BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to valves. More specifically, the subject matter disclosed herein relates to control of butterfly valve positioning. 
         [0002]    A typical butterfly valve is controlled by a pneumatic actuator. The actuator includes a piston positioned in a cylinder. The piston is connected to a linkage that converts linear motion of the piston in the cylinder to rotary motion of the linkage. The linkage is, in turn connected to a butterfly disc located in a duct, or other flow structure. Rotation of the linkage rotates the butterfly disc in the duct, thus opening or closing the valve. 
         [0003]    In a typical cylinder of a butterfly valve, the cylinder has an upstream cavity connected to a pneumatic supply and a downstream cavity also connected to the pneumatic supply. A controller directs the pneumatic supply to increase or decrease the pressure in the downstream cavity to drive motion of the piston in the cylinder to either open or close the valve, as the upstream cavity is maintained at a constant pressure. The typical arrangement also includes an intermediate cavity located between a downstream piston face and an upstream piston face, which is vented directly to ambient. The controller attempts to modulate the piston position by controlling the pressure in the downstream cavity. As the piston approaches a selected position, the controller adjusts the downstream cavity pressure to balance the upstream cavity pressure, thus stopping the piston from further movement. Unfortunately, balancing the two cavity pressures at the exact time the piston is in the selected position has proven to be difficult. The piston often overshoots the selected position, thus resulting in valve instability. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    In one embodiment, a butterfly valve includes a duct defining a flow pathway and a valve disc rotably disposed in the flow pathway. A pneumatic actuator includes an intermediate chamber housing an actuator piston and is operably connected to the valve disc to drive rotation of the valve disc. A downstream bleed conduit extends from the flow pathway downstream of the valve disc to the intermediate chamber to pressurize the intermediate chamber thereby reducing a rate of movement of the valve disc in the flow pathway. 
         [0005]    In another embodiment, a method of operating a butterfly valve includes pressurizing a pneumatic actuator by flowing air from a pneumatic source and translating a piston of the pneumatic actuator via the pressurization. A valve disc operably connected to the piston and located in a flow pathway is rotated by the translation of the piston. A flow of air is bled from the flow pathway downstream of the valve disc to an intermediate chamber of the pneumatic actuator, and the intermediate chamber is pressurized via the flow of air from the flow pathway to reduce a rate of movement of the valve disc in the flow pathway. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawing in which: 
           [0008]      FIG. 1  is a schematic of an embodiment of a pneumatic butterfly valve system; and 
           [0009]      FIG. 2  is a schematic of a vent portion of an embodiment of a pneumatic butterfly valve system. 
       
    
    
       [0010]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Shown in  FIG. 1  is an embodiment of a pneumatically-operated butterfly valve  10 , such as those utilized to regulate bleed airflow from, for example, a compressor  12 . A bleed duct  14  extends from the compressor  12  to a valve duct  66 , and a valve disc  16  is rotably positioned in the valve duct  66  such that rotation of the valve disc  16  decreases or increases flow allowable through the valve duct  66  and the bleed duct  14 . The valve disc  16  is connected to a pneumatic actuator  18 , which drives rotation of the valve disc  16 . The actuator  18  includes a cylinder  20  with a piston  22  slidably positioned in the cylinder  20 . The piston  22  is connected to the valve disc  16  via one or more linkages  24  to translate sliding motion of the piston  22  in the cylinder  20  into rotary motion of the valve disc  16 . 
         [0012]    Movement of the piston  22  in the cylinder  20  is determined by a difference in pressure between an upstream chamber  26  of upstream cylinder  60  and a downstream chamber  28  of downstream cylinder  62 . The upstream chamber  26  is defined by an upstream face  30  of the piston  22  and an inner wall  32  of the upstream cylinder  60 , while the downstream chamber  28  is defined by a downstream face  34  of the piston  22  and an inner wall  33  of the downstream cylinder  62 . Downstream cylinder  62  has a different diameter, in this embodiment, a larger diameter, than upstream cylinder  60 . The pressure in the upstream chamber  26  is maintained as a substantially constant pressure. To do so, an upstream tap  36  extends from the valve duct  66  into the upstream chamber  26  to supply an upstream airflow  38 , as needed, to the upstream chamber  26 . Upstream airflow  38  is also routed to a controller  40  via return conduit  42 . In some embodiments, the return conduit  42  includes a filter  44  to filter the upstream airflow  38  so that the controller  40  is not contaminated. In some embodiments, the controller  40  is an electronically-controlled pneumatic device. The pressure in the downstream chamber  28  is controlled via a downstream pneumatic inlet  46  connected to the controller  40 . Increasing an inlet flow  48  via the downstream pneumatic inlet  46  increases the pressure in the downstream chamber  28 . Pressure balance is achieved by maintaining a ratio of an upstream pressure in the upstream chamber  26  and a downstream pressure in the downstream chamber  28  equal to a ratio of the areas of the upstream cylinder  60  and the downstream cylinder  62 . The pressures in the upstream chamber  26  and the downstream chamber  28  when pressure balance is achieved may be referred to as “upstream balance pressure” and “downstream balance pressure”, respectively. When the pressure in the downstream chamber  28  is greater than the downstream balance pressure, the piston  22  is urged toward the return conduit  42 . The movement of the piston  22  moves the linkage  24  to rotate the valve disc  16  toward a fully opened position. Decreasing the pressure in the downstream chamber  28  such that the pressure in the upstream chamber  26  is greater than the upstream balance pressure urges the piston  22  toward the downstream pneumatic inlet  46 , which results in the movement of the valve disc  16  toward a fully closed position. 
         [0013]    The valve  10  disclosed herein includes a dampening mechanism to control the motion of the valve disc  16  between the fully opened position and the fully closed position to slow or stop the movement of the valve disc  16 . The piston  22  includes an intermediate chamber  50  located between the upstream face  30  and the downstream face  34  of the piston  22 . A downstream bleed conduit  52  extends from the bleed duct  14  at a location downstream of the valve disc  16 , to the intermediate chamber  50 . Duct flow  54  downstream of the valve disc  16 , flows from the valve duct  66  through the downstream bleed conduit  52  to pressurize the intermediate chamber  50 . In some embodiments, as shown in  FIG. 1 , the downstream bleed conduit  52  extends away from the valve duct  66  at an angle greater than 90 degrees relative to the duct flow  54  direction, to prevent ingestion of contaminants into the downstream bleed conduit  52 . The pressurization of the intermediate chamber  50  can slow or stop movement of the piston  22  during operation of the valve  10 , thus making the valve motion more controllable and precise. For example, the valve disc  16  may be set in a half-open position or other selected position between fully opened and fully closed. 
         [0014]    Referring now to  FIG. 2 , to control the amount of pressurization of the intermediate chamber  50 , a downstream vent conduit  56  extends from the downstream bleed conduit  52  and vents to ambient through a restriction  65 . The downstream bleed conduit  52  and the downstream vent conduit  56  are sized and configured to pressurize the intermediate chamber  50  to a selected pressure, for example, between about 70% and 95% of a downstream duct pressure, which is a pressure in the valve duct  66  downstream of the valve disc  16 . This degree of pressurization effectively slows the movement of the piston  22  and valve disc  16  to a selected rate of rotation, thereby making the operation of the valve more controllable. In other embodiments, the selected pressure of the intermediate chamber  50  is about 80% of the downstream duct pressure. In some embodiments, the downstream bleed conduit  52  includes a restriction  64 , such as a narrowing, at a location along its length, for example, at the valve duct  66 . Similarly, the downstream vent conduit  56  may include a restriction  64  at the downstream bleed conduit. The restrictions are provided to prevent pressure in the intermediate chamber  50  from equaling the downstream duct pressure. While the selected pressure is within the range above in some embodiments, it is to be appreciated that in other embodiments the selected pressure may be between the downstream duct pressure and ambient pressure, as determined by the relative restrictions in the downstream bleed conduit  52  and the downstream vent conduit  56 . Further, utilization of the downstream vent conduit  56  prevents a detrimental temperature increase in the intermediate chamber  50  which would result in deterioration of piston ring seals  58  located at the piston  22  to seal between the piston  22  and cylinder  20 . 
         [0015]    In some embodiments, the upstream tap  36 , the downstream bleed conduit  52  and/or the downstream vent conduit  56  are formed integral to the valve  10  by, for example, casting. Alternatively, the upstream tap  36 , the downstream bleed conduit  52  and/or the downstream vent conduit  56  are formed separately and joined to the valve  10  by welding, brazing, adhesives, or mechanical fasteners. 
         [0016]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while the various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
A butterfly valve includes a duct defining a flow pathway and a valve disc rotably disposed in the flow pathway. A pneumatic actuator includes an intermediate chamber housing an actuator piston and is operably connected to the valve disc to drive rotation of the valve disc. A downstream bleed conduit extends from the flow pathway downstream of the valve disc to the intermediate chamber to pressurize the intermediate chamber thereby reducing a rate of movement of the valve disc in the flow pathway.