Metal working – Method of mechanical manufacture – Heat exchanger or boiler making
Reexamination Certificate
1997-08-27
2001-06-19
Flanigan, Allen (Department: 3743)
Metal working
Method of mechanical manufacture
Heat exchanger or boiler making
C165S076000, C029S402030
Reexamination Certificate
active
06247231
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
This invention relates generally to the repair of heat exchanger tubing, such as in a steam generator. More particularly, this invention relates to a method of repairing such tubing in difficult to access geometries using laser welding techniques to achieve a partial replacement of the tubing.
BACKGROUND OF THE INVENTION
FIG. 1
illustrates a prior art steam generator
10
. The steam generator
10
includes a shell
12
, a tube bundle
14
positioned within the shell
12
, a tube sheet
16
supporting the tube bundle
14
, and a water box
18
positioned beneath the tube sheet
16
. The tube bundle
14
includes a set of “U” shaped tubes
20
. For clarity, only one “U” shaped tube
20
is shown in the figure, but it should be understood that the tube bundle
14
can be made up of thousands of individual tubes
20
. A division plate
22
divides the water box
18
into an inlet section
24
and an outlet section
26
.
The steam generator
10
receives hot reactor coolant in the water box
18
through nozzle
28
. From the inlet section
24
of the water box
18
, the coolant flows through tubes
20
to the outlet section
26
of the water box
18
and back to the reactor (not shown) through nozzle
30
. Secondary water enters the shell
12
through nozzle
40
and is heated by contact with the tubes
20
. As the secondary water is heated, it boils to generate steam, which exits the shell
12
at the top of the steam generator
10
through nozzles
42
and
44
. The steam thus generated is routed to a steam turbine (not shown) where it is expanded to drive an electrical generator (not shown).
The tubes
20
are connected to a tube sheet
16
by seal welding or by expanding the tubes within the tube apertures in the tube sheet
16
. Located at various heights in the shell
12
are tube support plates
50
containing apertures through which the tubes
20
pass. The apertures in the tube support plates
50
are slightly larger in diameter than the outside diameter of the tubes
20
so that the tubes can slide vertically within the support plates. This relative sliding capability is necessary to accommodate differential thermal expansion which occurs when the steam generator
10
is brought on-line and is slowly heated to operating temperature.
The steam generator tubes
20
are susceptible to several types of corrosion mechanisms that can ultimately lead to leakage or significant wall thinning. These corrosion mechanisms include primary water stress corrosion cracking, secondary side intergranular attack, secondary side intergranular stress corrosion cracking, and secondary side wastage. Primary side degradation typically occurs at locations of high tensile residual stress, such as expansion transition areas, inner row U-bends, and dented tube support locations. Secondary side degradation occurs at locations where impurities can concentrate, providing corrosion sites, such as tube-to-tubesheet crevices, tube support plate-to-tube interfaces, anti-vibration bars interfaces, and sludge pile regions (located at the tubesheet).
Current techniques to mitigate corrosion-induced problems include: plugging the degraded tubes, sleeving the degraded tubes, and replacing the steam generator. Plugging the degraded tubes takes the tube out of service, reducing the steam generator efficiency. The ability to plug tubes is based on the “plugging margin” that is calculated based on operating experience for each steam generator.
Sleeving is a more expensive mitigation technique; it involves welding a tube section or “sleeve” to the interior surface of a degraded region of the existing tube. Sleeving allows the tube to remain in service, with some reduction in flow rate. Sleeving is generally performed when the steam generator “plugging margin” is approached. The final option is to replace the steam generator. Replacement of the steam generator addresses the problem, but at a prohibitive cost.
The Electric Power Research Institute, the assignee of the present invention, has developed improved corrosion mitigation techniques that rely on cladding the inside surface of steam generator tubes with corrosion resistant weld metal using a nd:YAG laser to provide a permanent repair for steam generator tubing. This technology is described in U.S. Pat. Nos. 5,430,270 and 5,514,849, both of which are expressly incorporated by reference herein.
A majority of steam generator tube failures occur at the tubesheet region. During fabrication of the steam generator, tubes are mechanically or hydraulically expanded along the length of the tubesheet apertures to seal the area between the tube and the tubesheet. The intent is to prevent water and corrodants from creating corrosion products between the tube and the tubesheet. However, at the top surface of the tubesheet, a small change in tube diameter occurs (referred to as a “roll transition”) where the rolling process is completed. This area is highly stressed and subjected to secondary water environment of the steam generator, which contains a variety of contaminants. Sludge from the accumulation of secondary water impurities further compounds the problem by providing additional concentration of corrosion products. As a result, this region is very susceptible to stress-corrosion cracking.
Currently, sleeving is the principal means of repair for tube-to-tubesheet location failures. However, recent plant experience has shown that the sleeves may fail due to improper installation, insufficient heat treatment, cold work stresses, and other factors. Consequently, techniques have been developed to cut and remove sleeves and to install new sleeves.
In view of the foregoing, it would be highly desirable to provide a method for repairing heat exchanger tubes. More particularly, it would be highly desirable to provide a method for repairing heat exchanger tubes at the tube-to-tubesheet region of a steam generator, which is the most common location for steam generator tube failures. Ideally, such a method would rely upon existing repair equipment.
SUMMARY OF THE INVENTION
A method of repairing a heat exchanger tube in a steam generator includes the step of removing a degraded tube segment from an operative heat exchanger tube in a steam generator. A weld metal ring is then positioned between the operative heat exchanger tube and a new tube segment. The weld metal ring is then welded to the operative heat exchanger tube and the new tube segment to form a linking tube section. The linking tube section is then heat treated. The new tube segment is then expanded toward a steam generator tubesheet. Finally, the new tube segment is welded to the tube sheet.
The invention provides a method for repairing heat exchanger tubes at the tube-to-tubesheet region of a steam generator, which is the most common location for steam generator tube failures. The method relies upon existing laser welding repair equipment and can therefore be readily implemented. The steps of the method are sequenced so that residual stresses along the operative tube and at the linking tube section are minimized.
REFERENCES:
patent: 3579805 (1971-05-01), Kast
patent: 3806693 (1974-04-01), Miller
patent: 3962767 (1976-06-01), Byerley et al.
patent: 4573248 (1986-03-01), Hackett
patent: 4694549 (1987-09-01), Rabe
patent: 4723578 (1988-02-01), Mordarski et al.
patent: 5066846 (1991-11-01), Pirl
patent: 5479699 (1996-01-01), Snyder
patent: 5514849 (1996-05-01), Findlan et al.
Findlan Shane Joseph
Spanner Jack C.
Electric Power Research Institute
Flanigan Allen
Pennie & Edmonds LLP
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