Note: Descriptions are shown in the official language in which they were submitted.
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CREVICE CORROSION-RESISTANT LIQUID-COOLED ARMATURE BAR
CLIP-TO-STRAND CONNECTION AND RELATED METHOD
BACKGROUND OF THE INVENTION
This invention relates generally to the manufacture of generators and,
specifically, to
improving the joint between a hydraulic header clip or fitting and a liquid
cooled
armature bar.
The armature windings of large steam-turbine generators are generally water-
cooled.
The armature windings are composed of half coils or armature bars connected at
each
end through copper or stainless steel fittings and water-cooled connection
rings to
form continuous hydraulic circuits. The hydraulic winding circuits are
typically
connected to inlet and outlet water manifolds with plastic hoses that provide
electrical
isolation. The manifolds are connected to the stator water cooling system
which
cools, filters and deionizes the water and pumps the water back to the
armature
winding. The armature bars are composed of rectangular copper strands arranged
in
rectangular bundles. All of the strands may be hollow, or the bundle may
include a
mixture of solid and hollow strands. The hollow stands thus have a duct for
conducting cooling water. A braze alloy joins the strands to each other and to
a
waterbox header clip at each end of one of the armature bars. The clip
functions to
deliver water to and collect water from the hollow strands. The clip is
connected
through copper or stainless steel fittings to a second armature bar to form a
complete
armature coil element of the winding.
The braze joints between the strands and between the strands and the clip must
retain
hydraulic and electrical integrity for the expected lifetime of the winding.
The braze
alloy or filler metal that joins the strands and the strands to the clip is
currently a self-
fluxing copper-phosphorous alloy, also referred to as a BCuP alloy. This
family of
alloys typically contains 31/2 - 7 weight percent phosphorous. The surface of
the braze
joint is constantly exposed to a deionized, oxygenated water environment.
It has become evident that the factors required to support a crevice corrosion
mechanism in a clip-to-strand braze joint are phosphorous, copper, suitable
corrosion
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initiation sites and water. If any one of these factors are eliminated,
crevice corrosion
should not occur. Corrosion of the phosphorous-containing braze alloy and
adjoining
copper strand surfaces can occur under certain conditions, specifically if
critical
crevice geometry and crevice water chemistry requirements are met. The
corrosion
process can initiate if the joint surface contains any surface crevices,
pinholes, or
porosity at or near the surface of the joint, and if the critical water
chemistry
conditions that can support corrosion develops within these sites. The
corrosion
process can progress through the braze joint as long as critical crevice
geometry and
water chemistry conditions exist. In addition, porosity within the braze joint
can
accelerate the total apparent corrosion rate. Eventually, the path of
corrosion can
result in a water leak through the entire effective braze joint length,
compromising the
hydraulic integrity of the clip-to-strand joint.
U.S. Patent 5,796,189 discloses an arrangement where all of the strands are
cut to the
same length and the copper-phosphorous (BCuP) braze alloy is pre-placed flush
to the
ends of the strands. A braze alloy anti-wetting agent is used on the ends of
the hollow
strands to prevent plugging of the hollow strands and an inert purge gas is
used during
the brazing cycle. Use of the anti-wetting agent, although effective for
preventing
hollow strand plugging, can result in strand faying surface contamination and
a poor
effective braze joint length.
A recent pending and commonly owned US Patent 6,784,573 issued August 31,
2004,
teaches the use of extended hollow strands in combination with the use of a de-
oxidizing gas as the purge gas. The extended hollow strands eliminate the need
to use
an anti-wetting agent on the ends of the hollow strands and the de-oxidizing
purge gas
limits oxidation during the brazing cycle, de-oxidizes strand surfaces and the
braze
alloy prior to alloy melting, and improves braze alloy wetting and flow.
However,
this practice continues the use of a phosphorous-containing braze alloy and,
therefore,
the potential for crevice corrosion still exists, although it is greatly
minimized due to
the significant reduction, or the elimination in most cases, of surface
initiation sites.
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BRIEF DESCRIPTION OF THE INVENTION
This invention relates to a brazed connection of a liquid-cooled armature bar
strand
package to a hydraulic header clip that is not susceptible to crevice
corrosion
initiation, and a related method for manufacturing the connection.
Generally, the invention provides a non-crevice-corroding clip-to-strand braze
joint
using a braze alloy that is essentially phosphorous-free, one of the key
factors of the
known crevice corrosion mechanism. A braze alloy that is essentially
phosphorous-
free has a phosphorous content that is sufficiently low enough such that
phosphorous-
containing metallurgical phases that are susceptible to crevice corrosion
cannot form.
Generally, and for purposes of this discussion, alloys with less than 500 ppm
(or 0.05
weight percent) phosphorous are considered essentially phosphorous-free. The
benefits are expected to be improved generator availability and reliability.
Specifically, the invention employs extended hollow strands (relative to the
solid
strands) to avoid the need for a braze alloy anti-wetting agent on the ends of
the
hollow strands. Strips of a rolled, essentially phosphorous-free, copper-
silver braze
alloy are placed between the tiers of strands and between the strands and the
internal
surfaces of the header clip. Additional metal shims composed of copper or
other
suitable metal may be placed between the tiers to aid braze alloy retention.
To further
aid braze alloy retention, a temporary refractory sleeve may be placed around
the
strand package at the strand-to-clip interface and a spring-loaded chill clamp
is used.
The joint is brazed in a chamber under a controlled gas atmosphere to avoid
the need
for brazing fluxes. Optionally, a stick-form of the copper-silver alloy can be
used to
add additional filler metal to the braze joint during the brazing process.
The braze joint can be made with the axis of the armature bar in either a
horizontal or
a vertical orientation. The vertical orientation is preferred, however,
because it aids
alloy retention in the joint and permits pieces of the alloy to be more easily
pre-placed
on the surface of the assembly inside the hydraulic header clip, thereby
providing a
source of additional filler metal that will melt and flow over the surface to
create a
thicker layer of filler metal than would be possible with the armature bar in
a
horizontal orientation.
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The copper-silver alloy can contain one or more other elements, such as tin,
zinc or
nickel, that can result in solidus and liquidus modifications to suit specific
applications. In a less preferred embodiment of the manufacturing method, the
brazement may be made in an atmosphere of air or an inert gas such as nitrogen
or
argon, along with the use of brazing fluxes when it is desirable to avoid
brazing in a
chamber with a controlled atmosphere.
Accordingly, in one aspect, the invention relates to a brazed joint between an
armature bar strand package and an end fitting comprising a plurality of solid
strands
and a plurality of hollow strands arranged in a tiered array and forming the
strand
package, the plurality of hollow strands having free ends that extend axially
beyond
corresponding free ends of the solid strands; a cavity in the end fitting, the
free ends
of the plurality of hollow strands and the corresponding free ends of the
solid strands
extending through the opening and received in the cavity; and an essentially
phosphorous-free copper-silver braze alloy joining the free ends of the
plurality of
hollow strands and the corresponding free ends of the plurality of solid
strands to each
other and to interior surfaces of the end fitting.
In another aspect, the invention relates to a brazed joint between an armature
bar and
an end fitting comprising a cavity in the end fitting, accessed by an opening;
an array
of solid and hollow strands received in the opening and arranged in a tiered
array; and
an essentially phosphorous-free, copper-silver braze alloy joining the solid
and hollow
strands to each other and to internal surfaces of the end fitting, the braze
alloy
covering free ends of the solid strands but leaving free ends of the hollow
strands
open and unobstructed.
In still another aspect, the invention relates to a method of forming a brazed
joint
between an armature bar and an end fitting comprising: a) locating ends of a
plurality
of hollow strands and a plurality of solid strands within a cavity in an end
fitting such
that free ends of the hollow strands extend axially beyond free ends of the
solid
strands; and b) pre-placing an essentially phosphorous-free copper-silver
braze alloy
around and between the ends of the hollow strands and the solid strands such
that the
braze alloy does not extend axially beyond the free ends of the hollow
strands.
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In still another aspect, the invention relates to a brazed joint between an
armature bar
strand package and an end fitting comprising a plurality of strands arranged
in a tiered
array and forming the strand package; a cavity in the end fitting, the free
ends of the
plurality of strands extending through the opening and received in the cavity;
and an
essentially phosphorous-free copper-silver braze alloy joining the free ends
of the
plurality of strands to each other and to interior surfaces of the end
fitting.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a side elevation of an armature bar and header clip assembly;
FIGURE 2 is a side section, taken along the line 2-2 in Figure 3, of a header
clip-to-
strand connection in accordance with an exemplary embodiment of the invention;
FIGURE 3 is an end elevation of the connection shown in Figure 2; and
FIGURE 4 is a partial end elevation of a clip-to strand connection with metal
shims
between the tiers.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to Figure 1, a liquid-cooled stator winding used in a typical
liquid-
cooled generator includes a plurality of armature bars 10 (one shown), the
center
portion 12 of which is adapted to pass through radially extending slots in a
stator core
(not shown), terminating at opposite ends in hydraulic end fittings or header
clips 14
and 16, respectively, typically formed of an electrically conductive material
such as
copper. Inlet hoses (not shown) connect the header clips 14, 16 to an inlet or
outlet
coolant header (also not shown).
With reference also to Figure 2, the armature bar 10 is composed of many small
rectangular solid and hollow copper strands 18, 20, respectively (Figure 2),
that are
brazed to the interior of the header clips 14, 16 as further described below.
It will be
appreciated that the strands 18, 20 may also be constructed of metals other
than
copper, such as copper-nickel alloys or stainless steel. Since the clips 16,
20 are
identical, only clip 16 will be described in detail.
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As best seen in Figures 2 and 3, the solid and hollow copper strands 18, 20
are
disposed in side-by-side and superposed relation one to the other, in a
generally
rectangular, multi-tier array. The array may be compressed within the
hydraulic end
fitting or header clip 16 by means of a side plug 22 (Figure 3) fitted within
a similarly
shaped "window" cut-out of the header clip. The rows or tiers of strands 18,
20
within the stator bar are brazed to one another as well as to the interior
surfaces 24 of
the end fitting using a filler metal or braze alloy barrier coat 26. The braze
alloy 26
preferably comprises an essentially phosphorous-free copper-silver alloy in
rolled
strip form. The latter facilitates placement of the alloy between the tiers of
strands
and between the strands and the internal surfaces of the end fitting or header
clip.
The particular configuration of solid strands 18 and hollow strands 20 within
the
package may vary. For example, there may be a 1 to 1 ratio of solid strands to
hollow
strands, up to a ratio of, for example, 6 to 1 or more, depending on the
capability of
the bar design to remove heat during generator operation. Conversely, the
package
may contain all hollow strands. The arrangement of solid and hollow strands
within
the array may vary as well. Thus, while a two-tier array is shown in Figures 2
and 3,
it will be appreciated that four and six or more tiers are possible.
The free ends of the hollow strands 20 extend axially beyond the corresponding
free
ends of the solid strands 18, projecting into an open cavity or manifold 28.
The
differential lengths of the solid and hollow strands may be achieved by any
suitable
means including the use of a specialized tool to shorten the solid strands.
The filler
metal or braze alloy 26 is pre-placed within the header clip 16 so as to
surround the
enclosed ends of the hollow and solid strands, but not to extend axially
beyond the
hollow strands 20. As best seen in Figure 2, the braze alloy barrier coat 26
extends
axially along and between all sides of each of the strands 18, 20 in the
array, and also
covers the ends (or faying surfaces) of the solid strands 18 while leaving the
ends of
the hollow strands 20 open and unobstructed for free flow of coolant through
the
hollow strands.
With reference to Figure 4, a multi-tier arrangement is illustrated showing
how metal
shims 30 composed of copper or other suitable metal may be placed between the
tiers
to aid braze alloy retention. These shims 30 extend axially to a location
substantially
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flush with the back wall of the clip. A temporary refractory sleeve (not
shown) may
be placed around the strand package at the strand-to-clip interface. In
addition, a
spring-loaded chill clamp (not shown) may be used to further aid braze alloy
retention. Specifically, the chill clamp would be applied about the strands
just behind
the clip 16 to establish a temperature differential between the strands inside
the clip
and the strands outside the clip. The temperature differential aids in
containing the
flow of braze alloy to areas inside the clip, i.e., it substantially prevents
the escape of
braze alloy along the strands beyond the back wall 32 of the clip. The joint
assembly
may be brazed in a controlled gas atmosphere (for example, a gas containing
nitrogen,
hydrogen and trace amounts of oxygen; or 100% hydrogen), in a chamber to avoid
the
need for a flux. If desired, a stick-form of the copper-silver alloy may be
used to add
additional filler material to the braze joint during the brazing process.
When heated to its melting temperature, the braze alloy 26 flows and fills in
the
spaces between the solid and hollow strands 18, 20 and between the strands and
the
interior surfaces 24 of the header clip, including at the opening of the
header clip into
which the strands are inserted. At its melting temperature, the alloy 26
remains
sufficiently viscous that it does not flow substantially to the free ends of
the hollow
strands 20. In other words, the extended length of the hollow strands 20
provides a
safety margin in that the excess alloy material will not flow out as far as
the ends of
the hollow strands, precluding the possibility of plugging the cooling
passages in the
hollow strands.
The braze joint can be made with the axis of the armature bar in a horizontal
or a
vertical orientation. The vertical orientation is preferred because it aids
alloy
retention in the joint and permits pieces of the alloy to be pre-placed on the
surface of
the assembly inside the hydraulic header clip, thereby providing a source of
additional
filler metal that will melt and flow over the surface resulting in a thicker
layer of filler
metal than would be possible with the armature bar in a horizontal
orientation.
The essentially phosphorous-free copper-silver alloy can contain one or more
other
elements, such as tin, zinc or nickel, that can result in solidus and liquid
modifications
to suit the application. In a less preferred embodiment of the manufacturing
method,
the brazement may be made in an atmosphere of air or an inert gas, such as
nitrogen
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or argon, with the use of brazing fluxes to avoid the need to braze in a
chamber with a
controlled atmosphere.
While there have been described herein what are considered to be preferred and
exemplary embodiments of the present invention, other modifications of these
embodiments falling within the scope of the invention described herein shall
be
apparent to those skilled in the art.
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