Note: Descriptions are shown in the official language in which they were submitted.
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NUCLEAR STEAM GENER~ R TUBESHEET SHIELD
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- BACKGROUND OF THE INVEMTION
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This invention relates to U-tube type heat exchangers,
and more particularly, to nuclear steam generators. In U-tube
type heat exchanges a first heat exchange fluid is passed into a
shell within which a bundle of upstanding U-shaped tubes is
disposed, with the ends of the tubes being secured in a
tubesheet adjacent the bottom of the heat exchanyer.
Simultaneously a second heat exchange fluid is passed through
the tubes. As the first fluid passes over the outer surfaces of
`- the tubes it comes in an indirect heat exchange relation with
the second fluid that is passing through the tubes. When the
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second fluid is at a higher temperature than the first fluid,
the first fluid is thereby heated, ana a portion of the first
fluid is converted to vapor. Heated first fluid rises within
the heat exchanger, and the vapor is thereafter separated from
the li~uid phase of the heated first fluid. The liquid phase of
the heated first fluid is recirculated within the shell, being
returned to the lower portion of the steam generator through a
downcomer space defined between the inner wall of the heat
exchanger shell and the outer wall of a shroud disposed around
the tube bundle.
The thermal effectiveness of a U-tube heat exchanger,
such as a nuclear steam generator, can sometimes be improved by
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incorporating an integral preheater into -the unit. When an
in-tegral preheater is employed, relati~ely cold feedwater is
introduced in-to the heat exchanger just above -the tubeshee-t.
Feedwater temperature normally is within a range of from 115C.
-to approximately 190C., whereas the tubeshee-t temperature may
be approximately 260C. When the relatively cold feedwater
impinges upon the tubesheet, i-t can shock the tubesheet and
result in fatigue cracking of the tubesheet.
It has previously been suggested that a plate be
disposed above the tubesheet in the preheater zone in order to
shield the tubesheet from the incoming feed water. However,
when plate-type shields are used, cold feed water can leak from
the preheater zone to the boiling zone through clearances
existing between the tubes and tube holes formed through the
shield. If the tubes are contact rolled into the plate-type
shield to preclude such leakage, residual tube stresses can be
imposed on the tubing. Furthermore, additional stresses, due to
misalignment of tubes, radial differential thermal expansion
between the tubesheet and the plate~type shield, and/or out-of-
plane bending of the tubesheet because of differential pressureacross the tubesheet, can also be introduced when tubes are
rolled into a plate-type shield. Another disadvantage of a
plate-type shield is that uncertain ther~al/hydraulic conditions
- can exist under the plate-type shield.
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The presen-t invention provides a shield for -the upper
I face of -the tubesheet which precludes thermal shock of the
tubesheet and also avoids leakage and stress problems attendant
to plate-type shields.
UMM RY OF THE INVENTION
~ In accordance wi-th an illustrative embodiment
`: demonstrating features and advantages of the present invention,
there is provided a nuclear steam generator of the type in which
a plurality of U-shaped tubes are connected at opposite ends to
a tubesheet and extend hetween inlet and outlet chambers, and
the steam generator includes an integral preheater zone adjacent
;~ downflow legs of the U-shaped tubes. The improvement comprises
a thermal shield disposed adjacent an upper face of the
tubesheet within the preheater zone, the shield including
ductile cladding material applied directly to the upper face of
the tubesheet with the downflow legs of the U-shaped tubes
extending through the cladding into the tubesheet. The ductile
cladding acts as a thermal shield to prevent incoming feed fluid
from thermally shocking the tubesheet, steady-state and
transient thermal stress in ligaments of the tubesheet being ~!
primarily confined to the cladding material. Because of its
ductility and fatigue properties, the cladding absorbs such
stresses.
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D SCRIPTION OF THE DR~WINC,S
FIG. ] is an elevational view of a nucJear steam gen
erator incorporating the -thermal shield of the present invention;
and
FI~. 2 is a sectional view of a portion of the steam
generator of FIG. l.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1 there is shown a nuclear steam
generator 10 which includes a cylindrical shell 12 within which
a bundle of U-shaped tubes 14 is disposed. Tubes 14 include
upflow legs 16 communicatiny with tubesheet 18 at one end
and downflow legs 20 communicating with tubesheet 18 at another
end thereof. A header 22 is attached to the underside of tube-
sheet 18, and together with the underside of tubesheet 18 and
a partition plate 24 define an inlet chamber 26 and an outlet
chamber 28. An inlet 30 is provided for introducing a heat
exchange fluid, such as heavy water, into inlet chamber 26;
this fluid is referred to as tubeside fluia. Similarly, an
outlet 32 is provided for removal of tubeside heat exchange
fluid from outlet chamher 28. An inlet 34 is provided for
introducing another heat exchange fluid, referred to as
shell-side fluid, into shell 12. The shellside heat exchange
fluid, which could be water, for example, passes over the outer
surfaces of tubes 14, and a por-tion of this fluid is vapor-
ized. The heated shellside fluid thereafter rises into the
upper section 36 above section 12 within which separators (not
shown) are disposed for separating the vapor from the heated
-- fluid. The vapor is ultimately removed from the steam gener-
ator 10 through outlet 3~, while the liquid phase is recircu-
lated through downcomer space 37 to boiling zone 39.
Turniny to FIG. 2, a more detailed sectional view
of the cylindrical shell 12 of steam generator 10 is shown.
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Adjacen-t inle~ 3~ is a prehea-ter or econornizer zone ~0. The
preheater zone ~0 is defined by -the inner wall of a generally
semi-cylindrical sec-tion of shroud ~2, a section ~3 of the upper
face of tubesheet 18, and a first plate 4~. Plate 4~ extends
across a diametrial tube-free zone ~6 formed between upflow legs
16 and downflow legs 20 of U-tubes 14, and is welded to shroud
42 along its side edges. A second plate 9~ is also disposed
within zone 46, being closer to upflow legs 16 than is first
plate 44. Plate 48 is also welded to shroud 42, but extends
beyond shroud 42 to shell 12.
E'eed water at a -temæerature within a range of from
115C. to approximately 190C is introduced through inlet 34
into preheater zone ~0. The tubesheet 18 and shell 12 are at a
temperature of approximately 260~C. during normal full load op-
eration of the steam generator 10. However, it should be- under-
stood that the temperatures of the tubesheet and the feedwater
can vary during transient conditions, and these "normal" temp-
eratures are used herein only to illustrate possible temperature
differences between the feedwater and the tubeshee-t. A thermal
; 20 shield 50 is disposed above section ~3 of the upper face of
tubesheet 18 in the preheater zone 40. Thermal shield 50 comprises
a weld deposit of ductile Ni-Cr-Fe alloy having a finished thick-
ness of approximately 6.35mm. The weld deposit, or cladding,
is applied to the tubesheet before holes are formed in the tube-
sheet for receiving the ends of U-tubes 1~. Therefore a tight
fit is assured between the thermal shield 50 and the tubes 1~.
Since the cladding is applied directly to the face of the tubesheet,
thereby eliminating any space between the cladding and the tubeshee~
face, lea]cage of feed water from the preheater zone ~0 to the boil-
ing zone 39 is not possible. Although in the preferred embodimenta Ni-Cr-Fe alloy ma-terial is used as cladding, other weld material
haviny similar ductility and fatigue properties can be employed.
In operation relatlvely cold feedw~ter is introduced
through inlet 34 into the preheater zone 40 of steam generator
10. Relatively hot fluid, such as heavy water, is introduced
thro~lgh inlet 30 into chamber 26. The hot fluid rises through
upflow l.egs 16 of U-tubes 14, coming .in contact with the colder
shellside fluid and giving of a portion of its hea-t there-to.
The tubeside fluid is then returned through downflow legs 20
and empties into chamber 28 from which it is removed through
outlet 32. The incoming feed water falls within shell.12 on
to ~.hermal shi.eld 50 disposed above section 43 o:E the upper
face of tubeshee-t 18 in preheater zone 40. Shield 50 acts to
prevent the relatively cold shellside fluid from thermally
shocking tubesheet 18. After the colder fluid is preheated
:~ within zone 40, it rises and continues to absorb addltional
heat as it contacts the outer surfaces of U-tubes 14. Upon ris-
. ing into upper section 36, vapor is separated from the heated
.~ shellside fluid, and is later removed through outlet 38. Sep-
arated shellside liquid is recirculated through downcomer space
37 to boiling zone 39.
A latitude of modification, change and substitution
is intended in the foregoing disclosure and in some instances
some features of the invention will be employed without a
corresponding use of other features. Accordingly, it is
appropriate that the appended claims be construed broadly and
in a manner consisten-t with the spirit and scope of the inven-
tion herein.
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