US20120027568A1

US20120027568A1 - Low-pressure steam turbine and method for operating thereof

Info

Publication number: US20120027568A1
Application number: US13/193,867
Authority: US
Inventors: Brian Robert Haller
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2010-07-30
Filing date: 2011-07-29
Publication date: 2012-02-02
Also published as: EP2412922A1; ITMI20101447A1; JP2012031864A; CN102418565A

Abstract

The disclosure relates to a multi-stage low-pressure steam turbine and a method for operating thereof. The steam turbine includes a last stage in which the leading edge of each vane of the last stage is skewed so as to form a W shaped K-distribution across the span of the vanes. This shape allows for efficient last stage operation at low last stage exit velocities. The disclosure includes a method for operating such a steam turbine at a last stage exit velocity between 125 m/s and 150 m/s.

Description

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 to Italian Patent Application No. MI2010A001447 filed in Italy on Jul. 30, 2010, the entire content of which is hereby incorporated by reference in its entirety.

FIELD

The present disclosure relates to low-pressure steam turbines for addressing exhaust losses through last stage design.

BACKGROUND INFORMATION

In the field of low-pressure steam turbines there is a desire to reduce exhaust losses in order increase turbine net efficiency. One way of achieving this is to increase the efficiency of the last turbine stage.
As described in http://www.powermag.com/issues/cover stories/The-long-and-short-of-last-stage-blades 483 p3.html (14 Jul. 2010) it is known that above last stage exit velocities of approximately 190 m/s (600 ft/sec), exhaust losses in low-pressure steam turbine can increase in a squared relationship with velocity. Although there may seem to be a general benefit in reducing exit velocities, when reduced to below approximately 170 m/s (550 ft/sec), net efficiency can decrease due, for example, to the increasing influence of reverse vortices at the blade root. This conclusion is further supported by, for example, K. Kavney et al “Steam turbine 34.5 Inch Low-Pressure Section Upgrades” GE Energy GER-4269 (08/06), where, on page 7, it concludes that generally exhaust losses are at their minimum when the exhaust velocity is about 600 ft/s (190 m/s).
As described in EP 1260674 (A1) turbine stages can be classified as being either impulse or reaction stages depending on the blade root reaction degree. In this context, blade root reaction degree can be defined as the ratio of the heat drop (variation of enthalpy) across a moving blade to the total heat drop across a turbine stage. Impulse blade stages, for example, can have a blade root reaction degree of between 0 to 10%. For reaction blade stages, the blade root reaction degree can rise to about 50%. Stages with blade root degrees of between 10% and 50% can be classified as low-reaction type stages. Methods of achieving a particular blade root reaction degree are both varied and known and can be achieved, for example, by modifying the blades lean or sweep.
Depending on the design of a stage, the blade root reaction degree may influence overall blade efficiency. For example, U.S. Application Publication No. 2004/0071544 describes an impulse type stage that has improved efficiency through adaptation of the reaction degree.

SUMMARY

A multi-stage low-pressure steam turbine according to the disclosure having a last stage, comprising a plurality of stationary vanes circumferentially distributed to form a vane row wherein each vane has an airfoil with a span extending from a radially extending a base to a tip of the airfoil; a plurality of blades circumferentially mounted and distributed on a rotatable rotor of the steam turbine; and a K ratio defined as a ratio of vane throat to pitch, where each of the vanes includes a leading edge that is skewed so as to form a W shaped K-distribution across the span of the vanes.
A method is disclosed for operating a steam turbine including a stage having a plurality of stationary vanes circumferentially distributed to form a vane row wherein each vane has an airfoil with a span extending from a radially extending a base to a tip of the airfoil; a plurality of blades circumferentially mounted and distributed on a rotatable rotor of the steam turbine; and a K ratio defined as the ratio of vane throat to pitch, where each of the vanes includes a leading edge that is skewed so as to form a W shaped K-distribution across the span of the vanes; the method comprising configuring the area of the exit region of the last stage for operation at an exit velocity of between 125 m/s and 150 m/s; and adjusting the feed flow rate through the steam turbine such that the exit velocity is between 125 m/s and 150 m/s.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, exemplary embodiments of the disclosure are described more fully hereinafter with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of part of an exemplary embodiment of low-pressure steam turbine;

FIGS. 2 a and 2 b show perspective views of an exemplary embodiment of last stage vane of the steam turbine of FIG. 1;

FIG. 3 shows a top view of two last stage vanes of an exemplary embodiment of vane row of the steam turbine of FIG. 2; and

FIG. 4 is a graph showing the K-distribution across the span of an exemplary embodiment of vane.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiments disclosed herein.
A low-pressure steam turbine last stage is disclosed that can overcome losses due to reverse vortices resulting from low last stage exit velocity.
The disclosure is based on providing a low-pressure steam turbine with last stage vanes that have leading edges skewed so as to form a W shaped K-distribution across the span of each of the vanes. This configuration provides a means to adapt the last stage for efficient operation at low exit velocities.
In exemplary embodiments, the last stage can be configured as a low reaction stage through'a combination of vane 14 and blade 12 parameters including the tangential lean angle 30, the tilt angle of the trailing edge in the meridional plane 32 and the vane pitch 26. A reaction degree configuration between impulse type blading and high root reaction e.g. >50% provides further means of delaying efficiency collapse while operating at low exit velocities.
An exemplary method is provided for operating at low exit velocities between 125 m/s and 150 m/s. This can be achieved by adjusting the steam rate through a steam turbine with a configured exit annulus area that has skewed vane leading edges that form a W shaped K-distribution across the span of each vane and can have a last stage root reaction degree of between 15%-50%.
Other exemplary embodiments further include vanes with straight trailing edges.
FIG. 1 shows an exemplary low-pressure multiple stage turbine steam turbine 10. Each stage 16 of the low-pressure steam turbine 10 includes a plurality of stationary vanes 14 that are circumferentially distributed on the inner casing 15 to form a vane row, and a plurality of blades 12 that are circumferentially mounted and distributed on a rotating rotor 17. The pressure of the steam exiting the last stage 18 of the low-pressure steam turbine 10 defines the steam turbine as a low-pressure steam turbine 10. Typically, at steady state conditions, the last stage 18 exit pressure of a low-pressure steam turbine 10 ranges from atmospheric pressure to a low vacuum.
Immediately downstream of the last stage 18, the inner casing 15 forms an annulus (not shown). The area of the annulus, together with the volumetric flow rate of steam exiting the last stage 18 defines the exit velocity of the steam turbine 10.
As shown in FIGS. 2 a and 2 b, each vane 14 has an airfoil 31 with a leading edge 20 and a span 36 defined as a radial extension between a base 38 and a tip 34 of the airfoil 31. As shown in FIG. 3, the distance between adjacent vanes 14 taken from the trailing edge 22 of one of the vanes 14 to the face of an adjacent vane 14 defines a throat 24. The distance between leading edges 20 of two adjacent vanes 14 defines the pitch 26. The ratio of the throat to pitch further defines a K-value.
In an exemplary embodiment, the leading edge 20 of each vane 14 is skewed so as to form, as shown in FIG. 2 b and FIG. 4, a W shaped K-distribution across the span 36 of the vanes 14.
In an exemplary embodiment, the skewed vanes 14 can be configured to have a root reaction degree, in operation, between 15% and 50%, for example, between 35% and 45%. As shown in FIG. 2 a, in a vane 14 of an exemplary embodiment, the trailing edge 22 has a title angle in the meridional plane 32 and, as shown in FIG. 2 b, a tangential lean angle 30. The root reaction degree of exemplary embodiments can be defined by a combination of vane 14 and blade 12 parameters including the tangential lean angle 30, the tilt angle of the trailing edge in the meridional plane 32 and the vane pitch 26.
In an exemplary embodiment, the tangential lean angle 30 can be between 16 degrees and 25 degrees, for example, about 19 degrees.
In an exemplary embodiment, the tilt angle 32 can be between 3 degrees and 13 degrees, for example, about 8 degrees.
In an exemplary embodiment, as shown in FIG. 2 b, each vane 14 further has a tapering axial width across the span 36.
In an exemplary embodiment, each vane 14 further has a straight trailing edge 22.
Another exemplary embodiment relates to a method for operating a steam turbine 10 of any of the previously described steam turbines 10. The method involves configuring the area of the exit region of the last stage 18 for operation at an exit velocity of between 125 m/s and 150 m/s and adjusting the feed flow rate through the steam turbine 10 such that the exit velocity is between 125 m/s and 150 m/s.
Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

REFERENCE SIGNS

10 Low-pressure steam turbine
12 Blade
14 Vane
15 Inner Casing
16 Stage
17 Rotor
18 Last stage
Leading edge
22 Trailing edge
24 Throat
26 Pitch
30 Tangential lean angle
31 Airfoil
32 Tilt angle in the meridional plane
Tip
36 Span
38 Base

Claims

1. A multi-stage low-pressure steam turbine having a last stage, comprising:

a plurality of stationary vanes circumferentially distributed to form a vane row wherein each vane has an airfoil with a span extending radially from a base to a tip of the airfoil;

a plurality of blades circumferentially mounted and distributed on a rotatable rotor of the steam turbine; and

a K ratio defined as a ratio of vane throat to pitch, where each of the vanes include a leading edge that is skewed so as to form a W shaped K-distribution across the span of the vanes.

2. The steam turbine of claim 1, wherein the last stage is configured to have a root reaction degree, in operation, of between 15% and 50%.

3. The steam turbine of claim 2, wherein the root reaction degree is between 35% and 45%.

4. The steam turbine of claim 2, wherein the airfoil of each vane comprises:

a tangential lean angle; and

a trailing edge tilt angle in a meridional plane, wherein the root reaction degree is defined by the lean angle, the tilt angle and a vane pitch.

5. The steam turbine of claim 4, wherein the tangential lean angle is between 16 degrees and 25 degrees.

6. The steam turbine of claim 5, wherein the tangential lean angle is 19 degrees.

7. The steam turbine of claim 2, wherein the tilt angle is between 3 degrees and 13 degrees.

8. The steam turbine of claim 7, wherein the tilt angle is 8 degrees.

9. The steam turbine of claim 1, wherein each vane has a tapering axial width across the span.

10. The steam turbine of claim 1, wherein each vane has a straight trailing edge.

11. The steam turbine of claim 3, wherein the airfoil of each vane comprises:

a tangential lean angle; and

12. The steam turbine of claim 3, wherein the tilt angle is between 3 degrees and 13 degrees.

13. The steam turbine of claim 4, wherein the tilt angle is between 3 degrees and 13 degrees.

14. The steam turbine of claim 8, wherein each vane further has a tapering axial width across the span.

15. The steam turbine of claim 2, wherein each vane further has a straight trailing edge.

16. The steam turbine of claim 3, wherein each vane further has a straight trailing edge.

17. The steam turbine of claim 4, wherein each vane further has a straight trailing edge.

18. The steam turbine of claim 5, wherein each vane further has a straight trailing edge.

19. The steam turbine of claim 6, wherein each vane further has a straight trailing edge.

20. A method for operating a steam turbine which includes a last stage having a plurality of stationary vanes circumferentially distributed to form a vane row wherein each vane has an airfoil with a span extending radially from a base to a tip of the airfoil, a plurality of blades circumferentially mounted and distributed on a rotatable rotor of the steam turbine, and a K ratio defined as the ratio of vane throat to pitch, where each of the vanes includes a leading edge that is skewed so as to form a W shaped K-distribution across the span of the vanes, the method comprising:

configuring an area of an exit region of the last stage for operation at an exit velocity of between 125 m/s and 150 m/s; and

adjusting a feed flow rate through the steam turbine such that the exit velocity is between 125 m/s and 150 m/s.