Nozzle segment, steam turbine with diaphragms for a plurality of nozzle segments, and method of assembling the same

Abstract

The present invention relates to a one-piece or monolithic nozzle segment (30) having an airfoil (33). In one aspect of the invention, a steam turbine has a casing (17) supporting a plurality of nozzle segments (30) forming a diaphragm (22) having an airfoil (33) disposed in a flow path (23) through which a working fluid flows. The diaphragm (22) coaxially surrounds a rotational axis (A) of the steam turbine (15) and comprises a plurality of individual nozzle segments (30). The nozzle segments (30) and the casing (17) of the steam turbine (15) have substantially equal coefficients of thermal expansion. The casing (17) and the nozzle segments (30) are made of different materials, particularly different types of martensitic steel. In yet another aspect of the invention, each nozzle segment (30) is designed to withstand 100 MPa or more, or 125 MPa or more, or 150 MPa or more, under tensile stress. 5 The core (37) comprises martensitic steel having a minimum creep rupture strength at a temperature of 580°C that satisfies the condition that it will not rupture for more than 100 hours.

Description

[Technical Field]

[0001] The present invention relates to a nozzle segment of a diaphragm, a steam turbine having a casing and a diaphragm attached thereto, and a method of assembling the diaphragm. [Background technology]

[0002] U.S. Patent Application Publication No. 2006 / 0245923 discloses a turbine nozzle segment configuration including a first ring segment, a second ring segment, and a plurality of airfoils extending therebetween, the nozzle segment being fabricated from a solid ring.

[0003] Nozzle boxes made up of individual nozzle segments, each having a plurality of airfoils, are known and are described in U.S. Patent No. 7,207,773. The working fluid flows through the nozzle box in an axial direction parallel to the axis of rotation of the turbine.

[0004] U.S. Patent No. 4,776,765 describes a technique for reducing solid particle erosion by providing a protective means on at least a portion of the suction side of the nozzle partition. The nozzle can be made of martensitic chromium stainless steel, and a surface coating can be provided as the protective means.

[0005] Examples of other known nozzle configurations are described in U.S. Patent Nos. 4,025,229, 5,807,074, 6,631,858, 6,754,956, and U.S. Patent Application Publication No. 2003 / 0103845.

[0006] U.S. Patent No. 4,948,333 discloses a diaphragm supported by a turbine casing for redirecting a radial working fluid flow. The diaphragm includes two rings extending coaxially with the turbine's axis of rotation, with airfoils supported between them. The airfoils redirect the working fluid flow, which is substantially radially oriented upstream of the diaphragm, to one that includes a circumferential working fluid flow component about the axis of rotation. European Patent No. 3,412,872 also discloses a similar diaphragm.

[0007] Another device for redirecting the flow of a working fluid from a radial to an axial direction is known and is described in US Pat. No. 7,670,109.

[0008] One type of steam turbine includes a diaphragm for directing the flow of a working fluid to a first stage of turbine rotor blades connected to a rotating rotor. The rotor includes an axially extending shaft defining an axis of rotation. The diaphragm is also referred to as a "nozzle assembly." The diaphragm includes a plurality of airfoils, which may be referred to as "nozzles."

[0009] At least some steam turbines have previously suffered from diaphragm failures, such as partial cracking, bending, or even fracture of airfoils. To reduce these failures and improve component reliability and lifespan, measures have been taken to reduce thermal stresses in the components, such as by increasing the radial and axial clearances at the connections between the diaphragm and the casing of the steam turbine. However, under some operating conditions, the increased clearances can result in reduced efficiency due to leakage of working fluid.

[0010] It is therefore desirable to provide a steam turbine with a diaphragm that provides high reliability and longevity and simplifies assembly, particularly for steam turbines configured for working fluid operating temperatures above 570°C (the ultra-supercritical temperature range of the working fluid). [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent Application Publication No. 2006 / 0245923 Summary of the Invention

[0012] This problem is solved by a nozzle segment according to claim 1, a steam turbine according to claim 11 and a method for assembling a diaphragm to a casing according to claim 16.

[0013] A steam turbine includes a casing enclosing one or more turbine pressure sections having multiple rows of stator vanes coupled to the casing and rotor blades coupled to a rotor of the steam turbine.

[0014] A diaphragm is mounted on the casing, particularly downstream of the inlet passage and upstream of the first turbine pressure section, to direct the working fluid to the one or more turbine pressure sections. The diaphragm is particularly configured to direct the flow of the working fluid toward the rotor blades. In one embodiment, the diaphragm is ring-shaped and coaxially surrounds the rotational axis of the rotor of the steam turbine.

[0015] A diaphragm for a steam turbine may include separate diaphragm sections, each of which may extend substantially semicircularly around the axis of rotation of the steam turbine, and each diaphragm section may be attached to a portion of the casing (e.g., an assigned casing half) of the steam turbine.

[0016] In one aspect of the present invention, a nozzle segment for a steam turbine diaphragm is provided. Each nozzle segment includes a first ring segment and a second ring segment extending parallel to each other and spaced apart axially. The first and second ring segments support a plurality of airfoils extending from the first ring segment to the second ring segment, defining a nozzle opening between two immediately adjacent airfoils. The first ring segment can be referred to as a root, and the second ring segment can be referred to as a shroud. Preferably, the shroud is thinner than the root as viewed or measured axially. For example, each nozzle segment can include 8 to 12 airfoils. The diaphragm can include eight or more nozzle segments. However, the number of nozzle segments can vary in different embodiments.

[0017] The first and second ring segments of each nozzle segment extend along an arc about the axis of rotation of the steam turbine, and all of the nozzle segments together form an annular diaphragm.

[0018] In one aspect of the invention, the coefficient of thermal expansion of each nozzle segment is substantially equal to the coefficient of thermal expansion of a steam turbine casing supporting the nozzle segment, and preferably differs from the coefficient of thermal expansion of the casing by no more than 5%, no more than 3%, or no more than 2% at temperatures up to 600°C.

[0019] In a preferred embodiment of the present invention, each nozzle segment is subjected to a tensile stress of 100 MPa or more, or 125 MPa or more, or 150 MPa or more for 10 minutes. 5 The core comprises a martensitic steel having a minimum creep rupture strength at 580°C that satisfies the condition of not rupturing for more than 1000 hours. Creep rupture strength is determined by measuring the duration of a material probe under a given tensile stress until rupture occurs. This characteristic can be used independently of the difference in thermal expansion coefficients between the casing and the nozzle segments.

[0020] In particular, the martensitic steel of the core may be X17CrMoVNbB9-1 (commonly known as "B steel") or X22CrMoV12-1 grade 1.4923 type steel. Other steels, such as X10CrWMoVNb9-2 grade 1.4901, X14CrMoVNbN10-2, 9Cr-3W-3Co-VNbBN, or X13CrMoCoVNbNB9-2-1, may also be used to manufacture the nozzle segment 30, so long as their coefficient of thermal expansion is substantially equal to that of the casing.

[0021] In a preferred embodiment, the airfoil, first ring segment, and second ring segment of each nozzle segment are integrally or monolithically fabricated from the same material without any seams or joints. For example, each nozzle segment can be machined from a single, solid initial workpiece. During machining, material can be removed from the solid workpiece (e.g., by milling or erosion) to obtain the desired configuration of the nozzle segment. Alternatively, each nozzle segment can be manufactured by additive manufacturing techniques. In this embodiment, each nozzle segment does not include any welded joints, adhesive joints, form-fit joints, or material bonds, particularly between the airfoil and ring segment.

[0022] Alternatively, in another embodiment, the airfoil and ring segments may be manufactured separately and then connected to form the nozzle segment, and in particular the connection between the airfoil and ring segment may be established by a welded joint.

[0023] Advantageously, the core of each nozzle segment is coated on one or more surface regions with a surface coating that can make the nozzle segment less susceptible to high temperature oxidation and solid particle erosion compared to the material of the nozzle segment core.

[0024] The surface coating may include one or more of chromium, carbon, and nickel. In one embodiment, the surface coating may include chromium carbide (Cr3C2), nickel chromium (NiCr), or a combination thereof.Alternatively, the surface coating may comprise one or more of titanium, aluminum, and nitrogen, and in one embodiment, may comprise titanium aluminum nitride (TiAlN).

[0025] The surface area to which the surface coating is applied is preferably the surface of the airfoil. The surface coating may partially or completely cover only the surface of the airfoil. Alternatively, the surface coating may also cover at least a portion of the first or second ring segment of the nozzle segment, preferably the surface area exposed to the flow of working fluid.

[0026] The surface coating can be applied to one or more surface regions of the nozzle segment by thermal spraying, preferably by high-velocity oxygen-fuel (HVOF) or high-velocity air-fuel (HVAF) spraying. For example, the coating material in powder form can be fed into a burner and projected by a high-velocity gas jet onto one or more surface regions to be coated. The surface regions to be coated may be roughened prior to application of the coating material to improve adhesion.

[0027] By providing a nozzle segment that includes multiple airfoils, larger units can be handled during diaphragm assembly and disassembly, and the nozzle segment is less susceptible to excitation caused by working fluid flow compared to diaphragm configurations with individual airfoils.

[0028] The material of the casing supporting the nozzle segments is different from the material of the nozzle segments, in particular the material of the nozzle segments has a higher creep strength than the material of the casing supporting the nozzle segments.

[0029] Nozzle segments are subject to length fluctuations due to temperature changes. This can lead to mechanical stresses that can shorten the diaphragm's lifespan or cause it to fail during operation. The thermal expansion coefficients of the nozzle segments and the casing are substantially equal in the relevant temperature range, particularly above 570°C and up to 650°C, allowing the nozzle segments to expand or contract in the same way as the casing. This reduces mechanical stresses on the components. Clearances between directly adjacent nozzle segments and between the nozzle segments and the casing's support structures, which are provided during installation, can be minimized. This results in improved reliability as well as steam turbine efficiency.

[0030] The casing supporting the nozzle segments can be made of a different martensitic steel than the martensitic steel used for the core of the nozzle segments. For example, the martensitic steel used for the casing can be GX12CrMoVNbN9-1 material number 1.4955 as specified in EN 10213 "Pressure Casting Steel."

[0031] In a preferred embodiment of the steam turbine, each nozzle segment corresponds to an embodiment of a nozzle segment according to the first aspect of the invention.

[0032] A preferred embodiment of the steam turbine includes two opposing casing grooves that open on opposite sides. The casing grooves form a support structure for the casing configured to support the nozzle segments. The casing grooves can extend coaxially or circumferentially around the rotational axis of the steam turbine. Each nozzle segment can have a first casing groove engaged with a first ring segment and a second casing groove engaged with a second ring segment. The casing grooves are spaced apart axially from one another, such that the airfoils of the nozzle segments are positioned in gaps between the casing grooves within the working fluid flowpath.

[0033] The casing of the steam turbine may include a first casing half and a second casing half. One group of nozzle segments may be disposed in the first casing half to form a first diaphragm section. Another group of nozzle segments may be disposed in the second casing half to form a second diaphragm section. The two separate diaphragm sections facilitate easy assembly and disassembly of the casing halves of the steam turbine casing.

[0034] Preferably, at least the outermost nozzle segment of each diaphragm section is fixed against movement along the extension direction of the casing groove by one or more clamping elements. The clamping elements can each form a form-fit and / or force-fit connection between the outermost nozzle segment and the casing half. The outermost nozzle segment is the nozzle segment in each casing half that is directly adjacent to the dividing surface where the first casing half and the second casing half are connected. By fixing the outermost nozzle segment, the intermediate nozzle segment of each diaphragm section is also held between the two outermost nozzle segments of each diaphragm section.

[0035] In one embodiment of the steam turbine, each nozzle segment can have a ring segment groove in one of the ring segments, preferably the first ring segment. The casing of the steam turbine, particularly both casing halves, has arc-shaped protrusions that engage with the ring segment grooves of the respective nozzle segments. Thus, the arc-shaped protrusions of the first casing half engage with the ring segment grooves of the nozzle segments to form a first diaphragm section, and one arc-shaped protrusion of the second casing half engages with the ring segment grooves of the nozzle segments to form a second diaphragm section. Preferably, each arc-shaped protrusion extends radially from one side wall of the casing groove relative to the rotational axis of the steam turbine.

[0036] The diaphragm or diaphragm section may be assembled with a casing or casing half of a steam turbine as described below.

[0037] For assembly, nozzle segments are provided, each having a first ring segment, a second ring segment, and a plurality of airfoils extending from the first ring segment to the second ring segment. A steam turbine casing is provided, having a first casing half and a second casing half. The first casing half may be the upper casing half and the second casing half may be the lower casing half, or vice versa. Each casing half is provided with a semicircular first casing groove and a semicircular second casing groove arranged opposite each other.

[0038] The prepared nozzle segment grooves are used to form the first diaphragm section in the first casing half. To do this, one of the nozzle segments is inserted into the opposing casing groove and moved to the desired position. The nozzle segment is fixed in the desired position by suitable clamping means, such as a clamp or brake strip inserted between the nozzle segment and the wall of the first or second casing groove. The remaining nozzle segments of the first diaphragm section are then similarly inserted into the casing grooves of the first casing half. If necessary or advantageous, clearance shims may be placed between adjacent nozzle segments of the first diaphragm section.

[0039] The second diaphragm section is assembled in the second casing half in the same manner as the first diaphragm section.

[0040] Preferably, the outermost nozzle segments of each diaphragm section are secured to their respective casing halves by one or more fastening elements, e.g., fastening pins. The outermost nozzle segments of each diaphragm section are the two nozzle segments directly adjacent to the dividing plane between the first and second casing halves. The casing halves are joined together along the dividing plane. Preferably, the dividing plane extends horizontally.

[0041] Once the outermost nozzle segments of each diaphragm section have been secured by fastening means, in particular fastening pins, the clearance shims can be subsequently removed so that the nozzle segments of each diaphragm section are positioned adjacent to one another with a predetermined clearance.

[0042] If desired, the fastening elements, particularly the fastening pins, can be machined or processed to a desired outer contour that matches the outer contour of the adjacent nozzle segments so as not to interfere with or disrupt the connection of the two diaphragm sections when the first and second casing halves are coupled together. When the casing halves are coupled together, the diaphragm sections preferably form a closed circular ring that is coaxially disposed about the rotational axis of the steam turbine.

[0043] Preferred embodiments of the steam turbine and method are disclosed in the dependent claims, the detailed description and the drawings. Preferred embodiments of the invention will now be described in detail with reference to the accompanying drawings. [Brief explanation of the drawings]

[0044] [Figure 1] 1 illustrates a cross-sectional view along the axis of rotation of an embodiment of a steam turbine having a casing and a diaphragm attached to the casing within a working fluid flow path. [Figure 2] FIG. 2 is an enlarged view of section II of FIG. 1, showing the arrangement of the diaphragm on the casing. [Figure 3]FIG. 10 is a perspective view of an embodiment of a diaphragm formed by a plurality of arcuate nozzle segments. [Figure 4] FIG. 4 is a perspective view of an embodiment of the nozzle segment of FIG. 3. [Figure 5] 2 is a schematic diagram of a first casing half having a first diaphragm section and a second casing half having a second diaphragm section. FIG. [Figure 6] 1A to 1C are diagrams illustrating an assembly process for assembling a nozzle segment into a casing groove of a steam turbine casing. [Figure 7] 1A to 1C are diagrams illustrating an assembly process for assembling a nozzle segment into a casing groove of a steam turbine casing. [Figure 8] 1A to 1C are diagrams illustrating an assembly process for assembling a nozzle segment into a casing groove of a steam turbine casing. [Figure 9] 3 is a flow diagram of an embodiment of an assembly method for assembling a diaphragm to a casing of a steam turbine. [Figure 10] The temperature-dependent thermal expansion coefficients of various steels are shown. [Figure 11] 1 shows a schematic cross-section of an airfoil of a nozzle segment. [Figure 12] 1 shows a schematic cross-section of an airfoil of a nozzle segment. DETAILED DESCRIPTION OF THE INVENTION

[0045] 1 illustrates an embodiment of a steam turbine 15 in cross section along a rotation axis A. The rotation axis A is defined by a shaft 16 that is rotatably supported in a casing 17 of the steam turbine 15. In a preferred embodiment, the casing 17 comprises a first casing half 17a and a second casing half 17b that are joined together along a preferably horizontally extending dividing plane P. The dividing plane P is illustrated schematically in FIGS. 3 and 5.

[0046] A steam turbine includes one or more pressure sections, and may have multiple pressure sections, such as a high-pressure section and an intermediate-pressure section. Each pressure section includes stationary vanes 18 arranged in a ring about the axis of rotation A and coupled to a casing 17. The rotating blades 19 and shaft 16 of each pressure section are part of the rotor 20 of the steam turbine.

[0047] A working fluid flows along a fluid path within the casing 17, and the stationary vanes 18 and the rotating blades 19 are disposed within the fluid path of the working fluid to drive the rotor 20. The working fluid is used to rotate the rotor 20 about a rotation axis A.

[0048] In this specification, the axial direction D is a direction parallel to the rotation axis A. Any radial direction of the rotation axis A is called the radial direction. A direction along a circular path around the rotation axis A or the axial direction D is called the circumferential direction C.

[0049] The steam turbine includes an inlet passage 21, also referred to as an inlet scroll, upstream of the first pressure section. The inlet passage 21 extends in a circumferential direction C about the axis of rotation A inside the casing 17. A diaphragm 22 is positioned to guide the flow of working fluid through a fluid connecting passage 23 downstream of the inlet passage 21 and upstream of one or more pressure sections. The inlet passage 21, the diaphragm 22, and the fluid connecting passage 23 are partially illustrated in the enlarged view of FIG. 2 corresponding to the section designated II in FIG. 1. The flow of working fluid upstream of the diaphragm 22 is substantially radial toward the axis of rotation A. The diaphragm 22 is configured to redirect this flow to include a flow direction component in the circumferential direction C.

[0050] The fluid connecting passage 23 fluidly connects the inlet passage 21 to one or more pressure sections of the steam turbine 15. Adjacent to the fluid connecting passage 23, the casing 17 includes a first casing groove 24 and a second casing groove 25 arranged at a distance from each other in the axial direction D. The casing grooves 24, 25 are aligned with each other such that the open sides of the casing grooves 24, 25 face each other in the axial direction D. The fluid connecting passage 23 extends between the casing grooves 24, 25. The casing grooves 24, 25 extend coaxially with the rotation axis A. They are configured to support the diaphragm 22 such that the diaphragm 22 extends coaxially around the rotation axis A of the steam turbine 15.

[0051] 3 and 4, the diaphragm 22 includes a plurality of nozzle segments 30. Each nozzle segment 30 extends in an arc in a circumferential direction C about the axis of rotation A. All of the nozzle segments 30 together form a closed ring.

[0052] In the preferred embodiment, the diaphragm 22 includes eight nozzle segments 30. However, the number of nozzle segments 30 on the diaphragm 22 can vary and may be more or less in other embodiments.

[0053] As specifically shown in Figure 4, each nozzle segment 30 includes a first ring segment 31 and a second ring segment 32. The two ring segments are axially spaced apart from one another. A plurality of airfoils 33 extend between the first ring segment 31 and the second ring segment 32, and the ring segments 31, 32 are connected to one another by the plurality of airfoils 33 to form an integral or monolithic nozzle segment 30. The number of airfoils 33 in each nozzle segment 30 can vary, and in this example, each nozzle segment can include 8 to 12 airfoils 33.

[0054] Two immediately adjacent airfoils 33 of nozzle segment 30 define an opening 34 in diaphragm 22 through which working fluid flows. As best seen in FIG. 2 , airfoils 33 and opening 34 are positioned within fluid connecting passage 23 such that working fluid flows from inlet passage 21 through opening 34 in diaphragm 22 toward one or more pressure sections of steam turbine 15.

[0055] As can be seen from Figures 3 and 4, adjacent nozzle segments 30 have a first face 35 mating with the circumferential end of the first ring segment 31 and a second end face 36 at the circumferential end of the second ring segment 32. The end faces 35, 36 at each circumferential end of the nozzle segments 30 preferably extend within a common mid-plane S. This mid-plane S may be aligned parallel to the axial direction D in one dimension and may be inclined relative to the circumferential direction C. This means that the mid-plane S is not oriented perpendicular to the circumferential direction C, but rather includes an acute angle α with the circumferential direction C, as shown schematically in Figures 4 and 7. The angle α may be equal for all mid-planes S between two adjacent nozzle segments 30.

[0056] The inclined end faces 35, 36 provide overlapping regions where the first ring segment 31 and the second ring segment 32 overlap, respectively. The overlapping regions are located inside the first casing groove 24 and the second casing groove 25, respectively.

[0057] A schematic diagram of the casing 17, which includes a first casing half 17a and a second casing half 17b, is shown in Figure 5. The semicircular portions of the casing grooves 24, 25 are provided in the first casing half 17a, and the other semicircular portions of the casing grooves 24, 25 are provided in the second casing half 17b. One group of nozzle segments 30 located in the first casing half 17a forms the first diaphragm section 22a, and the nozzle segments 30 located in the second casing half 17b form the second diaphragm section 22b. Each diaphragm section 22a, 22b extends substantially semicircularly. In a fully assembled state, the two diaphragm sections 22a, 22b form a ring-shaped diaphragm 22 coaxially arranged around the rotation axis A. In this assembled state, the two casing halves 17a, 17b are connected to each other along a parting plane P.

[0058] The casing 17, as well as the casing halves 17a, 17b in this example, are made from a steel alloy including martensitic steel. Preferably, at least the support structure of the casing halves 17a, 17b, including the casing grooves 24, 25, includes or is made of martensitic steel. The martensitic steel used for the casing 17 is preferably Stg9T type steel. The temperature-dependent normalized thermal expansion coefficient of Stg9T type steel is shown in FIG. 10.

[0059] Due to the different requirements for the mechanical properties of the casing 17 and the diaphragm 22, the steel grade used for the casing 17 is not suitable for manufacturing the diaphragm 22. In previous steam turbines, the diaphragm 22 was manufactured from the steel grade X10CrNiW17-13-3, especially for applications where the working fluid temperature exceeded 570°C (ultra-supercritical operating conditions). However, additional measures had to be taken to combine this austenitic material with the casing, such as inserting an intermediate layer (e.g., a 617 alloy weld layer) in the second casing groove 25 to accommodate the different mechanical properties of the steel grades used for the diaphragm 22 and the casing 17. This additional intermediate layer prevented or at least reduced failures due to mechanical stresses, especially failures due to different thermal expansion coefficients (see FIG. 10).

[0060] The present invention addresses this problem by using a material for the manufacture of the diaphragm 22 or nozzle segment 30, respectively, that is compatible with the martensitic steel grade used to manufacture the casing 17.

[0061] In the present invention, the steel contained in or from which the nozzle segments 30 are made has a coefficient of thermal expansion substantially equal to the coefficient of thermal expansion of the steel contained in or from which the casing 17 is made (at least in the support structure of the diaphragm 22 with the casing grooves 24, 25), except that the steel type used for the casing 17 is not suitable for manufacturing the nozzle segments 30.

[0062] In one embodiment, the steel used to manufacture the nozzle segments 30 is a martensitic steel having higher mechanical strength, particularly higher tensile strength and / or creep strength, than the martensitic steel used in the casing 17. Preferably, X17CrMoVNbB9-1 (commonly known as "B steel") or St12T type steel is used to manufacture the nozzle segments 30. In a preferred embodiment, the martensitic steel used to manufacture the nozzle segments 30 has a minimum creep strength at a temperature of 580°C. The minimum creep strength of this core martensitic steel is such that the time to creep rupture under a tensile stress of 100 MPa or more, or 125 MPa or more, or 150 MPa or more is at least 10 minutes. 5 The condition of time is satisfied at a temperature of 580°C. Other steels such as X10CrWMoVNb9-2 material no. 1.4901, X14CrMoVNbN10-2, 9Cr-3W-3Co-VNbBN or X13CrMoCoVNbNB9-2-1 may also be used to manufacture the nozzle segment 30 as long as their coefficient of thermal expansion is substantially equal to that of the casing.

[0063] Figure 10 shows that the thermal expansion coefficient of steel B is substantially equal to that of steel Stg9T at least in the temperature range up to 600°C. Specifically, in this temperature range, as shown in Figure 10, the difference between the thermal expansion coefficient of the casing material and the thermal expansion coefficient of the nozzle segment material is less than 0.05, preferably less than 0.02.

[0064] In a preferred embodiment, each nozzle segment 30 has a core 37 made of a martensitic steel (e.g., B steel) having a minimum creep strength. One or more surface regions of each nozzle segment 30 may be covered with a surface coating 38. The surface region of each nozzle segment 30 covered with the surface coating 38 may be the surface of the airfoil 33, as shown in Figures 11 and 12. The surface coating 38 may have a uniform thickness (Figure 11), or the surface coating may have a varying thickness (Figure 12). In the latter case, the surface coating 38 may be thicker in areas of the airfoil 33 that are more susceptible to wear, particularly at and near the leading edge, and thinner in the area of ​​the trailing edge 33 of each airfoil. Note that Figures 11 and 12 are schematic and not to scale.

[0065] The surface coating 38 may include one or more of chromium, carbon, and nickel. Preferably, the surface coating 38 includes one or more of chromium carbide (Cr3C2) and nickel chromium (NiCr). It may be applied by thermal spraying to one or more surface regions of each nozzle segment 30, particularly the surfaces of the airfoils 33. For example, high velocity oxygen-fuel (HVOF) or high velocity air-fuel (HVAF) thermal spraying may be used. The material for the surface coating 38 may be supplied in powder form and sprayed at high velocity by a thermal spray device onto the surface regions to be coated.

[0066] Each nozzle segment 30, and in this example the core 37 of each nozzle segment 30, is made from the same continuous material (particularly B steel) without seams or joints. For example, each nozzle segment forms an integral unit. Preferably, there are no welded joints, adhesive joints, bolted joints, etc. between the airfoils 33 and the first and second ring segments 31, 32. Alternatively, the airfoils 33 of each nozzle segment 30 may be joined to the ring segments 31, 32 by welding or other suitable methods.

[0067] As is apparent from FIG. 4 , the axial dimension D of the first ring segment 31 may be greater than the dimension of the second ring segment 32. In this example, one or more radially facing side surfaces of the first ring segment 31, preferably the side surface facing distally from the axis of rotation A, are provided with ring segment grooves 42. When the nozzle segment 30 is inserted into the first and second casing grooves 24, 25 of each casing half 17 a, 17 b, protrusions 43 extending from the side surfaces defining the first casing groove 24 engage with the ring segment grooves 42, as most clearly seen in FIGS. 2 and 7 . This form fit can also be used to secure the nozzle segment 30 in the circumferential direction C during assembly, as will be explained in more detail with reference to FIGS. 6-9 .

[0068] FIG. 9 is a flow chart of one embodiment of a method for assembling diaphragm 22 to casing 17 of steam turbine 15 .

[0069] In a first step 100, the required number of nozzle segments 30 (e.g., eight nozzle segments 30) and two casing halves 17a and 17b are prepared. Subsequently, in a second step 101, one of the nozzle segments 30 is inserted into the casing groove 24, 25 of the first casing half 17a. The inserted nozzle segment 30 is fixed using a clamping element 44 to form a force fit between the inserted nozzle segment 30 and the first casing half 17a. In this embodiment, the clamping element 44 has the form of a clamp or braking strip 45 that is arranged between the bottom of the ring segment groove 42 and the free end of the protrusion 43, and creates a clamping effect when the nozzle segment 30 is moved onto the braking strip 45 or when the braking strip 45 is inserted from one end into the gap between the protrusion 43 and the bottom of the ring segment groove 42 (see FIG. 7).

[0070] A single clamping element 44 can be used to form a press fit between the first casing half 17a and two immediately adjacent nozzle segments 30. Specifically, the damping strip 45 forming the clamping element 44 can have a section that is positioned within the ring segment groove 42 of one nozzle segment 30 and another section that extends therefrom, as shown in FIG. 7 . The next adjacent nozzle segment 30 can be moved onto the accessible section of the damping strip 45 to create the desired clamping effect. The clamping element 44 or damping strip 45 is used to hold the inserted nozzle segment 30 in the desired position during assembly. They do not need to be removed and may remain on the completed casing 17.

[0071] If it is desired to create a predetermined clearance between two adjacent nozzle segments, a clearance shim 46 may be placed in the first casing groove 24 in a second step 101. The clearance shim 46 may have a plate-like configuration and extend along a mid-plane S. Two adjacent nozzle segments 30 may abut against the clearance shim 46 from both sides.

[0072] In a third step 102, it is checked whether the first diaphragm section 22a is complete, i.e., whether all of the nozzle segments 30 forming the first diaphragm section 22a have been inserted into the casing grooves 24, 25 of the first casing half 17a. If so, the method proceeds to a fourth step 103 (OK branch from the third step 102). If not, the method repeats the second step 101 again, inserting and fixing the next nozzle segment 30 of this first diaphragm section 22a (NOK branch from the third step 102).

[0073] In a fourth step 103, the two outermost nozzle segments 30 are each fastened with a clamping element 47 (in this example, a clamping pin 48). The clamping element 47 is inserted by a force fit into a clamping area at the end of the first casing groove 24 adjacent to the dividing plane P. This clamping area 49 is formed by a casing cavity 50 provided at the bottom of the first casing groove 24 and an aligned segment cavity 51 provided in the first ring segment 31 of the outermost nozzle segment. The segment cavity 51 generally opens on the side of the first ring segment 31 facing distally from the airfoil 33. In this example, the casing cavity 50 and the segment cavity 51 define a cross section perpendicular to the circumferential direction C that coincides with the cross section of the clamping element 47. In this example, this cross section is circular.

[0074] The clamping pins 48 are pressed into the openings defined by the cooperating or aligning cavities 50, 51 so that a snug press fit is formed. Alternatively / in addition, a material bond can be provided between the clamping pins 48 and the surfaces defining the casing cavity 50 and / or the segment cavities 51. This material bond can be created by gluing and / or welding.

[0075] In this manner, both outermost nozzle segments 30 located immediately adjacent to the dividing plane P are secured within the first casing half 17a. If clearance shims 46 have been inserted between adjacent nozzle segments, these can then be removed.

[0076] After the clamping pin 48 is inserted, the end of the clamping pin 48 may be machined or otherwise removed so that it does not extend beyond the edge defined by the first surface 35 and the adjacent casing surface 52 where the opening of the casing cavity 50 is located, as shown in FIG. 8. In this example, the end of the clamping pin 48 is removed to form a chamfer with two inclined surface areas, one of which extends parallel to the first surface 35 and the other of which extends parallel to the bottom of the first casing groove 24. In this way, the connection between the two diaphragm sections 22 a, 22 b is not disturbed by the clamping pin 48 securing the respective outermost nozzle segments 30.

[0077] Alternatively, the clamping element 47 or clamping pin 48 may have the required shape or contour at its end before being inserted into the clamping area 49 .

[0078] Removal of the clearance shims 46 and processing of the ends of the clamping pins can be performed any time during assembly after the outermost nozzle segment 30 is secured and before the casing halves 17a, 17b are joined together.

[0079] After assembling first diaphragm section 22a to first casing half 17a, second diaphragm section 22b is assembled to second casing half 17b in fifth step 104, sixth step 105, and seventh step 106 of the method. Steps 104-106 correspond to steps 101-103.

[0080] Finally, after assembly of both diaphragm sections 22a, 22b is complete, in an eighth step 107, the casing halves 17a, 17b are joined together.

[0081] The present invention relates to a one-piece or monolithic nozzle segment 30 having an airfoil 33. In one aspect of the invention, a steam turbine has a casing 17 supporting a plurality of nozzle segments 30 forming a diaphragm 22 with airfoils 33 located in a flow path 23 through which a working fluid flows. The diaphragm 22 coaxially surrounds the rotational axis A of the steam turbine 15 and comprises a plurality of individual nozzle segments 30. The nozzle segments 30 and the casing 17 of the steam turbine 15 have substantially equal coefficients of thermal expansion. The casing 17 and the nozzle segments 30 are made of dissimilar materials, particularly different types of martensitic steel. In yet another aspect of the invention, each nozzle segment 30 is configured to withstand 100 MPa or more, or 125 MPa or more, or 150 MPa or more, under tensile stress. 5 The core 37 includes a martensitic steel having a minimum creep strength that satisfies the condition of at least 100 hours at a temperature of 580°C. [Explanation of symbols]

[0082] 15 Steam turbine 16 shaft 17 Casing 17a First casing half 17b Second casing half 18 Stator blade 19 Moving blade 20 rotors 21 Inlet channel 22 diaphragm 23 fluid connection channel 24 First casing groove 25 Second casing groove 30 nozzle segments 31 First Ring Segment 32 Second Ring Segment 33 Airfoil 34 Aperture 35 First Side 36 Second Side 37 cores 38 Surface Film 42 Ring segment groove 43 Protrusion 44 clamping elements 45 Braking Strips 46 Clearance shim 47 Fastening element 48 Fastening pin 49 Tightening area 50 casing cavity 51 Segment Cavity 52 Casing surface 100 First step 101 Second step 102 Third step 103 Fourth step 104 Fifth step 105 Sixth Step 106 Seventh Step 107 Eighth Step α angle A rotation axis C circumferential direction D-axis direction P split plane S intermediate plane

Claims

1. Nozzle segments (30) for a diaphragm (22) of a steam turbine (15), the nozzle segments (30) configured to be mounted to a casing (17) of the steam turbine (15), each nozzle segment (30) comprising a first ring segment (31), a second ring segment (32) extending parallel to the first ring segment (31), and a plurality of airfoils (33) extending between the first ring segment (31) and the second ring segment (32); Each nozzle segment (30) has a core (37) containing martensitic steel, and the thermal expansion coefficient of each nozzle segment (30) differs by no more than 5% from the thermal expansion coefficient of a casing (17) of a steam turbine (15) in a temperature range of 600°C or less.

2. The martensitic steel is subjected to tensile stress of 100 MPa or more, 125 MPa or more, or 150 MPa or more for 10 ５ 2. The nozzle segment of claim 1, having a minimum creep rupture strength at a temperature of 580°C that satisfies the condition that it will not rupture for more than 100 hours.

3. 3. The nozzle segment (30) of claim 1 or claim 2, wherein the airfoil (33), first ring segment (31), and second ring segment (32) are machined integrally from the same solid material workpiece without seams or joints.

4. 3. The nozzle segment of claim 1 or claim 2, wherein the airfoil (33), first ring segment (31) and second ring segment (32) are made separately and then connected to one another.

5. A nozzle segment according to any one of claims 1 to 4, wherein the core (37) is made of X17CrMoVNbB9-1.

6. 6. The nozzle segment of claim 1, wherein one or more surface regions of the nozzle segment include a surface coating, the surface coating comprising one or more of chromium, carbon, and nickel, or one or more of titanium, aluminum, and nitrogen.

7. The surface coating is chromium carbide (Cr ３ C ２ 7. The nozzle segment of claim 6, comprising one or more of nickel chromium (NiCr), nickel chromium (NiCr), and titanium aluminum nitride (TiAlN).

8. A steam turbine (15), a casing (17) enclosing one or more turbine pressure section vanes (18) and rotor blades (19) coupled to the casing (17); a diaphragm (22) attached to the casing (17) and including a plurality of nozzle segments (30), each of the nozzle segments (30) being a nozzle segment according to any one of claims 1 to 7; and A steam turbine (15) comprising:

9. The steam turbine of claim 8, wherein at least a support structure of a casing (17) supporting the diaphragm (22) is made of a material different from the material of the nozzle segments (30).

10. The steam turbine of claim 9, wherein the creep rupture strength of the material of the nozzle segments (30) is greater than the creep rupture strength of the material of the supporting structure of the casing (17).

11. A steam turbine according to any one of claims 8 to 10, wherein the nozzle segments (40) are arranged in two opposing casing grooves (24, 25).

12. 11. The steam turbine of claim 8, wherein the casing (17) comprises a first casing half (17a) and a second casing half (17b), wherein a group of nozzle segments (30) attached to the first casing half (17a) form a first diaphragm section (22a), and a group of nozzle segments (30) attached to the second casing half (17b) form a second diaphragm section (22b).

13. A method of assembling a diaphragm (22) to a casing (17) of a steam turbine (15), comprising the steps of: (a) providing a plurality of monolithic nozzle segments (30), each including a first ring segment (31), a second ring segment (32) extending parallel to the first ring segment (31), and a plurality of airfoils (33) extending between the first ring segment (31) and the second ring segment (32), wherein the thermal expansion coefficient of each nozzle segment (30) differs by no more than 5% from the thermal expansion coefficient of a casing (17) of a steam turbine (15) in a temperature range of 600°C or less; (b) preparing a first casing half (17a) and a second casing half (17b) of a turbine casing (17), the first casing half (17a) and the second casing half (17b) having a semicircular first casing groove (24) and a semicircular second casing groove (25) respectively arranged opposite to each other; (c) inserting one of the nozzle segments (30) into the first and second casing grooves (24, 25) of the first casing half (17a) and fixing the inserted nozzle segment (30) by one or more clamping elements (44) arranged in one of the casing grooves (24, 25); (d) repeating the previous step (c) with other nozzle segments (30) to form a first semicircular diaphragm section (22a) from the plurality of nozzle segments (30) in the first casing half (17a); (e) repeating the previous steps (c) and (d) on a second casing half (17b) to form a second semicircular diaphragm section (22b) from the plurality of nozzle segments (30) in the second casing half (17a); A method comprising:

Images