JP2009216089A - Low pressure section steam turbine bucket - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve rigidity, damping characteristics, and vibration stress by using long blades. <P>SOLUTION: The bucket has blades 118 of which length is about 45 inches or more. The bucket 102 includes a dovetail part 121 disposed close to a radially inward position of the bucket, a tip shroud disposed close to a radially outward position of the bucket, and an intermediate span shroud 610 disposed at a radially intermediate position between the radially inward position and outward position. The bucket 102 is formed of titanium base alloy comprising aluminum by about 2-6.25 wt.%, vanadium by about 3.5 wt.% or less, tin by about 2.25 wt.% or less, zirconium by about 2.25 wt.% or less, molybdenum by about 1.75-5.0 wt.%, chrome by about 2.25 wt.% or less, silicon by about 0.7 wt.% or less, iron by about 2.3 wt.% or less, and titanium for the rest. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-strength bucket used in the final stage of a steam turbine engine. In particular, the present invention relates to the use of a titanium-based alloy to produce a high strength last stage turbine bucket having a blade length of about 45 inches or greater.

  In general, it is known that the performance of a steam turbine is greatly influenced by the design and performance of a subsequent bucket operating at reduced steam pressure. Ideally, the last stage bucket should effectively utilize the expansion of the steam until it is reduced to the turbine exhaust pressure, and the kinetic energy of the steam flow exiting the last stage should be as small as possible.

  The requirements for using steam turbine buckets are complex and demanding. In particular, the last stage bucket is constantly exposed to various harsh operating conditions such as high humidity and a corrosive environment caused by carryover from the boiler. Such conditions can cause serious corrosion and pitting problems in the bucket material, especially for long last stage turbine buckets with blade lengths of 40 inches or longer. Therefore, research and development have been continued for the last stage bucket for turbines in order to improve the efficiency under severe operating conditions. This is because even a slight increase in the efficiency and durability of the bucket can have significant economic benefits over the life of the steam turbine engine.

  The final stage turbine bucket is exposed to a wide range of flows, loads and large dynamic forces. Therefore, from the standpoint of mechanical strength and durability, the main factors affecting the final design of the bucket profile include the effective length of the bucket, the pitch diameter and the operating speed in the flow region during operation. Damping, bucket fatigue and corrosion resistance at the maximum predicted operating conditions of the constituent materials also play an important role in the final bucket design and manufacturing process.

In the development of larger last stage turbine buckets, for example buckets with a blade length of about 40 inches or more, the inertial load often exceeds the strength performance of conventional bucket materials such as Ti ₆ Al ₄ V and iron-based alloys, so another design Problems are imposed. Steam turbine buckets, especially last stage buckets with long blades, are subject to large tensile loads and are subject to repeated stresses that, when combined with a corrosive environment, can damage the buckets during long periods of use. Is big. Furthermore, the last stage steam is usually "wet", i.e. contains more saturated steam. As a result, water droplet impact erosion of the bucket material often occurs at the final stage. Such erosion shortens the useful life of the bucket and reduces the overall efficiency of the steam turbine.

  Heretofore, it has been difficult to find bucket materials that can meet all the mechanical requirements of various end uses, especially mechanical designs that use long wing buckets, i.e., buckets with wing lengths of about 40 inches or more. Long buckets inevitably require high strength and have the potential to cause erosion and pitting as described above. The high stress inherent in long blade design also increases the risk of stress corrosion cracking at high operating temperatures. This is because the high strength required for the bucket material tends to easily cause stress cracking at an operating temperature of about 400 ° F. The effects of pitting corrosion and corrosion fatigue increase as the stress applied to the last stage bucket having a long blade length increases. Alloys selected to simply meet the basic mechanical design requirements of another turbine stage often do not meet the minimum mechanical strength and erosion resistance requirements of the final stage bucket.

  In order to solve the problems associated with the long blade length of the final stage turbine bucket, various approaches have been conventionally taken depending on the end use requirements. A single bucket material may be appropriate if the usage requirements are less stringent. However, in order to increase erosion resistance, the bucket usually needs to be cured by local heat treatment (for example, flame quenching or induction quenching) at the leading edge to provide sufficient erosion resistance. Alternatively, an erosion resistant shield material (such as stellite) can be brazed and attached to the bucket by gas-tungsten arc welding or electron beam welding. Most of these conventional physical deposition methods cause some degradation in the weld heat affected zone over time, and the bucket eventually breaks in place. In addition, there is always a risk of poor welding, which in some cases can result in scrapping the entire bucket assembly.

  In other known manufacturing or repair methods for steam turbine buckets, the insert is welded to the leading edge of the bucket blade, and then a portion of the insert is cured to provide a hardened surface outside the leading edge. However, in this case as well, the curing process does not normally extend to the joint between the insert and the blade itself. Thus, a portion of the insert remains uncured and is subject to premature failure or erosion under severe operating conditions.

  A method of forming a bimetallic structure that satisfies many requirements of high stress last stage buckets for use in steam turbines is disclosed in US Pat. No. 5,351,395 to Crawmer et al. . In the method described in Patent Document 1, the erosion-resistant insert material is attached while both the bucket and the insert material are in a state having substantially optimum weldability. After welding, the bimetal assembly is heat treated to optimize the bucket characteristics with little distortion. The insert material can also be hardened after machining (by conventional methods such as flame quenching) to improve erosion resistance. However, it has been confirmed that the bimetal structure disclosed in Patent Document 1 does not solve all the pitting corrosion and corrosion problems inherent in the final stage turbine bucket, particularly a bucket having a blade length of about 40 inches or more.

US Pat. No. 5,351,395 US Pat. No. 5,067,766 US Pat. No. 5,267,834 US Pat. No. 5,277,549 US Pat. No. 5,299,915 US Pat. No. 5,393,200 US Pat. No. 5,480,285 US Pat. No. 6,575,700 US Pat. No. 6,682,306 US Pat. No. 6,814,543 US Pat. No. 6,893,216 US Pat. No. 7,097,428 US Pat. No. 7,195,455 US Patent Application Publication No. 2007/0292265

AMIR MUJEZINOVIC, "Bigger Blades Cut Costs" Modern Power Systems, Feb. 2003, p.25, 27. MICHAEL BOSS, "Steam Turbine Technology Heats Up", PEI Magazine, April 2003, p.77, 79, 81.

  Accordingly, there is a need in the industry for a last stage bucket having a long blade length, excellent stiffness, excellent damping characteristics and low vibration stress.

  In one aspect of the invention, a bucket for use in a low pressure section of a steam turbine is provided. The bucket is formed so that the blade length is about 45 inches or more. The bucket has a dovetail portion disposed near the radially inner position of the bucket, a tip shroud disposed near the radially outer position of the bucket, and a radius between the radially inner position and the outer position of the bucket. An intermediate span shroud disposed at an intermediate position in the direction. The bucket is about 2 wt.% To about 6.25 wt.% Aluminum, about 3.5 wt.% Or less vanadium, about 2.25 wt.% Or less tin, about 2.25 wt.% Or less zirconium, about 1. Titanium base consisting of 75 wt% to about 5.0 wt% molybdenum, up to about 2.25 wt% chromium, up to about 0.7 wt% silicon, up to about 2.3 wt% iron and the balance titanium Made of alloy.

  In another aspect, a steam turbine is provided that includes a low pressure turbine section having a plurality of final stage buckets disposed about a turbine wheel. The last stage bucket has a blade length of about 45 inches or more. The at least one final stage bucket includes a dovetail portion disposed near a radially inner position of the bucket, a tip shroud disposed near a radially outer position of the bucket, and a radially inner position and an outer position of the bucket. And an intermediate span shroud disposed at a radially intermediate position between the two. The final stage bucket comprises about 2% to about 6.25% aluminum, about 3.5% or less vanadium, about 2.25% or less tin, about 2.25% or less zirconium, 1.75% to about 5.0% by weight molybdenum, about 2.25% or less chromium, about 0.7% or less silicon, about 2.3% or less iron and the balance titanium It is made of a titanium-based alloy.

It is a partially broken perspective view of a steam turbine. It is a perspective view of the bucket of one embodiment of the present invention. It is an expansion perspective view of a shaft insertion curved surface dovetail of one embodiment of the present invention. FIG. 3 is a perspective view of an example of a tip shroud that can be used in the bucket of FIG. 2. It is a perspective view which shows the mutual relationship of the front-end | tip shroud adjacent. FIG. 3 is a perspective view of an intermediate span shroud that can be used with the bucket of FIG. 2. It is a perspective view which shows the mutual relationship of an adjacent intermediate span shroud.

  FIG. 1 is a partially broken perspective view of a steam turbine 10 including a rotor 12 with a shaft 14 and a low pressure (LP) turbine 16. The LP turbine 16 includes a plurality of rotor wheels 18 that are axially spaced. A plurality of buckets 20 are mechanically coupled to each rotor wheel 18. More specifically, the buckets 20 are arranged as a plurality of rows extending circumferentially around each rotor wheel 18. The plurality of fixed nozzles 22 extends in the circumferential direction around the shaft 14 and is positioned between adjacent bucket (20) rows in the axial direction. The nozzle 22 cooperates with the bucket 20 to form a turbine stage and defines a portion of the steam flow path through the turbine 10.

  During operation, the steam 24 flows into the inlet 26 of the turbine 10 and is directed to the nozzle 22. The nozzle 22 directs the steam 24 toward the bucket 20 toward the downstream. The steam 24 passes through the remaining stages and applies a force to the bucket 20 to rotate the rotor 12. At least one end of the turbine 10 extends axially from the rotor 12 and can be attached to a load or machine (not shown), such as a generator and / or another turbine. Thus, a large steam turbine apparatus may actually include a plurality of turbines all coaxially connected to the same shaft 14. Such an apparatus can be configured, for example, by connecting a high-pressure turbine to an intermediate-pressure turbine and connecting the intermediate-pressure turbine to a low-pressure turbine.

  In FIG. 1, as an embodiment, the low-pressure turbine is shown in five stages. The five stages are called L0, L1, L2, L3, and L4. L4 is the first stage and is the smallest (radial direction) of the five stages. L3 is the second stage and is the next stage in the axial direction. L2 is the third stage and is shown in the middle of the fifth stage. L1 is the fourth and last previous stage. L0 is the last stage and is the maximum (radial direction). Note that the fifth stage is only an example, and the number of low-pressure turbines may be more or less than five.

  FIG. 2 is a perspective view of a turbine bucket 20 that can be used in the turbine 10. Bucket 20 includes a blade portion 102 that includes a trailing edge 104 and a leading edge 106, with steam generally flowing from leading edge 106 to trailing edge 104. Bucket 20 also includes a first concave sidewall 108 and a second convex sidewall 110. The first sidewall 108 and the second sidewall 110 are connected axially at the trailing edge 104 and the leading edge 106 and extend radially between the rotor blade root 112 and the rotor blade tip 114. Blade chord distance is the distance measured from the trailing edge 104 to the leading edge 106 at any point along the radial length 118 of the blade 102. In one embodiment, the radial length 118, or wing length, is about 45 inches. In another embodiment, the radial length 118 is about 40 to about 50 inches or more. Although the radial length 118 is described herein as being equal to about 45 inches, the radial length 118 can be any suitable length depending on the particular application. The root 112 includes a dovetail 121 that is used to incorporate the bucket 20 into the rotor disk along the shaft 14.

  FIG. 3 is an enlarged view of the dovetail 121. In one embodiment, dovetail 121 is a shaft insertion curved dovetail that engages a mating slot formed in the rotor disk. In one embodiment, the dovetail 121 has four convex protrusions 302. In another embodiment, the dovetail 121 may have more or fewer convex protrusions. An axially-inserted curved dovetail is preferred to obtain an appropriate distribution of mean and local stresses, over-speed protection and an appropriate low cycle fatigue (LCF) margin.

  FIG. 4 is an enlarged view of bucket tip 4 having an integrated tip shroud 410 of one embodiment. The tip shroud 410 improves the rigidity and damping characteristics of the bucket 20. A seal rib 420 can be disposed on the outer surface of the tip shroud. The rib 420 functions as a sealing means that restricts steam flow beyond the outer portion of the bucket 20. The ribs 420 can be a single rib, or can be comprised of multiple ribs, multiple straight or chevron teeth, or one or more teeth of different dimensions (eg, labyrinth type seals).

  FIG. 5 shows the tip shroud 410 initially assembled. Tip shroud 410 is designed to have a gap 510 between adjacent tip shrouds during initial assembly and / or at zero speed. As can be seen from the figure, the ribs 420 are also slightly offset from each other in the non-rotating state. As the turbine wheel rotates, the bucket 20 begins to twist back. As the rotational speed approaches an operating level (eg, about 1800 to about 3600 RPM), the bucket twist returns due to centrifugal force, the gap 510 is closed, and the ribs 420 are aligned with each other. When the shrouds are closely meshed with each other, it is possible to improve the bucket rigidity, improve the bucket damping performance, and improve the sealing performance at the radially outer position of the bucket 20.

  6 and 7 show an intermediate span shroud 610 located between the tip shroud 410 and the root portion 112. The intermediate span shroud 610 is located on the convex and concave side walls of the bucket 20. At zero speed, there is a gap between adjacent intermediate span shrouds of adjacent buckets. The gap closes as the bucket twists back as the turbine wheel begins to rotate and approaches operating speed. The intermediate span shroud is aerodynamically shaped to reduce windage loss and improve overall efficiency. If the intermediate span shrouds contact each other while the bucket twists back, the bucket stiffness and damping characteristics also improve. As the bucket returns to twist, the tip shroud 410 and the intermediate span shroud 610 contact their respective adjacent shrouds. The plurality of buckets 20 operate as one connected structure and exhibit superior stiffness and damping characteristics compared to a disjoint design. Another advantage is that the rotor vibration stress is reduced.

  As an example, the bucket of the present invention can be formed of a titanium alloy having a ratio (% by weight) shown in Table 1.

本発明のバケットを形成するのに用いるチタン基合金、即ち翼長が約４５インチ以上のバケットに用いる合金は、室温での最小極限引張強さが１４５ｋｓｉ、室温での０．２％降伏強さが１３０ｋｓｉ、４００°Ｆでの最小極限引張強さが１２５ｋｓｉ、４００°Ｆでの最小０．２％降伏強さが１１０ｋｓｉである。合金は、β又はαβ構造のどちらかを示し、最小破壊靱性が約５０ｋｓｉ・ｉｎｃｈ^１／２に達することが好ましい。

The titanium-based alloy used to form the bucket of the present invention, i.e., the bucket used with a blade length of about 45 inches or more, has a minimum ultimate tensile strength at room temperature of 145 ksi and a 0.2% yield strength at room temperature. Has a minimum ultimate tensile strength at 130 ksi and 400 ° F. of 125 ksi and a minimum 0.2% yield strength at 400 ° F. of 110 ksi. Preferably, the alloy exhibits either a β or αβ structure and a minimum fracture toughness reaches about 50 ksi · inch ^1/2 .

  Various steam turbine buckets having a blade length of about 45 inches were formed in accordance with the present invention using the above titanium alloy composition ranges. As previously mentioned, the final bucket profile and the particular alloy used will depend on a number of design factors such as the effective length of the bucket, the pitch diameter and the operating speed of the bucket in the flow region during operation. Damping, bucket fatigue, and alloy corrosion resistance at the highest predicted operating conditions also play an important role in the final design of the bucket using a titanium-based alloy that falls within the preferred composition range described above. An example of a profile of a longer blade last stage bucket that can be formed of a titanium alloy in accordance with the present invention is shown in US Pat. No. 5,393,200, “Patent for turbine last stage” assigned to the present applicant. Are listed.

  After formation, each bucket according to an embodiment of the invention was stress relieved and the bucket surface was machined to a final profile using normal finishing and heat treatment steps. Conventional mechanical strength tests and corrosion resistance tests were performed on various buckets with blade lengths of about 45 inches or more within the range of the nominal operating temperature of the last stage steam turbine to the highest predicted operating temperature. The titanium-based alloy material used in the bucket according to the present invention exhibited excellent corrosion resistance and above-average strength characteristics.

  A typical method for manufacturing a titanium-based steam turbine bucket according to the present invention includes the following steps. First, a titanium billet made of the above alloy composition is formed into a bucket and forged using a conventional screw press, hammer forging and / or hydraulic press. If desired, the forged buckets can be heat treated and quenched to relieve stress and develop mechanical strength characteristics. Depending on the specific end use, the bucket can be aged using conventional means and machined to a final operable shape (usually machining is done on all sides, ie 360 °).

  Although the above method was developed for relatively long buckets, such as last stage steam turbine buckets with blade lengths of about 45 inches or more, depending on the specific bucket design and end use conditions, the titanium alloy composition is within the above composition range. The method can be adjusted by changing within.

The bucket of the embodiment of the present invention is preferably used in the final stage of the low pressure section of the steam turbine. However, the bucket can also be used for another stage or another section (eg, a high or medium pressure section). The preferred span length of bucket 20 is about 45 inches, and this radial length can provide a final stage exhaust annulus area of about 112 ft ² (about 10.4 m ² ). This expanded (improved) exhaust annular area can reduce the loss of kinetic energy experienced when steam exits the final stage bucket. This loss reduction improves turbine efficiency.

  According to the present invention, an excellent bucket for a steam turbine is provided. The bucket of the present invention is preferably used in the final stage of the low pressure section of the steam turbine. The bucket's integral tip shroud and intermediate span damper provide excellent stiffness and damping characteristics. The shaft insertion curved dovetail also improves the distribution of mean stress and local stress at the dovetail boundary.

  While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the scope of the claims.

DESCRIPTION OF SYMBOLS 10 Steam turbine 12 Rotor 14 Shaft 16 Low pressure turbine 18 Rotor wheel 20 Bucket 22 Nozzle 24 Steam 26 Inlet 102 Blade 104 Trailing edge 106 Leading edge 108 Concave side wall 110 Convex side wall 112 Rotor blade route 114 Rotor blade tip 118 Radial length 121 Dovetail 302 Convex protrusion 410 Tip shroud 420 Seal rib 510 Gap 610 Intermediate span shroud

Claims (9)

In the bucket (20) used in the low pressure section of the steam turbine (10), the bucket (20) is formed such that the blade length (118) is about 45 inches or greater,
A dovetail portion (121) disposed near the radially inner position of the bucket;
A tip shroud (410) disposed near a radially outer position of the bucket;
An intermediate span shroud (610) disposed at a radially intermediate position between the radially inner and outer positions;
The bucket (20) is about 2% to about 6.25% aluminum, about 3.5% or less vanadium, about 2.25% or less tin, about 2.25% or less zirconium, From about 1.75 wt% to about 5.0 wt% molybdenum, up to about 2.25 wt% chromium, up to about 0.7 wt% silicon, up to about 2.3 wt% iron and the balance titanium. Formed of titanium-based alloy
Bucket (20).   The bucket according to claim 1, wherein the dovetail portion (121) comprises an axially inserted curved dovetail.   The bucket of claim 1, wherein the bucket comprises a last stage bucket. In a steam turbine including a low pressure turbine section, the low pressure turbine section includes a plurality of final stage buckets (20) having a blade length of about 45 inches or more disposed about the turbine wheel;
At least one of the last stage buckets
A dovetail portion (121) disposed near the radially inner position of the last stage bucket;
A tip shroud (410) disposed near the radially outer position of the last stage bucket;
An intermediate span shroud (610) disposed at a radially intermediate position between the radially inner and outer positions;
Each final stage bucket comprises from about 2% to about 6.25% aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium; From about 1.75 wt% to about 5.0 wt% molybdenum, up to about 2.25 wt% chromium, up to about 0.7 wt% silicon, up to about 2.3 wt% iron and the balance titanium. Formed of titanium-based alloy
Steam turbine. The steam turbine of claim 4, wherein the plurality of final stage buckets provide an exhaust annular area of about 112 ft ² or greater.   The steam turbine of claim 4, wherein the plurality of final stage buckets rotate at an operating speed of about 1,800 rpm to about 3,600 rpm.   The tip shrouds (410) of the plurality of last stage buckets are shaped to have a gap between the tip shrouds of adjacent last stage buckets, and the turbine wheel rotates at a predetermined speed or more, and the turbine wheel rotates. The steam turbine of claim 4, wherein the gap closes when the twists of the plurality of final stage buckets return.   The intermediate span shrouds (610) of the plurality of final stage buckets are shaped to have a gap between the intermediate span shrouds of adjacent final stage buckets, and the turbine wheel rotates at a predetermined speed or more, and the turbine wheel The steam turbine according to claim 4, wherein the gap is closed when the twist of the last stage bucket is returned by rotation.   The steam turbine according to claim 4, wherein the dovetail portion (121) comprises an axial insertion curved dovetail.

Images