HIGHLY CORROSION-RESISTANT MOVABLE BLADE ASSEMBLY FOR A STEAM TURBINE, IN PARTICULAR A GEOTHERMAL IMPULSE TURBINE

TECHNICAL FIELD

The present inventibn relates to a highly corrosion-resistant movable blade assembly for a steam turbine, in particular a geothermal impulse turbine, and to a steam turbine, in particular a geothermal impulse turbine, featuring such a movable blade assembly.

BACKGROUND ART

As is known, in steam turbines, energy is transferred from the steam to the shaft by successively expanding the steam, which, as it expands in the nozzles forming the fixed part of the turbine, increases in speed to convert the thermal energy of the steam to kinetic energy, which is transmitted to the shaft by movable blades fitted to the periphery of the shaft.

To make the best use of the energy in the steam, a steam turbine normally comprises a number of successive

stages which, in axial-flow turbines, are arranged coaxially with the turbine (machine) axis, so that the steam discharged from one stage flows directly and more or less axially into the next. The movable part of each stage (movable blades) comprises a number of blades arranged radially about the shaft and fixed to \the shaft by fasteners of various types (inverted-pine-shaped or hammer-headed fasteners) ; and the ends or tops of the blades are connected to one another by shroud rings fixed to the tops of the blades, for example, by riveting (upsetting) pegs formed on the tops of the blades or formed in one piece with the blades themselves.

One known application of steam turbines is in geothermal energy generating systems, in which the fluid evolving in the turbine is defined by endogenous natural steam, i.e. steam generated directly in the earth as opposed to steam produced by conventional fossil- or nuclear-fuelled boilers or by heat-recovery boilers as in combination-cycle systems.

Unlike 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. 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.

This applies in particular to the stages in which the steam expanding in the turbine passes from the superheated to the saturated state, and results in severe corrosion/erosion of the blades, and in salt deposits (typically sulphide and chloride) , especially beneath the movable blade shroud rings, thus resulting in subdeposit corrosion.

As a result, the working life of the movable blades, particularly those at the phase-passage turbine stages, is reduced, and operating cost is increased, due to frequent replacement of the blades and stoppage of the system for maintenance work.

The blades and shroud rings are normally made of martensitic stainless steel, such as AISI 403 or similar, which, being a good compromise between mechanical and erosion resistance characteristics, is widely used in steam turbines, even in geothermal systems .

Steam turbines with movable blades and shroud rings made of martensitic stainless steel, however,

have a 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.

The problem is further compounded by the formation of deposits of various types (depending on the chemical nature of the endogenous steam evolving in the turbine) , which accumulate particularly on the underside of the shroud rings of the movable blades, thus resulting in subdeposit corrosion phenomena. DISCLOSURE OF INVENTION It is an object of the present invention to provide a highly corrosion-resistant movable blade assembly for a steam turbine, in particular a geothermal impulse turbine, designed to eliminate the aforementioned drawbacks of the known art. According to the present invention, there is provided a highly corrosion-resistant movable blade assembly for a steam turbine, in particular a geothermal impulse turbine, comprising an array of rotor blades carried by a shaft rotating about an axis; the assembly being characterized in that each blade extends between one end attached to the shaft, and a free end terminating with a top portion with no mechanical connection to the top portions of the

adjacent blades; and in that the blades are made of a nickel-based metal alloy preferably also containing chromium, iron, niobium, and molybdenum.

In a preferred embodiment, the metal alloy contains over roughly 45% by weight of nickel; over roughly 15% by weight of chromium; over roughly 3% by weight of niobium; and over roughly 2% by weight of molybdenum.

In addition to significant quantities of iron, niobium, and molybdenum, the metal alloy also advantageously contains smaller quantities of aluminium and titanium, in particular roughly 0.1 to 1% by weight of aluminium, and roughly 0.5 to 1.5% by weight of titanium. BRIEF DESCRIPTION OF THE DRAWINGS

A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a geothermal steam turbine of substantially known configuration;

Figure 2 shows a schematic partial longitudinal section of one stage of the Figure 1 turbine featuring a movable blade assembly in accordance with the invention; Figure 3 shows a partial view of a movable blade assembly in accordance with the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Number 1 in Figure 1 indicates a substantially

known steam turbine, which is therefore only shown schematically. More specifically, turbine 1 is an axial impulse turbine of a geothermal power generating system. Turbine 1 substantially comprises a casing 2 having an inlet 3 and an outlet 4 for a stream of endogenous natural steam extracted from the earth. Casing 2 houses a stator 5 integral with casing 2; and a rotor 6 connected integrally to a drive shaft 7 extending through casing 2 along a rotation axis A.

As usual, casing 2 houses a number of successive stages 10, each defined by an array 11 of fixed stator blades 12 integral with casing 2 and projecting substantially radially from an inner wall 13 of casing 2, and by an array 14 of movable rotor blades 15 carried by and rotating integrally with shaft 7.

Figure 2 shows, purely schematically, a stage 10, in particular an impulse stage 10, of turbine 1.

Stage 10 comprises a movable blade assembly 20 defined by blades 15, which are arranged substantially in a ring about shaft 7 and project radially from shaft 7 towards wall 13 of casing 2.

With reference also to Figure 3, blades 15 have a substantially concave profile, and may be of any known type, e.g. cylindrical with a constant section, tapered and/or twisted.

Each blade 15 is fixed in known manner directly to shaft 7 or, as shown in Figure 2, to a supporting wheel

21 integral with shaft 7. Each blade 15 extends between a fastening end 22, having a connecting portion 23 for connection to shaft 7, and a free end 24 opposite fastening end 22 and terminating with a top portion 25 in no way connected mechanically to the top portions 25 of the other blades 15.

In other words, no mechanical connection, in particular no shroud ring, is provided connecting the top portions 25 of rotor blades 15. Eliminating the shroud ring poses no vibration problems of blades 15 if they are appropriately shaped and sized and exhibit no resonance phenomena with excitation frequencies.

In accordance with the invention, assembly 20 and, in particular, blades 15 are made of a nickel-based metal alloy (i.e. in which nickel is the predominant component, and which contains roughly over 45% by weight of nickel) with a high chromium content (roughly over 15% by weight) , and also containing iron and significant quantities of niobium (roughly over 3% by weight) and molybdenum (roughly over 2% by weight) . The metal alloys used in accordance with the present invention are therefore substantially nickel-chromium alloys containing significant quantities of iron, niobium and molybdenum, together with smaller quantities of aluminium (roughly 0.1 to 1% by weight) and titanium (roughly 0.5 to 1.5% by weight).

Purely by way of example, nickel-based alloys in the Special Metals Corporation INCONEL ^® group are

particularly indicated, and, in terms of corrosion resistance in geothermal applications, have proved far superior to conventional steel, in particular AISI 403 i martensitic steel.

The INCONEL ^® name includes a group of nickel-based alloys which can be age hardened, exhibit good creep resistance up to roughly 700°C, and are highly resistant to corrosion in general and to stress corrosion in particular.

It is understood, however, that similar materials to the alloys in the INCONEL ^® group may be used in accordance with the invention.

Examples of materials used to make blades 15 and assembly 20 in general (main components only) are shown in Table 1, and particularly advantageous materials in Table 2.

TABLE 1

TABLE 2

The advantages of the i present invention over known solutions will be clear from the foregoing description

- the movable blade assembly and, specifically, the rotor blades made in accordance with the invention are far more corrosion resistant than blades made of conventional materials;

- the materials used also have better mechanical

characteristics, so that, for a given degree of erosion, resistance is improved and, hence, the working life of the component parts increased; eliminating the shroud ring (and any other member mechanically connecting the top portions of the blades) eliminates areas in which deposits (defined by substances in the steam) can accumulate on the rotor;

- also eliminated are the gaps, subject to gap corrosion, between the top portions of the blades and the shroud rings;

- deposits defined by substances in the steam are projected radially outwards onto the stator, and accumulate in particular on stator portions directly facing the top portions of the movable blades, so that subsequent deposits only accumulate up to a certain point, beyond which they \are removed mechanically by the top portions of the movable blades which act as scraping tools as they rotate; the movable blade assembly according to the invention may also be used on steam turbines originally designed for conventional solutions, with only minor alterations to the original fixed parts.

Clearly, changes may be made to the assembly as described and illustrated herein without, however, departing from the scope of the invention as defined in the accompanying Claims.

CLAIMS

1) A highly corrosion-resistant movable blade assembly (20) for a steam turbine, in particular a geothermal impulse turbine, comprising an array (14) of rotor blades (15) carried by a shaft (7) rotating about an axis (A) ; the assembly being characterized in that each blade (15) extends between one end (22) attached to the shaft (7), and a free end (24) terminating with a top portion (25) with no mechanical connection to the top portions (25) of the ot:her blades (15); and in that the blades (15) are made of a nickel-based metal alloy.

2) An assembly as claimed in Claim 1, characterized in that the blades (15) are made of a nickel-based metal alloy containing chromium, iron, niobium, and molybdenum.

3) An assembly as claimed in Claim 1 or 2, characterized in that the metal alloy contains over roughly 45% by weight of nickel, and over roughly 15% by weight of chromium.

4) An assembly as claimed in one of the foregoing Claims, characterized in tjiat the metal alloy contains over roughly 3% by weight of niobium, and over roughly 2% by weight of molybdenum. 5) An assembly as claimed in one of the foregoing Claims, characterized in that the metal alloy contains significant quantities of iron, niobium, and molybdenum, together with smaller quantities of