Patent Application
20150129166
This invention relates to tube support devices, which are also called tube stakes that are inserted into tube bundles in heat exchangers and other fluid-handling equipment.
Heat exchangers and other fluid-handling equipment are often fitted with tube bundles that are fabricated by the attachment of a collection of tubes to one or two tubesheets. The tubes in the bundle may be arranged in two primary layouts: rectangular (also referred to as inline arrangement) and triangular (also referred to as staggered arrangement). The rectangular arrangement leads to tubes that are aligned vertically and horizontally while the triangular arrangement sets three adjacent tubes at the vertices of an equilateral triangle. The tube bundle may be fitted with a series of baffles that provides support to the tubes while also diverting the shellside flow across the tube bundle in a serpentine manner. The firm attachment of tubes to the tubesheets enables the use of two separate fluids: one on the shellside and the second on the tubeside. Such an exchanger, therefore, allows energy transfer across the tube wall thickness from one fluid to the other.
The shellside fluid, for the most part, flows orthogonal to the tubes in the bundle. Such a crosswise flow tends to create unstable eddies on the downstream side of some or all tubes. Under some conditions, such eddies may lead to vibration of the tubes. Depending on the intensity of these vibrations, some tubes may experience excessive displacements from their original (i.e., with no flow condition) locations and eventually lead to tube failures. Tube damage owing to flow-induced vibration can occur very quickly (several days or weeks) or take several years. Also, a properly designed exchanger may experience such failures owing to a recent increase in shellside flow rate, the flow conditions, tube wall corrosion. etc. Tube failures often require isolation of the exchanger for repair thereby causing costly downtime (e.g., production losses) and the associated repair costs as well as operational risks.
In their U.S. Pat. Nos. 7,032,655, 7,128,130, and 7,267,164, Wanni et al. provide background on the state-of-the-art tube support devices available on the marketplace for vibration mitigation in tube bundles. Those patents are incorporated by reference here for their disclosures where necessary to provide background to the present invention. Such prior art also discuss in detail the weaknesses of those technologies available in the marketplace. While the technologies presented by Wanni et al. solve most of the problems they had described, the new technologies proposed by Wanni et al. also have several limitations. First, the dimples located on one end of the tube support as well as the protrusions along the remaining length require substantial amount of metal deforming (starting from a flat thin strip of metal), especially, for the rectangular tube arrangement which has the widest tube lanes. As a result, proper forming of dimples would be difficult for harder metallurgies, such as Titanium and Duplex Steel, where penetrating cracks are difficult to avoid. Second, fabrication of tube support requires a very high pressing force (in excess of 200 tons representing expensive machinery), especially, when a single pressing operation is used. The use of multiple pressings along the length of the tube support decreases the required pressing force, but this lowers the technical quality of the stake and also increases the production time substantially. Third, fabrication of these tube supports requires the use of high-precision die sets that are very expensive and they will wear out and can be damaged with repeated use. Fourth, these tube supports cannot properly support those tubes that are located very close to the shell inner diameter when U-tube bundles are involved; this is because some of the tubes can be located only ⅛″ (3.2 mm) away from the shell inner diameter, but the dimples have a thickness of ⅜″ (9.5 mm) in this region. Fifth, the dimples at the outer end of the tube support do not provide an adequately robust locking with the nearby tubes so that the stake can pivot around this outer end and rotate under some conditions, especially when the tube support has a somewhat loose fit within the tube bundle. Sixth, the equipment used for the purpose of fabricating the tube supports with the dimples and protrusions does not lend itself easily to achieve an adjustable overall thickness for the tube support. Seventh, once the tube support is formed, its overall thickness, that determines the extent of vibration mitigation, cannot be adjusted in any way. This could lead to inadequate tube support in certain regions of a tube bundle or additional tube support must be used to achieve an acceptable level of vibration mitigation.
In their U.S. Pat. Nos. 7,506684 and 7,793,708, Wanni et al. provide an alternate stake design that is limited only to tube bundles having the rectangular tube arrangement.
In a basic form the present invention is directed to a tube support device for a tube bundle that has a plurality of elongated, longitudinally extending tubes arranged in rows, with tube lanes extending substantially perpendicularly to the tube rows and separating adjacent rows of tubes. The support device takes the form of an elongated, longitudinally extending tube stake inserted in one of the tube lanes.
The tube stake that is essential to my invention has at least one protrusion formed along its length, the protrusion having a forward end and a rearward end. The elongated stake is inserted in a tube lane so that a first tube is located at the forward end of the protrusion and a second, next adjacent tube of the bundle is positioned at the rearward end of the protrusion. The protrusion is sized so that, in tube contacting position, it exerts a force on the tubes that at least has a component that is substantially parallel to the longitudinal axis of the stake, so that the opposed faces move the contacted tubes away from each other and stress the tubes. In this manner the tubes are made more rigid and their vibration during the flow of fluid across them is reduced.
The support device, or tube stake, is the subject of the invention, as is the combination of the tube stake and the tube bundle. In both inventions the tube stake can be formed from a single strip of material, usually metal, or two strips of metal joined along their primary surfaces in back-to-back position, The protrusions can be pressed from a single strip of metal or welded or otherwise affixed to the surface of the tube stake. Whether the stake is a single or double strip, one or more protrusions can be formed on one or opposed surfaces of the stake and, if there are protrusions on opposed surfaces, the protrusions can be aligned in back-to-back position or, more advantageously, spaced alternately along opposed surfaces of the supporting device.
The tube support or stake device of the present invention provides direct and robust support to the tubes on both sides flanking the stake. In its preferred embodiment the stake has an effective thickness that is greater than the spacing between the tubes to account for the clearance between the tubes and the holes in the baffles as well as to account for the slight warping of the tubes. The length of the stake may be selected to satisfy the number of tubes requiring such support based on the estimated vibration damage potential. In fact, in some regions, such support may be needed across the entire tube bundle while some other regions may require support only to a depth of several tube rows. If necessary, the stake may extend across the entire bundle cross-section, but insertion of the stake would be somewhat easier if the stake length is limited to a maximum equal to the bundle radius. Since the stake deflects all of the tubes that it comes in contact with, for the most part stakes are inserted only in alternate tube lanes; however, if additional tube support is deemed necessary, on a rare basis, stakes play be inserted in adjacent tube lanes as well.
Preferably, the material for the stakes 10, 20, 40 and 70 may be selected as the same material as that of the tubes in the tube bundle. However, an alternate metallurgy may be used based on the recommendation of a materials expert.
According to the present invention, a tube support device or a tube stake (“stake”), which is useful in mitigating flow-induced vibration in tube bundles, comprises an elongated member of a strip or a pair of elongated members that are nested together, which is intended to be inserted in a tube lane between the tubes of a tube bundle in a heat exchanger or other similar equipment. The stake is fabricated by deforming or folding of a thin strip of metal in a manner each will provide one or two flat surfaces to come in contact with the tubes flanking the tube lane in which the stake is inserted. The overall thickness of the stake (a single strip or a pair of nested strips) is greater than the gap between tubes (i.e., the tube lane width) so that the insertion of the stake would result in a displacement of each tube in a direction orthogonal to the flat surfaces of the stake. Such displacements of the tubes provide stiffening and superior support to these tubes.
The stakes may be used in all tube configurations. Preferably, the 90- and 45-degree (also referred to as the rectangular and rotated rectangular tube arrangements, respectively) tube layouts shall use a stake consisting of a pair of nested strips even though a single-strip design is also viable. On the other hand, the 30- and 60-degree (also referred to as the triangular and rotated triangular tube arrangements, respectively) layouts shall use the single-strip design.
The present invention also provides a robust design for vibration mitigation in the U-bend region of tube bundles made of U-tubes as well. This design may also be used in new bundles as a low-cost and superior design alternative to the technologies used currently in the marketplace.
The stakes described in this invention may be fabricated by conventional pressing with a set of dies. Preferably, a pair of rollers may be used to conveniently form each strip so that simple equipment may be used with a minimum force necessary to carry out the needed forming action.
The present invention allows alleviation of most of the problems inherent in the technologies available on the marketplace. Namely, since the stake can be formed by primarily bending or folding and also uses a reduced pressing depth, cracking of the strip material can be avoided. The stakes are provided with special designs on the outer end to enable robust locking on to up to six tubes so that pivoting of the stake is virtually impossible. The stakes can be easily trimmed at the end so that they will not interfere with the shell inner surface when U-tube bundles are involved.
While the present invention has been described with respect to preferred embodiments thereof, it will be apparent to those of skill in this art that certain alterations and modifications of those embodiments without departing from the spirit of my invention, which is to be limited only to scope, including equivalents, of the following, appended claims.
