Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TUBE SUPPORT SYSTEM FOR NUCLEAR STEAM GENERATORS
FIELD AND BACKGROUND OF INVENTION
[0001] The present invention relates generally to nuclear steam
generators, and
in particular to a new and useful tube support system and method for use in
nuclear
steam generators which employ tube support plates to retain the tube array
spacing
within the steam generator.
[0002] The pressurized steam generators, or heat exchangers,
associated with
nuclear power stations transfer the reactor-produced heat from the primary
coolant
to the secondary coolant, which in turn drives the plant turbines. These steam
generators may be as long as 75 feet and have an outside diameter of about 12
feet.
Within one of these steam generators, straight tubes, through which the
primary
coolant flows, may be 5/8 inch in outside diameter, but have an effective
length of as
long as 52 feet between the tube-end mountings and the opposing faces of the
tube
sheets. Typically, there may be a bundle of more than 15,000 tubes in one of
these
steam generators. It is clear that there is a need to provide structural
support for
these tubes, such as a tube support plate, in the span between the tube sheets
to
ensure tube separation, adequate rigidity, and the like.
[0003] U.S. patent 4,503,903 describes apparatus and a method for
providing
radial support of a tube support plate within a heat exchanger, such as a U-
tube
steam generator having an inner shell and an outer shell. The apparatus is
rigidly
attached to the inner shell, and is used to centrally locate the tube support
plate
within the inner shell.
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[0004] U.S. patent 5,497,827 describes apparatus and method for radially
holding
a tube support within a U-tube steam generator. Abutments radially separate an
inner bundle envelope, or inner shell, from an outer pressure envelope. Each
abutment is fixed to the inner bundle envelope by welding, and contacts the
inner
face of the pressure envelope. The abutments maintain the different coaxial
envelopes of the steam generator and the assembly of the bundle by spacer
plates
in the radial directions. This is done to avoid relative displacements and
shocks
between the envelopes and the bundle in the case of external stresses, such as
those accompanying an earthquake. In one variant, elastic pressure used to
make
contact with a spacer plate is obtained by a spiral spring. The spring is
located
internal to the pressure envelope.
[0005] U.S. Patent 4,204,305 describes a nuclear steam generator commonly
referred to as a Once Through Steam Generator (OTSG). An OTSG contains a tube
bundle consisting of straight tubes. The tubes are laterally supported at
several
points along their lengths by tube support plates. The tubes pass through tube
support plate holes having three bights or flow passages, and also having
three tube
contact surfaces for the purpose of laterally supporting the tubes. It is
generally
recognized that after a heat exchanger is assembled, the tubes will contact
one or
two of the inwardly protruding lands of the tube support plate holes. This
contact
provides lateral support to the tube bundle to sustain lateral forces such as
seismic
loads, as well as provides support to mitigate tube vibration during normal
operation.
[0006] U.S. patent 6,914,955 B2 describes a tube support plate suitable for
use
in the aforementioned OTSG.
[0007] For a general description of the characteristics of nuclear steam
generators, the reader is referred to Chapter 48 of Steam/Its Generation and
Use,
41st Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., 2005.
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SUMMARY OF INVENTION
[0008] The present invention is drawn to an improved method and apparatus
for
supporting tubes in a steam generator.
[0009] According to the invention, there is provided a tube bundle support
system
and method which advantageously permits tube support plates to be installed in
an
aligned configuration that is compatible with normal fabrication processes. A
controlled misalignment is then imposed on one or more tube support plates as
the
steam generator heats up, i.e. in the hot condition. The tube support plates
are
made from a material having a lower coefficient of thermal expansion than the
shroud that surrounds the tubes. As a result, radial clearances open adjacent
to the
tube support plate as the steam generator heats up. These radial clearances
provide space for lateral shifting or displacement of the individual tube
support plates
by an associated tube support plate displacement system.
[0010] Each tube support displacement system advantageously has only one
part
located inside the steam generator shell, thereby minimizing the potential of
loose
parts.
[0011] The method and apparatus can be readily retrofit to existing steam
generators, since few internal alterations are required. Conversely, the
invention
can be easily removed, restoring the steam generator to its original
condition.
[0012] The normal load paths used for the transmission of seismic loads
between
tubes, supports, shroud and shell are advantageously unaltered.
[0013] Accordingly, one aspect of the invention is drawn to a method of
assembling and operating a steam generator having a plurality of tubes in a
spaced
parallel relation for flow of a fluid there through and the tubes transfer
heat with a
fluid flowing over the tubes, and also having a plurality of tube support
plates
disposed transverse to the tubes. The method of assembling and operating the
steam generator includes the steps of 1) aligning the tube support plates; 2)
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inserting the tubes through the aligned tube support plates; and 3) while
heating up
the steam generator, displacing at least one support plate out of alignment in
a
lateral direction transverse to the tubes, thereby increasing tube support
effectiveness.
[0014] The method may include displacing adjacent support plates in the
same
lateral direction transverse to the tubes.
[0015] The method may include displacing only every other support plate in
the
same lateral direction transverse to the tubes.
[0016] The method may include displacing alternating support plates in a
first
lateral direction transverse to the tubes and displacing the remaining support
plates
in a lateral direction transverse to the tubes and opposite the first
direction.
[0017] The method may include displacing a first plurality of support
plates in a
first lateral direction transverse to the tubes and a remaining plurality of
support
plates in a lateral direction transverse to the tubes and opposite the first
direction.
[0018] The method may include displacing one or more tube support plates,
in
the same or varying amounts and directions, and providing one or more
displacements for any individual tube support plate.
[0019] Another aspect of the invention is drawn to a tube support system
for use
in a heat exchanger having a plurality of tubes in spaced parallel relation
for flow of
fluid there through in indirect heat transfer relation with a fluid flowing
there over, and
also having a cylindrical shroud that is disposed within a cylindrical
pressure shell
and surrounds the tubes. The tube support system includes a tube support plate
disposed transverse to the tubes that is made of a material having a lower
coefficient
of thermal expansion than the shroud. The tube support system also includes
means for displacing the tube support plate in a lateral direction transverse
to the
tubes. The means for displacing the tube support plate includes a restraining
mechanism connected to the shell at one end, and a linking bar connected to
the
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tube support plate at the other end. The linking bar may be installed slack or
with
some tension, and as the radial clearance between the tube support plate and
the
shroud that surrounds the tubes opens up during heating, the tension or
increasing
tension in the linking bar pulls on the tube support plate, thereby displacing
the tube
support plate.
[0020] Yet another aspect of the invention is drawn to a tube support
displacement system for use in a heat exchanger having a plurality of tubes in
spaced parallel relation for flow of fluid there through in indirect heat
transfer relation
with a fluid flowing there over, the heat exchanger further having tube
support plates
arranged transverse to the tubes and a cylindrical shroud, the shroud disposed
within a cylindrical pressure shell and surrounding the tubes. The tube
support
displacement system may include within the linkage system a threaded
engagement
or other adjustable mechanism to change the length or amount of tension in the
linkage during its installation, or afterwards.
[0021] The tube support plate displacement system can be used to provide
controlled misalignments on one or more tube support plates, in the same or
varying
amounts and directions, and with one or more apparatus being provided for any
individual tube support plate.
[0022] The various features of novelty which characterize the invention are
pointed out with particularity in the claims annexed to and forming part of
this
disclosure. For a better understanding of the present invention, and the
operating
advantages attained by its use, reference is made to the accompanying drawings
and descriptive matter, forming a part of this disclosure, in which a
preferred
embodiment of the invention is illustrated.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the accompanying drawings, forming a part of this specification,
and in
which like reference numbers are used to refer to the same or functionally
similar
elements:
[0024] FIG. 1 is a sectional side view of a once-through steam generator
whereon the principles of the invention may be practiced;
[0025] FIG. 2 is a side view of one embodiment of a linkage mechanism of
the
tube support displacement system according to the present invention;
[0026] FIG. 3 is a partial sectional plan view of the linkage mechanism of
the tube
support plate displacement system according to the present invention; and
[0027] FIG. 4 is a sectional side view of a tube support plate arrangement
incorporating a plurality of tube support plate displacement systems
according to the present invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] FIG. 1 depicts a prior art once-through steam generator 10
comprising a
vertically elongated, cylindrical pressure vessel or shell 11 closed at its
opposite
ends by an upper head 12 and a lower head 13.
[0029] The upper head includes an upper tube sheet 14, a primary coolant
inlet
15, a manway 16 and a hand hole 17. The manway 16 and the hand hole 17 are
used for inspection and repair during times when the steam generator 10 is not
in
operation. The lower head 13 includes drain 18, a coolant outlet 20, a hand
hole 21,
a manway 22 and a lower tube sheet 23.
[0030] The steam generator 10 is supported on a conical or cylindrical
skirt 24
which engages the outer surface of the lower head 13 in order to support the
steam
generator 10 above structural flooring 25.
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[0031] The overall length of a typical steam generator of the sort under
consideration is about 75 feet between the flooring 25 and the upper extreme
end of
the primary coolant inlet 15. The overall diameter of the unit 10 moreover, is
in
excess of 12 feet.
[0032] Within the shell 11, a lower cylindrical tube shroud, wrapper or
baffle 26
encloses a bundle of heat exchanger tubes 27, a portion of which is
illustrated in
FIG. 1. In a steam generator of the type under consideration moreover, the
number
of tubes enclosed within the shroud 26 is in excess of 15,000, each of the
tubes
having an outside diameter of 5/8 inch. It has been found that Alloy 690 is a
preferred tube material for use in steam generators of the type described. The
individual tubes 27 in the tube bundle each are anchored in respective holes
formed
in the upper and lower tube sheets 14 and 23 through belling, expanding or
seal
welding the tube ends within the tube sheets.
[0033] The lower shroud 26 is aligned within the shell 11 by means of
shroud
alignment pins. The lower shroud 26 is secured by bolts to the lower tubesheet
23
or by welding to lugs projecting from the lower end of the shell 11. The lower
edge
of the shroud 26 has a group of rectangular water ports 30 or, alternatively,
a single
full circumferential opening (not shown) to accommodate the inlet feedwater
flow to
the riser chamber 19. The upper end of the shroud 26 also establishes fluid
communication between the riser chamber 19 within the shroud 26 and annular
downcomer space 31 that is formed between the outer surface of the lower
shroud
26 and the inner surface of the cylindrical shell 11 through a gap or steam
bleed port
32.
[0034] A support rod system 28 is secured at the uppermost support plate
45B,
and consists of threaded segments spanning between the lower tubesheet 23 and
the lowest support plate 45A and thereafter between all support plates 45 up
to the
uppermost support plate 45B.
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[0035] A hollow, toroid shaped secondary coolant feedwater inlet header 34
circumscribes the outer surface of the shell 11. The header 34 is in fluid
communication with the annular downcomer space 31 through an array of radially
disposed feedwater inlet nozzles 35. As shown by the direction of the FIG. 1
arrows,
feedwater flows from the header 34 into the steam generator unit 10 byway of
the
nozzles 35 and 36. The feedwater is discharged from the nozzles downwardly
through the annular downcomer 31 end through the water ports 30 into the riser
chamber 19. Within the riser chamber 19, the secondary coolant feedwater flows
upwardly within the shroud 26 in a direction that is counter to the downward
flow of
the primary coolant within the tubes 27. An annular plate 37, welded between
the
inner surface of the shell 11 and the outer surface of the bottom edge of an
upper
cylindrical shroud, baffle or wrapper 33 insures that feedwater entering the
downcomer 31 will flow downwardly toward the water ports 30 in the direction
indicated by the arrows. The secondary fluid absorbs heat from the primary
fluid
through the tubes 27 in the tube bundle and rises to steam within the chamber
19
that is defined by the shrouds 26 and 33.
[0036] The upper shroud 33, also aligned with the shell 11 by means of
alignment
pins (not shown in FIG. 1), is fixed in an appropriate position because it is
welded to
the shell 11 through the plate 37, immediately below steam outlet nozzles 40.
The
upper shroud 33, furthermore, enshrouds about one third of the tubes 27 of the
bundle.
[0037] An auxiliary feedwater header 41 is in fluid communication with the
upper
portion of the tube bundle through one or more nozzles 42 that penetrate the
shell
11 and the upper shroud 33. This auxiliary feedwater system is used, for
example,
to fill the steam generator 10 in the unlikely event that there is an
interruption in the
feedwater flow from the header 34. As mentioned above, the feedwater, or
secondary coolant that flows upwardly along the tubes 27 in the direction
shown by
the arrows rises into steam. In the illustrative embodiment, moreover, this
steam is
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superheated before it reaches the top edge of the upper shroud 33. This
superheated steam flows in the direction shown by the arrow, over the top of
the
shroud 33 and downwardly through an annular outlet passageway 43 that is
formed
between the outer surface of the upper cylindrical shroud 33 and the inner
surface of
the shell 11. The steam in the passageway 43 leaves the steam generator 10
through steam outlet nozzles 40 which are in communication with the passageway
43. In this foregoing manner, the secondary coolant is raised from the feed
water
inlet temperature through to a superheated steam temperature at the outlet
nozzles
40. The annular plate 37 prevents the steam from mixing with the incoming
feedwater in the downcomer 31. The primary coolant, in giving up this heat to
the
secondary coolant, flows from a nuclear reactor (not shown) to the primary
coolant
inlet 15 in the upper head 12, through individual tubes 27 in the heat
exchanger tube
bundle, into the lower head 13 and is discharged through the outlet 20 to
complete a
loop back to the nuclear reactor which generates the heat from which useful
work is
ultimately extracted.
[0038] To
facilitate fabrication, and specifically the insertion of tubes 27 during
the fabrication process, the tube support plates 45 are generally aligned with
each
other, and also with the upper and lower tube sheets. The alignment of the
tube
support plates 45 is maintained by tube support plate alignment blocks 104,
shown
in FIGs. 3 - 4, situated around the perimeter of the tube support plates
between the
tube support plates and the inner surface of the shroud or baffle 26, 33. The
tube
support plate alignment blocks 104 are attached to the shroud 26, 33, or a
tube
support plate 45, but not to both, and fill most, or all, of the available
clearance
between the tube support plates 45 and shroud 26, 33 at discrete locations
around
the tube support plate perimeter. The shroud, which is generally a large
continuous
cylinder, is laterally supported within the OTSG shell 11 by shroud alignment
pins
106, shown in FIGs. 3 and 4. This support arrangement provides a lateral load
path
from the tubes 27, through the tube support plates 45, to the shroud 26, 33,
which is
supported by the shell 11.
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[0039]
Turning now to the present invention and referring to FIGs. 2 ¨ 4, there is
provided a tube bundle support system 100 and method for precisely aligning
tube
support plates 45 during fabrication, with minimal clearances between
components,
and then imposing a controlled misalignment as the steam generator heats up.
Tube support plates 45 are advantageously installed in an aligned
configuration that
is compatible with normal fabrication processes.
Displacement to cause
misalignment is produced, only when the heat exchanger is heated. Displacement
to nnisalign tube support plates 45 in the hot condition can advantageously
mitigate
tube vibration due to either cross flow or axial flow excitation mechanisms.
[0040]
Misalignment between the different elevations of tube support plates 45 is
partially accomplished during heat up by making the tube support plates 45
from a
material having a lower coefficient of thermal expansion than the shroud 26,
33.
Radial clearances 102, shown in FIG. 4, between tube support plates 45 and the
shroud 26, 33, open at the positions of the tube support plate alignment
blocks 104
as the steam generator heats up. These radial clearances provide space to
facilitate
lateral shifting or displacement of the individual tube support plates 45.
[0041] As
described in greater detail below, lateral shifting or displacement is
achieved by means of a tube support plate displacement system 100 having
linking
bars 112 which, when in tension, pull on the sides of respective tube support
plates
45. The difference in thermal expansion between the shroud 11, which is
preferably
made of carbon steel, and tube support plates 45, which are preferably made of
410S stainless steel, provides enough operational clearance to allow for
effective
lateral displacement of tube support plate 45, thereby mitigating flow induced
vibration of tubes 27. Radial clearances 102 may be reduced to zero due to the
linking bar force.
[0042]
Tube support plate alignment blocks 104 may be installed with an initial
clearance to facilitate tube support plate motion in the hot condition.
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[0043] As shown in FIG. 4, by alternating the tension direction of the
consecutive
tube support plates at different elevations, for example, 45C, 45D, 45F, and
45F, the
desired tube support plate misalignment and the loading of tubes 27 within the
tube
support plate holes 116 can be achieved. Generally the tube support plate 45
is
made of a material with a thermal expansion coefficient sufficiently less than
that of
the shroud, so that the resulting radial gap between a tube 27 and its support
hole
116 that would result at operating temperatures would be greater than the gap
between the tube 27 and its support hole 116 at operating temperatures.
[0044] It may not be necessary to laterally misalign the tube support
plates 45 at
all elevations of the upright heat exchanger. It may, for example, be
acceptable to
shift every other tube support plate 45 in the same direction while
restraining the
remaining tube support plates 45 in their neutral positions to achieve the
desired
misalignment. Also, there may be more than one tube support displacement
system
100 per tube support plate elevation. The tube support displacement system 100
can thus be used to variably displace the plurality of tube support plates, in
one or
more of a plurality of different directions, to provide controlled
misalignments on one
or more tube support plates, in the same or varying amounts and directions,
and
with one or more apparatus being provided for any individual tube support
plate.
[0045] Referring now to FIGs. 2 and 3, there is shown the linking bar 112
having
a connecting end 124 and with a threaded end 126. The threaded end 126 of the
linking bar 112 extends through a restraining plate or disc 113 and threadably
engages a restraining nut 115. As shown in FIG 3, the restraining disc 113 is
in
contact with a lip within a handhole 132 provided on the shell 11.
[0046] As shown in FIGs. 3 - 4, the tube support displacement system 100 is
used to impose lateral displacements to tube support plates 45. The
restraining nut
115 is threadably engaged with the linking bar 112. The threaded end 126 of
the
linking bar 112 and the restraining nut 115 is accessible for adjusting
tension in the
linking bar 112, or for adjusting the length of the linking bar between the
restraining
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plate or disc 113 and the connecting end 124 engaged with the tube support
plate
45. The shell 11 is provided with a hand hole 132 for access to the
restraining nut
115. When not in use, the hand hole 132 is sealed by a bolted and gasketed
hand
hole cover 134.
[0047] In
a preferred embodiment, the connecting end 124 of the linking bar 112,
as illustrated in Figs. 3 and 4, may comprise a finger that extends into a
vacant tube
hole in the tube support plate 45. Alternatively, the connecting end 124 of
the linking
bar 112 may comprise any of the following constructions, alone or in
combination at
one or more tube support plate 45 elevations:
[0048] (a) a
threaded end which is threadably engaged with a drilled and
tapped hole provided into an edge or surface of the tube support plate 45;
[0049] (b) an
end welded to an edge or surface of the tube support plate
45;
[0050] (c) an
end hooked or engaged to one or more tubes 27 extending
through the tube support plate 45, with or without an internal stabilizer core
inserted
into the one or more tubes 27;
[0051] (d) an
end hooked or engaged into a new or existing hole in the
tube support plate 45, other than a hole normally used for receiving a tube
27;
[0052] (e) an
end which is connected to a lip, protrusion, depression,
deformation or flange on the tube support plate 45, or to a support rod 28
extending
through the tube support plate 45; and
[0053] (f) an
end which is clamped tightly to top and bottom surfaces of
the tube support plate 45 so as to sandwich the tube support plate 45
therebetween.
[0054]
When the shell / shroud / tube support plate assembly heats up, the higher
coefficient of thermal expansion of the shell 11 and shroud 26, 33 material
relative to
the material of tube support plate 45 will cause a dilation of the shroud 26,
33
relative to the tube support plate 45. As shown in FIG. 4, in this hot
condition, the
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linking bar 112 will cause a lateral displacement or offset 136 of the tube
support
plate 45 relative to the initially centered position 138 within the shroud 26,
33. The
tensile force in the linking bar 112 will either be reacted by contact with
tubes 27, or
by contact with both tubes 27 and tube support plate alignment block(s) 104 on
the
opposite side of the tube support plate 45. In either case, tube contact
forces are
achieved, thereby providing the desired effect of increased tube support
effectiveness.
[0055] Control of the tube-to-support plate contact forces in the hot
condition is
achieved by controlling the initial cold condition tension in the linking bar
112 or its
effective length. The tension or length is adjustable through hand hole 132
which
provides access to the restraining nuts 115 of the linking bar 112. In the
cold
shutdown condition, the hand hole cover 132 can be removed to gain access to
the
linking bar 112, and the restraining nuts 115 can be adjusted by turning the
restraining nuts 115 to obtain the desired length or tension.
[0056] As shown in FIG. 4, by alternating the tension direction for
consecutive
tube support plates at different elevations, e.g. 45C, 45D, and 45E, the
desired tube
support plate misalignment and the loading of tubes 27 within tube support
plate
holes can be achieved. It may not be necessary to laterally misalign all tube
support
plate elevations. It may, for example, be acceptable to shift every other
plate in the
same direction, while restraining the remaining plates in their neutral
positions to
achieve the desired misalignment. Also, there may be more than one tube
support
plate displacement system 100 per tube support plate elevation.
[0057] Additionally, the contact forces between tubes 27 and tube support
plates
45 may be controlled by limiting the tension, or length, of the linking rod
112. These
can be controlled by either selecting a material for the tube support plate 45
with a
desired coefficient of thermal expansion, such that the tension is limited by
the
maximum radial clearance in the hot condition between the tube support plate
45
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and the tube support plate alignment blocks 104, or, alternatively, by
adjusting the
effective length of the linking bar 112.
[0058] The linking bar 112 may comprise bar of any desired cross-section,
or
cable, or chain or tubular structures, with materials selected to meet
applicable
pressure, temperature and stress criteria. Advantages of the invention
include:
[0059] The tube support plates 45 are installed in an aligned configuration
that is
compatible with normal fabrication processes. The desired misalignment occurs
only when heating the heat exchanger.
[0060] The misaligned tube support plates 45 in the hot condition can
mitigate
tube vibration due to either cross flow or axial flow excitation mechanisms.
[0061] Tube to tube support plate contact loads in the hot condition are
controlled
by controlling the linking bar effective length or tension, the tube support
plate
displacement, or a combination thereof.
[0062] The normal load paths used for the transmission of seismic loads
between
tubes 27, tube support plates 45, shroud 26, 33 and shell 11 are unaltered.
[0063] Tube support plate displacement system 110 has only three parts,
linking
bar 112, restraining plate or disc 113, and restraining nuts 115 which are
threadably
engaged, and are internal to the steam generator shell 11, thereby minimizing
the
potential of loose parts.
[0064] The hardware for linking bar 112 tension or length adjustment is
readily
accessible.
[0065] The linking bar connecting end 124 is situated within the shroud
opening
130 and the threaded end 126 is situated within the hand hole 132. The linking
bar
112 is threadably engaged with the restraining components 113, 115 thereby
preventing each from becoming a loose part.
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[0066] Linking bar tension is reacted against the shell 11, which is a
stiff anchor
point, as opposed to a reaction against the shroud 26, 33 which is relatively
flexible.
[0067] The design is capable of being retrofitted to existing designs,
since few
internal alterations are required. Conversely, the tube support plate
displacement
system 150 can be easily removed, restoring the support arrangement to its
original
condition.
[0068] The tube support plate alignment blocks 104 may be installed with an
initial clearance to facilitate tube support plate displacement during heat up
of the
heat exchanger.
[0069] While specific embodiments and/or details of the invention have been
shown and described above to illustrate the application of the principles of
the
invention, it is understood that this invention may be embodied as more fully
described in the claims, or as otherwise known by those skilled in the art
(including
any and all equivalents), without departing from such principles.
