Inter-blade seal

Abstract: Turbines having rotor blades (3, 5) fixed in dovetail grooves (14) in the rotor periphery have bridge pieces (17) which provide in effect a false floor to the inter-stage recess (8, 11) to close off the machining recess from the fluid path. This invention provides a method of fixing this bridge piece which is simple, positive and avoids loading the rotor disc rims with additional centrifugal force. The dovetail groove (14) for the blade root is continued into a raised portion of the rotor recess periphery. This raised portion is grooved circumferentially (21) and the stem (19) of the 'T' section bridge piece fits snugly in the groove. A dovetail section key (23) is inserted through the blade root dovetail groove (14) and into the raised portion of the rotor, passing through a hole in the stem (19) of the bridge piece. The latter is thus positively located and locked in position.

Description: This invention relates to turbines, which term is used in this specification to mean rotary machinery which is driven by or which drives a working fluid such as gas or steam by means of rings of blades mounted on a rotor to intercept the working fluid in an annular passage. The invention is of particular application to gas turbine compressors for providing compressed air to the fuel combustion process, but it is not limited to this application. A gas turbine compressor has a rotor with rings of blades spaced along the axis so as to interleave with rings of fixed blades extending inwardly from the compressor casing. This invention is concerned with the rotor blades and the smooth flow of fluid (air) from each ring of blades to the next. On a compressor rotor the moving blades are often fitted with dovetail roots which engage complementary grooves in the respective rotor disc so holding the blades against the very large centrifugal forces that arise. The dovetail section blade root may be fitted to the rotor in circumferential grooves, or, more commonly, grooves which are axial or skewed to the rotor axis according to the particular aerofoil section of the blade. The machining of the dovetail grooves in the rotor, where these grooves are axial or skewed to the axis, requires circumferential recesses in the rotor in front of each ring of blades and to a radial depth below that of the roots of the downstream blades. The term "in front of" is taken to mean "upstream of" and correspondingly "behind" is taken to mean "downstream of". In a compressor, as the flow moves downstream and the gas is compressed, the annular passage is made smaller by increasing the rotor diameter and reducing the height of the blades. In a conventional machine this stepping of the rotor diameter assists the machining in that the inter-stage recess has to be only marginally deeper than the downstream blade root. In any event the inter-stage recess would take the gas flow out of the direct path between the aerofoil sections of the blades, the resulting turbulence causing losses. This is normally prevented by providing bridge pieces which effectively close off the recess and provide a smooth inner boundary to the flow, limiting it to the annular region of the blade aerofoil section. One method of fitting such bridge pieces is to fit platform-like pieces in grooves in the upstream and downstream faces of the rotor discs close to the rim of the discs. The bridge pieces are fitted into these grooves through a machined window and then slid around the grooves to their required positions. Allowance is made for expansion of the bridge pieces both axially and circumferentially and consequently seals are required. Additionally stop plates and springs may be required to be fitted to prevent uncontrolled circumferential sliding of the bridge pieces. A further disadvantage of this method of providing bridge pieces is that the grooves on the faces of the rotor discs for receiving the bridge pieces necessitate an increase in the axial spacing of the stages beyond that required for entry of the blade root on assembly. Again, the centrifugal force on the bridge pieces is carried as an additional load on the disc rim which is already highly stressed by the forces in the rotor blades. An object of the present invention is to provide a turbine having bridge pieces which very largely avoid the above disadvantages. Another example of the prior art in this field is patent number GB1,236,920. This shows a compressor rotor drum carrying a succession of axially spaced-apart rotor blade stages. Each rotor blade has a dovetail-type root which is held in complementary-shaped dovetail grooves in the periphery of the rotor drum, the dovetail grooves extending axially of the rotor drum. The periphery of the rotor drum is recessed between adjacent rotor blade stages, each recess having a central high-level section with a low-level section on each side adjacent the blade stages. The high-level section of each recess is provided with dovetail grooves which may be taken as extensions of the dovetail grooves housing the blade roots. The boundary of the flow passage between adjacent rotor blade stages is defined by bridge pieces which extend between adjacent rotor blade stages. The bridge pieces have radially and axially extending wall portions which are keyed into the dovetail grooves in the high level sections of the recesses. According to the present invention, there is provided a turbine or compressor for a working fluid, including a rotor rotatable about an axis, a plurality of stages of blades mounted on the rotor in axially spaced succession, each blade having a root of dovetail cross section mounted in a complementary dovetail groove extending axially, or skewed to the axial plane, in the periphery of the rotor, the rotor periphery having a recess between at least some adjacent blade stages, the recess having a stepped profile that includes a high level section and a low level section, the dovetail groove extending into the high level section, the low level section providing axial and radial clearance for a cutting tool to access the dovetail section groove, a circumferentially extending series of bridge pieces between adjacent blade stages, the bridge pieces having platform surfaces providing an inner boundary to a flow path of the working fluid, and for each bridge piece, a key for keying the bridge piece to the high level section of the recess, each key having a cross-section at least partially conforming to the dovetail. groove and lying within the extension of the dovetail groove in the high level section, characterised by each bridge piece having a circumferentially extending wall section supporting the platform surface and engaging a circumferential groove in the high level section of the recess to restrain axial movement of the bridge piece. Preferably, the wall section has a hole through which the key extends, the engagement of the key and the hole preventing movement of the bridge piece radially and circumferentially. The key may be of dovetail cross-section, an end of the key engaging the bridge piece wall section and being relieved on its radially inner face to provide a shoulder on the key which abuts the bridge piece wall section axially and prevents axial movement of the key. The wall section and the platform may together provide a 'T' section in an axial plane, the stem of the 'T' lying approximately midway along the axial extent of the recess. The bridge piece may comprise two wall sections supporting the platform, the two wall sections being spaced apart in respective grooves in the high level section of the recess and the key extending through holes in both the wall sections. The joins between adjacent bridge pieces in a series may be angled to the axial direction. A turbine compressor having bridge pieces in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an axial section of part of the compressor rotor and parts of two rotor blades in successive stages; Figure 2 is a plan view, i.e. looking radially inwards, of the region between the blades of Figure 1; and Figure 3 is a perspective view of a dovetail locking piece shown in Figures 1 and 2, the perspective axes being indicated separately. Referring to the drawings, the rotor 1 is fitted with successive rings of blades of which 3 indicates a blade in a particular stage and 5 indicates a blade in the next downstream stage. The upper part of these blades is of aerofoil section and the lower part, below approximately the broken lines 6 and 7, is of dovetail section in known manner, the axis of the dovetail running at an angle of 20° or so to the rotor axis. This skewing of the blade and its root may be seen more clearly in Figure 2. In a conventional compressor the recess between successive blade stages is of uniform or continuously varying diameter and extends to a depth below the innermost extremity of the blade roots. The rotor is usually in the form of separate discs keyed to a shaft and each blade stage is mounted on its own rotor disc. The recess is therefore partially formed by the opposing faces of the disc rims and these downstream and upstream faces are grooved close to the rim periphery to receive bridge plates as described above. In the present embodiment of the invention, applicable for example to a one piece rotor body for successive rows of rotor blades or a rotor made up of discs on a shaft or a composite of both, the upstream part 8 extending for rather more than half the axial extent of the recess and having a diameter 'A' preferably just less than that of the downstream blade root extremity 9. The downstream part, 11, of the profile has a diameter 'B' just less than that of the upstream blade root extremity 13. The dovetail groove 14 for the root of blade 3 in the rotor 1 (i.e. in the rotor disc carrying blade 3) has a cross-section as shown by the outline of the projection 15 in Figure 2. This dovetail groove 14 extends right through the rim of the disc (below the line 6 in Figure 1) and is continuous through the rotor forming the upstream part 8 of the recess. The cutter with which this groove 14 is cut can thus pass through the rotor beneath the line 6, through the rotor in the upstream part 8 of the recess, and is then free to be removed from the downstream part 11 of the recess. The axial extent of the part 11 of the recess is greater than the maximum diameter of the cutter, i.e. the width of the groove 14 in Figure 2. While it is convenient that, in the lower-pressure stages of the compressor the blade root extremity (e.g. in Figure 1) is clear of diameter 'A' for ease of insertion of the cutter for blade 5, this will not be so in the higher-pressure stages. The cutter can nevertheless then be lowered radially into the downstream part 11 of the recess if the rotor diameter at blade root 9 is less than diameter 'A' of the upstream recess. The whole extent of the recess 8 and 11 is required to be closed off to gas flow which is required to pass smoothly from the aerofoil section of blade 3 to the aerofoil section of blade 5. A platform-like bridge piece 17 extends between the two blades, forming a lower boundary to the gas flow. The platform part is supported by a circumferentially extending wall section 19. The bridge piece 17 extends circumferentially in an arcuate manner and may cover several blade pitches. The bridge piece is of 'T' section, as shown in Figure 1, the platform forming the cross-bar of the 'T' and the wall section 19 the stem of the 'T' which fits into a circumferential groove 21 in the upstream recess rotor. The 'T' section is preferably balanced for centrifugal forces in a rotor axial plane (i.e. the plane of Figure 1) and therefore the stem 19 is approximately central in the recess. The upstream portion of the rotor in the recess thus extends for more than half of the recess axial extent to allow room for the groove 21. Movement of the bridge piece is prevented by a dovetail locking piece or key 23 which is shown in perspective view in Figure 3. This locking piece has the same cross-section as the root of blade 3 and thus fits snugly in the dovetail groove 14 in that portion within the recess part 8. The downstream end 25 of the locking piece passes through a hole in the stem 19 and extends into an end part of the dovetail groove 14 in a wall 27 formed by the groove 21. In order that the stem 19 has sufficient 'body' under the locking piece 23, rear (i.e. downstream) end 25 of the locking piece is relieved as shown. This has the advantage of providing a shoulder or abutment face 29 on the locking piece which engages the stem 19 and prevents the locking piece escaping downstream into the recess portion 11. The hole in the stem 19 may be rectangular, i.e. not engaging the locking piece on the 'dovetail surfaces' 31. The lower part of these surfaces may also be removed to make vertical surfaces 33 conforming to the side walls of the rectangular hole in the stem 19. Since the dovetail groove 14 is skewed to the rotor axis the ends of the locking piece 23 are correspondingly angled to conform to the circumferential planes of the rotor and root of the blade 3. While the term "dovetail" has been used to describe the form of the blade root section and its complementary groove, the term is intended to cover any wedge formation. The method of assembly is as follows (a) The bridge piece 17 is fitted into position with the hole in the stem 19 in line with the dovetail slot 14. (b) The locking dovetail piece 23 is entered into the front (i.e. upstream) end of the dovetail slot and passed through until the reduced nose section projects through the 'T' stem and slightly into the downstream recess 11. (c) The dovetail root of blade 3 is now entered into dovetail slot 14 and pushed into position in abutment with the bridge piece 17. The locking piece 23 is thus prevented from moving upstream. (d) This procedure is repeated until all of the bridge pieces 17, locking dovetail pieces 23 and blades 3 are in position. (e) The blades 3 are then locked in position by the next (upstream) ring of bridge pieces and the process repeated through all the stages. The leading (upstream) stage blades may be fixed by locking tabs or by staking. The following advantages are provided by the design described above: (1) Each bridge piece is individually located and locked in position. (2) Axial clearances are maintained by the accurate location of the stem of the bridge piece in its groove. (3) Stop plates and springs to prevent circumferential drifting of the bridge pieces - a feature of conventional designs - are not required. (4) The bridge pieces can be light in weight compared to conventional designs. (5) The accurate location of the bridge pieces by means of the stem and groove fitting permits tight fitting seals to be inserted between the bridge pieces and the rotor discs and between adjacent bridge pieces. (6) The bridge pieces act as axial locking devices on the adjacent stages of moving blades. (7) The centrifugal force on the bridge piece is carried on the main part of the rotor body and not as an additional load on the highly loaded bladed disc as in conventional designs. (8) No locking tabs, requiring bending over, are needed on any stage except possibly the front face of the first stage. (9) The centrifugal force of the bridge piece is carried on the locking piece extension in double shear. (10) The dovetail locking piece is radially supported along its full length including the section after the groove. Certain refinements and modifications may be found desirable. Thus: (1) The junction between adjacent bridge pieces can be angled to reduce turbulence created by the joint passing the fixed blade tips and hence reduce vibratory excitation of the fixed blades. (2) Careful attention to the masses of the individual bridge and locking pieces can lead to reduction of potential unbalance forces. (3) The ends of the dovetail locking pieces can be drilled and tapped for the insertion of an extraction tool. (4) The wings of the bridge piece are preferably balanced for centrifugal force taking account of the difference in radii and the relative length of the wings. This balancing is done in both axial and circumferential planes. (5) The bridge pieces may be provided with double stems fitting into respective circumferential grooves. This would put the locking piece in multiple shear, giving lower overall attachment shear stresses.

Claims: A turbine or compressor for a working fluid, including a rotor (1) rotatable about an axis, a plurality of stages of blades (3, 5) mounted on the rotor in axially spaced succession, each blade having a root of dovetail cross section mounted in a complementary dovetail groove (14) extending axially, or skewed to the axial plane, in the periphery of the rotor, the rotor periphery having a recess (11) between at least some adjacent blade stages, the recess having a stepped profile that includes a high level section and a low level section, the dovetail groove (14) extending into the high level section, the low level section providing axial and radial clearance for a cutting tool to access the dovetail section groove, a circumferentially extending series of bridge pieces (17) between adjacent blade stages, the bridge pieces having platform surfaces providing an inner boundary to a flow path of the working fluid, and for each bridge piece, a key (23) for keying the bridge piece to the high level section of the recess, each key having a cross-section at least partially conforming to the dovetail groove and lying within the extension of the dovetail groove in the high level section, characterised by each bridge piece having a circumferentially extending wall section (19) supporting the platform surface and engaging a circumferential groove (21) in the high level section of the recess to restrain axial movement of the bridge piece. A turbine or compressor according to Claim 1, wherein the wall section (19) has a hole through which the key extends, the engagement of the key and the hole preventing movement of the bridge piece radially and circumferentially. A turbine or compressor according to Claim 2, wherein the key is of dovetail cross-section and has an end which engages the bridge piece wall section, the key end being relieved on a radially inner face to provide a shoulder (29) on the key which abuts the bridge piece wall section axially and prevents axial movement of the key. A turbine or compressor according to Claim 3, wherein the wall section and the platform together provide a 'T' section in an axial plane, the stem of the 'T' lying approximately midway along the axial extent of the recess. A turbine or compressor according to any preceding claim, wherein the high level section of the rotor recess profile lies at the upstream side of the recess. A turbine or compressor according to Claim 2 or Claim 3, wherein the bridge piece (17) comprises two wall sections supporting the platform, the two wall sections being spaced apart in respective grooves in the high level section of the recess and the key extending through holes in both wall sections. A turbine or compressor according to any preceding claim, wherein the joins between adjacent bridge pieces in a series thereof are angled to the axial direction. A turbine or compressor according to any preceding claim, wherein each key is adapted for attachment of a tool to extract the key along the dovetail section groove when the adjacent blade root is removed.

Inventors: ROYLE ERIC EDWARD, ROYLE, ERIC EDWARD
Applicants: ABB ALSTOM POWER UK LTD, ABB ALSTOM POWER UK LTD.
Application year: 1995
Main classification: F01D5
IPC codes: F01D11, F04D29, F01D5