Abstract:
An apparatus for supplying relatively cool feedwater to a heated pressure vessel, while moderating the thermal gradients within the apparatus and the pressure vessel. The feedwater apparatus is generally comprised of a feedwater inlet nozzle, thermal sleeve and sparger assembly which is structured to provide a thermal barrier and to lengthen the path of heat conduction through the feedwater inlet nozzle; to insure adequate support for the thermal sleeve and the sparger; to improve feedwater flow through the thermal sleeve and the sparger; and to facilitate the inspection and repair of the welds used to structure the feedwater apparatus.

Description:
FIELD AND BACKGROUND OF INVENTION 
     The present invention relates, in general, to steam generator pressure vessels and, in particular, to an apparatus for supplying relatively cool feedwater to a heated pressure vessel while moderating the thermal gradients therein and in the vessel. 
     The present invention is particularly suitable for the type of steam generators that are associated with nuclear power plants. In this regard, such steam generators may be viewed as comprising a vertically oriented and substantially closed vessel within which a primary fluid which has been heated by circulation through the reactor core and a vaporizable fluid, in the form of feedwater, are made to flow in indirect heat exchange relationship, such that heat is transferred from the heated fluid to the feedwater. Moreover, in accordance with conventional practice, the steam generator vessel contains a bundle of heat exchange tubes with the ends of each of the heat exchange tubes being suitably retained within a pair of tube sheets. The steam generator vessel is generally substantially cylindrical in configuration, and has a tube sheet suitably mounted therewithin, such as to be positioned adjacent but spaced from each of the ends of the steam generator vessel. Each of the heat exchange tubes in the bundle is in turn suitably supported from the steam generator vessel so as to extend longitudinally therewithin, with the respective ends thereof emplaced in a corresponding one of the aforesaid pair of tube sheets. A cylindrical baffle or shroud is disposed about the bundle of heat exchange tubes to divide the steam generator vessel interior into an annular down flow passageway and an axially disposed evaporator chamber containing the bundle of heat exchange tubes. A plurality of feedwater inlet nozzles communicates with the annular down flow passageway. The feedwater inlet nozzles are generally formed as an integral part of the steam generator vessel, and are spaced at a common elevation around the steam generator vessel. 
     The heated primary fluid enters the steam generator vessel through a primary fluid inlet and is made to flow through the heat exchange tubes of the bundle, and thence discharged out of the steam generator vessel through a primary fluid outlet, to be conveyed through the remainder of the reactor coolant system. The feedwater is introduced through the feedwater inlet nozzles, and is made to flow down the annular passageway until the tube sheet near the bottom of the annular passageway causes the feedwater to reverse direction, passing in heat transfer relationship with the outside of the heat exchange tubes while flowing upwardly through the inside of the shroud. While the feedwater is circulating in heat transfer relationship with the heat exchange tubes of the bundle, heat is transferred from the heated primary fluid in the tubes to the feedwater surrounding the tubes causing a portion of the feedwater to be converted to steam. The steam then rises and is discharged from the steam generator vessel through one or more steam outlets for circulation through typical generating equipment to produce electricity in a manner well known in the art. 
     The feedwater inlet nozzle is fed by a supply conduit which is connected thereto for discharge into a thermal sleeve that extends within and through the feedwater inlet nozzle and has one end generally formed with or connected to a sparger, the latter distributes the feedwater downwardly through the annular passageway. The thermal sleeve acts as a shield to reduce the temperature gradients between the relatively cool feedwater flowing therethrough, as compared to the heated feedwater inlet nozzle and steam generator vessel. 
     The relatively large temperature gradients extending through the feedwater inlet nozzle from the warm steam generator vessel to the relatively cool feedwater tend to produce thermal stresses. Thermal gradients, and the thermal stresses resulting therefrom, are particularly aggravated as a result of changes in the feedwater flow through the inlet nozzle of this type steam generator, under certain operating conditions such as during the reactor start-up as well as during changes in the reactor power output. It is during these changes in feedwater flow that there occurs thermal cycling of the feedwater inlet nozzle and the thermal sleeve. Such thermal cycling may induce fatigue failure in the dissimilar metal weld which fixedly secures the thermal sleeve, through a transition ring, to the feedwater inlet nozzle. In fact, due to restricted access to this thermal sleeve weld region, it is difficult to detect and eliminate weld flaws. Moreover, since the nozzle is usually made of low alloy steel, it corrodes much faster than the thermal sleeve which is made of corrosion-resistant material. Thus, the feedwater inlet nozzle side of this dissimilar metal weld will be severely thinned. Obviously, this corrosion problem could be eliminated if the feedwater inlet nozzle were made of the same expensive corrosion-resistant material as that of the thermal sleeve. However, the material cost of such a modification would be high because of the heavy section size of the feedwater inlet nozzle. When the cantilever thermal sleeve and sparger unit is subjected to a bending moment by feedwater injection and pressure difference or the occurrence of an earthquake, significant bending and axial stresses will occur at the thinned cross section on the feedwater inlet nozzle side of the dissimilar metal weld. As a result, the thermal sleeve may develop fatigue cracks, and the ensuing leaks of feedwater may flow around the outer surface of the thermal sleeve, and come in direct contact with the feedwater inlet nozzle and hence cause undesirable cooling which may lead to thermal stresses in the area of the feedwater inlet nozzle and the surrounding wall portion of the steam generator vessel. The thermal stresses imposed on the feedwater inlet nozzle and the surrounding wall portion of the steam generator vessel will reduce the life expectancy of this equipment, if the undesirable cooling is not eliminated. Therefore, repair of the thermal sleeve is required whenever such leaks occur. However, the repair of the thermal sleeve has proven to be a difficult task, because of the restricted access to the dissimilar metal weld which is used to secure the thermal sleeve to the feedwater inlet nozzle. 
     Accordingly, this prior art feedwater inlet nozzle, thermal sleeve and sparger assembly has encountered limitations as to, the operating conditions of the feedwater system with respect to reactor start-ups and changes in reactor power output, and also with respect to feedwater flow-induced vibration and fretting of the thermal sleeve, and further with respect to the repair of the thermal sleeve. Thus, there is a need to provide industry with solutions to these problems. 
     SUMMARY OF INVENTION 
     These difficulties are overcome, to a large extent, through the practice of the present invention which provides an improved apparatus for supplying feedwater to a nuclear type steam generator pressure vessel. The apparatus is generally comprised of a feedwater inlet nozzle, a thermal sleeve and a sparger, and is structured to supply relatively cool feedwater as compared to its heated self and the heated pressure vessel, while moderating the thermal gradients across the feedwater inlet nozzle and the surrounding wall portion of the pressure vessel; reducing the feedwater flow-induced vibration and fretting of the thermal sleeve; improving the structural support of the thermal sleeve and sparger; and facilitating the repair of the thermal sleeve. 
     Accordingly, there is provided a feedwater source including a conduit to supply the feedwater to the thermal sleeve which extends through the bore of the feedwater inlet nozzle and through an inlet in the steam pressure vessel wall. The thermal sleeve, which is fixedly supported by the feedwater nozzle, conveys the feedwater to the sparger located in the steam pressure vessel. The underside of the sparger includes a plurality spray holes which inject the feedwater downward into an annular passageway formed between the pressure vessel wall and a shroud that defines the evaporator chamber. The downstream end of the sparger is closed off by a generally flat plate which acts to deflect the feedwater toward the spray holes. The deflector plate can either be formed as an integral part of the sparger or be welded thereto. The deflector plate is advantageously sloped at an angle of 45 degrees measured clockwise from the longitudinal axis of the sparger so as to smoothen the flow of feedwater through the thermal sleeve and the sparger, thereby lengthening the life expectancy of the apparatus by reducing the flow-induced vibration and fretting. 
     The feedwater nozzle has its inlet face welded to the discharge end of the feedwater supply conduit, and also to the thermal sleeve as one of the two points used to support the sleeve. The other of the two points used to support the thermal sleeve is a weld between the outlet end of the feedwater nozzle and the thermal sleeve. This two-point support arrangement acts to increase the mechanical strength of the feedwater apparatus and, particularly, that of the thermal sleeve and sparger assembly, with a concomitant reduction in stress corrosion. The welds providing the two-point support for the thermal sleeve and sparger assembly are dissimilar welds to accommodate cost restraints requiring that the feedwater nozzle be made out of a metal composition that is less resistant to corrosion than that used in the making of the thermal sleeve. As a result, the feedwater nozzle side of the dissimilar weld will eventually become severely thinned and require repair. The feedwater apparatus is advantageously structured in that all of the welds, including the two dissimilar welds used to fixedly attach the thermal sleeve to the feedwater nozzle are readily accessible for inspection and repair. 
     The feedwater inlet nozzle has a cylindrically-shaped inner surface which defines a bore extending therethrough. The feedwater nozzle has an inlet and an outlet end portion wherein the bore is sized to obtain a tight fit or, alternatively, an interference fit between the inner surface of these nozzle portions and the outer surface of the correspondingly adjacent portions of the thermal sleeve. The feedwater nozzle inner surface which lies intermediate of the tight-fitting nozzle end portions is configured to form a recess therein and to cooperate with the recessed walls and the outer surface of the thermal sleeve to define an annular chamber therebetween. The chamber is provided with one or more threaded passageway openings extending through the body of the feedwater nozzle. A threaded plug is also provided to shut off the passageway opening. The chamber extends over a major length of the feedwater nozzle bore and is filled with a dry gaseous medium, for example, dry nitrogen or dry air, thereby forming a thermal barrier between the relatively cool feedwater flowing through the thermal sleeve and the heated surrounding portions of the feedwater nozzle and pressure vessel wall, and thus moderating the thermal gradients and the thermal stresses resulting therefrom. The use of dry nitrogen gas is preferred since it reduces stress erosion in the chamber. 
     A collar is coaxially disposed around the feedwater inlet nozzle intermediate the inlet and outlet portions thereof. The collar is normally formed as an integral part of the feedwater nozzle, and has a downstream end portion welded to the pressure vessel wall and an upstream end portion abutting a flanged ring which is provided with a plurality of circumferentially spaced apertures. The pressure vessel wall includes a plurality of apertures circumferentially spaced around the vessel wall inlet and penetrating the wall. These apertures correspond in number and arrangement to the apertures provided in the flanged ring. Fastening means that are generally in the form of threaded studs and lock nuts are used to clamp the flanged ring against the collar so as to forcibly and further secure the feedwater inlet nozzle to the pressure vessel wall. The collar includes an annular portion which is located intermediate of the downstream and upstream end portions of the collar. The annular portion of the collar is advantageously configured with a plurality of circumferentially spaced grooves that serve to lengthen the path of heat conduction, and thereby reduce the thermal gradients and the thermal stresses resulting therefrom. The land segments formed between the grooves provide the force transfer path used to rigidly secure feedwater inlet nozzle to the pressure vessel wall. 
    
    
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood and its advantages will be more appreciated from the detailed description of the preferred embodiment, especially when read with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic sectional side view of a feedwater apparatus comprised of a feedwater inlet y-forging nozzle, thermal sleeve and sparger assembly known in the art; 
     FIG. 2 is a schematic sectional side view of a feedwater apparatus comprised of a feedwater inlet nozzle, thermal sleeve and sparger assembly which incorporates the present invention; 
     FIG. 3 is a schematic sectional side view of the feedwater inlet nozzle shown in FIG. 2; and 
     FIG. 4 is an end view of the feedwater inlet nozzle taken along line  4 — 4  of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG.1 of the drawings, there is shown a prior art feedwater apparatus  10 , with the partial cross section of the wall  12  of a vertically extending, substantially cylindrically-shaped steam generator pressure vessel. The feedwater apparatus  10  extends within and through the bore  14  of an inlet  16  formed through the wall  12  of the pressure vessel, and is generally comprised of a feedwater inlet nozzle  18 , a thermal sleeve  20  and a sparger  22 . The pressure vessel wall  12  is provided with a plurality of apertures  24  circumferentially spaced around the inlet  16  and penetrating the outside of the vessel wall  12 . The outlet end of the feedwater inlet nozzle  18  is located adjacent to the vessel wall inlet  16 , and includes a collar  26  which is welded to a retaining ring  28  abutting the steam generator vessel wall  12 . A flanged ring  30  rests on the shoulder  32  configured by the collar  26  and is axially aligned with the bore  14  of the vessel wall inlet  16 . The flanged ring  30  is provided with a plurality of apertures  34  which correspond in number and arrangement to the apertures  24  which penetrate the steam generator vessel wall  12 . Fastening means, which are generally in the form of threaded studs  36  and lock nuts  38 , are provided to clamp the flanged ring  30  against the collar  26 , thereby forcibly securing the feedwater inlet nozzle  18  to the steam generator wall  12 . A weld  40  connects the inlet end of the feedwater nozzle  18  to a feedwater supply conduit  42 . 
     The thermal sleeve  20  has its downstream end formed as an integral part of or, alternatively, welded to the sparger  22 , and its upstream end connected by a dissimilar metal weld  44  to a transition ring, not shown, with the latter, in turn, being welded to the feedwater inlet nozzle  18 . The outer surface of the inlet end portion of the thermal sleeve  20  is narrowly spaced from the inner surface of the feedwater inlet nozzle  18  to define therebetween a constricted annular passage  48  opening into the bore  14  of the steam generator vessel wall inlet  16 . Water will fill the annular passage  48  during operation. 
     The sparger  22  includes a plurality of spray holes  50  that direct the relatively cool feedwater downward through an annular passageway  52  formed between the heated steam generator vessel wall  12  and a heated shroud  54  that defines a conventional evaporator chamber, not shown. 
     Although the steam generator vessel is generally protected from the thermal stresses caused by temperature differences, the feedwater inlet nozzle  18  and the surrounding or nearby portion of the vessel wall  12  and, more particularly, the weld juncture  46  between the thermal sleeve  20  and the feedwater inlet nozzle  18  continue to be limiting factors for this prior art feedwater apparatus. In fact, and as shown in FIG. 1, because of the narrowness of the constricted passage  48 , there is limited access to the dissimilar weld  44  which connects the thermal sleeve  20  through a transition ring, not shown, to the feedwater inlet nozzle  18 , thus, making it difficult to detect and eliminate flaws in the dissimilar weld  44 . Also, the weld  44  will be severely thinned, since the transition ring of the feedwater inlet nozzle  18  is usually made of low alloy steel and corrodes much faster than the thermal sleeve  20 , which is typically made of corrosion-resistant material. Therefore, when the cantilever thermal sleeve  20  and sparger  22  components of the feedwater apparatus  10  are subjected to a bending moment created by feedwater injection and pressure differences or by an earthquake, significant bending and axial stresses on the thinned cross section may occur at the location of the dissimilar metal weld  44 . As a result, the thermal sleeve  20  may develop fatigue cracks and the ensuing leaks of feedwater may flow around the outer surface of the thermal sleeve  20 , and come in direct contact with the feedwater inlet nozzle  18 . This, in turn, can lead to significant thermal stresses in the feedwater inlet nozzle  18  and the adjacent wall  12  portion of the steam generator pressure vessel. Repair of the thermal sleeve  20  is required whenever such leakage of feedwater occurs, since the significant thermal stresses imposed on the relatively hot feedwater inlet nozzle  18  and the surrounding wall portion of the steam generator by the leakage of the relatively cool feedwater being supplied by the conduit  42  will reduce the life expectancy of the equipment. 
     Turning now to the preferred embodiment of the present invention as depicted in FIGS. 2,  3 , and  4 , wherein like reference numerals are used to refer to the same or functionally similar elements. 
     In FIG. 2 there is shown a feedwater apparatus  110  incorporating the present invention, and a partial cross section of the wall  112  of a vertically extending, substantially cylindrically-shaped steam generator pressure vessel. The feedwater apparatus  110  extends within and through the cylindrically-shaped bore  114  of an inlet  116  formed through the wall  112  of the pressure vessel. The feedwater apparatus  110  is generally comprised of a feedwater inlet nozzle  118 , a thermal sleeve  120  and a sparger  122 . The steam generator vessel wall  112  includes a plurality of apertures  124  circumferentially spaced around the inlet  116  and penetrating the outside of the vessel wall  112 . The feedwater inlet nozzle  118 , also shown at FIGS. 3 and 4, has an inlet portion  126  and an outlet portion  128 . A collar  130  is located between the inlet portion  126  and the outlet portion  128  of the feedwater nozzle  118 , and is normally formed as an integral part of the nozzle  118 . The outlet portion  128  of the nozzle  118  lies within the bore  114  and its outer surface is spaced from the inner surface of the pressure vessel inlet  116 , to define therebetween a constricted or narrow annular cavity  132  opening into the remainder of the bore  114 . The downstream end portion  131  of the collar  130  is welded to the steam generator vessel wall  112 , and the upstream end portion  133  of the collar  130  abuts a flanged ring  134 , which is provided with a plurality of apertures  136  that correspond in number and arrangement to the apertures  124  which penetrate the steam generator vessel wall  112 . Fastening means, which are generally in the form of threaded studs  138  and lock nuts  140 , are provided to clamp the flanged ring  134  against the collar  130 , thereby forcibly and rigidly securing the feedwater inlet nozzle  118  to the steam generator vessel wall  112 . 
     In accordance with the present invention, the rim  142  of the collar  130  includes an annular portion  143  situated between the downstream and upstream end portions  131  and  133  of the collar  130 , and configured with a plurality of circumferentially spaced grooves  144  which serve to lengthen the path for heat conduction thereby reducing the thermal gradients and the thermal stresses resulting therefrom. The land segments  146  located between the grooves  144  provide the force transfer path between the flanged ring  134  and the pressure vessel wall  112 . The threaded studs  138  pass through the corresponding apertures  124  and  136  and cooperate with the lock nuts  140  to forcibly and rigidly secure the feedwater inlet nozzle  118  to the vessel wall  112 . 
     The inner surface of the feedwater inlet nozzle  118  defines a cylindrically-shaped bore  148 . The portions of the bore  148  which lie within the nozzle inlet portion  126  and the nozzle outlet portion  128  are sized to obtain a tight or, alternatively, an interference fit between the inner surface of the nozzle inlet portion  126  and the outer surface of the thermal sleeve inlet. portion  156 , and between the inner surface of the nozzle outlet portion  128  and the outer surface of the thermal sleeve outlet portion  157 . 
     The nozzle inner surface, which lies intermediate of the respective inner surfaces of the tight or interference fitting nozzle portions  126  and  128 , is configured to form a recess  147  therein and-to cooperate with the recessed walls  149  and the outer surface of the thermal sleeve  120  to define an enclosed annular chamber  150  therebetween. The chamber  150  is provided with a passageway opening  152  extending through the body of the feedwater inlet nozzle  118 . The opening  152  is preferably threaded to accommodate the closing thereof with a threaded plug  154 , as shown at FIG.  3 . 
     In accordance with the present invention, a dry gaseous medium, for example, dry nitrogen or dry air is introduced through the passageway opening  152  into the comparatively lengthy chamber  150  which, when filled, is closed off with the plug  154 . Dry nitrogen gas is the preferred medium for filling the chamber  150  since it can reduce erosion. The annular chamber  150  covers a major lengthwise portion of the feedwater nozzle  118  and the dry gaseous medium, which fills the annular chamber  150 , forms a thermal barrier between the relatively cool feedwater flowing through the thermal sleeve  120  and the surrounding portions of the heated feedwater inlet nozzle  118  and pressure vessel wall  112 , and thus acts to moderate the thermal gradients and the thermal stresses resulting therefrom. 
     The inlet portion  156  of the thermal sleeve  120  extends from within the outlet end portion  158  of the feedwater supply conduit  160  through the bore  148  of the feedwater inlet nozzle  118  and through the pressure vessel wall inlet  116 . The outlet end of the thermal sleeve  120  is welded to the inlet end of the sparger  122 . Alternatively, the sparger  122  may be formed as an integral part of the thermal sleeve  120 . The outer surface of the thermal sleeve  120  is in tight or, alternatively, interference fit engagement with the inner surface of outlet end portion  158  of the feedwater supply conduit  160 . 
     In accordance with the present invention, the thermal sleeve  120  extends within the outlet portion  158  of the feedwater supply conduit  160  and the inlet portion  126  of the feedwater inlet nozzle  118  in tight or interference fit engagement and is fixedly connected by a first dissimilar weld  162  to the inlet end  164  of the feedwater inlet nozzle  118  and the outlet end  165  of the feedwater supply conduit  158 , and is further fixedly connected by a second dissimilar weld  166  to the outlet end  168  of the feedwater inlet nozzle  118 . The welds  162  and  166  are referred to as dissimilar welds since they are used to join components of different metal composition as in the case of the nozzle  118  and the thermal sleeve  120 . The two-point support provided by the tight engagement and the dissimilar welds  162  and  166  for the thermal sleeve  120  and sparger  122  assembly acts to increase the mechanical strength of the feedwater apparatus  110  and, particularly, that of the thermal sleeve  120  and sparger  122  assembly, with a concomitant reduction in stress corrosion. 
     Moreover, the present invention provides full access to the welds used to structure the feedwater apparatus  110 , thereby facilitating the inspection and repair of such welds. Furthermore, the construct of the feedwater apparatus  110  allows for the thermal sleeve second dissimilar weld  166  to be placed within the bore  114  of the inlet  116  of the steam generator vessel wall  112 , rather than having to locate this weld in the constricted annular cavity  132 , as in the case of the prior art feedwater apparatus  10 , shown in FIG. 1, where the dissimilar weld  44  had to be placed in the constricted passage  50 . As a result of providing full access to all of its welds, the construct of the present invention assures the integrity of such welds. 
     The underside of the outlet end portion  170  of the sparger  122  includes a plurality of spray holes  172  which produce the desired spray pattern, while directing the relatively cool feedwater downward through an annular passageway  174  formed between the steam generator vessel wall  112  and a shroud  176  that defines a conventional evaporator chamber, not shown. The direction of the downward sprayed feedwater is generally away from the vessel wall  112  so as to avoid local temperature variations, and thereby prevent thermal cycling of the steam generator vessel wall  112 . 
     In accordance with the present invention, the downstream end  178  of the sparger  122  is advantageously formed with a downward sloped deflector plate  180  which acts to direct the feedwater toward the spray holes  172 . The defector plate  180  can be welded to the downstream end  178  of the sparger  122 , as shown in FIG. 2, or it can be formed as an integral part of the sparger  122 . The deflector plate  180  extends at an angle of 45 degrees measured clockwise from the longitudinal axis  182  of the sparger  122 . The 45 degree slope of the deflector plate  180  acts to smoothen the feedwater flow and, thus, reduces the flow-induced vibration and fretting. 
     Although the present invention has been described above with reference to particular means, materials and embodiments, it is to be understood that this invention may be varied in many ways without departing from the spirit and scope thereof, and therefore is not limited to these disclosed particulars but extends instead to all equivalents within the scope of the following claims.