GB2400144A

GB2400144A - Sealing between turbine blade platforms

Info

Publication number: GB2400144A
Application number: GB0406252A
Authority: GB
Inventors: Richard Stephen Allton
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2003-03-19
Filing date: 2004-03-19
Publication date: 2004-10-06
Anticipated expiration: 2024-03-19
Also published as: GB0306228D0; GB2400144B; GB0406252D0

Abstract

A turbine assembly comprises an annular row of turbine blades 6 connected to a turbine rotor by means of turbine blade root portions 4, sealing means 10 is provided between confronting edges of the platforms of adjacent turbine blades to inhibit the passage of gases therepast, one of the confronting edges of each adjacent pair of platform portions has a recess 9 therein to define a passage for receiving the sealing means 10. The sealing means comprises elongate sealing elements having cross-sectional profiles that fit in the passages, the elongate sealing elements being flexible in directions transverse of their longitudinal extents, each sealing element comprises at its outer periphery a longitudinal succession of platform contacting members. The sealing means may be in the form of a coil (fig. 3) or beads (40 in fig. 4A).

Description

2400 1 44

TURBINE ASSEMBLIES

Technical Field

The present invention relates to axial flow turbine assemblies and in particular to the reduction of fluid leakage between a turbine blade and an adjacent turbine blade by use of sealing elements between adjacent turbine blade platforms. The invention has relevance to gas turbine engines and other types of machine utilising axial flow turbines.

0 Background Art

A known type of gas turbine rotor blade has a root portion used to connect the turbine blade to the turbine rotor disc or drum, an aerofoil portion and a platform portion between the aerofoil portion and the root portion. The platform portion gives a smoother inner diameter of the annular flow passage for the working gas and thus t5 improves aerodynamic efficiency of the turbine blade. It is normal to provide a means of sealing between the confronting edges of the platforms of adjacent turbine blades, i.e., an inter-platform sealing element, in order to inhibit the passage of gases either radially inwardly or radially outwardly past the platforms, thereby reducing turbulence in the flow passage and increasing efficiency.

Where the edges of adjacent platform portions are substantially straight, then a well- known method of providing a seal is to have slots in the confronting platform edge faces and to position solid metal strips in the slots. These are commonly called "strip seals". Unfortunately, the strip seal solution is not easily applied where the shape of adjacent platform edges is non-linear because solid metal strips are not sufficiently yielding to follow changes of direction in the slots.

One known way of providing a seal in such cases is to spot-weld a number of relatively thin flexible strips to the undersides of the platforms. Free edges of these strips deflect so radially outwards when the turbine rotor is rotating to seal the gap between the platform portions of adjacent turbine blades. However, there are drawbacks to the use of such strips. Firstly, they can be damaged during maintenance of the turbine and so require À c À À À c À À À À . À À À 1 À À . . À À À replacement. Secondly, some proposed turbine blade materials are unsuitable for welding. There is therefore a need for an improved means of reducing gas leakage between confronting non-linear edges of turbine blade platforms. Such improved means should preferably be easy to install and not be subject to damage during blade removal.

Summary of the Invention

The present invention provides a turbine assembly comprising an annular row of turbine blades connected to a turbine rotor by means of turbine blade root portions and lo having platform portions radially outward of the root portions, sealing means being provided between confronting edges of the platforms of adjacent turbine blades to inhibit the passage of gases therepast, at least one of the confronting edges of each adjacent pair of platform portions having a recess therein to receive the sealing means and the sealing means comprising elongate sealing elements having cross-sectional is profiles that fit in the recesses, the elongate sealing elements being flexible in directions transverse of their longitudinal extents, each sealing element comprising at its outer periphery a longitudinal succession of platform-contacting members.

It is conceivable that only one edge of each platform may be provided with a recess, i.e., one of the confronting edges of adjacent platforms has a recess and the other one does not. In this case, during operation of the turbine, centrifugal forces will cause one side of the flexible sealing element to be in contact with the recess in one of the platform edges and the other side of the flexible sealing element to be in contact with the un-recessed edge of the adjacent platform. Normally, however, both edges of each platform will be recessed to receive the flexible sealing elements, so that during operation of the turbine, the sealing elements contact the interior surfaces of both recesses.

It is preferred that the cross-sectional profiles of the elongate sealing elements are generally round in overall shape. Although the crosssectional profiles of the recesses in the platform edges may be rectangular or v-shaped, it is preferred that they are complementary to the overall cross-sectional profiles of the transversely flexible c À c c e.e c, c *e, cee À .e

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À À - 3 sealing elements, e.g., the recess profiles may be part-circular.

In any case, the recesses in the confronting platform edges define passages into which the flexible sealing elements can be removably inserted once the turbine blades have been connected to the turbine disc or rotor by their root portions. The flexibility of the proposed sealing elements in directions transverse of their longitudinal extents enables unitary sealing elements to be inserted into the passages so that when the turbine is in use, centrifugal forces acting on the sealing elements effect a sealing action over substantially the whole length of each platform edge. It is envisaged that this property lo will be particularly valuable where - as is often the case - the confronting edges of adjacent blade platform portions are not straight but exhibit a so-called "platform interlock" or the like, such that the passages into which the sealing elements are inserted have one or more curved or angled bends part way along their lengths: the transversely flexible sealing elements will accommodate themselves to the bend or bends in the passages.

In one embodiment of the invention, the sealing elements consist of wire coils that look like so-called "coil-bound" helical springs, i.e., confronting edges of the wire in successive turns of the coil are in contact with each other. In this case, the above mentioned longitudinal succession of platform-contacting members at each sealing element's outer periphery is the successive turns of wire in the coil. To achieve more efficient sealing against flow of gas through the coil, the wire may have a square section instead of a round section, so that mutually contacting edges of the wire have a greater contact area.

It should be understood that although the above wire coil sealing elements look like coil-bound helical springs, it is not necessary for the wire of which they consist to have the resilient torsion properties usually associated with helical springs. Consequently, most of the well- known high temperature alloys normally used in gas turbines would be suitable candidates from which to make the wire coils.

Gas leakage through the flexible sealing element can be reduced if it is formed from À a a À I a À a a a a À ., # c a a # # two or more coaxial helical wire coils, one coil being nested inside the other. The winding direction of any nested pair of coils can be opposite sense or the same sense.

In the latter case, the turns of the inner one of the pair of coils can cover the junctions between the turns of the outer one.

The skilled person will recognise that where a wire coil extends around a bend in a passage, gas leakage will be greater at that point than at points where the wire coil is completely straight. This is because when the wire coil bends, the turns on one side of the coil will separate from their neighbours and allow gas to leak through at that point.

lo This gas leakage may be reduced if the wire coil is produced from wire whose cross- sectional profile has diametrically opposed concave and convex surfaces. More particularly, when the wire is wound up into a helical coil, the radially inner and outer surfaces of each turn are preferably flat but the two faces that bear against adjacent turns are concave and convex respectively. If two adjacent turns in a coil are considered then it will be readily appreciated that the convex surface of one turn nests in, and bears against, the concave surface of the other. This in itself will reduce leakage between successive turns of the coil. Furthermore, when the wire coil bends, the wire in successive turns on the inside of the bend will exhibit a rotational sliding action over itself and the wire in successive turns on the outside of the bend will not to separate from their neighbours to the same extent as for a square or circular-section wire and the leak paths will therefore have smaller dimensions.

In an alternative embodiment of the invention, the sealing elements consist of a succession of discrete bead-like members of generally parttoroidal fomm whose end faces are held in contact with or in close proximity to each other with each other, e.g., by being threaded onto a flexible central wire or the like, each sealing element thereby looking somewhat like a row of beads. In this case, the above-mentioned longitudinal succession of platfomm-contacting members at each sealing element's outer periphery is the successive bead-like members in the row of such members. To facilitate transverse flexing ofthe sealing elements as previously specified, the mutually confronting end surfaces of the bead-like members can comprise part-sphencal curved complementary male and female bearing surfaces. To achieve more efficient sealing against flow of gas past the À e'. r À À Àf ' t À' . e 4 À Be. ,4e I lee q À e e e - 5 individual bead-like members, their peripheral surfaces may comprise a series of substfmtially longitudinally extending flutes or ridges, whereby in conjunction with each platform, the outer surface of each bead-like member acts in a manner analogous to a labyrinth-type seal.

It is envisaged that the bead-like members may be made from a refractory ceramic material, such as silicon nitride (Si3N4), perhaps fibrereinforced if necessary. The central wire or the like, onto which the bead-like members are threaded, may comprise a single strand of wire, or several strands twisted together in the manner of thin wire lo rope. The wire strands may consist of a suitable high temperature alloy, such as a nickel-based alloy or superalloy.

Brief Description of the Drawings

Figure 1 is a cross-sectional end view of a turbine blade sealing assembly according to one embodiment of the present invention; Figure 2 is a radially inward looking plan view of the turbine blade sealing assembly of Figure 2; Figure 3 is an enlarged partial side view of a flexible sealing element in accordance with the invention, the flexible sealing element being in the form of a coil-bound wire coil; Figure 4A is an enlarged longitudinal-sectional view of part of a flexible sealing element in accordance with the invention, the flexible sealing element being in the form of a row of bead-like members; Figure 4B is a cross- sectional view of one of the bead-like members in Figure 4A; and Figure 4C is a cross-sectional view of an alternative form of a bead-like member suitable for forming part of a flexible sealing element in accordance with the invention.

Detailede Description of Some Exemplary Embodiments Figures 1 and 2 show an axial flow turbine assembly 1 with a pair of circumferentially adjacent turbine blades 2 retained in a turbine rotor 3, which may be a disc or a drum.

The two blades form part of an annular row of such blades extending around the circumference of the turbine rotor 3. Each turbine blade 2 has a root portion 4 that 11' t À r I I I r I I I t t t I I 1 1 C 1 1 1 1 1 1 1 1 1 1 1 ' 1 C 1 connects the turbine blade to the turbine rotor 3, a platform portion 5 radially outward of the root portion, and an aerofoil portion 6 radially outward of the platform portion.

To inhibit the passage of gases inwards or outwards past confronting edges 7 of adjacent platforms 5, it is necessary to provide a means of sealing between the confronting edges 7. To this end, the confronting edges 7 of each adjacent pair of platform portions 5 are provided with recesses 9 to receive elongate sealing elements 12. These sealing elements 12 have cross-sectional profiles that fit in the passages 10 defined by the recesses, and are flexible in directions transverse of their longitudinal lo extents. In Figure 2, the recesses 9, the passage 10 and the outer diameter of the sealing element 12 are all shown in dashed lines. As will be described more fully below, the outer periphery of each sealing element comprises a longitudinal succession of platform-contacting members to effect the required seal between the adjacent platforms.

As shown, the cross-sectional profiles of the elongate sealing elements 12 are preferably of a generally circular shape. Although the crosssectional profiles of the recesses 9 in the platform edges may be rectangular or v-shaped, it is preferred that they are complementary to the cross-sectional profiles of the flexible sealing elements, so they are shown here as particular. The recesses 9 in the confronting platform edges 7 thereby define circular passages 10 into which the flexible sealing elements 12 can be removably inserted once the turbine blades 2 have been connected to the turbine disc or rotor 3 by their root portions 4. As shown in Figure 2, the transverse flexibility of the sealing elements 12 enables them to effect a sealing action over substantially the whole length of each platform edge 7 when the turbine is in use, due to centrifugal forces acting on the sealing elements and forcing them into intimate contact with the curved surfaces of the recesses.

As seen in Figure 2, the platform portion 5 of each turbine blade 2 has a pair of circumferentially opposed non-linear side edge faces 7 and a pair of end faces 8 which are orientated generally transverse to the longitudinal axis of the rotor. To accommodate the large camber of the aerofoil portion 6, each edge 7 is formed from a 8 1 4 8 e 4, e 4. , . , 8. - 7

first length 7a and a second length 7b such that each edge 7 has a shallow-angled V- or chevron-shape when viewed from a radially outer position. Although the first and second edge lengths 7a and 7b are shown as making a definite angle shape with respect to each other, it would also be possible to blend the first and second lengths into each other by including the same angle in the arc of a curve.

Due to the non-linear platform edges, the passages 10 into which the flexible sealing elements are inserted are not straight because they each have a bend 11 at the junction of the first and second platform edge portions 7a, 7b. However, because of their lo flexibility, sealing elements 12 will accommodate themselves to the bends 11 in the passages.

When the turbine is at rest, the fit of the elongate sealing elements 12 in the passages should be just sufficiently loose to facilitate their insertion into the passages, but is within this constraint, the gap between the outside surfaces of the sealing elements and the interior surfaces of the passages should be as small as possible, to facilitate sealing over a maximum surface area when the turbine is in operation.

As shown more clearly in Figure 3, in this embodiment of the invention the sealing to elements 12 consist of wire coils that look like so-called "coil-bound" helical springs, i.e., confronting faces 30, 31 ofthe wire in successive turns A, B. C, etc., of the coil are in contact with each other. The successive turns of wire in the coil form the above- mentioned longitudinal succession of platform-contacting members at each sealing element's outer periphery. In the present case, to achieve more efficient sealing against flow of gas through the coil, the wire from which the coil is wound has a square section, so that mutually contacting faces 30, 31 of the wire have a greater contact area than would be the case with a round section wire. The wire itself may be made from a high- temperature nickel alloy or the like.

so To reduce gas leakage through the flexible sealing element 12 it may be formed from two or more coaxial helical wire coils (not shown), one coil being nested inside the other. If both coils are wound in the same sense, the turns of the inner one of the pair À Be. eee. ee. ce ce - 8 of coils can cover the junctions between the turns of the outer one.

Figure 2 illustrates that when a sealing element 12 in the form of a wire coil extends around the bend 11 in the passage, the turns on the outside of the bend will separate from their neighbours at 13, so allowing an increased leakage flow at that point.

Provided the rest of the coil is restricts leakage flow sufficiently, increased leakage flow at point 13 will not be a serious problem. However, if necessary, gas leakage may be further reduced by producing the coil from wire whose cross-sectional profile has diametrically opposed concave and convex surfaces. Consequently, when the wire is lo wound up into a helical coil, the two faces 30, 31 (Figure 3) that bear against adjacent turns are concave and convex respectively. If two adjacent turns in a coil are considered, the convex surface of one turn nests in, and bears against, the concave surface of the other. This measure will in itself reduce leakage between successive turns of the coil, but when the wire coil bends, the wire in successive turns on the inside of the bend will exhibit a rotational sliding action over itself and the wire in successive turns on the outside of the bend will not separate from their neighbours to the same extent as for a square or circular-section wire and the leak paths will therefore have smaller dimensions.

Figure 4A illustrates an alternative embodiment of the invention, in which the sealing elements 12 consist of a succession of discrete bead-like members 40, of generally part- toroidal form, whose end-faces 41, 42 are held in contact with or closely proximate each other by virtue of being threaded onto a flexible central wire 43, which may be a thin wire rope made of several wire strands twisted together. For convenience of illustration only two of the row of bead-like members 40 are shown, though more would of course be needed. In this embodiment, the above-mentioned longitudinal succession of platform- contacting members at each sealing element's outer periphery is the successive bead-like members 40. Transverse flexing of the sealing elements is facilitated because the mutually confronting end-faces 41, 42 of the bead-like members so 40 comprise curved complementary male and female part-spherical bearing surfaces, which are capable of a pivoting/sliding action with respect to each other.

À 1 It will be seen from Figure 4A that the bead-like members 40 are retained in position on the wire 43 because each end of the length of wire 43 is provided with a ferrule or the like to engage an end face of the adjacent bead-like member.

s As shown in Figure 4B, the cross-sections of the bead-like members 40 may be circular, but to achieve more efficient sealing against leakage flow past the individual bead-like members, their peripheral surfaces may comprise a series of substantially longitudinally extending flutes 44 or ridges 45, as shown in Figure 4C. In this way, the outer surface of each bead-like member acts in conjunction with each platform in a lo manner analogous to a labyrinth-type seal.

The bead-like members 40 may comprise silicon nitride (Si3N4) or another refractory ceramic material, which may be fibre-reinforced if necessary. The flexible central wire 43 may comprise a suitable high temperature alloy, such as a nickel-based alloy or Is superalloy.

It will be evident to the skilled person that it will be necessary to dimension the bead- like members 40 so that they can be inserted into the passages 10 and pushed around the bends (if any) in the passages, such as the bend 11 in Figure 2. It may also be necessary to position the bead-like members so that the interfaces between adjacent members coincide suitably with the position(s) of the bends in the passages 10.

Although the sealing elements 12 described above will not provide a gastight seal, it is believed they will reduce the amount of gas leakage between the radially adjacent turbine blades 2 for any given pressure differential, compared to known strip seals.

It will be necessary to provide some means of retaining the sealing elements 12 within the passages 10 during operation of the turbine. For example, one end of the passages may be narrowed or occluded to prevent movement of the sealing elements in that so direction, and on at least the other side of the turbine blade row, the well-known principle of the cover-plate may be utilised, this comprising a segmented annular plate that is fixed over the axial ends 8 of the blade platforms 5 and the blade root shank :e À:: e': À : ee. : - 10 portions 14. Such a cover plate is needed anyway to prevent axial leakage of air under the blade platforms If one of the turbine blades 2 needs to be replaced for any reason, the sealing element 12 is simply removed and then reinserted into the passage 10 when the new turbine blade has been connected to the turbine rotor 3.

Claims

e. À. .. ct #e tee' - 11 CLAIMS

1. A turbine assembly comprising an annular row of turbine blades connected to a turbine rotor by means of turbine blade root portions and having platform portions radially outward of the root portions, sealing means being provided between confronting edges of the platforms of adjacent turbine blades to inhibit the passage of gases therepast, at least one of the confronting edges of each adjacent pair of platform portions having a recess therein to define a passage for receiving the sealing means and the sealing means comprising elongate sealing elements having cross-sectional profiles that fit in the passages, the elongate sealing elements being flexible in directions transverse of their longitudinal extents, each sealing element comprising at its outer periphery a longitudinal succession of platform-contacting members.

2. A turbine assembly according to claim 1, in which only one edge of each platform is provided with a recess, whereby during operation of the turbine, centrifugal forces cause one side of the flexible sealing element to be in contact with the recess in one of the platform edges and the other side of the flexible sealing element to be in contact with an un-recessed edge of the adjacent platform, thereby to effect a sealing action over substantially the whole length of each platform edge.

3. A turbine assembly according to claim 1, in which both edges of each platform are recessed to receive the flexible sealing elements, so that during operation of the turbine, centrifugal forces cause the sealing elements contact the interior surfaces of both recesses, thereby to effect a sealing action over substantially the whole length of each platform edge.

4. A turbine assembly according to any preceding claim, in which the overall cross- sectional profiles of the elongate sealing elements are generally round in shape and the overall cross-sectional profiles of the recesses in the platform edges are complementary to those of the sealing elements.

5. A turbine assembly according to any preceding claim, in which the confronting edges of À e.. 46e e. ce - 12 adjacent blade platform portions are non-linear, such that the passages for the sealing elements have one or more curved or angled bends part way along their lengths: the transversely flexible sealing elements accommodating themselves to the bend or bends in the passages.

6. A turbine assembly according to any preceding claim, in which the sealing elements consist of wire coils, confronting edges of the wire in successive turns of the coils being in contact with each other, the longitudinal succession of platform-contacting members at each sealing element's outer periphery being the successive turns of wire in the coil.

7. A turbine assembly according to claim 6, in which the wire has a round section.

8. A turbine assembly according to claim 6, in which the wire has a square section.

9. A turbine assembly according to any one of claims 6 to 8, in which the flexible sealing elements are each formed from a pair of coaxial helical wire coils, one coil being nested inside the other.

10. A turbine assembly according to claim 9, in which the pair of nested coils are wound in opposing senses.

11. A turbine assembly according to claim 9, in which the pair of nested coils are wound in the same sense.

12. A turbine assembly according to claim 11, in which the turns of the inner one of the pair of coils cover the junctions between the turns ofthe outer coil.

13. A turbine assembly according to any one of claims 6 to 12, in which the or each wire coil is produced from wire whose cross-sectional profile has diametrically opposed concave and convex surfaces, such that when the wire is wound up into a helical coil, the convex surface of one turn nests in, and bears against, the concave surface of the adjacent turn. .

C t À C À À À À e À À - 13

14. A turbine assembly according to any one of claims 1 to 5, in which the sealing elements consist of a row of discrete bead-like members of generally part-toroidal form, end-faces of the bead-like members being held in contact with or in close proximity to each other.

15. A turbine assembly according to claim 14, in which the bead-like members are held in position by a flexible wire or the like passing through their centres, the above-mentioned longitudinal succession of platform-contacting members at each sealing element's outer I periphery being the row of bead-like members.

16. A turbine assembly according to claim 14 or claim 15, in which mutually confronting end surfaces of the bead-like members comprise curved complementary male and female bearing surfaces.

17. A turbine assembly according to any one of claims 14 to 16, in which the peripheral surfaces of the rows of bead-like members comprise a series of substantially longitudinally extending flutes or ridges which, during operation of the turbine, seal against adjacent surfaces of the platforms.

18. A turbine assembly according to any one of claims 14 to 17, in which the bead-like members are made from a refractory ceramic material.

19. A turbine assembly according to any one of claims 14 to 18, in which the central wire comprises a single strand of wire, or several strands twisted together in the manner of thin wire rope.

20. A turbine assembly according to claim 19, in which the wire consists of a nickel-based alloy or superalloy.

21. A turbine blade assembly substantially as herein described with reference to the accompanying drawings.

e' e.e cete a. .e - 14

22. An inter-platform sealing element for use in a turbine blade assembly according to any one of the preceding claims.