GB2033492A - Interconnecting turbine blades - Google Patents

Abstract

Adjacent turbine rotor blades are connected together by pretensioned tie-rods 17 having swivel joints 18 to allow relative angular movement between the tie-rods and the blades. Each joint may comprise a hemispherical member seated in a socket in the respective blade, each rod end being screw-threaded and engaged in a threaded aperture in the respective hemispherical member. <IMAGE>

Description

SPECIFICATION Turbine rotor This invention relates to turbine rotors of the axial-flow type.

The rotor blades of the low-pressure or of the last stage of a turbine are subjected during operation to a variety of disturbing forces and it is well known to employ stiffening arrangements between adjacent blades, to reduce or eliminate unduly large amplitudes of vibration, which may cause fatigue failure of individual blades. Such stiffening means may comprise a bridging arrangement between adjacent blades and may include bracing ties, lacing wires and coverbands similar to those described in British Patents Nos. 1,037,951, 1,499,586 and 1,509,185.

On large diameter rotors, such as those employed on the low-pressure final stage of steam turbines, relatively thin aerofoil blade sections are necessary at the blade ends, in order to achieve high efficiency and such sections are relatively weak in torsion. Moreover, the blades are formed with a high degree of twist.

Some form of stiffening or bridging between adjacent blade tips is necessary in such rotors subject to the following factors:- (1) The centrifugal forces are high and the distance between adjacent blade tips is so relatively large that the provision of a stiffening or bridging tie arrangement is difficult 6 owing to the stresses involved.

(2) Because of the high level of centrifugal forces and the elasticity of the materials employed in the rotor blades the rotor increases in diameter and the blades increase in length when the rotor runs-up to speed. The distance between adjacent blades also increases and thus any stiffening or bridging arrangement must necessarily be capable of accommodating the circumferential strain imposed.

(3) Any stiffening or bridging tie arrangement employed is required to have a fairly small mass in order to avoid the imposition of excessively high centrifugal forces on the blades.

(4) With highly twisted blades centrifugal forces produce torsional moments which partially untwist the blades so that the stiffening or bridging tie means must be capable of either accommodating the untwisting of the blades or of resisting such untwisting.

In a turbine rotor according to the invention the blades are coupled together at or near their tips by tie-rods each of which extends between the immediately adjacent leading and trailing edges respectively of immediately adjacent blades each tie-rod comprising swivel joint means permitting angular movement between the tie-rod and the blades and each tierod being under tension and comprising screw means by which the tie rod can be tensioned, the tensions in every two adjacent tie-rods imposing on the blade to which they are commonly connected a torsional load opposite in sense to that in which the blade is urged to untwist by forces produced by rotation of the rotor.

Said swivel joint means may comprise two swivel joints respectively adjacent said edges of the blades.

The two swivel joints may be positioned at the respective sides of the blades remote from the centre of the tie-rod.

Alternatively, the swivel means may be positioned between the blades.

The screw means may, for example, comprise screwed end portions on the tie-rod engaging screwed apertures in respective members of the swivel joints.

Alternatively, the tie-rod may comprise two parts the screw means comprising portions of those parts in screwed engagement.

Turbine rotors will now be described to illustrate the invention by way of example with reference to the accompanying drawings in which: Figure 1 is an elevation of a rotor of a steam turbine; Figure 2 is a development of part of the rotor shown in Fig. 1 showing sections transversely of the blades.

Figs. 1 and 2 show a turbine rotor having a hub 10 and a circumferential row of blades 11. Each blade has reinforced, thickened regions 1 3 and 1 4 adjacent leading and trailing blade edges respectively, and near the outer end or tip 1 2 of the blade. The blades have apertures 1 5 leading to hemispherical sockets 1 6 formed in the reinforced sections 13, 14 and immediately adjacent blades are connected together by tie-rods 1 7 adjacent their leading and trailing edges.

The tie-rods 1 7 have screw-threaded end portions 1 7A on to which are screwed hemispherical ball-members 18, which seat within the sockets 1 6 to form socket swivel joints.

During assembly of the rotor each tie-rod is pretensioned by tightening the ball members 18, which are provided with slots for this purpose. The end of the screw-threaded portion 1 7A is then peened over to provide a secure, permanent fixing for the swivel joint.

The degree of pretensioning is such as to eliminate clearances and ensure proper seating of the ball-and-socket surfaces.

The pretensioning of the tie-rods 1 7 imposes torsional loads on the blades 11. Each blade 11 is subjected to the tension loads in the two tie-rods 1 7 connected to the blade 11. The torsional load on the blade 11 is in a sense opposite to that in which the blade is urged to untwist by forces produced by rotation of the rotor.

Untwisting of the blades is therefore resisted by the tie-rods 1 7. The tensions in the tie-rods 1 7 increase as the speed of the rotor increases so that the increasing forces urging the blades to untwist are opposed by the increasing tensions in the tie-rods.

The blades 11 are designed so that before the tie-rods 1 7 are tensioned during assembly, the twist in the blades 11 is slightly less than the maximum desired value. The tie rods 1 7 are tensioned so as to increase the twist in each blade 11 so as to bring it close to or equal to the desired value.

In addition, the tie-rods 1 7 are pretensioned so that the leading edges of the blades 11 are as close as practicable to a common plane transverse to the axis of rotation of the rotor.

This is an important consideration, since any blade having a leading edge which is ahead of its fellows will suffer additional or even excessive erosion and wear.

The screwed tie-rods 1 7 afford considerable flexibility of procedure in the manufacture of the rotor. Clearances are automatically taken up as the tie-rods are tightened. The number of turns of the "screw" may be a notionally constant number; but instead, or in addition, the tie-rods, or some of them, may be pretensioned to a predetermined value or values.

The spacing between immediately adjacent blades 11 in the finished rotor is less critical generally than the setting of the leading edges in a common plane. However, the blade spacing may also be regulated at least within a tolerance band quite readily and whatever criteria are adhered to tie-rods of but a single size suffice to enable the rotor to be assembled and completed to satisfactory tolerances.

The precise sequence of tensioning of the tierods is a matter of choice based on skill and experience.

The ball-and-socket swivel joints allow relative angular movement between the tie-rods and the blades 11. This occurs at least during construction of the rotor as the tie-rods 1 7 are tensioned by adjustment of their screw means. This reduces or eliminates residual bending stress in the tie-rods 1 7 in the completed rotor and also reduces or eliminates corresponding unwanted residual stresses in the blades 11.

When the rotor is in operation, the tie-rods may or may not need to move angularly relatively to the blades. However, if forces increase to a sufficient value so as to overcome the friction at the swivel joints, articulation can occur and excessive loads are thus avoided by angular accommodation of the blades and tie-rods.

The swivel joints are inherently self-aligning and excessive point loading cannot arise by contrast with prior proposals involving the abutment of plain surfaces of blades and tie rod under load which, if disturbed angularly can produce severe point loading and dam age. Also, stressing of the tie-rods.

The tension of the tie-rods and the provision of swivel joints ensure that the tie-rods are fully effective throughout the whole of the operating speed range of the rotor. Each tierod and swivel assembly is always fully engaged with the two adjacent blades.

The assembly of the rotor as a whole is greatly facilitated by the invention.

The hemispherical members 1 8 may be made of a material having anti-fretting properties to avoid fretting fatigue due to any slight relative movement that may be set up under buffetting conditions for example in the rotor blades during operation. By this means greater freedom of choice of blade and tie material is permitted, without regard to fretting fatigue resistance.

Instead of the ball and socket joints described above a different combination of joints or a different kind of swivel joint may be used which allows reltive angular movement between the tie-rods and the blades, and between adjacent blades.

For example, one member 1 8 may be integral with or welded or otherwise secured to the remainder of the tie-rod 1 7 without provision for screw adjustment, only the other member 1 8 being adjustable by screw action.

In another alternative the tie-rod may comprise two swivel joints secured to respective portions of the tie-rod which portions are in adjustable screw-threaded engagement and the swivel joints are secured to respective blades.

In other forms of rotor in accordance with the invention, the blades may be interconnected by a further series of means interconnecting the blades. Such further series may comprise tie-rods similar to those already described; or it may comprise different means altogether. Such further series would be arranged generally at a different radius in relation to the axis of rotation of the rotor.

In another form of rotor, each blade may have a tip sealing formation such as an integral platform which acts to reduce flow of steam past the blade tip.

In the rotor described in detail with reference to the drawings, the ends of each tie-rod 1 7 are at the same radius in relation to the axis of rotation of the rotor. However, in a modification the ends may be at different radii.

Claims (1)

1. A turbine rotor comprising twisted blades coupled together at or near their tips by tie-rods each of which extends between the immediately adjacent leading and trailing edges respectively of immediately adjacent blades, each tie-rod comprising swivel joint means permitting relative angular movement between the rod and the blades, and each tie rod being under tension and comprising screw means by which the tie-rod can be tensioned, the tensions in every two tie-rods imposing on the blade to which they are commonly connected a torsional load in the same sense as that in which the blade is already twisted.

2. A rotor according to claim 1, in which said swivel joint means comprises two swivel joints respectively adjacent said edges of the blades.

3. A rotor according to claim 2, in which each of two immediately adjacent blades has an aperture through which the tie-rod connecting the blades extends, and in which each swivel joint comprises a part-spherical member engaging the respective blade in a correspondingly shaped socket formed in the blade on the side remote from the centre of the tierod.

4. A rotor according to claim 3, in which each socket is formed in a thickened region of the blade.

5. A rotor according to claim 1, in which said swivel joint means is positioned between the immediately adjacent blades.

6. A rotor according to any preceding claim, in which said screw means comprises a screwed rod portion of the tie-rod in screwed relationship with a screw threaded aperture in a member of swivel joint means.

7. A rotor according to claim 3 or 4, in which each part-spherical member has a screw-threaded aperture in which is engaged a respective screwed end portion of a common rod forming the remainder of the tie-rod in each case.

8. A rotor according to any claim of claims 1 to 5, in which the tie-rod in each case comprises two parts in screw-threaded engagement forming said screw means, the two parts being connected to respective members of respective swivel joints forming said swivel joint means.

9. A rotor according to any preceding claim, in which said tie-rods form a first means interconnecting the blades and in which there is a second means interconnecting the blades at a radius with respect to the rotor rotational axis different from the radius at which said first means is positioned.

10. A rotor according to claim 9, in which the second means comprises tie-rods similar to those of the first means.

11. A rotor according to any preceding claim, in which the ends of each tie-rod are at the same radius in relation to the rotor rotational axis.

1 3. A rotor according to any claim of claims 1 to 11, in which the ends of each tierod are at different radii in relation to the rotor rotational axis.

14. A turbine rotor substantially as described herein with reference to Figs. 1 and 2 of the drawings.