Abstract:
A control valve which includes a discrete, multi-stage, multi-path valve trim, located in concentric cylinders, along the outlet axis of the valve, with potential to characterize flow resistance at different opening points.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 62/111,584 entitled AXIAL RESISTANCE VALVE TRIM DESIGN filed Feb. 3, 2015. 
     
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND 
       [0003]    1. Technical Field 
         [0004]    The present disclosure relates generally to flow control devices and, more particularly, to a control valve which includes a discrete, multi-stage, multi-path valve trim, located in concentric cylinders, along the outlet axis of the valve, with potential to characterize flow resistance at different opening points. 
         [0005]    2. Description of the Related Art 
         [0006]    There is currently known in the prior art a type of control valve commonly referred to as a pintle valve wherein the short extension of a needle-valve tip is used to facilitate the control of fluid through the valve. An exemplary pintle-type fluid flow control device or control valve is described with particularity in Applicant&#39;s U.S. Pat. No. 5,803,119 entitled FLUID FLOW CONTROL DEVICE issued Sep. 8, 1998, the disclosure of which is incorporated herein by reference. 
         [0007]    In a first embodiment of the control valve described in the &#39;119 Patent, a cylindrical plug is moveable axially within a complimentary cylinder. The plug includes a fluid flow path extending axially therein. As the plug is moved axially relative to the cylinder, the area of path entry available for fluid flow and the length of the fluid flow path is varied. As a result, as the plug moves further out of the cylinder, a corresponding increase in fluid flow is obtained in the control valve. In a second embodiment, the plug is devoid of the aforementioned flow path, and is reciprocally movable within the bore of a tapered, annular sleeve. The sleeve is fitted into an external, solid cylinder. The outer surface of the sleeve is provided with a series of axially extending passageways, each of which fluidly communicates with the bore of the sleeve via a plurality of axially spaced and radially directed openings extending through the thickness of the sleeve. As the plug is lifted, successive openings along each passageway are exposed so that flow increases through the sleeve. 
         [0008]    The flow control device or control valve constructed in accordance with the present invention represents an improvement over the control valve described in the &#39;119 Patent, and in particular the second embodiment thereof, by virtue of its inclusion of more intricate and complex flow passages which each have a single entry and exit path. The increased intricacy/complexity of the flow passages within the control valve of the present invention provides for a better flow range/rangeability therethrough. These, as well as other features and advantages of the present invention, will be described in more detail below. 
       BRIEF SUMMARY 
       [0009]    In accordance with the present disclosure, there is provided a control valve having a multi-stage, multi-path trim installed therein. Traditional multi-stage, multi-path valves use a series of concentric cages or stacked disks flow elements to control the flow of fluid inside the valve. The cages or stacked disks contain flow paths that are arranged perpendicular to the valve axis. In the control valve of the present invention, the flow paths are arranged parallel to the valve axis. The present control valve includes an external sleeve valve plug which throttles a flow control element comprising an annular, generally cylindrical cage which is disposed within an outer liner or seat ring. The cage defines a first or upper portion which, in one embodiment, is of a reduced diameter in comparison to a second or lower portion thereof. Fluid enters the upper portion of the cage perpendicular to the axis of the valve, and then turns to flow along the axis of the valve, the fluid traveling within feed slots formed within the upper portion of the cage. Each feed slot defines a portion of a corresponding one of the flow paths, and typically has a generally linear, non-tortuous configuration. Additionally, each of the feed slots has a dedicated, single entry or feed opening, each feed opening in the upper portion of the cage thus being connected to a discrete flow path. It is thus the feed openings in the upper portion of the cage that are throttled by the sleeve plug of the valve. 
         [0010]    In the control valve, each of the feed slots transitions into a corresponding resistance path formed within the lower portion of the cage. As such, each flow path thus comprises the combination of a feed slot, and its corresponding resistance path. Each resistance path has a tortuous configuration, and a dedicated, single exit or outlet opening, thus resulting in each flow path having one fluid entry or inlet point, and one fluid exit or outlet point. Each flow path also features multiple stages of pressure reduction as facilitated by the resistance path thereof as formed in the outside of the lower portion of the cage. The resistance path of each flow path may also feature changes in depth or width which provide for expansion of the flow as it moves toward the corresponding outlet opening. The flow paths discharge into the bore of the cage and out of the valve. The aforementioned outer liner encapsulates the flow paths and isolates the fluid from the valve body. 
         [0011]    The design of the present control valve permits considerable flexibility in the arrangement of the flow paths. The flow paths in this design can be arranged with different types of flow paths to vary the fluid resistance at different opening points. High resistance flow paths may be used near the valve seat, and low resistance or even open-hole ports can be used when the valve is full open. Open-hole ports can also be applied on top of the flow control element to provide a bypass flow path in case the axial flow paths become blocked with particulate. 
         [0012]    Another feature of the design is that the ligament between stages in each flow path can be varied to provide resistance to erosion by extending the length of the flow control element. In conventional multi-stage cage or stacked disk designs, this is only possible by increasing the diameter of the flow control element. Increasing the diameter increases weight by the square of the difference, whereas increases in length increase weight linearly. 
         [0013]    An additional feature of the design is that it can be used with an upstream seat, also known as external sleeve trim. In conventional multi-stage cage or stacked disk designs the valve plug is installed inside the flow control element. In flow-to-close applications, the fluid exiting the flow control element discharges on the valve plug, causing premature erosion. The arrangement of the valve trim of the present invention is better suited to external sleeve trim designs than conventional multi-stage cage or stacked disk designs. 
         [0014]    Still further, the control valve of the present disclosure provides superior erosion resistance compared to traditional single-stage or even multi-stage designs. The number of pressure reduction stages can be significantly higher than in traditional designs, which reduces fluid velocities which cause erosion. The width of the ligaments inside the flow paths can be significantly greater, which provides longer life in erosive services. The design can be used with an upstream seat/external sleeve trim, which relocates the plug out of areas of high velocity. 
         [0015]    The present disclosure is best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    These, as well as other features of the present disclosure, will become more apparent upon reference to the drawings wherein: 
           [0017]      FIG. 1  is a partial cross-sectional view of a control valve constructed in accordance with a first embodiment of the present disclosure, illustrating the sleeve plug thereof in an open position; 
           [0018]      FIG. 2  is a cross-sectional view similar to  FIG. 1 , but depicting only the sleeve plug and flow control element of the valve, with the sleeve plug being in the open position; 
           [0019]      FIG. 3  is a partial cross-sectional view of the control valve constructed in accordance with the first embodiment of the present disclosure, illustrating the sleeve plug thereof in a closed position; 
           [0020]      FIG. 4  is a cross-sectional view similar to  FIG. 3 , but depicting only the sleeve plug and flow control element of the valve, with the sleeve plug being in the closed position; 
           [0021]      FIG. 5  is a side-elevational view the flow control element of the control valve shown in  FIGS. 1-4 ; 
           [0022]      FIG. 6  is a side-elevational view of the flow control element shown in  FIG. 5 , the outer sleeve or liner of the flow control element being transparent to facilitate the depiction of the internal cage thereof; 
           [0023]      FIG. 7  is a perspective view of the flow control element shown in  FIGS. 5 and 6 , the outer sleeve or liner of the flow control element being transparent to facilitate the depiction of the internal cage thereof; 
           [0024]      FIG. 8  is a partial cross-sectional view of a control valve constructed in accordance with a second embodiment of the present disclosure, illustrating the plug thereof in an open position; 
           [0025]      FIGS. 9 and 10  are cross-sectional views similar to  FIG. 8 , but depicting only the plug and flow control element of the valve, with the plug being in the open position; 
           [0026]      FIGS. 11 and 12  are cross-sectional views similar to  FIG. 8 , but depicting only the plug and flow control element of the valve, with the plug being in the closed position; and 
           [0027]      FIG. 13  is a perspective, cross-sectional view of the flow control element shown in  FIGS. 8-12 , further depicting the plug in its open position. 
       
    
    
       [0028]    Common reference numerals are used throughout the drawings and detailed description to indicate like elements. 
       DETAILED DESCRIPTION 
       [0029]    Referring now to the drawings wherein the showings are for purposes of illustrating preferred embodiments only, and not for purposes of limiting the same,  FIGS. 1-4  are cross-sectional views of a control valve  10  constructed in accordance with the present disclosure. As will be described in more detail below, the valve  10  is shown in  FIGS. 1 and 2  in an open position, and in  FIGS. 3 and 4  in a closed or shut-off position. 
         [0030]    The control valve  10  includes a valve body  12  which defines an inflow passage  14  and an outflow passage  16 . The inflow and outflow passages  14 ,  16  each fluidly communicate with an interior chamber or a valve gallery  18  partially defined by the body  12 . In addition to the body  12 , the valve  10  includes a bonnet  20  which is attached to the body  12  and partially encloses the gallery  18 . As seen in  FIGS. 1 and 3 , the attachment of the bonnet  20  to the body  12  is preferably facilitated through the use of mechanical fasteners comprising a nut and bolt combination, though other attachment methods are contemplated to be within the spirit and scope of the present disclosure. The bonnet  20  defines a bore  22  which extends axially therethrough and communicates with the gallery  18 . The bore  22  is of variable diameter and, as viewed from the perspective shown in  FIGS. 1 and 3 , includes a narrow middle section interposed between wider upper and lower sections, the purpose of which will be described in more detail below. An actuating valve stem  24  extends into the bore  22  and is reciprocally moveable therein to selectively actuate the control valve  10 . 
         [0031]    Disposed within the gallery  18  of the body  12  is a flow control element  26  (see  FIGS. 5-7 ) which provides flow resistance as fluid flows from the inflow passage  14  to the outflow passage  16 . The flow control element  26  generally includes an annular, generally cylindrical cage  28  which is disposed within an outer sleeve or liner  30 . The flow control element  26  includes a plurality of flow paths positioned between the inflow passage  14  and the outflow passage  16 , wherein the flow paths are specifically configured and adapted to reduce the pressure of the fluid flowing therethrough from the inflow passage  14  toward the outflow passage  16 . The flow paths are arranged substantially parallel to a valve axis  32  and in a multi-staged configuration, such that the flow paths are selectively throttled via an external sleeve valve plug or simply sleeve plug  34  which is transitional between an open position and a closed position as will be described in more detail below. 
         [0032]    Referring first to  FIGS. 1 and 2 , the sleeve plug  34  is shown in the open position, which allows fluid to flow into the flow paths of the flow control element  26 . The sleeve plug  34  may be selectively transitioned from the open position to the closed position, as depicted in  FIGS. 3 and 4 , to prevent fluid from entering the flow paths of the flow control element  26 . The sleeve plug  34  is coupled to the valve stem  24  and includes stem attachment portion  36  and a throttling portion  38  extending from the stem attachment portion  36 . The stem attachment portion  36  includes an outer end surface  40  and an inner end surface  42 . The stem attachment portion  36  is attached to the valve stem  24  such that the valve stem  24  preferably does not extend beyond the inner end surface  42 . The throttling portion  38  is an annular structure extending from the stem attachment portion  36  and terminating to define a distal rim  44 . The throttling portion  38  further includes an annular outer surface  46  and an opposing annular inner surface  48 . The inner end surface  42  of the stem attachment portion  36  and the annular inner surface  48  of the throttling portion  38  collectively define a sleeve recess  50  within which the flow control element  26  is received when the sleeve plug  34  is in the closed position. 
         [0033]    Referring now to  FIGS. 5-7 , the flow control element  26  is depicted, with  FIG. 5  providing a side elevational view of the flow control element  26 , and  FIGS. 6 and 7  showing the flow control element  26  with the outer liner  30  being transparent to facilitate the depiction of the inner cage  28 . As will be described in more detail below, the outer liner  30  and cage  28  are formed separately and then joined while in a fuseable state to ultimately form the flow control element  26 . 
         [0034]    The outer liner  30  defines a liner axis  52  and includes a pair of opposing end walls  54 ,  56 , as well as an outer surface  58  and an inner surface  60  defining a liner opening  62  extending along the liner axis  52 . The liner opening  62  is substantially complimentary in shape to external configuration of the cage  28 . The outer liner  30  includes an inlet section  64  having a plurality of inlet openings  66  extending between the outer and inner surfaces  58 ,  60  generally perpendicular to the liner axis  52 . The plurality of inlet openings  66  are arranged at varying locations along the liner axis  52 . In the exemplary embodiment, the inlet openings  66  are arranged in a helical configuration, with adjacent openings  66  being slightly offset from each other in an axial direction. It is contemplated that the inlet openings  66  may be arranged in a single-helix arrangement, double-helix arrangement, or other suitable arrangements as may be implemented based on the desired functionality of the control valve  10 . 
         [0035]    The outer surface  58  of the outer liner  30  at the inlet section  64  is of a substantially uniform diameter, which is smaller than the outer diameter of the remaining portions of the outer liner  30 . The outer diameter of the inlet section  64  is also slightly smaller than the inner diameter of the throttling portion  38  of sleeve plug  34  to allow the inlet section  64  to be received within the sleeve recess  50  when the plug  34  is in the closed position. 
         [0036]    The outer liner  30  may include one or more liner bypass openings  68  formed between the inlet openings  66  and the end wall  54 , wherein the liner bypass openings  68  extend from the outer surface  58  to the inner surface  60 . The purpose of the liner bypass openings  68  will be discussed in more detail below. 
         [0037]    The inlet section  64  of the outer liner  30  transitions into a fusto-conical liner seating surface  70  specifically configured and adapted to interface with the sleeve plug  34  when the sleeve plug  34  is in the closed position. The liner seating surface  70  flares outwardly from the inlet section  64 , with the diameter of the liner seating surface  70  increasing as the liner seating surface  70  extends away from the inlet section  64 . 
         [0038]    The liner seating surface  70  transitions into an extension section  72 , which is of substantially uniform diameter and extends between the liner seating surface  70  and a flange  74 , which defines a maximum outer diameter. The flange  74  transitions into an outlet section  76  which has an outer diameter smaller than that of the flange  74 . 
         [0039]    The inner surface  60  of the outer liner  30  is stepped and defines a first diameter, a second diameter, and a shoulder  75  (see  FIGS. 2 and 4 ) therebetween. In an exemplary embodiment, the smaller first diameter extends from the end wall  54  to the flange  74 , with the shoulder  75  being located in the flange  74 . The larger second diameter extends from the shoulder  75  to the end wall  56 . 
         [0040]    The cage  28  defines a cage axis  78  and includes an outer surface  80  and an inner surface  82  which defines a cage bore  84  extending along the cage axis  78  between opposed end walls  86 ,  88  (see  FIG. 6 ) of the cage  28 . The cage  28  further defines a first or upper portion  90  which, in one embodiment, is of a reduced diameter in comparison to a second or lower portion  92  thereof. A cage shoulder  94  separates the first portion  90  from the second portion  92 . A plurality of substantially linear feed slots  96  are formed in the first portion  90  of the cage  28 , with each feed slot  96  extending into the cage  28  from the outer surface  80 , and further extending in spaced, generally parallel relation to the cage axis  78 . The feed slots  96  extend only partially between the outer surface  80  and the inner surface  82 , and thus are not in direct fluid communication with the cage bore  84 . Each feed slot  96  includes an inlet end portion  98  in alignment with and in communication with a corresponding inlet opening  66  formed on the outer liner  30  when the cage  28  is inserted within the outer liner  30 . The inlet end portions  98  of the various feed slots  96  are preferably formed at different locations along the cage axis  78 , which results in feed slots  96  having different lengths. The “length” of each feed slot  96  is defined as the distance between the cage shoulder  94  and the distal tip of the inlet end portion  98 . Since the inlet end portions  98  are in communication with the helically arranged inlet openings  66 , the lengths of the feed slots  96  are incrementally variable in a circumferential direction. For instance, looking at the cage depicted in  FIG. 7 , slot  96   a  is of a first length, adjacent slot  96   b  is of a second length greater than the first length, and slot  96   c  is of a third length greater than the second length, and so on. The feed slots  96  in the exemplary embodiment are of substantially similar width and are substantially evenly spaced around the circumference of the first portion  90  of the cage  28 . However, it is understood that in other embodiments, the feed slots  96  may have variable widths and/or depths, and may further be unevenly spaced around the circumference of the first portion  90  of the cage  28  without departing from the spirit and scope of the present invention. Furthermore, it is also contemplated that other implementations of the cage  28  may include non-linear (e.g., tortuous) feed slots. 
         [0041]    The second portion  92  of the cage  28  includes a plurality of tortuous resistance paths  100  in communication with respective ones of the plurality of feed slots  96  via a respective connecting slot  102  formed in the shoulder  94 . Each resistance path  100  extends into the cage  28  from the outer surface  80  of the second portion  92  of the cage  28  along an axis perpendicular to the cage axis  78  to define a resistance path depth. The resistance paths  100  extend only partially into the cage  28 , and thus, the resistance paths  100  are not in direct fluid communication with the cage bore  84 . The resistance paths  100  are tortuous or serpentine in configuration, and thus include both axial and circumferential components. In particular, adjacent axial components are connected via an intervening circumferential component. The tortuous or serpentine configuration of the resistance paths  100  provides multiple stages of pressure reduction as fluid flows therethrough. 
         [0042]    Each resistance path  100  is in fluid communication with a discharge opening  104 , which extends radially through the cage  28  between the outer and inner surfaces  80 ,  82 . As can be seen in  FIGS. 1-4 , the exemplary discharge openings  104  are angled downwardly, and thus are offset from an axis perpendicular to the cage axis  78 . However, it is understood that the discharge openings  104  may be perpendicular to the cage axis  78  without departing from the spirit and scope of the present invention. The center of the discharge opening  104 , as formed on the outer surface  80  of the cage  28 , is spaced from the shoulder  94  by a discharge opening distance. In the exemplary embodiment, a first group of discharge openings  104  are formed at a first discharge opening distance and a second group of discharge openings  104  are formed at a second discharge opening distance. The discharge openings  104  are arranged around the circumference of the cage in an alternating pattern, i.e., a discharge opening  104  from the first group is positioned between a pair of discharge openings  104  from the second group. 
         [0043]    As shown in  FIGS. 1-4 , the depth of the resistance path  100  increases from the shoulder  94  to the discharge opening  104 . The increase in resistance path depth allows for a greater reduction in pressure as the fluid flows therethrough. The width of the resistance paths  100  may also vary to further enhance the pressure reducing capabilities of the control valve  10 , and in particular the flow control element  26  thereof. Although the exemplary embodiment includes resistance paths  100  with a variable depth, it is understood that in other embodiments, the resistance paths may have a substantially uniform depth and/or width along the length thereof. 
         [0044]    The inner surface  82  of the cage  28  includes a first section  106  that is of substantially uniform diameter, wherein the first section  106  transitions into a second section  108  that is of a gradually increasing diameter. 
         [0045]    The cage  28  may additionally include one or more cage bypass openings  110  formed in the first upper portion  90  between the feed slots  96  and the end wall  86 , with the bypass openings  110  extending between the outer and inner surfaces  80 ,  82  thereof. The cage bypass openings  110  are aligned with and in fluid communication with respective ones of the liner bypass openings  68  when the cage  28  is inserted within the outer liner  30 . As will be described in more detail below, the bypass openings  110  are configured to allow fluid to enter the cage bore  84  without having to travel through the feed slots  96  and resistance paths  100 , as may be the case in the event of blockage of the feed slots  96  or resistance paths  100 . 
         [0046]    The cage  28  and outer liner  30  may be formed separately and joined when each is in a semi-soft or workable state (e.g., a “green” state), which allows the newly formed assembly (i.e., the combined cage  28  and outer liner  30 ) to meld together to form a unitary structure. In particular, the first portion  90  of the cage  28  is inserted into the outlet section  76  of the outer liner  30 , with the cage  28  being advanced into the liner  30  until the cage shoulder  94  rests against the liner shoulder  75 . The cage  28  and outer liner  30  are preferably formed such that the corresponding end walls  54 ,  86  and  56 ,  88  and are substantially flush with each other when the cage  28  is completely inserted into the liner  30 . The cage  28  is rotationally aligned with the liner  30  such that the inlet openings  66  are aligned with corresponding ones of the feed slot inlet end portions  98 , and the liner bypass openings  68  are aligned with corresponding ones of the cage bypass openings  110 . A first end cap  112  is placed over the inlet section  64  of the liner  30  and first portion  90  of the cage  28  and a second end cap  114  is placed over the outlet section  76  of the liner  30  and the second portion  92  of the cage  28 . The first end cap  112  is annular in shape and includes an opening that is complimentary in size to the cage bore  84  at the end wall  86  of the cage  28 . Likewise, the second end cap  114  includes an opening that preferably compliments the tapered cage bore  84  at end wall  88 . The second end cap  114  may additionally a flange  116  which extends over the end wall  88 , and a cylindrical section  118  extending axially from the flange  116 . 
         [0047]    When the cage  28  is completely inserted within the outer liner  30  and the first and second end caps  112 ,  114  are placed in their respective positions, the entire assembly may be heated as allows the components to fuse together and form a substantially rigid, uniform structure upon cooling of the assembly. 
         [0048]    After the flow control element  26  is assembled, it may be inserted into the valve body  12  to reside within the gallery  18 . The valve body  12  may be adapted to allow the downstream portion of the flow control element  26  to be advanced into the gallery until the flange  74  engages with a shoulder formed on the valve body  12 . A valve body insert  122  may be placed over the upstream end of the flow control element  26  to secure the flow control element  26  within the gallery  18 . Various seals and/or packing elements known in the art for mitigating the unwanted bleeding or leakage of fluid from the valve  10  may be employed as needed. In this regard, those of ordinary skill in the art that many of the components of the control valve  10  described above and/or shown in  FIGS. 1 and 3  other than for the flow control element  26  and sleeve plug  34  are exemplary only, and that flow control element  26  and sleeve plug  34  may be integrated into valves of differing construction than that shown and described above. 
         [0049]    In use, with the control valve  10  being in the closed position (see  FIGS. 3 and 4 ), the distal rim  44  of the sleeve plug  34  is seated against the liner seating surface  70  of the outer liner  30 . In this position, the sleeve plug  34  covers all of the inlet openings  66  formed on the outer liner  30 , and thus fluid is prevented from flowing through the flow control element  26 . 
         [0050]    As the control valve  10 , and in particular the sleeve plug  34 , transitions from the closed position toward the open position, the distal rim  44  of the sleeve plug  34  is moved from the liner seating surface  70  to expose at least one inlet opening  66 , while at least some of the remaining openings  66  are still covered by the sleeve plug  34 . In this respect, with the sleeve plug  34  being in a partially-open position, fluid may enter the exposed inlet opening(s)  66  and flow into the corresponding feed slot(s)  96  associated with the exposed inlet opening(s)  66 . The fluid continues through the feed slot(s)  96  and transitions into the corresponding connecting slot(s)  102  and continues through the corresponding tortuous resistance path(s)  100 . As the fluid flows through the resistance path(s)  100 , the pressure of the fluid is reduced. When the fluid reaches the discharge opening(s)  104 , the fluid is discharged into the cage bore  84 , and continues on to the outflow passage  16 . 
         [0051]    When the valve  10  is close to its fully open position, every inlet opening  66  is exposed, thereby allowing fluid to enter each of the feed slots  96  and corresponding resistance paths  100 . However, the bypass openings  68  remain covered, thereby preventing fluid from passing through the bypass openings  68  and into the cage bore  84 . Thus, when the valve  10  is almost in its fully open position, all fluid passing through the outflow passage  16  will have passed through the feed slots  96  and the corresponding resistance paths  100 . 
         [0052]    The movement of the control valve  10 , and in particular the sleeve plug  34  thereof, to the fully open position exposes the bypass openings  68 . The uncovering or exposure of the bypass openings  68  allows fluid flow through the aligned bypass openings  68 ,  110  and directly into the cage bore  84 , effectively “bypassing” the feed slots  96 , resistance paths  100  and intervening connecting slots  102 . As will be recognized, fluid entering the cage bore  84  from the bypass openings  68 ,  110  is able to flow directly into the outflow passage  16 . 
         [0053]    According to one embodiment, the control valve  10  creates a pressure balance on opposed sides of the sleeve plug  34 . In particular, the valve stem  24  includes a stem passageway  120  that on one end communicates with the gallery  18 , and particularly, the cage bore  84 , and on the other end communicates with the bore  22  formed in the bonnet  20 . Thus, as pressurized fluid enters the cage bore  84 , the fluid may fill the cage bore  84  and flow through the stem passageway  120  and into the bonnet bore  22  to balance the pressure on either side of the sleeve plug  34 . 
         [0054]    Referring now to  FIGS. 8-13 , there is depicted another embodiment of a control valve  210 , which generally include a valve body  212 , a bonnet  220 , and a flow control element  226 . The primary distinction between the first embodiment described above and shown in  FIGS. 1-7 , and the second embodiment described below and shown in  FIGS. 8-13 , relates to the flow control element  226 , and thus, the following discussion will focus on the flow control element  226 . 
         [0055]    The flow control element  226  includes three primary components, namely, an inlet element  228 , a resistance element  232 , and an outer liner or sleeve  230 . The inlet element  228  includes a first end portion  234  coupled to an actuating valve stem  236 , with the first end portion  234  being received within a recess  238  formed within the valve stem  236 . The first end portion  234  transitions into an inlet element seating surface  240 , which is frusto-conical in shape, with a tapered diameter which decreases in a direction extending away from the first end portion  234 . An inlet region  242  extends from the inlet element seating surface  240 , with the inlet region  242  having a plurality of inlet openings  244  extending from an outer surface  246  of the inlet element  228  to an inner surface  248  of the inlet element  228 , which along with a chamber end wall  250 , defines an inner chamber  252 . In an exemplary embodiment, and referring specifically to  FIG. 11 , the inlet region  242  includes five levels of openings, with openings  244   a  being on a first level, openings  244   b  being on a second level, openings  244   c  being on a third level, openings  244   d  being on a fourth level, and openings  244   e  being on a fifth level. The openings in the first, third and fifth levels are axially aligned with each other and slightly offset from the openings formed on the second and fourth levels, the openings on the second and fourth levels being axially aligned with each other. 
         [0056]    The inlet element  228  further includes a stem  254  extending from the chamber end wall  250 . A plurality of drainage openings  256  are in communication with the inner chamber  252  and extend from the chamber end wall  250  and through the stem  254  to the outer surface  246  of the inlet element  228 . The drainage openings  256  have an angled configuration, wherein the draining openings  256  are neither perpendicular nor parallel to an inlet element axis  258 . In the exemplary embodiment, the inlet element  228  includes three drainage openings  256 , with each drainage opening  256  being considerably larger than the inlet openings  244 . However, the size, number and arrangement of the drainage openings  256  may vary. 
         [0057]    The resistance element  232  includes a first end portion  260  (e.g., an upstream end portion) and a second end portion  262  (e.g., a downstream end portion), wherein the first end portion  260  includes a resistance element seating surface  264  that is complimentary in shape to the inlet element seating surface  240  such that when the flow control element  226  is in the closed position, the inlet element seating surface  240  is seated against the resistance element seating surface  264 . The resistance element  232  further includes an outer surface  266  and an opposing inner surface  268 , with the inner surface  268  defining a resistance element bore  270  having a wide upstream region and a narrow downstream region. The outer surface  266  defines a resistance element shoulder  272  (see  FIG. 9 ) adjacent the first end portion  260 . 
         [0058]    A plurality of resistance paths  274  are formed in the resistance element  232 , with the resistance paths  274  extending into the resistance element  232  from the outer surface  266  thereof. The resistance paths  274  extend only partially into the resistance element  232 , and thus, the resistance paths  274  are not in direct fluid communication with the resistance element bore  270 . The resistance paths  274  are tortuous or serpentine in configuration, and thus comprise both axial and circumferential components. In particular, adjacent axial components are connected via an intervening circumferential component. The tortuous or serpentine configuration of the resistance paths  274  provides multiple stages of pressure reduction as fluid flows therethrough. 
         [0059]    Each resistance path  274  is in fluid communication with a path inlet opening  276  on one end, and a path discharge opening  278  on the other end. The path inlet opening  276  is in communication with the upstream region of the resistance element bore  270 , while the path discharge opening  278  is in communication with the downstream region of the resistance element bore  270 . According to one embodiment, the path inlet openings  276  are formed at various axial locations on the resistance element  232 . For instance, in the exemplary embodiment, the resistance element  232  includes path inlet openings  276   a  and  276   b , which are axially offset from each other. In particular, opening  276   a  is spaced farther from the resistance element seating surface  264  than opening  276   b . The purpose of the axial offset is to allow for a selective and incremental throttling of the resistance paths  274 , as will be discussed in more detail below. 
         [0060]    The exemplary path discharge openings  278  are angled downwardly, and thus are offset from an axis perpendicular to the resistance element axis  280 . However, it is understood that the path discharge openings  278  may be perpendicular to the resistance element axis  280  without departing from the spirit and scope of the present invention. In the exemplary embodiment, a first group of discharge openings are formed at a first discharge opening level and a second group of discharge openings are formed at a second discharge opening level. 
         [0061]    The depth of the resistance path  274  (e.g., the distance by which the resistance path extends into the resistance element  232 ) may increase from the path inlet opening  276  to the path discharge opening  278 . The increase in resistance path depth allows for a greater reduction in pressure as the fluid flows therethrough. Although the exemplary embodiment includes resistance paths  274  with a variable depth, it is understood that in other embodiments, the resistance paths  274  may each have a substantially uniform depth along the length thereof. 
         [0062]    The outer liner  230  includes a sleeve-like body that is cylindrical in shape and which is sized to fit over the outer surface  266  of the resistance element  232 . In particular, the outer liner  230  includes an outer surface  282  and an inner surface  284  defining a liner bore sized to receive the resistance element  232 . A lip  286  is formed on the outer liner  230 , with the lip  286  being configured to engage with the resistance element shoulder  272  to secure the resistance element  232  within the outer liner  230 . The external configuration of the outer liner  230  is comprised of a first region  288 , a second region  290  and a third region  292 , wherein the first region  288  is of a first diameter, the second region  290  is of a second diameter larger than the first diameter, and the third region  292  is of a third diameter less than the second diameter which results in the second region  290  defining a radially extending flange. The flange rests against a complimentary shoulder formed in the valve body  212  when the flow control element  226  is placed in the valve body  212 . 
         [0063]    An end cap  294  is secured to the third region  292  of the outer liner  230  and the second end portion  262  of the resistance element  232 , with the end cap  294  having an end cap bore  296  which is complimentary in shape to the resistance element bore  270 . 
         [0064]    The resistance element  232 , outer liner  230  and end cap  294  may be formed separately and joined when each is in a semi-soft or workable state (e.g., a “green” state), which allows the newly formed assembly (i.e., the combined resistance element  232 , outer liner  230 , and end cap  294 ) to meld together to form a unitary structure. In particular, the first end portion  260  of the resistance element  232  is inserted into the third region  292  of the outer liner  230 , with the resistance element  232  being advanced into the liner  230  until the resistance element shoulder  272  rests against the liner lip  286 . The resistance element  232  and outer liner  230  are preferably formed such that the respective end walls are substantially flush with each other when the resistance element  232  is completely inserted into the liner  230 . When the resistance element  232  is completely inserted within the outer liner  230  and the end cap  294  is in place, the entire assembly may be heated which allows the components to fuse together and form a substantially rigid, uniform structure upon cooling of the assembly. 
         [0065]    Once the resistance element  232 , outer liner  230  and end cap  294  are joined, the inlet element  228  may be inserted into the fused assembly, with the stem  254  of the inlet element  228  being advanced through the resistance element bore  270  and received within the narrow downstream region of the bore  270 . 
         [0066]    In use, the flow control element  226  may be selectively transitioned between closed and open positions to allow for selectively throttling of the inlet element  228  relative to the resistance element  232  for controlling the number of inlet openings  244  and resistance paths  274  exposed to the fluid flow. When the inlet element  228  is in the closed position, the inlet element seating surface  240  is positioned against the resistance element seating surface  264 , and all of the inlet openings  244  are recessed within the resistance element  232 . Therefore, fluid is prevented from flowing through the flow control element  226 . 
         [0067]    As the valve stem  236  is moved away from the resistance element  232  and outer liner  230 , the inlet element  228  coupled to the valve stem  236  begins to move out of the resistance element  232 . In the exemplary embodiment, the fifth level of inlet openings  244   e  are the first openings  244  which will be exposed to the fluid flow, while the first level of inlet openings  244   a  will be the last openings  244  exposed to the fluid flow. If the inlet element  228  is positioned relative to the resistance element  232  such that some inlet openings are exposed to the fluid, while other inlet openings remain covered by the resistance element, the inlet element  228  is considered to be in a partially open position.  FIG. 9  shows the inlet element  228  in a partially open position. As fluid enters the exposed inlet openings  244 , the fluid passes into the inner chamber  252  of the inlet element  228 . The fluid drains out of the inner chamber  252  view the drainage openings  256  and into a throttling chamber  298  that is in fluid communication with the drainage openings  256  and is collectively defined by the inlet element  228  and the resistance element  232 . In particular, the throttling chamber  298  is effectively that portion of the wide upstream region of the resistance element bore  270  that is not occupied by the inlet element  228 . Accordingly, the size of the throttling chamber  298  varies as the inlet element  228  moves between the closed and open positions. In particular, the size of the throttling chamber  298  increases as the inlet element  228  moves from the closed position toward the open position. Conversely, the size of the throttling chamber  298  decreases as the inlet element  228  moves from the open position toward the closed position. Movement of the inlet element  228  relative to the resistance element  232  also incrementally exposes the path inlet openings  276  to the fluid flow. In the partially open position, at least one path inlet opening  276  is exposed to the fluid flow, while at least one path inlet opening  276  remains covered by the inlet element  228 , and is thus isolated from the fluid flow. The exposed path inlet opening  276  receives the fluid from the throttling chamber  298  and communicates the fluid to the corresponding resistance path  274 . The fluid flows through the resistance path  274  and passes through the path discharge opening  278  and into the resistance element bore  270 . From the bore  270 , the fluid flows through the bore  296  of the end cap  294  and into the outflow passage  16 . 
         [0068]    When the inlet element  228  is in the fully open position, the inlet element  228  has been moved out of the resistance element  232  by a distance which exposes all of the inlet openings  244  to the fluid flow and all of the path inlet openings  276  to the throttling chamber  298 . In this respect, when the inlet element  228  is in the fully open position, fluid may flow through all of the inlet openings  244  and through all of the resistance paths  274 . 
         [0069]    As the inlet element  228  moves from the open position toward the closed position, the inlet element  228  moves back into the resistance element  232 , with the inlet openings  244  becoming incrementally covered by the resistance element  232  and the path inlet openings  276  becoming incrementally covered by the inlet element  228 . When the inlet element  228  reaches the closed position, all of the inlet openings  244  are positioned within and covered by the resistance element  232  and the path inlet openings  276  are covered by the inlet element  228  so as to prevent fluid from flowing through the flow control element  226 . 
         [0070]    Though not shown, another embodiment of the present disclosure is contemplated wherein the control valve is outfitted with a flow control element similar to the flow control element  26 , but differing from the standpoint that both the cage  28  and outer liner  30  will have more uniformly cylindrical configurations, as opposed to be provided with the aforementioned various regions or sections of differing outer diameter and separated by various shoulders. 
         [0071]    This disclosure provides exemplary embodiments of the present disclosure. The scope of the present disclosure is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process may be implemented by one of skill in the art in view of this disclosure.