axial misalignment which may have occurred between the sealing strip and the tips or covers of the bucket assembly.

Still further objects and advantages of the invention will be apparent from an understanding of the principles taught by the operation of the invention. The latter understanding will be gained from the following detailed description of a preferred form of the invention.

Summary of the Invention In a preferred embodiment, a steam turbine includes a rotor and bucket assembly wherein a plurality of rotor buckets is circumferentially aligned and affixed to a shaft. A plurality of covers is associated with the outer radial tip portions of the buckets such that each cover connects a pair of adjacent bucket tips together. Ribs extend radially outward from the circumferential or radially outer surface of the covers. The plurality of ribs, corresponding to the covers, are tangentially aligned with respect to each other. The ribs' radially extensive edges are in close proximity with the inner surface of a shell, which surrounds the rotor and bucket assembly, and present the only impediment to the flow of steam through the radial clearance gap between the outer surface of the cover and the inner surface of the shell. The ribs are effective to form a substantially continuous, radially extending ring which provides a seal between the sets of the rotor buckets.

Brief Description of the Drawings While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as the invention, the invention will be better under stood from the following description taken in connection with the accompanying drawings in which:: Figure 1 illustrates a cutaway portion of a radial side view of an apparatus constructed in accordance with the teachings of the prior art and includes a spill strip and a radial moisture removal slot; Figure 2 illustrates a cutaway portion-of a side view of an apparatus constructed in accordance with the principles of this invention, and includes a ribbed cover; Figure 3 illustrates a partial axial view of one turbine stage which includes a number of buckets and the covers associated with those buckets; Figure 4 shows a radial top inward view of a partial section of the buckets, covers and ribs; Figure 5 illustrates a radial cross-sectional view of three ribs; and, Figure 6 illustrates an alternative embodi ment wherein the ribs do not extend over the outer radial tip portions of the buckets.

Detailed Description of the Drawings Figure 1 generally illustrates an apparatus constructed in accordance with the principles taught by the prior art. The flow of steam is designated by an arrow in both Figures 1 and 2. U.S. Patent No. 4,335,600, issued to Wu et al., illustrates a cutaway view of a steam turbine as Figure 1 therein, and such disclosure is incorporated herein by reference thereto. Only a partial cutaway, radial side view is shown in Figures 1 and 2, but it is to be understood that the turbine includes a rotor and bucket assembly of which only the radially outer portion is illustrated herein.A better understanding of the turbine stage can be gained by viewing Figure 3 which illustrates a rotor 11 with buckets 1 3 affixed to a rotor shaft 1 5. Figure 3 is a partial axial view of a segment of the turbine stage which extends 360" around rotor shaft 1 5. As used herein, this latter view represents the front view of the turbine stage, therefore all drawings herein are referenced from this perspective. The conventional turbine comprises a number of stages, but it is to be understood that although only one stage is illustrated herein, the principles of this invention could be applied to most stages of the turbine.

In Figure 1, the stage, which includes bucket 12, is surrounded by a coaxial shell 14. A nozzle partition 10 is upstream of a bucket 1 2 and is part of the turbine stage.

Nozzle partition 10 directs the flow of steam onto the blade of a bucket 1 2. Shell 14 has a radially inner surface 16 and a radial moisture removal slot 1 8 therethrough. The slot removes the water film which flows axially along surface 1 6 before the water is deflected by a sealing strip 20 towards tho rotating bucket 1 2. As stated earlier, sealing strip 20 is effective to liniit the flow of steam axially around the radially extensive tip portions of bucket 1 2 through radial clearance gap 22 but would deflect water droplets onto the high velocity tip portions of bucket 1 2 if slot 18 was not immediately upstream of the strip.It should be noted that clearance gap 22 is the radial distance between sealing strip 20 and the tip portions of the plurality of buckets of which bucket 1 2 is a member. Gap 22 circumferentially and coaxially surrounds the rotor and bucket assembly. Although sealing strip 20 assists in reducing steam flow through clearance space 22, some steam flow escapes with the condensate of the steam through removal slot 1 8. Slot 18 is necessary because it removes the water film before the flow is deflected into the axial working passage of the turbine, and consequently limits the erosion of the buckets in that stage and allows the rotor and buckets to rotate as freely as possible.

Figure 2 is substantially the same view, i.e., the cutaway, side view of a turbine stage as shown in Figure 1. However, Figure 2 illustrates an apparatus constructed in accordance with the principles of the present invention. A nozzle partition 30, upstream from a bucket 32, directs the steam onto the buckets of the stage of which bucket 32 is a part. A shell 34 coaxially surrounds the rotor and bucket assembly and has an inner surface 35. To limit the flow of steam around the radially extensive tip portions of bucket 32, only one rib 36 extends radially outward from the radially outer surface of a cover (the cover is not visible from the viewpoint of Figure 2). The radial extension of the rib is illustrated in Figure 3 wherein rib 1 7 extends beyond the radially extensive portion 1 9 of bucket 1 3.

Returning to Figure 2, the radially extensive edge of rib 36 is in close proximity with surface 35. A radial-clearance gap 38 has substantially the same dimensions as clearance gap 22 illustrated in Figure 1. By way of example, the dimension of the radial clearance gap is on the order of three tenths of an inch for the last stage of a low-pressure turbine having an active bucket length of 33 1/2 inches.

During the operation of the last stage of a low pressure turbine, the film of water which axially flows over surface 35 in Figure 2, is virtually unimpeded because of the elimination of the sealing strip and since the only impediment to the flow of steam through the gap is the ribs which extend radially beyond the outer surface of the cover. Gap 38 is large enough to pass the normal flow of water along surface 35 during the normal operation of the turbine stage. Ultimately, in both the structures illustrated in Figures 1 and 2, the condensate of the steam is directed by some means to the interior space of the exhaust hood of the turbine, which is not illustrated in the Figures, but is downstream of the last turbine stage, i.e., towards the right of Figures 1 and 2.

Figure 4 illustrates a radial top view of a pair of buckets 40, 42, connected together at their respective outer radial tips by a cover 44. A detailed description of cover 44, its relationship with the tips of the buckets and the operating characteristics of that cover with respect to the turbine as a whole can be found in a United States Patent No.

3,302,925, by V.S. Musick, the disclosure in that patent is incorporated herein by reference thereto.

Cover 44 has a rib 46 extending from its radially outer surface 45. The rib is similar to rib 36 and rib 1 7 as illustrated in Figures 2 and 3, respectively. Rib 46 extends radially outward from the circumferential surface defined by the plurality of covers which connect a corresponding plurality of bucket tips together in one stage of a turbine. As illustrated in Figure 4, rib 46 is tangentially aligned with a rib 48 on an adjacent cover 50. Similarly, rib 46 is tangentially aligned with a rib 52 which is a part of an adjacent cover 54.

In a preferred embodiment of this invention, the leading end 60 of rib 46 is in close proximity to the trailing end 62 of rib 48. The leading and trailing designations relate to the direction of rotation as shown by an arrow in Figure 4. In a similar fashion, the trailing end of rib 46 is in close proximity to the leading end of rib 52.

Rib 46, in combination with ribs 52, 48 and the other ribs corresponding to the plurality of covers, form a substantially continuous, radially extending ring effective to provide a seal between the respective stages of the turbine. The seal is unique because the water film on the inner surface of the shell is substantially not disturbed during its flow towards the exhaust hood. The ribs form a substantially circumferential ring 21, as illustrated in Figure 3, around the turbine stage just as the radially outer surfaces of-the covers form a circumferential band or surface about the stage.When a ribbed cover is used on the last stages of a low pressure turbine unit, it is not necessary to remove the film of condensate which accumulates and axially flows along the inner surface of the turbine shell, since the rib is the only impediment to the flow of steam through the radial clearance gap. Hence, the moisture removal slot is unnecessary and therefore can be eliminated.

Since the dimensions of radial clearance gap 38 are similar to the dimensions of radial clearance gap 22 of prior art assemblies, (shown in Figures 2 and 1 respectively), an improvement in the efficiency of the turbine stage is noted by the estimated savings of .6% of the total steam flow through the stage.

The .6% represents the estimated loss of steam flow which passes through radial moisture removal slot 18 (illustrated in Figure 1).

The conservation of .68 of the steam flow increases the efficiency of the turbine stage and turbine as a whole.

In its preferred embodiment, rib 46 is an integral part of cover 44. Since the buckets in this turbine stage may expand radially due to thermal expansion or move radially due to mechanical reactions sometimes experienced during the operation of the turbine, the rib may be made of a relatively abradible material which will "rub off" if the rotor and bucket assembly should experience a slight deviation in rotation from the normal axis and "rub" against the shell. In this view, the ribs could be of an abradable material as compared to the shell's material. As stated earlier, the centerline of the turbine stage moves during operation due to thermal expansion of the rotor. The single ribbed cover apparatus described herein is not influenced by the axial movement of the stage's centerline.Also, the plurality of ribs is effective to provide a seal for any turbine stage which has water flowing along the inner surface of the shell surrounding that turbine stage.

Figure 5 illustrates several cross-sectional views of a rib constructed in accordance with the principles of this invention.

The geometric configuration of the rib is an important consideration because the amount of steam flow through the radial clearance gap is related to that rib's configuration. The radially extensive edge of the rib is preferably relatively narrow as compared with the base of the rib proximate the cover. Other features relate to the height of the rib versus the width of its base, the height versus the steady state radial clearance gap distance, and the width of the rib's radially extensive edge versus the steady state radial clearance gap distance.

Ratios of 2.0, 1.7 and 0.10, respectively, have been theoretically proposed for the optimum performance of a rib as a sealing means in a turbine with 33 1/2 inch active bucket length. When the turbine stage is in operation, these geometric features of a single rib force the steam flowing through the radial clearance gap into a smaller radial space than physically present. This phenomenon is part of the vena-contracta theory and is relatively well known in the fluid mechanics art. The single rib decreases the flow of steam through the radial clearance gap.The cross-sectional configurations of a rib which performs opti mally are based upon the study of fluid flow through an orifice and other sealing devices in accordance with the principles of fluid mechanics, therefore the ratios presented above are only indicative of a range of values, To be specific, the height versus the base width may vary with other structural parameters of the buckets themselves but herein a ratio between 1.7 and 2.0 was found to be adequate. The height versus the gap distance may be greater than or equal to 1.7 with 2.0 a nominal goal, and the edge width versus the gap distance may be less than or equal to 0.1. A single rib extending above each cover is important because a greater number of ribs may not conserve as much steam flow through the radial clearance gap as only one rib per cover.Also a single rib which does not radially extend beyond the outer radial tip portions of the buckets does not conserve the flow of steam as described herein.

Three radial cross sectional views of ribs which may be utilized in accordance with the principles of this invention are illustrated in Figure 5. The illustrated ribs are not the only ribs which could be constructed in accordance with the principles described above, but the examples are illustrative of the type of rib which operates efficiently in the environment described herein. Ribs 65a, b and c extend above the outer radial cover surfaces 64a, b and c, respectively, as illustrated in Figures 5a, b and c. The flow of steam is shown by the arrow in Figure 5 and is representative of the direction of flow in Figures 5a, b and c. In Figure 5a, rib 65a has a trapezoidal crosssectional configuration with a downstream face being angled approximately 40 from a horizontal reference plane.It has been found that declination angles exceeding 40e are favorable in this exemplary setting. Figure 5b illustrates rib 65b as including a relatively wide, cross-sectional base proximate to surface 64b which progressively narrows from that relatively wide base to its radially extensive edge. The top edges of ribs 65a, b and c are truncated. Rib 65c, illustrated in Figure 5c, has a relatively straight wall surface normal to the direction of the flow of steam, a truncated radially extensive edge and a relatively wide base proximate surface 64c, therefore its cross-sectional view narrows relatively progressively from its base to its radially extensive edge. It should be noted that these three cross-sectional configurations of a rib are not the only ones which apply the principles taught by this invention.A person of ordinary skill in the art could detail many different shapes and configurations of a rib which radially extends from the outer surface of a cover and operates as described hereinabove.

Figure 6 shows an alternative embodiment of the present invention. A cover 70 connects the tip of a rotor bucket 72 to the tip of rotor bucket 74. A cover 76 and cover 77 connect adjacent buckets to buckets 74 and 72, respectively. A radially extending rib 78 projects above the outer surface of cover 70 and is tangentially aligned with respect to rib 8a which is integral with cover 76 and with respect to rib 81 which is an integral part of cover 77. As shown in Figure 6, the trailing end of rib 80, is not proximate the leading end of rib 78, i.e., the rib's trailing end terminates proximate the outer tip portion of the rotor bucket. A space 82 separates the trailing end of rib 80 from the leading end of rib 78.Hence, rib 78 does not project over the tip portion of bucket 74 but terminates proximate thereto and rib 80 similarly terminates proximate the tip portion of adjacent bucket 74. A similar space may be present between corresponding ribs on adjacent covers as illustrated. Steam flow around the radially extensive tip portion of the rotor buckets and through the space is relatively small in this alternative embodiment because space 82 and similar spaces along the outer circumference of the stage comprise a relatively small part of the substantially continuous, radially extending ring formed by the plurality of ribs associated with the plurality of covers in the turbine stage. The steam flow through space 82 is substantially limited when the rotor and bucket assembly is in operation.

This invention may be utilized with covers which are connected to the buckets by laterally extending tenons which mate with lateral holes in outer tips of the buckets, i.e., the specific covers illustrated herein. The covers illustrated herein are typically called side entry covers and are clearly described in U.S. Patent No. 3,302,925, incorporated herein as previously noted. Other types of covers, may also utilize a rib as described herein. This invention may also be practiced by connecting a limited number of buckets together in a group in one stage of a turbine yet not connecting all the grouped buckets together. Although there are breaks or gaps in the relatively continuous radially extending ring formed by the ribs, the breaks rotate at such a rate that axial steam flow through the breaks are relatively minimal. This invention may be practiced by having the ribs and covers only associated with the buckets. In this manner, the covers and ribs form an integral part of the buckets.

An apparatus constructed in accordance with the principles of this invention is useful in retrofitting large steam-turbines because the covers and ribs specifically detailed in this specification can be added to the stages of a low pressure turbine wherein the steam is saturated and a moisture removal slot is not feasibly installed in the existing turbine shell.

The appended claims are intended to cover all the above modifications and other apparatus which fall within the scope of the present invention.

Claims

1. In a steam turbine, a rotor and bucket assembly-surrounded by a shell, said assembly including a plurality of rotor buckets circumferentially aligned and affixed to a rotor and a corresponding plurality of bucket covers at the radially outer tips thereof, each said cover connecting the outer radial tips of a pair of adjacent buckets together, each of said covers comprising: only one rib extending radially outward from the radially outer surface of each of said covers and extending beyond said radially outer tips of said buckets, the radially extensive edge of said rib being in close proximity with the inner surface of said shell, said rib being the only impediment to the flow of steam through the radial clearance gap between said outer surfaces and said inner surface of said shell, and said rib being tangentially aligned with respect to adjacent ribs on adjacent covers; and said rib, in combination with the ribs on said plurality of covers, forming a substantially continuous, radially extending ring effective to provide a seal between the respective stages of said turbine.

2. A rotor and bucket assembly as recited in claim 1 wherein said rib is integral with its corresponding cover.

3. A rotor and bucket assembly as recited in claim 1 or 2 wherein said rib consists of an abradible material.

4. A rotor bucket assembly as recited in claim 1,2 or 3 wherein said rib has a wide, cross-sectional base portion proximate to said radially outer surface of its corresponding cover and said rib's cross-section progressively narrows from said base portion to said radially extensive edge.

5. In a steam turbine, a rotor and bucket assembly surrounded by a shell, said assembly comprising: a rotor shaft; at least one set of plurality of rotor buckets affixed to said shaft and circumferentially aligned thereon; a plurality of covers, each cover effective to connect the outer radial tip portions of a pair of adjacent buckets together;; each cover having only one integral rib which extends radially outward from its radially outer surface and extends beyond said radially outer tips of said buckets, each rib being tangentially aligned with respect to adjacent ribs on adjacent covers, and the radially extensive edges of the plurality of ribs corresponding to said plurality of covers being in close proximity with the inner surface of said shell, said plurality of ribs being the only impediment to the flow of steam through the radial clearance gap between said outer surface of said covers and said inner surface of said shell, and said plurality of ribs effective to form a seal between the sets of said rotor buckets when said assembly is in operation.

6. A rotor and bucket assembly as recited in claim 5 wherein said rib has a wide, crosssectional base portion proximate to said radially outer surface of its corresponding cover and said rib's cross-section progressively narrows from said base portion to said radially extensive edge.

7. In a steam turbine, a rotor and bucket assembly surrounded by a shell, said assembly comprising: a rotor shaft; at least one set of plurality of rotor buckets affixed to said shaft and circumferentially aligned thereon; a plurality of covers, each said cover associated with the outer radial tip portions of a corresponding bucket; a plurality of ribs, each rib being integral with a corresponding cover and extending radially outward from the radially outer surface of said corresponding cover, each rib being tangentially aligned with respect to adjacent ribs on adjacent covers, and the radially extensive edges of said plurality of ribs being in close proximity with the radially inner surface of said shell, and said plurality of ribs effective to form a substantially continuous, radially extending ring which provides a seal between the sets of said rotor buckets.

8. A rotor and bucket assembly as recited in claim 7 wherein said rib has a wide, crosssectional base portion proximate to said radially outer surface of its corresponding cover and said rib's cross-section progressively narrows from said base portion to said radially extensive edge.

9. In a steam turbine, a rotor and bucket assembly surrounded by a shell, said assembly comprising: a rotor shaft; at least one set of plurality of rotor buckets affixed to said shaft and circumferentially aligned at an axial position thereon, the outer radial tip portions of each of said buckets having at least two lateral holes therethrough; a plurality of covers, each cover having at least a pair of oppositely extending lateral tenons, and each cover effective to connect said outer radial tip portions of a pair of adjacent buckets together by matingly joining each laterally extending tenon with a corresponding lateral hole in said pair of buckets;; each cover having only one integral rib which extends radially outward from its radially outer surface and extends beyond said radially outer tips of said buckets, each of said ribs being tangentially aligned with respect to adjacent ribs on adjacent covers, and the radially extensive edges of the plurality of ribs corresponding to said plurality of covers being in close proximity with the inner surface of said shell, said ribs effective to form a substantially continuous, radially extending ring which provides a seal between the sets of said rotor buckets when in operation.

10. In a steam turbine, a rotor and bucket assembly surrounded by a shell, said assembly comprising: a rotor shaft; at least one set of plurality of rotor buckets affixed to said shaft and circumferentially aligned thereon; a plurality of covers, each cover effective to connect the outer radial tip portions of a pair of adjacent buckets together;; each cover having only one integral rib which extends radially outward from its radially outer surface and extends beyond said radially outer tips of said buckets, each rib being tangentially aligned with respect to adjacent ribs on adjacent covers, and the radially extensive edges of the plurality of ribs corresponding to said plurality of covers being in close proximity with the inner surface of said shell, each of said ribs having a leading end which terminates proximate to the outer radial tip portions of one of said pair of buckets and each rib having a trailing end which terminates proximate to the outer radial tip portions of the other bucket of said pair of buckets, said plurality of ribs being the only impediment to the flow of steam through the radial clearance gap between said outer surface of said covers and said inner surface of said shell, and said plurality of ribs effective to form a seal between the sets of said rotor buckets when said assembly is in operation.

11. A rotor and bucket assembly as recited in claim 10 wherein said rib has a wide, cross-sectional base portion proximate to said radially outer surface of its corresponding cover and said rib's cross-section progressively narrows from said base portion to said radially extensive edge.

12. A rotor and bucket assembly substantially as herein described with reference to and as shown in Figures 2 to 5a or Figure 5b or Figure Sc or Figure k of the accompanying drawings.

1 3. A steam turbine including an assembly as claimed in any preceding claim.