Patent Publication Number: US-6701764-B2

Title: Method of expanding an intermediate portion of a tube using an outward radial force

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of creating an expanded region on a tube, and more specifically, to a method of using an inserted material to create an expanded region on cooling tubes for a catalytic combustor for a combustion turbine so that the cooling tubes maintain contact with one another and dampen vibration. 
     2. Background Information 
     Combustion turbines, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades, and therefore the shaft, to rotate. The shaft may be connected to a generator. 
     Typically, the combustor assembly creates a working gas at a temperature between 2,500 to 2,900 degrees Fahrenheit (1371 to 1593 degrees centigrade). At high temperatures, particularly above about 1,500 degrees centigrade, the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO 2 , collectively known as NOx. The formation rate of NOx increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NOx will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by premixing all of the fuel with all of the of air available for combustion (referred to as low NOx lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NOx production is reduced. 
     Lean pre-mixed flames are generally less stabile than non-well-mixed flames, as the high temperature regions of non-well-mixed flames add to a flame&#39;s stability. One method of stabilizing lean premixed flames is to react some of the fuel/air mixture in conjunction with a catalyst prior to the combustion zone. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or catalyst bed, causing a pre-reaction of a portion of the mixture and creating radicals which aid in stabilizing combustion at a downstream location within the combustor assembly. 
     Prior art catalytic combustors completely mix the fuel and the air prior to the catalyst. This provides a fuel lean mixture to the catalyst. However, with a fuel lean mixture, typical catalyst materials are not active at compressor discharge temperatures. As such, a preburner is required to heat the air prior to the catalyst adding cost and complexity to the design as well as generating NOx emissions, See e.g., U.S. Pat. No. 5,826,429. It is, therefore, desirable to have a combustor assembly that bums a fuel lean mixture, so that NOx is reduced, but passes a fuel rich mixture through the catalyst bed so that a preburner is not required. The preburner can be eliminated because the fuel rich mixture contains sufficient mixture strength, without being preheated, to activate the catalyst and create the necessary radicals to maintain a steady flame, when subjected to compressor discharge temperatures. As shown in U.S. patent application Ser. No. 09-670,035, which is incorporated by reference, this is accomplished by splitting the flow of compressed air through the combustor. One flow stream is mixed with fuel, as a fuel rich mixture, and passed over the catalyst bed. The other flow stream may be used to cool the catalyst bed. 
     One disadvantage of using a catalyst is that the catalyst is subject to degradation when exposed to high temperatures. High temperatures may be created by the reaction between the catalyst and the fuel, pre-ignition within the catalyst bed, and/or flashback ignition from the downstream combustion zone extending into the catalyst bed. To reduce the temperature within the catalyst bed, prior art included a plurality of closely-oriented, parallel cooling tubes. These cooling tubes were susceptible to vibration because they were cantilevered, being connected to a tube sheet at their upstream ends. The inner surface of the cooling tubes were free of the catalyst material and allowed a portion of the compressed air to pass, unreacted, through the cooling tubes. The fuel/air mixture passed over the tubes, and reacted with, the catalyst bed. Then, the compressed air and the fuel/air mixture were combined. The compressed air absorbed heat created by the reaction of the fuel with the catalyst and/or any ignition or flashback within the catalyst bed. See U.S. patent application Ser. No. 09-670,035. 
     The disadvantage of such cooling systems was susceptibility of the tubular configuration to vibration damage resulting from: (1) flow of cooling air inside of the tubes, (2) flow of the fuel/air mixture passing over the tubes transverse and longitudinal to the tube bundle, and (3) other system/engine vibrations. Such vibration has caused problems in the power generation field, including but not limited to: degradation of connecting joints (e.g. brazing of the cooling conduits to the tubesheet); deformations due to tube to tube or tube to support structure impacting; and premature ignition, known as backflash, which results from irregular and reverse flow around and through the cooling conduits. Moreover, vibration of the cooling conduits or tubes, must be eliminated to prevent insufficient cooling, improper fuel reactions and even physical damage to the structural elements of the combustor. 
     Nonuniform tube expansion and overall tube expansion has been achieved by mechanical methods as propelling a ball through the overall tube length, pressing a pointed die in the end of tube to flare the end, and expanding a collet within the tube body. Each of these prior methods of tube expansion has its own shortcomings and none can achieve localized, uniform expansion. The collet approach is limited in that uniform expansion is not achieved and localized cracking of the tube wall may result. Pressing a pointed die in the end of the tube, if exactly centered, can produce a simple conical flare at the end of a tube but cannot achieve more complex shapes such as bulges. Propelling a ball through the tube has been successfully used in overall tube expansion but is ineffective in localized bulging or flaring of tubes. 
     None of the existing methods of tube expansion can achieve the localized and uniform tubular expansions at an intermediate portion of the tube necessary to suppress vibration of the parallel cooling conduits within a catalytic combustor. 
     There is, therefore, a need for an effective method of making uniform, localized expanded regions, or “bulges,” on the intermediate portions of a cooling tube for a catalytic reactor assembly of a combustion turbine. 
     There is further a need for a method of assembling the catalytic combustor so that the plurality of bulged cooling tubes contact one another thus suppressing vibration and minimizing degradation of the assembly. 
     SUMMARY OF THE INVENTION 
     These needs, and others, are met by the instant invention, which provides a method to create uniform localized expansions on the intermediate portion of a cooling tube. In turn, the tubes, whether assembled so that the expansion on one tube contacts the expansions on adjacent tubes, or so that the expansions on one tube are staggered with respect to the expansions on adjacent tubes thus contacting the unexpanded regions of that tube, create a dampening device by maintaining tube to tube contact and minimizing vibration. 
     The preferred method of expanding tubes utilizes a combination of localized softening of the tube by applying an annealing heat treatment followed by internal pressurization of a fluid to create an outward radial force. One way of providing such internal pressurization is hydraulically, by filling a tube with hydraulic fluid, sealing it, and then applying pressure using a pump. To avoid cracking the tube from work hardening, this technique may be repeated multiple times, reannealing the tube, and gradually applying greater pressure with each iteration until the desired bulge is formed. Work hardening is the phenomenon in which steel hardens due to cold working or working the steel when it is cool or unannealed. As the steel stretches and hardens it becomes more susceptible to cracking thus necessitating reheating or reannealing between internal pressurization steps. To further refine the process and add precision to the shape and size of the bulges, the tube may be placed in a rigid die having a machined cavity corresponding to the desired bulge. 
     Alternative hydraulic pressure methods may be employed to bulge the tube. One such method would be to immerse a portion of an annealed tube which has been filled with water and sealed, into a cryogenic liquid, such as liquid nitrogen, until the water freezes. As the water freezes, the fluid water is compressed, thereby increasing pressure in the tube. Also, if the tube remains in contact with the cryogenic liquid, ice may form within the annealed portion of the tube. As the water freezes and expands, the annealed portion of the tube is expanded. Expansion could also be achieved by other methods of internal pressurization, including but not limited to pneumatic pressurization and heat treatment of a solid insert with a higher coefficient of thermal expansion. 
     This method of forming expanded regions may also be performed after the tubes are attached to the tube sheet. That is, an intermediate portion of each tube is first given an annealing heat treatment and then the tubes are attached to the tube sheet as is known in the prior art, forming a tube sheet assembly. Each tube has one end plugged and is then filled with water. The other end of each tube is then plugged. One end of the tube sheet assembly is then dipped in a cryogenic fluid. As the water in the tube freezes, the annealed portion of each tube will bulge until it contacts an adjacent tube. Thus, because the tubes expand to each other, the size of each expansion does not need to be rigidly controlled. 
     It is an object of this invention to provide a method of forming at least one generally uniform, localized expansion on a cooling tube for a catalytic combustor. 
     It is further an object of this invention to provide a method of forming various expansion lengths, widths and heights on a cooling tube for a catalytic combustor. 
     A still further object of this invention is to provide a method of assembling a catalytic combustor assembly so that the cooling tubes, having an expanded region, contact one another, thus suppressing vibration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is an isometric view showing a catalytic combustor having six modules housing a plurality of cooling tubes disposed about a central axis in a generally hexagonal orientation. 
     FIG. 2A is a side view of a pair of cooling tubes for a catalytic combustor module each having expanded regions which are structured to contact each other. FIG. 2B is a side view of a pair of cooling tubes for a catalytic combustor module having staggered expanded sections so that the expanded regions on one tube contact the narrow regions on an adjacent tube. 
     FIGS. 3A-3D show one embodiment of the present method. More specifically, FIG. 3A shows the tube being annealed, FIG. 3B shows a tube plugged on one end being filled with a fluid, FIG. 3C shows a tube plugged on both ends being inserted into a die, and FIG. 3D shows the tube connected to a pump. 
     FIGS. 4A-4C show another embodiment of the present method. More specifically, FIG. 4A shows the tube being annealed, FIG. 4B shows a tube plugged on one end being filled with water, FIG. 4C shows a tube plugged on both ends being inserted into a cryogenic liquid bath. 
     FIGS. 5A-5C show another embodiment of the present method that may be used on a core. FIG. 5A shows the tubes being annealed. FIG. 5B shows plugged tubes coupled to a tube sheet being filled with water while the side walls, the inner shell and the inner wall of the core are attached to the tube sheet. FIG. 5C shows the core after being dipped into a cryogenic liquid bath. 
     FIGS. 6A-6C show one embodiment of the present method. More specifically, FIG. 6A shows a mass of solid material having a different coefficient of thermal expansion being inserted into the tube. FIG. 6B shows both the tube and the mass being heated. FIG. 6C shows the tube in a die/vacuum chamber. 
     FIG. 7A shows the method of tube expansion employing a laser to anneal a local region in the form of a narrow ridge along the longitudinal axis of the tube. FIG. 7B also shows the tube disposed in a die/vacuum chamber. 
     FIG. 8 is a schematic of the tube disposed in an axial centrifuge. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, a catalytic reactor assembly is often separated into modules  50  that are disposed about a central axis  100 . Each module  50  includes an outer shell  24 , an inner shell  26 , a tube sheet  28 , a fuel inlet  37 , an inner wall  32  and sidewalls  52 ,  54 . A plurality of cooling tubes  30  are enclosed by inner shell  26 , inner wall  32  and sidewalls  52 ,  54 . The tubes  30  have a first end  46 , an intermediate portion  47 , and a second end  48  (FIG.  2 ). As used herein, the intermediate portion  47  is located anywhere between, and spaced from, the first and second ends  46 ,  48 . The rhomboid tube sheet  28  is coupled to the inner shell  26 , inner wall  32  and sidewalls  52 ,  54  of the upstream end of the module  50  by a fastening process (e.g. brazing). The tube sheet  28  is perforated and supports a plurality of cooling tubes  30 . The tube sheet  28 , the tubes  30 , the inner shell  26 , the inner wall  32  and sidewalls  52 ,  54  form the core  56  of the module  50 . As shown, six modules  50  form a generally hexagonal cluster about the central axis  100 . Of course, any number of modules  50  of various shapes could be used. 
     The outer shell  24  and the inner shell  26  form a first plenum. This plenum is open to an air source. Typically, the catalytic reactor assembly  1  is part of a combustor assembly for a compressor-turbine. The combustor assembly is in fluid communication with compressed air from the compressor. A portion of compressed air flows through the first plenum. Fuel lines  37  supply fuel to the first plenum. When the fuel is mixed with air in the first plenum, a fuel/air mixture is created. 
     The inner shell  26 , sidewalls  52 ,  54 , inner wall  32 , and the tube sheet  28  form a fuel/air plenum. The cooling tubes  30  extend through the fuel/air plenum. The inner surface of the fuel/air plenum, including the outer side of the cooling tubes  30 , is coated with a catalytic material  30   a  (FIG.  2 ). The first plenum and the fuel/air plenum are in fluid communication. In operation, the fuel/air mixture travels from the first plenum to the fuel/air plenum, where the fuel reacts with the catalytic material  30   a . Another portion of compressed air from the compressor passes through the cooling tubes  30  and absorbs heat from the catalytic reaction. 
     To minimize vibration of the cooling tubes  30 , a vibration dampening device  120 , as shown in FIG. 2, can be used. The dampening device  120  consists of at least one expanded region  140  and at least one narrow region  160  on one or more of the tubes  30 . The narrow region  160 , in most of the embodiments, is simply the unexpanded part of the tube  30  or the nominal tube circumference. The expanded region  140  permits the plurality of closely oriented and parallel tubes  30  to remain in contact with one another, thus suppressing vibration. The expansion region  140  may have different shapes, as detailed below. In a first embodiment of the apparatus, the expansion region  140  may be a localized expansion  130  of the nominal tube circumference with a gradual transition region  135  between the nominal tube circumference and the center of expansion. As used herein, a “localized expansion” indicates that the entire circumference, or outer periphery, of a tube is expanded as opposed to just a portion of the circumference. The gradual transition  135  and subtle expansion profile  130  are necessary to promote even flow through the module  50  and prevent an excessive pressure drop. The expanded region  140  does not extend the entire length of the tube  30  and there may be more than one expanded region on each tube  30 . The catalyst material  30   a  may cover the entire tube  30  or only the narrow regions  160 , in which case the contacting expanded regions  140  need not coated. Because the tubes  30  contact each other at the expanded regions  140 , the expanded portions at the point of contact are not exposed to the fuel/air mixture. As such, it may be more cost efficient to not coat the expanded regions  140  with the catalyst material  30   a.    
     Each tube  30  may have an expansion  140  at the intermediate portion  47  and an expansion  140  at the tube second end  48 , which is the downstream end. Both expansions  47 ,  48  help to generate the desired flow path around the tubes  30  and the desired minimal pressure drop within the module  50 . The tubes  30  downstream ends  48  are expanded and each of the expanded regions  140  of one tube  30  contact the expanded regions  140  of the adjacent tubes  30 . The catalyst  30   a  is only covering the unexpanded or narrow regions  160  of the tube  30 . A flow path  138  between the tubes  30  is created between the contacting localized expansions  130  of the tubes  30  at each location where the narrow region  160  of one tube  30  is opposite the narrow region  160  of the adjacent tube  30 . 
     As shown in FIGS. 3A-3D, the method to create the expanded regions  140  on each tube  30  is to prepare the tube  30  for expansion, insert a material  201  into the tube  30 , and then use the material  201  to exert a uniform, generally outwardly radial force until a portion of the prepared tube  30  bulges to form the expanded region  140 . There are at least three procedures by which the material  201  can be used to create the uniform, generally outwardly radial force. First, while using a sealed tube, a fluid material  401  can be pressurized to increase the pressure inside the tube  30 . Second, again with a sealed tube and when the material is water  402 , the water  402  may be frozen. Because water  402  expands as it freezes, the volume inside the tube will increase. Third, a solid material  500  having a greater coefficient of thermal expansion may be placed in the tube and heated. 
     Preparation of the tube includes the steps of locally softening a first region  140   a  on the intermediate portion  47  of the tube  30  by applying an annealing heat treatment. The heat treatment is applied by a heat source  200 , typically a flame. When a fluid material  401  is used, the tube  30  must be sealed with a first and second plug  146 ,  148 . Thus, after annealing the tube  30 , one end of the tube  30  is plugged with a first plug  146 . Next the fluid material  401  is inserted into the tube  30 . As shown in the figure, the fluid material  401  is a liquid, however, the fluid material  401  may also be a gas. The fluid material  401  is selected from the group including air, water, hydraulic fluid, and non-Newtonian fluids. After the fluid material  401  is inserted, the second plug  148  is then placed on the tube  30 . Either the first plug  146  or the second plug  148  includes a valve means  149 . At this point the tube  30  is prepared. 
     In a first embodiment of the method, the tube  30  is expanded by pressurizing the fluid material  401 . As shown in FIG. 3C, the tube  30  is inserted into a rigid die  700  having a machined cavity  750  corresponding to the desired size and shape for the expanded region  140 . That is, where a localized expansion  130  is desired, the cavity  750  extends around the entire tube  30 . Where, as detailed below, an expanded region  140  having a different shape is desired, the cavity  750  may be machined to that shape. The rigid die  700  permits precise shape and dimension of the expansions  140  as well as uniformity among the expansions  140  of different tubes  30 . A pump  600  is attached to the valve means  149 . Thus, the pump  600  is in fluid communication with the fluid material  401  in the tube  30 . Next, the pump  600  is used to increase the pressure within the tube  30 . As the pressure within the tube  30  increases, the first region  140   a  which has been annealed is expanded. The entire process, the heating step, and/or just the pressurizing step, can be repeated as many times as necessary to gradually form the desired expanded region  140  while avoiding cracking the tube  30  due to work hardening. By way of example, a ten inch long catalytic combustor cooling tube  30  having a nominal initial tube diameter of 0.187 inch requires repetition of this method two times to expand the tube  30  from the initial diameter to a desired expanded region  140  diameter of 0.244 inch. This is the necessary amount of expansion required for the expanded region  140  of one tube  30  to contact the expansions  140  of the adjacent tubes  30 . 
     A second embodiment of the method is shown in FIGS. 4A-4C. The tube  30  is prepared as before, that is, heated and plugged and filled as shown in FIGS. 4A and 4B. In this embodiment, the tube must be filled with water  402 , as opposed to other fluid materials. Also, in this embodiment, the second plug  148  does not need a valve means  149 . After the tube  30  is filled with water  402  and plugged, the tube  30  is partially or entirely immersed in a cryogenic fluid bath  450  for several minutes until the water  402  freezes (FIG.  4 C). Preferably, the cryogenic fluid is liquid nitrogen. The water  402  has a coefficient of thermal expansion which is different from the tube  30 . As the water  402  freezes, the ice expands. Initially, the expansion of the ice increases the pressure of the water  402  in the tube  30 . As the pressure increases, the first region  140   a  on the tube  30  expands. This can be accomplished by partially submerging the tube  30  in the cryogenic fluid bath  450 . Alternatively, the tube may be left partially submerged, or may be entirely submerged, in the cryogenic fluid bath  450  until ice forms within the first region  140   a . As the ice expands in the first region  140   a  an outwardly radial force is created which expands the softened region  140   a  of the tube  30  to form the desired expanded region  140 . After the expansion process is complete, the plugs  146 ,  148  are removed and the water  402  is emptied. This embodiment has the advantage of not requiring a pump  600 . 
     The embodiment of this method using water/ice may also be practiced where the tubes  30  are connected to a tube sheet  28 . As shown in FIGS. 5A, the tubes  30  are coupled to the tube sheet  28  as is known in the prior art. As shown in FIGS. 5A and 5B, the tubes  30  are prepared as detailed above, that is, annealed, filled with water and plugged. As shown in FIG. 5B, the tube sheet  28  may then be coupled to the inner shell  26 , the inner wall  32 , and the side walls  52 ,  54  to form a core  56 . Thus, core  56  forms an enclosure, having one open end, around the tubes  30 . As shown in FIG. 5C, the core  56  is then dipped, either partially or entirely, into a cryogenic fluid bath  450 . As the water freezes, the pressure within the tubes  30  is increased, the soft first region  140   a  expands. The soft first region  140   a  on each tube will expand until the soft first region  140   a  contacts another tube  30  or the inner shell  26 , the inner wall  32 , and/or the side walls  52 ,  54 . As such, when using this embodiment of the method, the size and shape of the expanded regions  140  do not have to precisely match each other as contact between the tubes  30  is assured during the freezing process. After the expansion process is complete, the plugs  146 ,  148  are removed and the water  402  is emptied. 
     In another embodiment of the method, shown in FIGS. 6A and 6B, the tube  30  is heated, as before, and a mass of solid material  500  having a coefficient of thermal expansion which is different from, that is, greater than, the coefficient of thermal expansion of the material used to form the tube  30  is inserted into the tube  30 . As used herein, the word “solid” refers to the state of matter of the material, as opposed to the geometry of the mass  500 . That is, the solid mass  500  may have any shape, e.g. a hollow cylinder, so long as the material is a solid. The mass of solid material  500  is shaped to fit snugly within the tube  30 . Typically, the tube  30  is made from steel or a steel alloy. The solid material  500  may be a bimetal  502 , like NiTi, or a memory metal. The solid material  500  is placed within the soft first region  140   a . The solid material  500  and the soft first region  140   a  are then heated by a heat source  506 . Because the solid material  500  has a greater coefficient of thermal expansion, or, in the case of a shape memory, an inclination to change to an alternate shape when heated, the solid material expands more than the tube  30  creating an outward radial force. Accordingly, the soft first region  140   a  forms an expanded region  140 . This embodiment of the method may also utilize a vacuum chamber  550  to assist in bulging the expanded region  140 . That is, the tube  30  could also be sealed and placed in a vacuum chamber  550 , as shown in FIG.  6 B. Furthermore, as shown in FIG. 6C, the vacuum chamber  550  may include a die  560  to precisely control the expanded region  140  size, location and shape. 
     This method may also be used to form expanded regions  140  having a shape other than a circumferential localized expansion  130 . For example, as seen in FIG. 7A, during the preparation of the tube  30 , a laser  800  may be used to anneal a local portion  840  of the tube  30 . As used herein, a “local portion” is a relatively thin area, such as an arc of about ten degrees, extending, generally, in the axial direction. Following the annealing step, the tube  30  is sealed and a pump  600  is attached at the inlet valve  149 . As with the embodiment shown in FIG. 6C, the tube  30  is then be placed in a vacuum chamber  550 , having a die  560 . Here the die cavity  562  corresponds to the desired ridge like shape for the expanded regions  140 . As the pump  600  increases internal pressure, the vacuum chamber  550  reduces ambient pressure thus causing the annealed region  840  to expand. 
     As shown in FIG. 8, the radial force used to expand the tube  30  may also be created by centrifugal force. That is, after the tube  30  is prepared, the plugs  146 ,  148  are attached to a axial rotating device  900  that spins the tube  30  about the longitudinal axis of the tube  30 . The tube  30  would be attached to a rotating device  900 , such as the electrical motors  902 , using extended cuffs  904  so that only the soft first region  140   a  to be expanded would be exposed. That is, a cuff  904  is located on at least one side of the first region. The mass of the exposed tube  30  along with the mass of the material  201  within the tube  30 , when rotated at high rpm and subjected to sufficient centrifugal forces causes the first region  140   a  to yield outwardly, forming an expanded region  140  in the tube  30 . 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, similar processing methods could be applied to geometries other than circular tubes  30  such as square tubes or rectangular tubes or even to items other than tubes such as spheres or boxes that could be locally heat treated and pressurized. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.