Patent Number: 043205283
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the drawings of the invention in detail and more particularly to FIG. 1, there is shown at 10 a steam generator or heat exchanger. The external shell or envelope 12 of said heat exchanger is a pressure vessel. Inside this external shell 12 are a large number of tubes 14 which are the tubes that carry the primary fluid within the primary system of said heat exchanger. Said tubes 14 pass through support plates 16 which are located along the length of said tubes 14 and which encircle each tube 14 so as to form a means for separating one tube from the next and allowing each tube to remain in a fixed position within the tube bundle. Said support plates 16 are in turn contained within a cylindrical iron wrapper 18. The tubes 14 are typically made of a nickel alloy such as inconel, and number on the order of 10,000, although the configuration of the heat exchanger and corresponding number of tubes will vary from manufacturer to manufacturer. The tubes 14 usually range from 5/8 inch to 7/8 inch in outer diameter and are approximately 50 mils in thickness. The support plates 16, in most current heat exchangers, are made of carbon steel and are approximately 3/4 inch to 1 inch thick. The tubes 14 are connected at their bottom end to an apertured plate or tube sheet 20. In normal operation, the primary fluid 2 comes from a heat source such as a nuclear reactor and enters said heat exchanger 10 through a primary entrance nozzle 24. The fluid enters into the area between the bottom of the pressure vessel external shell 12 and the tube sheet 20. A separating wall 22 separates the inlet side 25 of the heat exchanger 10 from the outlet side 27. The primary fluid 2 which comes from a heat source such as a nuclear reactor carries heat with it as it is forced through the various tubes 14 and up through the heat exchanger 10. The heat exchanger 10 illustrated in FIG. 1 is of the U-bend type, where the tubes 14 run most of the length of the heat exchanger 10 and are bent at the top to form a U-shaped configuration. The U-shaped tubes 14 are attached at their bottom to the tube sheet 20 which is mounted to the back of the external shell 12 of the heat exchanger 10, and thereby define the primary system of the heat exchanger 10. Upon reaching the uppermost portion of the tubes 14, the primary fluid 2 starts back down the opposite side of the tubes 14 and exits the heat exchanger 10 through the primary outlet nozzle 26 on the outlet side 27 of the heat exchanger 10. Heat which is carried by the primary fluid 2 is transferred to the secondary fluid 4 while the primary fluid 2 is circulating through tubes 14. Said secondary fluid 4 enters the heat exchanger 10 through secondary inlets 42 and 44 located in the external shell 12 and is located in the area surrounding said tubes 14 and within the external shell 12. Sufficient heat is transferred to the secondary fluid 4 so that the primary fluid 2 exiting the primary outlet nozzle 26 is at a substantially lower temperature than it was when it entered the heat exchanger through primary inlet nozzle 24. The secondary fluid 4 absorbs heat carried by the primary fluid 2 and said secondary fluid 4 becomes steam during the heat absorption process. Said steam passes through separators 30 which remove excess moisture from said steam, and then exits through the steam outlet 32 at the top of the heat exchanger 10. The high pressure steam can then be used to drive a turbine. The secondary fluid 4, secondary inlets 42 and 44, separators 30, and steam outlet 32 define the secondary system of the heat exchanger 10. The primary fluid 2 can be water. A gas such as helium or another liquid such as liquid sodium can also be used for the primary fluid. The secondary fluid 4 is usually water. Referring to FIG. 2, said support plates 16 contain apertures or crevices 38 through which said tubes 14 run. It is at the site of the apertures or crevices 38 that one of the problems which the present invention is intended to solve first occurs. In those heat exchangers in which the support plates 16 are made of steel, the elevated temperatures and water environment promote the oxidation of the support plates 16 and magnetite is formed from the steel on the exposed surfaces. As previously described, magnetite, which is a ceramic material and is relatively "spongy", occupies a greater spatial volume than the steel which has been oxidized to form the magnetite. As shown in FIG. 3, as the steel support plate 16 is oxidized to magnetite 40, and the magnetite 40 builds up at the area where the tubing 14 is surrounded by the support plate 16, the crevice or aperture 38 between the support plate 16 and tubing 14 is reduced, and magnetite 40 eventually fills the aperture 38 between the support plate 16 and the tubing 14. As further shown in FIG. 3, the phenomena known as "denting" or "pinching" takes place. The tubing 14 is constricted by the increasing volume of the magnetite 40, and can be damaged and/or cracked. The movement of fluid through the tubing 14 can be substantially impeded at the site of this restriction. Although magnetite 40 will also be created on other surfaces of support plate 16, conventional cleaning methods, such as those described by M. F. Obrecht, et al in his paper, Supra might be satisfactory to handle the problems at these other areas on the support plates 16. The magnetite 40 within the aperture 38, which causes the denting and deformation of tube 14, is not easily susceptible to the cleaning methods disclosed in the prior art. The chemical solvents cannot easily reach into this area. Conventional chemical cleaning methods utilizing more or less accepted chemical cleaning formulations are so slow as to endanger the integrity of the heat exchanger system. If these chemicals are left long enough to be effective against the magnetite, they will also attack the basic structural elements of the heat exchanger as well. Conventional chemical methods known in the prior art are also ineffective in removing the magnetite 40 at the aperture 38 because the cleaning fluid cannot be adequately circulated or agitated to continually bring a fresh supply of cleaning fluid to the site to be cleaned. The present invention involves the process of and apparatus for removing the buildup of products of corrosion, oxidation, sedimentation, and comparable chemical reactions from various portions of heat exchanger systems such as the location wherein the primary heat exchanger tubes come in contact with support plates for those tubes. The process involves immersing the surfaces to be cleaned in a chemical solvent capable of attacking said buildup of products of corrosion, oxidation, sedimentation and comparable chemical reactions at a relatively slow rate. The solvent is then heated to desired temperatures adjacent said surfaces to be cleaned. Finally, the process involves generating a source of sonic energy to be used in conjunction with said chemical solvent and directing said sonic energy through said chemical solvent and to said surfaces to be cleaned at specific frequencies whereby cavitation of said sonic energy is combined with said chemical solvent so as to enhance and accelerate the removal of said buildup of products of corrosion, oxidation, sedimentation and comparable chemical reactions. The present invention solves the problem of removing the magnetite 40 from the apertures or crevices 38 between said support plates 16 and said tubes 14. Referring to FIG. 4, a chemical solvent 80 is placed inside the heat exchanger 10 and within the exterior shell 12. Sufficient chemical solvent is put into the heat exchanger to cover said tubes 14 and said support plates 16, as shown in FIG. 4. One chemical solvent which can be used is the combination of 8% solution of sodium salt of ethylenediaminetetracetic acid (EDTA), plus 4% solution of citric acid plus an effective amount of a standard corrosion inhibitor (such as 0.6% of OSI-1 corrosion inhibitor sold by Halliburton Services). Said chemical solvent 80 can be heated to a desired temperature, which is between 120.degree. F. and 220.degree. F. A preferred heating method would be the utilization of the primary circulating system to circulate a heated fluid through the tubes 14 until the solvent has reached its desired temperature. Once achieved, that temperature can be maintained by adding heat through the primary system. Alternatively, the chemical solvent 80 can be heated externally and then the heated solvent 80 can be added to the secondary system inside the heat exchanger 10. This method is less desirable than the preferred method because it requires the heating and circulating of a potentially hazardous and corrosive substance. Further, utilizing a benign heating fluid through the tubes 14 in the primary system provides the additional benefit of inducing a convection flow of the chemical solvent 80 at the interfaces of the tubes 14 and the support plates 16. Care should be taken that the temperatures at the interfaces of the tubes 14 and the support plates 16 during the cleaning process does not exceed the desired levels since undue heating adversely affects the efficiency of the sonic cleaning process. Sonic energy is generated from transducers 50 which contain a face 51 and a rear portion 53. Referring to FIGS. 4 and 5, the preferred placement of the sonic transducers 50 is shown in the form of a ring 52 of such transducers encircling the wrapper 18 which in turn encircles the support plates 16 and tubes 14. The wrapper 18 significantly reduces the effectiveness of sonic energy generated by the sonic transducers 50. Further, a problem is created because the thin fluid layer of chemical solvent 80 which is trapped between the transducer face 51 and the wrapper 18 cavitates or boils due to the heat generated by the transducer 50 and this is turn decouples the transducer 50 from the wrapper 18. This problem is solved by either of the following means. The first and preferred method shown in FIG. 5b, involves placing a thin layer of high boiling point fluid 90 between the transducer face 51 and the wrapper 18. The fluid 90, such as oil, can be placed in a container 92 such as a flexible plastic bag which is approximately 1/8 inch thick, and will remain in place by pressure between the face of the transducer 50 and the wrapper 18. The combination of this coupling fluid 90 and the container 92 for the fluid 90 should have the same acoustic impedance as the metal wrapper 18 in order to have good sonic transmission. The transducers 50 are held firmly against the fluid filled container 92 or metal wrapper 18 by mechanical means such as a support wedge 99 placed between the rear portion of the transducer 53 and the internal vertical portion of the shell 12, or by direct mechanical or magnetic attachment to the metal wrapper 18. In the second method, shown in FIG. 5a, windows 94 whose dimensions are approximately the size of the transducer face 50 are cut in the wrapper 18 portion in front of each transducer 50. After the cleaning process has been completed, these windows 94 are sealed by replacing the metal removed on cutting the window 94 in the wrapper 18 and welding the piece of metal back in place. When the windows 94 are cut slightly smaller than the face of the transducer 51, the transducer can be held in place against the metal wrapper 18 by direct mechanical or magnetic attachment to the metal wrapper 18, or by mechanical means such as a support wedge 99 placed between the rear portion of the transducer 53 and the internal vertical portion of the shell 12. When the window 94 is cut slightly larger than the face of the transducer 51, part of the transducer 50 can be placed through the metal wrapper and will remain in place in this fashion. The ring 52 of transducers 50 is energized to radiate sonic energy in the frequency spectrum between 2 KHZ and 200 KHZ. The choice of these frequencies permits improved coupling of the sonic energy into the chemical solvent 80 and to the sites of interest such as the aperture 38 between the support plates 16 and tubes 14. The optimum cleaning interval for any heat exchanger can be experimentally determined, but it is believed that approximately 24 hours of sonic irradiation should be adequate to clean the first or uppermost plate. Sonic irradiation can be extended for longer periods as necessary. Results of experimental tests have shown that over a 24 hour cleaning period negligible adverse effects from the chemical solvent 80 are experienced by the other components. Some experiments suggest that the cleaning process may be accomplished in somewhat less time and, in any given heat exchanger, it may be possible to visually observe the progress of the cleaning, at least insofar as the uppermost support plate is concerned, since it might be subject to visual monitoring. As each plate 16 is cleaned, the fluid level is dropped as is the ring 52 of transducers 50 and the process is repeated. This procedure, has, however, the effect of exposing at least the lower portions of the vessel to the chemical solvent for longer periods of time. In view of the longer, but "passive" exposure to the solvent, as one proceeds toward the bottom of the tank the period of time during which the sonic transducers are operated at each fluid level is progressively reduced. It has been experimentally determined that using the chemical solvent 80 alone without sonic energy irradiation would require approximately 8 days to achieve a similar cleaning effect as is achieved by the present invention in only one day. Therefore, the adverse affects of the solvent 80 on the components of the heat exchanger are substantially reduced due to the significant decrease in time that the solvent 80 must remain inside the heat exchanger. The embodiment of the present invention described above requires the use of a ring of sonic transducers around the outer circumference of the metal wrapper 18 of the heat exchanger 10. As each support plate and tube is cleaned, the cleaning solvent level 80 is lowered to a few inches above the next support plate and the ring 52 of sonic transducers 50 is lowered to be in alignment with the next support plate to be cleaned, as shown in FIG. 5. A key point in this process is that the level of chemical solvent must be only a few inches above the surface area to be cleaned. If the level is much higher, the effectiveness of the sonic energy in creating the cavitation at the site to be cleaned is significantly reduced. In order to create cavitation at the site to be cleaned, the transducers must be able to generate a power output greater than about 0.2 watts per square centimeter at room temperature. This power density limitation on the transducers is demonstrated in the textbook "Sonics--Techniques For The Use Of Sound And Ultrasound In Engineering And Science, by Theodor F. Huetter and Richard H. Bolt, Fourth Edition published in 1965," pages 228 to 232. Referring specifically to FIG. 6.13 on page 230 of said textbook, in order to produce cavitation in degassed water at room temperature, the transducer must generate approximately 0.2 watts per cubic centimeter. As shown by the chart, if the transducer has a power output greater than about 0.2 watts per square centimeter, cavitation will be produced over a broad frequency range. An alternative embodiment of the present invention is shown in FIG. 6 wherein the ring 52" of transducers 50" is wholly exterior to the heat exchanger 10 and is placed around the outer circumference of the external shell 12 of the heat exchanger 10. In this embodiment, the actual cleaning procedure would be substantially similar to that of the preferred embodiment described above except that the ring 52" of transducers 50" is mounted on the outside and must be "coupled" to the interior of the vessel. The heat exchanger 10 is filled with the chemical solvent 80 which is heated to the desired temperature. The ring 52" of transducers 50" is placed at the height of the uppermost support plate 16 and is energized. The sonic energy is transmitted to the interior through a sonic coupler 58 which may include a fluid held in place by seals 60. As each support plate and tube is cleaned, the cleaning solvent level 80 is lowered to just above the next support plate and the ring 52" of sonic transducers 50" is lowered to be in alignment with the next support plate to be cleaned. The patent to Ostrofsky, U.S. Pat. No. 3,295,596 illustrates a particular coupler apparatus which would be employed. The embodiment of the present invention is designed to be used with those heat exchangers where interior access is either severely limited or is considered too hazardous. The rings 52, 52" of transducers 50, 50" can be successively repositioned in the vertical direction during the cleaning process. At each repositioning, the fluid level is lowered to a height above the transducer ring sufficient to support and maintain the efficient transmission of sonic radiation to the surfaces to be cleaned. As shown, the tubes and plates are cleaned in increments. It may be sufficient that each increment includes one of the support plates and that a suitable interval of time is employed to irradiate the plate. The time required for each of the plates can, of course, be experimentally determined. However, it is believed that although the sonic energy is primarily directed at a particular plate and its tube intersections, the adjacent plates will also benefit from the sonic energy and the cleaning of those plates will proceed, as well. The time required for the later increments may be progressively less, so that by the time the lowermost plate is reached, the required cleaning time for this plate will be substantially less than for the others. The total time during which the lowermost portions of the heat exchanger have been immersed in the solvent bath will, nevertheless, be substantially less than required through the use of solvents alone. Because the cleaning action of the solvent 80 is intensified, it is possible to use a chemical solvent at greater concentrations for shorter cleaning time. Depending upon the construction of the heat exchanger and the materials used in its fabrication, some optimum combination of solvent strength and cleaning time can be devised to minimize the unwanted effects of the solvents on the structural components. Many of the special fluid properties necessary to maximize the efficiency of the sonic cleaning process, can be achieved in the compounding of the chemical solvent. The solvent should be active at relatively low temperatures (below 200.degree. F.) and be substantially immune to the effects of sonic cavitation. Further, the solvent should optimize those properties which support high cavitation energy levels such as high surface tension, low vapor pressure and low viscosity. The utilization of sonic energy in the cleaning process not only has a direct effect on the scale, corrosion products and magnetite, but also enhances the effect of the chemical solvent by agitating and circulating the solvent in the regions being cleaned. This agitation tends to carry away "saturated solvent" and waste products, and brings fresh solvent to the region so that the solvent does not lose its effectiveness. While the process of cleaning the particular surfaces of the heat exchanger has been described, the presence of the sludge pile, and its effect on the cleaning process has not been considered heretofore. Because the sludge pile does contain a large quantity of loose sediment, magnetite, copper and other corrosion products, the fluid agitation caused by the sonic cavitation may stir up the sludge and its presence may actually interfere with the cleaning action of the solvent upon the structural parts. It may therefore be desirable to initiate a preliminary cleaning process in an attempt to remove the sludge pile before any other cleaning is attempted. For this operation, it may be preferable to have transducers mounted to the exterior shell 12 of the heat exchanger 10 and to use a fairly concentrated and relatively strong chemical solvent which just covers the sludge pile only and is not brought in contact with the remaining structural elements. Is is also possible that through the application of sonic energy alone, the sludge pile can be "stirred up" sufficiently to enable a flushing operation to carry away a substantial portion of the sludge pile, without the need for chemical solvent action. If the removal of the sludge pile is not to be undertaken, it may be necessary to provide some physical isolation of the sludge pile from the cleaning solvent so as not to contaminate and/or neutralize the chemical solvent before it has had a change to work on the structures to be cleaned. In this event, it may be necessary to provide a blanketing layer of an appropriate liquid which will effectively isolate the sludge pile from the chemical solvent bath. Another alternative embodiment of the present invention is shown in FIG. 7 wherein the ring 52' of transducers 50' are placed inside the heat exchanger 10 and inside the metal wrapper 18, and over the bundle of and substantially parallel to the tubes 14. The effectiveness of this placement may be limited if the vessel is quite deep. Very deep vessels might not be optimally served. However, for those heat exchangers in which the embodiment can be successfully employed, it offers the advantages of both easier installation and removal. Turning next to FIG. 8 and FIG. 9, there is shown an additional alternative embodiment of the present invention. As shown, individual sonic transducers 70 are placed within selected tubes 14 of the primary system. Energizing these transducers 70 can concentrate the sonic energy in the immediate vicinity of the tubes 14. By appropriate positioning of a transducer 70, along the axis of the tube, the energy can be successively directed to the deposits at each of the support plates 16, in turn. This application of the present invention can also be used to clean tubes which are badly corroded internally or which are dented. The cleaning of these tubes would prevent further tube damage and would eliminate the need to remove tubes from service by plugging them at the tube sheet 20. Access to the interior of the tubes can be achieved either from the manifold area at the primary inlet 24 and primary outlet 26, or, selected tubes can be cut and later repaired when the cleaning process has been concluded. These transducers 70 mounted interior to the tubes 14 could be employed in conjunction with other transducers which could be either mounted on the exterior wall 12 of the heat exchanger 10 or mounted on the metal wrapper 18 to operate in a cooperating and coordinated fashion. Alternatively, if relatively unrestricted access can be gained to the interior of the heat exchanger, some transducer elements can be attached directly to support plates 16. As shown in FIG. 8, it is also possible to utilize pressure sensitive transducers 72 at various locations within the vessel to determine the magnitude of sonic energy at selected locations. This monitoring capability can increase the efficiency of the cleaning process since the sonic transducers 70 can then be selectively or differentially driven to maximize the cleaning action at desired locations. Other variations and modifications will appear to those skilled in the art in terms of instrumentation, directing the sonic energy and using measurements of water pressure and frequency to determine the energy level at any given point within the heat exchanger system. Where time is a critical factor, as in the cleaning of the heat exchanger portion of a nuclear reactor, the present invention provides time savings that are appreciable and significant. Of course, the present invention is not intended to be restricted to any particular form or arrangement, or any specific embodiment disclosed herein, or any specific use, since the same may be modified in various particulars or relations without departing from the spirit or scope of the claimed invention hereinabove shown and described of which the methods shown are intended only for illustration and for disclosure of an operative embodiment and not to show all of the various forms of modification in which the invention might be embodied. The invention has been described in considerable detail in order to comply with the patent laws by providing a full public disclosure of at least one of its forms. However, such detailed description is not intended in any way to limit the broad features or principles of the invention, or the scope of patent monopoly to be granted.