Abstract:
A steam nozzle flow restrictor for a steam generator that restricts steam exit flow during a steam line break, but has a low pressure drop during normal operation. The flow restrictor has a support web suspended from the steam outlet nozzle with the web having a central opening concentric with the central axis of the nozzle. A shaft is slidably supported within the central opening of the web and has an orifice plate that is suspended within the steam generator at one end of the shaft, spaced from the steam nozzle. The orifice plate closes against the underside of the nozzle upon encountering increased steam exit flow as a result of a steam line break.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This invention relates generally to steam generators and more particularly to a flow restrictor for a steam outlet nozzle of a steam generator. 
         [0003]    2. Description of Related Art 
         [0004]    A pressurized water nuclear reactor steam generator typically 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 divider 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 a 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 the wrapper on the inside and a feedwater ring disposed above the U-like curvature end of the tube bundle. 
         [0005]    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 and through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator secondary side, i.e., the side of the generator interfacing with the outside of the tube bundle above the tube sheet, through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. In one embodiment, upon entering the steam generator, the feedwater mixes with water returning from moisture separators. This mixture called the downcomer flow, is conducted down the annular chamber adjacent the shell until the tube sheet located at the bottom of the annular chamber causes the water to change 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 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 through a steam exit nozzle and is typically circulated through a turbine to generate electricity in a manner well known in the art. 
         [0006]    Nuclear steam generators typically have flow limiters in the steam exit nozzles to limit the amount of steam that will be exhausted into the containment in the unlikely event of a steam line break. While the flow limiters are an important safety feature they add a penalty that can detract from the efficiency of the generators. The full power pressure drop in the steam nozzle flow limiter of some steam generators can be nearly 20 psi. A pressure drop reduction to 5 psi or less in the steam nozzle flow limiter can have a significant benefit to the outlet steam pressure, and thus favorably increase plant efficiency and revenues. In addition, a reduction in the pressure drop reduces moisture carryover by up to 0.05 percent, which benefits turbine longevity. 
         [0007]    Accordingly, a new flow limiter design is desired that will have a significantly lower pressure drop without sacrificing performance. 
         [0008]    Further, such a new steam nozzle flow limiter design is desired that will have significantly lower moisture carryover. 
       SUMMARY 
       [0009]    These and other objects are achieved by the steam generator described hereafter having a steam outlet with an interior wall defining a steam outlet conduit with a central axis. A support web is suspended from the steam outlet conduit and has a central opening concentric with the central axis. A shaft is slidably supported and axially movable within the central opening and has a stop at one end that prevents the one end from slipping into the opening and an orifice plate at another end. The orifice plate extends perpendicular to the central axis and is spaced from the steam outlet conduit when the stop at the one end rests against the central opening. When the shaft moves to an extent of its travel in a direction to move the stop away from the support web central opening, the orifice plate rests against an input end of the steam conduit. The orifice plate is in a closed position when the shaft moves to the extent of its travel and the orifice plate has one or more openings that restrict flow of steam through the steam outlet conduit as compared to the opening available to the steam outlet conduit when the shaft is in an open position with the stop resting against the central opening. A biasing mechanism maintains the shaft in an open position during normal steam generator operation, but its force is overcome by the increase flow of steam experienced upon a steam line break. 
         [0010]    Preferably, the openings in the orifice plate are a plurality of holes that are symmetrically formed about the axis. Desirably, the openings are formed from two concentric series of holes that are evenly spaced around the axis. 
         [0011]    In one embodiment, the steam outlet conduit has an inlet with a contour and the orifice plate has a complimentary contour. In another embodiment the axis extends in the vertical direction and the biasing mechanism is gravity. Alternatively, the axis need not extend in a vertical direction and the biasing mechanism is a spring supported between the support web and the orifice plate. Desirably, the spring is a helical spring wound on the shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    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: 
           [0013]      FIG. 1  is a perspective view, partially cut away, of a vertical tube and shell steam generator; 
           [0014]      FIG. 2  is a perspective view taken from the underside of the flow restrictor of the embodiment described hereafter with the orifice plate in an open position; 
           [0015]      FIG. 3  is a perspective view of the moveable elements of the flow restrictor described herein taken from the underside of the orifice plate; 
           [0016]      FIG. 4  is a perspective view of the flow restrictor described hereafter taken from the underside with the orifice plate in a closed position; and 
           [0017]      FIG. 5  is an isometric view of the flow restrictor described hereafter with the orifice plate in an open position. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    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 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  13 . A divider 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  12 . 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 . 
         [0019]    The tube bundle  12  is encircled by a wrapper  36  which forms an annular passage  38  between the wrapper  36  and the shell and cone portions  14  and  20 , respectively. 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 larger tubes  44 . Swirl vanes  46  are disposed within the larger tubes  44  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 top surface of the lower deck plate  40 . After flowing through the centrifugal separator, the steam passes through a secondary separator  48  before reaching a steam outlet nozzle  50  centrally disposed in the dished head  16 . 
         [0020]    The feedwater inlet structure of this generator includes a feedwater inlet nozzle  52  having a generally horizontal portion called a feedring  54  and a plurality of discharge nozzles  56  elevated above the feedring. Feedwater, which is supplied through the feedwater inlet nozzle  52 , passes through the feedwater ring  54  and exits through discharge nozzles  56  and, in one prior art embodiment, mixes with water which was separated from the steam and is being recirculated. The mixture then flows down from above the lower deck plate  40  into the annular, downcomer passage  38 . The water then enters the tube bundle  12  at the lower portion of the wrapper  36  and flows among and up the tube bundle where it is heated to generate steam. The moisture laden steam then passes through the openings  42  in the lower deck plate, to and through the riser tubes  44  and vanes  46  which form the primary separators, and to and through the secondary separators  48  to the steam outlet nozzle  50 . From the steam outlet nozzle the dried steam vapor is typically conveyed to a turbine/generator for the production of electricity. The steam exiting the turbine is condensed and recycled back to the feedwater inlet nozzle in a closed cycle to continuously remove heat from the reactor primary coolant loop. The continuous removal of heat from the reactor core in this fashion assists in controlling the temperature of the core. 
         [0021]    A large break in the steam line between the steam outlet nozzle  50  and the turbine can actually affect the ability of the tube bundle  12  to transfer heat from the primary coolant. Therefore, in the unlikely event of such a break, it is desirable to restrict the amount of steam that can escape from the closed cycle secondary loop. For this purpose, a steam nozzle flow restrictor is employed as part of or in place of the steam outlet nozzle  50 . In its preferred form, the steam nozzle flow restrictor should provide little, if any, resistance to steam flow during normal steam generator operation while impeding the loss of steam from the steam generator in the unlikely event of a major steam line break. 
         [0022]    Such a steam nozzle flow restrictor for a nuclear steam generator constructed in accordance with a preferred embodiment described herein is illustrated in  FIGS. 2-5 .  FIG. 2  provides a perspective view of the steam nozzle flow restrictor  58  taken from the underside from which the several moveable components of the flow restrictor can be best observed. The steam nozzle flow restrictor  58  includes a supporting web  60  having three or more radial arms  62  that extend from the interior walls of the steam outlet nozzle  50  or in an insert within the nozzle. The web&#39;s radial arms are equal distantly spaced around the interior walls of the nozzle and have a central hub  64  with an axially extending through hole  66 . The flow restrictor assembly  58  further includes a flow restrictor orifice plate  68  that has a number of axially extending through holes  70 . The through holes  70  are arranged in concentric circles with the holes in each circle having approximately the same diameter and the holes in the interior circle having a smaller diameter than the holes in the exterior circle. Though two such circles are shown, it should be appreciated that the number of holes and the number of circles will depend on the volume of steam that the restrictor will be designed to release to avoid over pressurization of the generator. The orifice plate  68  has a centrally disposed, axially extending shaft  72  that is affixed to an upper side of the orifice plate. The shaft  72  extends axially through the hole  66  in the central hub  64  and is capped at its distal end with a radially extending land that forms a stop  74 , seen in  FIG. 3 , that prevents the shaft from dropping through the web&#39;s central hole  66 .  FIG. 4  shows the flow restrictor of  FIG. 2  with the orifice plate and shaft in a closed position while  FIG. 5  shows an isometric front view of the flow restrictor shown in  FIGS. 2-4  with the orifice plate in the open position. 
         [0023]    During a steam line break, the additional lifting force caused by rapid steam flow causes the flow restrictor orifice plate assembly, comprising the flow restrictor orifice plate  68 , the shaft  72  and the stop  74 , to move upward, thereby causing the orifice plate  68  to seat against the underside of the supporting web  60 . The relocation of the flow restricting orifice plate  68  to the high velocity zone at the underside of the web  60  limits steam flow during peak break conditions and enhances safe shutdown of the reactor. In the closed position shown in  FIG. 4 , the outer periphery of orifice plate holes  70  are substantially sealed off by the underside of the steam outlet nozzle  50  allowing a limited flow through the smaller series of holes  70  in the inner circle on the orifice plate  68 , which limits the pressure buildup within the generator to within design limits. 
         [0024]      FIG. 2  shows the flow restrictor assembly  58  during normal operation. The central rod shaft  72  supports the flow restrictor orifice plate  68  from the web  60  and allows generally unrestricted flow into the steam outlet nozzle  50 .  FIG. 5  is a top view of the flow restrictor assembly  58 , showing the stop block  74  connected to the central shaft  72  and resting on the hub  64  with the flow restrictor orifice plate  68  in the down (unactuated) position. 
         [0025]      FIG. 4  illustrates the flow restrictor  58  after actuation by a steam line break. During a steam line break, the increased flow through the steam outlet nozzle  50  results in a lifting force on the lower face of the restrictor orifice plate  68 , overcoming the assembly dead weight, such that the flow restrictor plate is pressed against the web  60  or the nozzle  50 . The central shaft  72  slides through the hole  66  in the web  60 , guiding the flow restrictor orifice plate  68  to the flow restricted position. 
         [0026]    A number of optional features may be included to meet different functional criteria. For example, the biasing mechanism for biasing the shaft  72  in the open position may comprise a helical spring  76  wound around the shaft  72  between the orifice plate  68  and the web central hub  64 . This alternative will enable the flow restrictor assembly  58  to be employed in the horizontal position. Additionally, a latching element, such as the leaf spring  78 , that is cantilevered from the shaft  72  ( FIG. 3 ), can be employed to latch the flow restrictor assembly  58  in the closed position once it is actuated. Other possible alternatives include legs which extend out from the orifice plate in a radial direction to prevent lateral motion of the lower end of the restrictor assembly; gusseting of the flow restrictor orifice plate to the central shaft; use of a bolted attachment between the central shaft and the orifice plate; and different configurations of the orifice plate attachment, such as the use of multiple plates. 
         [0027]    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 equivalents thereof.