Heat exchanger tube support bar

FIELD AND BACKGROUND OF INVENTION

[0001] The present invention relates generally to the field of heat exchanger tube supports, and in particular to a new and useful tube support bar for restraining and positioning the U-bends of water tubes within a nuclear steam generator.

[0002] In a pressurized water nuclear power station, steam generators, which are large heat exchangers, transfer heat, produced via nuclear reactions in the reactor core, from a primary water coolant to a secondary water coolant that drives the steam turbine. The primary coolant is pressurized, which allows the primary water coolant to be heated in the reactor core with little or no boiling. For example, in a light water reactor, the primary coolant is pressurized to about 2250 psia and heated to about 600 deg F. in the reactor core. From the reactor, the primary water coolant flows to a steam generator, where it transfers heat to the secondary coolant. In a U-tube, or recirculating steam generator, the primary coolant enters at the bottom of the steam generator, flows through tubes having an inverted U-shape transferring heat to the secondary coolant, and then exits at the bottom of the steam generator. The secondary coolant is pressurized only to a pressure below that of the primary side, and boils as it flows along the outside of the tubes, thereby producing the steam needed to drive the turbine. Nuclear steam generators must be capable of handling large quantities of two-phase secondary coolant moving at high flow rates, and are therefore very large structures. For example, a nuclear U-tube steam generator can weigh more than 450 tons, with a diameter exceeding 12 feet and an overall length of greater than 70 feet. It may contain as many as 9,000 or more of the long, small diameter, thin-walled U-shaped tubes. For a general description of the characteristics of nuclear steam generators, the reader is referred to Chapters 47 and 52 of <i>Steam/Its Generation and Use, </i>40th Edition, The Babcock &amp; Wilcox Company, Barberton, Ohio, U.S.A., ©1992, the text of which is hereby incorporated by reference as though fully set forth herein.

[0003] Heat exchangers such as nuclear steam generators require tube restraints or supports, to position the tubes and to restrain the tubes against flow induced vibration forces. In the U-bend region of a nuclear steam generator, a large flow of steam and water mixture passes upwards through the tube array, in a general direction which locally is normal to the axis of the individual U-bend tubes. This large two phase flow is able to cause excitation of the U-bend tubes via the turbulent and other flow forces imparted by the flow. As a result, the tubes tend to vibrate in both the out-of-plane and in-plane directions relative to the U-bend plane. Typically this restraint function is provided by an array of flat U-bend support bars. While such flat bars provide positive restraint in the U-bend out-of-plane direction, they provide restraint only by friction in the in-plane direction.

[0004] One known type of nuclear steam generator U-bend support, depicted in FIG. 1, and in greater detail in FIG. 2, is manufactured by Babcock &amp; Wilcox Canada Ltd. FIG. 1 shows a nuclear steam generator <b>100</b> having a plurality of U-bend tubes <b>110</b>. The U-bend tubes <b>110</b> are arranged in layers, with each layer having multiple tubes all positioned within the plane of their respective U-bends. Each layer incorporates a set of tubes of successively larger radius which are nested to create the layer of tubes in the particular plane. For purposes of illustration, however, FIG. shows only a limited number of U-bend tubes <b>110</b>, and FIG. 2 shows only the outermost tubes of the center U-bend layers. The straight leg portions of the U-bend tubes <b>110</b> are supported at several locations by vertically spaced apart tube support plates <b>200</b>, as shown in FIG. 1.

[0005] As shown in FIG. 1, and in greater detail in FIG. 2, the U-bend tubes <b>110</b> are positioned and restrained in the U-bend region by a U-bend support assembly <b>120</b> which includes a number of U-bend support bar arrays <b>180</b>. Each U-bend support bar array <b>180</b> is comprised of flat U-bend support bars <b>160</b>, which are positioned in sets between layers of tubes within the U-bend region of the steam generator. The flat U-bend support bars <b>160</b> fan out from the center of the U-bend such that individual bar sets are assembled into a U-bend support bar array <b>180</b>, or “fan” bar array, in which the lower ends of the individual bars are interconnected. As shown in FIG. 2, the inner ends of the flat U-bend support bars <b>160</b> of a particular U-bend support bar array <b>180</b>, are interconnected by a mechanical or welded joint <b>210</b>. Each U-bend support bar array <b>180</b> incorporates about 4 to 12 of the flat U-bend support bars <b>160</b>. The flat U-bend support bars <b>160</b> are positioned so as to provide support to the U-bend tubes <b>110</b> at certain points along the arc of each U-bend tube in the array. The angular separation of the flat U-bend support bars <b>160</b> depends upon the U-bend size and flow conditions, and the flat U-bend support bars <b>160</b> are located to minimize unsupported tube lengths. The individual flat U-bend support bars <b>160</b> are typically made of stainless steel, and are about 1″ to 1.5″ wide and about 0.1″ to 0.2″ thick. A U-bend support assembly <b>120</b> may incorporate 100 to 200 of the fan-shaped U-bend support bar arrays <b>180</b>, with one such array located between each plane of U-bend tubes. The outer ends of the flat U-bend support bars <b>160</b> are collected, restrained and supported by arch bar support structures <b>170</b> located adjacent the steam generator U-bend. Each arch bar support structure <b>170</b> positions the flat U-bend support bars <b>160</b> of a U-bend support bar array <b>180</b>, carrying the weight of the bars and redistributing the weight of the U-bend support assembly <b>120</b> back to the peripheral layer of U-bend tubes.

[0006] The U-bend support bar arrays <b>180</b> position the planes of U-bend tubes <b>110</b> in space, and most importantly, restrain the individual U-bend tubes against flow induced vibration. Restraint against out-of-plane motion is provided by the physical presence of the flat U-bend support bars <b>160</b>, which are situated immediately adjacent to the U-bend tubes <b>110</b>. The bar-to-tube clearance is purposely quite small, with the bar-to-tube diametral clearance varying from about 0 to 0.010″ or more. The flat U-bend support bars <b>160</b>, with their small bar-to-tube clearances, thus prevent significant motion of the tubes in the out-of-plane direction <b>140</b>. In the in-plane direction <b>130</b>, however, the U-bend tubes <b>110</b> are not positively restrained, but instead depend solely upon friction between the U-bend tubes <b>110</b> and the flat U-bend support bars <b>160</b> to restrict and dampen the flow induced motion of the tubes in their in-plane direction. Depending on the design details and flow conditions, the effect of the friction in providing in-plane restraint may not be fully adequate in providing in-plane restraint.

SUMMARY OF INVENTION

[0007] The present invention is drawn to an improved heat exchanger tube support bar which is particularly suited for the U-bend region of a U-tube nuclear steam generator. The bar is configured so that it has scalloped pockets on opposite surfaces of the bar, which can be positioned to engage and restrain the tubes during steam generator operation. Each pocket is arranged so that it provides a support surface for a tube. As the tube is supported on each side by one of these pockets with close clearance, the tube is positively constrained in its out-of-plane direction by virtue of the small tube-to-bar clearance, and positively supported in the in-plane direction by the contour of the pocket as it engages with the U-tube.

[0008] The support bar has a thin profile width section whereby the bar may be positioned within an existing tube array in two different orientations, i.e. with either the diagonal or vertical cross sectional axes of the bar positioned parallel to the plane of the tubes. In the diagonal orientation, the bar may be moved, as for installation, within the space between layers of tubes without restriction. With the pocketed bars correctly positioned, and with the pocketed bars rotated to the vertical orientation, pockets along the diagonally opposite shoulder areas of the bars are able to positively position the tubes in both in-plane and out-of-plane directions for purposes of providing restraint of the tubes against flow induced vibration.

[0009] Accordingly, one aspect/object of present invention is to provide an improved heat exchanger tube support bar providing support in both the in-plane and out-of-plane directions.

[0010] It is a further aspect/object of the present invention to provide an improved U-bend support bar which reduces the susceptibility of the tubes in the U-bend region to flow induced vibration and tube fretting at the support locations.

[0011] It is an object of one embodiment of the invention to provide a U-bend support bar to serve in place of one or more of the U-bend support bars within a particular U-bend support bar array, either as a retrofit or during new equipment manufacture.

[0012] It is an object of an alternate embodiment of the invention to provide an auxiliary U-bend support bar for installation within an existing U-bend support bar array, either as a retrofit or during new equipment manufacture.

[0013] Accordingly one aspect of the invention comprises a support bar, for use in a heat exchanger having rows of tubes arranged in a plurality of parallel tube planes defining an in-plane direction and an out-of-plane direction, the support bar having an elongated body with first and second sides, and first and second ends; a first shoulder, extending along the first side adjacent the first end, and having a first plurality of pockets adapted for receiving a first row of tubes; a second shoulder, extending along the second side adjacent the second end, and having a second plurality of pockets adapted for receiving a second row of tubes; and wherein the pockets are designed to have a small clearance with the tubes after installation thereby restraining the tubes against motion in both the in-plane direction and the out-of-plane direction.

[0014] Another aspect of the invention comprises a support bar, for use in the U-bend region of a heat exchanger having rows of U-bend tubes arranged in a plurality of parallel U-bend tube planes defining an in-plane direction, and an out-of-plane direction, the support bar having an elongated body with first and second sides, and first and second ends, the body having a cross sectional form generally in the shape of a parallelogram having a short diagonal and a long diagonal, a center located at the intersection of the short diagonal and the long diagonal, a first axis passing through the center, a first pair of parallel surfaces located on opposite sides of the first axis, a second axis passing through the center, and a second pair of parallel surfaces located on opposite sides of the second axis; a first shoulder, extending along the first side adjacent the first end, and having a first plurality of pockets adapted to restrain a first row of U-bend tubes against motion in both the in-plane direction and the out-of-plane direction in the U-bend region; and a second shoulder, extending along the second side adjacent the second end, and having a second plurality of pockets longitudinally offset from the first plurality of pockets and adapted to restrain a second row of U-bend tubes against motion in both the in-plane direction and the out-of-plane direction in the U-bend region; and wherein the first and second shoulders each have shoulder sides parallel to the second axis, and inner and outer shoulder end surfaces perpendicular to the first axis.

[0015] 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.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same:

[0017]FIG. 1 is a sectional front elevation view of a nuclear steam generator where the pocketed tube support bars of the invention may be used;

[0018]FIG. 2 is a partially cut away perspective view of a known U-bend support assembly;

[0019]FIG. 3 is a perspective view of an end section of the improved tube support bar of the present invention;

[0020]FIG. 4 is a partial plan view of the improved tube support bar of the present invention;

[0021]FIG. 5 is a cross sectional view of the improved tube support bar of the present invention;

[0022]FIG. 6 is partially cut away perspective view of a U-bend support assembly using the improved tube support bar of the present invention according to an embodiment as an auxiliary support bar.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Referring to the drawings in which reference numbers are used to refer to the same or functionally similar elements, FIG. 3 shows a portion of an improved tube support bar <b>260</b> according to the present invention.

[0024] The detailed features of the pocketed tube support bar <b>260</b> are shown in FIGS. <b>3</b>-<b>5</b>. FIGS. 3 and 5 both show that the general shape of the cross section of the pocketed tube support bar <b>260</b> resembles the letter “Z.” In the following description the central and extremity portions of the Z-shape will be referred to as the body and the shoulders, respectively. The body is roughly in the shape of a parallelogram having a long diagonal and a short diagonal, shorter than the long diagonal, wherein the diagonals meet at the center of tube support bar <b>260</b>.

[0025] As shown in FIG. 5 the cross sectional shape of the pocketed tube support bar <b>260</b> is defined primarily by two axes passing through the center of the pocketed tube support bar <b>260</b>; a first axis <b>20</b>, shown in, but not limited to, the vertical orientation, and a second, diagonal axis <b>30</b> at an angle to axis <b>20</b>.

[0026] The body of the pocketed tube support bar <b>260</b> incorporates a first pair of surfaces, <b>90</b> and <b>90</b>′, aligned in parallel with, and arranged on opposites sides of, the first axis <b>20</b>, and also incorporates a second pair of surfaces, <b>80</b> and <b>80</b>′, aligned in parallel with, and arranged on opposites sides of, the second, diagonal axis <b>30</b>. The body of the pocketed tube support bar <b>260</b> thus has the general shape of a long, thin parallelogram having a first side defined by surfaces <b>80</b>-<b>90</b> and a second side defined by surfaces <b>80</b>′-<b>90</b>′. The body has first and second ends <b>50</b> and <b>50</b>′ located at the ends of sides <b>80</b> and <b>80</b>′ respectively.

[0027] Surfaces <b>80</b> and <b>80</b>′ intersect surfaces <b>90</b> and <b>90</b>′, respectively, on opposite sides of and adjacent to the center of the pocketed tube support bar <b>260</b>. The intersections of surfaces <b>80</b>-<b>90</b> and <b>80</b>′-<b>90</b>′ are each provided with a blend radius <b>40</b> and <b>40</b>′, with the distance between the arcs of the radius being the same as the distance between the planes formed by the pairs of surfaces <b>80</b> and <b>80</b>′, and <b>90</b> and <b>90</b>′ respectively.

[0028] A line <b>32</b> which falls along axis <b>31</b> perpendicular to the second, diagonal axis <b>30</b>, or along axis <b>21</b> perpendicular to first axis <b>20</b>, or any direction between axes <b>21</b> and <b>31</b>, and which passes through the center of the pocketed tube support bar <b>260</b> to connect the first side of the body (surfaces <b>80</b>-<b>40</b>-<b>90</b>) with the second side of the body (surfaces <b>80</b>′-<b>40</b>′-<b>90</b>′), sets the thickness of the pocketed tube support bar <b>260</b>, and defines the thickness required for pocketed tube support bar <b>260</b> to serve as a flat tube support bar <b>160</b>. This thickness is the same in all directions along axis <b>21</b> or axis <b>31</b> or any direction between axes <b>21</b> and <b>31</b>. In this manner the pocketed tube support bar <b>260</b> can serve as a flat bar tube restraint with any bar orientation, i.e. with axis <b>20</b> or <b>30</b>, or any axis in between, being parallel to the tube plane.

[0029] The thickness of the shoulders of the pocketed tube support bar <b>260</b> is defined by pairs of parallel surfaces <b>70</b>-<b>80</b>′ and <b>70</b>′-<b>80</b>. Surfaces <b>70</b>-<b>80</b>′ and <b>70</b>′-<b>80</b> are respectively co-planar, aligned parallel with and located on opposite sides of the diagonal axis <b>30</b>. The distance between surfaces <b>70</b>-<b>80</b>′ and <b>70</b>′-<b>80</b> in a direction perpendicular to the diagonal axis <b>30</b> is also equivalent to the maximum thickness of the pocketed tube support bar <b>260</b>, which is set to equal the spacing between successive layers of tubes <b>110</b>, less a small clearance.

[0030] The shoulders each have outer end surfaces <b>50</b> and <b>50</b>′, and inner end surfaces <b>55</b> and <b>55</b>′, which are perpendicular to the first axis <b>20</b>, and are therefore parallel to each other. The shoulders of pocketed tube support bar <b>260</b> may also incorporate parallel shoulder side surfaces <b>85</b> and <b>85</b>′ aligned in parallel with, and arranged on opposites sides of, the axis <b>20</b> as shown in FIGS. 3 and 5.

[0031] As shown in FIGS. <b>3</b>-<b>5</b>, there are pockets or scallops <b>60</b> and <b>60</b>′ within the shoulders of the pocketed tube support bar <b>260</b>. Pockets <b>60</b> and <b>60</b>′ are arranged on the opposite sides of the pocketed tube support bar <b>260</b> such that when the pocketed tube support bar <b>260</b> is rotated so that surfaces <b>90</b> and <b>90</b>′ are parallel with the plane of the tubing, and more or less in contact with the tubes <b>110</b>, the tubes <b>110</b> are nested in the individual pockets along the length of the pocketed tube support bar <b>260</b>.

[0032]FIG. 4 provides a plan view of the pocketed tube support bar <b>260</b> looking in the direction parallel to the axes of the tubes <b>110</b> and perpendicular to the pocketed tube support bar <b>260</b>. The figure shows two partial layers of tubes <b>110</b> in cross section, with the first layer nested into pockets <b>60</b> and the second layer nested into pockets <b>60</b>′ on the opposite side of the pocketed tube support bar <b>260</b>.

[0033] In one embodiment, the pocketed tube support bars <b>260</b> are designed to be positioned among the U-bend tubes within the U-bend region of a U-tube steam generator <b>100</b>, and are oriented so the axes <b>20</b> of the bar cross sections are parallel to the space between layers of U-bend tubes. For this orientation the pockets of the pocketed U-bend support bars <b>260</b> would be nested to the respective tubes to provide positive restraint of the U-bend tubes in the in-plane direction <b>130</b> as well as the out-of-plane direction <b>140</b>. The pocketed support bars <b>260</b> thus provide positive U-bend tube restraint in the in-plane direction, unlike flat bars, which provide restraint in the in-plane direction only by way of tube-to-bar friction.

[0034] In the above embodiment the outer ends of the pocketed tube support bars <b>260</b> are affixed, directly or indirectly, to an existing external support structure such as arch bars <b>170</b>, either with or without being affixed to a special external structure for the pocketed tube support bars <b>260</b> themselves.

[0035] In another embodiment, the pocketed tube support bar <b>260</b> of the present invention can be used within an existing U-bend support assembly <b>120</b>, as shown in FIG. 6, to provide additional support within the U-bend region of the tubes <b>110</b>, and may also be used to replace existing support bars within a U-bend support assembly <b>120</b>. For example, pocketed tube support bars <b>260</b> could be distributed within U-bend support bar arrays <b>180</b> with one bar placed between each layer of U-bend tubes. One or more arrays of such pocketed U-bend support bars <b>260</b> may be used. The longitudinal orientation of a pocketed U-bend support bar <b>260</b> may be radial, or some other direction with a radial component. The length of the pocketed U-bend support bar <b>260</b> may extend inwards from the periphery of the tube bundle to a point part way toward the smallest radius tube, as shown in FIG. 6, or may extend all of the way to the smallest radius tube. A U-bend support assembly <b>120</b> may also include pocketed U-bend support bars <b>260</b> sandwiched between two layers of tubes, and in each space between successive layers of tubes, so that pocketed U-bend support bars <b>260</b> are situated between each of the inter-tube layers extending from the middle of the bundle outward for some fraction, or for all of the inter-tube layers of the bundle.

[0036] The location, orientation and shape of the array of pockets as well as the cross sectional dimensions of the bars must be matched to the intended installation location. For a radial bar installation, in a steam generator with a uniform tube array, a uniform array of correctly sized pockets will be appropriate. For a non-radial orientation or a non-uniformly spaced tube array, the pocket array must be specifically tailored to suit the intended installation. FIG. 4 depicts an embodiment suitable for use in a staggered tube bundle, in which the pockets are longitudinally offset on opposite sides of tube support bar <b>260</b>. For application to an in-line tube bundle, the pockets are instead longitudinally aligned, rather than offset, on opposite sides of tube support bar <b>260</b>. While the pockets <b>60</b> and <b>60</b>′ are shown in the figures to be of cylindrical shape, each pocket could be formed with three or more flat surfaces, with the flat surfaces being parallel to the adjacent tube surface.

[0037] The pocketed tube support bars <b>260</b> are designed to be installed into an existing, fully-assembled steam generator tube bundle, either in addition to, or in place of, conventional flat U-bend support bars <b>160</b>. Such retrofit assembly is not possible for other scalloped bars, corrugated bars or other bars shaped to provide restraint in the in-plane direction. By virtue of their unique design, the pocketed tube support bars <b>260</b> are suited for use in retrofit applications within a fully-assembled steam generator tube bundle, either before or after entering service. The pocketed tube support bars <b>260</b> are, however, equally suited for use in original equipment applications, layered between tubes as they are inserted into a new steam generator during manufacture.

[0038] The pocketed tube support bars <b>260</b> may be made with a variety of configurations and details as required for the specific application. For example, in a nuclear steam generator application in the U-bend region, tube support bars <b>260</b> may be of various widths as required for a particular design, and can range from about 1″ or less to about 2″ for some applications. The thickness of the tube support bar <b>260</b> is dictated by the space between adjacent layers of tubes, together with desired tube-to-bar clearance, and may vary from 0.1″ or less to about 0.4″. As a further example, pocketed tube support bars <b>260</b> may be made from 400 Series or 300 Series stainless steels, or possibly of other high alloy material or a low alloy steel, but other materials may also be suitable.

[0039] 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. The present invention is not limited to the U-bend region of U-tube steam generators, and can be applied to provide in-plane and out-of-plane support to the straight-leg portion of the U-tubes of a U-tube steam generator.

[0040] The present invention is also not limited to U-tube steam generators, and can be used to provide in-plane and out-of-plane support to the tubes of a variety of heat exchangers including spiral tube heat exchangers or straight tube heat exchangers, such as shell-and-tube heat exchangers, and for a variety of applications in the process, energy and other industries.