US20100074741A1

US20100074741A1 - Highly corrosion-resistant fixed blade assembly for a steam turbine, in particular a geothermal impulse turbine

Info

Publication number: US20100074741A1
Application number: US12/522,154
Authority: US
Inventors: Luciano Cozza; Mario Zannoni
Original assignee: Ansaldo Energia SpA
Current assignee: Ansaldo Energia SpA
Priority date: 2007-01-04
Filing date: 2007-01-04
Publication date: 2010-03-25
Also published as: EP2115273A1; WO2008081485A1; JP2010515849A

Abstract

A highly corrosion-resistant fixed blade assembly (20) for a steam turbine, in particular a geothermal impulse turbine, is formed by an array (11) of stator blades (12), supported at their respective longitudinally opposite ends (27, 28) by an outer (21) and an inner ring (22). Each blade (12), made with a nickel-based metal alloy, for example, an alloy of the Hastelloy® family or a similar material, is integrally connected to the rings (21, 22) and is supported by the rings via respective mechanical joints (31, 32) devoid of structural welds, so as to be removable from the rings without breaking the rings.

Description

TECHNICAL FIELD

The present invention concerns a highly corrosion-resistant fixed blade assembly for a steam turbine, in particular a geothermal impulse turbine; the invention also concerns a steam turbine, in particular a geothermal impulse turbine, equipped with this fixed blade assembly.

BACKGROUND ART

As is known, in steam turbines the transfer of energy from the steam to the shaft takes place through a successive series of expansions of the steam that, expanding in the nozzles that constitute the fixed part of the turbine, increases in speed, therefore transforming the steam's heat energy into kinetic energy; the kinetic energy is transmitted to the shaft through mobile blades mounted on the circumference of the shaft itself.
In order to use the energy within the steam in an efficient manner, a steam turbine is normally composed of a number of successive stages, which for axial-flow turbines are arranged coaxially with the axis of rotation of the mobile blades (machine axis), so that the steam discharged from one stage flows directly and more or less axially into the next.
In impulse turbines, each stage includes a mobile blade assembly, formed by an array of blades carried integrally by the shaft, and a fixed blade assembly, usually called a diaphragm, facing the mobile blade assembly and formed by an array of fixed blades, opportunely shaped and arranged radially with respect to the machine axis. The fixed blades are supported by a pair of rings, radially internal and radially external respectively and normally composed of a pair of coupled half-rings; each blade is welded at both ends to the support rings.
It is also known that steam turbines find one of their possible applications in geothermal power generation plants. In these plants, the fluid that evolves in the turbine consists of endogenous steam of natural origin, i.e. steam generated directly in the earth, instead of steam produced by conventional boilers fed with fossil or nuclear fuels, or by heat-recovery boilers as in combination-cycle systems.
With respect to boiler-generated steam, endogenous steam is characterized not only by much lower than normal thermodynamic conditions (substantially pressure and temperature), but also by uncontrolled chemistry which depends strongly on the site from which the steam is extracted.
Therefore, unlike other types of systems, in which dedicated systems for processing the condensate used to produce the steam ensure optimum steam characteristics, the steam supplied to the turbine in geothermal systems is normally less than optimum, especially as regards chemical aggression. In addition to noncondensable gases, endogenous steam in fact normally contains various substances in various forms which impart aggressive characteristics to it which are totally absent in industrial steam, and which result in the formation of deposits on the steam turbine blades.
These effects are particularly noticeable in the stages in which the steam expanding in the turbine passes from the superheated state to the saturated state, and results in severe corrosion/erosion on the fixed blades and on the mobile blades.
The blades (fixed and mobile) are normally made of martensitic stainless steels, for example, the AISI 403 type or similar. These materials are a good compromise between mechanical characteristics and erosion resistance characteristics, and are therefore widely used in steam turbines, even in geothermal systems.
Nevertheless, these materials have the problem of poor resistance to corrosion and stress corrosion, particularly in the presence of chloride, as in the case of geothermal system steam turbines, and particularly at the stages where the steam passes from the superheated to the wet state, where the condensation mechanism enriches the concentration of impurities in the steam.
It follows, in particular, that the service life for the fixed blades is relatively short, especially for the blades in the turbine stages in which the change of phase takes place, with consequently high operating costs related to the need to proceed with substitution of the blades relatively frequently; clearly, the plant stoppage times for carrying out the maintenance operations also generate significant costs due to lost production.
Secondly, since, as previously pointed out, the fixed blades in impulse turbines are welded at both of their ends to support rings (or half-rings), in the case of deterioration of a blade, it becomes necessary to substitute the entire diaphragm. In fact, in the known solutions, any mechanical joint systems for the ends of the fixed blades to support rings only serve for the positioning or centring of the blades, but do not have a structural function and are intrinsically unable to support the blades independently, always requiring structural welding to ensure the fixing of the blades.

DISCLOSURE OF INVENTION

One object of the present invention is that of providing a highly corrosion-resistant fixed blade assembly for a steam turbine, in particular a geothermal impulse turbine, which is devoid of the above-described drawbacks of known art.
The present invention therefore concerns a highly corrosion-resistant fixed blade assembly for a steam turbine, in particular a geothermal impulse turbine, as essentially defined in the attached claim 1 and, for its preferred aspects, in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described further by way of example in the following non-limiting embodiment, with reference to the attached FIGURE, which is a partial schematic view in longitudinal cross-section of a geothermal steam turbine equipped with fixed blade assembly in accordance with the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the FIGURE, a steam turbine, in itself generally known and therefore only schematically and partially represented, is designated by reference numeral 1. In particular, the turbine is an axial impulse turbine, used in a geothermal power generation plant. The direction of the steam in the turbine 1 is schematically indicated by arrow 3.
In general terms, the turbine 1 includes a casing 2 containing a stator 5, integrally carried by the casing 2, and a rotor disc 6, integrally connected to a drive shaft 7 running through the casing 2 along a rotation axis A; as usual, the casing 2 houses a number of successive stages 10. Each stage 10 is defined by an array 11 of fixed stator blades 12, integral with the casing 2 and projecting from an internal wall 13 of the casing 2 in a substantially radial direction with respect to axis A, and an array 14 of mobile rotor blades 15, carried by the shaft 7 and integrally rotating with the shaft 7.
Each stage 10 includes a fixed blade assembly 20, constituting a diaphragm of the stage 10 and formed by blades 12 and by two support rings 21 and 22, respectively externally radial and internally radial, substantially coaxial around the axis A and preferably formed by respective pairs of half- rings 23 and 24 coupled close together; the blades 12, having substantially known geometry and profile, are substantially arranged in a radial pattern around the axis A and radially extend inside the wall 13 of the casing 2 towards the shaft 7 between the rings 21 and 22.
The outer ring 21 (formed by half-rings 23) is fixed, in a known manner, to the wall 13; the inner ring 22 (formed by half-rings 24) carries a sealing group 25, of known type, facing the shaft 7.
Each blade 12 extends along a longitudinal axis L between two longitudinally opposite ends 27 and 28 provided with respective connection portions 29 and 30 that project transversally from the ends 27 and 28 of the blade 12; the connection portions 29 and 30 are made as an integral part of the blades 12 in the same material as the blades 12.
Each blade 12 is integrally connected to the rings 21 and 22 (and specifically to a half-ring 23 and a half-ring 24) and is supported by the rings 21 and 22 via mechanical joints 31 and 32 devoid of structural welds, so as to be removable from the rings 21 and 22 (or from the half-rings 23 and 24) without breaking the rings 21 and 22. The joints 31 and 32 have undercuts 33 that longitudinally constrain the blade 12 to the rings 21 and 22, along the longitudinal axis L of the blade 12.
The term “structural welds” is intended to indicate welds capable of ensuring firm connection of components under normal operating conditions, and specifically capable of ensuring that the rings 21 and 22 effectively support the blades 12 in use.
In particular, each blade 12 is integrally connected to the half- rings 23 and 24 and is supported by the half-rings 23 and and thus by the rings 21 and 22 via mechanical joint members 35 arranged at the ends 27 and 28 of the blade 12; each blade 12 is supported by the rings 21 and 22 exclusively via the joint members 35, and not via welding. The structural support function of the blades 12 is exclusively carried out by the joints 31 and 32; in other terms, the blades 12 do not form a monolithic body with the half- rings 23 and 24, but are instead separable from the half- rings 23 and 24.
The joint members 35 of each blade 12 comprise a pair of male elements 36 that engage with respective female elements 37 having a shape matching the shape of the male elements 36. The male elements 36 are inserted in the respective female elements 37 with clearances that are eliminated by wedges or shims (known and not shown for simplicity); the final coupling between the male elements 36 and the respective female elements 37 is therefore devoid of play (zero-clearance). Each blade 12 carries a male element 36 and a female element 37 arranged at the ends 27 and 28 of the blade 12.
In the shown example, the connection portions 29 facing towards the outer ring 21 carry male elements 36, shaped like a hammerhead for example, which engage respective female elements 37, consisting of respective seats formed in the outer ring 21; instead, the connection portions 30 facing towards the inner ring 22 are provided with respective female elements 37 or seats, which are engaged by respective male elements 36, these also, for example, shaped like a hammerhead, carried by the inner ring 22.
The blades 12, including the connection portions 29 and 30, and preferable also the half- rings 23 and 24, are made by casting with a nickel-based metal alloy, for example, of the CX2MW type according to ASTM A494, similar to the alloy with the commercial name of Hastelloy C22®, or another alloy of similar characteristics.
The metal alloys utilized in accordance with the present invention are essentially nickel-chromium-molybdenum-tungsten-iron alloys, having nickel as the predominant component, with high chromium content and containing significant quantities of molybdenum, together with tungsten and iron.
In particular, the alloy utilized contains nickel in quantities greater than approximately 55% by weight and preferably greater than approximately 58% by weight, chromium in quantities approximately between 20% and 22.5% by weight, molybdenum in quantities approximately between 12.5% and 14.5% by weight, tungsten in quantities approximately between 2.5% and 3.5% by weight and iron in quantities approximately between 2% and 6% by weight.
With regards to corrosion resistance in a geothermal environment, these alloys demonstrate a distinctly superior behaviour to those of conventional steels and, in particular, to AISI 403 type martensitic steels.
The advantages of the present invention with respect to known solutions clearly emerge from that which has been explained:

- the fixed blade assembly 20 and specifically the stator blades 12 made in accordance with the invention have significantly superior corrosion resistance with respect to blades made with traditional materials,
- the stator blades 12 can be substituted with reutilization of the support rings 21 and 22 (or the half-rings 23 and 24), with clearly evident advantages in terms of cost and simplicity of maintenance operations,
- the solution of the invention can also be utilized in steam turbines originally designed for conventional solutions, with limited need for operations on the original fixed parts.

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

Claims

1. A highly corrosion-resistant fixed blade assembly (20) for a steam turbine, in particular a geothermal impulse turbine, comprising an array (11) of stator blades (12) substantially arranged in a radial pattern around an axis (A) and supported at respective longitudinally opposite ends (27, 28) by an outer support ring (21) and an inner support ring (22); wherein each blade (12) of the assembly (20) is firmly connected to the rings (21, 22) and is supported by the rings via respective mechanical joints (31, 32) devoid of structural welds, so as to be removable from the rings without breaking the rings.

2. An assembly according to claim 1, wherein the blades (12) are made of a nickel-based metal alloy.

3. An assembly according to claim 1, wherein each blade (12) is integrally connected to the rings (21, 22) and is supported by the rings (21, 22) via mechanical joint members (35) arranged at the ends (27, 28) of the blade.

4. An assembly according to claim 3, wherein each blade (12) is supported by the rings (21, 22) exclusively via the joint members (35).

5. An assembly according to claim 3, wherein the joint members (35) comprise a pair of male elements (36) that engage with respective female elements (37) having a shape matching the shape of the male elements (36).

6. An assembly according to claim 5, wherein the male elements (36) are inserted in the respective female elements (37) with clearances that are eliminated by wedges or shims.

7. An assembly according to claim 5, wherein each blade (12) carries a male element (36) and a female element (37) arranged at the respective ends (27 and 28) of the blade.

8. An assembly according to claim 1, wherein each blade (12) has two connection portions (29, 30) arranged at the ends (27, 28) of the blade and which project transversally from the ends of the blade.

9. An assembly according to claim 8, wherein the connection portions (29 and 30) respectively carry a male element (36) that engages a female element (37) formed in a first ring (21), and a female element (37) that is engaged by a male element (36) carried b a second ring (22).

10. An assembly according to claim 8, wherein the connection portions (29 and 30) are made as an integral part of the blades (12) in the same material as the blades.

11. An assembly according to claim 1, wherein the blades (12) are made h casting with a nickel-based metal alloy containing chromium, molybdenum, tungsten and iron.

12. An assembly according to claim 1, wherein the blades (12) are made with a metal alloy of the CX2MW type according, to ASTM A494 or another alloy of similar characteristics.

13. An assembly according to claim 1, wherein the blades (12) are made by casting with a nickel-based metal alloy of the Hastelloy® family or a similar material.

14. A steam turbine (1), in particular a geothermal impulse turbine, including a stator (5) and a rotor disc (6) integrally connected to a shaft (7) rotating around an axis (A) of rotation, wherein, the stator (5) of the turbine (1) includes a fixed blade assembly (20) according to claim 1.