IMPELLER OF A PELTON TURBINE

A PELTON TURBINE WHEEL The present invention relates to a turbine wheel of a Pelton turbine with a number of buckets arranged on a disc-shaped wheel and consisting of a bucket root formed in one piece with the disc-shaped wheel and a bucket body made of several pieces. In the manufacture of Pelton wheels, it is known to make them in one piece from cast material, but these known manufacturing techniques are not completely satisfactory. The complex geometry of the wheel often gives rise to difficulties in the casting process, and in particular with large wheels, the casting process is made very difficult and the risk of casting flaws is high, which is detrimental to the life expectancy of the wheel. Furthermore, such casting flaws are sometimes difficult to detect with known testing methods, and in particular in the case of large Pelton wheels which have undetected casting flaws, there is a very great risk of a break, especially in the area of the bucket root. It has therefore already been suggested to replace cast wheels with forged wheels, which provide better opportunities for preventing material flaws. However, it should be noted that in general, it is extremely expensive to manufacture such forged wheels, especially in the case of wheels that must be made, i.e. forged, in one piece with the buckets. It must be stated that the cost and difficulties of making large Pelton wheels from forged material rises greatly as their diameter increases, so that it is practically impossible to manufacture Pelton wheels forged completely from one piece at an economic and competitive price. Therefore, various suggestions have been made for making such turbine wheels of large dimensions under economically justifiable conditions, even when at least some forged elements are used. For example, AT-PS 394 093 and DE-PS 39 38 357 contain an Pelton wheel structure , whereby a Pelton wheel consisting of individual buckets combined into a star-shaped cluster is centered via two cylindrical collars of the bucket feet by at least two wheel discs lying symmetrically to the centre plane, and an appropriate clamping means for mounting the bucket feet is provided between the two wheel discs. According to a modified embodiment of a turbine wheel of a Pelton turbine, such as described in EP-B 0 522 336 or the corresponding German patent applications DE-A 41 27 622 and DE-A 41 43 378, the turbine wheel is produced by welding the roots of the individual buckets into an annular rim and, using annular welds, to weld the inside of the rim to the outer edge of the wheel disc. According to another modified embodiment described in EP-A 0 346 681, it is known to design the turbine wheel of a Pelton turbine with processes for fastening the individual buckets, whereby the individual buckets in the area of their roots are fixed to the processes of the wheel by means of appropriate screw joints. All these known embodiments thus provide that the buckets are fixed to the wheel or to the wheel-shaped disc by means of clamping, welding or bolting in the area of the bucket root, which constitutes the most stressed part of such a Pelton wheel. It is obvious that especially in this most stressed part of the bucket root, where there are bound to be joints, there is a great risk of breaking or defects, or of excessively rapid and extensive wear and tear. In addition, an arrangement for producing metal workpieces with an auxiliary arrangement has become known, for example from EP-A 0 496 181 and the corresponding US-PS 5 233 150, whereby, for example, a Pelton wheel is made on a forged hub, which serves as a base body or as a disc-shaped wheel, by built-up welded layers of the buckets on the hub. It is obvious that such build-up welding of the buckets, with their usually complicated geometry, is extremely costly, and that such welding, which must be performed with complicated control devices, is very time- consuming. In addition, in that case, too, the bucket roots, i.e. the places where the individual buckets are joined to the disc or the disc-shaped wheel, constitute especially stressed areas between the forged hub and the buckets made by multi-layered build-up welding, so that the above mentioned disadvantages cannot be avoided with that version either. A turbine wheel for a Pelton turbine of the type mentioned above was disclosed in DE-1 063 540 and DE-1 086 640. The first document describes two versions of Pelton wheels, in which the bucket root is designed in one piece with the turbine wheel. The first version provides a one-piece bucket body welded to the bucket root via curved or pointed border surfaces. In the second version, the multi-piece bucket body has a middle part forming one piece with the bucket root, and two side parts. The second document describes a Pelton wheel which is substantially the same as the second version described in the first document, but in which the one-piece middle part of the wheel is provided with a process to accommodate centrifugal forces. In practical terms, the formation of a turbine wheel with one-piece bucket roots and a one-piece bucket body cannot be achieved because there is not enough room to produce the complex connection and in particular a welded connection. A Pelton wheel with multi-part buckets and a middle part made in one piece with the bucket roots has the advantage that such a turbine wheel — because of its narrower width — can be manufactured, in particular by forging, at a reasonable cost. However, the 1 5 material requirements and the cost of manufacturing the bucket bodies would be high. The objective of the present invention is to further improve a turbine wheel of the above type for a Pelton turbine, whereby it is possible to manufacture a large-scale wheel for a Pelton turbine with simple process steps and at a correspondingly lower cost without having to provide a joint in the particularly stressed area of the bucket root, i.e. the immediate connection between the bucket and the disc-shaped wheel. The objective can be achieved with a turbine wheel according to the invention for a Pelton turbine, substantially characterized in that each bucket root is provided with a frontal surface that is level in peripheral direction and on each side with side surfaces that are level in radial direction, that the separately manufactured bucket bodies are made in three parts, that the middle part of the bucket body is provided with a level frontal surface and two level side surfaces, that the frontal surface of this middle part is connected with the peripherally level frontal surface of the same width of the bucket root, and that the side parts, each of which has a level side surface, are connected with a side surface of the middle part as well as with the associated radially level side surface of the bucket root. Since the wheel is formed in one piece with the bucket roots, the particularly stressed area of the bucket roots can be made especially strong without, for example, weakening this area by providing special connections, such as screw joints, between the wheel and the individual buckets. In addition, it is obvious that the formation of the disc-shaped wheel in one piece with the bucket roots leads only to a minor enlargement of the disc-shaped wheel, whereby the area of the bucket roots has a correspondingly simple geometric configuration, so that the extra work required in producing the wheel together with the bucket roots is minor or negligible in comparison with producing only the wheel. The invention also provides that the bucket bodies, which have a correspondingly simple geometric configuration, are made separately and in 3 parts and are connected with the bucket roots made in one piece with the wheel. Thus, the work required in manufacturing a turbine wheel according to the invention for a Pelton turbine is considerably less because the bucket bodies, which themselves have a complex geometric configuration, can be made separately and — because they are comparatively small components — easily and cost-effectively, while the dimensions of the disc-shaped wheel made as one piece with the bucket roots are only slightly increased, and this wheel has a comparatively simple overall structure that can be produced easily and therefore cost-effectively. After manufacturing the individual elements, the bucket bodies can be fixed in a simple manner to the disc-shaped wheel that also includes the bucket roots, thus to produce a turbine wheel for a Pelton turbine, whereby it is obvious that the design according to the invention does not have to provide separate connections in the area of the particularly stressed bucket roots. Thanks to the construction according to the invention and the tougher design, the strength of the wheel can be increased, which means that its R.P.M.s and thus its performance can be increased. The special shape of the connecting surfaces between the individual elements of the bucket bodies and between the bucket bodies and the bucket roots ensures that not only is it possible to manufacture the turbine wheel of high-quality materials in the bucket root areas of particular stress, but also that a correspondingly large connection area between the bucket root and the separately made bucket body can be achieved. In general, when multi-part designs of bucket bodies are used, the size of individual parts can be smaller, leading not only to advantages in terms of a more cost-effective production, but also in terms of the higher quality of the individual parts by reducing the possibility of material flaws. According to a particularly advantageous embodiment it is provided that the bucket roots have a length in radial direction that extends from the root end up to halfway of the cup-shaped recesses of the buckets, so that when the dimensions of the disc-shaped wheel and the bucket roots integrated therein are slightly increased, and when the overall geometry is simple, the entire turbine wheel can be produced at a cost-effective price without having to weaken the particularly stressed connection between the bucket roots and the wheel, while at the same time, the wheel is easily accessible for work in the bucket root area. Furthermore, by selecting the actual length of the bucket root up to a certain dimension of the hollow or cup-shaped recess, the achievable or permissible states of stress in the buckets or bucket elements can also be taken into account. At the same time, it is possible to achieve a high degree of accuracy for the shape of the entire turbine and better accessibility for tests to be performed during manufacturing. To achieve a simple and reliable connection between the bucket body or any individual components thereof and the bucket root, the invention provides that a rigid connection of a bucket root with a bucket body is produced by means of welding. Due to the fact that the bucket body or its individual components are of small design, and thanks to the one-piece design of the bucket roots and the disc-shaped wheel, a reliable and secure connection of the individual components can be achieved, whereby in the case of a welding connection, for example, the welding is done under an inert-gas blanket. Furthermore, the connections between the bucket root and the bucket body can be tested in a simple manner, especially by means of known test procedures. In accordance with an alternative embodiment for a simple connection, the invention provides a detachable connection of a bucket root and a bucket body by means of a screw joint or key joint, whereby its is preferably arranged that in case of a screw joint or key joint between a bucket root and a bucket body a sealing element or seal weld is provided between the bucket root and the bucket body to ensure a tight and strong connection. As already indicated above, forged parts of a turbine wheel for a Pelton turbine are characterized mainly by improved mechanical characteristics and are less prone to flaws, or that flaws which may occur can be more easily corrected. The invention therefore provides that the wheel is made from a forged material in one piece with the bucket roots, in accordance with a particularly preferred embodiment. Due to the fact that the bucket roots only cause a minor enlargement of the dimensions of the disc-shaped wheel and usually have a simple geometric configuration, the higher cost of forming forged elements can be minimized, and the particularly stressed area of the connection between the bucket roots and 1 5 the disc-shaped wheel can be produced of a correspondingly strong material that can be worked without causing flaws, and in a particularly economic manner. According to a particularly preferred embodiment, the bucket bodies are made of a cast or forged material, whereby it is suggested as especially preferable that a bucket body designed in several pieces is at least partly made of forged material. As already mentioned several times, the bucket bodies themselves have relatively small dimensions, so that the slightly higher costs of manufacturing them from forged material is justified in every case, considering their resulting greater strength in comparison with bucket bodies made of cast material. In addition, especially in case of a multi-piece bucket body, the individual elements can be made of different materials, and the parts less subject to stress, for example the lateral areas or side walls, can be made of cheaper cast material, while the middle parts for example can be made of forged material. The invention is described below by means of embodiments of a turbine wheel for a Pelton turbine, with reference to the schematic drawings in which Fig- 1 shows the schematic lateral view of a turbine wheel according to the invention for a Pelton turbine; Fig. 2 shows a view along line 11-11 in Fig. 1, at an enlarged scale; Fig. 3 shows a partial view, enlarged again, of the connection between a bucket body and the bucket root; and Fig. 4 shows a section along line IV-IV in Fig. 3, whereby Fig. 3 provides a view in the direction of arrow III in Fig. 4. In Fig. 1, a turbine wheel for a Pelton turbine is given the general reference number 1, whereby a number of buckets 3 is arranged on a disc- shaped wheel 2. As indicated schematically in Fig. 1, the turbine wheel 1 is formed in one piece with the bucket roots 4 which are adjacent to the bucket bodies 5, whereby a connecting line between the bucket roots 4 and the associated bucket bodies 5 is schematically indicated for some buckets 3 by means of a peripherally level frontal surface 6 of the bucket roots 4. The running direction of the turbine wheel in Fig. 1 is schematically indicated by arrow 7. The enlarged representation according to Fig. 2 shows that the disc¬ shaped wheel 2, shown partly in section, is formed as one piece with the bucket roots 4, whereby in the embodiment according to Fig. 2, the bucket roots 4 are adjacent to bucket bodies composed of a number of elements. Here, it is provided that a middle section 8 of the bucket bodies is connected via a level frontal surface 11 with the frontal face 6 of bucket root 4, whereby in addition, lateral sections 9 with level side surfaces 12 are connected with the side surfaces 13 of bucket root 4 as well as with the level side surfaces 10 of the middle section 8 of the bucket body. For example, the connection or fixing can be accomplished by means of welding the individual elements 8, 9 and 4 along the connection lines defined by border surfaces or side surfaces 6, 11 and 10, 12, 13. It is shown further that the bucket roots 4 extend over a partial area of the cup-shaped recess of bucket 3. It can be provided that the disc-shaped wheel 2 with the bucket roots 4, forming one piece with same, is made of a forged material to ensure high bucket bodies, which make up bucket 3, can also be made of forged material. As an alternative, it can also be provided that, for example, the less stressed lateral sections or side walls 9 are made of a cast material that is cheaper to produce, while the middle section 8, whose width is substantially that of the bucket root 4, is also made of a forged material. Should it be preferred to connect the elements 8, 9 with the bucket root 4 by means of a screw joint or key joint instead of welded connections along the connection lines, an appropriate seal or superficial seal weld must be provided in the area of the connection lines. The further enlargement of the embodiment in Fig. 3 and 4 shows that the disc-shaped wheel is formed as one piece with the bucket roots 4, to which the middle section 8 and the lateral sections 9 are connected — via the connection lines defined by the side surfaces — with the bucket root 4, in each case to form a bucket 3. The additional lines indicated in Fig. 2, 3, 4, partly as dotted lines, show the complex geometric configuration of bucket 3.