FIELD OF INVENTION
The present invention relates to a method of manufacturing a replaceable seal assembly for fitting on an existing damaged rotor after machining and for repairing the same. More particularly, the invention relates to a special fitted seal assembly between relatively rotating members, for turbo machinery sealing.
BACKGROUND OF INVENTION
This invention relates to the method of formation of a replaceable seal assembly and more particularly, to replaceable sleeve for turbo machinery sealing. Rotary machines, such as steam and gas turbines, used for power generation and mechanical drive applications are generally large machines consisting of multiple turbine stages. In turbines, high pressure fluid flowing through the turbine stages must pass through a series of stationary and rotating components, and seals between the stationary and rotating components are used to control leakage. The efficiency of the turbine is directly dependent on the ability of the seals to prevent leakage, e.g., between the rotor and stator. To minimize the leakage the clearances at seal between rotating and stationary element are kept minimum. So in case of uneven thermal expansion or distortion and high vibrations triggered by some malfunctions, rubbing occurrences are more at seal location. The instances of rubbing between rotating and stationary element, has following major implications:

• Changed run-out condition of rotor due to uneven heating and rubbing which deteriorate rotor balance condition.
• Localized metallurgical changes in the microstructure of rotor due to local uneven heating which may lead to early cracking and fatigue failure during operations.
• Changed local hardness values and residual stress on rotor surface where rubbing took place which lead to higher thermal sensitivity of rotor and poor vibrational behavior during operation.
Thus far in turbo machines seals have been formed as an integral part of the associated shaft in a unitized construction. So in case of rubbing at seal locations, the shaft also had to be replaced along with its sealing elements. Since such shafts represent very substantial investments, it is beneficial to provide a shaft seal assembly which may easily be replaced in the event of such damage.
Prior attempts to satisfy the replaceable seal requirements are disclosed in U.5.Pat.No.3,046,648 where in the pressure seal including a honeycomb section which is suitably formed and secured to a retaining ring to provide a separate rather than integral sealing member. The retainer ring is preferably press-fitted to a receiving member within a suitable receiving recess; but is in all instances intended to be removable and replaceable. For reasons which will be hereinafter explained, this system suffers in the relaxation of fit between the retaining ring and shaft for high temperature and high speed applications. In addition shrink-fitting of the retaining

ring onto shaft at such hot steam path portion is not feasible due to following reasons:
• High shrinkage stresses get generated in shrunk on ring as very high amount of shrinkage is required to compensate for sudden heating and expansion of sleeve duringinitial thermal transient and to secure sleeve on to rotor against high centrifugal forces at operating condition.
• With very high operating stresses and high temperature, creeping occurs at rapid rate which ultimately results in loss of shrinkage of sleeve and subsequent operational trouble.
A second such prior art system, disclosed in U.S.Pat.No.3,846,899 for formation of replaceable labyrinth seals. Each groove on the rotating shaft has an overhanging shoulder at one of its side sand a predetermined width. Disposed within each groove is a plurality of circumferentially abutting tooth segments having bases of a predetermined width smaller than the width of the grooves. The segments are mounted within the groove with their bases partially underlying the overhanging shoulders, and a retaining metal deposit such as thermally sprayed or brazed metal, etc. is disposed upon the bases and within the remaining groove width. For reasons which will be hereinafter explained, this system suffers in the tendency of the rotor material cracking at deposit/brazing material location unless the spraying or brazing process is conducted under highly controlled environment and precise deposition parameters. In addition after material deposition, stress relieving heat treatment may

also be needed which could have further complex implications on rotor shape and uncertainty of rotor performance.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of manufacturing a replaceable seal assembly for fitting on an existing damaged rotor after machining and for repairing the same, which is reliable and simple to implement and is capable of providing a separate sealing member.
Another objection of the invention is to propose a method of manufacturing a replaceable seal assembly for fitting on an existing damaged rotor after machining and for repairing the same, whereby as a separate unit the seal is easily installed and readily replaced.
SUMMARY OF THE INVENTION
Accordingly, it is the objective of the invention to provide a replaceable seal for turbo machines which overcomes the draw backs of the prior art, which is simple to implement and yet reliable to sustain severe operational requirement of rotor.
Briefly stated, the present invention discloses a special fitted sleeve which is suitably formed and secured to the rotor to provide a separate rather than integral sealing

member. As a separate unit the seal is more easily installed and more readily replaced. The special featured fitted sleeve is conceptualized which achieves following objectives:
• It maintains true location on rotor by virtue of its fit during initial assembly.
• During operation, the special shaped sleeve tends to secure its radial position onto rotor under the action of centrifugal force and maintain its true condition unlike shrink fitted ring which tends to relieve under centrifuging action.
The special fitted sleeve features and its dimensional parameters are shown inFig.6. For a given length of seal on the rotor, parametric design of fitted sleeve can be optimized to minimize the stresses and deflection of sleeve to meet the desired service requirement.
According to a feature of the invention, there is provided a special fitted sleeve for repairing an existing damaged rotor at sealing portion due to rubbing during operation. The repair methodology is carried out by, identifying the length of rotor to be machined after visual inspection, measurement of run-out, hardness and residual stress at the seal portion of rotor.
Rotor diameter is machined minimally in order to achieve rotor run-out, hardness and residual stress values within acceptable range based on rotor material properties. The dimensional parameters of specially fitted sleeve are optimized with reference to operating condition. The dimensional parameters of fitted sleeve are finalized

considering the physical assembly constraints on the rotor and the minimum sleeve thickness selected shall be greaterthan rotor diameter machined for hardness and run-out improvement.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1 - Shows a partial cross section of front portion of a steam
turbine with labyrinth seal integral part of shaft;
FIG. 2 - Shows a partial cross sectional view of a replaceable seal
according to prior art (U.S.Pat.No.3,046,648)
FIG. 3 - Shows a sectional view of a replaceable labyrinth seal
according to prior art (U.s.Pat.No.3,846,899)
FIG.4 - Shows a assembly of turbine rotor and fitted sleeve according
to the present invention.
FIG.5 - Shows a cross section of the special fitted sleeve to be
mounted on the machined turbine rotor according to the
present invention; and
FIG.6 - Shows a machined turbine rotor for fitting the replace able
sleeve, according to the present invention;

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 1shows a partial cross section of front portion of the reaction type steam turbine. It will be understood however that the concepts of the present invention is equally applicable to any type of turbo machine such as gas turbine or compressor.
Referring to FIG 1, where in turbine outer casing 1 houses steam chamber 3 with control stage 4 inside it. The sealing of high pressure steam after control stage is achieved by means of stages of labyrinth fins 5 on rotor 2 and as well as inside stationary steam chamber 3. The labyrinth fins 5 have been formed directly on the rotor 2. So in case of rubbing due to malfunction at seal, the rotor 2 also had to be replaced along with its sealing elements.
Referring to FIG 2, a prior art replaceable seal shows a stationary housing 24, a rotatable member 22 and seal 20. The seal 20 is shown mounted on a rotatable shaft 22 and disposed in sealing relation to a stationary member 24through which the rotatable shaft extends. As shown the seal ring 20 is press fitted to rotating member 22 within a suitable receiving recess. For high temperature and high speed applications shrink fit between the seal ring 20 and rotating member 22 tends to relax which results in loss of contact between seal ring 20 and rotating member 22 and subsequent operational problems.

Referring to FIG 3, a prior art replaceable labyrinth seal as above in which each groove 15 has a predetermined width 16 and a locator member, in this embodiment comprising an overhanging shoulder 17 defining one of its walls. Disposed within each groove 15 is a row of labyrinth seal teeth comprising a plurality of abutting tooth segments 18. The tooth segment shave thickened base portions 20, disposed with in the grooves; and the tooth segments extend there from radially outwardly from the shaft and into proximity with the shroud 14. The base portions of the tooth segments have predetermined widths smaller than width 16 of the grooves 15. For the purpose of retaining the plurality of tooth segments 18inproper position within groove 15 during operation of the rotatable shaft, a retaining body of deposited metal 22 is disposed over a portion of the bases 20 of the tooth segments, the deposit substantially filling the remaining width of the grooves not filled by the tooth segment bases 20. The deposit 22 may be formed by thermal spraying or brazing or by other means. For reasons which will be hereinafter explained, this system suffers from warping of fin segment rings, rotor deposit material cracking and base rotor material property deterioration in heat affected zone unless the process is conducted under highly controlled environment and with precise deposit process parameters. In addition after depositing, stress relieving heat treatment and finish machining also may be needed to achieve required rotor shape and ensure reliable rotor performance.

Referring to FIG 4, shows a cross section of a one form of replaceable sleeve with labyrinth seal configuration according to present invention is disclosed in the environment of a steam turbine. It will be appreciated, however that the concepts of the present invention can be applied in any situation wherein a seal between rotating shaft and stationary part embracing the shaft is subjected to conditions conducive to damage of the shaft due to rubbing during operation thereto and the requirement of replaceable sealing ring.
FIG. 5 is cross section of the special fitted sleeve 6 to be mounted on the machined turbine rotor 2 according to the present invention. The sleeve 6 is provided with the locating steps 7, 8 shown in fig. 6 at both the ends. The dimensional parameters of specially fitted sleeve i.e. a, b, c, d, e, D as shown in fig. 6 can be optimized using finite element analysis or analytically with following criteria with reference to operating condition:
• Minimum value of center deflection of sleeve.
• Minimum stresses in the radial locating steps of both fitted sleeve and rotor on the front and rear end.
FIG.6 is machined turbine rotor 1 for fitting the replaceable sleeve 6, according to the present invention. Rotatable shaft 2 is provided with grooves 9, 10 at both ends substantially parallel with one another. The cross sectional shape of the groove of the shaft 2 preferably conforms to that of the associated locating step of the sleeve6.

The sleeve 6with locating steps 7 & 8 at both the ends, is fitted to the shaft 2withthe cooperation of grooves on the shaft 9 & 10. The tolerances on inner diameter of sleeve 6 and outer diameter of shaft 2 are selected such that the tolerances results in suitable clearance fit for facilitating assembly. The locating steps 7 & 8 on sleeve 6 have predetermined widths which also have clearance fit with the groove on shaft 2.
According to the object of the present invention, the locating steps 7 & 8 of sleeve 6 are underlying the overhanging shoulders 11 & 12 of shaft 2, for the purpose of securing the sleeve against the high centrifugal forces associated with rotation of shaft 2. For the purpose of retaining the fitted sleeve 6 in proper position on to the shaft 2 during operation, diametrically anti rotation screws 13 are positioned between sleeve 6 and shaft 2.
After locking the sleeve 6 onto the shaft 2, the outer diameter of sleeve 6 is machined to suit to plurality of seal design which includes but not limited to labyrinth seals, abradable seals, brush seals, honeycomb seals, the rotary part of the corresponding seal is machined on the outer surface of sleeve 6.
To take care of axial thrust load on the sleeve during operation, the sleeve 6 is screwed to the shaft 2 by means of fixing screws 15. The size and number of axial sleeve fixing screws 6 are decided based on the axial thrust load on to sleeve.

The axial clearances ‘t1 & t2’ between fitted sleeve and rotor at leading and trailing end are decided based on transient thermal expansion of fitted sleeve. The provision of axial clearance would allow free expansion of fitted sleeve with reference to its axial fixity and prevent axial restraint loading of axial fixing screws. Under the action of centrifugal force at operating condition, the radial fit provided at leading edge of fitted sleeve tries to seal high pressure steam escaping to low pressure through clearance space between sleeve and rotor.
The fitted sleeve is provided with pressure relief holes 14 on the trailing low pressure end to prevent any pressure build up in case of leaking high pressure steam through the radial fit at leading edge.

WE CLAIM
1. A method for manufacturing a replaceable seal assembly for fitting after machining
on an existing damaged rotor and for repairing the same, the said method
comprising;
machining the damaged rotor shaft (2) to a minimum to remove run out of the rotor;
providing a special sleeve (6) with locating steps (7, 8) on it and matching grooves (9, 10) on shaft (2) to maintain the radial position of sleeve with respect to shaft;
fitting the sleeve (6) on to the machined shaft/rotor (2);
fixing the sleeve (6) on to the shaft (2) by fixing screws (15) to combat axial thrust load on the sleeve (6) during operation;
providing an axial clearance between fitted sleeve (6) and rotor (2) at leading and trailing end to allow free expansion of fitted sleeve (6); wherein the radial fit provided at leading edge of fitted sleeve (6) configures to seal high pressure steam escaping to low pressure through clearance space between sleeve (6) and rotor (2) under the action of centrifugal force at operating condition.
2. The method as claimed in claim 1, wherein the step of fitting the sleeve (6) on to
the shaft (2) comprising sliding fit for simple and easy assembly/disassembly.

3. The method of replacing a damaged sleeve (6) comprising the steps of
removing the fixing screws (15) between rotor (2) and damaged sleeve (6); removing the damaged sleeve (6);
fitting a new sleeve having locating steps to match with grooves of the shaft (2); wherein the sleeve (6) is fixed on the rotor (2) by means of screws (15).
4. The method as claimed in claim 1, can be implemented as design for plurality of
sealing configurations in turbo machine applications.