Abstract:
A uniform leakage flow device for the slip point of piping systems, and particularly in reactor pressure vessels, selectively imposes a steady, uniform flow of fluid through the slip joint between two adjacent pipe surfaces to thereby eliminate the detrimental flow-induced vibration associated with the unsteady and non-uniform leakage of fluid through the slip joint field. The uniform leakage flow device comprises a plurality of axial grooves that are formed in the wall surface of the slip joint.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a non-provisional application claiming priority to provisional patent application Ser. No. 60/834,929 filed Aug. 2, 2006. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to tubular jet pumps used in various industries to transport and/or circulate cooling liquid in heat-generating systems, such as nuclear reactors and hydroelectric generation systems. More particularly, this invention relates to means for uniformly controlling the leakage flow rate through a slip joint to thereby eliminate detrimental vibration in the slip joints of such jet pumps. 
         [0004]    2. Description of Related Art 
         [0005]    Pipes, tubes and cylinders are used to transport a variety of fluids, such as water, oil, and liquid chemicals in various industries including the nuclear industry, the electric power industry, such as for internal components of heat exchangers, the hydroelectric power generation industry, the petroleum industry, such as piping used in refining of oil, the chemical industry, such as the piping used in processes for making chemical based products, and the space industry, for spacecraft heat exchangers and other similar devices. 
         [0006]    Oftentimes, the piping components in such industrial systems are submerged in the same fluids which the piping is transporting. As an example, the tubular components that make up a jet pump assembly are housed within a nuclear reactor pressure vessel and reside in the fluid that the jet pump is used to transport. That is, the jet pump assembly transports the cooling water to the reactor core, but the jet pump assembly itself is also submerged in that same fluid. The pipes and tubes that comprise such submerged systems are supported within the surrounding structures by support or restraining apparatus. The surrounding structures (e.g., a reactor vessel) may be of a different material, such as carbon steel (reactor pressure vessel), than the material that the piping is made of, such as stainless steel (jet pump assembly) with different thermal coefficients of expansion. In order to accommodate the different amounts of axial thermal expansion that will occur between the tubes and the surrounding support structure at higher operating temperatures, designers install slip joints along the piping to minimize thermal stress build up within the tubes. 
         [0007]    Recent engineering experience has shown that if a sufficient pressure gradient exists across these slip joint interfaces, the connecting tubular components may incur detrimental flow-induced vibration, and failure results from either excessive wear or fatigue of the piping material or support/restraining apparatus. One exemplar system where such failure occurs is the jet pump assemblies used in nuclear reactors. 
         [0008]    A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends with typically a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV and a core shroud, typically surrounds the core and is supported by shroud support structure. The shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. A space, or annulus, is located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud. A plurality of jet pumps are positioned within the annulus. An typical example of such reactor cores is disclosed in U.S. Pat. No. 4,675,149 to Perry, et al. 
         [0009]    In a BWR, the hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. Examples of such jet pump assemblies are disclosed in U.S. Pat. No. 6,587,535 to Erbes, et al. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against opposing contacts within the restrainer bracket by a gravity-actuated wedge and two set screws. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. 
         [0010]    The lower portion of the jet pump, known as the diffuser, is coupled to the inlet mixer by a slip joint. This construction facilitates the disassembly and repair of the jet pump. The slip joint between the jet pump inlet mixer and the jet pump diffuser collar has about a 0.015 inch diametral operating clearance which accommodates the relative axial thermal expansion movement between the upper and lower parts of the jet pump and permits leakage flow from the driving pressure inside the pump. A limited amount of leakage may be beneficial to clean the joint of corrosion product build up. 
         [0011]    Excessive leakage flow, however, can cause oscillation motion in the slip joint, which is a source of detrimental vibration excitation in the jet pump assembly. The slip joint leakage flow rate can increase due to single loop operation, increased core flow, or deposition of jet pump detritus, or crud, in the slip joint. Additional detrimental conditions that may lead to damaging vibration between the inlet mixer and diffuser of the jet pump assembly are well-known, such as loss of the set screw support in a jet pump assembly as described in U.S. Pat. No. 6,394,765 to Erbes, et al. 
         [0012]    In addition to affected set screw gaps, thermal and pressure displacements of the shroud and the pressure vessel can diminish alignment interaction loads in the jet pump assembly which are beneficial in restraining vibration. The resultant increased vibration levels and corresponding vibration loads on the piping and supports can cause jet pump component degradation from wear and fatigue. 
         [0013]    High levels of flow-induced vibration (FIV) are possible in some jet pump designs at some abnormal operational conditions having increased leakage flow rates. Reducing leakage flow through the slip joint prevents or reduces oscillatory slip joint motion and suppresses FIV. Prior efforts to reduce the leakage flow rate in jet pump slip joints have been disclosed in U.S. Pat. No. 6,394,765, which discloses an external clamp apparatus for laterally stabilizing the slip joint; U.S. Pat. No. 6,438,192 to Erbes, et al., which discloses a split ring seal and latch assembly positioned at the upper end of the diffuser tube to stabilize the inlet mixer; U.S. Pat. No. 6,450,774 to Erbes, et al., which discloses a device for producing a lateral support load on the slip joint by causing an ovate deformation in the diffuser when attaching it to the inlet mixer; and U.S. Pat. No. 6,587,535 to Erbes, et al., which discloses a labyrinth seal in the slip joint for reducing slip joint leakage flow. 
         [0014]    Each of the previously disclosed inventions has demonstrated some characteristic which has rendered the device or method insufficient in producing effective reduction of slip joint-induced vibration, and none has been directed to providing a means for selectively controlling leakage flow rate through the slip joint to ameliorate the damaging effects of excessive leakage flow rate through the slip joint. In addition, those devices and methods that impose a lateral force on the slip joint also prevent axial movement in the slip joint, which does not properly allow for thermal expansion in the slip joint. 
         [0015]    It would be advantageous in the industry to provide a device for reducing or eliminating non-uniform, or unsteady, leakage flow rates though a slip joint by selectively controlling the uniform leakage of fluid through a slip joint in order to control, and thereby eliminate, the detrimental vibration and other damaging conditions that occur in unsteady slip joints, i.e., those slip joints through which non-uniform leakage flow occurs. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    In accordance with the present invention, a uniform leakage flow device is provided in a slip joint between intercoupled pipes to selectively control the amount of fluid allowed to leak through the slip joint, thereby selectively eliminating the amount of detrimental vibration or oscillation that otherwise occurs at the slip joint. The present invention may be adapted for use in any slip joint between intercoupled pipes, but is disclosed herein with respect to jet pump assemblies of the type used in reactor pressure vessel, by way of example. 
         [0017]    The uniform leakage flow device of the present invention comprises forming a selected number of axial grooves in the slip joint between an inlet mixer and the diffuser. The axial grooves may be formed in either the outer wall surface of the inlet mixer at the slip joint or in the inner wall surface of the diffuser at the slip joint. The axial grooves are machined into the wall surface either at initial assembly of the jet pump components, or after operation of the reactor pressure vessel has taken place. Methods for effecting formation of the axial grooves both prior to and after operation of the reactor pressure vessel are disclosed. 
         [0018]    The axial grooves are formed in a precise manner to selectively control and make uniform the flow of fluid leakage through the slip joint. The number, size and positioning of the axial grooves in the wall surface is determined by the application of equations as disclosed herein. 
         [0019]    In known slip joint structures, mechanical devices have been invented to eliminate or reduce the slip joint leakage flow and in turn eliminate the detrimental slip joint flow induced vibration mechanism. The present invention is directed to a completely different approach by providing a means to directly destroy the flow induced vibration mechanism by allowing the leakage flow to pass uniformly through the slip joint by way of machined axial grooves. In the presence of these axial grooves, the slip joint leakage flow will always be uniform and even around the circumference of the slip joint. 
         [0020]    Further, in known slip joint structures, both the total leakage flow and the leakage flow around the annulus and within the slip joint varied depending on the relative position of the mating parts. Non-uniform slip joint leakage flow, in both the total flow and peripheral flow, incurred time-varying lateral forces on the mating parts of the slip joint. These time-varying lateral forces incurred relative motion of the mating parts. This relative motion lead to increased non-uniform flow and higher fluid forces that, in turn lead to higher levels of vibration and wear of the mating jet pump components. This spiraling condition lead to higher and higher vibration levels of the mating parts. The axial grooves of the present invention impose uniform leakage flow at the slip joint location. The imposed uniform leakage flow does not change in its total amount, nor periphery within the slip joint annulus due to relative motion of the slip joint mating parts. The uniform leakage flow cannot incur net, time-varying lateral fluid forces that cause detrimental vibration and relative lateral motion of the slip joint mating parts. 
         [0021]    The placement of axial grooves in the slip joint reduces or eliminates non-uniform or unsteady leakage flow through the slip joint and thereby reduces or eliminates vibration levels associated with non-uniform or unsteady slip joint leakage flow. The present invention provides the further advantage of allowing axial movement between the jet pump components and supporting structure to accommodate thermal expansion in the jet pump assembly, and does not impose undue stress in the inlet mixer. The present invention also eliminates the use of structures, such as clamps and the like, which present a potential detriment of having loose parts moving in the jet pump assembly, recirculation system or reactor core. These and other advantages of the present invention will become more clear in light of the following detailed description. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0022]    In the drawings, which illustrate what currently is considered to be the best mode for carrying out the invention: 
           [0023]      FIG. 1  is a partial, schematic view of a nuclear reactor, shown in cutaway, illustrating a conventional jet pump assembly positioned in the annulus of the reactor; 
           [0024]      FIG. 2  is an enlarged view in cross section of a slip joint between the inlet mixer and diffuser of a jet pump; 
           [0025]      FIG. 3  is an enlarged view in cross section of a slip joint in which the uniform leakage flow rate device of the present invention is provided; 
           [0026]      FIG. 4  is a cross section view taken at line  3 - 3  of  FIG. 3  illustrating the size, number and positioning of axial grooves in the inlet mixer outer wall; 
           [0027]      FIG. 5  is an enlarged view in cross section of an alternative embodiment of the invention where the axial grooves are formed in the inner wall of the diffuser; and 
           [0028]      FIG. 6  is a cross section view taken at line  5 - 5  of  FIG. 5  illustrating the size, number and position of axial grooves in the diffuser wall. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIG. 1  is a schematic illustration of a portion of a conventional reactor pressure vessel (RPV)  20  for a boiling water reactor. Such reactors are previously described in U.S. Pat. No. 4,675,149 and U.S. Pat. No. 6,587,535, the disclosures of which are incorporated herein. The RPV  20  has a generally cylindrical shape and is closed at one end by a bottom head (not shown) and at its other end by removable top head (not shown). A top guide (not shown) is spaced above a core plate  22  within RPV  20 . A shroud  24  surrounds the core plate  22  and is supported by a shroud support structure  26 . An annulus  28  is formed between the shroud  24  and sidewall  30  of the RPV  20 . 
         [0030]    An inlet nozzle  32  extends through the sidewall  30  of the RPV  20  and is coupled to a jet pump assembly  34 . The jet pump assembly  34  includes a riser pipe  38  and a plurality of inlet mixers  42  connected to the riser pipe  38  by a transition assembly  44 . A diffuser  46  is connected to and positioned below each of the inlet mixers. A slip joint  48  couples each inlet mixer  42  to a corresponding diffuser  46 . 
         [0031]      FIG. 2  is illustrates in an enlarged cross sectional view the relative positioning of an inlet mixer  42  and diffuser  46 . It can be seen that the inlet mixer  42  is generally cylindrical and has an outer wall surface  50 . The inlet mixer  42  has an open end  58  which is received in an open end  60  of the generally cylindrical diffuser  46 . The diffuser  46  has an inner wall surface  52  positioned adjacent to the outer wall surface  50  of the inlet mixer  42 . An operational clearance  54  exists at an interface  56  between the outer wall surface  50  of the inlet mixer  42  and the inner wall surface  52  of the diffuser  46 . When fluid is pumped through the inlet mixer  42  into the diffuser  46 , in the direction of arrow  62 , leakage of some of the fluid occurs through the clearance  54  in the slip joint  48 , as shown by arrow  64 . 
         [0032]    Leakage flow from within the jet pump at the slip joint  48  interface between the inlet mixer  42  and the diffuser  46  can become unsteady and non-uniform due to relative lateral motion between the two mating parts, the inlet mixer  42  and diffuser  46 . This unsteady slip joint leakage flow is the source of a detrimental vibration excitation in the jet pump assembly  34 . High levels of flow induced vibration (FIV) are possible in some jet pump designs at some abnormal operational conditions having increased unsteady slip joint leakage flow rates. Changing the leakage flow characteristics from unsteady flow to steady axial flow through the slip joint can prevent oscillatory slip joint motion and eliminate detrimental, high level FIV. 
         [0033]    Thus,  FIG. 3  illustrates a first embodiment of the invention where axial grooves  60  are formed in the slip joint  48  interface between the outer wall surface  50  of the inlet mixer  42  and the inner wall surface  52  of the diffuser  48 . In this particular embodiment, the axial grooves  60  are formed in the outer wall surface  50  of the inlet mixer  42 . As seen more clearly in  FIG. 4 , a plurality of axial grooves  60  may be formed about the circumference of the inlet mixer  42 . In an alternative embodiment shown in  FIGS. 5 and 6 , the axial grooves  60  may be formed in the inner wall surface  52  of the diffuser. 
         [0034]    The number of axial grooves that may be formed in the wall surface (of either the inner mixer or diffuser) may vary, but should number at least four. In a particularly suitable embodiment of the invention, the number of axial grooves formed in the wall surface may be twelve. The number of axial grooves may exceed twelve in number, however. 
         [0035]    In a particularly suitable embodiment, the depth (d) of the axial grooves is from two to four times the distance (g) of the radial rap or operational clearance  54  defined between the outer wall surface  50  of the inlet mixer and the inner wall surface  52  of the diffuser  46 . The width (w) of the axial groove  60  depends on the additional slip joint leakage flow area introduced by the axial grooves  60 . The additional slip joint leakage area, defined as the sum of the areas (w×d) of each axial groove, should be approximately equal to the original slip joint leakage area, or operational clearance  54 . The width of the axial grooves can be calculated with the above information and the known outside diameter (D) of the inlet mixer  42 . 
         [0036]    The number, width and depth of the axial grooves required to produce a uniform and steady leakage flow through the slip joint can be calculated with the following equations, where A is the slip joint leakage area, N is the number of axial grooves, D is the outer diameter of the inlet mixer, w is the width of the axial groove, d is the depth of the axial groove and g is the distance of the radial gap. In the equations illustrated below, the number “3” indicates an exemplar equation where the depth of the groove is three times the measurement (g) of the radial gap. 
         [0000]        A= 3.14159 ×D×g=N×w&#39;d=N×w× 3 ×g    Equation 1: 
         [0000]        w =3.14159 ×D×g /( N× 3 ×g )=1.0472 ×D/N    Equation 2: 
         [0037]    The uniform leakage flow device of the present invention may be formed in the slip joint either when the jet pump assembly is new (i.e., non-irradiated) and being positioned in the RPV, or the invention can be formed as a retrofit to an existing RPV. In the first method of formation, the axial grooves are machined into the wall surface of the slip joint, (either in the inlet mixer or the diffuser) prior to coupling of the inlet mixer to the diffuser in assembly of the jet pump. 
         [0038]    In the later method of installing the uniform leakage flow device of the invention after the RPV has been in operation, the inlet mixer is removed from the diffuser by means known in the industry. However, because the jet pump has been irradiated during operation of the RPV, the components, comprising the inlet mixer and or diffuser, must be shielded within a water source to protect the workers who are handling the jet pump components. The axial grooves are machined in the wall surface of the inlet mixer or diffuser at the slip joint using tools that may be used underwater. When the axial grooves have been machined into the wall surface of the slip joint, the inlet mixer is re-coupled with the diffuser as is known in the art. 
         [0039]    The uniform leakage flow device described herein produces a selected steady and uniform flow of fluid leakage through the slip joint to control detrimental vibration and oscillation in the jet pump assembly. The present invention also enables axial movement of the jet pump components due to varying thermal expansion rates in the components, while maintaining a comprehensive seal at the slip joint. The number and positioning of the axial grooves may vary depending on the particular installation specifications and can be adapted to any variety of piping systems. Therefore, reference herein to particular embodiments and structures of the invention is by way of example only and not by way of limitation.