Abstract:
A U-tube steam generator having a dual system for collecting loose parts and sludge. A loose parts collector having a water overflow edge is disposed between a feedwater inlet and a tube bundle of the steam generator. A sludge collector having a water outlet that is disposed downstream of the overflow edge of the loose parts collector and maintains a pressure differential between a water inlet of the sludge collector and the water outlet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to application Ser. No. 11/563,742 filed Nov. 28, 2006. This application is a Continuation of Provisional Application No. 60/977,406, filed Oct. 4, 2007 from which this application claims priority. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to steam generators for nuclear power plants and more particularly, to vertical, U-tube steam generators having both loose parts and sludge collectors. 
     2. Description of Related Art 
     A nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet and a channel head forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and a wrapper on the inside, and a feedwater ring disposed above the U-like curvature end of the tube bundle. 
     The primary fluid having been heated by circulation through the reactor enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator secondary side through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. Upon entering the steam generator, the feedwater mixes with water returning from steam separators, called the recirculation stream. This mixture, called the downcomer flow is conducted down the annular chamber adjacent to the shell until the tube sheet near the bottom of the annular chamber causes the water to reverse direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam, and the steam vapor then exits the steam generator and is typically circulated through electrical generating equipment to generate electricity in a manner well known in the art. 
     Since the primary fluid contains radioactive materials and is isolated from the feedwater only by the U-tube walls, the U-tube walls form part of the primary boundary for isolating these radioactive materials. It is, therefore, important that the U-tubes be maintained defect-free so that no breaks will occur in the U-tubes that will cause radioactive materials from the primary fluid to enter the secondary side; an undesirable result. 
     Loose parts enter the steam generator through the feedwater stream and can cause damage to the heat transfer tubes. Sludge can also enter the steam generator through the feedwater stream and can also cause damage to the heat transfer tubes. The size of sludge particles are in the range of micrometers while loose parts are in the range of inches. Sludge tends to deposit on tube surfaces and eventually lead to chemical concentrations that cause tubing corrosion. The damage caused by loose parts and sludge can result in having to plug or repair the damaged tubes to avoid contamination of the secondary fluid. In extreme cases, the damage can lead to a tube leak and forced outage with significant expense to the plant. Therefore, it is important to prevent foreign objects and minimize sludge from entering the steam generator and/or to remove the loose parts and sludge from the steam generator before tube damage occurs. 
     Prior attempts to prevent steam generator loose parts from reaching the tube bundle have focused on a sieving action. For example, spray nozzles with small holes have been attached to the feedwater distribution ring to trap loose parts. Although such spray nozzles have succeeded in trapping larger parts, small parts may pass through the holes in the nozzles due to their size. Such loose parts, e.g., pieces of metal rope or rods, have caused tube damage in operating steam generators. 
     Co-pending application Ser. No. 11/563,742 filed on Nov. 28, 2006 (NSD 2005-013) proposes an improved loose parts collector that employs a weir having a vertical wall that surrounds at least a portion of a lower deck plate of the steam generator. The weir has a radially inwardly extending lip affixed to the upper end of the wall that traps loose parts as the combined feedwater and recirculation stream flows over the weir and down the downcomer between the wrapper and the shell. This proposed arrangement provides a substantial improvement to the collection of loose parts. 
     Previously, sludge collectors had been used which generally provided collector boxes that sat on top of the lower deck plate. The collector boxes typically had centrally disposed water inlet holes and peripherally disposed water outlet holes around the circumference of the collector box lid. The sludge collector box would draw a portion of the feedwater from the feedwater ring and the recirculation stream into the collector box through the water inlet holes on the lid. The water passes through the collector towards the periphery of the box in extremely slow motion providing the particles time to settle to the floor of the collector box while the water continues exiting the collector box through the outlet holes at the edge of the lid of the collector box. The water enters the collector due to a pressure differential between the inlet and outlet holes on the lid of the collector box. The pressure differential is due to the fact that the fluid flow near the inlet holes is relatively quiescent compared to the flow over the outlet holes where the remaining portion of the feedwater from the feedwater ring and the recirculation stream is rushing into the downcomer annulus at a relatively high velocity. Therefore, fluid static pressure is relatively high at the inlet holes and low at the outlet holes and a pressure differential is developed between them that is the driving force to draw the water into the box of the sludge collector. 
     The loose parts collector proposed by application Ser. No. 11/563,742, filed Nov. 28, 2006 (NSD 2005-013) consisting of the weir having a circular vertical skirt around the edge of the lower deck together with a horizontally inwardly extending lip complicates and to a degree compromises the operation of the sludge collector. The water pool on top of the lower deck plate receives both recirculated water and feedwater. The recirculated water comes from the separated water from the primary separator. The separated water is at the saturation temperature. The feedwater discharges into the water pool via spread tubes or J-tubes of the feedwater ring. The feedwater is subcooled, about 100° F. below the saturation temperature. The separated, saturated water will mix with the subcooled feedwater in the water pool. The mixed water is still subcooled (about 20° F. below the temperature of the saturated water) and flows over the edge of the lower deck plate and downward along the downcomer annulus. 
     The water mass in the water pool over the lower deck plate is large compared to the mass of the feedwater flow and the recirculated, saturated water flow. Thus, the fluid flow in the water pool and on the lower deck plate is generally slow except for the limited local zones where water enters and leaves the pool. Therefore, loose parts will not travel far prior to falling on lower deck plate. The weir will therefore retain the fallen loose parts. However, the weir also restricts flow at the outlet of the sludge collector and, thus, pressure is essentially uniform on top of the lower deck. In other words, both the center and edge are at the same high pressure. Thus, because of the weir of the loose part collector, the pressure differential between the inlet and the outlet holes of the sludge collector disappears and thus there would be no flow within the sludge collector box when combined with such a loose parts collector. In other words, the installation of the loose parts collector described in co-pending application Ser. No. 11/563,742 (NSD 2005-013) has the potential to destroy the function of the sludge collector. 
     Accordingly, it is an object of this invention to provide an improved loose parts collector and sludge collector that will not impede the performance of either. 
     Furthermore, it is an object of this invention to provide such a dual sludge and loose parts collector that will efficiently collect both loose parts and sludge. 
     Additionally, it is an object of this invention to provide such a loose parts and sludge collector that will not impede the efficiency of the steam generator. 
     SUMMARY OF THE INVENTION 
     This invention achieves the foregoing objectives by providing a dual system that will collect sludge as well as loose parts that enter a steam generator with each function sharing the pool of water that is supported on the lower deck plate of the steam generator. This steam generator includes a feedwater inlet and a tube bundle spaced from the feedwater inlet. The loose parts collector is disposed between the feed water inlet and the tube bundle with the loose parts collector having a water overflow edge which partially retains the pool of water on the lower deck plate fed from the feedwater inlet. The sludge collector shares, in part, the pool of water and has a water outlet disposed downstream of the overflow edge of the loose parts collector. 
     In one preferred embodiment the loose parts collector includes a vertical wall that extends in an upward direction from the lower deck plate of the steam generator and surrounds the feedwater pool fed by the feedwater inlet. The sludge collector water outlet is disposed downstream of an interior side of the vertical wall. Desirably the loose parts collector is positioned on top of the sludge collector. 
     In another embodiment, the sludge collector comprises a vertical wall which extends upward from a lower deck plate of the steam generator and surrounds a pool of water fed by the feedwater ring. The sludge collector also comprises a cover that is supported spaced above the lower deck plate with the cover having a number of access openings disposed inwardly of the vertical wall. The access openings form an inlet for the feedwater to enter the sludge collector. The peripheral edge of the cover is desirably radially spaced from the vertical wall at least a number of circumferential locations. The cover as a vertically extending ledge that surrounds at least a portion of the cover and terminates in an upward direction to form the overflow edge. At least portions of the vertically extending ledge are spaced from an interior side of the vertical wall to form a conduit therebetween that forms the water outlet of the sludge collector. Preferably, the overflow edge has a radially, inwardly extending lip that forms a weir for the collection of loose parts. In one preferred embodiment the vertical wall and the vertically extending ledge define an annular passage that forms the sludge collector water outlet. In still another embodiment the vertical wall and the vertically extending ledge are formed as one member with the sludge collector water outlet extending therethrough. In the latter embodiment the sludge collector water outlet may be formed by a series of tubes extending through the one member and circumferentially spaced around the periphery of the lower deck plate. 
     In still another embodiment the invention includes a nuclear reactor power generation facility having a steam generator that includes the afore described dual system for the collection of loose parts and sludge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view, partially cut away, of a vertical steam generator; 
         FIG. 2  is a cross-section of the upper portion of the vertical steam generator illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic of the lower deck plate portion of a steam generator that illustrates the operation of a conventional sludge collector; 
         FIG. 4  is a schematic of a portion of the area just above the lower deck plate of a steam generator that illustrates the operation of the loose parts collector of application Ser. No. 11/563,742 (NSD 2005-013); 
         FIG. 5  is a schematic of the area around the lower deck plate portion of a steam generator illustrating the affect of combining the sludge collector of  FIG. 3  with the loose parts collector of  FIG. 4 ; 
         FIG. 6  is a schematic of the area around the lower deck plate portion of a steam generator illustrating the improvement of this invention; 
         FIG. 7  is a schematic of the area around the lower deck plate portion of a steam generator illustrating a second embodiment of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIG. 1  shows a steam or vapor generator  10  that utilizes a plurality of U-shaped tubes which form a tube bundle  12  to provide the heating surface required to transfer heat from a primary fluid to vaporize or boil a secondary fluid. The steam generator  10  comprises a vessel having a vertically oriented tubular lower shell portion  14  and a top enclosure or dished head  16  enclosing the upper end and a generally hemispherical shaped channel head  18  enclosing the lower end. The lower shell portion  14  is smaller in diameter than the upper shell portion  15  and a frustoconical-shaped transition  20  connects the upper and lower portions. A tube sheet  22  is attached to the channel head  18  and has a plurality of holes  24  disposed therein to receive ends of the U-shaped tubes. A dividing plate  26  is centrally disposed within the channel head  18  to divide the channel head into two compartments  28  and  30 , which serve as headers for the tube bundle. Compartment  30  is the primary fluid inlet compartment and has a primary fluid inlet nozzle  32  in fluid communication therewith. Compartment  28  is the primary fluid outlet compartment and has a primary fluid outlet nozzle  34  in fluid communication therewith. Thus, primary fluid, i.e., the reactor coolant, which enters fluid compartment  30  is caused to flow through the tube bundle  12  and out through outlet nozzle  34 . 
     The tube bundle  12  is encircled by a wrapper  36 , which forms an annular passage  38  between the wrapper  36  and the lower shell  14  and frustoconical transition cone  20 . The top of the wrapper  36  is covered by a lower deck plate  40  which includes a plurality of openings  42  in fluid communication with a plurality of riser tubes  44 . Swirl vanes  46  are disposed within the riser tubes to cause steam flowing therethrough to spin and centrifugally remove some of the moisture contained within the steam as it flows through this primary centrifugal separator. The water separated from the steam in this primary separator is returned to the water pool  80  (see  FIG. 2 ). After flowing through the primary centrifugal separator, the steam passes though a secondary separator  48  before reaching a steam outlet nozzle  50  centrally disposed in the dished head  16 . 
     The feedwater inlet structure of this generator includes a feedwater inlet nozzle  52  having a generally horizontal portion called a feedring  54  and discharge nozzles  56  elevated above the feedring. Feedwater, which is supplied through the feedwater inlet nozzle  52 , passes through the feedring  54 , and exits through discharge nozzles  56  and mixes with water which was separated from the steam and is being recirculated. The mixture then flows down over the lower deck plate  40  and into the annular passage  38 . The water then enters the tube bundle at the lower portion of the wrapper  36  and flows among and up the tube bundle where it is heated to generate steam. 
       FIG. 2  is a cross-sectional view of the upper portion of the steam generator shown in  FIG. 1 . The same reference characters are employed to designate the corresponding components in the several figures. The generator illustrated in  FIGS. 1 and 2  includes the loose parts collector weir  60  described in U.S. patent application Ser. No. 11/563,742 (NSD 2005-013). The loose parts collector weir  60  is a nearly cylindrical wall structure that is interior to the upper drum, i.e., the interior volume above the lower deck plate  40  of the steam generator  10 , to retain loose parts along the transit path from the feedwater discharge nozzle  56  to the tube bundle  12 . The weir  60  is a vertical, or nearly vertical structure formed as an integral part of or affixed to the lower deck plate  40 , such as by welding, at or near the periphery of the lower deck plate  40  and circumscribes the lower deck plate surface, preferably at its near peripheral location. Desirably, the loose parts collector weir  60  includes an inwardly projecting lip which, along with the weir  60  and the lower deck plate  40  to which it is attached form a pocket  70  that captures the loose parts without the possibility of re-entrainment until the parts are removed from the steam generator  10  during a normal outage. As water flows from the top of the lower deck  40  towards the downcomer annulus  38 , loose parts will tend to be deposited onto the lower deck plate aided by gravity and be retained by the loose parts collector weir  60 . 
     Some operating generators have sludge collectors  68  integrated with the lower deck plate  40 .  FIG. 3  is a schematic of a portion of the steam generator  10  illustrating the operation of a traditional sludge collector  68 . The sludge collector  68  is generally a box that sits on top of the lower deck plate  40 . The lower deck plate  40  forms the bottom of the box and a cover  69  (also previously referred to as a lid) is supported above and spaced from the lower deck plate  40 . The cover  69  has centrally disposed water inlet holes  76  that receive a portion of the recirculated water from the primary moisture separator  72  and some of the feedwater from the feedwater discharge nozzles  56 . The water that enters the feedwater sludge collector box  68  is discharged from peripheral openings  78  around the circumference of the cover  69  where it joins the remainder of the recirculated water and new feedwater and flows over the wrapper  36  into the downcomer passage  38 . The sludge collector draws water into the collector box  68  through the inlet holes  76  on the cover  69  of the collector where the water passes through the collector at an extremely slow speed which permits the sludge particles to settle on the floor of the collector while the water continues its passage to the outlet holes  78  in the cover  69  of the collector. The water is drawn into the collector as a result of a pressure differential between the inlet  76  and outlet  78  holes on the cover  69 . This pressure differential is developed because the fluid flow near the inlet  76  is relatively quiescent when compared to the flow over the outlet hole  78  where the water is rushing into the downcomer annulus  38  at a relatively high velocity. Therefore, fluid static pressure is relatively high at the inlet holes  76  and low at the outlet holes  78  resulting in a pressure differential between the inlet and outlet that forms the driving force to draw the water into the box of the sludge collector  68 . 
       FIG. 4  is a schematic of the portion of the steam generator  10  shown in  FIG. 3  showing the loose parts collector  60  previously described.  FIG. 5  is a schematic of a portion of a steam generator  10  shown in  FIG. 4  combining the loose parts collector  60  shown in  FIG. 4  with the traditional sludge collector shown in  FIG. 3 .  FIG. 5  shows the loose parts collector  60  on top of the sludge collector  68 . Because of the weir of the loose parts collector, the pressure differential between inlet  76  and outlet holes  78  disappears. Thus, there would be no flow within the sludge collector  68  in the arrangement shown in  FIG. 5 . In other words, the installation of a loose parts collector in combination with the sludge collector as applied by the prior art, destroys the function of the sludge collector  68 . 
       FIG. 6  illustrates a schematic of the portion of the steam generator previously shown in  FIGS. 3 ,  4  and  5  with the improvement provided by this invention that enables sludge to be collected in combination with a loose parts collector as previously described. The key to restoring the pressure differential so that water is drawn into the sludge collector  68  and at the same time enabling the weir  60  to perform its function to stop loose parts from entering the downcomer  38  is to have the water outlet holes  78  of the sludge collector  68  downstream of the weir  60  as illustrated in  FIG. 6 . The sludge collector  68  of this invention is formed from an outside wall  64  that circumscribes an interior portion and is attached to the upper surface of the deck plate  40  and extends upward to a predetermined height approximately equal to the height of the weir  60  of the prior art. The weir  60  of this invention further comprises a concentric interior wall  58  that is spaced from the outer wall  64  and extends from and is attached to the cover  69  of the sludge collector  68  to approximately the height of the outside wall  64 . The weir lip  62  extends from the outside wall  64  radially inward over the inside wall  58 , to which it is attached, and is cantilevered inward as in the prior art to prevent loose parts from being drawn over the weir. A sludge water outlet  78  is formed in the lip  62  between the inside wall  58  and the outside wall  64  of the sludge collector  68  so that the sludge outlet water is drawn into the fast moving stream of the water passing over the weir into the downcomer passage  38 . The outside wall  64  extends the sludge collector  68  all the way to and through the lip  62  of the weir  60 . This extended rectangular box appears as a circular ring in three dimensions. Such an extension reestablishes the pressure differential and enables the water to be drawn into the sludge collector  68  at the water inlet  76  previously shown. Thus, with this improvement sludge particles can settle in the sludge collector  68  as water moves through the collector. 
       FIG. 7  depicts an alternative design for restoring the pressure differential. In this embodiment the coupling of the sludge collector pool with the water passing over the weir is achieved through the use of conduits or tubes that are circumferentially spaced around the cover  69  of the sludge collector  68  and extend vertically through the weir lip  62  of the loose part collector weir  60 . 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalence thereof.