AU720250B2

AU720250B2 - Sealing device for axial flow turbine

Info

Publication number: AU720250B2
Application number: AU18633/99A
Authority: AU
Inventors: Manabu Komiyama; Minoru Matsuda; Toshio Suzuki
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1998-03-06
Filing date: 1999-03-05
Publication date: 2000-05-25
Anticipated expiration: 2019-03-05
Also published as: CN1228505A; AU1863399A; CN1160509C; KR100309970B1; CA2264996A1; JPH11257014A; KR19990077657A; CA2264996C

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): Kabushiki Kaisha Toshiba ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

INVENTION TITLE: Sealing device for axial flow turbine The following statement is a full description of this invention, including the best method of performing it known to me/us:la BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a sealing device for reducing the leakage of working fluid through the gap between the rotor and a sealing ring attached to the inner ring of the nozzle diaphragm of the stationary member of a stage of an axial flow turbine.

Description of the Related Art Generally, an axial flow turbine, such as a steam turbine or a gas turbine, has stationary nozzles and a rotor. The nozzles receive a working fluid, such as steam, from the high-pressure upstream side and makes the working fluid expand in the low-pressure downstream side to convert heat energy into mechanical energy. The rotor is driven by the mechanical energy for rotation at a high rotating speed to generate rotational energy, and a power generator is driven by the rotational energy to generate power.

In the stage of the axial flow turbine, the working fluid leaks through the gap between the rotating part and the stationary part when the same flows through the passage of the stage and bypasses the nozzles and passages between the moving blades, which reducing the efficiency of the axial flow turbine. A 25 leakage loss resulting from the leakage of the working fluid through the gap between the rotor and the inner circumference of the inner ring of the nozzle diaphragm is one of the causes reducing the efficiency of the axial flow turbine. A sealing ring fixedly provided with sealing fins is fitted in the inner ring of the S• 30 nozzle diaphragm to reduce the leakage loss. The sealing ring reduces the gap to suppress the leakage of the working fluid through the gap. Although it is preferable to reduce the gap to a minimum to suppress the leakage, the sealing fins come into contact with the rotor and cause an excessive vibration of the turbine shaft if the gap is excessively small. Therefore, a relatively big gap is formed to avoid contact between the rotor and the sealing fins. The turbine shaft is liable to vibrate excessively when the turbine is in a starting operation, when load on the turbine changes and when the turbine is in a stopping operation.

Therefore, the rotor comes into contact with the sealing fins, the edges of the sealing fins are abraded, the size of the gap increases and, consequently, leakage loss increases to reduce the efficiency of the turbine.

Fig. 6 shows a sealing device for an axial flow turbine as a first example of the related art. A stage of the axial flow turbine has a nozzle diaphragm 30 having an outer ring 2 fixed to a casing 1 and an inner ring 3, a plurality of nozzles 4 formed by a cascade, a rotor 5 supported for rotation in a central region of the casing 1, a wheel 6 formed on the rotor 5, and moving blades 7 attached to the wheel 6. A groove 8 is formed in the inner surface of the inner ring 3 of the nozzle diaphragm 30, and a sealing ring 9 consisting of a plurality of segments 9a is fitted in the groove 8. Sealing fins 10 are fixed to the inner surface of the sealing ring 9 so that a gap a of the least allowable size is formed between the rotor 5 and the sealing fins 10. In this stage of the axial flow turbine, the size of the gap a between the rotor 5 and the sealing fins 10 is small to allow only a small leakage of the working fluid b through the gap a. Since the size of the gap a is small, the rotating rotor 5 comes into contact with the sealing fins 10 toabrade the edges of the sealing fins 10 and, consequently, the size of the gap a is increased, and the leakage of the 25 working fluid increases to reduce the efficiency of the stage. The contact between the rotor 5 and the sealing fins 10 causes an excessive vibration of the rotor 5, which makes the stable operation :of the axial flow turbine impossible. Therefore, other sealing devices shown in Figs. 7 and 8 as a second and a third examples of S" 30 the related art, respectively, have been employed to prevent the reduction of the efficiency of the axial flow turbine and the excessive vibration of the rotor due to the abrasion of the sealing fins.

Referring to Fig. 7, the sealing device employed in a turbine is provided with bellows 11 attached to the outer surface of a plurality of segments 9a of a sealing ring 9, and the bellows 11 open into pressure holes 12 formed in an inner ring 3 of a nozzle 3 diaphragm 30. The gap a between a rotor 5 and sealing fins is increased when the axial flow turbine is in a starting operation, when the load on the axial flow turbine changes and when the axial flow turbine is in a stopping operation. While the turbine is operating under the rated load, a high-pressure fluid is supplied through the pressure holes 12 into the bellows 11 to shift the plurality of segments 9a of the sealing ring 9 radially inward, whereby the size of the gap a is reduced.

Referring to Figs. 8A and 8B, the sealing device employed in a turbine has a sealing ring 9 consisting of a plurality of segments 9a. Holes 13 are formed in the end surfaces of the segments 9a of the sealing ring 9 so as to extend perpendicularly to the corresponding end surfaces, and a spring 14 is inserted in the holes 13 of the adjacent ends of the segments 9a of the sealing ring 9.

Fig. 8A shows a state where the operating speed of the axial flow turbine has been increased to a normal operating speed and the axial flow turbine is operating under a load between a middle load and a rated load. Since the pressure on the upstream side of the sealing ring 9 and the pressure on the downstream side of the same are different from each other, an upstream pressure Fa on the upstream side of the sealing ring 9 acts on the outer circumference of the sealing ring 9, and a downstream pressure Fb lower than the upstream pressure Fa 25 acts on the inner circumference of the sealing ring 9 provided with sealing fins 10. Consequently, the segments 9a of the sealing ring 9 are shifted radially inward by the pressure difference and the size of the gap a is reduced to a minimum.

When the axial flow turbine is in a stopping operation, the S: 30 springs 14 bias the adjacent segments 9a of the sealing ring 9 away from each other as indicated by the arrows c in Fig. 8B, so that the segments 9a of the sealing ring 9 are shifted radially outward to increase the size of the gap a between the rotor 5 and the fins 10 of the sealing ring 9 as shown in Fig. 8A. Since the segments 9a forming a lower half of the sealing ring 9 tend to move downward by gravity, those segments 9a of the sealing ring 9 are supported by gravity springs In the sealing device shown in Fig. 7, the bellows 11 are liable to be broken by repeated stress induced therein by the frequent start and stop of the turbine or by the frequent variation of the load on the turbine. Therefore, the bellows 11 must have a high strength, and the bellows 11 having a high strength increases the size of the sealing device.

In the sealing device shown in Figs. 8A and 8B, when the segments 9a of the sealing ring 9 move radially inward during the operations of increase and decrease of the load on the turbine while the turbine is operating under a low-load condition, the segments 9a of the sealing ring 9 are liable to move irregularly because the segments 9a of the sealing ring 9 are not held fixedly and, consequently, unnecessary repeated load and high-temperature creep load act on the springs 14. Therefore, the axial flow turbine cannot operate for a long time under a low-load condition. Since the segments 9a of the sealing ring 9 tend to move downward by gravity, the segments 9a forming the lower half of the sealing ring 9 are supported by the gravity springs 15. Since the gravity springs 15 pushes the segments 9a of the sealing ring 9 vertically upward while the same segments 9a of the sealing ring 9 move radially inward, the segments .9a of the sealing ring 9 do not move simultaneously and are liable to move irregularly. Those problems in the related art must be solved.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a highly reliable sealing device for an axial flow turbine having a rotor and a sealing ring provided with sealing fins, ca- 30 pable of preventing the excessive vibration of the rotor attributable the contact between the rotor and the sealing fins, of suppressing the leakage of the working fluid, and of functioning properly even if the axial flow turbine is operated for a long time under a low-load condition.

According to one aspect of the present invention, a sealing device for preventing the leakage of working fluid in an axial flow turbine having a rotor with a shaft, and a nozzle diaphragm with an inner ring disposed so as to surround the shaft of the rotor comprises: a sealing ring formed of a plurality of segments around the shaft of the rotor, each of the segments having a part adapted to be fitted in a holding groove such that each of the segments can radially move relative to the rotor, a first pressing means for pressing the segments of the sealing ring radially inward; and a second pressing means for pressing the segments of the sealing ring radially outward.

Since the segments of the sealing ring are pressed radially inward and radially outward with respect to the rotor by the first and the second pressing means, the segments of the sealing ring do not move irregularly or vibrate and do move smoothly in radial directions with respect to the rotor even if the pressure difference between the radially outer and the radially inner surfaces of the segments of the sealing ring varies with the variation of the load on the axial flow turbine.

Preferably, respective pressing forces of the first and the second pressing means are adjusted so that the segments of the sealing ring are shifted radially inward with respect to the rotor by the pressure difference between the radially outer and the radially inner surfaces of the segments of the sealing ring while the axial flow turbine is in rated operation, and the segments of the sealing ring are shifted radially outward with respect to the rotor by the pressure difference between the radially outer and the ra- 25 dially inner surfaces of the segments of the sealing ring while the axial flow turbine is in operation under a low load.

Since the segments of the sealing ring are shifted toward the rotor while the axial flow turbine is in operation under rated conditions, and the same are shifted away from the rotor while 30 the axial flow turbine is in operation under a low load, the gap between the inner ring of the nozzle diaphragm and the rotor can satisfactorily be sealed while the axial flow turbine in operation tunder rated conditions, and the vibration of the rotor due to contact between the sealing ring and the rotor can be avoided while the axial flow turbine is in operation under a low load.

Preferably, the first pressing means includes a plate spring which is interposed between a bottom surface of the holding groove and an outer surface of the segment of the sealing ring.

Preferably, the segment of the sealing ring is provided in its end surface with a groove extending between an upstream side with respect to the flowing direction of the working fluid and a downstream side with respect to the flowing direction of the working fluid, the holding groove has a brim, and the second pressing means includes a curved plate spring which rests on the through groove and the brim.

Preferably, the curved plate spring extends between the grooves of the adjacent segments of the sealing ring.

Preferably, the segment of the sealing ring is provided in its middle part with a through hole extending between an upstream side with respect to the flowing direction of the working fluid and a downstream side with respect to the flowing direction of the working fluid, the holding groove has a brim, and the second pressing means includes a curved spring which is inserted in the through hole and rests on the through hole and the brim.

Preferably, the rotor has a horizontal axis, and the respective pressing forces of the first and the second pressing means are adjusted so as to compensate the differences in pressing forces acting on the segments of the sealing ring attributable to the respective own weights of the segments of the sealing ring.

25 Preferably, the rotor has a horizontal axis, and only the segments forming a lower half of the sealing ring are pressed by the first sealing means so as to compensate the difference in pressing forces acting on the segments of the sealing ring attributable to the respective own weights of the segments of the 30 sealing ring.

Preferably, the rotor has a horizontal axis, and the respective pressing forces of the plate springs pressing the seg- *i *"*ments forming an upper half of the sealing ring and the segments forming the lower half of the sealing ring are adjusted so as to compensate the differences in pressing forces acting on the segments of the sealing ring attributable to the respective own weights of the segments of the sealing ring.

Preferably, the second pressing means includes normally flat plate-shaped member which curves gradually when its temperature rises, the plate-shaped member is received in a groove formed in an end surface of the segment so as to extend between an upstream side with respect to the flowing direction of the working fluid and a downstream side with respect to the flowing direction of the working fluid, or inserted in a through hole formed in a middle part of the segment of the sealing ring so as to extend between an upstream side with respect to the flowing direction of the working fluid and a downstream side with respect to the flowing direction of the working fluid, and the plate-shaped member curves as the temperature thereof rises while the axial flow turbine is in operation under a low load to shift the segment of the sealing ring radially outward with respect to the rotor.

Preferably, the plate-shaped member is formed of a pair of plate-shaped, elastic members differing from each other in quality.

Preferably, the plate-shaped member consists of a member of an expandable material and a member of a shape memory alloy.

Preferably, the segment of the sealing ring has an outer surface, the holding groove has an inner surface, the maximum size of a radial gap between the outer surface of the segment of the sealing ring and the inner surface of the holding groove is equal to a radial distance by which the segment of the sealing ring moves radially when the load on the axial flow turbine in- S. creases or decreases.

.9Preferably, surfaces of the inner ring of the nozzle diaphragm with which the segments of the sealing ring come into 30 contact are coated with a corrosion-resistant coating or are finished by a surface hardening treatment.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: Fig. 1A is a schematic sectional view of a sealing device in a first embodiment according to the present invention for preventing the leakage of a working fluid in an axial flow turbine; Fig. 1B is a fragmentary front view of a sealing ring employed in the axial flow turbine; Fig. 1C is a schematic perspective view of a segment of the sealing ring; Figs. 2A and 2B are schematic sectional views of the sealing device shown in Fig. 1A of assistance in explaining the radial shifting of segments forming the sealing ring; Fig. 2C is a diagram of assistance in explaining the radial shifting of the segments forming the sealing ring; Fig. 3A is a schematic front view of the sealing device shown in Fig. 1A; Fig. 3B is a perspective view of a flat plate spring included in the sealing device shown in Fig. 1A; Fig. 3C is a perspective view of a curved plate spring included in the sealing device shown in Fig. 1A; Fig. 4A is a schematic sectional view of a sealing device in a second embodiment according to the present invention for preventing the leakage of a working fluid in an axial flow turbine; Fig. 4B is a fragmentary front view of a sealing ring employed in the axial flow turbine; 4C is a schematic perspective view of a segment 25 forming the sealing ring; Figs. 5A and 5B are schematic sectional views of a sealing device in a third embodiment according to the present invention; Fig. 5C is a diagram of assistance in explaining the radial shift of segments forming a sealing ring; 30 Fig. 6 is a schematic sectional view of a sealing device for preventing the leakage of a working fluid in an axial flow turbine, as a first example of the related art; Fig. 7 is a schematic sectional view of a sealing device for preventing the leakage of a working fluid in an axial flow turbine, as a second example of the related art; and Figs. 8A and 8B are a schematic sectional view and a schematic front view, respectively, of a sealing device for preventing the leakage of a working fluid in an axial flow turbine, as a third example of the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A sealing device in a first embodiment according to the present invention will be described hereinafter. Referring to Fig.

1A, an inner ring 3 of a nozzle diaphragm 30 is provided in its inner circumference with a holding groove 8, and a plurality of segments 9a of a sealing ring 9 are arranged in a circumferential arrangement with their outer walls received in the holding groove 8. Sealing fins 10 are attached fixedly to the inner surfaces of the segments 9a facing a rotor 5 included in an axial flow turbine.

A gap a is formed between the circumferential surface of the rotor and the sealing fins 10. A working fluid leaks through the gap a as indicated by the arrow b. Each segment 9a of the sealing ring 9 is provided with an axial through groove 16 and a curved plate spring 17 is inserted in the axial through groove 16. The curved plate spring 17 has opposite ends resting on an upstream brim 18 and a down stream brim 19, respectively, of the holding groove 8 to press the segment 9a radially outward. The curved plate spring 17 is in contact with the upstream brim 18 of the holding groove 8 of the inner ring 3 of the nozzle diaphragm the outer circumferential surface of the axial through groove 16 S. and the downstream brim 19 of the holding groove 8. The curved plate spring 17 is convex radially outward so as to be in contact with the outer circumferential surface of the groove 16 to bias the segment 9a of the sealing ring 9 radially outward. Flat plate springs 20 are placed between the outer circumferences of the segment 9a of the sealing ring 9 and the bottom surface of the holding groove 8 so as to bias the segments 9a of the sealing ring 9 radially inward, toward the shaft 5a of the rotor 5. As shown in Fig. 1B, the axial through grooves 16 are formed in opposite end parts 21a and 21b of outer parts of the adjacent segments 9a of the sealing ring 9 so as to extend between the upstream side and the downstream side of the segments 9a. As shown in Fig. 1A, the curved plate spring 17 is inserted in the axial through groove 16.

As shown in Fig. 2A, the size a 1 of the gap a is relatively large while the axial flow turbine is stopped or the same is in operation under a low load. In this state, the pressure difference Fr between a pressure acting on the outer circumference of the segments 9a and a pressure acting on the inner circumference of the segments 9a is very small, the pressing force Fp of the curved plate spring 17 is greater than the pressing force Fs of the flat plate spring 20 and, consequently, the segments 9a of the sealing ring 9 are shifted radially outward.

As shown in Fig. 2B, the size a2 of the gap a is relatively small while the axial flow turbine is in operation under a rated load. In this state, the pressure difference Fr between a pressure acting on the outer circumference of the segments 9a and a pressure acting on the inner circumference of the segments 9a is large, the sum of the pressure difference Fr and the pressing force Fs of the flat plate spring 20 exceeds the pressing force Fp of the curved plate spring 17 and, consequently, the segments 9a of the sealing ring 9 are shifted radially inward. As shown in Fig.

2A, the curved plate spring 17 assumes a curved shape of a big height HI and hence the segment 9a of the sealing ring 9 is pressed at the outer circumferential surface of the axial through groove 16 while the axial flow turbine is stopped or the same is in operation under a low load. As shown in Fig. 2B, the curved spring 17 is compressed in a shape of a small height H2 by the pressure difference Fr acting on the segment 9a while the axial flow turbine is in operation under a rated load. The size of the gap a is determined so that the segment 9a is able to move radially by a distance k al I a2) when the load on the axial flow turbine increases or decreases. The gap a of such a size sup- 30 presses the leakage of the working fluid and the vibration of the rotor 5 attributable to the contact between the shaft 5a of the rotor 5 and the sealing fins 10 of the segments 9a of the sealing ring 9.

Fig. 2C is a diagram showing the variation of the size of the gap a. The vibration of the rotor 5 increases during the start and the operation under a low load and hence the size of the gap a increases to a 1 to prevent the vibration of the shaft 5a of the rotor 5 due to the contact between the rotor 5 and the sealing fins and the abrasion of the sealing fins 10. While the axial flow turbine is in steady operation under a middle load or a rated load, the sealing fins 10 are abraded scarcely by the shaft 5a of the rotor 5. Therefore, the size of the gap a decreases to a2 to suppress the leakage of the working fluid, whereby the efficiency of the axial flow turbine can be improved. When the segments 9a of the sealing ring 9 are shifted radially inward and the size of the gap a is decreased while the axial flow turbine is operating under a middle load, the segments 9a of the sealing ring 9 are liable to move irregularly. This problem can be solved by the first embodiment because the curved plate spring 17 supported at its opposite ends on the outer brim 18 and the inner brim 19 of the holding groove 8 exerts a pressure radially outward to the segment 9a, while the flat plate spring 20 exerts a pressure radially inward to the segment 9a to suppress the unsteady vibrations of the segments 9a of the sealing ring 9.

As shown in Figs. 2A and 2B, the inner end surface of the segment 9a of the sealing ring 9 is pressed against the brims 18 and 19 on the upstream and downstream side of the holding groove 8 of the inner ring 3 of the nozzle diaphragm 30 by the pressure of the working fluid while the axial flow turbine is in op- S: eration. If the inner end surface of the segment 9a is roughened oooo by corrosion or abrasion, the friction between the segment 9a and the inner ring 3 changes and, consequently, the balance of forces acting on the segment 9a to move the segment 9a radially relative to the rotor 5 changes. In the first embodiment, the respective contact surfaces of the inner ring 3 of the nozzle diaphragm 30 and the segments 9a of the sealing ring 9 are coated 30 with a corrosion-resistant coating or are finished by a surface hardening treatment to ensure that the segments 9a of the sealing ring 9 are able to move smoothly and the variation of the frictional characteristic of the segments 9a with time can be suppressed. When the surfaces of the segments 9a of the sealing ring 9 are thus finished by a surface treatment, the size of the gap between the sealing fins 10 and the shaft 5a of the rotor 5 can properly be controlled during the increase and the decrease of the load on the axial flow turbine. Consequently, the leakage of the working fluid and the vibration of the shaft 5a of the rotor 5 due to contact between the sealing fins 10 and the shaft 5a of the rotor 5 can be suppressed.

Fig. 3A is a schematic front view of the sealing device shown in Fig. 1A, Fig. 3B is a perspective view of the flat plate spring 20 biasing the segment 9a radially inward and Fig. 3C is a perspective view of the curved plate spring 17 biasing the segment 9a radially outward. As shown in Fig. 3A, the sealing ring 9 is divided into the plurality of segments 9a, usually, four to twelve segments 9a. A vertically downward force Fj, the own weight of the segment 9a, acts on each segment 9a. Therefore, the segments 9a forming an upper half of the sealing ring 9 tend to move vertically toward the rotor 5, and those forming a lower half of the sealing ring 9 tend to move vertically away from the rotor 5. Therefore, the pressing force Fs of the upper flat plate springs 20 pressing the segments 9a forming the upper half of the sealing ring 9 and the pressing force Fs of the lower flat plate springs 20 pressing the segments 9a forming the lower half of the sealing ring 9 must be different from each other to enable all of the segments 9a of the sealing ring 9 to move simultaneously radially inward to reduce the size of the gap a. Therefore, the upper and the lower flat plate springs 20 are formed so as to be different from each other in length L, width W or thickness T so that the upper and the lower flat plate springs 20 have different spring constants, respectively. The upper flat plate springs 20 exerting pressure on the segments 9a forming the upper half of the sealing ring 9 are formed so that the length L, the width W or the thickness T thereof is smaller than that of the lower flat plate springs 20 exerting pressure on the segments 9a forming the lower half of the sealing ring 9 to equalize the force acting radially inward on the segments 9a forming the upper half of the sealing ring 9 and the force acting radially inward on the segments 9a forming the lower half of the sealing ring 9. Similarly, the upper curved plate springs 17 pressing the segments 9a forming the upper half of the sealing ring 9 radially outward and the lower curved plate springs 17 pressing the segments 9a forming the lower half of the sealing ring 9 radially outward may be formed so as to be different from each other in length L, width W or thickness T so that the upper and the lower curved plate springs 17 have different spring constants, respectively.

Although the upper and the lower flat plate springs 20 are formed so that the length L, the width W or the thickness of the upper flat plate springs 20 is smaller than that of the lower flat plate springs 20 in the first embodiment, the upper flat plate springs 20 may be omitted and only the lower flat plate springs 20 may be employed.

Thus, in the sealing device in the first embodiment, all of the segments 9a of the sealing ring 9 are caused to start moving radially inward by the pressure difference between the outer and the inner circumference of each segment 9a substantially simultaneously, and the segments 9a do not move irregularly when the axial flow turbine is operating under a middle load. Accordingly, troubles attributable to the excessive vibration of the rotor 5 can be avoided and the leakage of the working fluid due to the abrasion of the sealing fins 10 can be suppressed.

A sealing device in a second embodiment according to the present invention will be described hereinafter. This embodiment has axial through holes instead of the axial through grooves of the first embodiment. Referring to Fig. 4A, an inner ring 3 of a nozzle diaphragm 30 is provided in its inner circumference with a holding groove 8, and a plurality of segments 9a forming a sealing ring 9 are arranged in a circumferential arrangement with their outer walls inserted in the holding groove 8. Sealing fins 10 are attached fixedly to the inner surfaces of the inner wall of the segments 9a of the sealing ring 9 facing a rotor 5 included in an axial 30 flow turbine. A gap a is formed between the circumferential surface of the rotor 5 and the sealing fins 10. A working fluid leaks through the gap a as indicated by the arrow b. As shown in Fig. 4B, each segment 9a of the sealing ring 9 is provided in its outer wall 23 with a plurality of axial through holes 22 extending between the upstream side and the downstream side of the sealing ring 9 with respect to the flowing direction of the working fluid in a circumferential arrangement. A curved plate spring 17 is inserted in each of the through holes 22 so that the opposite ends thereof rest on an upstream brim 18 and a downstream brim 19 of the holding groove 8, respectively, and a middle part thereof is in contact with the radially outer surface of the through hole 22.

The curved plate springs 17 exert pressure radially outward to the segments 9a of the sealing ring 9. The curved plate springs 17 are inserted in the axial through holes 22. Flat plate springs are placed between the outer circumferences of the segment 9a of the sealing ring 9 and the bottom surface of the holding groove 8 so as to bias the segments 9a of the sealing ring 9 radially inward, toward the shaft 5a of the rotor 5. The function of the curved plate springs 17 inserted in the axial through holes 22 is the same as that of the curved plate springs 17 employed in the first embodiment. The curved plate springs 17 increase the size of the gap a while the axial flow turbine is in a period of operation immediately after start and in operation under a low load. Thus, the rotor 5 will not vibrate excessively, and the abrasion of the sealing fins 10 can be prevented. The size of the gap a decreases while the axial flow turbine is in operation under a load in the range of a middle load to a rated load and hence the leakage of the working fluid can be suppressed. Since the segments 9a of the sealing ring 9 move smoothly radially inward when the axial flow turbine provided with the sealing device in the second embodiment is in operation under a middle load, the troubles at- 25 tributable to the excessive vibration of the rotor 5 can be prevented, and the increase of leakage of the working fluid due to the abrasion of the sealing fins 10 can be prevented.

A sealing device in a third embodiment according to the present invention for an axial flow turbine will be described 30 hereinafter. Fig. 5A is a sectional view of the sealing device in a state where the axial flow turbine is stopped and cold or in operation under a rated load and the size of the gap a is small, Fig.

5B is a sectional view of the sealing device in a state where the axial flow turbine is in a warming-up operation under no load or in operation under a low load and the size of the gap a is large.

The sealing device in the third embodiment employs plate-shaped elastic members 24 each consisting of two plates of different qualities instead of the curved plate springs 17 employed in the first embodiment shown in Figs. 1 to 3. Each of the elastic members 24 is formed by combining a plate-shaped first member 24a of an expanding material and a plate-shaped second member 24b of a shape memory alloy.

While the axial flow turbine is stopped and cold, both the first plate-shaped member 24a and the second plate-shaped member 24b of the elastic member 24 are flat as shown in Fig. Consequently, segments 9a of a sealing ring 9 are pressed radially inward with respect to a rotor 5 and a gap a is in a small size a2. When the axial flow turbine is in a warming-up operation under no load or in operation under a low load, the temperature of the elastic member 24 rises and the first plate-shaped member 24a of the elastic member 24 expands in a memorized shape (original shape) of the second plate-shaped member 24b. Consequently, the elastic member 24 curves in a shape convex radially outward on an upstream brim 18 and a downstream brim 19 of a holding groove 8 of an inner ring 3 of a nozzle diaphragm to shift the segment 9a of the sealing ring 9 radially outward, whereby the size of the gap a increases to a size a 1 as shown in Fig. 5B. When the axial flow turbine is in operation under a rated load, the pressure difference Fr between the outer and the inner circumference of each segment 9a exceeds the force of the 2 second plate-shaped member 24b resisting an action to deform 25 its original shape and the elastic member 24 is pressed in the shape of a fiat plate as shown in Fig. 5A. Consequently, each segment 9a of the sealing ring 9 is shifted radially inward and the size of the gap a is reduced to the size a2.

Fig. 5C is a diagram showing the variation of the size of 30 the gap a when the elastic elements 24 each formed by combining the two plate-shaped members of different qualities are used.

while the axial flow turbine is stopped and cold or while the axial flow turbine is disassembled or assembled, the elastic members 24 are cold and the size of the gap a is small. The sealing fins are liable to come into contact with the rotor 5 due to the thermal deformation of the turbine casing and such or the unbalanced rotation of the rotor 5 when the axial flow turbine is in a warming-up operation, the operating speed of the axial flow turbine is increasing or decreasing, or the axial flow turbine is in operation under a low load. However, since the size of the gap a increased because the elastic members 24 are curved in a radially outward convex shape by the increase of the temperature around the segments 9a of the sealing ring 9, the vibration of the shaft 5a of the rotor 5 attributable to the contact between the sealing fins and the shaft 5a of the rotor 5 can be avoided. The size of the gap a decreases while the axial flow turbine is in operation under a rated load because the first plate-shaped members 24a of the elastic members 24 are pressed to become flat by the pressure difference between the outer and the inner circumference of each segment 9a. The size of the gap a can be increase to the size a 1 by using the elastic members 24 when the axial flow turbine is in operation under a low load in which contact between the sealing fins 10 and the shaft 5a of the rotor 5 is liable to occur, and hence the vibration of the shaft 5a of the rotor 5 attributable to contact between the sealing fins 10 and the shaft 5a of the rotor 5 can be avoided. The size of the gap a in a state where the axial flow turbine is in operation under a rated load can be the same size of the gap a in a state where the axial flow turbine is cold and stopped or being disassembled or assembled. Therefore, the size o of the gap a while the axial flow turbine is in rated operation can be managed on the basis of the measurement of the gap a measured when the axial flow turbine is stopped and the casing thereof is opened. As a result, it is possible to suppress the leakage of the working fluid and the vibration of the shaft 5a of the rotor attributable to contact between the sealing fins 10 and the shaft of the rotor 30 As is apparent from the foregoing description, according to the present invention, the size of the gap between the rotor and the sealing fins of the sealing ring held on the inner ring of the stationary nozzle diaphragm of a stage of the axial flow turbine can be reduced while the axial flow turbine is in rated operation, whereby the leakage of the working fluid can be suppressed and the efficiency of the stage of the axial flow turbine can be improved. Since the size of the gap between the sealing fins and 17 the shaft of the rotor can be increased while the axial flow turbine is in unsteady operation under a low load, the vibration of the shaft of the rotor due to contact between the shaft of the rotor and the sealing fins of the sealing ring can be suppressed.

Although the invention has been described in its preferred embodiments with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

S

lo

Claims

1. A sealing device for preventing the leakage of a work- ing fluid in an axial flow turbine having a rotor with a shaft, and a nozzle diaphragm with an inner ring surrounding the shaft of the rotor, said sealing device comprising: a sealing ring formed of a plurality of segments around the shaft of the rotor, each of the segments having a part adapted to be fitted in a holding groove of the inner ring such that each of the segments can radially move relative to the shaft of the rotor; first pressing means for pressing the segments of the sealing ring radially inward; and second pressing means for pressing the segments of the sealing ring radially outward.

2. The sealing device according to claim 1, wherein re- spective pressing forces of the first and second pressing means are adjusted so that the segments of the sealing ring are shifted radially inward with respect to the rotor by a pressure difference between radially outer and radially inner surfaces of the seg- ments of the sealing ring while the axial flow turbine is in rated operation, and the segments of the sealing ring are shifted radi- ally outward with respect to the rotor by a pressure difference between the radially outer and radially inner surfaces of the seg- ments of the sealing ring while the axial flow turbine is in opera- tion under a low load.

3. The sealing device according to claim 1, wherein the first pressing means includes a plate spring which is interposed between a bottom surface of the holding groove and an outer surface of the segment of the sealing ring.

4. The sealing device according to claim 1, wherein the segment of the sealing ring is provided in its end surface with a through groove extending between an upstream side with respect to a flowing direction of the working fluid and a downstream side with respect to the flowing direction of the working fluid, the holding groove has a brim, and the second pressing means in- cludes a curved plate spring which rests on the through groove and the brim. The sealing device according to claim 4, wherein the curved plate spring extends between the through grooves of the adjacent segments of the sealing ring.

6. The sealing device according to claim 1, wherein the segment of the sealing ring is provided in its middle part with a through hole extending between an upstream side with respect to a flowing direction of the working fluid and a downstream side with respect to the flowing direction of the working fluid, the holding groove has a brim, and the second pressing means in- cludes a curved plate spring which is inserted in the through hole and rests on the through hole and the brim.

7. The sealing device according to claim 1, wherein the rotor has a horizontal axis, and respective pressing forces of the first and second pressing means are adjusted so as to compen- sate a difference in pressing forces acting on the segments of the sealing ring attributable to respective own weights of the seg- ments of the sealing ring.

8. The sealing device according to claim 1, wherein the rotor has a horizontal axis, and only the segments forming a low- er half of the sealing ring are pressed by the first sealing means so as to compensate a difference in pressing forces acting on the segments of the sealing ring attributable to respective own weights of the segments of the sealing ring. rotor9. The sealing device according to claim 3, wherein the rotor has a horizontal axis, and respective pressing forces of the plate springs pressing the segments forming an upper half of the sealing ring and the segments forming the lower half of the seal- ing ring are adjusted so as to compensate a difference in pressing forces acting on the segments of the sealing ring attributable to respective own weights of the segments of the sealing ring.

10. The sealing device according to claim 1, wherein the second pressing means includes a normally flat plate-shaped member which curves gradually when a temperature thereof rises, the plate-shaped member is received in a through groove formed in an end surface of the segment of the sealing ring so as to extend between an upstream side with respect to a flowing di- rection of the working fluid and a downstream side with respect to the flowing direction of the working fluid, or inserted in a through hole formed in a middle part of the segment of the seal- ing ring so as to extend between the upstream side with respect to the flowing direction of the working fluid and the downstream side with respect to the flowing direction of the working fluid, and the plate-shaped member curves as the temperature thereof rises while the axial flow turbine is in operation under a low load to shift the segment of the sealing ring radially outward with respect to the rotor.

11. The sealing device according to claim 10, wherein the plate-shaped member is formed of a pair of plate-shaped, elastic members differing from each other in quality.

12. The sealing device according to claim 11, wherein the plate-shaped member consists of a member of an expandable material and a member of a shape memory alloy.

13. The sealing device according to claim 1, wherein the segment of the sealing ring has an outer surface, the holding groove has an inner surface, the maximum size of a radial gap between the outer surface of the segment of the sealing ring and the inner surface of the holding groove is equal to a radial dis- tance by which the segment of the sealing ring moves radially when the load on the axial flow turbine increases or decreases.

14. The sealing device according to claim 1, wherein a surface of the inner ring of the nozzle diaphragm with which the tool segment of the sealing ring comes into contact is coated with a corrosion-resistant coating or is finished by a surface hardening treatment. S. S Q:\opER\GCP\18633.C 4/1/00 -21- A sealing device substantially as hereinbefore described with reference to Figures I to of the drawings. DATED this 4th day of January, 2000 K.ABUSHIKI KAISHA TOSHIBA By its Patent Attorneys DAVIES COLLISON CAVE FN~j ro~