PELTON-TYPE TURBINE BUCKET AND METHOD FOR MANUFACTURING THE SAME

Description

[0001] The present invention relates to a Pelton turbine with increased service life and a method for manufacturing the turbine. In particular, the present invention relates to the turbine buckets with a coated substrate and a method for providing and coating the substrate.

[0002] Erosion-resistant coatings are often used on hydropower turbine components, particularly on Pelton turbine runners, which are susceptible to erosion by sediments contained in the headwater.

[0003] For example, EP 3 647 585 B1 discloses a component of a hydroelectric turbine comprising a wedge-shaped substrate with a tip section and at least two side sections, wherein an erosion-resistant coating is applied to the wedge-shaped substrate, and wherein the erosion-resistant coating applied to the tip section is at least twice as thick as the erosion-resistant coating applied to the side sections, and wherein the erosion-resistant coating has a density of at least 99.5%. The erosion-resistant coating is applied using the high-velocity air-fuel (HVAF) process. In particular, EP 3 647 585 B1 discloses buckets of a Pelton turbine runner constructed in this manner.

[0004] The object of the invention is to provide a Pelton-type turbine impeller having an alternative design and being manufactured by an alternative method, wherein the resistance of the impeller to erosion is comparable to the resistance of the impellers known from the prior art.

[0005] The object is achieved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention can be found in the subclaims.

[0006] The invention is explained below with reference to the figures. The figures show in detail: Fig.1 Bucket of a Pelton turbine according to the prior art Fig.2 Degraded main cutting edge Fig.3 Main cutting edge of a bucket according to the invention Fig.4 Main cutting edge of a bucket according to the invention Fig.5 Main cutting edge of a bucket according to the invention

[0007] Figure 1shows a schematic section through a bucket of a Pelton-type turbine according to the prior art. The bucket comprises a substrate, designated 1. The substrate is made of steel. The main cutting edge, designated 2, is located centrally in the bucket. The main cutting edge splits the water jet directed onto the turbine into two partial jets, which are then redirected by the two bucket halves. The main cutting edge is therefore often referred to as a "splitter." Figure 1 The cut shown is perpendicular to the direction of the main cutting edge. To protect the cup against erosion, the inside of the cup is coated with an erosion-resistant coating, which is designated by 3. As can be seen from Figure 1 As can be seen, the substrate already has essentially the same contour as the coated cup.

[0008] Figure 2shows the main cutting edge 2 degraded by erosion from Figure 1 Since abrasion is most severe at the tip of the cutting edge, where the undivided water jet impinges, the coating is removed there first, followed by the underlying substrate. The coating remaining on the flanks is removed less quickly than the substrate, resulting in the formation of a characteristic groove. It is clear that such a profile of the main cutting edge leads to significant efficiency losses, so that an impeller with such degradation must be replaced.

[0009] The described degradation process can be slowed by applying as much resistant material as possible at the point of greatest abrasion. In the solution according to EP 3 647 585 B1, this is achieved by ensuring that the erosion-resistant coating applied to the tip section is at least twice as thick as the erosion-resistant coating applied to the side sections. An undesirable side effect is that this increases the tip radius at the main cutting edge, leading to reduced initial turbine efficiency.

[0010] The inventors therefore sought a way to arrange as much resistant material as possible at the most stressed area without increasing the tip radius accordingly. They realized that this could be achieved by giving the substrate a contour prior to coating that resembles the contour of a degraded cup (see Figure 2 ) corresponds.

[0011] Figure 3shows the main cutting edge of a cup according to the invention. The substrate has a wedge-shaped configuration in the region of the main cutting edge, wherein the end of the wedge does not terminate in a single tip, but rather there is a depression or groove arranged between two tips at this point. In other words: the wedge-shaped substrate comprises a first and a second tip section and a total of four side sections in the region of the main cutting edge, wherein a depression extends between the tip sections, and wherein two side sections are assigned to each tip section, and wherein two of the side sections are arranged on the outside of the wedge-shaped substrate, and the two remaining side sections are oriented on the inside towards the depression. It is clear that the lines formed by the end points of the two tip sections run parallel to the cutting line of the main cutting edge.The depression forms a "valley" between the tip sections.

[0012] When coating such a substrate, the depression is first filled with erosion-resistant material. The coating process is then continued in this area until the coating material has formed a raised portion there, in order to form the tip of the main cutting edge. To achieve this, the surface of the coating in the area of ​​the deepest point of the depression must protrude at least slightly above the surface of the coating at the two tip sections. However, it is advantageous if the raised portion is as strong as possible. Subsequently, the outer side sections and the rest of the Pelton cup are coated. The outer side sections and the rest of the Pelton cup could just as well be coated first, i.e., before coating the depression as described above. Figure 3It is evident that the tip radius of a main cutting edge produced in this way is comparatively small in relation to the amount of erosion-resistant material at this point. Furthermore, the bond between the coating and the substrate in this highly stressed area is very strong, as the recess enables a positive fit.

[0013] Figure 4shows the main cutting edge according to the invention, with certain characteristic dimensions marked. These dimensions relate to the layer thickness of the coating at specific locations on the substrate, with the coating thickness being measured in a direction perpendicular to the substrate surface at the respective location. DA indicates the thickness of the coating in the area of ​​an outer side section. DI indicates the thickness of the coating in the area of ​​an inner side section. Ds indicates the thickness of the coating at a tip section. A main cutting edge according to the invention is characterized in that the following relationship is satisfied for the stated coating thicknesses: D A < D S ≤ D I

[0014] Typical layer thicknesses in the area of ​​the outer side sections are in a range of 0.3 mm ≤ DA ≤ 0.7 mm.

[0015] Figure 5shows the main cutting edge according to the invention, with certain further characteristic dimensions indicated. W is the distance between the two tip sections of the substrate, this distance being measured perpendicular to the axis of symmetry of the cutting edge and perpendicular to the cutting line. H is the height of the tip sections in relation to the deepest point of the recess. H is removed parallel to the axis of symmetry of the cutting edge. D is the thickness of the coating at the point where the axis of symmetry of the main cutting edge intersects the substrate, i.e. in the center of the recess, which usually has its deepest point there.

[0016] For many common geometries of Pelton buckets, the dimensions mentioned lie in the following advantageous value ranges according to the invention: W = 3 − 7 mm H = 0,1 − 0,7 mm D ≥ 1 mm

[0017] The precise shape of the recess is not crucial. The simplest way to do this is to mill it into the substrate, e.g., using a cove milling cutter. After the substrate with the recess has been prepared, the erosion-resistant coating is applied. For suitable materials and compositions of the erosion-resistant coating, see sections

[0027] and

[0028] of EP 3 647 585 B1. A combination of tungsten carbide particles with a CoCr matrix is ​​considered particularly suitable. According to the invention, the coating can be applied using HVOF or HVAF. HVOF is considered to be particularly suitable. Details of these processes can be found in EP 3 647 585 B1.

[0018] The manufacturing process according to the invention comprises the following steps: S1: Providing a substrate 1 with a recess arranged between two tip sections, and wherein two side sections are arranged on the outside of the tip sections; S2: Applying an erosion-resistant coating 3 to the substrate 1 in the region of the recess, filling the recess with erosion-resistant material, and then continuing the coating process in this region until the coating material has formed a raised portion there; S3: Applying an erosion-resistant coating 3 to the substrate in the region of the outer side sections.

[0019] It is clear that step S1 must be executed before steps S2 and S3. The order of steps S2 and S3 can then be arbitrary.

[0020] Finally, it should be noted that the coating process described can be performed on separate buckets if the buckets are mounted as a whole on a hub during the manufacture of a Pelton impeller. However, if the buckets are manufactured integrally with the hub, or if only parts of the buckets are mounted on the hub, with the remaining buckets being manufactured integrally with the hub, then the coating process described is performed when the entire bucket is attached to the hub. List of reference symbols

[0021] 1Substrate 2Main cutting edge or chip 3Coating

Claims

1. Bucket of a turbine of the Pelton type comprising a substrate (1), a main cutting edge (2) and an erosion-resistant coating (3) covering the substrate (1), wherein the substrate (1) has a wedge-shaped configuration in the region of the main cutting edge (2), characterised in that the substrate (1) comprises a first and a second tip section and a total of four side sections in the region of the main cutting edge (2), wherein a depression extends between the tip portions, and wherein two side sections are associated with each tip section, and wherein two of the side sections are arranged on the outside of the wedge-shaped substrate and the two remaining side sections are orientated inwards towards the depression, and wherein the coating (3) forms an elevation in the region of the depression in order to form the tip of the main cutting edge (2).

2. The bucket according to claim 1, wherein the two tip sections are arranged such that they have a distance W of 3 to 7 mm.

3. The bucket according to claim 1 or 2, wherein the two tip sections and the depression are formed such that the tip sections have a height H with respect to a deepest point of the depression which is between 0.1 mm and 0.7 mm.

4. The bucket according to any one of the preceding claims, wherein the coating (3) has a thickness D of at least 1 mm in the centre of the depression.

5. Bucket according to one of the preceding claims, wherein the coating (3) consists of a matrix of CoCr with embedded tungsten carbide particles.

6. A method of manufacturing a bucket of a Pelton type turbine according to any one of the preceding claims, the method comprising the following steps: S1: providing a substrate (1) having a depression arranged between two tip sections, and wherein two side portions are arranged on the outside of the tip sections; S2: Application of an erosion-resistant coating (3) to the substrate (1) in the region of the recess, the latter being filled with erosion-resistant material and the coating process then being continued in this region until the coating material has formed an elevation there; S3: Application of an erosion-resistant coating (3) to the substrate (1) in the region of the outer side sections.