Abstract:
A dual piston valve actuator operable to provide an initial thrust force that is greater than a subsequent thrust force for moving a gate valve from a closed position to an open position. The actuator may include a first piston and a second piston movably disposed within a bore of the first piston. The first piston may define a first piston area for proving the initial thrust force, and the second piston may define a second piston area (that is less than the first piston area) for providing the subsequent thrust force. Movement of the first piston is limited relative to the second piston for moving the gate valve using the initial thrust force an initial portion of the valve stroke, while moving the gave valve using the subsequent thrust force the remaining portion of the valve stroke.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/509,024, filed Jul. 18, 2011, the content of which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the invention generally relate to a valve actuator. More particularly, embodiments of the invention relate to a dual piston actuator for actuating gate valves. 
         [0004]    2. Description of the Related Art 
         [0005]    Various designs of valve actuators exist that operate to open and close valves in a variety of uses. The petroleum industry utilizes these actuators to operate gate valves that incorporate a sliding gate within a valve body to selectively block fluid flow through piping or tubing. Positioning gate valves along the piping or tubing at various locations controls and directs the flow of fluids therethrough. 
         [0006]    A general operation of a gate valve includes moving a valve stem extending from the valve body to moves a gate axially within the valve body between an open position and a closed position. When the gate is in the open position, fluid may flow unobstructed through the valve body. When the gate is in the closed position, the gate blocks fluid flow through the valve body. Typically, an actuator piston is used to impart the axial movement to the valve stem to move the gate to the open position. The actuator also includes a spring to continuously bias the valve stem into the closed position. Thus, force applied to the actuator piston from either a hydraulic or pneumatic source, depending on the type of actuator, must overcome the bias of the spring to move the gate to the open position. In addition to the spring bias, the actuator must be capable of developing sufficient thrust to overcome any static gate sealing, initial drag forces on the gate, and the fluid pressure within the valve body opposing movement of the gate and the valve stem. 
         [0007]    Generally, a high thrust force is required only to break any static gate sealing and overcome the initial drag on the gate, but a lesser force is required to continue movement of the gate through the remaining stroke to the open position. The excessive thrust during the remaining stroke may provide undue stress and/or fatigue on the valve stem. Conventional hydraulic or pneumatic actuators include fixed diameter actuator pistons that are adjustable in the application of thrust by the fluid pressure supplied to the actuator. Adjusting the fluid supply to the actuator during operation adds complexity to the gate valve operation. Further, depending on the size of the gate valve and/or the amount of thrust force needed, larger actuators require larger volumes of operating fluid and thus resources to move the actuator piston and the gate valve through the full stroke. 
         [0008]    Therefore, there is a need for a new and improved valve actuator operable to provide a high initial thrust force and a subsequent reduced thrust force for operating a valve. 
       SUMMARY OF THE INVENTION 
       [0009]    In one embodiment, an actuator may comprise a housing; a first piston movably disposed in the housing, wherein the first piston comprises an outer shoulder for engagement with an inner shoulder of the housing to limit movement of the first piston in a first direction; and a second piston movably disposed in a bore of the first piston, wherein the second piston is movable in the first direction after engagement between with the outer shoulder of the first piston and the inner shoulder of the housing. 
         [0010]    In one embodiment, a method of operating an actuator may comprise supplying pressurized fluid to first chamber of an actuator housing, thereby moving a first piston and a second piston in a first direction a pre-determined distance, wherein the second piston is movably disposed within a bore of the first piston; prohibiting movement of the first piston in the first direction beyond the pre-determined distance; and supplying pressurized fluid to a second chamber of the actuator housing, thereby moving the second piston in the first direction beyond the pre-determined distance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    So that the manner in which the above recited features of the invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0012]      FIG. 1  is a sectional view of an actuator and a gate valve in a closed position according to one embodiment. 
           [0013]      FIG. 2  is a sectional view of the actuator and the gate valve in a partially open position according to one embodiment. 
           [0014]      FIG. 3  is a sectional view of the actuator and the gate valve in an open position according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a sectional view of an actuator  100  coupled to a gate valve  200 . The gate valve  200  is illustrated in a closed position, such that fluid flow through the gate valve  200  is blocked. Although described herein with respect to the operation of the gate valve  200 , the actuator  100  may be used with other types of valves and/or actuator-powered systems. 
         [0016]    The actuator  100  may be coupled to a bonnet assembly of the gate valve  200 , the bonnet assembly comprising a bonnet housing  210 , a bonnet cap  215 , and a seal/bearing/wiper sleeve  217 . The bonnet cap  215  may be threadedly coupled to the outer diameter, upper end potion of the bonnet housing  210 . One or more set screws may be disposed through the bonnet cap  215  and into engagement with the bonnet housing  210  to prevent uncoupling. The sleeve  217  may be enclosed within an upper bore of the bonnet housing  210  by the bonnet cap  215 . One or more seals, such as o-rings, may be provided between the sleeve  217  and the inner diameter of the bonnet housing  210 . 
         [0017]    A valve stem  230  is movably disposed through the bonnet assembly and includes a shoulder  235  for engagement with an inner shoulder of the bonnet housing  210  to prevent removal of the valve stem  230 , to provide a sealing interface, and/or to stop the travel of the valve gate  240  when in the closed position. The bonnet assembly is coupled to a valve body  220 , such as by a thread, a weld, and/or a bolt/screw connection. A valve gate  240  is coupled to the valve stem  230  for axial movement within the valve body  220  to open and close fluid flow through the valve body  220  via an opening  245  of the valve gate  240 . 
         [0018]    A housing of the actuator  100  comprises a bottom plate  115 , a spring housing  110 , a piston housing  120 , and a top cap  130 . The bottom plate  115  may be coupled to the bonnet cap  215  by a thread, a weld, and/or a bolt/screw connection. The actuator  100  housing components may similarly be coupled to each other. The bottom plate  115  may be connected to the bottom end of the spring housing  110 , the piston housing  120  may be connected to the upper end of the spring housing  110 , and the top cap  130  may be connected to the upper end of the piston housing  120 . In one embodiment, the actuator  100  housing components may be threadedly coupled together with set screws engagements to prevent uncoupling. In one embodiment, one or more of the actuator  100  housing components may be formed integrally and/or from multiple pieces. 
         [0019]    Within the spring housing  110  is a spring  117  (other similar biasing members may be used) disposed between an upper retaining plate  118  and a lower retaining plate  119 . The lower retaining plate  119  may be coupled to the bottom plate  115  and disposed around the bonnet cap  115 . The upper retaining plate  118  is coupled to a retaining nut  125 , which may be threadedly connected to the upper end of the valve stem  230 . An inner shoulder of the upper retaining plate  118  may be biased into engagement with an outer shoulder of the retaining nut  125  by the spring  117  to secure the components together. The upper retaining plate  118  may be rotatable relative to the retaining nut  125  to compensate for any transverse forces provided by the spring  117  during compression as described herein. The spring  117  also biases the valve stem  230  (via the upper retaining plate  118  and the retaining nut  125 ) in an upward direction and/or away from the valve body  220  until the shoulder  235  of the valve stem  230  engages the bonnet housing  210 . The valve gate  240  is thereby moved into the closed position as illustrated in  FIG. 1 . 
         [0020]    Within the piston housing  120  is a first piston  140 , a second piston  150 , and a plate member  145 . The second piston  150  may be concentrically disposed within and movable relative to the first piston  140 . The plate member  145  is coupled to the upper end of the first piston  140  (such as by a threaded connection) and includes a bore through which a top shaft  135  is disposed for connection to the upper end of the second piston  150 . The remaining portion of the top shaft  135  is disposed through the bore of the top cap  130  and out of the upper end of the actuator  100  housing, visible by an operator or other users. The top cap  135  includes an inlet  131  and an outlet  132  for supplying and/or returning pressurized fluid to and from the actuator  100  to actuate the first and second pistons  140 ,  150 . One or more seals, such as o-rings, may be provided between the outer diameter of the first piston  140  and the inner diameter of the piston housing  120 . One or more seals, such as o-rings, may also be provided between the outer diameter of the second piston  150  and the inner diameter of the first piston  140 . One or more seals, such as o-rings, may further be provided between the top cap  130  and the piston housing  120  interfaces, as well as the top cap  130  and the top shaft  135  interfaces. 
         [0021]    To actuate the gate valve  200  into the open position, pressurized fluid may be supplied through inlet  131  to a first (sealed) chamber  160  formed by the top cap  130 , the piston housing  120 , the first piston  140 , and the plate member  145 . Pressurized fluid in the first chamber  160  acts on a first piston area defined by the upper surfaces of the first piston  140  and the plate member  145  to overcome the force of the spring  117  and any other forces such as static gate sealing forces, initial drag forces, and/or pressure forces within the valve body  220  acting against the valve stem  230  and/or the gate  240 . The pressurized fluid acting on the first piston area provides an initial thrust force that moves the first piston  140  and the plate member  145  into contact with an upper surface of the second piston  150  (if not already contacting). The initial thrust force moves the first piston  140 , the plate member  145 , and the second piston  150  relative to the actuator  100  housing and into engagement with the retaining nut  125 . A guide member  127  may be provided for directing the second piston  150  into contact with the upper end of the retaining nut  125 . Upon contact, the initial thrust force continues to move the second piston  150  and the retaining nut  125  against at least the spring  117  force, to thereby move the valve stem  230  and the valve gate  240  axially within the valve body  220 . 
         [0022]    The initial thrust force may be required to initiate movement of the valve gate  240  and overcome the combination of the spring  117  force, static gate sealing forces, initial drag forces, and/or pressure forces within the valve body  220  acting against the valve stem  230  and/or the valve gate  240 . However, after the initial stroke of the valve gate  240 , the subsequent thrust force required to move the gate valve  240  through the complete valve stroke to the open position may only need to overcome at least the force of the spring  117 , and possibly pressure forces within the valve body  220 . The actuator  100  is therefore operable to provide a greater, initial thrust force for a pre-determined distance and/or amount of valve stroke, and a lesser, subsequent thrust force for another pre-determined distance and/or the remainder of the valve stroke. 
         [0023]    As illustrated in  FIG. 2 , the initial thrust force (provided by the pressurized fluid in the first chamber  160  acting on the first piston area defined by the upper surfaces of the first piston  140  and the plate member  145 ) moves the first and second pistons  140 ,  150  together in a first, downward direction and/or toward the valve body  220 . The first and second pistons  140 ,  150  move together until an outer shoulder  141  of the first piston  140  contacts an inner shoulder  121  (or other stop-type member) of the piston housing  120 . The shoulder engagement prohibits further movement of the first piston  140  and the plate member  145  in the first direction beyond the pre-determined distance and/or initial stroke. The second piston  150 , however, may continue to move in the first direction beyond the pre-determined distance as described with respect to  FIG. 3 . 
         [0024]    As illustrated in  FIG. 3 , pressurized fluid supplied to the first chamber  160  is also communicated to a second (sealed) chamber  165  via one or more ports  157  disposed through the plate member  145 . The second chamber  165  is formed by the first piston  140  bore, the plate member  145 , and the second piston  150 . Pressurized fluid in the second chamber  165  acts on a second piston area defined by the upper surface of the second piston  150  and/or any corresponding planar portion of the top shaft  135 , which provides a subsequent, lesser thrust force to move the valve stem  230  and the valve gate  240  through the remaining stroke to the open position. The second piston  150  moves the retaining nut  125  (thereby further compressing the spring  117  via the upper retaining plate  118 ), the valve stem  230 , and the valve gate  240  through the remaining valve stroke until the valve  200  is in the open position (e.g. when the opening  245  of the valve gate  240  is in alignment with a bore of the valve body  220  to allow unobstructed fluid flow through the valve body  220 ) and/or when the bottom end of the retaining nut  125  engages the upper end of the top cap  215 . The top shaft  135  is also moved through the top cap  130  and the plate member  145  by its connection to the second piston  150 . As the gate valve  200  is actuated to the open position, the portion of the top shaft  135  that extends from the upper end of the actuator  100  housing is reduced and moves into the housing to provide a visual indication to an operator or other user of the operational position of the actuator  100  and/or the gate valve  200 . The top shaft  135  may provide an indication that the actuator  100  and/or gate valve  200  are in the closed position when the top shaft  135  is fully extended from the actuator  100  housing. The top shaft  135  may provide an indication that the actuator  100  and/or gate valve  200  are in the open position when the top shaft  135  is almost fully inserted into actuator  100  housing. 
         [0025]    In one embodiment, the first piston area defined by the upper surfaces of the first piston  140  and/or the plate member  145  is greater than the second piston area defined by the upper surfaces of the second piston  150  and/or any corresponding planar portion of the top shaft  135 . In one embodiment, the one or more components defining the first piston area may be integral with each other to provide a single continuous first piston area surface. In one embodiment, the one or more components defining the second piston area may be integral with each other to provide a single continuous second piston area surface. In one embodiment, the initial thrust force provided by pressurized fluid in the first chamber  160  acting on the first piston area is greater than the subsequent thrust force provided by pressurized fluid in the second chamber  165  acting on the second piston area. The first and/or second piston areas may be dimensioned to provide the necessary thrust forces during the initial and subsequent strokes of the actuator  100  and/or gate valve  200 . 
         [0026]    In one embodiment, the volume of fluid required to actuate the gate valve  200  using the actuator  100  is significantly reduced by the reduction in volume from the transition of the first chamber  160  to the second chamber  165 , as compared to using the first piston  240  only to stroke the gate valve  200  the entire distance from the closed position to the open position. In one example, the swept volume, e.g. the volume of fluid required to actuate a similar size but single-piston actuator through full stroke, may be reduced up to about 50 percent or more using the actuator  100 . In one example, a conventional single piston actuator for operating a 7-8 inch, 10-15 kpsi valve may have a swept volume of about 4-3.5 liters, whereas the actuator  100  may be configured with a swept volume of less than about 2-2.5 liters for operating the same valve. The sizes and dimensions (e.g. inner and/or outer diameter, length, wall thickness, etc.) of the actuator  100  housing, the first piston  140 , the second piston  150 , and/or the plate member  145  may be configured to provide the thrust forces and/or swept volumes for operating various types and sizes of valves. 
         [0027]    To move the gate valve  200  to the closed position, the pressurized fluid in the chambers  160 ,  165  may be removed through the outlet  132  until at least the spring  117  force is sufficient to overcome any thrust force provided by the first and/or second pistons  140 ,  150 . The spring  117  force moves the valve stem  230  (via the upper retaining plate  118  and the retaining nut  125 ) and the valve gate  240  in an opposite, upward direction and/or toward the top cap  130 . The spring  117  force may also move the first and/or second pistons  140 ,  150  (via the retaining nut  125 ) in the opposite, upward direction and/or toward the top cap  130 , thereby collapsing the chambers  160 ,  165 . The shoulder  235  on the valve stem  230  may engage the inner shoulder on the bonnet housing  210  to limit or stop the movement by the spring  117  force. At this point, the gate valve  240  may be in the closed position as illustrated in  FIG. 1 . The operation of the actuator  100  and/or the gate valve  200  may be repeated as desired. 
         [0028]    While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.