Metal working – Method of mechanical manufacture – Impeller making
Reexamination Certificate
2000-01-07
2001-12-25
Rosenbaum, I Cuda (Department: 3726)
Metal working
Method of mechanical manufacture
Impeller making
C029S889720, C029S402080
Reexamination Certificate
active
06332272
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of turbine blades, and more particularly to the field of the repair of the tip portion of turbine blades.
FIG. 1
illustrates a turbine blade
10
as is known in the prior art for use in power generating turbines, such as in the first row of blades of a gas or combustion turbine. Turbine blade
10
includes a blade root
12
, an airfoil portion
14
, and a tip portion
16
. The blade root
12
is designed to be inserted into and retained by a disc on a rotating shaft (not shown) of the turbine. Airfoil portion
14
is shaped to extract energy from combustion gases passing over the airfoil portion
14
, thereby imparting rotating mechanical energy to the turbine shaft. For modern gas turbine engines, airfoil portion
14
is designed to include one or more cooling passages formed below the surface of the airfoil for the passage of cooling air necessary to insure the integrity of the blade material in the hot combustion gas environment. Such cooling passages may be formed in a forged blade by a drilling process or may be formed directly in a cast material blade. For cast turbine blades, the cooling passages are formed by supporting a ceramic core within the volume of the mold as the material of the blade is cast. In order to support the ceramic core in its proper position during the casting process, it is necessary to extend a portion of the core to the edge of the casting, thereby creating one or more openings in the tip portion
16
of the as-cast blade. These openings must then be sealed during the fabrication of the blade in order to assure the proper flow of the cooling air within the turbine blade
10
. If the size of the opening is sufficiently small, it may be sealed by a weld plug
18
formed on the tip
16
of the blade
10
. For larger openings it may be necessary to cover the opening with a cap such as one or more plates
20
in order to seal the opening. U.S. Pat. No. 4,073,599 issued on Feb. 14, 1978, to Allen et al. describes such a blade tip closure design, and it is incorporated by reference herein. Plates
20
are mechanically restrained by the structure of the blade tip
16
and are held in position and sealed by one or more brazed joints
21
. It may be appreciated that the assembly and brazing of plates
20
can be a difficult and expensive process. Furthermore, in spite of efforts to maintain the core in its proper position during the casting process, many cast blades are rejected due to a minimum wall violation caused by unintended movement of the core resulting in an end of a cooling passage being located proximate a surface of a tip end of the airfoil of the blade.
Turbine blade
10
is designed to rotate within a casing (not shown). It is important for the blade tip
16
to fit precisely within the casing in order to minimize the passage of combustion gases around the blade tip
16
, since such bypass gases impart no energy to the airfoil section
14
. The blade tip
16
is provided with a squealer
22
which is a raised lip extending around the periphery of the blade tip
16
. Squealer
22
gets its name from the sound that is produced in the event of a mechanical interference between the blade tip
16
and the casing. Ideally the squealer
22
is sized to fit within the casing without rubbing but with a minimum of space there between.
It is known that turbine blades
10
may develop one or more cracks
24
near the tip
16
of the blade
10
due to low cycle fatigue stresses imparted on the blade tip
16
during the operation of the turbine. If a crack
24
extends beyond a critical dimension, the turbine blade
10
must be removed from service and/or repaired in order to prevent catastrophic failure of the blade and turbine. It can be appreciated that a crack
24
may be repaired by removing the material adjacent to the crack
24
to form a crack repair volume, and then filling the crack repair volume with weld metal. However, the presence of braze joint
21
utilized to secure plates
20
in position can complicate the repair process, since weld integrity is adversely affected when applied over a braze material.
In light of the limitations of the prior art designs, it is desirable to provide a method for repairing a cracked hollow turbine blade which overcomes the problems associated with the presence of braze material in the proximity of the cracked area. It is also desired to provide a method of manufacturing a hollow turbine blade that precludes the possibility for a repair in the area of a braze joint. Furthermore, it is desired to provide a turbine blade having improved level of performance to prevent the occurrence of cracks near the blade tip.
SUMMARY OF THE INVENTION
These and other objects of the invention are met in a method of repairing a turbine blade, the blade having a plurality of cooling passages formed therein extending to a tip of the blade, the blade further having a cap brazed over an end of a cooling passage at the tip and a squealer portion extending beyond the cap, the method including the steps of: removing the squealer portion, cap, and all braze material from the blade to form a repair surface on the tip; forming a replacement cap sized to span the plurality of ends of the cooling passages; attaching the replacement cap to the repair surface by welding to seal the cooling passage ends; forming a replacement squealer portion by welding
REFERENCES:
patent: 4073599 (1978-02-01), Allen et al.
patent: 4487550 (1984-12-01), Horvath
patent: 5106010 (1992-04-01), Stueber et al.
patent: 5374319 (1994-12-01), Stueber et al.
patent: 5822852 (1998-10-01), Bewlay et al.
patent: 6049978 (2000-04-01), Arnold
Foster Michael F.
Junkin John E.
Lovelace Daniel
Sinnott Zachary
Smith Kevin D.
Cuda Rosenbaum I
Siemens Westinghouse Power Corporation
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