JP2010515849A - High corrosion resistant fixed blade assembly for steam turbines, especially geothermal impulse turbines - Google Patents

Abstract

  A highly corrosion-resistant stationary blade assembly (20) for a steam turbine, in particular a geothermal impulse turbine, is supported by an outer ring (21) and an inner ring (22) at longitudinally opposite ends (27, 28), respectively. Formed by an array (11) of blades (12); Each blade (12) formed of a nickel-based alloy, for example an Hastelloy® type alloy or similar material, is integrally connected to the ring (21, 22) and has no structural welds at all. It is supported by these rings via mechanical joints (31, 32).

Description

  The present invention relates to a highly corrosion-resistant fixed blade assembly for a steam turbine, particularly a geothermal impulse turbine, and more particularly to a steam turbine provided with the fixed blade assembly, particularly a geothermal impulse turbine.

  As is well known, in a steam turbine, the transfer of energy from the steam to the shaft extends to the nozzles that make up the stationary part of the turbine, increasing the speed and thus converting the heat energy of the steam into kinetic energy. Occurs through a series of dilations. Kinetic energy is transmitted to the shaft via movable blades attached around the shaft itself.

  In order to use energy in an efficient way inside the steam, the steam turbine is usually composed of a number of successive stages, whereas the axial turbine is arranged coaxially with the rotating axis of the moving blade (machine axis) Thus, the steam discharged from one stage flows directly into the next stage somewhat in the axial direction.

  In an impulse turbine, each stage is formed by a movable blade assembly formed by an array of blades conveyed integrally by a shaft and an array of stationary blades, commonly referred to as a diaphragm, opposite to the movable blade assembly, to the machine shaft. It has a stationary blade assembly that is suitably shaped and arranged in the radial direction. The stationary blades are supported by a pair of radially inner and radially outer rings, usually composed of a pair of joined half rings, each blade welded to a support ring at both ends.

  It is also well known that steam turbines find one of their possible applications in geothermal power plants. In these plants, the fluid released to the turbine is composed of naturally occurring endogenous steam, i.e., steam generated directly in the ground, into a conventional boiler or combined cycle system supplied with coal or nuclear fuel. It is an alternative to the steam produced by some heat recovery boilers.

With respect to steam generated in a boiler, endogenous steam is not only characterized at significantly lower conditions than the standard thermodynamic conditions (substantially pressure and temperature), but also highly dependent on the position at which the steam is extracted. It is also characterized by a chemical action that is not.
Thus, unlike other types of systems where a dedicated system for treating the condensate used to generate steam ensures optimal steam characteristics, steam supplied to a turbine in a geothermal system is typically It is by no means optimal for chemical infringement. In addition to non-condensable gases, endogenous steam effectively imparts aggressive properties that are not entirely present in industrial steam, resulting in various forms in various forms that form deposits on steam turbine blades. Contains substances as standard.

  These effects are particularly noticeable at the stage where steam expanding into the turbine passes from superheat to saturation, resulting in severe erosion / erosion on the stationary and moving blades.

These blades (fixed blades and movable blades) are typically formed of martensitic stainless steel, such as AISI 403 type or the like. These materials have a good compromise between mechanical and corrosion resistance and are therefore widely used in steam turbines even in geothermal systems.
Nonetheless, these materials can also move from a superheated state to a wet state where the condensation mechanism increases the concentration of impurities in the steam, especially when chloride is present, as in the case of geothermal system steam turbines. In the transition stage, there is a problem that resistance to corrosion and stress corrosion is weak.

  In particular, the service life for fixed blades is relatively short, especially for blades in turbine stages where phase changes occur, resulting in operational costs related to the need to replace blades relatively frequently. It will be expensive. Clearly, plant downtime for performing maintenance operations also generates significant costs due to manufacturing waste.

  Second, as indicated above, in the case of blade degradation, the fixed blades of the impulse turbine are welded at their ends to support the ring (half ring), requiring the entire diaphragm to be replaced. Become. In fact, in a well-known solution, any mechanical coupling system for both ends of the stationary blade to support the ring not only replaces the positioning or centering of the blade, but also has no structural function, Originally it cannot support the blades independently and always requires structural welding to ensure the blades are secured.

  One object of the present invention is to provide a high corrosion resistant fixed blade assembly for a steam turbine, in particular a geothermal impulse turbine, which does not have any of the problems of the prior art.

  The invention therefore relates to a highly corrosion-resistant fixed blade assembly for a steam turbine, in particular a geothermal impulse turbine, as defined essentially in the appended claim 1 and in the dependent claims for preferred embodiments thereof.

FIG. 1 is a partial schematic view of a longitudinal cross-sectional view of a geothermal steam turbine with a fixed blade assembly according to the invention.

  The invention will be further described by way of example in the following non-limiting embodiments with reference to the accompanying drawings.

  In the figure, the steam turbine is generally well known per se and is therefore schematically and partially illustrated and indicated by reference numeral 1. In particular, the turbine 1 is an axial-flow impulse turbine used in a geothermal power plant. The direction of steam in the turbine 1 is schematically indicated by the arrow 3.

  In general terms, the turbine 1 includes a casing 2 that includes a stator 5 that is integrally mounted by a casing 2, and a rotor disk 6 that is integrally connected to a drive shaft 7 that passes through the casing 2 along the rotation axis A. As usual, the casing 2 accommodates a number of successive steps 10. Each stage 10 is mounted with a shaft 7 and rotates integrally with the shaft 7 and an array 11 of fixed stator blades 12 that are integrated with the casing 2 and project from the inner wall 13 of the casing 2 in a substantially radial direction with respect to the axis A. Defined by an array 14 of movable rotor blades 15.

  Each stage 10 constitutes a diaphragm of stage 10 and is formed by blades 12 and two support rings 21 and 22, which are respectively radially outward and radially inward and substantially coaxial about axis A, It has a stationary blade assembly 20 formed by a pair of half-rings 23 and 24, preferably in close proximity. The blade 12 has a substantially well-known geometric shape and profile, is arranged substantially in a radial pattern around the axis A, and faces the shaft 7 between the rings 21 and 22 inside the wall 13 of the casing 2. It extends in the radial direction.

  The outer ring 21 formed by the half ring 23 is fixed to the wall 13 by a known method, and the inner ring 22 formed by the half ring 23 is mounted with a known type sealing body 25 facing the shaft 7.

  Each blade 12 has a longitudinal axis L between two ends 27 and 28 with respective connecting portions 29 and 30 projecting laterally from two longitudinally opposite ends 27 and 28 of the blade 12. Extend along. These connecting portions 29 and 30 are formed as an integral part of the blade 12 in the same material as the blade 12.

  Each blade 12 is integrally connected to rings 21 and 22 (and specifically to half ring 23 and half ring 24) and is connected by rings 21 and 22 via mechanical joints 31 and 32 without any structural welds. It is supported and can therefore be removed from the rings 21 and 22 (or the half rings 23 and 24) without breaking the rings 21 and 22. The joints 31 and 32 have an undercut 33 that fixes the blade 12 to the rings 21 and 22 in the longitudinal direction along the longitudinal axis L of the blade 12.

  The term “structural weld” is intended to indicate a weld that can ensure a stable connection of the components under normal operating conditions, and in particular, the rings 21 and 22 effectively use the blade 12 during use. It can be ensured to support.

  In particular, each blade 12 is integrally connected to the half rings 23 and 24 and is connected by the rings 21 and 22 and thus by the half rings 23 and 24 via mechanical joint members 35 arranged at the ends 27 and 28 of the blade 12. Supported, each blade 12 is supported by the rings 21 and 22 via the joint member 35 without welding. The structural support function of the blade 12 is performed exclusively by the joints 31 and 32, in other words, the blade 12 does not form a monolithic structure with the half rings 23 and 24, but instead from the half rings 23 and 24. Separable.

  The joint member 35 of each blade 12 includes a pair of male elements 36 that engage with respective female elements 37 having a shape that matches the shape of the male element 36. The male elements 36 are inserted into the respective female elements 37 with gaps removed by wedges or shims (well known and not shown for simplicity), so that the male elements 36 and the respective female molds are inserted. The final coupling with the element 37 is therefore free of play (zero clearance). Each blade 12 is fitted with a male element 36 and a female element 37 which are arranged at the ends 27 and 28 of the blade 12.

  In the example shown in the figure, the connection 29 facing the outer ring 21 is fitted with a male element 36, which is associated with an individual female element 37, for example made of a respective sheet formed on the outer ring 21. Instead, the connecting portion 30 facing the inner ring 22 is provided with a respective female element 37 or seat, which is engaged by an individual male element 36. The latter is also shaped, for example, in the shape of a hammer that is worn by the inner ring 22.

  The blade 12 with the connections 29 and 30, and preferably the half rings 23 and 24, are formed, for example, by casting with a CX2MW type nickel-based alloy according to ASTM A494, which is a Hastelloy C22®. Similar to an alloy with a commercial name or another alloy with similar properties.

  The alloy utilized in accordance with the present invention is basically a nickel-chromium-molybdenum-tungsten-iron alloy with nickel as the major component, with a high content of chromium, a significant amount of molybdenum, and tungsten. Including iron.

  In particular, the alloys utilized are in an amount greater than about 55 wt.%, Preferably greater than about 58 wt.% Nickel, about 20 to 22.5 wt.% Chromium, about 12.5 to 14.5 wt. % Molybdenum, about 2.5 to 3.5 wt% tungsten, and about 2 to 6 wt% iron.

  With respect to the corrosion resistance of the geothermal environment, these alloys perform clearly better than conventional steels, especially those of AISI 403 type martensitic steel.

The advantages of the present invention over known solutions will become apparent from the description given below.
The stationary blade assembly 20, and in particular the stator blade 12 formed according to the invention, has a very good corrosion resistance relative to blades made of conventional materials.
The stator blade 12 can be replaced by reusing the support rings 21 and 22 (or half rings 23 and 24), and has quite obvious advantages in terms of cost and ease of maintenance work.
The inventive solution has a limited need for operation on the original fixed part and can also be utilized in a steam turbine originally designed for a conventional solution.

  Finally, it is clearly understood that changes and modifications may be made to what is described and illustrated herein without departing from the scope of the invention as defined by the appended claims.

Claims (14)

Stator blades (12) arranged in a radial pattern substantially around the axis (A) and supported at longitudinally opposite ends (27, 28) by outer support ring (21) and inner support ring (22), respectively. An array (11) of
Each blade (12) is fixedly connected to the ring (21, 22) and is supported by these rings via respective mechanical joints (31, 32) without any structural welds and therefore without breaking the ring Removable from the ring,
High corrosion resistant fixed blade assembly (20) for a steam turbine, in particular a geothermal impulse turbine.   The assembly of claim 1, wherein the blade (12) is formed from a nickel-based alloy.   Each blade (12) is integrally connected to the ring (21, 22) and is connected by a ring (21, 22) via a mechanical coupling member (35) located at the end (27, 28) of the blade. The assembly according to claim 1 or 2, wherein the assembly is supported.   The assembly according to claim 3, wherein each blade (12) is supported by the ring (21, 22) exclusively via the coupling member (35).   The joint member (35) includes a pair of male elements (36) that engage respective female elements (37) having a shape that matches the shape of the male element (36). An assembly according to claim 3 or claim 4.   The assembly according to claim 5, wherein said male element (36) is inserted into each said female element (37) with a gap removed by a wedge or shim.   7. A blade according to claim 5 or claim 6, wherein each blade (12) is fitted with the male element (36) and the female element (37) arranged at respective ends (21 and 28) of the blade. Assembly.   8. Each of the blades (12) has two connections (29, 30) arranged at the blade ends (27, 28) and projecting laterally from the blade ends. The assembly described in.   The connection (29 and 30) is mounted by the second ring (22) and the male element (36) that engages the female element (37) formed in the first ring (21). 9. The assembly according to claim 8, wherein each female element (37) engaged by a male element (36) is mounted.   The assembly according to claim 8 or 9, wherein the connection (29 and 30) is formed as an integral part of the blade (12) in the same material as the blade (12).   The assembly according to any one of the preceding claims, wherein the blade (12) is formed by casting with a nickel-based alloy comprising chromium, molybdenum, tungsten and iron.   The assembly according to any one of the preceding claims, wherein the blade (12) is formed of a CX2MW type alloy according to ASTM A494 or another alloy of similar properties.   13. An assembly according to any one of the preceding claims, wherein the blade (12) is formed by casting with a nickel-based alloy of Hastelloy® type or similar material. A stator (5) and a rotor disk (6) integrally connected to a shaft (7) rotating about a rotation axis (A), the stator (5) being any one of claims 1 to 13. A fixed blade assembly (20) according to
Steam turbine (1), especially a geothermal impulse turbine.

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