US20120163969A1

US20120163969A1 - Turbine including exhaust hood

Info

Publication number: US20120163969A1
Application number: US12/977,115
Authority: US
Inventors: Chaitanya Venkata Rama Krishna Ongole; Subhrajit Dey; Dale William Ladoon; Manisekaran Santhanakrishnan; Hiteshkumar Rameshchandra Mistry
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2010-12-23
Filing date: 2010-12-23
Publication date: 2012-06-28
Also published as: RU2011152290A; FR2969692A1; JP2012132455A; DE102011056638A1

Abstract

A turbine includes an inner casing, a rotor extending in a longitudinal direction, and rows of buckets transversely disposed on the rotor. A bearing cone covering a portion of the rotor and a flow guide extending from the inner casing define an annular passage for flow of exhaust gases. Exhaust gas movement is facilitated by a guide cap having a streamlined surface and situated in a downstream direction of the flow guide, a tip leakage flow injection channel to inject exhaust gases at the inner surface of the flow guide, an incline of the casing of the turbine surrounding the last stage buckets relative to the longitudinal axis, or use of a first portion of the annular passage with a substantially constant surface area and a second portion of the annular passage with a progressively increasing surface area.

Description

BACKGROUND

The present invention relates to steam turbines. More specifically, the present invention relates to embodiments to provide improved exhaust hood performance in a steam turbine.
Steam turbines are widely used for power generation and primarily include a casing, a rotor extending in a longitudinal axis of a steam turbine, and a plurality of rows of buckets transversely disposed on the rotor. During operation, exhaust gases leave a last row of buckets and flow through an annular passage. Typically, the annular passage is defined by a steam guide which extends from the casing and a bearing cone that surrounds a portion of the rotor. Depending on various configurations of the steam turbine, the annular passage acts as an exhaust hood to diffuse the exhaust gases and direct the exhaust gases towards a condenser.
Particularly in a downward exhaust hood configuration, the exhaust gases are required to turn 180 degrees after leaving the annular passage and prior to reaching the condenser. Due to the simultaneous diffusion and turning in the annular passage, the high velocity exhaust gases experience turbulence and/or flow separation at the walls of the annular passage. This leads to the formation of vortices, decreases pressure recovery in the annular passage, and affects the overall performance of the steam turbine.
Various prior art solutions have been proposed to improve the pressure recovery in a downward exhaust hood. For example, a set of adjustable guide vanes may be provided on the bearing cone to guide the exhaust gases. The adjustable guide vanes change the cross-sectional area of the annular passage and improve pressure recovery in the annular passage. Further, the geometry of the annular passage may be modified to increase pressure recovery. However, geometry modifications lead to a complex and uneven construction of the exhaust hoods.
In light of the foregoing, there exists a need for an improved exhaust hood.

BRIEF DESCRIPTION

In accordance with one embodiment of the present invention, a turbine includes an outer casing surrounding an inner casing, a rotor enclosed by the inner casing that extends along a longitudinal axis of the turbine, and multiple rows of buckets transversely disposed on the rotor. The turbine further includes a bearing cone that surrounds a portion of the rotor and a flow guide that extends from the inner casing such that an annular passage for flow of exhaust gases is defined by the flow guide and the bearing cone. The bearing cone and the flow guide form an inner wall and an outer wall of the annular passage respectively. A guide cap having a streamlined surface is provided in a downstream direction of the flow guide.
In accordance with another embodiment of the present invention, the turbine includes one or more tip leakage flow injection channels which inject the exhaust gases at the inner surface of the flow guide. The injection channels are provided to energize flow of the exhaust gases in the annular passage and prevent the boundary layer separation at the inner surface of the flow guide.
In accordance with another embodiment of the present invention, the casing of the turbine surrounding the last stage buckets is inclined relative to the longitudinal axis of the turbine. The angle of inclination may range from about 5 degrees to 15 degrees relative to the longitudinal axis.
In yet another embodiment of the present invention, a first portion of the annular passage includes a substantially constant surface area and a second portion of the annular passage includes a progressively increasing surface area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view through a portion of a turbine with a downward flow exhaust hood, according to an embodiment of the present invention;

FIG. 2 is a longitudinal cross-sectional view through a portion of a turbine with a downward flow exhaust hood, according to another embodiment of the present invention;

FIG. 3 is a longitudinal cross-sectional view through a portion of a turbine with a downward flow exhaust hood, according to yet another embodiment of the present invention;

FIG. 4 is a longitudinal cross-sectional view through a portion of a turbine with a downward flow exhaust hood, according to yet another embodiment of the present invention; and

FIG. 5 is a longitudinal cross-sectional view through a portion of a turbine with a downward flow exhaust hood, according to yet another embodiment of the present invention.

DETAILED DESCRIPTION

Illustrative embodiments of the invention will now be described more fully hereinafter with reference to the accompanying drawings. The invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
Embodiments presented herein enable the guiding of exhaust gases in a downward exhaust hood configuration of a turbine.
Turning now to the drawings, and referring first to FIG. 1, an exemplary embodiment is shown in accordance with certain aspects of the present invention. FIG. 1 illustrates a longitudinal cross-sectional view through a portion of a turbine 100 with a downward flow exhaust hood 102. In an embodiment of the present invention, the turbine 100 may include a steam turbine and the exhaust gases thereof include steam.
Generally, the turbine 100 includes an outer casing 104 and an inner casing 106 such that the outer casing 104 encloses the inner casing 106. A rotor 108 is enclosed by the inner casing 106 and extends along a longitudinal axis AA′ of the turbine 100. The inner casing 106 and the rotor 108 form a flow path 110 for a working fluid such that the working fluid flows from a high pressure region to a low pressure region within the turbine 100. Further, multiple buckets 112 are transversely disposed with respect to the longitudinal axis AA′, and mechanically coupled to the rotor 108. More specifically, the buckets 112 are arranged in multiple rows which are arranged circumferentially around the rotor 108. Moreover, multiple nozzles 114 may extend from the inner casing 106 to circumferentially surround the rotor 108 and are axially positioned between the adjacent rows of the buckets 112. The buckets 112 and nozzles 114 work together and form multiple turbine stages, thus defining a portion of the flow path 110 leading to the exhaust hood 102.
The exhaust hood 102 may include an annular passage 116, such that the exhaust gases leaving the last row of buckets 112, are diffused in the annular passage 116. In an embodiment of the present invention, the annular passage 116 is defined by a flow guide 118 and a bearing cone 120 wherein the flow guide 118 and the bearing cone 120 may form the outer and inner walls, respectively, of the annular passage 116. As illustrated in FIG. 1, the flow guide 118 may extend from the inner casing 106. Further, the bearing cone 120 may surround a portion of the rotor 108. Both the flow guide 118 and the bearing cone 120 extend by 360 degrees about the longitudinal axis AA′. Alternatively, the flow guide 118 and the bearing cone 120 may include two halves, upper and lower halves, joined at flanges located longitudinally along a plane extending from the longitudinal axis AA′.
In an embodiment of the present invention, a guide cap 124 is provided in a downstream direction of the flow guide 118 (that is, in a location such that the exhaust gases pass over the guide cap 124 after passing through the annular passage 116 on the exhaust gas path towards an outlet 22 of turbine 100). In one embodiment of the present invention, the guide cap 124 may be integrally formed with the flow guide 118. In another embodiment of the present invention, the guide cap 124 may be a separate component which is attached to the flow guide 118 by welding, riveting, or fastening, for example. In one embodiment of the present invention, the guide cap 124 may have an airfoil shape.
During operation of the turbine 100, the exhaust gases turn by more than 180 degrees after leaving the annular passage 116 and are directed towards a condenser (not shown) through an outlet 122. Particularly at a top portion of the turbine 100, the exhaust gases exit the last stage turbine buckets 112 in an axial direction substantially parallel to the longitudinal axis AA′ of the turbine 100. Subsequently, the exhaust gases turn by almost 90 degrees to a radial direction in the annular passage 116, as illustrated by an arrow 126. In the annular passage 116, the exhaust gases may diffuse and cause pressure recovery. Subsequently, the exhaust gases turn by another 90 degrees over the guide cap 124, as illustrated by an arrow 128. Finally, the guide cap 124 turns the exhaust gases towards the condenser in a downwards direction. The guide cap 124 may have a substantially streamlined surface 130. As used herein, a “streamlined surface” is a surface which is contoured in a manner to enable a smooth flow of the exhaust gases after exiting the flow guide 118. Thus, formation of vortices and re-circulation regions in the exhaust hood 102, while taking the turn illustrated by the arrow 128, is substantially reduced.
FIG. 2 is a longitudinal cross-sectional view through a portion of a turbine 100 with a downward flow exhaust hood 102, according to another embodiment of the present invention. As illustrated in FIG. 2, the annular passage 116 may have a first portion 202 of a substantially constant surface area which is followed by a second portion 204 with a progressively increasing surface area. In one embodiment of the present invention, in the first portion 202 a radius of curvature of the flow guide 118 is substantially equal to a radius of curvature of the bearing cone 120. Whereas in the second portion 204 of the annular passage 116, the radius of curvature of the flow guide 118 is smaller than the radius of curvature of the bearing cone 120. Consequently, the exhaust gases smoothly turn from the axial direction to the radial direction in the first portion 202, and then diffuse in the second portion 204 of the annular passage 116. By allowing the exhaust gases to turn with minimal or no diffusion in the first portion 202 followed by the diffusion in the second portion 204, flow separation in the annular passage 116 is substantially reduced.
FIG. 3 is a longitudinal cross-sectional view through a portion of a turbine 100 with a downward flow exhaust hood 102, according to yet another embodiment of the present invention. Flow separation may still occur when the exhaust gases are decelerated by frictional forces acting at an inner surface 302 of the flow guide 118. In addition, the exhaust gases may also encounter an adverse pressure gradient in the annular passage 116 which is stronger than the kinetic energy of the exhaust gases. These conditions result in boundary layer separation at the inner surface 302 of the flow guide 118, and formation of eddies and vortices. Consequently, the pressure recovery in the exhaust hood 102 is lowered, thereby reducing the overall efficiency of the turbine 100.
The boundary layer separation of exhaust gases may be substantially prevented by injecting high momentum exhaust gases near the inner surface 302. This energizes the boundary layer flow and prevents the flow separation. As illustrated in FIG. 3, one or more tip leakage flow injection channels such as pipes 304 are configured to inject a flow of the high momentum exhaust gases at the inner surface 302 of the flow guide 118. Alternatively, the tip leakage flow injection channels may include one or more slots provided in the flow guide 118. In one embodiment of the present invention, the high momentum exhaust gases are shown as being injected at a shoulder portion 306 of the flow guide 118. In one embodiment, the high momentum exhaust gases comprise bypassed flow of the exhaust gases from various turbine stages, which do not directly contribute to the turbine output.
As described above, the turbine 100 includes buckets 112 which may be mechanically coupled to the rotor 108, and arranged circumferentially around a longitudinal axis AA′ of the turbine 100 to form multiple turbine stages along with a set of fixed nozzles 114. Multiple turbine stages are axially arranged on the rotor 108 with a predetermined axial clearance between any two turbine stages. Further, as illustrated in FIG. 4, an optimal radial clearance C1 is typically provided between a tip 402 of the bucket 112 and the inner casing 106. In conventional embodiments, the optimal radial clearance C1 is substantially equal to a baseline clearance, which is between 0.3% and 0.6% of the bucket length for a given turbine stage. Based on experimental results and analysis for the optimal radial clearance C1, it is observed that a flow separation point may occur at a shoulder portion 404 of the flow guide 118. In an embodiment of the present invention, an optimal radial clearance C2 is provided between a tip 402′ of the bucket 112 and the inner casing 106. The optimal radial clearance C2 is increased, in accordance with one embodiment of the present invention, to energize the boundary layer flow at the shoulder portion 404 of the flow guide 118. In one embodiment of the present invention, this facilitates the flow of the exhaust gases through the annular passage 116 and improves the pressure recovery from the exhaust gases, thereby increasing the overall efficiency of the turbine 100. In one embodiment of the present invention, wherein the optimal radial clearance C2 is increased by about 8% to 15% relative to the optimal radial clearance C1 and the tip leakage flow injection channel of FIG. 3 is used, the overall efficiency of the turbine 100 is expected to increase by about 18%.
FIG. 5 is a longitudinal cross-sectional view through a portion of a turbine 100 with a downward flow exhaust hood 102, according to yet another embodiment of the present invention. As illustrated in FIG. 5, the turbine 100 includes the last stage turbine buckets 112. In an embodiment of the present invention, a portion of the inner casing 106 for the last stage turbine buckets 112 is inclined relative to the longitudinal axis AA′ of the turbine 100. This reduces a radius of curvature of the flow guide 118 which may subsequently reduce an overall axial footprint of the turbine by about 1 ft. to 2 ft., as shown by dotted lines. The reduction of the axial footprint may further decrease the materials and manufacturing cost of the exhaust hood 102 of the turbine 100. Additionally, the reduction of the axial footprint may reduce the overall weight of the turbine 100. In a more specific embodiment of the present invention, an angle α of inclination of the inner casing 106 for the last stage turbine buckets 112 is within a range from about 5 degrees to 15 degrees relative to the longitudinal axis AA′ of the turbine 100. In an even more specific embodiment of the present invention, the angle α of inclination may be in the range from about 6 degrees to 10 degrees relative to the longitudinal axis AA′ of the turbine 100.
The present invention has been described in terms of several embodiments solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.

Claims

1. A turbine comprising:

an outer casing;

an inner casing enclosed by the outer casing;

a rotor enclosed by the inner casing and extending along a longitudinal axis of the turbine;

a plurality of rows of buckets transversely disposed on the rotor;

a bearing cone surrounding at least a portion of the rotor;

a flow guide extending from the inner casing;

an annular passage for flow of exhaust gases defined by the flow guide and the bearing cone, wherein the bearing cone and the flow guide form an inner wall and an outer wall of the annular passage respectively; and

a guide cap having a streamlined surface provided in a downstream direction of the flow guide.

2. The turbine according to claim 1, wherein the guide cap comprises an airfoil shape.

3. The turbine according to claim 1, wherein the exhaust gases comprise steam.

4. A turbine comprising:

an outer casing;

an inner casing enclosed by the outer casing;

a plurality of rows of buckets transversely disposed on the rotor;

a bearing cone surrounding at least a portion of the rotor;

a flow guide extending from the inner casing;

at least one tip leakage flow injection channel configured to inject flow of exhaust gases at the inner surface of the flow guide to prevent boundary layer separation of the exhaust gases at the inner surface of the flow guide.

5. The turbine according to claim 4, wherein the at least one tip leakage flow injection channel comprises at least one pipe.

6. The turbine according to claim 4, wherein the at least one tip leakage flow injection channel comprises at least one slot in the flow guide.

7. The turbine according to claim 4, wherein the at least one tip leakage flow injection channel is situated at a shoulder portion of the flow guide.

8. The turbine according to claim 4, wherein the exhaust gases comprise steam.

9. The turbine of claim 4, wherein one or more rows of buckets are provided at an optimal radial clearance from the inner casing to energize flow of exhaust gases in the annular passage, wherein the optimal radial clearance is increased by about 8% to 15% relative to a baseline clearance.

10. A turbine comprising:

a rotor extending along a longitudinal axis of the turbine;

a plurality of rows of buckets transversely disposed on the rotor; and

a casing surrounding the plurality of rows of buckets,

wherein a portion of the casing for a last row of buckets is inclined at an angle in a range of 5 degrees to 15 degrees with respect to the longitudinal axis.

11. The turbine according to claim 10, wherein the angle is in a range of 6 degrees to 10 degrees with respect to the longitudinal axis.

12. The turbine according to claim 10, wherein the exhaust gases comprises steam.

13. A turbine comprising:

an outer casing;

an inner casing enclosed by the outer casing;

a plurality of rows of buckets transversely disposed on the rotor;

a bearing cone surrounding at least a portion of the rotor;

a flow guide extending from the inner casing; and

an annular passage for flow of exhaust gases defined by the flow guide and the bearing cone, wherein the bearing cone and the flow guide form an inner wall and an outer wall of the annular passage respectively,

wherein a first portion of the annular passage comprises a substantially constant surface area and a second portion of the annular passage comprises a progressively increasing surface area.

14. The turbine according to claim 13, wherein in the first portion a radius of curvature of the flow guide is substantially equal to a radius of curvature of the bearing cone.

15. The turbine according to claim 14, wherein in the second portion the radius of curvature of the flow guide is smaller than the radius of curvature of the bearing cone.

16. The turbine according to claim 13, wherein the exhaust gases comprises steam.