DESCRIPTION

The present invention relates 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 said 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

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. Further details and specific embodiments will refer to the attached drawings, in which:

- Figure 1 is a perspective view of a stage of a steam turbine according to an embodiment of the present invention;

- Figure 2 is a sectional view of the stage of figure 1 along the plane B-8; - Figure 2b is a schematic detail C from the sectional view of figure 2;

- Figure 2c is a sectional view of a detail of the stage of figure 1 ; and

- Figures 3a, 3b and 3c are perspective views of respective steps of the method for manufacturing a stage of a steam turbine according to an embodiment of the present invention. DETAILED DESCRIPTION

The following description of exemplary embodiments refer to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

Therefore, a stage of a steam turbine will be described by referring to the attached figures, in which will be indicated with the number 1 .

The stage 1 has a central axis "A". The stage has a central zone 1a and a peripheral zone 1b with respect to the central axis "A". In other words, the central zone 1a can be considered an internal part of the stage 1 , while the peripheral zone 1 b can be understood as an external part of the stage 1 with respect to the central axis "A". The flow of steam inside the turbine is directed substantially along the central axis "A".

The stage 1 is provided with a plurality of blades 6. Each blade 6 projects radially from the central zone 1a to the peripheral zone 1 b. Additionally, each blade 6 has an external surface 7, which is defined by an airfoil whose geometrical parameters are chosen depending on the specific application.

At least one of the blades 6, preferably several blades 6 and more preferably all of them, have an opening 8 on the external surface 7. Indeed, the blades 6 are also provided with a cavity 9 located in an internal zone. In other words, the blades 6 are hollow. In detail, the cavity 9 extends along at least a portion of the radial length of the blade 6, preferably along the full radial length of the blade 6. Each opening 8 likewise extends along at least a portion of the radial length of the blade 6. In the context of the present disclosure, by "radial length" is meant the length of the blade 6 along a radial direction, namely a direction perpendicular to the central axis "A" of the stage 1 and projecting from it. The opening 8 is configured so as to place the cavity 9 in fluid communication with a volume outside the blade 6.

More particularly, that the cavity 9 inside the blade 6 has an internal surface 10. The internal surface 10 is 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 to figure 2, please note that the cavity 9 appears jagged only because the plane B-B, shown in figure 1 , is transversal to the blade 8. With reference to the section of the blade 6 shown in figure 2c, please note that the cavity 9 shown therein is a ruled surface as described above. The stage 1 is provided with at least one channel 5, which can be located in the peripheral zone 1a and/or in the central zone 1 b of the stage 1 . With additional detail, the channel 5 can be placed in fluid connection with an internal zone of the turbine where the stage 1 is installed. More particularly, the channel 5 is placed in fluid communication with the cavities 9 of the blades 6.

The channel 5 itself 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 openings 8, into the cavities 9 and then into the channel 5, thereby removing condensed steam from the external surface 7 of the blades 6.

According to one embodiment of the invention, the stage 1 comprises a plurality of sectors 2. In particular, each sector 2 is geometrically a circular sector, i.e. a sector of a circle or, more precisely, of a circular ring. Each sector 2 comprises a central 2b and a peripheral portion 2a, as well as a plurality of the above mentioned blades 6. Each blade 6 is attached to the central 2b and to the peripheral portion 2a. The stage 1 also comprises a central 3 and a peripheral guide 4. Moreover, the sectors 2 are sealed to the guides 3, 4. Specifically, the central 2b and the peripheral portion 2a are attached each to the respective guide 3, 4.

With greater detail, both portions 2a, 2b are provided with a profiled rail 1 1 which fits into the respective guide 3, 4. Indeed, the above mentioned channels 5 are defined between the sectors 2 and the guides 3, 4. Specifically, the peripheral portion 2a is coupled to the peripheral guide 4, thereby defining an outer channel 5. The central portion 2b is coupled with the central guide 3, thereby defining an inner channel 5. In order to isolate the channels 5 from the environment inside the turbine, appropriate channel seals 12 are provided between the sector 2 and the guides 3, 4.

These channel seals 12, schematically shown in figure 2b, comprise a core 13 of rigid material, preferably metal, more preferably steel. The channel seal 12 may also comprise a coating 14. The coating 14 can preferably be made of a ceramic, composite or plastic material. With additional detail, in this arrangement the core 13 is sandwiched between two or more layers of coating 14. Alternatively, the channel 5 can be made airtight by welding the sector 2 directly to the guides 3, 4.

As noted above, an embodiment of the present invention may or may not have both channels 5, but has at least one of them. Furthermore, even if two channels 5 are present they may or may not be used during normal operation.

Please note that, according to several embodiment of the invention, each sector 2 is machined out of a single block of material. In other words, each sector 2 is built as a single piece. Advantageously, this allows to build a stage 1 of a turbine in which there is no welding between the blades 6 and the central 2b or the peripheral portion 2a.

In an embodiment of the present invention, the stage 1 comprises four sectors 2, each having an angular opening of 90° with respect to the central axis "A". In another embodiment of the invention, the stage 1 comprises two sectors 2 each having an angular opening of 180°. Other embodiments are possible, comprising different numbers of sectors 2 which have different angular openings.

Preferably, the guides 3, 4 have an angular opening of 180°. Therefore, the stage 1 preferably comprises two central guides 3 and two peripheral guides 4. The method according to an embodiment of the present invention therefore comprises the steps of machining a block of material to define a sector 2. Preferably, several sectors 2 are machined. More preferably, the machining is done by milling a block of material. Even more preferably, all sectors 2 of a stage 1 are milled out of a respective single block of material. During this step, the external shape of each sector 2 is defined, including the blades 6 with their respective external surfaces 7. The cavity 9 is then cut into the external surface 7 of at least one of the blades 6. Specifically, according to the preferred embodiment of the invention the cavity 9 is cut by wire electric discharge machining. For this reason, the resulting internal surface 10 of the cavity 9 is a ruled surface. The opening 8 is also machined during this step, preferably by die-sink electric discharge machining.

The central guide 3, as well as the peripheral guide 4, are then manufactured. Any suitable known manufacturing technique can be employed, therefore this step will not be further detailed. Preferably, the guides 3, 4 are manufactured with a slightly different curvature so that, when they are joined to the sectors 2, they are slightly deformed. As explained above, in this embodiment the guides 2, 4 have an angular opening of 180°.

The sector 2 is then slid into the guides 3, 4. Specifically, two sectors 2 are slid on the central guide 3, as shown in figure 3a. Specifically, the profiled rail 1 1 from the central portion 2b of each sector 2 is inserted into the central guide 3. During this phase the central guide 3, which is not perfectly circular, is deformed elastically by the rail 1 1 . In this way, the elastic deformation acts as a preload between the sector 2 and the central guide 3, so as to prevent an unwanted relative motion between the two components.

If a channel seal 12 is present between the central guide 3 and the sectors 2, it is installed during this phase.

The sectors 2 are 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, preferably metal and more preferably steel. The inter-sector seal may also comprise a coating of deformable material, preferably rubber or plastic. The peripheral guide 4 is then placed over the two sectors 2. With greater detail, the peripheral guide 4 is deformed by the sectors 2 in the same way described above with reference to the central guide 3.

The sectors 2 are then sealed to the guides 3, 4. If channel seals 12 are employed this step can be skipped. Otherwise, this step is performed by welding the sector 2 to the guides 3, 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 in figure 3c. By joining two half-stage, the above described stage 1 can 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 sectors 2 can be joined before they are inserted into the guides 3, 4. Additionally, another embodiment is possible in which a sector 2 has an angular opening of 180°, thereby avoiding the need for the joining step.