GB2091140A

GB2091140A - Turbine blades repair

Info

Publication number: GB2091140A
Application number: GB8201051A
Authority: GB
Original assignee: Refurbished Turbine Components Ltd
Current assignee: Refurbished Turbine Components Ltd
Priority date: 1981-01-15
Filing date: 1982-01-14
Publication date: 1982-07-28
Also published as: GB2091140B

Abstract

A method of repairing turbine blades of the type in which an array of blades, for example a rotor, have a shroud (12) secured to the outer ends thereof, each blade (10) being provided with a tennon (11) cooperating with the shroud (12) to secure the blade (10) to the shroud (12). The tennons (11) are prone to erosion and damage during use of the turbine, and a method of repair comprises the steps of forming a new integral tennon (11a) and a root part (14), removing the shroud (12) from the blade (10), removing the eroded or damaged tennon (11) and a part of the blade to accommodate the root part (14), securing the new integral tennon (11a) and root part (14) to the blade (10) and securing the shroud (12) to the new tennon (11a). <IMAGE>

Description

SPECIFICATION Turbine blade repair The present invention relates to a method of repairing turbine blades of the type in which an array of turbine blades (arranged in a rotor, for example) has a shroud extending around the outer ends of the blades, each blade being provided with at least one tennon adapted to cooperate with the shroud in a manner such that each blade is secured to the shroud, such turbine blades will hereinafter be referred to as turbine blades of the type described.

Turbine blades of the type described which may be used, for example, in steam turbines, are in use subject to wear and damage.

Since the outer end of the turbine blade in a rotor assembly is the part which has the greatest linear speed in use, it is the outer end that is most prone to damage or erosion. Erosion generally occurs because of contact between the turbine blades with particles carried by the steam and also with the steam itself.

At the high pressure end of a steam turbine, for example, the blades are generally subjected to damage from particles of solid matter carried by the steam due to contact between the blades and the particles. At the low pressure end of the turbine, the steam is at a lower temperature having been cooled during passage through the turbine and water condenses therefrom more readily. The steam is, therefore, wetter and the transfer of energy by impact on the turbine blades of the steam itself is greater at the low pressure end of the turbine than it is at the high pressure end. Once again it is predominantly the outer ends of the blades which are especially prone to erosion.

Another part of the blades of the type described which are, in use, subjected to the greatest wear and erosion is the attachment of the blades to the shroud, which attachment usually comprises tennons extending substantially radially outwardly from each blade, which tennons extend into apertures or recesses in the shroud.

After continued use, the tennons tend to become eroded which can lead to separation of the turbine blade from the shroud.

The shroud is a fairly simple component and a new shroud can be manufactured without great cost.

The problem arises that if a shroud requires replacement it is difficult, if not impossible, to fit a new shroud to a turbine blade assembly such as a rotor if the tennons of some or all of the turbine blades are in an eroded state.

It has in the past, where a turbine blade of the type described is found to be worn but repairable, been the practice to build up the tennons that have been eroded by a welding process followed by machining to reform the built up tennon to the correct shape so that it may once again be located in its recess or aperture in the shroud.

Such a method of repair has not been found to be entirely satisfactory since a tennon built up by welding in such a manner has been found to be weaker than the oriainal tennon and the presence of a weakness at a position in a rotor assembly which in use of the turbine is subject to great stress can lead to premature damage and failure of the tubine as a whole.

Not only is the above described method of repair unsatisfactory, but it may also be a difficult and hence expensive method of repairing the tennon since, following the step of building up the tennon by welding metal in a fused state, the reshaping of the built up tennon to the correct shape by machining, for example, is difficult since the tennon is not conveniently located for easy machining, thus this part of the process can be time consuming and hence expensive.

It is an object of the present invention to provide an improved method of repairing turbine blades of the type described.

According to the present invention we provide a method of repairing turbine blades of the type described comprising the steps of forming a new ten non to the required shape integrally with a root part, removing the existing tennon or remains thereof from the turbine blade and sufficient material from the turbine blade to accommodate said root part, and securing the integral tennon and root part to the blade.

The securing of the root part to the blade may be enabled by a welding or a brazing operation.

The root part is preferably of elongate form, from the outer end of the turbine blade in a direction towards its inner end, each root part provides elongate surfaces which, on fitting of the root part into the turbine blade, are juxtaposed with surfaces of the turbine blade, along which juxtaposed surfaces the root part may be secured by welding or a brazing operation.

The root part may be so formed such that the juxtaposed surfaces of the blade and root part provide a mechanical key between the root part and the blade.

To enable such a mechanical key the root part may be tapered so that the root part is of increased width in a direction radially innermost of the turbine blade since in use of the turbine the centrifugal force is laterally outwardly on the root part of the tennon.

The above described method of repairing turbine blades of the type described offers significant advantages over the known method.

The new tennon and root part can easily be preformed from a single piece of material by a machining process thus providing an integral homogeneous article unlike the previous known method of repairing turbine blades in which a blade machined from drawn material is provided with a tennon of cast material from the welding process.

Furthermore, the method of repair of the present invention may enable in some cases repair of the turbine blades in situ, i.e. without removal of the blade from the rotor, thus significantly decreasing the time required to effect repair.

The provision of the root part not only provides a far greater surface area along which the tennon and root part can be joined to the turbine blade, but, since it extends from the extreme outer edge towards the inner end of the turbine blade, the securing of the root part to the turbine blade takes place in a position subject to less stress than that applied to the extreme edge of the turbine blade which greatly enhances the chance of a good working life of the replaced tennon.

After the fixing of the root part to the turbine blade which as aforementioned is preferably a welding process, the blade may be machined in the area of the weld to the correct shape and also subjected to a heat treatment process to remove any undesirable stresses caused by the welding or pressing of the root part into the turbine blade, or by the peening operation used to secure the shroud to the blade.

Astill further advantage of the method of repairing turbine blades of the present invention is that more than one tennon may be provided on a single root part thereby considerably decreasing the time necessary for repairing the blade which can be of great advantage when it is required that the turbine is out of commission for as short a time as possible.

The invention will now be described in more detail by way of example only with reference to the accompanying drawings wherein Figure I is a perspective view of a turbine blade having a tennon; Figure 2 is a diagrammatic view illustrating the securing of the turbine blades to a shroud; Figure 3 is a side elevation of an integral tennon and root part; Figure 4 is a plan view of the tennon and root part shown in Figure 3; Figure 5 is a front elevation of the tennon and root part; Figure 6 is a view of a turbine blade ready to accept a new tennon and root part; Figure 7 illustrates the turbine blade shown in Figure 6 and a tennon and root part being placed into its position for welding.

Figure 1 illustrates a turbine blade 10 having a tennon 11, which tennon 11 after assembly of the turbine is secured to a shroud 12 as shown in Figure 2, the means of securing the blade 10 to the shroud 12 comprising a through bore formed in the shroud through which the tennon 11 extends, the end of the tennon 11 then being subjected to a peening process in order to firmly secure the end of each blade 10 to the shroud 12.

Alternative methods of fixing may be used such as welding, brazing or any other suitable method.

After continual use of the turbine it is quite common for wear to take place, the greatest wear taking place at the outer ends of the turbine blades due to their higher linear speed and the tennon 11 and shroud 12 may become severely eroded to such an extent that one or more of the blades may separate from the shroud. This may be caused by total erosion ofthetennon 11 or partial erosion which so weakens the tennon that it shears along the end surface 13 of the blade 10. Similarly, erosion may take place in the shroud and the peened end of the tennon 11 thus enabling relative movement to occur between the blade 10 and the shroud 12.

When such wear takes place, it is necessary to repair the turbine, and in particular the tennons 11 on turbine blades subject to wear, and sometimes the shroud itself may be replaced.

The method of repair of the present invention may, in some circumstances, allow repair of the turbine blades in situ, or alternatively, where necessary, the blades needing repair may be completely removed from the turbine.

In order to repair the blade 10, an integral tennon and root part, as shown in Figures 3,4 and 5, is formed, the newtennon 1 lea being identical in shape to the original tennon 11 and the root part 14 has a first pair of opposed surfaces 15 and 16 which are tapered and convergent towards the end at which the tennon 11 a is situated and are also tapered so that they are convergent in a direction remote from that side of the turbine blade subject to the greater force of impact during use of the turbine.

A second pair of opposed surfaces 17 and 18 are so formed that they will conform as near as possible to the surfaces 19 and 20 respectively of the turbine blade 10.

It is essential therefore that the root part 14 has a dimension between the surfaces 17 and 18 at least as thick as the turbine blade 10 so that after securing of the root part 14 to the turbine blade 10, the surfaces 17 and 18 of the root part 14 may conform as near as possible to the surfaces 19 and 20, and, if they stand proud, may be machined to conform to the shape of the turbine blade.

The first pair of opposed surfaces 15 and 16 are tapered into directions as afordescribed in order that the tennon and its root part 14 are mechanically keyed in to a corresponding slot formed in the turbine blade during the process of repair.

The turbine blade during use of the turbine will be subjected to considerable forces and the root part 14 is preferably so shaped that the forces acting on the turbine blade would tend to enhance the bond between the new root part 14 and the turbine blade 10.

During repair of the turbine blade a piece is removed as shown in Figure 6 corresponding to the shape of the root part 14 and the root part 14, together with the tennon 11a is inserted in position as shown in Figure 7.

It is preferable that the root part 14 is secured to the blade 10 by welding or brazing on the cooperating edges between the root part 14 and the blade 10. However, it is envisaged that if the slot cut in the blades 10 to receive the root 14 was machined to a sufficiently accurate degree, the root part 14 could be press-fitted into the blade 10 and held in place purely by a mechanical provided by its shape and dimensions, or possible adhesive may also be used.

Following the welding or brazing, should such an operation be necessary, of the root part 14 to the turbine 10, the area of the weld, together with the surfaces 17 and 18 ofthe root part 14, may be subjected to a further machining operation in order that the shape of the surfaces 19 and 20 on each side of the turbine blade 10 are returned to their original form and the blade, or at least the area where repair has been carried out, may then be subjected to a heat treatment and/or other process designed to relieve any stress that may have been allowed to develop in the blade during a welding operation for example.

The root part 14 may be of any suitable shape and, when welded or brazed to the turbine blade, it will be appreciated that the mechanical key formed by the shape as shown in the drawings is not essential, the root part 14 preferably being formed with generally rounded edges which assists in preventing areas where the application of stress may have a detrimental effect on the structure as a whole.

The shape of the root part 14 may be tapered in the opposite direction or the sides 15 and 16 may be substantially parallel to each other, the criteria being that a sufficient surface area along which it may be secured to the turbine blade 10 is provided in order that the integral root part 14 and the tennon 1 1a may be satisfactorily secured to the turbine blade 10.

After the root part 14, together with its integral tennon 1 Ia, have been secured to the turbine blade 10 and all such blades that need repair in the turbine have been so repaired, the shroud 12 or a new shroud may be placed in position, the ends of the tennons 1 1a would then be subjected to a peening process or alternativley could be welded or brazed to the shroud 12.

In a case where the tennons 1 1a are subjected to a peening process, any work hardening caused by the peening process is less likely to affect the welding of the root part 14to the blade 10 since it is in a position more remote than would have been the case of a tennon being built up by a welding operation.

Furthermore, the material from which the new tennon 11a and root part 14 is made may be a machined material unlike the material from which a tennon being built up purely by a welding operation i.e. cast material, the machined material therefore being compatible with the existing turbine blade 10 to provide a more homogeneous member and therefore beng less likely to incur damage.

Claims

1. A method of repairing a turbine blade of the type in which an array of such turbine blades has a shroud extending around the outer ends of the blades, each blade being provided with at least one tennon adapted to cooperate with the shroud in a manner such that each blade is secured to the shroud, said method comprising the steps of a. forming a new integral tennon and root part, b. removing the shroud from a damaged blade, c. removing the existing tennon from said blade, d. removing a part of the blade to accommodate said root part, e. securing the new integral tennon and root part to the blade, f. securing the shroud to the tennon.

2. The method of repairing a turbine blade as claimed in claim 1 wherein the step of securing the tennon and root part to the blade is carried out by a brazing process.

3. A method of repairing a turbine blade as claimed in claim 1 wherein the step of securing the tennon and root part to the turbine blade is carried out using a welding process.

4. A method of repairing a turbine blade as claimed in any of the preceding claims wherein the root part is of elongate form and adapted to be secured to the turbine blade so as to extend from the outer end of the turbine blade in a direction towards its inner end, each root part providing elongate surfaces which, on fitting of the part into the turbine blade, are juxtaposed with surfaces of the turbine blade along which juxtaposed surfaces the root part is secured.

5. A method of repairing a turbine blade as claimed in any of the preceding claims wherein the root part and the part removed from the blade to accommodate the root part are shaped in a manner to provide a mechanical key between the blade and root part in a manner to resist forces occurring during use of the turbine.

6. A method of repairing a turbine blade as claimed in any of the preceding claims wherein a plurality of tennons are formed on each root part.

7. A method of repairing a turbine blade as claimed in any of the preceding claims wherein the method includes the further step of subjecting the turbine blade and the new tennon and root part secured thereto to a heat treatment process adapted to relieve residual stress.

8. Aturbine blade repaired in accordance with the method of any of the preceding claims.

9. A method of repairing a turbine blade substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

10. Aturbine blade prepared in accordance with the method substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11. A method of repairing a turbine blade including any novel step or novel combination of steps as described in the specification and/or illustrated in the accompanying drawings.