Abstract:
The present invention is directed for use in a valve assembly including a housing having an axis and defining an interior, with fluid provided in the housing and substantially filling the housing interior, a valve stem disposed in the housing interior and a piston attached to the valve stem, wherein the piston and valve stem are moveable along the housing axis within the housing interior between first and second positions. The present invention includes a valve actuation control device disposed in the housing interior, the valve actuation control device allowing the fluid to freely flow from one side of the piston to the other side during at least a portion of the piston&#39;s movement between the first and second positions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of co-pending provisional patent application Serial No. 60/435,635 entitled “Valve Actuation Velocity Control Device”, filed on Dec. 19, 2002, the entire disclosure of which is incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is directed toward process valves and other valves used in applications that require characterized control and, more particularly, toward a process valve having an improved actuation velocity control device for characterizing the stem velocity of the valve, or item, during its movement between opening and closing.  
         BACKGROUND OF THE INVENTION  
         [0003]    Valves that are utilized in nuclear power plants for main steam or main feed water isolation, or other critical applications (e.g., gas turbines, dampers, etc.), are generally opened and closed utilizing valve actuators. Valves utilized in such critical applications are normally used for emergency shut-off purposes. Thus, these valve actuators must be operable to close the valve in a rapid period of time, typically within three seconds or so. Opening the valve is typically less of a concern, since the primary purpose of the valve is for an emergency shut-off situation. However, if the valve is used for an emergency open purpose, the valve must be operable to open in a rapid period of time. Typical actuators utilized in such critical and non-critical applications include, but are not limited to, mechanical, electromechanical, hydraulic, pneumatic, and hydro-pneumatic powered actuators.  
           [0004]    Various regulations govern the operational requirements for valves utilized in nuclear power plants. One such operational requirement is directed toward characterizing the flow of system media through the valve during the closing stroke of the valve in a predetermined manner. Typically, the system media flow through the valve has been characterized by an exponentially decreasing flow curve as the valve progresses through its closing stroke. However, recent requirements governing nuclear power plant use have called for a constant, or straight line, decreasing system media flow curve during the closing stroke of the valve. Thus, the valve actuators utilized in nuclear power plants will need to operate to close the valve in such a manner that the system media flow through the valve during its closing stroke is characterized by a constant, straight line decreasing curve.  
           [0005]    The present invention is directed toward overcoming one or more of the above-mentioned problems.  
         SUMMARY OF THE INVENTION  
         [0006]    The control flow through the actuator can be characterized as follows. The present invention is directed for use in a valve assembly including a housing having an axis and defining an interior, with fluid provided in the housing and substantially filling the housing interior, a valve stem disposed in the housing interior and a piston attached to the valve stem, wherein the piston and valve stem are moveable along the housing axis within the housing interior between first and second positions. The present invention includes a valve actuation control device disposed in the housing interior, the valve actuation control device allowing the fluid to freely flow from one side of the piston to the other side during at least a portion of the piston&#39;s movement between the first and second positions.  
           [0007]    More specifically, the inventive valve actuation control device is configured to allow for the free flow of fluid from one side of the piston to the other during the first half of the valve closing stroke. Allowing the free flow of fluid during the first half of the valve closing stroke will provide for a high, initial stem velocity during the first half of the valve closing stroke. This, in turn, allows the system media flowing through the valve body to be characterized in a desired manner during the valve closing stroke, for example, as a constant, straight line decreasing curve.  
           [0008]    In one form, the valve actuation control device includes at least one tube disposed in the housing interior generally parallel to the housing axis. The at least one tube includes at least one aperture formed therein for allowing the fluid to freely flow from one side of the piston to the other side during at least a potion of the piston&#39;s movement between the first and second positions.  
           [0009]    The at least one tube extends between first and second cylinder heads which seal the valve housing at its axial ends. One end of the at least one tube is sealed against one of the cylinder heads, in the other end of the at least one tube opens into the housing interior via a port formed in the other cylinder head. Fluid flow through the cylinder head port will be controlled via an adjustable radial screw received in the cylinder head port. The at least one tube is received through a cooperating aperture(s) formed in the piston, allowing the piston to move without damaging the at least one tube.  
           [0010]    The at least one aperture is preferably formed in one half of the at least one tube which is along the first half of the valve closing stroke. In a preferred form, the at least one tube includes three tubes, and the at least one aperture includes three apertures.  
           [0011]    In another form of the present invention, the valve actuation control device includes inner and outer coaxial housings, with the piston and valve stem moveable along the housing axis within the inner housing interior between the first and second positions. The inner housing includes at least one slot formed therein for allowing the fluid to freely flow from one side of the piston to the other side during at least a portion of the piston&#39;s movement between the first and second positions. The outer housing may include at least one channel formed therein aligned with the at least one slot in the inner housing for further allowing the free flow of fluid from one side of the piston to the other side during at least a portion of the piston&#39;s movement between the first and second positions.  
           [0012]    The at least one slot and the at least channel are preferably formed in the inner and outer housings, respectively, in one half of the housings which is along the first half of the valve closing stroke. In a preferred form, the at least one channel includes four channels equiangularly spaced about the housing axis, and the at least one slot includes four slots also equiangularly spaced about the housing axis. One, or both, of the inner and outer housings may be rotatable in order to selectively align the at least one slot with the at least one channel.  
           [0013]    The valve actuation velocity control device of the present inventive assembly includes various objectives, including, but in no way limited to, the following:  
           [0014]    It is an object of the present invention to close a valve in a manner such that the system media flow through the valve is characterized as a constant, straight line decreasing curve during the valve closing stroke.  
           [0015]    It is another object of the present invention to close a valve with a high initial stem velocity.  
           [0016]    It is an additional object of the present invention to close a valve with a high initial stem velocity, and then a constant, slower stem velocity through the balance of the valve closing stroke.  
           [0017]    It is a further object of the present invention to operate parallel gate, wedge gate and globe valves, and other valves as applicable, using an actuation device that characterizes the stem velocity of the valve.  
           [0018]    It is yet a further object of the present invention to apply the inventive valve assembly to nuclear critical and non-critical applications.  
           [0019]    It is still a further object of the present invention to provide a valve with an actuation velocity control device which is adjustable to allow adjustment of the high initial stem velocity, and hence the system media flow characteristics, during the valve closing stroke.  
           [0020]    Other object, aspects and advantages of the present invention can be obtained from a study of the specification, the drawings, and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a graph illustrating the media flow characteristics for conventional valves, as well as the desired media flow characteristic using the inventive valve assembly (media flow vs. time—inner legend), and the relationship of the desired media flow characteristic with the stem velocity of the inventive valve assembly during the valve closing stroke (time vs. valve stroke—outer legend);  
         [0022]    [0022]FIGS. 2 a - c  are partial cross-sectional views of a valve assembly according to a first embodiment of the present invention, with the valve in open (FIG. 2 a ), half closed (half stroke) (FIG. 2 b ) and fully closed (full stroke) (FIG. 2 c ) positions;  
         [0023]    [0023]FIG. 3 is an enlarged view of the valve assembly shown in FIG. 2 b;    
         [0024]    [0024]FIGS. 4 a - c  are partial cross-sectional views of a valve assembly according to a second embodiment of the present invention, with the valve in open (FIG. 4 a ), half closed (half stroke) (FIG. 4 b ) and fully closed (full stroke) (FIG. 4 c ) positions;  
         [0025]    [0025]FIG. 5 is an enlarged partial view of the valve assembly shown in FIG. 4 a ; and  
         [0026]    [0026]FIG. 6 is a cross-sectional view taken along line A-A in FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    The present invention consists of a valve assembly which achieves at least the above-identified objectives. The inventive valve assembly includes a valve, a valve actuator and an actuation velocity control device which is designed to control the valve stem velocity to characterize the system media flow through the valve in a desired manner.  
         [0028]    [0028]FIG. 1 is a graph of system media flow vs. time (inner legend) for various valve closing speeds for typical process valves utilized in nuclear power plant applications. Curve  12  illustrates the media flow characteristics for a valve having a 2.75 second closing time; curve  14  illustrates the media flow characteristics for a valve having a 2.50 second closing time; curve  16  illustrates the media flow characteristics for a valve having a 2.25 second closing time; curve  18  illustrates the media flow characteristics for a valve having a 2.0 second closing time; and curve  20  illustrates the base media flow characteristics during the closing stroke of a typical process valve having approximately a 1.75 second closing time.  
         [0029]    As shown in FIG. 1, the media flow characteristics through the valves utilized to generate the curves  12 ,  14 ,  16 ,  18  and  20  are generally represented by an exponentially decreasing curve as the valve proceeds through its closing stroke. However, recent regulations pertaining particularly to nuclear power plants and other critical applications have required that the media flow characteristics through the valve during its closing stroke be represented by a constant, straight line decreasing media flow curve. The desired linear characteristic (media flow vs. time) of a constant, decreasing media flow during the valve closing stroke is illustrated by the dotted line  22  in FIG. 1. Thus, the inventive valve assembly needs to be able to close the valve in such a manner to achieve the desired media flow characteristic curve  22  during the valve closing stroke. The characteristic curve  22  represents a constant, decreasing media flow velocity through a valve having approximately a 2.75 second closing time. However, it should be understood that the inventive valve assembly may incorporate other closing times (shorter or longer) without departing from the spirit and scope of the present invention.  
         [0030]    The valve actuation velocity control device of the present invention has been found to achieve the desired media flow characteristic  22  by providing for a high, initial valve stem velocity during the closing stroke, and then providing for a constant, slower valve stem velocity through the balance of the closing stroke. The valve stem velocity representation is shown at  24  in FIG. 1, which is graphed utilizing the outer legend of time vs. valve stroke.  
         [0031]    Typically, valves have opening and closing strokes which encompass movement of approximately 10 inches during opening and closing. As shown in FIG. 1, during the first half of the closing stroke (line  24   a ), the valve stem has an initially high velocity, moving approximately 5 inches in approximately 0.75 seconds (≈6.67 in/sec). During the second half of the closing stroke (line  24   b ), the valve stem has a slower velocity, moving the remaining approximately 5 inches in approximately 2.75 seconds (≈1.82 in/sec). By controlling the stem velocity in a manner shown by line  24 , the desired media flow characteristic illustrated by line  22  can be achieved during the valve closing stroke.  
         [0032]    FIGS.  2 - 3  show graphic illustrations of a first embodiment of the inventive valve assembly, shown generally at  30 , which includes a valve  32  having the inventive valve actuation velocity control device  34 . FIG. 2 a  illustrates the valve assembly  30  in a fully open position; FIGS. 2 b  and  3  illustrate the valve assembly  30  in a half closed (half stroke) position; and FIG. 2 c  illustrates the valve assembly  30  in a fully closed (full stroke) position. For clarity purposes, the valve actuator, valve body and valve gate have been omitted from FIGS.  2 - 3 .  
         [0033]    As shown in FIGS.  2 - 3 , the valve  32  includes a generally cylindrical housing  36  having an axis  37  and defining an interior  38 . The valve  32  further includes a valve stem  40  to which a piston  42  is attached, with the valve stem  40  and piston  42  disposed within the housing interior  38 . A lower portion  44  of the stem  40  is attached to a valve gate (not shown) which is received within a valve body (also not shown) in a well-known manner. Movement of the stem  40  effectuates movement of the gate to open and close the port formed through the valve body as is known in the relevant art. The upper portion  46  of the stem  40  is connected to a conventional valve actuator (not shown) which effectuates movement of the stem  40  to move the valve  32  between its open and closed positions.  
         [0034]    The interior  38  of the cylindrical housing  36  is typically filled with hydraulic fluid which surrounds the stem  40  and piston  42 . The valve actuation velocity control device  34  is disposed within the housing interior  38 . The valve actuation velocity control device  34  includes tubes  48  positioned within the housing interior  38 . Hydraulic fluid provided in the housing interior  38  surrounds the tubes  48  and fills the interiors thereof. The tubes  48  each include openings  50  formed in an upper portion of the tubes  48 , with the lower ends of the tubes  48  being sealed against the cylinder head  52 . The upper ends of the tubes  48  open into ports  54  formed in the cylinder head  56 . The ports  54  open into the housing interior  38 , and fluid flow through the ports  54  may be controlled with adjustable radial screws  58  received in the cylinder head  56 .  
         [0035]    The valve assembly  30  further includes a conventional force compensating assembly  60  attached to the cylinder heads  52  and  56  and in fluid communication with the housing interior  38  via ports  62  and  64  formed in the cylinder heads  52  and  56 , respectively. Typically, during opening and closing of the valve  32 , the force compensating assembly  60  receives hydraulic fluid from within the housing interior  38  and will conventionally control the speed of the valve stem  40 . The inventive valve actuation velocity control device  34  provides for an initial high stem  40  velocity during the first half of the closing stroke, while the force compensating assembly  60  will be used to control the stem  40  velocity during the second half of the closing stroke.  
         [0036]    Prior to initiating a closing stroke, the valve  32  will typically be in the fully open position shown in FIG. 2 a , with the piston  42  positioned at the top of the housing interior  38 . As shown in FIGS.  2 - 3 , the piston  42  includes apertures formed therethrough to accommodate the tubes  48 . The tubes  48  should fit snugly within the apertures, while allowing the piston  42  to move without damaging the tubes  48 . As the closing stroke is initiated by the actuator (not shown), the piston  42  and stem  40  are moved downward to the position shown in FIGS. 2 b  and  3 , which illustrates the valve  32  in a half closed, or half stroke, position. As the piston  42  moves from the position shown in FIG. 2 a  to the position shown in FIGS. 2 b  and  3 , very little hydraulic fluid will be forced through the lower port  62  and into the force compensating assembly  60 . Most of the hydraulic fluid will be forced and flow from the bottom side of the piston  42  through the openings  50  in the tubes  48 , and up through tubes  48  and back into the housing interior  38  on the top side of the piston  42 . The hydraulic fluid may either flow back to the housing interior  38  through an adjacent opening  50  to the top side of the piston  42 , or may flow up the tubes  48  into the cylinder head ports  54  and subsequently to the top side of the piston  42 , as shown by the flow arrows in FIG. 3. The fluid flow is regulated by both the adjustable radial screws  58  received in the cylinder head  56 , and by the geometric aspects of the tubes  48  and the openings  50 . In this manner, during the first half of the valve closing stroke, the valve stem  40 , and hence the valve gate (not shown), will have a high velocity (line  24   a  in FIG. 1) as the hydraulic fluid will flow freely into and out of the tubes  48 . Preferably, the initial high velocity of the stem  40  is a constant velocity, as shown by line  24   a  in FIG. 1.  
         [0037]    While adjustable radial screws  58  are shown for controlling hydraulic fluid flow through the cylinder head ports  54 , it should be understood that the screws  58  may be replaced with a mechanically or electrically controlled oraficing system, possibly similar to system  60 , for more intricate control of the hydraulic fluid flow through the cylinder head ports  54 .  
         [0038]    During the second half of the valve closing stroke, as the stem  40  and piston  42  move from the position shown in FIGS. 2 b  and  3  to the position shown in FIG. 2 c , the openings  50  in the tubes  48  do not effect the velocity of the stem  40 , as no fluid is flowing through the tubes  48  to the top side of the piston  42 . The openings  50  in the tubes  48  will be located on the top side of the piston  42  during the second half of the valve closing stroke. The hydraulic fluid will be forced and flow into the cylindrical head port  62 , through the force compensating assembly  60 , and back to the housing interior  38  at the top side of the piston  42  through cylinder head port  64 . The force compensating assembly  60  will conventionally control the closing speed of the stem  40  during the second half of the valve closing stroke. Preferably, the force compensating assembly  60  will provide for a constant velocity of the stem  40  through the second half of the valve closing stroke (see line  24   b  in FIG. 1).  
         [0039]    Conversely, during the first half of the valve opening stroke, the openings  50  in the tubes  48  will have no effect on the velocity of the stem  40 , and the force compensating assembly  60  will conventionally control the stem velocity during the first half of the valve opening stroke as the hydraulic fluid will be forced through the force compensating assembly  60  via ports  64  and  62 . The openings  50  in the tubes  48  are disposed on the top side of the piston  42  during the first half of the valve opening stroke. During the second half of the valve opening stroke, the hydraulic fluid will flow in a manner reverse to that of the first half of the valve closing stroke, flowing through the tube openings  50  and port  54  down through the tubes  48 , and back out of the openings  50  on the bottom side of the piston  42 . The valve actuation velocity control device  34  may thus be utilized in controlling the speed of the valve stem  40  during the second half of the valve opening stroke.  
         [0040]    The main aspect of the invention shown in FIGS.  2 - 3  is to control the operating velocity of the valve stem  40  during the valve closing stroke. By adjustment of the radial screws  58  and the geometry of the tubes  48  and openings  50 , the closing stroke of the valve  32  can be characterized in a desired manner, for example, in a manner as illustrated by line  24  in FIG. 1. In turn, the system media flowing through the valve body (not shown) will be characterized in a desired manner during the valve closing stroke, for example, as indicated by the line  22  in FIG. 1. Controlling the operating velocity of the valve stem  40  during the valve opening stroke is typically less of a concern, since valves used in critical and nuclear applications are typically used for emergency shut-off purposes. However, one skilled in the art will readily appreciate that the valve actuation control device of the present invention can be implemented to control the valve stem  40  during the valve opening stroke without departing from the spirit and scope of the present invention.  
         [0041]    It should be understood that while three tubes  48 , each having three openings  50 , are shown in the assembly  30  of FIGS.  2 - 3 , the valve actuation velocity control device  34  may include any number of tubes  48  and/or openings  50  without departing from the spirit and scope of the present invention. In accordance with the present invention, the number, sizes and geometries of the tubes  48  and openings  50 , as well as the location of the openings  50  along the lengths of the tubes  48 , may be varied in order to achieve a desired media flow characteristic.  
         [0042]    FIGS.  4 - 6  show graphic illustrations of a second embodiment of the inventive valve assembly, shown generally at  90 . FIGS. 4 a  and  5  illustrate the valve assembly  90  in a fully open position; FIG. 4 b  illustrates the valve assembly  90  in a half closed (half stroke) position; and FIG. 4 c  illustrates the valve assembly  90  in a fully closed (full stroke) position. For clarity purposes, the valve actuator, valve body and valve gate have been omitted from FIGS.  4 - 6 .  
         [0043]    The assembly  90  includes a valve  92  having inner  94  and outer  96  cylindrical housings. The inner cylindrical housing  94  extends along the entire length of the valve  92 , while the outer cylindrical housing  96  extends along only an upper half of the valve  92 . The valve  92  further includes a valve stem  98  and a piston  100  attached to the valve stem  98  for movement therewith. Both the stem  98  and piston  100  are disposed within the housing interior  101  defined by the inner cylindrical housing  94 . The lower portion  102  of the valve stem  98  is connected to a valve gate (not shown) which is received in a valve body (also not shown) in a well-known manner. The upper portion  104  of the valve stem  98  is connected to a conventional valve actuator (not shown) which effectuates opening and closing of the valve  92  as is known in the relevant art. The assembly  90  further includes a force compensating assembly  106 , similar in construction to the force compensating assembly  60  shown in FIGS.  2 - 3 . The force compensating assembly  106  is fluidly connected to the housing interior  101  via ports  108  and  110  formed in cylinder heads  112  and  114 , respectively, in a conventional manner. The force compensating assembly  106  will control the valve stem  98  velocity during the second half of the valve closing stroke and the first half of the valve opening stroke in a conventional manner as previously described.  
         [0044]    The inner cylindrical housing  94  includes a plurality of slots  116  formed therein. As shown more clearly in FIG. 6, in one form the inner cylindrical housing  94  includes four slots  116  spaced equiangularly about the housing axis  117 . The outer cylindrical housing  96  includes four axial channels  118  formed therein, also equiangularly spaced about the housing axis  117 . The axial channels  118  are aligned with the slots  116 , one each, to provide paths for the hydraulic fluid as the piston  100  is moved along the housing interior  101  to open and close the valve  92 . The housing interior  101  is filled with hydraulic fluid. The hydraulic fluid surrounds the stem  98  and piston  100 , and fills the slots  116  and channels  118 . The inner  94  and outer  96  housings, and the slots  116  and channels  118  formed therein, together make up the valve actuation velocity control device  120 .  
         [0045]    Prior to initiating a closing stroke, the valve  92  will be in the fully open position shown in FIGS. 4 a  and  5 , with the piston  100  positioned at the top of the housing interior  101 . Upon initiation of the closing stroke, the actuator (not shown) moves the stem  98  and piston  100  downward from the position shown in FIGS. 4 a  and  5  to the position shown in FIG. 4 b , which illustrates the valve  92  in a half closed (half stroke) position. As the piston  100  is moved downward during the first half of the closing stroke, very little hydraulic fluid will enter the force compensating assembly  106  via port  108 . Most of the hydraulic fluid will flow from the bottom side of the piston  100  through the slots  116  and into the channels  118 , around the piston  100 , and back into the housing interior  101  at the top side of the piston  100 , as shown by the flow arrows in FIG. 5. Flow of fluid in this manner will continue until the piston  100  reaches the half stroke position shown in FIG. 4 b . The fluid flow is regulated by the sizes and geometric aspects of the slots  116  ad channels  118 , as well as the alignment of the slots  116  and channels  118  as will be described infra. In this manner, during the first half of the valve closing stroke, the valve stem  98 , and hence the valve gate (not shown), will have a high velocity (line  24   a  in FIG. 1) as the hydraulic fluid will flow freely through the slots  116  and channels  118 . Preferably, the initial high velocity of the stem  98  is a constant velocity, as shown by line  24   a  in FIG. 1.  
         [0046]    During the second half of the valve closing stroke, as the stem  98  and piston  100  move from the position shown in FIG. 4 b  to the position shown in FIG. 4 c , the slots  116  and channels  118  do not effect the velocity of the stem  98 , as no fluid is flowing through the slots  116  and channels  118  to the top side of the piston  100 . The slots  116  and channels  118  will be located on the top side of the piston  100  during the second half of the valve closing stroke. Rather, the hydraulic fluid is forced into the force compensating assembly  106  via port  108 , flows through the force compensating assembly  106  and back to the housing interior  101  at the top side of the piston  100  via port  110 . The force compensating assembly  106  conventionally controls the velocity of the stem  98  to have a constant velocity (see line  24   b  in FIG. 1) during the second or last half of the valve closing stroke.  
         [0047]    Conversely, during the first half of the valve opening stroke, as the piston  100  moves from the position shown in FIG. 4 c  to the position shown in FIG. 4 b , the speed of the stem  98  will be conventionally controlled by the force compensating assembly  106 , and the slots  116  and channels  118  will have no effect on the stem  98  velocity. The slots  116  and channels  118  are disposed on the top side of the piston  100  during the first half of the valve opening stroke. During the last half of the valve opening stroke, the hydraulic fluid will flow in a manner reverse to that of the first half of the valve closing stroke, as the slots  116  and channels  118  will allow fluid to flow from the top side of the piston  100  to the bottom side of the piston  100 . The valve actuation velocity control device  120  may thus be utilized in controlling the speed of the stem  98  during the second half of the valve opening stroke.  
         [0048]    The assembly  90  shown in FIGS.  4 - 6  will allow the valve stem  98  to have a high initial velocity during the first half of the valve closing stroke, since fluid will flow freely through the slots  116  and channels  118  from the bottom side to the top side of the piston  100 . During the last half of the valve closing stroke, the force compensating assembly  106  will conventionally control the stem velocity. Configuration of the assembly  90  in this manner allows the closing stroke of the valve  92  to be characterized in a desired manner, for example, in a manner as illustrated by line  24  in FIG. 1. In turn, the system media flowing through the valve body (not shown) will be characterized in a desired manner during the valve closing stroke, for example, as represented by the dotted line  22  in FIG. 1. Controlling the operating velocity of the valve stem  98  during the valve opening stroke is typically less of a concern, since valves used in critical and nuclear applications are typically used for emergency shut-off purposes. However, one skilled in the art will readily appreciate that the valve actuation control device of the present invention can be implemented to control the valve stem  98  during the valve opening stroke without departing from the spirit and scope of the present invention.  
         [0049]    While FIGS.  4 - 6  illustrate four slots  116  and four channels  118  utilized in the valve actuation velocity control device  120 , one skilled in the art will appreciate that any number of slots  116  and channels  118  may be formed in the housings  94  and  96 , respectively, without departing from the spirit and scope of the present invention. In accordance with the present invention, the number, sizes and geometries of the slots  116  and channels  118 , as well as the location of the slots  116  and channels  118  along the lengths of the inner  94  and outer  96  housings, respectively, may be varied to regulate the fluid flow, and thus the stem  98  closing speed, in order to achieve a desired media flow characteristic.  
         [0050]    Additionally, the outer housing  96  may be rotatable, as indicated by arrow  122 , to align either all of the channels  118  with the slots  116 , as shown in FIG. 6, or to align only a portion of the channels  118  with the slots  116  to achieve the desired media flow characteristic. Typically, the inner housing  94  will be non-rotatable so that a sufficient bottom seal may be formed between the inner housing  94  and the cylinder head  112 . However, if desired for a particular application, the inner housing  94  may also be rotatable to align the slots  116  and channels  118 . Further, it will be appreciated that for particular applications the channels  118  may be omitted as the hydraulic fluid will readily flow to the opposite side of the piston  100  via the slots  116 .  
         [0051]    It should be noted that only a few methods of providing for the free flow of hydraulic fluid from one side of the piston to the other during the valve closing stroke to control the valve stem velocity are described herein. One skilled in the art will readily appreciate that various other methods could be implemented in accordance with the teaching of the present invention. While the present invention has been described in connection with the drawings, it should be understood that various modifications may be made without departing from the spirit and scope of the present invention.