Fluid reaction surfaces (i.e. – impellers) – Specific working member mount – Blade received in well or slot
Reexamination Certificate
1999-12-10
2002-12-17
Ryznic, John E. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Specific working member mount
Blade received in well or slot
C029S889100
Reexamination Certificate
active
06494683
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the repair of turbine rotor wheel dovetails and particularly relates to repaired turbine rotor wheels and apparatus and methods for replacing a damaged rotor wheel dovetail with a forged replacement ring in which new dovetails are formed. The invention is particularly applicable to the repair of steam turbine rotors but is also applicable to gas turbine rotors.
BACKGROUND OF THE INVENTION
Steam turbines typically used for power generation are comprised of multiple stages, each having fixed partitions and a plurality of turbine buckets mounted on rotatable turbine wheels. The buckets are conventionally attached to the wheels by a dovetail connection. A number of different types of dovetails may be employed. For example, a finger-type dovetail is often used to secure the buckets and rotor wheel to one another. In that type of dovetail, the outer periphery of the rotor wheel has a plurality of axially spaced circumferentially extending stepped grooves for receiving complementary fingers on each of the bucket dovetails when the buckets are stacked about the rotor wheel. Pins are typically passed through registering openings of the dovetail fingers of each of the wheel and bucket dovetails to secure the buckets to the wheel. Another type of dovetail is a tangential entry dovetail. The turbine wheel and bucket dovetails have a generally complementary pine tree configuration. Also, in gas turbines, axial entry dovetails are sometimes employed. In any event, the dovetail connections between the buckets and wheels are highly stressed and, after years of operation, tend to wear out and crack. On low pressure steam turbine rotors, cracking occurs typically as a result of stress corrosion. In high pressure steam turbine rotors, cracking typically occurs as a result of creep rupture and/or low cycle fatigue. It will be appreciated that the magnitude of the stresses in the rotor wheel are very substantial at the radial location of the wheel dovetail because of stress concentration factors developed by the dovetail geometry. That is, peak stresses are significantly higher in the wheel dovetail as compared with locations radially inwardly which have significantly lower stresses. For example, the pin openings in the finger-type dovetail, and the machined areas of the wheel defining the fingers concentrate the stresses in the dovetail area and, over time, cause cracking as a result of one or more of the aforementioned failure mechanisms.
Because of the mass and the rotational speed of a turbine, e.g., typically on the order of 3600 rpm, significant damage to the turbine, its housing and surrounds, as well as injury to turbine operators, can occur should cracks develop in the wheel dovetail sufficiently to permit one or more of the buckets to fly off the rotor wheel. Prior to the present invention, the utility operator, upon inspection of the rotor and identification of a significant crack in one or more of the turbine wheels, particularly at the dovetail connections, had essentially two choices: first, the entire rotor could be replaced and, secondly, the damaged rotor wheel could be repaired by employing a conventional weld buildup process. The first option is costly and may involve considerable costly downtime before a new rotor is available for installation. For that reason, removal of the damaged dovetail from the rotor wheel and replacement of the removed dovetail with built-up weld material has been the principal choice as the method of repairing damaged turbine wheels.
In a typical weld buildup process, the rotor is first removed from the turbine and the buckets are removed from the turbine wheel. The damaged dovetail is then removed from the wheel and weld material is applied to the rim of the wheel in multiple passes to provide a weld build up which can later be machined to provide the dovetail. The weld material can be the same as or different from the material from which the rotor wheel is made. For example, in U.S. Pat. No. 4,940,390, a TIG welding process is used to deposit a weld metal of 12 Cr material onto Ni—Cr—Mo—V. 12 Cr material is much more resistant to stress corrosion cracking than Ni—Cr—Mo—V. However, welding processes in general are prone to defects such as porosity and slag inclusions in the weld metal and it is difficult to optimize the properties of the weld material when it is being deposited on the wheel.
There are, however, specific limitations on the buildup of weld material on a wheel which render turbine rotor wheel buildups as a method of repair only marginally satisfactory. On one hand, the weld buildup material desirably should be as resistant and as strong as possible to resist stress corrosion cracking in service. On the other hand, the weld material must be weldable to the base material, i.e., the forging of the rotor. To provide such weldable material, carbon and certain other elements must be kept relatively low to render the material weldable. This results in a dovetail lower in strength as compared with what could be achieved by supplying a replacement rotor forging. Therefore weld buildups inherently limit the capability to provide optimum material for resistance to stress corrosion cracking, creep rupture and cycle fatigue. The resulting weld buildup typically sacrifices tensile and yield strength to accommodate the need for a material weldable to the base material. This lower strength, together with the tendency for weld defects to grow during operation of the turbine, can limit the life expectancy of the repair to well below that of a replacement rotor forging.
Stress relief is also an important consideration in employing weld buildups for rotor wheel repair. Typically, the weld buildup is applied to the rotor, while the rotor axis lies in a horizontal position. However, to stress-relieve the rotor by application of heat, conventional methodology provides for hanging the rotor vertically, i.e., the rotor axis lies vertically. It was believed that the application of heat for stress relief purposes must be applied while the rotor is vertical to achieve uniformity of applied heat and uniform stress relief about the rotor. This involves substantial handling of the rotor with attendant risk of damage to the rotor.
Accordingly, there has developed a need for a repaired turbine rotor wheel dovetail wherein the material of the dovetails has the same as or increased resistance to failure mechanisms, such as stress corrosion cracking, creep rupture and cycle fatigue, as well as improved methods for repairing the dovetails of turbine rotor wheels.
BRIEF SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a dovetail repair for a turbine rotor which, instead of a weld buildup repair, provides a ring replacement, typically a forging, for the entirety of the damaged dovetail and which forged ring can be formed of the same or improved materials in comparison with a replacement rotor material wherein the same or improved resistance to the various failure mechanisms is obtained. The replacement ring, forged or cast, is beneficially virtually free of defects and which defects might otherwise be extant in repaired dovetail characterized by a weld buildup of the same or similar material. Also, the material of the replacement ring is not a function of the welding process or the weld material employed to secure the ring to the rim of the wheel body after the damaged dovetail has been removed. Consequently, the forged replacement ring can be formed of materials which provide optimum properties for resistance to one or more of the different types of failure mechanisms. For example, for rotors formed of Ni—Cr—Mo—V or Cr—Mo—V or 12 Cr, a 12 Cr material such as 12% CrCb or an Inconel-based material can be employed. The weld material can be any weld material which is compatible with both the base material and the forged ring material, for example, a 12 Cr—Ni—Mo. The nature of the weld material is less significant to the welding process employed in this invention because the weld i
Crawmer Gerald Richard
Emeterio Eloy Vincent
Nolan John Francis
General Electric Company
Nixon & Vanderhye
Ryznic John E.
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