STRUCTURE

DESCRIPTION TECHNICAL FIELD

Embodiments of the subject matter disclosed herein correspond to turbomachine blades with protective structure, turbomachines (in particular steam turbines), and methods of forming protective structure.

BACKGROUND ART

During operation, the blades of turbomachines are often subject to erosion.

In particular, the blades of steam turbines are often subject to liquid droplet erosion.

Structures for protecting turbomachine blades against erosion, corrosion or wear may consist of layers formed on the surface of the blades or inserts fit in the body of the blades.

Protective layers may be formed for example by laser cladding or by cold spraying. Both these technologies are effective. Anyway, laser cladding heats the blades and may cause for example residual stress in the blades and/or distortion of the blades and may foster "stress corrosion cracking", while cold spraying hits the blades and may cause for example erosion of the edges of the blades during formation of the layers. It is to be noted that laser cladding is more expensive than cold spraying, but is quicker.

SUMMARY

Therefore, there is a general need for improving the protective structures of turbomachine blades and the methods of forming them. This need is particularly high for blades of steam turbines in the fields of "Oil & Gas" (i.e. machines and plants for exploration, production, storage, refinement and distribution of oil and/or gas) and "Energy" (i.e. machines and plants for power generation). First embodiments of the subject matter disclosed herein relate to a turbomachine blade.

According to such embodiments, the turbomachine blade is provided with an airfoil portion having a leading edge and a trailing edge and first and second sides connecting the leading edge and the trailing edge, and with a protective structure comprising a protective edging and first and second protective stripes; the protective edging is located on the leading edge; the first protective stripe is located on the first side and adjacent to the edging; the second protective stripe is located on the second side and adjacent to the edging; the edging is formed by laser cladding or welding or plasma spraying or detonation spraying or wire arc spraying or flame spraying or high velocity oxyfuel coating spraying or warm spraying; the first and second stripes are formed by cold spraying.

Second embodiments of the subject matter disclosed herein relate to a turbomachine.

According to such embodiments, a turbomachine, in particular a steam turbine, comprises a plurality of turbomachine blades as set out above.

Third embodiments of the subject matter disclosed herein relate to a method of forming protective structure.

According to such embodiments, a method of forming a protective structure on a turbomachine blade comprises the following successive steps: (A) providing a turbomachine blade comprising an airfoil portion having a leading edge and a trailing edge and first and second sides connecting the leading edge and the trailing edge, (B) forming a protective edging on the leading edge by laser cladding, and (C) forming first and second protective stripes on the first and second sides adjacently to the edging by cold spraying through at least one spraying nozzle.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated herein and constitute an integral part of the present specification, illustrate exemplary embodiments of the present invention and, together with the detailed description, explain these embodiments. In the drawings:

Fig. 1 shows a schematic lateral view of an embodiment of a turbomachine blade before forming a protective structure; Fig. 2 shows a schematic lateral view of an embodiment of a turbomachine blade during formation of a protective structure;

Fig. 3 shows a schematic lateral view of an embodiment of a turbomachine blade after forming a protective structure;

Fig. 4 shows schematically a detail of Fig. 3; Fig. 5 shows schematically a cross-section view of Fig. 4;

Fig. 6 shows a schematic and partial cross-section view of an embodiment of a turbomachine blade before forming a protective structure;

Fig. 7 shows a schematic and partial cross-section view of an embodiment of a turbomachine blade during formation of a protective structure (first intermediate stage);

Fig. 8 shows a schematic and partial cross-section view of an embodiment of a turbomachine blade during formation of a protective structure (second intermediate stage);

Fig. 9 shows a schematic and partial cross-section view of an embodiment of a turbomachine blade after forming a protective structure; Fig. 10 shows a schematic lateral view of an embodiment of a turbomachine blade after forming a protective structure in a way alternative to one of Fig. 3;

Fig. 1 1 shows first possible schematic and partial cross-section view of an embodiment of a turbomachine blade after forming a protective structure; Fig. 12 shows second possible schematic and partial cross-section view of an embodiment of a turbomachine blade after forming a protective structure;

Fig. 13 shows third possible schematic and partial cross-section view of an embodiment of a turbomachine blade after forming a protective structure; and

Fig. 14 shows fourth possible schematic and partial cross-section view of an embodiment of a turbomachine blade after forming a protective structure.

DETAILED DESCRIPTION

The following description of exemplary embodiments refers to the accompanying drawings.

The following 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.

Fig. 1 shows a schematic lateral view of a turbomachine blade 1 before forming a protective structure. Blade 1 is provided with an airfoil portion 2 having a leading edge 3 and a trailing edge 4 as well as a first side 5 and a second side 6 (shown only in Fig. 5) connecting leading edge 3 and the trailing edge 4. Airfoil portion 2 has a tip region 21 and a base region 22; base region 22 of airfoil portion 2 is adjacent to base portion 10 of blade 1.

Fig. 3, Fig. 4 and Fig. 5 show blade 1 after forming a protective structure. The protective structure comprises a protective edging 7 (that may be called "bumper" or "buffer") as well as a first protective stripe 8 and a second protective stripe 9 (shown only in Fig. 5); protective edging 7 is located on leading edge 3; first protective stripe 8 is located on first side 5 and adjacent to edging 7; second protective stripe 9 is located, preferably only, on second side 6 and adjacent to edging 7. In the embodiment of Fig. 5, stripes 8 and 9 do not cover edging 7 and are located respectively only on sides 5 and 6.

In the embodiment of Fig. 9, stripes 8 and 9 are located respectively on sides 5 and 6, but cover also entirely edging 7; the thickness of stripes 8 and 9 reduces gradually on edging 7 and is zero (or close to zero) in the middle of edging 7. In the embodiment of Fig. 1 1 (similar to Fig. 5), the two stripes do not cover edging 171 and are located respectively only on the two sides of the airfoil.

In the embodiment of Fig. 12 (similar to Fig. 9), the two stripes are located respectively on the two sides of the airfoil, but cover also partially edging 172; the thickness of the stripes reduces gradually on the edging and is zero (or close to zero) in the middle of the edging.

In the embodiment of Fig. 13, the two stripes are located respectively on the two sides of the airfoil, but cover also entirely edging 173; the thickness of the stripes reduces gradually on the edging and is minimum and low in the middle of the edging. In the embodiment of Fig. 14, the two stripes are located respectively on the two sides of the airfoil, but cover also entirely edging 173; the thickness of the stripes reduces gradually on the edging and is minimum and high in the middle of the edging. According to these embodiments, the edging of the protective structure protrudes from the leading edge of the airfoil portion of the blade. Protrusion (at the end of blade manufacturing) varies depending on the manufacturing process (e.g. degree of erosion due to cold spraying). Considering the camber line (100 in figures 1 1-14) of the airfoil portion and its prolongation, protrusion may be in the range of e.g. from 0.05 to 1 mm.

The cross-section area of the edging (immediately after its formation, e.g. before cold spraying) should take into account the manufacturing process (e.g. degree of erosion due to cold spraying) and may be in the range from 1 mm ² to 20 mm ² , more typically in the range from 4 mm ² to 10 mm ² . The cross-section area of the edging (immediately after its formation, e.g. before cold spraying) should take into account the manufacturing process (e.g. degree of erosion due to cold spraying) and may look like a bulge.

The external shape of the airfoil portion of the blade at the end of blade manufacturing depends on the external shape of the airfoil portion of the blade before formation of the protective structure and on the overlying protective structure, in particular the protective edging and its erosion during formation of the protective stripes. Therefore, at least the leading edge of the airfoil portion of the blade before formation of the protective structure should be manufactured a bit recessed with respect to the ideal position of the leading edge of the airfoil portion of the blade at the end of blade manufacturing.

The solution according to the above embodiments is particular useful for thin blades, for example blades having a thin leading edge, for example in the range of 4-8 mm. As it will be explained better afterward, the edging of the protective structure is formed by laser cladding or welding or plasma spraying or detonation spraying or wire arc spraying or flame spraying or high velocity oxyfuel coating spraying or warm spraying (preferably by laser cladding), while the first and second stripes of the protective structure are formed by cold spraying. The airfoil portion of the blade may be made of iron, titanium, nickel base alloy or stainless steel; preferably, it is made of stainless steel, for example AISI420.

The edging of the protective structure may be made of cobalt base alloy or cobalt-chromium alloy; preferably, it is made of Stellite-type material, in particular Stellite 6 or Stellite 12 or Stellite 21 , or Ultimet or any of the materials described and claimed in patent application EP1403397 or any of the materials described and claimed in patent application EP1403398. The material of the edging is more resistant to solid particle erosion than the material of the airfoil in order to resist better to the mechanical erosion due to cold spraying. The first and second stripes of the protective structure may be made of cobalt base alloy or cobalt-chromium alloy; preferably, they are made of Stellite-type material, in particular Stellite 6 or Stellite 12 or Stellite 21 , or Ultimet or any of the materials described and claimed in patent application EP1403397 or any of the materials described and claimed in patent application EP1403398. The material of the first and second stripes is resistant to liquid droplet erosion. The material of the first and second stripes may be equal to or different from the material of the edging.

The edging of the protective structure and the first and second stripes of the protective structure may extend over part (i.e. from preferably 20% to preferably 50%) of the height of the airfoil portion of the blade (see e.g. Fig. 3 and Fig. 10). Typically, the head end of the stripes is at the tip of the airfoil portion and the tail end of stripes is at an intermediate position of the airfoil portion. Preferably the edging of the protective structure is long equal to or slightly more than the first and second stripes of the protective structure. The height of the airfoil portion being the portion of the blade extending from the tip to the root of the blade.

The first and second stripes of the protective structure may extend over part (i.e. from preferably 5% to preferably 50%) of the width of the airfoil portion of the blade (see e.g. Fig. 3 and Fig. 10). Typically, the stripes extend from leading edge of the airfoil portion of the blade. The width of the airfoil portion being the portion of the blade extending from the leading to the trailing edge.

Blades identical or similar to those that have just been described may advantageously be used in turbomachines in the fields of "Oil & Gas" and "Energy". In particular, thanks to their resistance to liquid droplet erosion, steam turbines are an ideal application.

A protective structure identical or similar to those that have just been described (see Fig. 3+4 and Fig. 10) may be formed on a turbomachine blade through the following successive steps:

A) providing a turbomachine blade comprising an airfoil portion having a leading edge and a trailing edge and first and second sides connecting the leading edge and the trailing edge, (see e.g. Fig. 1 and Fig. 6)

B) forming a protective edging on the leading edge by laser cladding,

(see e.g. Fig. 2 and Fig. 7)

and

C) forming first and second protective stripes on the first and second sides adjacently to the edging by cold spraying through at least one spraying nozzle, which is a very effective way of forming thin and well adhering layers of materials resistant to erosion.

(see e.g. Fig. 3 and Fig. 5 or Fig. 9)

In this way, the leading edge of the airfoil is protected by the edging and the cross-section of the airfoil is well covered through cold spraying both laterally and frontally.

This is particularly advantageous for thin blades, more in particular for blades having a thin leading edge; a "thin leading edge" may have a thickness in the range of e.g. 2- 10 mm - as the cross-section of the leading edge is similar to a semicircle, a "thin leading edge" may have a radius in the range of e.g. 1-5 mm. In this way, heating of the blades due to laser cladding is quite low and therefore residual stress in the blades and distortion of the blades and "stress corrosion cracking" in the blades are quite low.

Alternatively to laser cladding, one might use: welding or plasma spraying or detonation spraying or wire arc spraying or flame spraying or high velocity oxyfuel coating spraying or warm spraying.

The same spraying nozzle may be used for forming both protective stripes. Alternatively, a first spraying nozzle may be used for forming a first protective stripe and a second spraying nozzle may be used for forming a second protective stripe; in this case, the first and second protective nozzles may be formed at the same time.

The stripes of the protective structure may comprise a plurality of parallel (or substantially parallel) segments. In figures 4-9, segments 81 and 91 are transversal (specifically perpendicular) to the leading edge 3. In figures 10-14, segments 181 are parallel (or substantially parallel) to the leading edge 103. It is to be noted that these segments touch each other laterally even if Fig. 4 and Fig. 10 show them as distant.

The spraying nozzle moves at least from a side (e.g. 5 or 6) of the airfoil portion toward the edging (e.g. 7) for forming a stripe (e.g. 8 or 9) and rotates preferably by at least 40° about the leading edge (e.g. 3) in order to form a stripe (e.g. 8 or 9) adjacent to the leading edge (e.g. 3) and to the edging (e.g. 7). During the initial part of the motion of the spraying nozzle, the spraying jet is directed highly inclined (preferably perpendicular) to the surface of (the side of) the airfoil; when the spraying nozzle approaches the leading edge the spraying nozzle is gradually rotated so that the spraying jet remains directed highly inclined (preferably perpendicular) to the surface of (the front of) the airfoil. As a first alternative, step C may comprise (consider e.g. figures 6-9):

- a first sub-step C IA (see Fig. 8) wherein a spraying nozzle moves from a first side (e.g. 5) of the airfoil portion toward the edging (e.g. 70, 71 , 72) and rotates by at least 40° and preferably 80-90° thus forming a segment (e.g. 81) of a first stripe (e.g. 8), and immediately after

- a second sub-step C2A (see Fig. 9) wherein a spraying nozzle moves from a second side (e.g. 6) of the airfoil portion toward the edging (e.g. 70, 71 , 72) and rotates by at least 40° and preferably 80-90° thus forming a segment (e.g. 91) of a second stripe (9); preferably, during sub-steps CIA and C2A the spraying nozzle maintains a constant longitudinal position on the airfoil portion.

In this way, two separate segments are formed at the same level on the two sides of the airfoil portion. If sub-steps C IA and C2A are repeated at different levels several times two stripes are formed on the two sides of the airfoil portion, contemporaneously.

As a second alternative, step C may comprise:

- a first sub-step C1B wherein a spraying nozzle moves from a first side (e.g.

5) of the airfoil portion toward the edging (e.g. 70, 71 , 72) and rotates by at least 40° and preferably 80-90° thus forming a segment (e.g. 81) of a first stripe (e.g. 8), and immediately after

- a second sub-step C2B (see Fig. 9) wherein the same spraying nozzle rotates by at least 40° and preferably 80-90°, and moves away from the edging (e.g. 7) to the first side (e.g. 5) of the airfoil portion thus forming another segment

(e.g. 81) of the same first stripe (e.g. 8); preferably, during sub-steps CIB and C2B the spraying nozzle maintains a constant longitudinal position on the airfoil portion, and the spraying nozzle is slightly displaced longitudinally after sub-step C IB and before sub-step C2B.

In this way, two parallel segments are formed at different levels on the same side of the airfoil portion in two manufacturing operations.

If sub-steps C IB and C2B are repeated several times one stripe is formed on one side of the airfoil portion. Afterwards, another stripe is formed on the other side of the airfoil portion.

As a third alternative, step C may comprise: - a first sub-step C3 wherein a spraying nozzle moves from the first side (e.g.

5) of the airfoil portion toward the edging (e.g. 7) thus forming a segment (e.g. 81) of the first stripe (e.g. 8),

- a second sub-step C4 wherein the same spraying nozzle rotates about the leading edge (e.g. 3) by 80-180° thus forming a (typically thin) connecting segment,

- a third sub-step C5 wherein the same spraying nozzle moves away from the edging (e.g. 7) to the second side (e.g. 6) of the airfoil portion thus forming a segment (e.g. 91) of the second stripe (e.g. 9); preferably, during sub-steps C3, C4 and C5 the spraying nozzle maintains a constant longitudinal position on the airfoil portion.

In this way, two consecutive segments are formed at the same level on the two sides of the airfoil portion.

If sub-steps C3, C4 and C5 are repeated at different levels several times two stripes are formed on the two sides of the airfoil portion, contemporaneously. According to the third alternative, step C may comprise also the following sub- steps after sub -step C5 : - a fourth sub-step C6 wherein the same spraying nozzle moves from the second side (e.g. 6) of the airfoil portion toward the edging (e.g. 7) thus forming a segment of the second stripe (e.g. 9) adjacent to the previous one,

- a fifth sub-step C7 wherein the same spraying nozzle rotates about the leading edge (e.g. 3) by 80-180° thus forming a (typically thin) connecting segment,

- a sixth sub-step C8 wherein the same spraying nozzle moves away from the edging (e.g. 7) to the first side (e.g. 5) of the airfoil portion thus forming a segment of the first stripe adjacent (e.g. 8) to the previous one; the spraying nozzle being slightly displaced longitudinally after sub-step C5 and before sub-step C6.

Preferably, during sub-steps C6, C7 and C8 the spraying nozzle maintains a constant longitudinal position on the airfoil portion.

In this way, further two segments (adjacent to the first two segments) are formed at the same level on the two sides of the airfoil portion.

If sub-steps C3, C4, C5, C6, C7 and C8 are repeated at different levels several times two stripes are formed on the two sides of the airfoil portion, contemporaneously.