IMPROVED PRESSURE PULSE CLEANING METHOD

Abstract: Apparatus for loosening and removing sludge and other impurities from the interior of a heat exchanger vessel (5) which may be the secondary side (5) of a nuclear steam generator (1). The apparatus generally comprises a nozzle (111) having a first end that is detachably mountable into an access opening (53a, 53b) such as a sludge lance port (53a, 53b) in the secondary side of the generator, and a second end that extends into the interior of the secondary side (5) at a downward angle relative to the tubesheet (7) in the vessel (5) for minimizing the stresses applied to the heat exchanger tubes (22), and for uniformly reflecting the pulse back up toward the upper portions of the secondary side (5). The apparatus preferably further comprises a recirculation system (114) for purifying the water within the generator (1) while a pulse generator (60a, 60b) emits a succession of sludge and debris loosening pulses therein. An array of inlet and suction tubes (121a, 121b) are further provided so that the sludge and debris may be removed.

Description: This invention generally relates to a method of cleaning heat exchanger vessels, and is specifically concerned with an improved pressure pulse cleaning method for loosening and removing sludge and debris from the secondary side of a nuclear steam generator. BACKGROUND OF THE INVENTION Pressure pulse cleaning devices for cleaning the interior of the secondary side of a nuclear steam generator are known in the prior art, and have been disclosed in US-A- 4,655,846 and US-A- 4,699.665. Such devices generally comprise a gas-operated pressure pulse generator having an outlet that is mountable in communication with the interior of the secondary side of the steam generator. The purpose of these devices is to loosen and remove sludge and debris which accumulates on the tubesheet, heat exchanger tubes and support plates within the secondary side. In operation, the secondary side of the generator is first filled with water. Next the outlet of the gas-operated pressure pulse generator is placed into communication with the water, such as by a nozzle which may be formed from either a straight section of conduit oriented horizontally over the tubesheet of the generator, or a pipe having a 90 degree bend which is oriented vertically with respect to the tubesheet. Finally, pulses of gas pressurized to between 3,5 and 350 bar (50 and 5000 pounds per square inch) are generated out of the nozzle of the pressure pulse generator. The succession of pressure pulses create shock waves in the water surrounding the tubesheet, the heat exchanger tubes and support plates within the secondary side of the generator. These shock waves effectively loosen and remove sludge deposits and other debris that accumulates within the secondary side over protracted periods of time. While the cleaning devices disclosed in these patents represent a major advance in the state of the art, the applicants have found that there are limitations associated with these devices which limit their usefulness in cleaning nuclear steam generators. However, before these limitations may be fully appreciated, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary. In the secondary side of such steam generators, the legs of the U-shaped heat exchanger tubes extend through bores in a plurality of horizontally-oriented support plates vertically spaced from one another, while the ends of these tubes are mounted within bores located in the tubesheet. The relatively small, annular spaces between these heat exchanger tubes and the bores in the support plates and the bores in the tubesheet are known in the art as "crevice regions." Such crevice regions provide only a very limited flow path for the feed water that circulates throughout the secondary side of the steam generator. The consequent reduced flow of water through these regions results in a phenomenon known as "dry boiling" wherein the feed water is apt to boil so rapidly in the crevice regions between the heat exchanger tubes and the bores in the support plate and tubesheet that these areas can actually dry out for brief periods of time before they are again immersed by the surrounding feed water. This chronic drying-out of the crevice regions due to dry boiling causes impurities dissolved in the water to precipitate out in these regions. The precipitates ultimately create sludge and other debris which can obstruct the flow of feed water in the secondary side of the generator to an extent to where the steam output of the generator is seriously compromised. Moreover, the presence of such sludges is known to promote stress corrosion cracking in the heat exchanger tubes which, if not arrested, will ultimately allow water from the primary side of the generator to radioactively contaminate the water in the secondary side of the generator. To remove this sludge, many other types of cleaning devices were used prior to the advent of pressure pulse cleaning devices. Examples of such prior art cleaning devices include ultrasonic wave generators for vibrating the water in the steam generator to loosen such debris, and sludge lances that employ a high-powered jet of pressurized water to flush such debris out. However, such devices were only partially successful in achieving their goal due to the hardness of the magnetite deposits which form a major component of such sludges, and the very limited accessibility of the crevice regions of the steam generator. Since its inception, pressure pulse cleaning has been a very promising way in which to remove such stubborn deposits of sludges in such small spaces, since the shock waves generated by the gas operated pressure pulse operators are capable of applying a considerable loosening force to such sludges. However, the applicants have found that the devices disclosed in both US-A- 4,655,846 and US-A- 4,699,665 have fallen short of fulfilling their promise in several material respects. For example, research conducted by the applicants indicates that the orientation of the nozzle used to introduce the pulses of gas into the secondary side significantly affects the peak stresses applied to the tubes closest the nozzle, and that prior art nozzle geometries fell far short of minimizing these stresses. Still another shortcoming observed by the applicants was the lack of any means to remove dissolved ionic species from the water during such prior art cleaning processes. Such ionic species, if not removed, are capable of precipitating out in the form of new sludges after the termination of the pressure pulse cleaning process if no provision is made to remove them. Additionally, applicants observed that if no provision is made to remove fine particulate matter from the water during the pressure pulse cleaning method, these fine particles of sludge are capable of settling onto the tubesheet and densely depositing themselves into the crevice regions between the tubesheet and the legs of the heat exchanger tubes, thereby defeating one of the purposes of the cleaning method. Finally, the applicants have observed that the relatively rapid pulse frequency taught in the prior art does not give the nozzle and, manifold of the pulse generator sufficient time to fill back with water, and thus leaves pockets of shock-absorbing gas in the pulse generator which limits the efficacy of later generated pulses in generating sludge-loosening shock waves. Clearly, what is needed is an improved pressure pulse cleaning method which overcomes the limitations associated with prior art pressure pulse cleaning devices and which is imminently practical for use in the secondary side of a nuclear steam generator. DISCLOSURE OF THE INVENTION Generally speaking, the invention comprises a method of loosening and removing sludge and other impurities from the interior of a heat exchanger vessel of the type having one or more access openings that overcomes the limitations associated with the prior art. The method of the invention is defined in the appended claims. In operation, the canted end of the nozzles are spaced as far as possible from the nearest heat exchanger tubes so that the power level controller of the pulse generators can be adjusted to create shock waves in the water at the highest possible power level without generating pressures that would jeopardize the integrity of the heat exchanger tubes. When the method is used to loosen and remove the sludge and other impurities from the secondary side of a steam generator having a plurality of U-shaped heat exchanger tubes, the canted ends of the nozzles are aligned within the centrally disposed main tube lane. The canted tips of the nozzles and their orientation down the main tube lane in combination with the pulse flattener all allow the pulse generators to be operated at a maximum power level while exerting a minimum peak stress on the heat exchanger tube closest to the nozzle outlets. Moreover, the power level controller allows the operator to easily adjust the power of the pulses to compensate for power losses that result when the water level in the generator is raised. The method may further use a recirculation system having a pump for inducing a flow in the water and a particulate filter for removing loosened particles of sludge and debris from the water in the vessel. The recirculation system may further include a demineralizer bed for removing ionic species from the liquid in the steam generator. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a nuclear steam generator with portions of the exterior walls removed so that the interiors of both the primary and secondary sides may be seen; Figure 2 is a partial cross-sectional side view of the steam generator illustrated in Figure 1 along the line 2-2; Figure 3A is a cross-sectional plan view of the steam generator illustrated in Figure 2 along the line 3A-3A; Figure 3B is an enlarged view of the area circled in Figure 3A; Figure 3C is a cross-sectional side view of the portion of the support plate and heat exchanger tubing illustrated in Figure 3B along the line 3C-3C; Figure 4A is a plan view of a portion of a different type of support plate and tubing wherein trifoil broaching is used in lieu of circular bores; Figure 4B is a perspective view of the portion of the support plate and tubing illustrated in Figure 4A; Figure 5 is a cross-sectional side view of the steam generator illustrated in Figure 1 along the line 5-5 with an apparatus to perform the method of the invention installed therein; Figure 6A is an enlarged view of the pressure pulse generator assembly of Figure 5 along with a schematized representation of the pressurized gas source used to power the pressure pulse generator; Figure 6B is a cross-sectional side view of the air gun used in the pressure pulse generator; Figure 7 is a plan view of the steam generator illustrated in Figure 5 along the line 7-7; Figure 8 is a schematic view of the recirculation system used to implement the method of the invention; Figure 9 is a graph illustrating the diminishment of the pressure of the gas within the pressure pulse generator after the pulse generator is fired, and Figure 10 is a graph illustrating the relationship between the maximum stress experienced by the heat exchanger tubes in the steam generator, and the location of these tubes with respect to the tubesheet. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT General Overview of the Apparatus of the Invention With reference now to Figures 1 and 2, wherein like numerals designate like components throughout all of the several figures, the method of the invention is particularly adapted for removing sludge which accumulates within a nuclear steam generator 1. But before the application of the invention can be fully appreciated, some understanding of the general structure and maintenance problems associated with such steam generators 1 is necessary. Nuclear steam generators 1 generally include a primary side 3 and a secondary side 5 which are hydraulically isolated from one another by a tubesheet 7. The primary side 3 is bowl-shaped, and is divided into two, hydraulically isolated halves by means of a divider plate 8. One of the halves of the primary side 3 includes a water inlet 9 for receiving hot, radioactive water that has been circulated through the core barrel of a nuclear reactor (not shown), while the other half includes a water outlet 13 for discharging this water back to the core barrel. This hot, radioactive water circulates through the U-shaped heat exchanger tubes 22 contained within the secondary side 5 of the steam generator 1 from the inlet half of the primary side 3 to the outlet half (see flow arrows). In the art, the water-receiving half of the primary side 3 is called the inlet channel head 15, while the water-discharging half is called the outlet channel head 17. The secondary side 5 of the steam generator 1 includes an elongated tube bundle 20 formed from approximately 3500 U-shaped heat exchanger tubes 22. Each of the heat exchanger tubes 22 includes a hot leg, a U-bend 26 at its top, and a cold leg 28. The bottom end of the hot and cold legs 24, 28 of each heat exchanger tube 22 is securely mounted within bores in the tubesheet 7, and each of these legs terminates in an open end. The open ends of all the hot legs 24 communicate with the inlet channel head 15, while the open ends of all of the cold legs 28 communicate with the outlet channel head 17. As will be better understood presently, heat from the water in the primary side 3 circulating within the U-shaped heat exchanger tubes 22 is transferred to nonradioactive feed water in the secondary side 5 of the generator 1 in order to generate nonradioactive steam. With reference now to Figures 2, 3A, 3B and 3C, support plates 30 are provided to securely mount and uniformly space the heat exchanger tubes 22 within the secondary side 5. Each of the support plates 30 includes a plurality of bores 32 which are only slightly larger than the outer diameter of the heat exchanger tubes 22 extending therethrough. To facilitate a vertically-oriented circulation of the nonradioactive water within the secondary side 5, a plurality of circulation ports 34 is also provided in each of the support plates 30. Small annular spaces or crevices 37 exist between the outer surface of the heat exchanger tubes 22, and the inner surface of the bores 32. Although not specifically shown in any of the several figures, similar annular crevices 37 exist between the lower ends of both the hot and cold legs 24 and 28 of each of the heat exchanger tubes 22, and the bores of the tubesheet 7 in which they are mounted. In some types of nuclear steam generators, the openings in the support plates 30 are not circular, but instead are trifoil or quatrefoil-shaped as is illustrated in Figures 4A and 4B. In such support plates 30, the heat exchanger tubes 22 are supported along either three or four equidistally spaced points around their circumferences. Because such broached openings 38 leave relatively large gaps 40 at some points between the heat exchanger tubes 22 and the support plate 30, there is no need for separate circulation ports 34. With reference back to Figures 1 and 2, the top portion of the secondary side 5 of the steam generator 1 includes a steam drying assembly 44 for extracting the water out of the wet steam produced when the heat exchanger tubes 22 boil the nonradioactive water within the secondary side 5. The steam drying assembly 44 includes a primary separator bank 46 formed from a battery of swirl vane separators, as well as a secondary separator bank 48 that includes a configuration of vanes that define a tortuous path for moisture-laden steam to pass through. A steam outlet 49 is provided over the steam drying assembly 44 for conducting dried steam to the blades of a turbine (not shown) coupled to an electrical generator (not shown). In the middle of the lower portions of the secondary side 5, a tube wrapper 52 is provided between the tube bundle 22 and the outer shell of the steam generator 1 in order to provide a down comer path for water extracted from the wet steam that rises through the steam drying assembly 44. At the lower portion of the secondary side 5, a pair of opposing sludge lance ports 53a, 53b are provided in some models of steam generators to provide access for high pressure hoses that wash away much of the sludge which accumulates over the top of the tubesheets 7 during the operation of the generator 1. These opposing sludge lance ports 53a, 53b are typically centrally aligned between the hot and cold legs 24 and 28 of each of the heat exchanger tubes 22. It should be noted that in some steam generators, the sludge lance ports are not oppositely disposed 180 degrees from one another, but are only 90 degrees apart. Moreover, in other steam generators, only one such sludge lance port is provided. In the steam generator arts, the elongated areas between rows of tubes 22 on the tubesheet 7 are known as tube lanes 54, while the relatively wider, elongated area between the hot and cold legs of the most centrally-disposed heat exchanger tubes 22 is known as the central tube lane 55. These tube lanes 54 are typically between 25 and 50 mm (an inch or two) wide in steam generators whose tubes 22 are arranged in a square pitch, such as that shown in Figures 3A, 3B, and 3C. Narrower tube lanes 54 are present in steam generators whose heat exchanger tubes 22 are arranged in a denser, triangular pitch such as shown in Figures 4A and 4B. During the operation of such steam generators 1, it has been observed that the inability of secondary-side water to circulate as freely in the narrow crevices 37 or gaps 40 between the heat exchanger tubes 22, and the support plates 30 and tubesheets 7 can cause the non-radioactive water in these regions to boil completely out of these small spaces, a phenomenon which is known as "dry boiling." When such dry boiling occurs, any impurities in the secondary side water are deposited in these narrow crevices 37 or gaps 40. Such solid deposits tend to impede the already limited circulation of secondary side water through these crevices 37 and gaps 40 even more, thereby promoting even more dry boiling. This generates even more deposits in these regions and is one of the primary mechanisms for the generation of sludge which accumulates over the top of the tubesheet 7. Often the deposits created by such dry boiling are formed from relatively hard compounds of limited solubility, such as magnetite, which tends to stubbornly lock itself in such small crevices 37 and gaps 40. These deposits have been known to wedge themselves so tightly in the crevices 37 or gaps 40 between the heat exchanger tubes 22 and the bores 32 of the support plates 30 that the tube 22 can actually become dented at this region. The instant invention is both an apparatus and a method for dislodging and loosening such deposits, sludge and debris and removing them from the secondary side 5 of a steam generator 1. Apparatus for performing the method of the Invention With reference now to Figures 5, 6A, 6B, 7 and 8, the apparatus for performing the method of the invention generally comprises a pair of pressure pulse generator assemblies 60a, 60b mounted in the two sludge lance ports 53a, 53b, in combination with a recirculation system 114. Because both of these generator assemblies 60a, 60b are identical in all respects, the following description will be confined to generator assembly 60b in order to avoid unnecessary prolixity. With specific reference to Figures 6A and 6B, pulse generator assembly 60b includes an air gun 62 for instantaneously releasing a volume of pressurized gas, and a single port manifold 92 for directing this pressurized gas into a generally tubular nozzle 111 which is aligned along the central tube lane 55 of the steam generator 1. The air gun 62 includes a firing cylinder 64 that contains a pulse flattener 65 which together are dimensioned to store about 1442 cubic centimeters of pressurized gas. Air gun 62 further includes a trigger cylinder 66 which stores approximately 164 cubic centimeters of pressurized gas, and a plunger assembly 68 having an upper piston 70 and a lower piston 72 interconnected by means of a common connecting rod 74. The upper piston 70 can selectively open and close the firing cylinder 64, and the lower piston 72 is reciprocally movable within the trigger cylinder 66 as is indicated in phantom. The area of the lower piston 72 that is acted on by pressurized gas in trigger cylinder 66 is greater than the area of the upper piston 70 acted on by pressurized gas in the cylinder 64. The connecting rod 74 of the plunger 68 includes a centrally disposed bore 76 for conducting pressurized gas admitted into the trigger cylinder 66 into the firing cylinder 64. The pulse flattener 65 also includes a gas conducting bore 77 that is about 12.70 millimeters in diameter. Pressurized gas is admitted into the trigger cylinder 66 by means of a coupling 78 of a gas line 80 that is connected to a pressurized tank of nitrogen 84 by way of a commercially available pressure regulator 82. Gas conducting bores 86a and 86b are further provided in the walls of the trigger cylinder 66 between a solenoid operated valve 88 and the interior of the cylinder 66. The actuation of the solenoid operated valve 88 is controlled by means of an electronic firing circuit 90. In operation, pressurized gas of anywhere between approximately 1 and 11 megapascals is admitted into the trigger cylinder 66 by way of gas line 80. The pressure that this gas applies to the face of the lower piston 72 of the plunger 68 causes the plunger 68 to assume the position illustrated in Figure 6B, wherein the upper piston 70 sealingly engages the bottom edge of the firing cylinder 64. The sealing engagement between the piston 70 and firing cylinder 64 allows the firing cylinder 64 to be charged with pressurized gas that is conducted from the trigger cylinder 66 by way of bore 76 in the connecting rod 74, which in turn flows through the gas-conducting bore 77 in the pulse flattener 65. Such sealing engagement between the upper piston 70 and the firing cylinder 64 will be maintained throughout the entire charging period since the area of the lower piston 72 is larger than the area of the upper piston 70. After the firing cylinder 64 has been completely charged with pressurized gas between 1 and 11 megapascals, the pressure pulse generator 60b is actuated by firing circuit 90, which opens solenoid valve 88 and exposes gas passages 86a and 86b to the ambient atmosphere. The resulting escape of pressurized gas from the trigger cylinder 66 creates a disequilibrium in the pressures acting upon the lower and upper pistons 70, 72 of the plunger 68, causing it to assume the position illustrated in phantom in less than a millisecond. When the air gun 62 is thus fired, 164 cubic centimeters of pressurized gas are emitted around the 360 degree gap 91 between the lower edge of the firing cylinder 64 and the upper edge of the trigger cylinder 66, while the remaining 1262 cubic centimeters follows 2 or 3 milliseconds later through the gas conducting bore 77 of the pulse flattener 65. The two-stage emission of pressurized air out of firing cylinder 64 lowers the peak amplitude of the resulting shock wave in the secondary side, thereby advantageously lowering the peak stress experienced by the heat exchanger tubes 22 in the vicinity of the nozzle 111. The single port manifold 22 completely encloses the circumferential gap 91 of the air gun 62 that vents the pressurized gas from the firing cylinder 64. Upper and lower mounting flanges 94a, 94b are provided which are sealingly bolted to upper and lower mounting flanges 96a, 96b that circumscribe the cylinders 64, 66 of the air gun 62. The manifold 92 has a single outlet port 98 for directing the pulse Of pressurized gas generated by the air gun 62 into the nozzle 111. This port 98 terminates in a mounting flange 100 which is bolted onto one of the annular shoulders 102 of a tubular spool piece 104. The other annular shoulder 107 of the spool piece 104 is bolted around a circular port (not shown) of a mounting flange 109. The spool piece 104 and outlet port 98 are sufficiently long so that the body of the air gun 62 is spaced completely out of contact with the shell of the steam generator 1. This is important, as such spacing prevents the hard outer shell of the air gun 62 from vibrating against the shell of the generator 1 when it is fired. In the preferred embodiment, both the single port manifold 92 and spool piece 104 are formed from stainless steel approximately 12.70 millimeters thick to insure adequate strength. The mounting flange 109 is also preferably formed from 12.70 millimeters thick stainless steel, and has a series of bolt holes uniformly spaced around its circumference which register with bolt receiving holes (not shown) normally present around the sludge lance port 52b of the steam generator 1. Hence, the pulse generator assembly 62b can be mounted onto the secondary side 5 of the steam generator without the need for boring special holes in the generator shell. The nozzle 111 of the pressure pulse generator assembly 60b includes a tubular body 112. One end of the tubular body 112 is circumferentially welded around the port (not shown) of the mounting flange 109 so that all of the compressed air emitted through the outlet port 98 of the single port manifold 92 is directed through the nozzle 111. A complete-penetration weld is used to insure adequate strength. The other end of the tubular body 112 is welded onto a tip portion 113 which is canted 30 degrees with respect to the upper surface of the tubesheet 7. Because the 30 degree orientation of the tip portion 113 induces an upwardly directed movement along the nozzle 111 when the pulse generator 60b is fired, a gusset 113.5 is provided between the tubular body 112 of the nozzle and mounting flange 109. In the preferred embodiment, the body 112 of the nozzle 111 is formed from stainless steel about 12.70 millimeters thick, having inner and outer diameters of 50.8 and 63.5 millimeters, respectively. The nozzle 111 is preferably between 508 and 610 millimeters long, depending on the model of steam generator 1. In all cases, the tip portion 113 should extend beyond the tube wrapper 52. Finally, two vent holes 113.9 that are 6.35 millimeters in diameter and 25.4 millimeters apart are provided on the upper side of the tubular body 112 of the nozzle 111 to expedite the refilling of the nozzle 111 with water after each firing of the air gun 62 (as shown in Figure 7). The provision of such vent holes 113.9 does not divert any significant portion of the air and water blast from the air gun 62 upwardly. It has been found that a 30 degree downward inclination of the tip portion 113 is significantly more effective than either a straight, pipe-like nozzle configuration that is horizontal with respect to the tubesheet 7, or an elbow-like configuration where the tip 113 is vertically disposed over the tubesheet 7. The applicant believes that the greater efficiency associated with the 30 degree orientation of the nozzle tip 113 results from the fact that the blast of water and pressurized air emitted through the nozzle 111 obliquely hits a broad, near-center section of the tubesheet 7, which in turn advantageously reflects the shock wave upwardly toward the support plates 30 and over a broad cross-section of the secondary side. This effect seems to be complemented by the simultaneous, symmetrical blast of air and water from the pulse generator 60a located 180 degrees opposite from pulse generator 60b. The symmetrical and centrally oriented impingement of the two shock waves seems to create a uniform displacement of water in the upper portion of the secondary side 5, as may be best understood with reference to Figure 5. This is an important advantage, as one of the primary cleaning mechanisms at work in the upper regions of the secondary side 5 of the steam generator seems to be the near instantaneous and uniform vertical displacement of the water. Still another important advantage associated with the oblique orientation of the blast of air and water is that the peak stress on the heat exchanger tubes 22 in the vicinity of the tip 113 is lowered. By contrast, if the nozzle tip 113 were directed completely horizontally, no part of the blast would be widely reflected upwardly, and the force of the air and water blast would act orthogonally on the nearest tube 22. Similarly, if the blast were directed completely vertically toward the tubesheet 7, the impact area of the blast against the tubesheet would be narrower, and peak tube stresses would again be higher as the blast would be more concentrated. With reference now to Figures 6A, 7 and 8, the apparatus further includes a recirculation system 114 that is interconnected with the pressure pulse generator assembly 60b by inlet hose 115, a suction-inlet hose 121a, and a suction hose 121b. As is best seen in Figure 6A, inlet hose 115 extends through the circular mounting flange 109 of the pressure pulse generator assembly 60b by way of a fitting 117. At its distal end, the inlet hose 115 is aligned along the main tube lane 55 above nozzle 111 as is best seen in Figure 7. At its proximal end, the inlet hose 115 is connected to an inlet conduit 119b that is part of the recirculation system 114. Suction-inlet hose 121a and suction hose 121b likewise extend through the mounting flange 109 by way of fittings 123a, 123b. Inlet hose 115 is provided with a diverter valve 126a connected thereto by a T-joint 126.1 for diverting incoming water into suction-inlet hose 121a as shown. Suction-inlet hose 121a includes an isolation valve 126b as shown just below T-joint 126.2. When suction-inlet hose 121a is used as a suction hose, valves 126a and 126b are closed and opened, respectively. When suction-inlet hose 121b is used as an inlet hose, valves 126a and 126b are opened and closed, respectively. The distal ends of the hoses 121a, 121b lie on top of the tubesheet 7, and are aligned along the circumference of the tubesheet 7 in opposite directions, as may best be seen in Figure 7. Such an alignment of the inlet hose 115 and hoses 121a, 121b helps induce a circumferential flow of water around the tubesheet 7 when hose 121a is used as an inlet hose by shutting valve 126b and opening valve 126a. As will be discussed later, such a circumferential flow advantageously helps to maintain loosened sludge in suspension while the water in the secondary side is being recirculated through the particulate filters 145 and 147 of the recirculation system 114. The proximal ends of each of the hoses 121a, 121b are connected to the inlet ends of a T-joint 125. The outlet end of the T-joint 125 is in turn connected to the inlet of a diaphragm pump 127 by way of conduit 125.5b. The use of a diaphragm-type pump 127 is preferred at this point in the recirculation system 114 since the water withdrawn through the hoses 121a, 121b may have large particles of suspended sludge which, while easily handled by a diaphragm-type pump, could damage or even destroy a rotary or positive displacement-type pump. Figure 8 schematically illustrates the balance of the recirculation system 114. The suction-inlet hose 121a and suction hose 121b of each of the pressure pulse generator assemblies 60a, 60b are ultimately connected to the input of diaphragm pump 127. The output of the diaphragm pump 127 is in turn serially connected to first a tranquilizer 129 and then a flow meter 131. The tranquilizer 129 "evens out" the pulsations of water created by the diaphragm pump 127 and thus allows the flow meter 131 to display the average rate of the water flow out of the diaphragm pump 127. The output of the flow meter 131 is connected to the inlet of a surge tank 135 via conduit 133. In the preferred embodiment, the surge tank 135 has an approximately 1 cubic meter capacity. The outlet of the surge tank 135 is connected to the inlet of a flow pump 137 by way of a single conduit 139, while the output of the pump 137 is connected to the inlet of a cyclone separator 141 via conduit 143. In operation, the surge tank accumulates the flow of water generated by the diaphragm pump 127 and smoothly delivers this water to the inlet of the pump 137. The pump 137 in turn generates a sufficient pressure head in the recirculating water so that a substantial portion of the sludge suspended in the water will be centrifugally flung out of the water as it flows through the cyclone separator 141. Located downstream of the cyclone separator 141 is a one to three micron bag filter 145 that is serially connected to a one micron cartridge filter 147. These filters 145 and 147 remove any small particulate matter which still might be suspended in the water after it passes through the cyclone separator 141. Downstream of the filters 145 and 147 is a 2 cubic meter supply tank 151. Supply tank 151 includes an outlet conduit 153 that leads to the inlet of another flow pump 155. The outlet of the flow pump 155 is in turn connected to the inlet of an ionic remover or demineralizer bed 157. The purpose of the flow pump 155 is to establish enough pressure in the water so that it flows through the serially connected ion exchange columns (not shown) in the demineralizer bed 157 at an acceptably rapid flow rate. The purpose of the demineralizer bed 157 is to remove all ionic species from the water so that they will have no opportunity to reenter the secondary side 5 of the generator 1 and create new sludge deposits. Located downstream of the demineralizer bed 157 is a first T-joint 159 whose inlet is connected to conduit 161 as shown. An isolation valve 160a and a drain valve 160b are located downstream of the two outlets of the T-joint 159 as shown to allow the water used in the cleaning method to be drained into the decontamination facility of the utility. Located downstream of the T-joint 159 is another T-joint 163 whose inlet is also connected to conduit 161 as shown. Diverter valves 165a and 165b are located downstream of the outlet of T-joint 163 as indicated. Normally valve 165a is open and valve 165b is closed. However, if one desires to fill a second steam generator 1 with the filtered and polished water drained from a first steam generator in order to expedite the pressure pulse cleaning method, valves 165a and 165b can be partially closed and partially opened, respectively. Flowmeters 167a, 167b are located downstream of the valves 165a and 165b so that an appropriate bifurcation of the flow from conduit 161 can be had to effect such a simultaneous drain-fill step. Additionally, the conduit that valve 165b and flowmeter 167b are mounted in terminates in a quick connect coupling 167.5. To expedite such a simultaneous drain-fill step, valves 165a and 165b are mounted on a wheeled cart (not shown) and conduit 161 is formed from a flexible hose to form a portable coupling station 168. Downstream of the portable coupling station 168, inlet conduit 161 terminates in the inlet of a T-joint 169 that bifurcates the inlet flow of water between inlet conduits 119a and 119b. Water is supplied through the recirculation system 114 through deionized water supply 170, which may be the deionized water reservoir of the utility being serviced. Water supply 170 includes an outlet conduit 172 connected to the inlet of another flow pump 174. The outlet of the flow pump 174 is connected to another conduit 176 whose outlet is in turn connected to the supply tank 151. A check valve 178 is provided in conduit 176 to insure that water from the supply tank 151 cannot back up into the deionized water reservoir 170. Method of the Invention With reference now to Figures 5, 6A and 6B, the method of the invention is generally implemented by the previously described pressure pulse generator assemblies 60a, 60b in combination with the recirculation system 114. However, before these components of the apparatus of the invention are installed in and operated in a steam generator 1, several preliminary steps are carried out. In the first of these steps, the relative condition of the heat exchanger tubes 22 is preferably ascertained by an eddy current or ultrasonic inspection of a type well known in the art. Such an inspection will give the system operators information which they can use to infer the maximum amount of momentary pressures that the tubes 22 of a particular steam generator can safely withstand without any danger of yielding or without undergoing significant metal fatigue. In this regard, applicants have observed that heat exchanger tubes 22 in moderately good condition can withstand momentary pressures of up to approximately 131 megapascals without yielding or without incurring significant amounts of metal fatigue. By contrast, it is anticipated that relatively old heat exchanger tubes 22 whose walls have been significantly weakened by corrosion and fretting may only be able to withstand only 103 megapascals, while relatively new tubes which are relatively free of the adverse affects of corrosion or fretting may be able to withstand up to 207 megapascals without any adverse mechanical effects. After the tubes 22 have been inspected by an eddy current or ultrasonic probe to the extent necessary to ascertain the maximum amount of momentary pressure they can safely withstand, the secondary side 5 of the steam generator 1 is drained and all loose sludge that accumulates on top of the tube sheet 7 is removed by known methods, such as flushing or by sludge lancing. In the preferred embodiment, sludge lancing techniques such as those disclosed and claimed in U.S. Patents 4,079,701 and 4,676,201 are used. Generally speaking, such sludge lancing techniques involve the installation of a movable water nozzle in the sludge lance ports 53a, 53b in the secondary side 5 which washes the loose sludge out of the generator 1 by directing a high velocity stream of water down the tube lanes 54. After all of the loose sludge on top of the tubesheet 7 has thus been removed, the pressure pulse generator assemblies 60a, 60b are installed in the sludge lance ports 53a, 53b in the positions illustrated in the Figures 6A and 7. Specifically, the tubular body 112 of the nozzle 111 of each of the generator assemblies 60a, 60b is centrally aligned along the main tube lane 55 in a horizontal position as shown so that the canted nozzle tip 113 assumes a 30 degree orientation with respect to the flat, horizontal upper surface of the tubesheet 7. Next the recirculation system 114 is connected to each of the pulse generator assemblies 60a, 60b by coupling the inlet hose 115 of each to the flexible inlet conduits 119a and 119b, and the suction-inlet hose 121a and suction hose 121b of each to flexible suction conduits 125.5a, 125.5b via the T-joint 125 of each assembly 60a, 60b. Next, the recirculation system 114 is connected via conduit 172 to the supply 170 of deionized water from the utility, as is best seen in Figure 8. The flow pump 174 is then actuated in order to fill supply tank 151 approximately one-half full, which will occur when tank 151 receives about 950 Litres (250 gallons) of water. Once supply tank 151 is at least one-half full, flow pump 155 is actuated to commence the fill cycle. In the preferred method of the invention, pump 155 generates a flow of purified water of approximately 0.454 cubic meter per minute which is bifurcated to two 0.227 cubic meter per minute flows at T-joint 169 between inlet hose 119a and 119b on opposing sides of the generator 1 in order to fill the secondary side 5 of the steam generator 1. During the time that the secondary side 5 is being filled via pump 153, valves 165a and 165b are opened and closed so that the entire flow of water from pump 153 enters the generator 1. Additionally, valves 126a, 126b are opened and closed in each of the generator assemblies 60a, 60b in order to further bifurcate the 0.227 cubic meter per minute flow from inlet conduit 119a, 119b between the inlet hose 115 and the suction-inlet hose 121a of each of the generator assemblies 60a, 60b. As soon as the water level on the secondary side 5 becomes great enough to submerge both hoses 121a, 121b diaphragm pump 127 is actuated and adjusted to withdraw 0.189 cubic meter per minute a piece out of the secondary side 5. Since the flow pump 155 introduces 0.454 cubic meter per minute, while the diaphragm pump 127 withdraws 0.189 cubic meter per minute, the secondary side 5 is filled at a net flow rate of 0.265 cubic meter per minute. Additionally, since the suction-inlet hose 121b of each of the generator assemblies 60a, 60b is used at this time as a fill hose, whose output is circumferentially directed toward an opposing suction hose 121a, a peripheral flow of water is created around the circumference of the secondary side as is best seen in Figure 7. Such a peripheral flow of water is believed to help keep in suspension the relatively large amounts of sludge and debris that are initially dislodged from the interior of the secondary side 5 when the generator assemblies 60a, 60b are actuated which in turn allows the recirculation system 114 to remove the maximum amount of dislodged sludge and debris during the fill cycle of the method. After the water level in the secondary side 5 of the generator 1 rises to a level of at least 152 millimeters over the nozzles 111 of each of the pressure pulse generator assemblies 60a, 60b, the firing of the air gun 62 of each of the assemblies 60a, 60b commences. If the prior eddy current and ultrasonic testing indicates that the heat exchanger tubes 22 can withstand momentary pressures of approximately 131 megapascals without any deleterious affects, the gas pressure regulators 82 of each of the generator assemblies 60a, 60b is adjusted so that gas of a pressure of about 3 megapascals is initially admitted into the firing cylinders 64 of the air gun 62 of each. Such a gas pressure applies a peak stress to the tubes 22 which is safely below the 131 megapascals limit, as will be discussed in more detail hereinafter. The firing circuit 90 is then adjusted to fire the solenoid operated valve 88 of the trigger cylinder 66 every seven to ten seconds. The firing of the air gun 62 at seven to ten second intervals continues during the entire fill, recirculation and drain cycles of the method. While the generator assemblies 60a, 60b are capable of firing at shorter time intervals, a pulse firing frequency of seven to ten seconds is preferred because it gives the nitrogen gas emitted by the nozzle 111 sufficient time to clear the nozzle 111 and manifold 92 before the next pulse. If pockets of gas remain in the pulse generator 60b during subsequent air gun firings, then a significant amount of the shock to the water within the secondary side 5 would be absorbed by such bubbles, thereby interfering with the cleaning action. It is important to note that the gas pressure initially selected for use with the pressure pulse generator assembly 60a, 60b induces momentary pressures that are well below the maximum safe amount of momentary forces that the tubes 22 can actually withstand, for two reasons. First, as will be discussed in more detail hereinafter, the pressure of the gas used in the generator assembly 60a, 60b is slowly raised in proportion with the extent to which the secondary side 5 of the steam generator 1 is filled until it is approximately twice as great as the initially chosen value for gas pressure. Hence, when the initial gas pressure used when the water level is just above the nozzles 111 is approximately 3 megapascals, the final pressure of the gas used in the pressure pulse generator assembly 60a, 60b will be approximately 5.52 to 6.21 megapascals. Secondly, the gas pressure is chosen so that the maximum pressure used will induce momentary forces in the tubes 22 which are at least 30 and preferably 40 percent below the maximum megapascals indicated by the previously mentioned eddy current and ultrasonic inspection to provide a wide margin of safety. In making the selection of which gas pressure to use, applicants have discovered that there is a surprising, non-linear relationship between the pressure of the gas used in the air gun 62 of each pulse generator assembly 60a, 60b and the resulting peak stress on the tubes 22, as is evident from the following test results: Approximate Gas Pressure Approximate Peak Tube Stress 3 megapascals 38 megapascals 6 megapascals 83 megapascals 11 megapascals 212 megapascals In most circumstances, the firing of the air gun 62 of both the pulse generators will be synchronous in order to uniformly displace the water throughout the entire cross-section of the secondary side 5 of the generator 1. However, there may be instances where an asynchronous firing of the air guns 62 of the different assemblies may be desirable, such as in a steam generator where the sludge lance ports 53a, 53b are only 90 degrees apart from one another. In such a case, the asynchronous firing of the air guns 62 could possibly help to compensate for the non-opposing arrangement of the pulse generators 60a, 60b in the secondary side 5 imposed by the location of the 90 degree apart sludge lance ports 53a, 53b. Figure 9 illustrates how the pressure of the gas within the 1442 cubic centimeter firing cylinder 64 of the air gun 62 diminishes over time, and Figure 10 indicates the peak stress experienced by the column of tubes closest to the nozzle 111. Specifically, when the pressure of the gas within the firing cylinder 64 is 6 megapascals, and a 164 cubic centimeter pulse flattener 65 having a gas-conducting bore 13 millimeters in diameter is used, the gas leaves the cylinder 62 over a time period of approximately five milliseconds. Figure 10 shows that the peak stress experienced by the column of tubes 22 closest to the tip portion 113 of the nozzle 111 is between 83 and 90 megapascals, which again is safely below the 131 megapascals limit. If no pulse flattener 65 were used, the closest column of heat exchanger tubes 22 in the secondary side 5 to the tip portion 113 of the nozzle 111 would be considerably higher, as the gas would escape from the air gun in a considerably shorter time than 5 milliseconds. The filling of the secondary side 5 at a net rate of about 0.265 cubic meters per minute continues until the uppermost support plate 30 is immersed with water. In a typical steam generator, about 64 cubic meters of water must be introduced into the secondary side 5 before the water reaches such a level. At a net fill rate of about 0.265 cubic meters per minute, the fill cycle takes about four hours. During the fill cycle, the pressure of the gas introduced into the firing cylinder 64 of each air gun 62 is raised from approximately 3 megapascals to approximately 5.52 to 6.21 megapascals in direct proportion with the water level in the secondary side 5. The proportional increase in the pressure of the gas used in the air guns 62 substantially offsets the diminishment in the power of the pulses created thereby caused by the increasing static water pressure around the tip portion 113 of the nozzle 111 of each. As soon as the water level in the secondary side 5 is high enough to completely submerge the highest support plate 30, the recirculation cycle commences. If desired, valves 126a, 126b may be closed and opened, respectively, in order to convert the function of suctionfill hose 121a into a suction hose. Moreover, the flow rate of fill pump 155 is lowered from 0.454 cubic meters per minute to only 0.189 cubic meters per minute, while the withdrawal rate of the diaphragm type suction pump 127 is maintained at 0.189 cubic meter per minute. The net result of these adjustments is that water is recirculated through the secondary side 5 of the steam generator 1 at a rate of approximately 0.189 cubic meter per minute. This circulation rate is maintained for approximately 12-48 hours while the air guns 62 of each of the generator assemblies 60a, 60b are fired at a pressure of 6 megapascals every seven to ten seconds. After the termination of the recirculation cycle, the drain cycle of the method commences. This step is implemented by doubling the flow rate of the diaphragm-type suction pump 127 so that each of the hoses 121a, 121b of each pulse generator 60a, 60b will withdraw approximately 0.085 cubic meters per minute. Since the fill pump 155 continues to fill the secondary side 5 at a total rate of approximately 0.189 cubic meter per minute, the net drain rate is approximately 0.151 cubic meter per minute. As the secondary side 5 has about 64 cubic meters of water in it at the end of the recirculation cycle, the drain cycle takes about seven hours. During this period of time, it should be noted that the pressure of the gas introduced into the firing cylinders 64 of the air guns 62 of the generator assembly 60a, 60b is lowered from 5.52 megapascals to 2.76 megapascals in proportion with the level of the water in the secondary side 5. To expedite the cleaning method in a utility where two or more steam generators are to be cleaned, a second steam generator (not shown) may be filled with the filtered and polished water that flows out of the demineralizer 157 of the recirculation system 114 during the drain cycle of a first steam generator. This may be accomplished by wheeling the portable coupling station 168 over to a second generator where other pulse generator assemblies 60a, 60b have been installed, and coupling the outlet of flowmeter 167b to the inlet conduits 119a, 119b of the second generator. Next, diverter valves 165a and 165b are adjusted so that part of the filtered and polished water leaving the demineralizer 157 is shunted to the inlet conduits 119a, 119b of the second generator. In order to maintain the seven hour time period of the drain cycle for the first steam generator, the flow rate of the pump 155 is increased to approximately 0.644 cubic meters per minute. The valve 165a is adjusted so that the flow rate as indicated by flowmeter 167a remains approximately 0.189 cubic meters per minute. The balance of the 0.454 cubic meters per minute flow is shunted through valve 165b to the secondary side 5 of the second steam generator. The implementation of this additional step not only lowers the total amount of time required to clean a plurality of steam generators by as much as 50 percent, but further considerably reduces the amount of deionized and purified water that the utility must supply from source 170 to implement the cleaning method of the invention. As it requires approximately 64 cubic meters or 65,318 kilograms of water to clean a single steam generator 1, the savings in water alone are clearly significant. Moreover, by reducing the overall amount of time required to clean two generators, the amount of time that the operating personnel are exposed to potentially harmful radiation is considerably reduced. The portability of the valves 165a, 165b afforded by the portable conduit coupling station 168 plus the use of a flexible hose for conduit 161 greatly facilitates the implementation of such a combined drain-fill step in the method of the invention.

Claims: A pressure pulse cleaning method for loosening sludge and other impurities from the interior of a steam generator vessel (5) having a plurality of parallel steam generator tubes (22) comprising the steps of at least partially filling the steam generator vessel with liquid, and using pressure pulse generating means (60a, 60b) with associated nozzle means (111) for generating a succession of pulses to create shock waves in said liquid, characterized in that a pair of pressure pulse generators (60a, 60b) with associated nozzles (111) is installed at opposite locations of said heat exchanger vessel (5) with the distal end (113) of each nozzle (111) being located in the interior of said steam generator vessel and oriented at an oblique angle of about 30 degrees with respect to the tube sheet so as to direct said pulses toward the tube sheet (7) of said steam generator vessel (5). The method of claim 1, characterized in that each nozzle (111) is detachably mounted in an access opening (53a, 53b) in the steam generator vessel (5) by means of a mounting flange (109). The method of claim 1, characterized in that the longitudinal axis of said nozzles (111) is aligned with the gap (54) between two adjacent rows of steam generator tubes (22) to maximize the distance between the distal ends (113) of said nozzles and the closest steam generator tubes (22) thereto. The method of any of claims 1 to 3, characterized in that a recirculation flow is induced in said liquid through a filter (145, 147) for removing loosened particles of sludge and debris from the liquid in the vessel. The method of claim 4, characterized in that said flow is recirculated through a ionic remover (157) for removing dissolved ionic species in the liquid. The method of claim 4, characterized in that said flow is recirculated through at least one demineralizer bed (157). The method of any of claims 4 to 6, characterized in that said recirculated flow is conducted into the vessel (5) substantially parallel to the inner periphery of said vessel in order to induce a circumferential flow of liquid around the interior of said vessel that helps maintain particles of sludge and debris in suspension in the liquid.

Inventors: AULD, GREGG DANIEL, FRANKLIN, RICHARD DALE, MURRAY, DAVID EDWARD
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1989
Main classification: F22B37
IPC codes: F22B37, F28G7