Electric heating – Metal heating – For deposition welding
Reexamination Certificate
1999-08-18
2001-01-16
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For deposition welding
C219S125100, C219S13700R
Reexamination Certificate
active
06175093
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for applying a weld overlay to a wastage susceptible structure.
According to U.S. Pat. No. 5,356,248 to Hillestad, steam generating boilers, such as those used at electrical power generating plants, are generally large steel or masonry structures containing hundreds of vertically arranged steel tubes which are in thermal contact with a burning fuel, such as coal. The burning fuel heats water circulated through the tubes. The heated water or steam is used to drive turbines which generate electricity.
Additionally, according to the '248 patent, the tubes are generally organized in large panels or banks of parallel tubes which are connected together with a metal membrane or web continuously interposed between each pair of adjacent tubes in the bank to form a tube wall. In a steam generator, the tubes are generally oriented in a vertical direction and are communicated with headers at their ends. Due to their exposure to relatively high temperature water and steam, these tubes are susceptible to corrosion mechanisms that can ultimately lead to significant tube wall thinning or even leakage.
One approach to repairing or preventatively strengthening the tubes of a steam generator involves the deposition of weld metal at locations of the tubes observed or otherwise judged to have a reduced thickness less than a desired tube thickness. However, the build up of weld metal to thereby form a corrosion resistant weld overlay presents installation and structural challenges. For example, since a field applied weld overlay is typically applied during a shutdown of the steam generator, it is desirable to accomplish the set up and operation of the weld equipment as rapidly as possible so as to limit the revenue loss downtime of the steam generator. The speed of the weld deposition, however, must take into account the tube structure impact which the weld operation may have. For example, tube panels may tend to deform due to the difference in temperature between the furnace side of the tube being welded and the relatively cooler, insulated exterior side of the tube. Moreover, shrinkage of the weld overlay can sometimes engender a deformation of the tube panel, thereby causing increases in the stresses exerted on the structural framework supporting the tubes. Furthermore, residual stress from the welding operation can sometimes result in thermal fatigue cracking of the weld overlay and the tube portions underlying and adjacent the weld overlay region.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide a method for applying a weld overlay to a surface of a wastage susceptible chamber in a manner which reduces the deformation and stresses imposed on the surface in contrast to conventional weld overlay applications.
It is another object of the present invention to provide a method for repairing a surface of a wastage susceptible chamber such as a steam generator without significant disruption of the normal operation of the chamber.
It is an object of the present invention to provide a method for prolonging the useful life of certain components of a wastage susceptible chamber, such as the tubes of a steam generator.
According to one aspect of the present invention, there is provided a method of applying a weld overlay to a surface of a wastage susceptible chamber. The basic steps of the method include evaluating the post weld orientation of the surface of the wastage susceptible chamber resulting from the movement of the surface in response to the application of a weld overlay thereon and selecting characteristics of a weld overlay to be applied to the surface of the wastage susceptible chamber based upon the step of evaluating the post weld orientation of the surface of the wastage susceptible chamber. The basic steps of the method also include determining a series of weld torch engagement locations relative to the surface of the wastage susceptible chamber at which a weld torch is to be located for performing thereat welding operations which result in a weld overlay on the surface having characteristics corresponding to the predetermined weld overlay characteristics. A further basic step of the method includes positioning a weld torch at the weld torch engagement locations and controlling the weld torch to produce welds on the surface of the wastage susceptible chamber resulting in the weld overlay having characteristics corresponding to the predetermined weld overlay characteristics.
According to one refinement of the method for implementation of the method on a wastage susceptible chamber wherein the surface of the wastage susceptible chamber is supported at a plurality of support locations by a support frame each of which restrain the movement of the surface in response to the application of a weld overlay thereon, the step of evaluating the post weld orientation of the surface of the wastage susceptible chamber includes evaluating the post weld orientation of other locations of the surface which are spaced from the support locations.
In the event that the wastage susceptible chamber includes a plurality of tubes each operable to conduct fluid therethrough during thermally varying operation of the chamber and the surface is comprised of outer surface portions of adjacent tubes, the step of evaluating the post weld orientation of the surface of the wastage susceptible chamber may include evaluating the movement of the outer surface portions of the adjacent tubes in response to the application of a weld overlay thereon.
In another refinement of the method of the present inventions, the step of selecting characteristics of a weld overlay to be applied to the surface of the wastage susceptible chamber includes selecting the characteristic that the weld overlay is applied in at least two temporally spaced phases, the first phase of the application of the weld overlay including applying at least one first phase weld strip which induces predetermined stresses in the surface and applying at least one second weld strip which induces predetermined stresses in the surface which at least partially mitigate the predetermined stresses induces by the application of the at least one first phase weld strip.
REFERENCES:
patent: 3182179 (1965-05-01), Anderson, Sr.
patent: 3199174 (1965-08-01), Nilsson et al.
patent: 3535768 (1970-10-01), Pruitt et al.
patent: 5356248 (1994-10-01), Hillestad
patent: 5440099 (1995-08-01), Smith
patent: 5550347 (1996-08-01), Kasuya et al.
patent: 5798627 (1998-08-01), Gilliland et al.
Cauthon Randall T.
Dzedzy Donald
Fong Milton A.
Keegan William
ABB Alstpm Power Inc.
Shaw Clifford C.
Warnock Russell W.
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