System design and cooling method for LP steam turbines using last stage hybrid bucket
A steam turbine system for use in conjunction with hybrid last stage(s) LP buckets. The system is adapted to cool the bucket tip region during low VAN windage conditions whereby the beneficial design and efficiency outcomes of the use of hybrid blades can be realized.
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This invention relates to a steam turbine system design to be used in conjunction with hybrid composite buckets (blades) in the last stage(s) of a steam turbine, typically a low pressure (LP) steam turbine section.
Steam turbine blades operate in an environment where they are subject to high centrifugal loads and vibratory stresses. Vibratory stresses increase when blade natural frequencies become in resonance with running speed or other passing frequencies (upstream bucket or nozzle count, or other major per/rev features). The magnitude of vibratory stresses when a blade vibrates in resonance is proportional to the amount of damping present in the system (damping is comprised of material, aerodynamic and mechanical components, as well as the vibration stimulus level). For continuously coupled blades, the frequency of vibration is a function of the entire system of blades in a row, and not necessarily that of individual blades within the row.
Furthermore, for turbine buckets or blades, centrifugal loads are a function of the operating speed, the mass of the blade, and the radius from engine centerline where that mass is located. As the mass of the blade increases, the physical area or cross-sectional area must increase at lower radial heights to be able to carry the mass above it without exceeding the allowable stresses for the given material. This increasing section area of the blade at lower spans contributes to excessive flow blockage at the root and thus lower performance. The weight of the blade contributes to higher disk stresses and thus to potentially reduced reliability.
Several prior U.S. patents/applications relate to so-called “hybrid” blade designs where the weight of the airfoil is reduced by composing the airfoil as a combination of a metal and polymer filler material. Specifically, one or more pockets are formed in the airfoil portion and filled with the polymer filler material. These prior patents/applications include U.S. Pat. Nos. 6,854,959; 6,364,616; 6,139,278; 6,042,338; 5,931,641 and 5,720,597; application Ser. No. 10/900,222 filed Jul. 28, 2004 and application Ser. No. 10/913,407 filed Aug. 7, 2004; the disclosures of each of which are incorporated herein by this reference.
Another issue relating to the use of hybrid steam turbine blades, however, relates to cost as a function of temperatures experienced by such blades during use. In a double flow steam turbine, for example, there is significant windage heating of the last stage blade tip area during partial load and full speed conditions. The hood area behind the blades has a water spray system to cool the exhaust flow to the condenser. In this regard, in a typical design the water sprays are not used to cool the bucket but to cool the exhaust steam to keep the exhaust casing seal within its material temperature limits. Typically this is a urethane seal between the LP hood and the condenser opening. Even during the operation of the water sprays, the cooling flow does not migrate to the heated area near the blade tips and, thus, cooling of the blade tips is minimal. The blade tips during this condition can reach in excess of 500° F. whereas, during normal operation, the blade temperatures reach only about 150° F.
BRIEF DESCRIPTION OF THE INVENTIONThis invention expands the hybrid blade concept by providing a steam turbine system designed for use in conjunction with hybrid last stage(s) LP buckets. In particular, the present invention provides a system that can cool the bucket tip region during low VAN windage conditions whereby the beneficial design and efficiency outcomes of the use of hybrid blades can be realized.
Thus, the invention may be embodied in an axial flow steam turbine including: a rotor; a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm; and at least one injection assembly for injecting cooling media toward a vicinity of said last stage.
The invention may also be embodied in a method of cooling the last stage of an axial flow steam turbine including a rotor, the last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm, the method comprising: injecting cooling media toward a vicinity of said last stage.
In the illustrated example, radially inner and outer pockets 30, 32 are formed on the pressure side of the airfoil portion 28, separated by a relatively wide web or rib and a mid-span damper 36. More (or fewer) pockets can be included in the blade design.
The airfoil includes a main body or section 34 consisting essentially of metal. In this regard, the term “metal” includes “alloy” but for the purposes of describing the invention herein is not considered to mean a “metallic foam”. In the example embodiment described herein, the main body 34 is a monolithic metallic section, although the invention is not necessarily limited in this regard. The metallic section has a first mass density and radially extends generally from the blade root to the blade tip. The pockets or recesses 30,32 are defined in the airfoil where the metal is omitted or removed. In this regard, the main body or metallic section of the blade is forged, extruded or cast and the surface recesses may be formed by machining such as, for example, by chemical milling, electrochemical machining, electro-discharge machining or high speed machining.
If deemed necessary or desirable, the filler material 38 used to fill pocket 32 may have different properties, such as temperature resistance, as compared to filler material 40 used to fill pocket 30. The utilization of different filler sections, or more specifically filler materials, permits improved temperature capability of hybrid blades at reduced cost. Each material used could be formulated for specific locations on the bucket based on temperature characteristics of the filler materials and temperature capability requirements of the blades in any given stage. Using the more expensive, high temperature, materials in a limited location on the bucket makes the design of hybrid blades more feasible especially for those blades that experience high windage conditions.
The blades may be manufactured with one or more pockets filled with filler materials chosen to achieve the desired natural frequencies of the individual blades as well as the entire row of blades.
In a first method associated with this example embodiment, the pockets 30, 32 of blades 24 within a row of such blades are filled with filler materials chosen as a function of natural frequency. Thus, all of the pockets (from one to four or more) could be filled with a similar polymer filler material designed to achieve the desired natural frequencies of the individual blades as well as the entire row of blades. In another example, each blade would incorporate at least two different filler materials of, for example, different stiffness, to achieve the desired natural frequencies.
In a second method associated with this example embodiment, two or more groups of blades with recessed pocket(s) along the pressure side of the airfoil may be formed with different filler materials in the pockets of the blades of each group. By way of example, one group of blades may use a higher strength or “stiffer” material as the pocket filler, while the other group of blades may use a lower stiffness material. Alternatively, plural pockets in the blades of one group may be filled with plural polymer fillers, respectively, and the plural pockets of the other group may be filled with respectively different plural polymer fillers. Thus, for example, and with reference to blade in
The blade designs described above may be utilized to form a row of blades on a steam turbine rotor wheel as illustrated in
It is also possible to vary the pattern of blade group distribution, again so as to achieve the desired frequency characteristics. For example, a pattern AABBAA . . . or AABAAB . . . might also be employed.
In another example embodiment, the blades are manufactured with one or more pockets filled with urethane or silicon polymer filler materials chosen as a function of damping characteristics of the filler materials.
Again this may be accomplished in one of two methods. The first method would be to use one or more multiple fillers within the pockets of each blade (or pockets of blade), chosen to alter the damping coefficients of each of the blades as well as the damping response of the entire row of blades. Depending upon where the specific material properties are required, some pockets could be filled with either a highly damped material or a material that may meet some other specific requirement, not necessarily related to damping. In some areas of the blade, for example, erosion may be a concern; materials that are desirable for erosion prevention, however, may not be desirable for vibration reduction. In other areas, erosion may not be as much of an issue, and vibration damping may be the principal concern. In any event, by altering the damping characteristics to a greater or lesser extent, the magnitude of the system vibrations in the row of blades may be reduced to a tolerable level.
The second method associated with this example again involves the separation of the blades into two discrete groups, each of which incorporates different filler materials to adjust the damping coefficient of the blades within the respective groups. For example, all of the blades of one group would incorporate one or more fillers in the respective pockets, while all of the blades of the second group would incorporate a different choice of one or more fillers. The blades would be assembled in a mapped configuration like those described above, i.e., ABAB . . . or AABBAA . . . , etc. The mapped configuration results in mixed tuning of the set of blades via various damping responses of the blades in each group of blades to create a more damped blade row or set. This may also shift the frequencies of each blade to take even greater advantage of the mixed tuning concept.
Each of the above methods may lead to the removal of the typical mechanical damper at the mid-span of certain blade designs. This mid-span connection is a flow disturbance that leads to reduced turbine efficiency. In other words, by using appropriate filler materials with improved damping properties, the complete removal of the current mid-span damper 36 is possible.
As noted above, a typical hybrid bucket 24 consists of a metallic blade section 34 with a recessed pocket or through wall window 30, 32 that contains composite matrix filler 40, 38.
This hybrid blade design allows for several beneficial outcomes. It creates a lighter bucket which allows for longer or wider chord buckets. A longer bucket will allow for more steam flow, thereby increasing the turbine output. A lighter bucket also allows for wider chord buckets or buckets with improved aerodynamics, thereby in providing stage efficiency.
The hybrid bucket design also affords the ability to “mixed tune” the continuously coupled bucket stage to dampen the overall frequency response of the stage. Further, the hybrid bucket has the opportunity to reduce costs. The titanium currently used on the longest buckets that are produced is very costly, at up to 3× the cost of steel alloy. The hybrid bucket has the opportunity to replace titanium designs with a steel design with hybrid pocketing. There is also the opportunity of lengthening the useful life of the bucket stage by adding the hybrid bucket material thereby reducing stress levels in both the bucket and rotor. Additionally, one could arrange more than one stage with a hybrid design that would increase aeroefficiency or increase bucket length to produce more power. Even further, the hybrid bucket, being lighter allows for more flexibility in adjusting the IRD (inner hub or root diameter) of the bucket. Making the IRD larger for the same bucket allows for more annulus area should it be required in the thermodynamic/performance design. On a typical turbine moving the bucket outboard increases the pull load on the rotor significantly due to the exponential factor increasing the bucket pull load. Additionally, one could make a longer bucket while maintaining or reducing the IRD, both of which produce more annulus area. The new IGCC turbine design concepts require more annulus area due to the higher flow rates of that particular application. The larger hybrid bucket annulus area makes that possible without having to create more LP sections to pass the flow. This is not physically obtainable with current metallic buckets due to length (stress) limitations.
An objective of the invention is to produce a steam turbine system design to be used in conjunction with hybrid last stages LP buckets of the type generally described above. However, a couple of issues exist in making a hybrid system design achievable. One issue is that of the high temperature that is created during low VAN operation. As noted above, a significant issue in using hybrid bucket design, that is, composite or polymer material in a metallic blade, is the temperature condition during flow (low VAN) operation when the rotor is at full speed. During low flow operation the bucket tip region is in a windage condition that heats up the flow to significantly higher temperature than at steady state operation. Thus, the hybrid bucket system design must be able to overcome the temperature increase.
One way to make the hybrid bucket design feasible is to develop high temperature composite materials for use in a high temperature steam environment. See in this regard co-pending application Ser. No. 10/900,222, filed Jul. 28, 2004, the disclosure of which is incorporated herein by this reference. A second approach, as set forth in greater detail below, is to actively cool the bucket tip region during the low VAN windage condition.
In one example embodiment as illustrated in
According to a further feature of the invention, which may be combined with water spray(s) 44 or provided in the alternative, is to inject steam or water from the outer side wall 50 of the last stage diaphragm 52 as illustrated in
Referring more particularly to
Referring more particularly to the steam or water injection options,
As noted above, steam or water injection and/or steam extraction may also be provided on the downstream side of the nozzle diaphragm 52, upstream of the hybrid blades 24. Thus, as illustrated in
As illustrated in
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
Claims
1. An axial flow steam turbine including:
- a rotor;
- a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm; and
- at least one injection assembly for injecting cooling media toward a vicinity of said last stage.
2. An axial flow steam turbine as in claim 1, wherein the injection assembly comprises water sprays in an exhaust area downstream of the last stage buckets.
3. An axial flow steam turbine as in claim 1, wherein said injection assembly comprises a steam or water injection cavity defined in said outer ring and a steam or water injection opening directed towards said nozzles of said diaphragm.
4. An axial flow steam turbine as in claim 3, wherein said injection opening is disposed upstream of the nozzles with respect to a steam flow path through the last stage.
5. An axial flow steam turbine as in claim 3, wherein the injection cavity is downstream of the nozzles with respect to a steam flow path through the last stage.
6. An axial flow steam turbine as in claim 1, comprising a moisture extraction assembly for extracting moisture from a steam flow path through the last stage, downstream of the nozzles and upstream of the buckets with respect to said steam flow path.
7. An axial flow steam turbine as in claim 6, wherein moisture extraction assembly comprises a moisture extraction cavity defined in said outer ring and a moisture extraction groove having an opening having a scoop directed towards said nozzles of said diaphragm.
8. An axial flow steam turbine as in claim 1, wherein at least one said bucket comprises a) a shank portion; and b) an airfoil portion having an operating temperature range, a design rotational speed, a blade root attached to said shank portion, a blade tip, and a radial axis extending outward toward said blade tip and inward toward said blade root, and wherein said airfoil portion also includes: (1) a metallic section consisting essentially of metal and having a first mass density, wherein said metallic section radially extends from generally said blade root to generally said blade tip; and (2) at least one fiber composite section, having a second mass density less than said first mass density.
9. An axial flow steam turbine as in claim 8, wherein said metallic section and said at least one fiber composite section together define a generally airfoil shape at said design rotational speed.
10. An axial flow steam turbine as in claim 8, wherein said fiber composite section is disposed in a pocket defined in a pressure side of said metallic section.
11. In an axial flow steam turbine including a rotor and a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm, a method of cooling said last stage, comprising:
- injecting cooling media toward a vicinity of said last stage.
12. A method as in claim 11, wherein said injecting comprises spraying water in an exhaust area downstream of the last stage buckets.
13. A method as in claim 11, wherein said injection comprises injecting steam or water from a steam or water injection cavity defined in said outer ring through a steam or water injection opening directed towards said nozzles of said diaphragm.
14. A method as in claim 13, wherein said injection opening is disposed upstream of the nozzles with respect to a steam flow path through the last stage.
15. A method as in claim 13, wherein said injection opening is disposed downstream of the nozzles with respect to a steam flow path through the last stage.
16. A method as in claim 11, further comprising extracting moisture from a steam flow path through the last stage, downstream of the nozzles and upstream of the buckets with respect to said steam flow path.
17. A method as in claim 16, wherein moisture is extracted through a moisture extraction groove having an opening including a scoop directed towards said nozzles of said diaphragm, and into a moisture extraction cavity defined in said outer ring.
18. A method as in claim 11, wherein at least one said bucket comprises a) a shank portion; and b) an airfoil portion having an operating temperature range, a design rotational speed, a blade root attached to said shank portion, a blade tip, and a radial axis extending outward toward said blade tip and inward toward said blade root, and wherein said airfoil portion also includes: (1) a metallic section consisting essentially of metal and having a first mass density, wherein said metallic section radially extends from generally said blade root to generally said blade tip; and (2) at least one fiber composite section, having a second mass density less than said first mass density.
19. A method as in claim 18, wherein said metallic section and said at least one fiber composite section together define a generally airfoil shape at said design rotational speed.
20. A method as in claim 18, wherein said fiber composite section is disposed in a pocket defined in a pressure side of said metallic section.
Type: Application
Filed: Jun 14, 2006
Publication Date: Dec 20, 2007
Applicant: GENERAL ELECTRIC COMPANY (Schenectady, NY)
Inventors: Steven Sebastian Burdgick (Schenectady, NY), Christophe Lanaud (Delanson, NY), Peter Michael Finnigan (Clifton Park, NY), Wendy Wen-ling Lin (Niskayuna, NY)
Application Number: 11/452,403
International Classification: F01D 25/32 (20060101);