Abstract:
In accordance with the present invention, there is provided an axial drag control valve which includes an internal disk stack trim and an internal actuator. The fluid inlet and outlet of the valve are disclosed along a common axis, which is further shared with the actuator of the valve. The actuator moves along this particular axis to control the fluid flow rate, pressure, or temperature of the system. The valve actuator may be powered by air from an external source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to control valves, and more particularly to an axial control valve product that provides high capacity and low noise performance characteristics. 
     2. Description of the Related Art 
     As is known in the control valve industry, three well known types of conventional fluid valves include rotary stem valves, sliding stem valves, and sleeve valves. Rotary stem valves generally comprise a rotary shaft or stem which is maintained within a valve body. The rotation of the shaft may be used to facilitate the alignment of a radial port of the shaft with a fluid port of the valve body to open a valve passage. Conversely, the rotation of the shaft may facilitate a misalignment of the ports to effectively close the valve passage. In operation, a typical rotary valve shaft or stem must rotate about 90° relative to the valve body between the fully open and closed positions. There exists in the prior art other types of rotary valve designs which utilize alternative geometries requiring a shaft rotation that is less than 90°, such as three way or angled ball valves. 
     Rotary valves typically employ the use of seals, and often bearings, which are disposed between the rotary shaft and the valve body to prevent fluid from leaking from the valve body between the shaft and the valve body. In this regard, one of the primary drawbacks of rotary valves is that the significant movement of the shaft typically causes substantial wear to the seals and, if present, the bearings. Thus, the bearings and seals of a rotary valve must typically be replaced over time. Another drawback is that the seals, in order to function properly, also add friction between the valve body and the shaft. Substantial force is therefore typically necessary to overcome the seal friction and rotate the shaft. 
     A sliding stem valve typically operates on a principle similar to a piston, and includes a valve plug on a stem that slides linearly within a valve body. The valve plug bears against a seat or closes a passage when moved to a closed position, and is spaced from the seat or clears the passage when moved to an open position. The valve stem and the valve plug must usually move relative to the valve body a significant distance between the fully open and closed positions. Like rotary stem valves, sliding stem valves typically employ seals, and often guides, between the stem and the valve body to prevent fluid from leaking from the valve body between the stem and the valve body. In this regard, one of the primary drawbacks of sliding stem valves is that the significant linear movement of the stem causes wear on the seals, thus often necessitating that the seals be replaced over time. Another drawback is that the seals also create friction that must be overcome in order to move the linear stem valve between its open and closed positions. 
     Sleeve valves typically have a valve body defining an axial fluid flow passage. A stationary valve plug is usually fixed within the valve passage and carries or defines a valve seat positioned on an upstream end of the plug. A slideable valve sleeve is positioned in the valve passage and can be selectively moved between a fully closed position with a downstream end of the sleeve bearing against the valve seat, and a fully opened position with the downstream end of the sleeve being spaced a prescribed distance from the valve seat. Fluid can flow through the valve passage and the sleeve, around the valve plug, and an exit outlet of the valve. 
     Sleeve valves as known in the prior art typically have a number of prescribed performance characteristics, such as fluid flow rate, fluid pressure, valve flow coefficient, as well as inherent, installed, and linear flow characteristics. Various flow characteristics of sleeve valves can typically be determined or controlled by a number of factors, including the size and shape or contour of the upstream end of the valve plug, the shape of the plug body beyond or downstream of the upstream end, and the passageway or orifice size and contour surrounding the valve plug. Other valve features can be designed and shaped to affect valve flow or performance characteristics as well, including contours of the valve sleeve outlet opening or the like. Along these lines, designing a particular valve plug shape is an often used means to achieve a desired valve performance or flow characteristic. However, as a result, a typical sleeve valve for a given system often has a unique, non-replaceable valve sleeve and plug. Thus, if a different valve flow characteristic is desired for a particular valve or system, or if a valve seat or plug is damaged within a valve or system, it is often necessary to remove and replace the entire valve assembly within the system. In this regard, to change the load characteristics or the valve plug, it has typically been necessary in the prior art to swap the entire sleeve valve with a newer replacement valve. 
     The axial drag valve constructed in accordance with the present invention is adapted to overcome many of the deficiencies highlighted above in relation to known rotary, sliding stem, and sleeve valve designs. Various novel features of the present invention will be discussed in more detail below. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided multiple embodiments of an axial drag control valve which includes an internal disk stack trim and an internal actuator. The fluid inlet and outlet of the valve are disposed along a common axis, which is further shared with the actuator of the valve. The actuator moves along this particular axis to control the fluid flow rate, pressure, or temperature of the system. In exemplary embodiments of the present invention, the valve actuator may be powered by air from an external source. 
     The present invention is best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
         FIG. 1  is an exploded view of an axial drag valve constructed in accordance with a first embodiment of the present invention; 
         FIG. 2  is a partial cross-sectional view of the axial drag valve of the first embodiment shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 1 ; 
         FIG. 4  is a partial, cross-sectional view of a first potential variant of the axial drag valve of the first embodiment shown in  FIGS. 1 and 2 ; 
         FIG. 5  is a partial, cross-sectional view of a second potential variant of the axial drag valve of the first embodiment shown in  FIGS. 1 and 2 ; 
         FIG. 6  is an enlargement of the encircled region  6  shown in  FIG. 5 ; 
         FIG. 7  is a cross-sectional view of an axial drag valve constructed in accordance with a second embodiment of the present invention; 
         FIG. 8  is a perspective, exploded view of the piston and rack assembly of the axial drag valve of the second embodiment shown in  FIG. 7 ; 
         FIG. 9  is a cross-sectional view taken along line  9 - 9  of  FIG. 7 ; and 
         FIG. 10  is a cross-sectional view of an axial drag valve constructed in accordance with a third embodiment of the present invention. 
     
    
    
     Common reference numerals are used throughout the drawings and detailed description to indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for purposes of illustrating preferred embodiments of the present invention only, and not for purposes of limiting the same,  FIGS. 1 and 2  depict an axial drag valve  10  constructed in accordance with a first embodiment of the present invention. More particularly,  FIG. 1  is an exploded view of the valve  10 , with  FIG. 2  being a cross-sectional view thereof. As will be discussed in more detail below, the valve  10  is selectively moveable between a closed position and a fully open position, and is depicted in  FIG. 2  in its closed position. 
     The valve  10  comprises a housing  12 . The housing  12  itself comprises an inlet section  14  which defines an inlet passage  16  and an outlet passage region  18 . In addition to the inlet section  14 , the housing  12  includes an outlet section  20  which itself defines an outlet passage region  22 . The outlet passage region  18  and the outlet passage region  22  collectively define an outlet passage of the valve  10  when the inlet and outlet sections  14 ,  20  are rigidly attached to each other. As seen in  FIG. 2 , the attachment of the inlet and outlet sections  14 ,  20  to each other is facilitated by the use of fasteners  24 , such as bolts. However, those of ordinary skill in the art will recognize that a wide variety of different attachment methods may be used to effectuate the rigid attachment of the inlet and outlet sections  14 ,  20  to each other. However, in the valve  10 , it is contemplated that any attachment method used to facilitate the attachment of the inlet and outlet sections  14 ,  20  to each other will be adapted to allow for the periodic separation of the inlet section  14  from the outlet section  20  as may be needed to access the interior of the housing  12  to allow for maintenance on other parts and components of the valve  10  which will be described in more detail below. 
     As indicated above, the outlet passage of the valve  10  is collectively defined by the outlet passage regions  18 ,  22 . In this regard, the configuration of the outlet passage of the valve  10  is governed, in large measure, by the shapes of the inlet and outlet sections  14 ,  20 . However, those of ordinary skill in the art will recognize that the shapes of the inlet and outlet sections  14 ,  20 , and hence the configuration of the outlet passage as shown in  FIG. 2 , is exemplary only, and that alternative configurations for the outlet passage are contemplated to be within the spirit and scope of the present invention. 
     Disposed within the interior of the inlet section  14 , and more particularly within the inlet passage  16  defined thereby, is an annular inlet shield  26 . The inlet shield  26  includes a base portion  28  which is rigidly attached to the inlet section  14 , and more particularly to a portion of the inner surface thereof which defines the inlet passage  16 . In addition to the base portion  28 , the inlet shield  28  includes a wall portion  30  which is integrally connected to the base portion  28 . As seen in  FIG. 2 , the wall portion  30  extends axially within the inlet passage  16  in spaced relation to the inner surface of the inlet section  14  defining the inlet passage  16 . Thus, an annular gap  32  is defined between the wall portion  30  and the inlet section  14 . 
     The valve  10  further comprises an annular guide member  34  which, like the inlet shield  26 , is attached to a portion of the inner surface of the inlet section  14 . More particularly, as shown in  FIG. 2 , the guide member  34  is attached to the inner surface of the inlet section  14  proximate the outlet passage region  18  defined thereby. In this regard, the outlet passage region  18  is partially defined by a radially extending flange portion  36  of the guide member  34 . 
     The valve  10  further comprises a generally cylindrical, tubular flow control element  38  which is disposed within the outlet passage region  18  of the inlet section  14 . As seen in  FIGS. 1 and 2 , one end or annular rim defined by the flow control element  38  is abutted against one end of the guide member  34 , including the flange portion  36  thereof. In this regard, as seen in  FIG. 2 , disposed within the flange portion  36  of the guide member  34  is a continuous, annular channel which accommodates a sealing member  40 . The sealing member  40  is effectively captured and compressed between the guide member  34  and the corresponding end of the flow control element  38 . The opposite, remaining end or annular rim of the flow control element  38  (i.e., the end opposite that abutted against the guide member  34 ) is itself abutted against an internal end cap  42  of the valve  10 . As seen in  FIGS. 1 and 2 , the end cap  42  is suspended within the interior of the outlet section  20 , and more particularly within the outlet passage region  22  defined thereby, the outlet passage region  22  being partially defined by the end cap  42 . The end cap  42  may be integrally connected to the outlet section  20  by support struts  44  which extend therebetween in the manner shown in  FIG. 1 . Though not shown, it is also contemplated that the end cap  42  may be attached to the inlet section  14  of the housing  12  by support bars which extend through the outlet passage region  18 . Whether attached to the outlet section  20  and/or the inlet section  14 , the end cap  42  is positioned along the axis of the housing  12  so that the flow control element  38  is effectively captured and compressed between the end cap  42  and the guide member  34 . 
     In the valve  10 , it is contemplated that the flow control element  38  may comprise a stack of annular discs that collectively define a series of substantially radially directed passageways extending between the inner and outer radial surfaces or edges of the discs. As seen in  FIGS. 1 and 2 , a series of these radially directed passageways disposed within one end portion of the flow control element  38  may be configured as enlarged flow openings  46  which provide substantially unimpeded flow through the flow control element  38 . If the flow openings  46  are provided in the flow control element  38 , it is contemplated that the remainder of the radially directed passageways will each be tortuous and define a plurality of turns therewithin in order to reduce the velocity of fluid that is flowing through the flow control element  38 . In this regard, the flow control element  38  may alternatively be fabricated to omit the flow openings  46 , all the radially directed passageways thus being tortuous. An exemplary flow control element  38  is disclosed in commonly owned U.S. Pat. No. 5,687,763, the disclosure of which is incorporated herein by reference. 
     The valve  10  further comprises an elongate, tubular piston sleeve  48  which is slidably disposed within the interior of the housing  12 , and predominantly within the inlet section  14  thereof. The piston sleeve  48  is selectively moveable between a closed position (shown in  FIG. 2 ) and a fully open position which will be described in more detail below. As shown in  FIGS. 1 and 2 , the piston sleeve  48  includes a main body portion  50  which defines a flow passage  52  extending axially therethrough. Protruding radially outward from the outer surface of the approximate center of the main body portion  50  is a flange portion  54 . The flange portion  54  resides within a piston sleeve chamber  49  of the valve  10  which is collectively defined by a portion of the inner surface of the inlet section  14 , a portion of the outer surface of the main body portion  50 , and the guide member  34 . Disposed within the flange portion  54  is a spaced pair of continuous, circumferential grooves or channels, each of which accommodates a sealing member  56 . Additionally, extending laterally through the flange portion  54  is a probe bore  58 , the axis of which extends in generally parallel relation to the axis of the flow passage  52 . The use of the probe bore  58  will be described in more detail below. In the piston sleeve  48 , the opposed end portions of the main body portion  50  each have a beveled or tapered configuration, and extend to respective ones of opposed first and second rims  60 ,  62  of the main body portion  50 . 
     As indicated above, the piston sleeve  48  is reciprocally moveable within the interior of the housing  12  between closed and fully open positions. When the piston sleeve  48  is in its closed position as shown in  FIG. 2 , the beveled surface defined thereby and extending to the first rim  60  thereof is abutted and sealed against an annular sealing member  64  which is attached to the end cap  42 . When the piston sleeve  48  is sealed against the sealing member  64 , the inner ends of each of the fluid passageways defined by the flow control element  38 , including the flow openings  46 , are effectively covered by the main body portion  50  of the piston sleeve  48 , and in particular the section thereof extending between the flange portion  54  and the first rim  60 . Additionally, when the piston sleeve  48  is in its closed position, the flange portion  54  is disposed in the piston chamber  49  in close proximity to the guide member  34 , but space therefrom by a slight gap. Further, when the piston sleeve  48  is in its closed position, a small section of the main body portion  50  thereof extending to the second rim  62  overlaps the wall portion  30  of the inlet shield  26 , thus protruding slightly into the gap  32  defined between the wall section  30  and the inner surface of the inlet section  14 . 
     In the valve  10 , fluid initially enters the inlet passage  16  defined by the inlet section  14  of the housing  12 . More particularly, the fluid flows through the interior of the inlet shield  26  and into the flow passage  52  defined by the piston sleeve  48 . As will be recognized by those of ordinary skill in the art, when the piston sleeve  48  is in its closed position as described above and shown in  FIG. 2 , fluid flowing through the flow passage  52  is effectively blocked from flowing radially outwardly through the flow control element  38 , and thus into the outlet passage collectively defined by the outlet passage regions  18 ,  22  of the housing  12 . Rather, the fluid flow impinges directly against the above-described end cap  42 . 
     Though not shown, when the piston sleeve  48  is moved or actuated to its fully open position, the tapered surface extending to the first rim  60  is moved or withdrawn from its sealed engagement to the sealing member  56 , with the retraction of the piston sleeve  48  being continued until such time as the first rim  60  is generally aligned with the flange portion  36  of the guide member  34 . When the piston sleeve  48  is in such fully open position, more of the main body portion  50  thereof is advanced into the gap  32  in comparison to when the piston sleeve  48  is in the closed position. In this regard, the segment of the main body portion  50  which is advanced into the gap  32  when the piston sleeve  48  is in its fully open position is identified by the letter “S” shown in  FIG. 2 . As shown in  FIG. 2 , one end of the segment S extends to the tapered surface extending to the second rim  62 . As the piston sleeve  48  moves between its closed and fully open positions, the inner surface of the segment S is maintained in sliding contact with the outer surface of the wall portion  30  of the inlet shield  26 . As will also be recognized by those of ordinary skill in the art, when the piston sleeve  48  is moved from the closed position toward the fully open position, fluid is able to flow through the flow passage  52 , and thereafter radially outwardly through the flow control element  38  and into the outlet passage of the valve  10 . Since the fluid must flow through the flow control element  38  to reach the outlet passage, the energy of the fluid is effectively reduced due to the above-described functional attributes of the flow control element  38 . 
     The opening of the valve  10  may be effectuated without necessarily actuating the piston sleeve  48  to its fully open position described above. In this regard, in the valve  10 , the axial movement of the piston sleeve  48  away from the sealing member  64  may be regulated or controlled depending on the desired level of fluid energy dissipation. Along these lines, as will be recognized, the greater the amount of axial movement of the piston sleeve  48  away from the sealing member  64 , the greater the number of energy dissipating fluid passageways of the flow control element  38  that will be exposed to the incoming fluid flow via the flow passage  52 . In this regard, maximum energy dissipation of the inlet fluid is achieved when the piston sleeve  48  is moved to its fully open position. 
     In the valve  10 , it is desirable to prevent any rotation of the piston sleeve  48  as it moves between its closed and fully open positions. As indicated above, as the piston sleeve  48  moves between its closed and fully open positions, the inner surface of the segment S is maintained in sliding contact with the outer surface of the wall portion  30  of the inlet shield  26 . To assist in preventing rotation of the piston sleeve  48 , it is contemplated that the segment S of the main body portion  50  (i.e., that section of the main body portion  50  which overlaps the wall portion  30  of the inlet shield  26  when the piston sleeve  48  is in its fully open position) may be formed or machined to have a polygonal cross-sectional configuration as shown in  FIG. 3 . As a result of forming the segment S of the main body portion  50  with such polygonal cross-sectional configuration, the interference between the main body portion  50  and the wall portion  30  of the inlet shield  26  as the segment S slides along the outer surface thereof effectively prevents rotation of the piston sleeve  48  relative to the inlet shield  26 . However, those of ordinary skill in the art will recognize that the piston sleeve  48  need not necessarily be formed to have a polygonal segment as described above, and that alternative structures may be integrated into the valve  10  to prevent the rotation of the piston sleeve  48 . 
     In the valve  10 , the reciprocal axial movement of the piston sleeve  48  as needed to effectuate the movement thereof between its closed and fully open positions is facilitated by the selective application of air pressure to either side of the flange portion  54  of the piston sleeve  48 . In this regard, disposed within the inlet section  14  of the housing  12  is a first air passage  66  which, as seen in  FIG. 2 , communicates with the piston chamber  49  at the gap normally defined between the flange portion  54  and the guide member  34  when the piston sleeve  48  is in its closed position. In addition to the first air passage  66 , also formed in the inlet section  14  is a second air passage  68  which also communicates with the piston chamber  49  at a location between the flange portion  54  of the piston sleeve  48  and the inlet passage  16  or inlet shield  26 . The first and second air passages  66 ,  68  are adapted to selectively supply air to, or exhaust air from, the interior of the housing  12  as needed to effectuate the reciprocal axial movement of the piston sleeve  48  therewithin. More particularly, to facilitate the movement of the piston sleeve  48  to the closed position shown in  FIG. 2 , pressurized air is input into the piston chamber  49  via the second air passage  68 , such pressurized air acting against the flange portion  54  in a manner effectively forcing it toward the guide member  34 . The movement of the piston sleeve  48  toward the guide member  34  is discontinued as a result of the abutment of the piston sleeve  48  against the sealing member  64 . As will be recognized, when the second air passage  68  is pressurized as occurs to facilitate the actuation of the piston sleeve  48  to the closed position, the first air passage  66  acts as an exhaust port so that air captured between the flange portion  54  and the guide member  34  does not impede the movement of the flange portion  54  toward the guide member  34 . 
     Conversely, to facilitate the movement of the piston sleeve  48  to the fully open position, the first air passage  66  is pressurized so as to facilitate the input of air into the piston chamber  49  at gap defined between the flange portion  54  and the guide member  34 , such pressurized air acting against the flange portion  54  as results in its movement away from the guide member  34 . Such movement of the flange portion  54  effectively draws the main body portion  50  of the piston sleeve  48  away from the sealing members  64 . As will be recognized, when the first air passage  66  is pressurized to facilitate the movement of the piston sleeve  48  toward the fully open position, the second air passage  68  effectively functions as an exhaust port so that any air trapped between the flange portion  54  and the inlet section  14  of the housing  12  does not impede the movement of the piston sleeve  48  away from the sealing member  64 . Within the piston chamber  49 , pressurized air is prevented from migrating between the peripheral edge of the flange portion  54  and the inner surface of the inlet section  14  by the sliding, sealed engagement effectuated by the sealing members  56  disposed within the flange portion  54 . Further, as the piston sleeve moved between its closed and fully open positions, fluid is further prevented from migrating between the main body portion  50  of the piston sleeve  48  and the guide member  34  by the sliding seal created by a sealing member  70  disposed within a complimentary annular channel within the guide member  34 , as shown in  FIG. 2 . 
     In order to monitor and thus tightly regulate or control the position of the piston sleeve  48  relative to the sealing member  64 , the valve  10  is preferably provided with a position feedback device  66  which is oriented within the piston chamber, and extends between the first and second air passages  66 ,  68 . More particularly, the feedback device  72  includes an elongate, generally cylindrical probe portion  74 , one end of which is attached to the guide member  34 , and the other end of which is attached to the inlet section  14 . The probe portion  74  extends through the piston chamber  49  in generally parallel relation to the axis of the flow passage  52 . The probe portion  74  is further advanced through the complimentary probe bore  58  extending through the flange portion  54  of the piston sleeve  48 , thus allowing the flange portion  54  to be selectively slidably advanced along the probe portion  74 . In this regard, the movement of the flange portion  54  relative to the probe portion  74  is operative to allow the feedback device  72  to effectively monitor the relative position of the flange portion  54 , and hence the piston sleeve  48 . Since the probe portion  74  is always advanced through the flange portion  54  throughout the range of movement of the piston sleeve  48  between its closed and fully open positions, the interference between the probe portion  74  and the piston sleeve  48  supplements the anti-rotation effect described above. 
     Referring now to  FIG. 4 , in accordance with a first potential variant of the valve  10 , the previously described end cap  42  may be substituted with the end cap  76  shown in  FIG. 4 . The end cap  76  as shown in  FIG. 4  is attached to the inlet section  14  of the housing  12  by support bars  78  which extend through the outlet passage region  18 . Alternatively, though not shown, the end cap  76 , like the above-described end cap  42 , may be integrally connected to the outlet section  20  of the housing  12  by support struts similar to the aforementioned support struts  44  shown in  FIG. 1 . Whether attached to the inlet section  14  and/or the outlet section  20 , the end cap  76  is positioned along the axis of the housing  12 , and more particularly the axis of the flow passage  52  of the piston sleeve  48 , with the flow control element  38  effectively being captured and compressed between the end cap  76  and the guide member  34 . 
     The primary distinction between the end cap  76  and the above-described end cap  42  lies the inclusion of a plurality of flushing holes  80  within the end cap  76 . When the piston sleeve  48  is in an open position, one end of each of the flushing holes  80  fluidly communicates with the interior of the flow control element  38 , with the opposed, remaining end of each of the flushing holes  80  fluidly communicating with the outlet passage of the valve  10 , and in particular the outlet passage region  22  defined by the outlet section  20 . As seen in  FIG. 4 , when the piston sleeve  48  in its closed position, that end of each of the flushing holes  80  otherwise fluidly communicating with the interior of the flow control element  38  is effectively blocked by the first rim  60  of the piston sleeve  48 . When the flushing holes  80  are blocked by the piston sleeve  48 , the tapered surface of the main body portion  50  extending to the first rim  60  is abutted and sealed against a complimentary tapered surface defined by the end cap  76 . 
     During normal operation of the valve  10  including the end cap  76 , when the piston sleeve  48  is in its closed position, fluid flowing through the flow passage  52  is effectively blocked from entering the flow control element  38  by the main body portion  50  of the piston sleeve  48 , and is further blocked from directly entering the outlet passage by the end cap  76 . However, the fluid flowing through the flow passage  52  and impinging the end cap  76  sometimes has various particles or other contaminants therein which accumulate within the flow passage  52  proximate the end cap  76  as also shown in  FIG. 4 . As the piston sleeve  48  moves from its closed position toward its open position, the flushing holes  80  are unblocked prior to any of the inner ends of the fluid passageways of the flow control element  38  being unblocked by the continued axial movement of the piston sleeve  48 . As a result, fluid is able to flow directly from the flow passage  52 , through the flushing holes  80 , and into the outlet passage of the housing  12  prior to such fluid flowing through the flow control element  38 . The initial flow of fluid through the flushing holes  80  provides an effective flushing feature which causes any contaminant accumulation to be purged directly into the outlet passage rather than being channeled through the flow control element  38 . The flushing of the contaminant accumulation is desirable since it could otherwise clog or obstruct the fluid passageways extending through the flow control element  38  due to the typical size and configuration of such fluid passageways. 
     Referring now to  FIGS. 5 and 6 , in accordance with a second potential variant of the valve  10 , the previously described end cap  42  may be substituted with the end cap  82  shown in  FIGS. 5 and 6 . The end cap  82  comprises a first section  84  which is attached to the inlet section  14  of the housing  12  by support bars  86  which extend through the outlet passage region  18 . Alternatively, though not shown, the first section  84  of the end cap  82  may be integrally connected to the outlet section  20  of the housing  12  by support struts similar to the aforementioned support struts  44  shown in  FIG. 1 . Whether attached to the inlet section  14  and/or the outlet section  20 , the end cap  82  is positioned along the axis of the housing  12 , and more particularly the axis of the flow passage  52  of the piston sleeve  48 , with the flow control element  38  effectively being captured and compressed between the first section  84  and the guide member  34 . In this regard, as seen in  FIG. 6 , the first section  84  may include a continuous annular groove or channel formed therein which accommodates a sealing member  87 , such sealing member  87  creating a fluid tight seal between the first section  84  and the corresponding end of the flow control element  38 . 
     In addition to the first section  84 , the end cap  82  comprises a second section  88  which is moveably attached to the first section  84 . The second section  88  is selectively moveable between a blocked or closed position (as shown in  FIG. 5 ) whereat the second section  88  is operative to block the flow of fluid from the flow passage  52  to the outlet passage region  22  of the outlet passage when the piston sleeve  48  is in its closed position, and a flushing position (shown in  FIG. 6 ) which effectuates fluid flow from the flow passage  52  directly into the outlet passage region  22  of the outlet passage, thus bypassing the flow control element  38 . In the end cap  82 , the movement of the second section  88  from its closed position to its flushing position may occur when the piston sleeve  48  acts against the end cap  82  in a manner which will be described in more detail below. 
     In the end cap  82 , the second section  88  is normally biased to its closed position. More particularly, attached to the first section  84  of the end cap  82  is a plurality of support rods  90 . One end of each of the support rods  90  is attached (e.g., threadably connected) to the end cap  82 . Each of the support rods  90  is slidably advanced through a corresponding, complimentary aperture disposed within a peripheral portion of the second section  88 . Attached (e.g., threadably connected) to the end portion of each support rod  90  opposite that connected to the first section  84  is a retention member  92  such as a nut. Captured between each retention member  92  and the peripheral portion of the second section  88  is a biasing member  94  such as a biasing spring. In this regard, each of the support rods  90  extends axially through a respective one of the biasing members  94 . As seen in  FIG. 5  and as explained above, the biasing members  94  act against the second section  88  in a manner normally biasing the second section  88  to its closed position. 
     During normal operation of the valve  10  including the end cap  82 , when the piston sleeve  48  is in its closed position, fluid flowing through the flow passage  52  is effectively blocked from entering the flow control element  38  by the main body portion of the piston sleeve  48 , and is further blocked from directly entering the outlet passage by the second section  88  of the end cap  82  while in its closed position. As indicated above, the fluid flowing through the flow passage  52  and impinging the second section  88  of the end cap  82  sometimes has various particles or other contaminants therein which accumulate within the flow passage  52  proximate the end cap  82 . Using the position data regarding the piston sleeve  48  generated by the feedback device  72 , the piston sleeve  48  can be manually or automatically moved to a flushing position wherein the piston sleeve  48  moves axially toward the end cap  82  such that the first rim  60  of the piston sleeve contacts and exerts pressure against the distal rim defined by an annular skirt portion of the second section  88  in the manner shown in  FIG. 6 . The pressure exerted by the piston sleeve  48  against the second section  88  is sufficient to overcome the biasing force exerted by the biasing members  94 . As a result, the biasing members  94  are effectively compressed in the manner shown in  FIG. 6 , thus facilitating the movement of the second section  88  from its closed position to its flushing position. 
     When the second section  88  moves to its flushing position, fluid is able to pass through a plurality of flow openings  96  disposed in the annular skirt portion of the second section  88  in the manner also shown in  FIG. 6 . As a result, fluid is able to flow directly from the flow passage  52  into the outlet passage of the housing  12  prior to such fluid flowing through the flow control element  38 . In this regard, when the second section  88  is moved to its flushing position by the piston sleeve  48 , the inner ends of the fluid passageways of the flow control element  38  are still effectively blocked by the piston sleeve  48 . As seen in  FIG. 5 , the flow openings  96  are covered or blocked by the first section  84  of the end cap  82  when the second section  88  is in its closed position. In this regard, it is only after the second section  88  returns from its flushing position to its blocked position that the piston sleeve  48  moves axially toward its open position, thus unblocking the inner ends of the fluid passageways of the flow control element  38 . The initial flow of fluid through the flow openings  96  and directly into the outlet passage provides an effective flushing feature which causes any contaminant accumulation to be purged directly into the outlet passage rather than being channeled through the flow control element  38 . As indicated above, the flushing of the contaminant accumulation is desirable since it could otherwise clog or obstruct the fluid passageways of the flow control element  38 . 
     Referring now to  FIGS. 7-9 , there is shown a valve  100  constructed in accordance with a second embodiment of the present invention. The valve  100  comprises a housing  112  having an inlet section  114  which defines an inlet passage  116  and an outlet passage region  118 . In addition to the inlet section  114 , the housing  112  includes an outlet section  120  which itself defines an outlet passage region  122 . The outlet passage region  118  and the outlet passage region  122  collectively define an outlet passage of the valve  100  when the inlet and outlet sections  114 ,  120  are rigidly attached to each other. As seen in  FIG. 7 , the attachment of the inlet and outlet sections  114 ,  120  to each other is facilitated by the use of fasteners  124 , such as bolts. However, those of ordinary skill in the art will recognize that a wide variety of different attachment methods may be used to effectuate the rigid attachment of the inlet and outlet sections  114 ,  120  to each other. However, in the valve  100 , it is contemplated that any attachment method used to facilitate the attachment of the inlet and outlet sections  114 ,  120  to each other will be adapted to allow for the periodic separation of the inlet section  114  from the outlet section  120  as may be needed to access the interior of the housing  112  to allow for maintenance and other parts and components of the valve  100  which will be described in more detail below. 
     As indicated above, the outlet passage of the valve  100  is collectively defined by the outlet passage regions  118 ,  122 . In this regard, the configuration of the outlet passage of the valve  100  is governed, in large measure, by the shapes of the inlet and outlet sections  114 ,  120 . However, those of ordinary skill in the art will recognize that the shapes of the inlet and outlet sections  114 ,  120 , and hence the configuration of the outlet passage shown in  FIG. 7 , is exemplary only, and that alternative configurations for the outlet passage are contemplated to be within the spirit and scope of the present invention. 
     As further seen in  FIG. 7 , the outlet section  120  includes a piston sleeve portion  126  which is axially suspended within the outlet passage region  122 , and is integrally connected to an outer wall portion  128  of the outlet section  120  by a stem portion  130 . The piston sleeve portion  126  of the outlet section  120  defines an interior piston chamber  132 . The piston chamber  132  communicates with one end of a stem aperture  134  which extends through the stem portion  130  and outer wall portion  128  of the outlet section  120 , that end of the stem aperture  134  opposite that communicating with the piston chamber  132  itself communicating with the exterior of the housing  112 . As further seen in  FIG. 7 , the piston sleeve portion  126  is formed such that one end of the piston chamber  132  is enclosed, with the opposite end of the piston chamber  132  facing the inlet passage  116  being open. Fluidly communicating with the piston chamber  132  is one end of a flushing pipe  133 , the opposite end of which communicates with the exterior of the outlet section  120  of the housing  112 . As seen in  FIG. 7 , the flushing pipe  133  extends through the outlet passage region  122  of the outlet passage. The use of the flushing pipe  133  will be discussed in more detail below. 
     The valve  100  further comprises a generally cylindrical, tubular flow control element  136  which is disposed within the outlet passage region  118  of the inlet section  114 . As seen in  FIG. 7 , one end or annular rim defined by the flow control element  136  is abutted against a sealing member  137 , which is in turn abutted against the inlet section  114 . The opposite, remaining end or annular rim of the flow control element  136  is abutted against the piston sleeve portion  126  of the outlet section  120 . Thus, the sealing member  137  is effectively captured and compressed between the inlet section  114  and the flow control element  136 . Similarly, the flow control element  136  is effectively captured and compressed between the sealing member  137  and the outlet section  120 , while residing within the outlet passage region  118  of the outlet passage of the housing  112 . In the valve  100 , it is contemplated that the flow control element  136  may comprise a stack of annular discs that collectively define a series of substantially radially directed passageways extending between the inner and outer radial surfaces or edges of the discs. Each of the radially directed passageways has a plurality of turns formed therewithin in order to reduce the velocity of fluid that is flowing through the flow control element  136 . An exemplary flow control element  136  is disclosed in commonly owned U.S. Pat. No. 5,687,763 as indicated above. 
     The valve  100  further comprises an elongate, tubular piston sleeve  138  which is moveably disposed within the interior of the housing  112 , and more particularly within the piston chamber  132  defined by the piston sleeve portion  126  thereof. The piston sleeve  138  is selectively movable between a fully open position (shown in  FIG. 7 ) and a closed position which will be described in more detail below. As shown in  FIGS. 7-9 , the piston sleeve  138  includes a main body portion  140  which defines a flow passage  142  extending axially therethrough. Disposed within the main body portion  140  and extending therethough along an axis which is generally perpendicular to that defined by the flow passage  142  is an elongate opening  144 . The walls of the main body portion  140  which collectively define the opening  144  each include a piston rack  146  formed therein. In the piston sleeve  138 , the end portion of the main body portion  140  facing the inlet passage  116  has a beveled or tapered configuration, and extends to a first rim  148  of the main body portion  140 , the opposite end of the main body portion  140  defining a second rim  150 . 
     As indicated above, the piston sleeve  138  is reciprocally moveable within the piston chamber  132  between closed and fully opened positions. When the piston sleeve  138  is in its closed position, the beveled surface defined thereby and extending to the first rim  148  thereof is abutted and sealed against a complimentary sealing surface defined by the sealing member  137 . In this regard, when the piston sleeve  138  is in its closed position, the inner ends of each of the fluid passageways defined by the flow control element  136  are effectively covered by the main body portion  140  of the piston sleeve  138 , and in particular the section thereof extending between the opening  144  and the first rim  148 . 
     In the valve  100 , fluid initially enters the inlet passage  116  defined by the inlet section  114  of the housing  112 . As will be recognized by those of ordinary skill in the art, when the piston sleeve  138  is in its closed position as described above, fluid is able to flow into the flow passage  142  defined by the main body portion  140 , despite being effectively blocked from flowing radially outwardly through the flow control element  136 , and thus into the outlet passage collectively defined by the outlet passage regions  118 ,  122  of the housing  112 . The fluid flowing through the flow passage  142  is effectively blocked from entering the outlet passage of the housing  112  by the closed end of the piston sleeve portion  126  of the outlet section  120 . Any debris accumulating within the piston chamber  132  defined by the piston sleeve portion  126  may be effectively flushed therefrom via the flushing pipe  133 . 
     When the piston sleeve  138  is moved or actuated to its fully open position as shown in  FIG. 7 , the tapered surface extending to the first rim  148  is moved or withdrawn from its sealed engagement to the sealing member  137 , with the retraction of the piston sleeve  138  being continued until such time as the first rim  148  is generally aligned with that end of the flow control element  136  abutted against the piston sleeve portion  126  of the outlet section  120 . When the piston sleeve  138  is moved from the closed position toward the fully open position, fluid is able to flow through the inlet passage  116 , into the interior of the flow control element  136 , and thereafter radially outwardly through the flow control element  136  and into the outlet passage of the valve  100 . Since the fluid must flow through the flow control element  136  to reach the outlet passage, and in particular the outlet passage region  118  thereof, the energy of the fluid is effectively reduced due to the above-described functional attributes of the flow control element  136 . The opening of the valve  100  may be effectuated without necessarily actuating the piston sleeve  138  to its fully open position as shown in  FIG. 7 . In this regard, the axial movement of the piston sleeve  138  away from the sealing member  137  may be regulated or controlled depending on the desired level of fluid energy dissipation. Along these lines, as will be recognized, the greater the amount of axial movement of the piston sleeve  138  away from the sealing member  137 , the greater the number of energy dissipating fluid passageways of the flow control element  136  that will be exposed to the incoming fluid flow. In this regard, maximum energy dissipation of the inlet fluid is achieved when the piston sleeve  138  is moved to its fully open position. 
     As further seen in  FIGS. 7-9 , in the valve  100 , the movement or actuation of the piston sleeve  138  between its closed and fully open positions as described above is facilitated by an elongate rotary drive shaft  152 . The drive shaft  152  includes an external spline portion  154  which is formed proximate one end thereof. The drive shaft  152  is advanced through the stem aperture  134 , with the lower portion of the drive shaft  152  further being advanced through the opening  144  of the main body portion  140 . The spline portion  154  of the drive shaft  152  is sized and configured relative to the opening  144  such that the spline portion  154  engages or meshes with the piston racks  146  of the main body portion  140 . More particularly, due to the manner of meshed engagement between the spline portion  154  and the piston racks  146 , the rotation of the drive shaft  152  in a first, clockwise direction facilitates the movement of the piston sleeve  138  toward its closed position. Conversely, the rotation of the drive shaft  152  in a second, counter-clockwise direction facilitates the movement of the piston sleeve  138  toward its fully open position. The rotation of the drive shaft  152  in either the clockwise or counter-clockwise directions as needed to facilitate the movement of the piston sleeve  138  between its closed and fully open positions may be effectuated by a rotary actuation device  156  which is cooperatively engaged to that end of the drive shaft  152  disposed furthest from the spline portion  154 . As seen in  FIGS. 7 and 9 , the end of the drive shaft  152  disposed closest to the spline portion  154  is preferably rotatably nested within a complimentary recess defined by the piston sleeve portion  126  of the outlet section  120 . 
     During the movement or actuation of the piston sleeve  138  between its closed and open positions by the rotation of the drive shaft  152 , it is desirable that the movement of the piston sleeve  138  be constrained to movement along the axis of the flow passage  142 . To assist in this movement, it is contemplated that the valve  10  may be outfitted with a guide rod  158  which is shown in  FIGS. 7 and 8 . The guide rod  158  is attached to the closed end of the piston sleeve portion  126 , and protrudes axially into the piston chamber  132  defined thereby. As seen in  FIG. 7 , a cylindrically configured main body of the guide rod  158  is sized and configured to be slidably received into the flow passage  142 , and in particular that portion of the flow passage  152  defined by the segment of the main body portion  140  extending between the piston racks  146  and second rim  150 . It is contemplated that some portion of the main body of the guide rod  158  will remain within the flow passage  142  during the full range of movement of the piston sleeve  138  between its closed and fully open positions. Though not shown, it is contemplated that the valve  100  may be outfitted with a feedback device similar to the above-described feedback device  72  for purposes of monitoring the position of the piston sleeve  138  relative to the housing  112 . 
     Referring now to  FIG. 10 , there is shown a valve  200  constructed in accordance with a third embodiment of the present invention. The valve  200  comprises a housing  212  having an inlet section  214  which defines an inlet passage  216  and an outlet passage region  218 . In addition to the inlet section  214 , the housing  212  includes an outlet section  220  which itself defines an outlet passage region  222 . The outlet passage region  218  and the outlet passage region  222  collectively define an outlet passage of the valve  200  when the inlet and outlet sections  214 ,  220  are rigidly attached to each other. As seen in  FIG. 10 , the attachment of the inlet and outlet sections  214 ,  220  to each other is facilitated by the use of fasteners  224 , such as bolts. However, those of ordinary skill in the art will recognize that a wide variety of different attachment methods may be used to effectuate the rigid attachment of the inlet and outlet sections  214 ,  220  to each other. However, in the valve  200 , it is contemplated that any attachment method used to facilitate the attachment of the inlet and outlet sections  214 ,  220  to each other will be adapted to allow for the periodic separation of the inlet section  214  from the outlet section  220  as may be needed to access the interior of the housing  212  to allow for maintenance and other parts and components of the valve  200  which will be described in more detail below. 
     As indicated above, the outlet passage of the valve  200  is collectively defined by the outlet passage regions  218 ,  222 . In this regard, the configuration of the outlet passage of the valve  200  is governed, in large measure, by the shapes of the inlet and outlet sections  214 ,  220 . However, those of ordinary skill in the art will recognize that the shapes of the inlet and outlet sections  214 ,  220 , and hence the configuration of the outlet passage shown in  FIG. 10 , is exemplary only, and that alternative configurations for the outlet passage are contemplated to be within the spirit and scope of the present invention. 
     As further seen in  FIG. 10 , the outlet section  220  includes a piston sleeve portion  226  which is axially suspended within the outlet passage region  222 , and is connected to an outer wall portion  228  of the outlet section  220  by a stem portion  230 . The piston sleeve portion  226  of the outlet section  220  defines an interior piston chamber  232 . The piston chamber  232  communicates with one end of a stem aperture  234  which extends through the piston sleeve portion  226 , the stem portion  230 , and the outer wall portion  228  of the outlet section  220 , that end of the stem aperture  234  opposite that communicating with the piston chamber  232  itself communicating with the exterior of the housing  212 . As further seen in  FIG. 10 , the piston sleeve portion  226  is formed such that one end of the piston chamber  232  is enclosed, with the opposite end of the piston chamber  232  facing the inlet passage  216  being open. 
     The valve  200  further comprises a tubular, generally cylindrical flow control element  236  which is disposed within the outlet passage region  218  of the inlet section  214 . As seen in  FIG. 10 , one end or annular rim defined by the flow control element  236  is abutted against a sealing member  237 , which is in turn abutted against the inlet section  214 . The opposite, remaining end or annular rim of the flow control element  236  is abutted against an adapter ring  239 , which is in turn abutted against the piston sleeve portion  226  of the outlet section  120 . Thus, the sealing member  237  is effectively captured and compressed between the inlet section  214  and the flow control element  236 , with the adapter ring  239  being captured and compressed between the outlet section  220  and the flow control element  236 . Similarly, the flow control element  236  is effectively captured and compressed between the sealing member  237  and the adapter ring  239 , while residing within the outlet passage region  218  of the outlet passage of the housing  212 . Captured between portions of the adapter ring  239  and the flow control element  236  in an annular sealing member  241 . In the valve  200 , it is contemplated that the flow control element  236  may comprise a stack of annular discs that collectively define a series of substantially radially directed passageways extending between the inner and outer radial surfaces or edges of the discs. Each of the radially directed passageways has a plurality of turns formed therewithin in order to reduce the velocity of fluid that is flowing through the flow control element  236 . An exemplary flow control element  236  is disclosed in commonly owned U.S. Pat. No. 5,687,763 as indicated above. 
     The valve  200  further comprises an elongate, tubular piston sleeve  238  which is moveably disposed within the interior of the housing  212 , and more particularly within the piston chamber  232  defined by the piston sleeve portion  226  thereof. The piston sleeve  238  is selectively movable between a closed position (shown in  FIG. 10 ) and a fully open position which will be described in more detail below. The piston sleeve  238  includes a main body portion  240  having an annular skirt portion  242  protruding axially from one end thereof. Disposed within the main body portion  240  and extending therethrough along an axis which is generally perpendicular to that defined by the main body portion  240  is an elongate opening  244 . A portion of a wall of the main body portion  240  which partially defines the opening  244  includes a piston rack  246  formed therein. In the piston sleeve  238 , the skirt portion  242  defines a beveled or tapered sealing surface  248  adjacent the distal rim thereof. The piston sleeve  238  also defines an end surface  250  which is at the end opposite the distal rim defined by the skirt portion  232 . Disposed within the outer surface of the main body portion  240  in close proximity to the end surface  250  is a continuous groove or channel which accommodates a sealing member such as an O-ring  251 . 
     As indicated above, the piston sleeve  238  is reciprocally moveable within the piston chamber  232  between closed and fully open positions. When the piston sleeve  238  is in its closed position, the sealing surface  248  defined by the skirt portion  242  is abutted and sealed against a complimentary sealing surface defined by the sealing member  237 . In this regard, when the piston sleeve  238  is in its closed position, the inner ends of each of the fluid passageways defined by the flow control element  236  are effectively covered by the main body portion  240  of the piston sleeve  238 , and in particular the section thereof extending between the opening  244  and the sealing surface  248 . 
     The piston sleeve  238  further includes at least one, and preferably a plurality of balance holes  260  formed therein. One end of each of the balance holes  260  extends to that surface of the main body portion  240  circumvented by the skirt portion  242 , with the opposite end of each of the balance holes  260  extending to the end surface  250 . As will be recognized by those of ordinary skill in the art, the balance holes  260  effectively bypass and thus do not communicate with the opening  244  formed in the main body portion  240 . 
     In the valve  200 , fluid initially enters the inlet passage  216  defined by the inlet section  214  of the housing  212 . When the piston sleeve  238  is in its closed position as described above, fluid is able to flow into the interior of the skirt portion  242 , and through the balance holes  260  into the piston chamber  232 , despite being effectively blocked from flowing radially outwardly through the flow control element  236 , and thus into the outlet passage collectively defined by the outlet passage regions  218 ,  222  of the housing  212 . The fluid flowing into the interior of the skirt portion  242  is effectively blocked from entering the outlet passage of the housing  212  predominantly by the skirt portion  232  itself. 
     When the piston sleeve  238  is moved or actuated to its fully open position, the sealing surface  248  defined by the skirt portion  242  is moved or withdrawn from its sealed engagement to the sealing member  237 , with the retraction of the piston sleeve  238  being continued until such time as the distal rim of the skirt portion  242  is generally aligned with the sealing member  241 . When the piston sleeve  238  is moved from the closed position to the fully open position, fluid is able to flow through the inlet passage  216 , and thereafter radially outwardly through the flow control element  236  and into the outlet passage of the valve  200 . Since the fluid must flow through the flow control element  236  to reach the outlet passage, and in particular the outlet passage region  218  thereof, the energy of the fluid is effectively reduced due to the above-described functional attributes of the flow control element  236 . The opening of the valve  200  may be effectuated without necessarily actuating the piston sleeve  238  to its fully open position. In this regard, the axial movement of the piston sleeve  238  away from the sealing member  237  may be regulated or controlled depending on the desired level of fluid energy dissipation. Along these lines, as will be recognized, the greater the amount of axial movement of the piston sleeve  238  away from the sealing member  237 , the greater the number of energy dissipating fluid passageways of the flow control element  236  that will be exposed to the incoming fluid flow. Maximum energy dissipation of the inlet fluid is achieved when the piston sleeve  238  is moved to its fully open position. 
     As further seen in  FIG. 10 , in the valve  200 , the movement or actuation of the piston sleeve  238  between its closed and fully opened positions as described above is facilitated by an elongate rotary drive shaft  252 . The drive shaft  252  includes an external spline portion  254  which is formed on a central portion thereof. The drive shaft  252  is advanced through the stem aperture  234 , with the lower portion of the drive shaft  252  further being advanced through the opening  244  of the main body portion  240 . The spline portion  254  of the drive shaft  252  is sized and configured relative to the opening  244  such that the spline portion  254  engages or meshes with the piston rack  246  of the main body portion  240 . More particularly, due to the manner of meshed engagement between the spline portion  254  and the piston rack  246 , the rotation of the drive shaft  252  in a first, clockwise direction facilitates the movement of the piston sleeve  238  toward its closed position. Conversely, the rotation of the drive shaft  252  in a second, counter-clockwise direction facilitates the movement of the piston sleeve  238  toward its fully open position. The rotation of the drive shaft  252  in either the clockwise or counter-clockwise directions as needed to facilitate the movement of the piston sleeve  238  between its closed and fully opened positions may be effectuated by a rotary actuation device (not shown) which is cooperatively engaged to the drive shaft  252 . As seen in  FIG. 10 , the end of the drive shaft  252  disposed closest to the spline portion  254  is preferably rotatably nested within a complimentary recess partially defined by the piston sleeve portion  226  of the outlet section  220 . Additionally, a seal assembly or packing is preferably interposed between the drive shaft  252  and that surface of the outlet section  220  defining the stem aperture  234 . Though not shown, it is contemplated that the valve  200  may also be outfitted with a feedback device similar to the above-described feedback device  72  for purposes of monitoring the position of the piston sleeve  238  relative to the housing  212 . 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention 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.