Patent Application
20050279487
Application Ser. No. 10/848,903 filed 19 May 2003, Publication No. 20050006075A1, entitled “Anti-Vibration Tube Support” of A. S. Wanni, M. M. Calanog, T. M. Rudy, and R. C. Tomotaki relates to a different type of anti-vibration tube support.
This invention relates to tube support devices, commonly referred to as tube stakes which are useful with tube bundles in heat exchangers and similar fluid-handling equipment.
Tube bundle equipment such as shell and tube heat exchangers and similar items of fluid handling devices utilize tubes organized in bundles to conduct the fluids through the equipment. In such tube bundles, there is typically fluid flow both through the insides of the tubes and across the outsides of the tubes. The configuration of the tubes in the bundle is set by the tubesheets into which the tubes are set. One common configuration for the tubes is the rectangular formation with the tubes set in aligned rows with tube lanes (the straight paths between the tubes) between each pair or rows, aligned orthogonally to one another. In this formation, each tube is adjacent to eight other tubes except at the periphery of the tube bundle and is directly opposite a corresponding tube across the tube lane separating its row from the two adjacent rows. In the triangular tube formation, the tubes in alternate rows are aligned with one another so that each tube is adjacent to six other tubes (the two adjacent tubes in the same row and four tubes in the two adjacent rows).
Fluid flow patterns around the tubes as well as the changes in the temperature and density of the fluids which arise as they circulate and result in heat exchange between the two fluids flowing in and around the tubes may give rise to flow-induced vibrations of an organized or random oscillatory nature in the tube bundle. If these vibrations reach certain critical amplitudes, damage to the bundle may result. Tube vibration problems may be exacerbated if heat exchange equipment is retubed with tubes of a different material to the original tubes, for example, if relatively stiff materials are replaced with lighter weight tubes. Flow-induced vibration may also occur when equipment is put to more severe operating demands, for example, when other existing equipment is upgraded and a previously satisfactory heat exchanger, under new conditions, becomes subject to flow-induced vibrations. Vibration may even be encountered under certain conditions when an exchanger is still in the flow stream but without heat transfer taking place.
Besides good equipment design, other measures may be taken to reduce tube vibration. Tube support devices or tube stakes as these support devices are commonly known (and referred to in this specification) may be installed in the tube bundle in order to control flow-induced vibration and to prevent excessive movement of the tubes. A number of tube supports or tube stakes have been proposed and are commercially available. One type, described in U.S. Pat. No. 4,648,442 (Williams) has a U-shaped configuration in which the distance between the top and bottom surfaces of the channel is the same as the distance between adjacent rows in the tube bundle (i.e., is substantially the same as the tube lane dimension). This type of stake is inserted between the rows in the bundle and is secured at the end by an arcuate segment which engages a segment of a tube at the periphery of the tube bundle so as to lock the stake in place in its appropriate position between the rows in the bundle. Stakes of this type are typically made of a corrosion-resistant metal, for example, type 304 stainless steel with a thickness between 0.7 and 1.2 mm to provide both the necessary rigidity for the staked tube bundle as well as sufficient resilience in the U-shaped channel to allow the stakes to be inserted into the lanes between the tubes in the bundle.
Another form of anti-vibration tube stake is described in U.S. Pat. No. 4,919,199 (Hahn) which discloses a stake made in a soft V-configuration strip in which saddles are formed perpendicular to the longitudinal axis of the strip in the open ends of these V-shaped cross sections. The saddles are formed in the strip with a pitch (distance between saddles) equal to the tube pitch and with a radius which matches that of the tubes in the tube bundle so the saddles engage with the tubes on one side of the tube lane. The engagement between these tubes and the saddles locks the tube into place in the tube bundle. The resilient nature of the strip, coupled with the spring type action provided by the V-configuration, permits the arms of the V to open and reduce the effective overall width of the stake and enables the stake to engage the tubes on both sides of a tube lane so that the V-shaped stake is locked into place between the two rows of tubes.
A similar type of tube stake is described in U.S. Pat. No. 5,213,155 (Hahn) which discloses a U-shaped stake which is inserted between two tube lanes with the closed end of the U over one of the peripheral tubes in the bundle. Saddles are formed in the open ends of the V-shaped cross section to engage with opposite sides of the tubes in a single row in the bundle. The U-shaped stake is fastened in place around the tubes of the bundle by suitable fasteners extending between the two arms of the stake.
One problem with the pressed configuration of the type shown in U.S. Pat. No. 4,648,442 is that the stakes do not create a positive location for each individual tube, although the stake is locked into place in its selected tube lane. The tubes remain free to vibrate in one plane parallel to the tube lane and parallel to the stake. A different problem exists with the design shown in U.S. Pat. No. 5,213,155: although the tubes in rows encircled by the U-shaped stakes are fully supported, the tubes at the periphery of the tube bundle which are not directly encircled by one of the stakes i.e., retained within one of the closed ends of the U-shaped stakes (these are the outer tubes in alternate rows which are not encircled by the ends of the U-shaped stakes), are free to move and vibration in these tubes can be expected under certain conditions. In addition, because the corrugation of the tube support has a transition region before reaching its full depth, the two tubes adjacent to each of the outermost tubes do not receive any vibration mitigation either.
One disadvantage of the stake designs which use channel pressings to accommodate the distance between the tubes forming a single tube lane is that deep channel pressings are required or other measures necessary when the tube lane is relatively wide. A more complicated form of tube support is shown in U.S. Pat. No. 6,401,803 (Hahn). This stake uses two V-shaped pressings separated by compression springs which force the stakes against the tubes on opposite sides of the tube lane in order to dampen oscillatory vibrations. This form of stake is, however, quite expensive to manufacture. A unitary stake which will accommodate relatively wide tube lanes without the complication of separate parts therefore remains desirable.
According to the present invention, a tube support or tube stake is used with in-line tube arrangements (rectangular tube configurations) to mitigate the possibility of tube damage from flow-induced vibration in the tube bundle of the heat exchanger, condenser or other collection of tubes, for example, in devices such as nuclear reactors, electrical heaters, or any collection of parallel cylindrical shapes that has a fluid flow passing over them. The tube support comprises a flat, elongated member or strip which is intended to be inserted in a tube lane between the tubes of the tube bundle. Raised-tube-engaging zones which include transverse, arcuate tube-receiving saddles are disposed along the length of the strip at successive longitudinal locations corresponding to the tube positions in the bundle. These tube-engaging zones extend laterally out from each face of the member opposite one another at each location; they extend away from the medial plane of the member, so that the saddles receive and closely hold the tubes on opposite sides of the tube lane.
The tube supports may be formed by joining two strips in back-to-back fashion each having the tube-engaging zones pressed out on one face of the strip. In this form, a flat strip is formed with the tube-engaging zones extending out on only one face of the strip and two of these strips are then united in back-to-back fashion to form the support with the tube-engaging zones on the opposed faces of the strip. An alternative construction uses a flat strip which is slitted at each tube location to provide adjacent transverse regions across the strip which are formed into raised tube-engaging zones on opposed faces of the strip. The tube-engaging zones at a given transverse position extend in an alternate fashion from the two opposite faces of the strip relative to the zones in the same transverse position at each successive longitudinal location. In either form, the support can be seen as having flat (planar) sections uniting the sections with the tube-engaging zones while the tube-engaging zones, including the saddles, can be seen as being formed with only one plane of curvature (i.e., the strip is curved solely in the longitudinal direction and not in the transverse direction; in the transverse direction, the strip is flat at all points across the width of the strip). It is this feature which enables the support to be readily fabricated in very simple pressing operations with simple press forms or dies.
The tube supports are intended for use in the conventional rectangular (in-line) tube formations. The supports may be inserted into each tube lane or into alternate tube lanes. When inserted into each tube lane, as is preferred, the tubes receive support from supports on both sides with consequent improved support.
The tube supports may be conveniently and inexpensively fabricated by pressing with simple die forms equipped with suitably arranged protrusions and cavities to form the saddles or by the use of pairs of rollers which have protrusions and cavities (alternating between the top and bottom rollers of the set) to form the raised zones on the strip. Many of the known types of tube support do not lend themselves to this simple, economical and convenient method of fabrication.
The tube support or tube stake of the present invention is arranged to provide direct support for tubes which are adjacent to one another but on opposite sides of a tube lane. The tube support may be inserted between the tubes in the tube bundle along a tube lane between adjacent tube rows. Where the construction of the exchanger permits, the support may be made sufficiently long to extend from one side of the tube bundle to the other to provide support for the tubes across the entire width of the bundle; in this case, the length of the tube supports will vary according to the length of the tube lanes across the bundle. In many cases, however, the location of pass lanes in the bundle will create discontinuities in the lanes so that it will not be possible to insert the supports all the way across the bundle. In such cases, it may be possible to insert the supports into the bundle from different sides of the bundle at different locations along the length of the bundle so as to provide as much support as possible for the tubes. Thus, the supports may be inserted vertically at one or more locations and horizontally at other locations along the length of the bundle. In view of their simple and repetitive configuration, the present tube supports may be readily cut to the desired length to fit the bundle, whether extending entirely across it or only part of the way. The tube supports or tube stakes can be utilized to provide vibration mitigation in addition to the baffles in standard shell-and-tube-type heat exchangers or as the only support mechanism in axial flow bundles. When the supports are used in addition to standard baffles, a girdle band connecting the outer edge of all the supports at any axial location may be provided and this may be as simple as a cable passing through a hole in the end of each support strip. When the supports are used as the only support in an axial flow bundle, a more rigid girdle with firm attachment to the supports is preferably used, as described below, along with a separate baffle construction to direct the liquid flow appropriately.
Tube support 10 comprises an elongated flat member made up of two flat strips of metal 11, 12 welded together back-to-back by resistance welds. One weld is indicated at 11 and other welds are regularly spaced at other locations along the length of the support. Alternative methods of attachment between the two strips may, of course, be used, for example, rivets or screws although these will, in general, not be as economical or reliable as resistance or spot welding. The tube-engaging zones are created on each face of support 10 by forming the two strips 11, 12 to provide the transverse, arcuate tube-receiving saddles at successive locations along the member corresponding to the positions of the tubes. The tube-engaging zones each comprise (as indicated with respect to tube A) a pair of lateral extensions 14, 15 which extend laterally outwards away from the medial plane of the support member in opposite directions towards the adjacent tubes at that location. The ends of the lateral extensions are joined together by means of a transverse, arcuate tube-receiving saddle 16 which has a curvature corresponding or approximating to the diameter of the tube so that the tube is nested closely in the saddle and held in place. A corresponding tube-engaging zone is formed on the other face of the member, extending laterally outwards, away from the medial plane of the member in the direction of tube B, with a corresponding transverse tube-receiving saddle to hold tube B. Similar tube-engaging zones are provided for tubes C and D and so on along the length of the support at successive locations along the length of the member.
The tube supports are preferably inserted into the tube bundle so that the tubes receive support on both sides from supports inserted into each tube lane.
As an alternative to the fabrication of the support from two flat strips of metal, as described above, the support may be fabricated in the form shown in
The tube-engaging zones are formed in an alternating, complementary fashion with the saddles to provide support for the tubes. The first pair of opposed tube-engaging zones XA and XB, which provide support for tubes A and B are formed with two tube-engaging zones XA extending from one face of the strip to support tube A and one central zone XB interposed between the two side zones XA, extending from the opposite face of the strip to support tube B. At the next adjacent longitudinal location along the strip, the zones are formed similarly but at this location, the single, central tube-engaging zone XD is formed on the side of the strip which faces tube D (on the same side of the tube lane as tube A) with two side zones XC extending from the opposite face of the strip to support tube C. This alternating arrangement is repeated at successive longitudinal locations along the strip with the tube-engaging zones extending out alternately out from each face of the strip at each location and in the alternative manner at successive locations along the strip. For example, taking a case where the strip is slitted twice, the three tube-engaging zones at each longitudinal location can be formed as follows:
Row 1: UP−DOWN−UP
Row 1: DOWN−UP−DOWN,
Note: the designations “UP” and “DOWN” do not refer to true vertical directions but only to the relative directions from the medial plane and faces of the strip.
In this way, the forces acting on the strip at any single longitudinal location are balanced about the center line of the strip and the asymmetric arrangement at each location is compensated over the length of the strip so that the forces created by engagement of the strip with the tubes on both sides of the tube lane are in overall balance or substantially so as equal or approximately equal numbers of tube-engaging zones are formed on each face of the strip. Thus, a single strip of sufficient width can be formed into a tube support by slitting the strip longitudinally twice or more in the areas where the tube-engaging zones are to be formed to form three or more regions which can be extended laterally outwards to form the opposed tube-engaging zones.
The total depth (d) of the saddles (saddle peak to saddle valley) will be a compromise between the need for good tube support (which dictates a deep saddle) and the need for ready insertion into the bundle (which dictates a shallow saddle) and both will depend upon the diameter of the tubes and the tube spacing. Typically, the depth of the saddles will be from 1 to 5 mm, preferably 2 to 4 mm. The distance between the lowest points of the saddles at the point where tube engagement occurs should be about 0.25 to 2 mm greater than the tube spacing at this point in order to create a small deflection in the tubes to ensure reliable tube support. This larger value is needed especially if the strips are inserted into alternate tube lanes in an existing exchanger. If it is feasible to fabricate the tube support structure as seen in
In addition to the total depth of the support, the thickness and stiffness of the metal of the strip will be factors in fixing the final tube deflection when the supports are inserted into the bundle. Normally, with the metals of choice, a strip thickness of from 1 to 2 mm for each of the two strips making up the support will be satisfactory to provide adequate tube support and ability to resist the stresses of insertion into the bundle. If a single slit strip is used, its thickness may be increased as necessary.
When the tube supports are inserted into the tube bundle, the raised tube-engaging zones have to be pushed past the tubes until the support is in its proper place in the bundle, with each tube accommodated within its corresponding saddle. Each tube-engaging zone has to be pushed through the gap between each pair of opposed tubes until the support is in place. Because the total depth of the tube engaging zones (peak-to-valley including plate thickness) is preferably slightly greater than the inter-tube spacing, the tubes have to bend slightly to let the saddles pass; although this maintains the support in place when it is in its final position, it makes insertion that much more difficult as the resistance to bending of each row of tubes has to be overcome. The lateral extensions 14, 15 which pass into the saddles may be given a greater slope so as to facilitate insertion: if this is done, the lateral extensions will provide ramps which will more readily part the tubes as the support is inserted into the bundle.
Each tube support engages with tubes on opposite sides of a tube lane so that insertion of a support in each tube lane provides support for two rows of tubes within the outer periphery of the tube bundle. At the periphery of the bundle some tubes may receive support from a support which does not support a tube on the other side. This reduces the effective support given to those tubes but since the length of support extending out from the last pair of tubes within the bundle is relatively short, some effective support is given to these outer tubes on one side at least by the cantilevered end of the support. Support may, however, be provided by tie roads and additional support strips as shown in
While the frictional engagement between the supports and the tubes will provide for retention of the supports in the bundle, the tube supports are preferably fixed into place, either as shown in
The tube supports are suitably made of a metal which will resist corrosion in the environment of the tube bundle in which it is to be used. Normally, to resist corrosion in both water and other environments, stainless steel will be satisfactory although other metals such as titanium may also be used. Stainless SS 304 is suitable except when chloride corrosion is to be expected when duplex stainless steel will be preferred. The duplex stainless steels which contain various amounts of the alloying elements chromium, nickel and optionally molybdenum are characterized by a mixed microstructure with about equal proportions of ferrite and austenite (hence the common designator “Duplex”). The chemical composition based on high contents of chromium, nickel and molybdenum provides a high level of intergranular and pitting corrosion resistance. Additions of nitrogen promote structural hardening by interstitial solid solution mechanism, which raises the yield strength and ultimate strength values without impairing toughness. Moreover, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventional stainless steels. They are also notable for high thermal conductivity low coefficient of thermal expansion, good sulfide stress corrosion resistance and higher heat conductivity than austenitic steels as well as good workability and weldability. The duplex stainless steels are a family of grades, which range in corrosion performance depending on their alloy content. Normally, duplex grades such as 2304, 2205 will be adequate for heat exchanger service with the final selection to be made consistent with recognized corrosion resistance requirements. Which ever form of support device is used, the strip may be made up of two or more strips nesting closely against one another if additional thickness or modulus is required. It may become desirable in certain instances, for example, if forming the strips from titanium which resists deep forming operations, to confer the requisite depth on the strip (from the bottom of one saddle to the bottom of the opposing saddle) by forming the saddles slightly less deeply from thinner section strip and then superimposing two strips together to give the desired total thickness or saddle depth. So, in the case of the two-strip variant shown in
In the two-strip embodiment, an alternative means to provide an adjustment to the thickness of the support device is to place a shim plate between the two saddle strips and connect it to the two saddle strips by some mechanism as welding or riveting. The thickness of this shim strip can be varied as required to provide the correct dimension to span the channel in a manner to provide the needed support interference.
Insertion of the tube supports into the tube bundle may be facilitated by first inserting a metal bar with beveled edges having a thickness that is slightly greater than the total depth of the support (including the saddles or other raised zones) after which the support is inserted into place and the metal bar is slowly removed to ensure the proper locking in of the tubes and the tube support. The bar may also be used in a similar manner to facilitate removal of the supports. An alternative insertion technique uses an expandable hose which may be pressurized from inside to displace the exchanger tubes outwards while the support device is inserted near the hose. Suitable expandable hoses of this kind may be fabricated from an interior tube of a resilient polymer material such as nylon, rubber or other elastomeric material with a surrounding braided sleeve, e.g., of stainless steel or nylon, for improved regularity of operation and increased safety. The hose, which is preferably flat in its unpressurized state, has a diameter (or a thickness in the case of flat hose) chosen to be just less than the spacing between the exchanger tubes so that it can be inserted readily into a tube lane. The hose has one closed end with the open end being attached to a supply of pressurized fluid, either air, gas or liquid. In one form, the open end can simply have a union or connector enabling the hose to be connected to the fluid source and, later on, deflated or depressurized. In the case of a hose intended to be inflated by air pressure, for example, the connector may be in the form of a Schraeder connector. A pressure regulating valve should be included for safety reasons, to prevent overinflation. Alternatively, a hydraulic pump may be provided to form an integrated unit with its own dedicated pressurization. The hydraulic pump may be activated by hand, in the manner of a hydraulic jack or even by a motor if the additional complexity may be tolerated. Again, a pressure regulator may be provided for safety. In use, the closed end of the hose is slipped into the tube lane into which the support device is to be inserted and expanded by applying pressure to the interior; the hose expands outwards and displaces the tubes a small distance to facilitate the insertion of the support device, after which the pressure may be released to permit the hose to resume its normal diameter or thickness so that it may be withdrawn out of the tube lane, leaving the support device in place, engaged by the tubes on either side of the tube lane. The supports may be inserted at axial locations determined by experience or by vibration studies for the relevant equipment.
With the back-to-back form of construction, the tube-engaging zones can be formed by a single pressing operation in the transverse direction, fabricating several rows of saddles at a time, with successive pressings along the length of the support, in a simple press with a low pressing force. The use of two press rolls would, of course, represent the most economical option for large-scale manufacture but is not necessary and cheaper, simpler equipment could be used failing access to greater resources. The pressings can then be fastened together to form the final support. The unitary, slitted, formed strips will normally be made in two operations, first by punching out the slits and second by forming the saddles using a press with opposed dies. A single operation which will slit the strips, press out the opposing tube-engaging zones and form the saddles is not, however, excluded if suitable equipment is available. One advantage of the present tube supports of either type described above is that they can be formed by a simple pressing operation on a flat metal strip, without the necessity to make three-dimensional pressings. The tube-engaging zones are formed by a simple, lateral forming operation which does not require pressing the saddles into any complicated sections such as V-sections or channels.
This application claims the benefit of U.S. Ser. No. 60/580,984 filed Jun. 18, 2004.
| Number | Date | Country | |
|---|---|---|---|
| 60580984 | Jun 2004 | US |
