Method for manufacturing a stage of a steam turbine

A method for manufacturing a stage of a steam turbine comprising the steps of milling a block of material to define a sector having a plurality of blades, each blade having an external surface; machining an opening in the external surface of at least one of the blades; machining a cavity in fluid communication with the opening; the step of machining the cavity being performed by wire electric discharge machining.

BACKGROUND

Embodiments of the present invention relate to a method for manufacturing a stage of a steam turbine. Specifically, the method relates to the manufacturing of a stage having hollow blades.

In steam turbines, partial condensation of the steam occurs at their last stage or stages. In particular, condensation occurs on the airfoil portion of the stator blades of a so-called “condensing stage”, typically the last stage of the turbine. If droplets are generated as a consequence of condensation, they leave the static stator blades and they hit the rotating rotor blades; therefore, damages to the rotor blades may occur. In order to reduce the damages caused by the droplets, the rotation speed of the rotor blades may be reduced. However, in this way the efficiency of the turbine is also reduced.

Alternatively, in order to reduce any damage on the rotor blades, solutions exist for collecting the condensation before the generation of droplets. The most typical of these solutions consists in using hollow stator blades where condensation is likely to occur, providing holes and/or slots through the airfoil portion of the blades extending from the airfoil surface to the internal cavity, and sucking from the internal cavity so to that any condensation leaves the airfoil surface and enters the internal cavity. In this way, the release of droplets can be highly reduced.

A method for manufacturing such stage of a steam turbine is therefore known. Such method comprises the steps of machining an inner and an outer ring having each a respective channel. Each of these rings has an internal surface with a plurality of holes in fluid communication with the channel. A plurality of turbine blades is manufactured, each blade having a respective opening and a hollow cavity in fluid communication with the external environment through the opening.

The blades are then welded to the rings. Specifically, each hole in a single ring is placed in fluid communication with the cavity of a respective blade. As a result, in the assembled stage the condensed water can be extracted through the opening of a blade, thus flowing into the cavity and then into the channel of one of the two rings.

SUMMARY OF THE INVENTION

A disadvantage of the above described prior art is in the welding phase of the above described method. Indeed, this step has to be performed both manually and within strict tolerances. This results in increased assembly time and, consequently, increased costs.

A first aspect of the invention is therefore a method for manufacturing a stage of a steam turbine. The method comprises the steps of milling a block of material to define sector with a plurality of blades; machining an opening in the external surface of at least one of the blades and a cavity in fluid communication with the opening. The step of machining the cavity is performed by wire electric discharge machining.

An advantage of this method is that it overcomes the problem of the prior art, since there is no more need to weld the blades manually. Indeed, in this method the direct intervention of the technician is kept to a minimum.

A second aspect of the invention is a sector for the assembly of a stage of a steam turbine. This sector comprises a central and a peripheral portion and a plurality of blades. Each blade is attached to the central and to the peripheral portions. At least one of the blades has an opening on a respective external surface and a cavity in fluid communication with the opening. The sector is machined out of a single block of material.

A third aspect of the invention is a stage for a steam turbine comprising a plurality of the above referenced sectors and at least a central and a peripheral guide. The sectors are sealed to the guides.

DETAILED DESCRIPTION

Therefore, a stage of a steam turbine will be described by referring to the attached figures, in which will be indicated with the number1.

The stage1has a central axis “A”. The stage has a central zone1aand a peripheral zone1bwith respect to the central axis “A”. In other words, the central zone1acan be considered an internal part of the stage1, while the peripheral zone1bcan be understood as an external part of the stage1with respect to the central axis “A”. The flow of steam inside the turbine is directed substantially along the central axis “A”.

The stage1is provided with a plurality of blades6. Each blade6projects radially from the central zone1A to the peripheral zone1B. Additionally, each blade6has an external surface7, which is defined by an airfoil whose geometrical parameters are chosen depending on the specific application.

At least one of the blades6, maybe several blades6and more particularly all of them, have an opening8on the external surface7. Indeed, the blades6are also provided with a cavity9located in an internal zone. In other words, the blades6are hollow.

In detail, the cavity9extends along at least a portion of the radial length of the blade6, more particularly along the full radial length of the blade6. Each opening8likewise extends along at least a portion of the radial length of the blade6. In the context of the present disclosure, by “radial length” is meant the length of the blade6along a radial direction, namely a direction perpendicular to the central axis “A” of the stage1and projecting from it. The opening8is configured so as to place the cavity9in fluid communication with a volume outside the blade6.

More particularly, that the cavity9inside the blade6has an internal surface10. The internal surface10is a ruled surface. In the context of the present disclosure, the term “ruled surface” is defined as a surface in which every point belongs to at least a straight line that lies fully on the surface itself. In other words, a ruled surface can be described as the set of points swept by a moving straight line. Examples of ruled surfaces are cylinders, cones or hyperboloids. A sphere is not a ruled surface.

With reference toFIG. 2, please note that the cavity9appears jagged only because the plane B-B, shown inFIG. 1, is transversal to the blade6. With reference to the section of the blade6shown inFIG. 2C, please note that the cavity9shown therein is a ruled surface as described above.

The stage1is provided with at least one channel5, which can be located in the peripheral zone1A and/or in the central zone1B of the stage1. With additional detail, the channel5can be placed in fluid connection with an internal zone of the turbine where the stage1is installed. More particularly, the channel5is placed in fluid communication with the cavities9of the blades6.

The channel5itself can be placed in fluid connection with a low pressure zone (not shown) outside the turbine. In this way, part of the steam flow inside the turbine can be sucked through the openings8, into the cavities9and then into the channel5, thereby removing condensed steam from the external surface7of the blades6.

According to one embodiment of the invention, the stage1comprises a plurality of sectors2. In particular, each sector2is geometrically a circular sector, i.e. a sector of a circle or, more precisely, of a circular ring. Each sector2comprises a central2B and a peripheral portion2A, as well as a plurality of the above mentioned blades6. Each blade6is attached to the central2B and to the peripheral portion2A.

The stage1also comprises a central3and a peripheral guide4. Moreover, the sectors2are sealed to the guides3,4. Specifically, the central2B and the peripheral portion2A are attached each to the respective guide3,4.

With greater detail, both portions2A,2B are provided with a profiled rail11which fits into the respective guide3,4.

Indeed, the above mentioned channels5are defined between the sectors2and the guides3,4. Specifically, the peripheral portion2A is coupled to the peripheral guide4, thereby defining an outer channel5. The central portion2B is coupled with the central guide3, thereby defining an inner channel5. In order to isolate the channels5from the environment inside the turbine, appropriate channel seals12are provided between the sector2and the guides3,4.

These channel seals12, schematically shown inFIG. 2B, comprise a core13of rigid material, maybe metal, more particularly steel. The channel seal12may also comprise a coating14. The coating14can be made of a ceramic, composite or plastic material. With additional detail, in this arrangement the core13is sandwiched between two or more layers of coating14.

Alternatively, the channel5can be made airtight by welding the sector2directly to the guides3,4.

As noted above, an embodiment of the present invention may or may not have both channels5, but has at least one of them. Furthermore, even if two channels5are present they may or may not be used during normal operation.

Please note that, according to several embodiment of the invention, each sector2is machined out of a single block of material. In other words, each sector2is built as a single piece. More particularly, this allows to build a stage1of a turbine in which there is no welding between the blades6and the central2B or the peripheral portion2A.

In an embodiment of the present invention, the stage1comprises four sectors2, each having an angular opening of 90° with respect to the central axis “A”. In another embodiment of the invention, the stage1comprises two sectors2each having an angular opening of 180°. Other embodiments are possible, comprising different numbers of sectors2which have different angular openings.

More particularly, the guides3,4have an angular opening of 180°. Therefore, the stage1in an embodiment comprises two central guides3and two peripheral guides4.

The method according to an embodiment of the present invention therefore comprises the steps of machining a block of material to define a sector2. Particularly, several sectors2are machined. In an embodiment, the machining is done by milling a block of material. More particularly, all sectors2of a stage1are milled out of a respective single block of material. During this step, the external shape of each sector2is defined, including the blades6with their respective external surfaces7.

The cavity9is then cut into the external surface7of at least one of the blades6. Specifically, according to an embodiment of the invention the cavity9is cut by wire electric discharge machining. For this reason, the resulting internal surface10of the cavity9is a ruled surface. The opening8is also machined during this step, more particularly by die-sink electric discharge machining.

The central guide3, as well as the peripheral guide4, are then manufactured. Any suitable known manufacturing technique can be employed; therefore, this step will not be further detailed. In an embodiment, the guides3,4are manufactured with a slightly different curvature so that, when they are joined to the sectors2, they are slightly deformed. As explained above, in this embodiment the guides2,4have an angular opening of 180°.

The sector2is then slid into the guides3,4. Specifically, two sectors2are slid on the central guide3, as shown inFIG. 3a. Specifically, the profiled rail11from the central portion2B of each sector2is inserted into the central guide3. During this phase the central guide3, which is not perfectly circular, is deformed elastically by the rail11. In this way, the elastic deformation acts as a preload between the sector2and the central guide3, so as to prevent an unwanted relative motion between the two components.

If a channel seal12is present between the central guide3and the sectors2, it is installed during this phase.

The sectors2are then joined together. They can be either welded or a specific inter-sector seal (not shown in the drawings) may be employed. The inter-sector seal can comprise a core of rigid material, maybe metal and more particularly steel. The inter-sector seal may also comprise a coating of deformable material, possibly rubber or plastic.

The peripheral guide4is then placed over the two sectors2. With greater detail, the peripheral guide4is deformed by the sectors2in the same way described above with reference to the central guide3.

The sectors2are then sealed to the guides3,4. If channel seals12are employed this step can be skipped. Otherwise, this step is performed by welding the sector2to the guides3,4. This operation can be performed by machine welding, without the direct intervention of an operator.

In this way, a half-stage is obtained, such as the one that is shown inFIG. 3C. By joining two half-stage, the above described stage1can be assembled.

Finally, it is to be noted that the above described steps are ordered for ease of description only. Indeed, if necessary the order can be changed, for example the sectors2can be joined before they are inserted into the guides3,4. Additionally, another embodiment is possible in which a sector2has an angular opening of 180°, thereby avoiding the need for the joining step.