WO2019236062A1

WO2019236062A1 - Arrangement of a last stage with flow blockers and corresponding method for suppressing rotating flow instability cells

Info

Publication number: WO2019236062A1
Application number: PCT/US2018/036016
Authority: WO
Inventors: James Mccracken
Original assignee: Siemens Energy, Inc.
Priority date: 2018-06-05
Filing date: 2018-06-05
Publication date: 2019-12-12

Abstract

An arrangement of a last stage of rotating blades and stationary vanes of a turbomachine (10) is provided. The arrangement includes a last stage of rotating blades (60) mounted to a rotor (50), the rotor coaxial with a turbine casing (20) and a last stage of stationary vanes (70) axially adjacent to the last stage of rotating blades, the last stage of stationary vanes mounted to an outer conical surface (30). The arrangement also includes a plurality of flow blockers (150) distributed circumferentially around the outer conical surface and mounted to the outer conical surface, each flow blocker disposed in the annular space (160) between the last stage of rotating blades and an adjacent last stage of stationary vanes wherein the flow blockers suppress the formation of rotating flow instability cells distributed around the last stage of rotating blades during low mass flow conditions of the turbomachine. A corresponding method to suppress the formation of rotating flow instability cells distributed around a last stage of rotating blades of a turbomachine during low mass flow conditions comprises inserting the plurality of flow blockers into the blade path of the last stage of rotating blades during low mass flow conditions of the turbomachine, and retracting the plurality of flow blockers out of the blade path during normal operating conditions of the turbomachine.

Description

ARRANGEMENT OF A LAST STAGE WITH FLOW BLOCKERS AND CORRESPONDING METHOD FOR SUPPRESSING

ROTATING FLOW INSTABILITY CELLS

BACKGROUND

1. Field

[0001] The present disclosure relates generally to the field of turbomachines and more particularly, to an arrangement of the last stage of rotating blades and stationary vanes of a turbomachine.

2. Description of the Related Art

[0002] When a turbine is operating at very low mass flow, such as during start-up, shut down or very low load, conditions exist under which the aerodynamic phenomenon of rotating instability (RI) may occur. Rotating instability is mostly a concern for the last stage of rotating blades in the low-pressure section of a steam turbine. Rotating instability consists of a pattern of local pressure variation cells in the fluid flow which are distributed around the circumference of the blade row and rotate circumferentially relative to the stationary frame of the steam turbine engine. Certain patterns may result in excitation of the rotating blade leading to vibration amplitudes which can potentially be damaging to the blade. To avoid this situation, operating restrictions may be used, however, it is not desirable to do so due to the impact on the steam turbine’s flexibility and permitted operating range.

[0003] European Patent Application EP 2816199 describes a method to suppress RI cells using steam injected through a number of passages/nozzles located between the last stage of stationary vanes and rotating blades. This flow injection approach requires complex piping and steam which must be supplied at pressure and temperature conditions appropriate for the last stage of the low pressure turbine.

[0004] European Patent Application EP 2685050 describes an assembly of static vanes for axial flow turbines for low pressure steam turbines. The assembly includes a stage of vanes with certain vanes having an extension over part of the vane height that reaches into the annular space between the rotor and the casing interrupting the flow pattern which causes the undesirable blade excitation.

[0005] Japanese Patent Document JPH06173606 describes a steam turbine blade cascade where the trailing edges of certain blades which form the boundaries of a nozzle group are extended in order to reduce steam flowing in the circumferential direction.

[0006] Vane extensions, as is proposed in both EP 2685050 and JPH06173606, can have detrimental effects related to rotating blade erosion, forced excitation at harmonics of the number/spacing of the extended vanes, and reduced efficiency of the turbine stage resulting from the varying passage shape associated with the extension of certain stationary vanes.

[0007] Technical paper,‘Stator Conditioning Effects on Steam Turbine Rotating Instability’ by Zhang, Proceedings of the IMechE, Part A, Journal of Power and Energy, 2014 discusses rotating instability with regards to several geometrical features and aerodynamic design aspects with a focus on the last stage stationary vane. It is disclosed that shifting the stator row 50% closer to the rotating row suppressed the formation of rotating instability for the configuration analyzed.

[0008] In view of the prior art, a need remains for a solution to prevent the formation of RI cells which could potentially cause elevated blade vibration in the intended range of operating conditions of the steam turbine.

SUMMARY

[0009] Briefly described, aspects of the present disclosure relate to an arrangement of a last stage of rotating blades and stationary vanes of a turbomachine and a system to suppress the formation of rotating flow instability cells distributed around a last stage, predominantly in the axial space between the stationary and rotating blades of a turbine.

[0010] A first aspect provides an arrangement of a last stage of rotating blades and stationary vanes of a turbomachine comprising a last stage of rotating blades circumferentially distributed on a rotor, the rotor coaxial with a turbine casing with a last stage of stationary vanes axially adjacent to the last stage of rotating blades and mounted to an outer conical surface. A plurality of flow blockers is distributed circumferentially around the outer conical surface and mounted to the outer conical surface, each flow blocker is disposed in the annular space between the last stage of rotating blades and an adjacent last stage of stationary vanes. The flow blockers suppress the formation of rotating flow instability cells distributed around the last stage of rotating blades during low mass flow conditions of the turbomachine. [0011] A second aspect provides a system to suppress the formation of rotating flow instability cells distributed around a last stage of rotating blades of a turbine. The system includes a turbine casing which supports stationary vane assemblies and a rotor disposed coaxial with the turbine casing in which rotating blades are mounted, wherein the turbine casing and the rotor establish the inner and outer boundaries of a fluid flow path, and an arrangement as described above.

[0012] A third aspect provides a method to suppress the formation of rotating flow instability cells distributed around a last stage of rotating blades of a turbomachine during low mass flow conditions. The method includes the steps of distributing a plurality of flow blockers circumferentially around an outer conical surface of a stationary vane assembly in an annular space between a last stage of rotating blades and stationary vanes, mounting the plurality of flow blockers to the outer conical surface, inserting the plurality of flow blockers into the blade path of the last stage of rotating blades during low mass flow conditions of the turbomachine. The flow blockers suppress the formation of rotating flow instability cells distributed around the last stage of rotating blades during low mass flow conditions of the turbomachine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 illustrates a longitudinal view of the rotating blade and stationary vane stages of a turbine,

[0014] Figure 2 illustrates an enlarged longitudinal view of the last stage of rotating blades and stationary vanes of a turbine including a proposed flow blocker,

[0015] Figure 3 illustrates a partial perspective view of the last stage of rotating blades having a plurality of flow blockers disposed 90 degrees apart circumferentially, and [0016] Figure 4 illustrates a retractable embodiment of the proposed flow blocker.

DETAILED DESCRIPTION

[0017] To facilitate an understanding of embodiments, principles, and features of the present disclosure, they are explained hereinafter with reference to implementation in illustrative embodiments. Embodiments of the present disclosure, however, are not limited to use in the described systems or methods.

[0018] The components and materials described hereinafter as making up the various embodiments are intended to be illustrative and not restrictive. Many suitable components and materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of embodiments of the present disclosure.

[0019] Figure 1 illustrates a longitudinal view of the blade and vane stages of a turbine 10. In the context of the disclosure, the turbine is a steam turbine and will be referred to as such hereinafter, however one skilled in the art would understand that the turbine may be any turbomachine. The illustrated steam turbine 10 comprises a turbine casing 20 and a rotor 50 which establish a fluid flow path. In the exemplary embodiment of a steam turbine, the fluid flow is a flow of steam. In the figures, the fluid flow direction is indicated by an arrow. The fluid flow path is defined by portions of the casing 20, rotor 50, rotating blades 60 and stationary vane assemblies 80. A rotor 50 is disposed coaxially with the turbine casing 20 on which a plurality of rows of rotating blades 60 are mounted with a plurality of individual blades distributed circumferentially. Stationary vane assemblies 80 consist of a plurality of stationary vanes 70 which are distributed circumferentially around a conical outer surface 30 and an inner cylindrical surface 40 and are mounted on the casing 20. A stage may be defined as a pair of stationary vane assemblies and rotating blade rows. The last stage is the furthest downstream stage in the fluid flow direction. The last stage of blades 60 and vanes 70 also is typically characterized as having the longest (radially) blades and vanes of all the stages in a typical steam turbine. In an embodiment, the last stage of blades and vanes may be a last stage of a low pressure (LP) part of the steam turbine 10.

[0020] Referring now to Figure 2, an enlarged longitudinal view of a last stage 100 of rotating blades and stationary vanes of a steam turbine including a proposed flow blocker 150 is shown. Although only one representative rotating blade 60 and one representative stationary vane 70 of the last stage 100 are shown in Figure 1, the last stage 100 of vanes and blades include a plurality of uniform rotating blades and a plurality of uniform stationary vanes distributed circumferentially around the rotor 50 within the turbine casing 20. An annular space 160, as shown, exists between the last stage of stationary vanes 70 and rotating blades 60. Within this annular space 160, a plurality of circumferential flow blockers 150 may be disposed in order to suppress the formation of rotating flow instability cells during the low mass conditions of the steam turbine 10.

[0021] The plurality of flow blockers 150 may be distributed circumferentially around the outer conical surface 30 Regarding its shape, the flow blocker 150 may include geometries which are aerodynamic or less aerodynamically shaped profiles. Each flow blocker 150 includes a length (1), the length (1) describing a radial distance from the outer conical surface 30 into the annular space 160. In an embodiment, the length (1) encompasses 10-20% of the length of the adjacent rotating blade 60 extending radially inward from the rotating blade tip 140. In particular, the length (1) encompasses approximately 20% of the length of the adjacent rotating blade 60 extending radially inward from the rotating blade tip 140. Additionally, the flow blocker 150 may include a shape that tapers radially from the outer cylindrical surface 30 into the annular space 160.

[0022] In an embodiment, a number of flow blockers 150 distributed around the circumference of the outer conical surface 30 lies in a range of 4 to 8. Figure 3 illustrates a partial perspective view of the last stage of rotating blades 60 with flow blockers 150 spaced apart circumferentially. Using four flow blockers 150 distributed around the circumference of the outer conical surface 30 sufficiently suppresses the formation of the rotating instability cells. While the preferred range may be the range of 4 to 8 flow blockers, more or less than this range of flow blockers 150 distributed around the circumference may also be used to suppress the formation of the rotating instability cells. The plurality of flow blockers 150 may be equally spaced around the circumference of the outer conical surface 30 so that, for example, when the number of flow blockers is 4, each flow blocker 150 would be spaced 90 degrees apart as depicted in Figure 3. The flow blockers 150 may also be unequally spaced to ease implementation within the overall turbine geometry, or to reduce harmonic excitation effects.

[0023] In an embodiment, each flow blocker 150 is retractable such that during normal operating conditions when the rotating instability cells are less likely to form, the flow blocker 150 is essentially removed from the blade path, or the fluid flow path upstream from the rotating blade. This functionality would enable the flow blocker 150 to be inserted into the fluid flow path during low flow conditions and removed from the flow path during normal operation, when having the flow blocker 150 in the blade path may cause a loss in efficiency of the stage 100.

[0024] Referring now to Figure 4, an embodiment of a retractable flow blocker 150 is illustrated. In order to remove the flow blocker 150 from the fluid flow path, an actuator 170 may be utilized to move the flow blocker 150 so that its length lies along the outer cylindrical surface 30 and substantially out of the fluid flow path. In an embodiment, the flow blocker 150 may be rotated out of the flow path. In this embodiment, each flow blocker 150 is disposed in a passage between adjacent stationary vanes 70 so that when rotated they will move between the stationary vanes and into a slot 180 in the outer conical boundary 30. A component 190 to which the flow blocker 150 is attached may include an actuator point for controlling the flow blocker 150 by the actuator 170. The component 190 may include a pivot point about which the flow blocker 150 would pivot for insertion and retraction from the flow path, for example, into the slot 180. In operation, the flow blocker angle would be parallel to the trailing edge and flow angle of the stationary vane 70. [0025] Alternately, the flow blocker 150 may be removed from the flow path radially through an opening in the outer conical surface 30 with a similar approach of mounting guides and an actuator. In this embodiment, the flow blocker 150 may be positioned in the annular space 160 between the stationary vane 70 and the rotating blade 60 In operation, the flow blocker 150 may be aligned with the trailing edge of the stationary vanes 70

[0026] The proposed arrangement of flow blockers suppresses the formation of the RI pattern by implementing barriers in the area between the rotating blade and stationary vane. This is intended to prevent the circumferential arrangement and rotational velocity of the RI cells.

[0027] While embodiments of the present disclosure have been disclosed in exemplary forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions can be made therein without departing from the spirit and scope of the invention and its equivalents, as set forth in the following claims.

Claims

What is claimed is:

1. An arrangement of a last stage 100 of rotating blades and stationary vanes of a turbomachine 10, comprising:

a last stage 100 of rotating blades 60 circumferentially distributed on a rotor 50, the rotor 50 coaxial with a turbine casing 20;

a last stage 100 of stationary vanes 70 axially adjacent to the last stage 100 of rotating blades 60, the last stage of stationary vanes 70 mounted to an outer conical surface 30; and

a plurality of flow blockers 150 distributed circumferentially around the outer conical surface 30 and mounted to the outer conical surface 30, each flow blocker 150 disposed in the annular space 160 between the last stage 100 of rotating blades 60 and an adjacent last stage of stationary vanes 70,

wherein the flow blockers 150 suppress the formation of rotating flow instability cells distributed around the last stage 100 of rotating blades 60 during low mass flow conditions of the turbomachine 10.

2. The arrangement as claimed in claim 1, wherein the flow blocker 150 is retractable during normal operation of the turbomachine such that the length (1) of the flow blocker 150 lies along the outer conical surface 30 and substantially out of the fluid flow path.

3. The arrangement as claimed in claim 2, wherein the flow blocker 150 is rotated into a slot 180 located in a passage between adjacent stationary blades 60.

4. The arrangement as claimed in claim 1, wherein the flow blocker 150 is retractable during normal operation of the turbomachine such that the flow blocker is removed radially into a slot within the outer conical surface 30.

5. The arrangement as claimed in claim 1, wherein each flow blocker of the plurality of flow blockers 150 is aligned with the trailing edge and flow angle of an axially preceding stationary vane 70 of the last stage 100 of stationary vanes.

6. The arrangement as claimed in claim 1, wherein each flow blocker of the plurality of flow blockers 150 is aligned with the trailing edge and not aligned with the flow angle of the axially preceding stationary vane 70 of the last stage 100.

7. The arrangement as claimed in claim 1, wherein each flow blocker of the plurality of flow blockers 150 is not aligned with the trailing edge and not aligned with the flow angle of the axially preceding stationary vane 70 of the last stage 100.

8. The arrangement as claimed in claim 1, wherein each flow blocker 150 comprises an airfoil-shaped profile.

9. The arrangement as claimed in claim 1, wherein each flow blocker 150 comprises a plate with rounded edges.

10. The arrangement as claimed in claim 1, wherein each flow blocker 150 includes a length (1), the length (1) describing a distance extending radially from the outer conical surface 30 into the annular space 160, and wherein the length (1) encompasses 10-20% of the length of the adjacent rotating blade 60 extending from the rotating blade tip 140.

11. The arrangement as claimed in claim 10, wherein the length (1) of each flow blocker encompasses approximately 20% of the length of the adjacent rotating blade 60 extending from the rotating blade tip 140.

12. The arrangement as claimed in claim 1, wherein a number of the plurality of flow blockers 150 lies in range of 4-8.

13. The arrangement as claimed in claim 12, wherein the plurality of flow blockers 150 are uniformly distributed circumferentially around the outer conical surface 30.

14. The arrangement as claimed in claim 12, wherein the plurality of flow blockers 150 are non-uniformly distributed circumferentially around the outer conical surface 30.

15. A system to suppress the formation of rotating flow instability cells distributed around a last stage 100 of rotating blades of a turbine 10, comprising: a turbine casing 20 which supports stationary vane assemblies 80 and a rotor 50 disposed coaxial with the turbine casing 20 in which rotating blades 60 are mounted, wherein the turbine casing 20 and the rotor 50 establishes a fluid flow path defined by portions of casing 20, rotor 50, rotating blades 60 and stationary vane assemblies 80;comprising an inner cylindrical surface 40 and an outer conical surface 30, the turbine casing 20 establishing a fluid flow path, the fluid flow path bounded radially outward by the outer conical surface 30 and bounded radially inward by the inner cylindrical surface 40; and

an arrangement as claimed in claim 1 ;

16. The system as claimed in claim 13, wherein the turbine 10 is a steam turbine.

17. The system as claimed in claim 13, wherein the last stage 100 is a last stage of a low pressure steam turbine.

18. The system as claimed in claim 13, wherein a number of the plurality of flow blockers 150 lies in range of 4-8.

19. The system as claimed in claim 16, wherein the plurality of flow blockers 150 are uniformly distributed circumferentially around the outer conical surface 30.

20. The system as claimed in claim 16, wherein the plurality of flow blockers 150 are non-uniformly distributed circumferentially around the outer conical surface 30.

21. A method to suppress the formation of rotating flow instability cells distributed around a last stage 100 of rotating blades 60 of a turbomachine 10 during low mass flow conditions, comprising:

distributing a plurality of flow blockers 150 circumferentially around an outer conical surface 30 of a turbine vane assembly 80 or casing 20 in an annular space 160 between a last stage 100 of stationary vanes 70 and a last stage 100 of rotating blades 60;

mounting the plurality of flow blockers 150 to the outer conical surface 30; inserting the plurality of flow blockers 150 into the blade path of the last stage 100 of rotating blades 60 during low mass flow conditions of the turbomachine 10; and

retracting the plurality of flow blockers 150 out of the blade path during normal operating conditions of the turbomachine 10,

wherein the flow blockers 150 suppress the formation of rotating flow instability cells distributed around the last stage 100 or rotating blades 60 during low mass flow conditions of the turbomachine 10.

22. The method as claimed in claim 19, wherein a number of the plurality of flow blockers 150 lies in range of 4-8

23. The method as claimed in claim 19, wherein the turbomachine 10 is a steam turbine.

24. The method as claimed in claim 19, wherein the retracting is implemented by an actuator 170.