Abstract:
The present invention relates to a mechanism for preventing damage to the actuator of a hydraulic choke valve in the event of excessive backpressure on the outlet of the choke. The backpressure relief device is applicable to a choke valve which meters flow from a high pressure annular region around the axially reciprocable choke throttling member into a low pressure exit channel coaxial with the throttling member. In particular, a shearable means is used to connect the reciprocable control shank of the actuator to the throttling valve member of the choke valve. In the event of excessive pressure on the exit channel of the choke valve, the shearable means will shear, thereby fully opening the valve and permitting the excessive pressure to escape. The shearable means is readily replaced so that the valve easily can be put back into operating condition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to pending U.S. Patent Application Ser. No. 60/508,182 (Attorney Docket Number PC-P005V, filed Oct. 2, 2003 by Robert Schmidt, et al. and entitled “Shear Mechanism for Backpressure Relief in a Choke Valve.” 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an apparatus and method for providing relief to a hydraulic choke from exposure to excessive backpressure. More particularly, the present invention relates to pressure reducing valves and valve openings having shearable restraints.  
         [0004]     2. Description of the Related Art  
         [0005]     The present invention is applicable to hydraulic choke valves, which are a subclass of pressure reducing valves. Choke type pressure reducing valves generally function by causing a portion of the potential energy of a pressurized fluid to be dissipated through turbulence when the pressurized fluid is passed through a restrictive orifice. Typically, the orifice of a choke valve is selectably variable through reciprocation of its valving member toward and away from the valve seat, so that a desired combination of flow and exit pressure may be obtained.  
         [0006]     A choke valve is normally open and is designed for one-way flow. This construction differs from that of a relief valve, which is another type of one-way flow valve that is normally closed. The present invention is applicable to a choke valve that differs in construction from the most common arrangement of such valves in having its flow gate pressure-balanced. Because of the pressure balancing of the sealing plug, the actuating loads on the stem of the flow gate are considerably reduced compared to those of most choke valves of comparable capacity. However, the stem of the flow gate, which connects to the reciprocable control screw of the actuator used to reciprocate the flow gate for this type of choke, is not pressure balanced. The present invention has as its purpose the release of excessive backpressure induced axial loads on the flow gate stem in order to avoid overload of the stem or the actuator that is attached to the stem.  
         [0007]     The concept of a shearable release as a means of opening a relief valve on a one-time basis prior to rebuilding of the valve is disclosed in Allen U.S. Pat. No. 2,304,491, where a common nail is used as a shear pin. The sealing plug of the Allen valve is directly restrained against reciprocably unseating by the nail.  
         [0008]     West U.S. Pat. No. 4,587,987 discloses a relief valve very similar to that of Allen, but with an indirectly acting shear mechanism. West uses a four-bar linkage mechanism that has a shearable link interconnecting two of its arms. The shaft of the West sealing plug is restrained by abutting one bar of the linkage, where one end of that bar is held by the shearable link. When the link shears due to excessive reaction loads to the forces applied to the supporting bar by the valve plug, the plug unseats to release the pressure.  
         [0009]     Risinger U.S. Pat. No. 4,359,094 discloses a relief valve that has multiple directly acting releases for its sealing valving members, wherein the valving members are held by shear screws or shear pins. This particular construction is made to be inserted into a well bore to control a flow bypass.  
         [0010]     While the references above do show shear release means for opening relief valves that are closed, there is a need for a shear release means for pressure reducing valves that are either open or closed.  
       SUMMARY OF THE INVENTION  
       [0011]     The invention contemplates a simple, easy means to release excessive backpressure acting on the throttling and sealing member of a pressure reducing valve before overloading either the stem of the valve or its actuator. The shearable means of the present invention is applicable to pressure reducing valves that operate through reciprocation of their valving members and is useable with a wide variety of valve actuator types.  
         [0012]     One aspect of the present invention is a choke valve comprising: a body having an axial through hole and a radial entry port; a valve seat coaxially housed in the body axial through hole on a first side of said radial entry port; a pressure balanced valving member axially reciprocable within the body axial through hole between a first sealing position bearing against said valve seat and a second position spaced away from said seat; actuator means for applying reciprocatory motion to the pressure balanced valving member through an intermediate structure; and a shearable mechanism interconnecting the actuator means and the valving member, wherein the shearable mechanism is responsive to fluid pressure in excess of a predetermined value.  
         [0013]     Another aspect of the present invention is a choke valve comprising: a body having an axial through hole passing from a first side of the body to a second side of the body, an outlet passageway coaxially aligned with the through hole and positioned at the first side of the body, and a radial inlet port intersecting the through hole between the first and second sides of the body; a valve seat coaxially housed in the through hole between the inlet port and the outlet passageway; a valving member axially reciprocable within the through hole between a first position bearing against the valve seat and a second open position spaced away from the valve seat; an actuator attached to the second side of the body for reciprocably moving the valving member between the first position and the second position; and a shearable mechanism having a stem interconnecting the actuator and the valving member; whereby whenever the shearable mechanism is subjected to fluid pressure in excess of a predetermined value from the outlet passageway the shearable mechanism will shear and the stem and the valving member will move away from the valve seat.  
         [0014]     Yet another aspect of the present invention is a choke valve comprising: a body having an axial through hole passing from a first side of the body to a second side of the body, an outlet passageway coaxially aligned with the through hole and positioned at the first side of the body, and a radial inlet port intersecting the through hole between the first and second sides of the body; a valve seat coaxially housed in the through hole between the inlet port and the outlet passageway; a pressure balanced valving member axially reciprocable within the through hole between a first position bearing against the valve seat and a second open position spaced away from the valve seat; an actuator attached to the second side of the body, the actuator having an axially reciprocating actuator shank for reciprocably moving the valving member between the first position and the second position; and a shearable mechanism interconnecting the actuator shank and the valving member, the shearable mechanism comprising a stem, a first end of the stem positioned in a socket in one end of the actuator shank and a second end of the stem mounted to the valving member, and a shear pin passing through the stem proximal to first end of the stem and passing through a wall of the socket in the actuator shank when the first end of the stem is located in the socket at less than a full depth of the socket; whereby whenever the valving member is subjected to a fluid pressure in excess of a predetermined value from the outlet passageway the shear pin will shear and the stem will move further into the socket in the actuator shank thereby moving the valving member away from the valve seat.  
         [0015]     Still yet another aspect of the present invention is a choke valve comprising: a body having an axial through hole passing from a first side of the body to a second side of the body, an outlet passageway coaxially aligned with the through hole and positioned at the first side of the body, and a radial inlet port intersecting the through hole between the first and second sides of the body; a valve seat coaxially housed in the through hole between the inlet port and the outlet passageway; a pressure balanced valving member axially reciprocable within the through hole between a first position bearing against the valve seat and a second open position spaced away from the valve seat; an actuator attached to the second side of the body, the actuator having an axially reciprocating actuator shank for reciprocably moving the valving member between the first position and the second position; and a shearable mechanism interconnecting the actuator shank and the valving member, the shearable mechanism comprising a stem, a first end of the stem positioned in a socket in one end of the actuator shank at less than a full depth of the socket and a second end of the stem mounted to the valving member, a pair of split shear ring halves mounted in a mounting groove in the stem proximal to the first end of the stem, and a shear ring keeper device mounted over the shear ring halves to maintain the shear ring halves in position; whereby whenever the shearable mechanism is subjected to a fluid pressure in excess of a predetermined value from the outlet passageway the shear ring halves will shear and the stem will move further into the socket in the actuator shank thereby moving the valving member away from the valve seat.  
         [0016]     The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or redesigning the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1  is an oblique view of a hydraulic choke valve with its actuator, where the choke meters flow from a high pressure annular region around the axially reciprocable choke throttling member into a low pressure exit channel coaxial with the throttling member;  
         [0019]      FIG. 2  is an axial view of the choke and actuator of  FIG. 1  from the actuator side of the choke with the lid of the actuator removed;  
         [0020]      FIG. 3  is a longitudinal sectional view of the choke and actuator taken along the section line  3 - 3  of  FIG. 2 ;  
         [0021]      FIG. 4  is an enlarged view of the portion of  FIG. 3  showing the throttling member and its connection to both the reciprocable control screw of the actuator and the other components of the choke valve;  
         [0022]      FIG. 5  is an oblique exploded view of the throttling member, the operating shaft, the reciprocable control screw, and their interconnection means;  
         [0023]      FIG. 6  is a longitudinal sectional view of the throttling member assembly with the shearable overpressure protection mechanism intact;  
         [0024]      FIG. 7  is a longitudinal sectional view, corresponding to  FIG. 6 , of the throttling member assembly with the shearable overpressure protection mechanism sheared; and  
         [0025]      FIG. 8  is a longitudinal cross-sectional view of the choke valve and reciprocable actuator control screw corresponding to  FIG. 3 , but with the shearable overpressure protection mechanism sheared.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     The present invention provides a shear release mechanism for releasing excessive backpressure acting on the throttling and sealing member of a pressure reducing valve before overloading either the stem of the valve or its actuator. This shear release mechanism operates through the reciprocation of the valving member and is applicable in a wide variety of valve actuator types.  
         [0027]     Referring now to the drawings, and initially to  FIG. 1 , it is pointed out that like reference characters designate like or similar parts throughout the drawings. The Figures, or drawings, are not intended to be to scale. For example, purely for the sake of greater clarity in the drawings, wall thickness and spacing are not dimensioned as they actually exist in the assembled embodiment.  
         [0028]      FIG. 1  shows a partial longitudinal section of one embodiment of a hydraulic choke valve of the present invention. Although the materials of construction of the choke valve and its actuator may vary, typically they are constructed of a high strength low alloy steel, mild steel, or, in the case of O-rings and other elastomeric seals, Viton™ or nitrile rubber.  
         [0029]     FIGS.  1  to  3  illustrate an assembled hydraulic choke valve system  10  consisting of a hydraulic choke valve  11  and an electrically or manually powered actuator  100 . Herein, the term “hydraulic choke” is taken to refer to the fact that the device is used with a variety of fluids, such as drilling mud, salt water, oil, gas, and other chemicals which may be injected into a well. “Hydraulic” does not herein refer to the choke actuation means.  
         [0030]     The body  12  of the hydraulic choke valve  11  is a heavy walled steel right rectangular prism with an axial passage  25  extending completely through body  12 . The body  12  has a centrally positioned heavy walled projecting central cylindrical neck outlet branch  13  containing coaxially positioned axial passage  25  extending downwardly through neck  13 . The flow entry for the choke is inlet port  19 , and the flow exit is the righthand end of axial passage  25 , as shown in  FIG. 3 .  
         [0031]     Concentric transverse outlet flange  14  is positioned at the outer end of the outlet branch  13 . Body  12  also has a cylindrical actuator mounting neck  17  extending outwardly concentrically with the outlet neck  13  and the axial passage  25 , but on the opposed side of the body from the outlet neck. The outer end of actuator mounting neck  17  has concentric external male retention threads  18  by which most of the internal components of the valve may be retained.  
         [0032]     Both the inlet and outlet flow passages  19  and  25 , respectively, are provided with concentric mounting grooves  21  and  16  for metal ring gasket seals (not shown) and concentric hole circles  20  and  15  for the mounting of the choke to connecting piping by means of threaded studs and nuts (not shown). The outlet flow passage has a terminal flange. The bolted and ring gasketed inlet and outlet connections are mateable with standard American Petroleum Institute (API) flange connections typically used for high pressures in the oilfield.  
         [0033]      FIGS. 3 and 4  illustrate the internal arrangements of the choke  11 . Inlet flow passage  19  into body  12  is radial to the axis of the through axial passage  25  which extends from the actuator end to the outlet end of the body. Axial passage  25  has multiple coaxial bores along its length from the actuator end to the outlet end, with a coaxial enlarged counterbored annular, approximately cylindrical inlet distribution chamber  24  centrally located in body  12 . Entry chamber  25  is radially intersected by inlet flow passage  19 . The actuator end of axial passage has an outwardly facing transverse guide stop shoulder  26 . On the outlet side of the axial passage  25  just below inlet distribution chamber  24  is a transverse seat stop shoulder  27 . The bore of the outlet side of the axial passage  25  is reduced below the seat stop shoulder  27 , but is again enlarged adjacent the outlet end at transverse liner stop shoulder  28 .  
         [0034]     Mounted in the large bore of passage  25  of the choke body  12  and abutting seat stop shoulder  27  on the outlet end of distribution chamber  24  is right circular annular cylindrical choke seat  32 . The seat  32  is provided with a pair of external male O-rings  33 . Outlet liner  34  is a thin-walled right circular cylindrical tube having a short exterior upset portion at its outlet end joined to the rest of the liner tube by an external transverse shoulder. The outlet liner  34  is inserted with a close slip fit into the outlet bore of the axial passage  25  to have its inner end abut against the outer end of seat  32  and its intermediate transverse shoulder abut against shoulder  28 .  
         [0035]     The flow passage from the cavity  25  to the choke outlet is restricted by choke gate  40 . Choke gate  40  has an approximately right circular cylindrical shape with an axial through hole  41  having a counterbored enlargement on each end of the through hole. The end portions of the choke gate exterior cylindrical surface have short reduced diameter sections with conical transitions  42  to the central outer cylinder. The reduced diameter exterior end sections are a slip fit to the bore of seat  32 . The conical transitions  42  serve as sealing faces for the choke gate  40  and are able to seal against the interior portion of the adjacent transverse upper end of the seat  32 . Choke gate  40  is symmetrical about its transverse horizontal midplane, so that it may be inverted and a new sealing face  42  used when the first becomes leaky. Choke gate  40  has one or more internal flow passages  43  parallel to but offset from the longitudinal axis and connecting from one side to the other in order that it will not fluid lock and will be exposed to balanced opening forces when it is fully or nearly closed.  
         [0036]      FIGS. 4 through 6  illustrate the components of the shearable actuator subassembly  44 . The subassembly shown in  FIG. 5  consists of a stem  45 , a shear pin  49 , two split shear ring halves  52 , a shear ring keeper nut  76 , an axially reciprocable actuator screw  130 , and set screws  78 . In actual practice, either the shear pin  49  or the split shear ring halves are sufficient to provide a shear mechanism for backpressure relief in the choke valve, although both the shear pin and the split shear ring halves are illustrated in FIGS.  4  to  6 . The shearable actuator subassembly  44  using only one of the shearable components (i.e., either the shear pin or the split shear ring) can readily be made from the drawings and description herein by one skilled in the art.  
         [0037]     Choke gate  40  is located on stem  45 , which is piloted into the upper counterbored pocket of the gate through hole  41 , and attached thereto by means of Allen screw  50  that extends through axial through hole  41  in the choke gate.  
         [0038]     Stem  45  is a stepped cylindrical rod extending upwardly towards linear actuator  100 . The enlarged choke gate end of stem  45  is centrally drilled and tapped to threadedly engage with screw  50  for retaining the stem  45  and the choke gate  40  together. The shank  46  of stem  45  on its upper actuator end has an annular male groove  47  of rectangular cross-section and, adjacent the transverse end of the stem, a diametrical through hole  48  for mounting a closely fitting cylindrical shear pin  49 . The shear pin  49 , as illustrate in  FIGS. 4 and 6 , is not only used as part of a shearable mechanism in the shearable actuator subassembly  44 , but the shear pin also serves as an antirotational device to keep the stem  45  and the choke gate  40  from rotating.  
         [0039]     The portions of the choke exposed to high velocity flow (such as the gate  40 , the seat  32 , the outlet liner  34 , and possibly the gate guide  55 ) will typically be constructed of sintered tungsten carbide, a ceramic material, or will be hardfaced with a suitable wear resistant material, such as Stellite 3™.  
         [0040]     The two split shear ring halves  52  are each 180° or slightly less than 180° segments of a right circular cylindrical ring which closely fits into annular male shear ring mounting groove  47  of stem  45 . The split ring halves  52  extend radially outwardly by approximately the depth of their mounting groove  47 . The shear ring is in halves so that it can be installed and removed readily from groove  47 .  
         [0041]     Gate guide  55 , also referred to as an operator nose, is a thin walled cylindrical tubular structure with a short enlarged cylindrical upper actuator end joined to the main body and having a downwardly facing intermediate exterior transverse shoulder at that transition in outer diameters. The lower exterior end of the gate guide  55  is also slightly reduced in diameter adjacent the inlet distribution chamber  24  of the choke body  12 . The central external cylindrical section of gate guide  55  has multiple male O-ring grooves on its exterior at approximately mid length containing O-rings  60  which seal between the exterior of guide  55  and the upper bore of axial passage  25  of the choke body  12 . The main bore through gate guide  55  is slightly enlarged at approximately midlength. The lower interior cylindrical face of gate guide  55  has multiple female O-ring grooves containing O-rings  61  that seal between the gate guide and the exterior cylindrical surface of the choke gate  40 . At approximately one fourth of the length of gate guide  55  down from its upper actuator end, a thick interior transverse bulkhead  56  with a coaxial through hole  57  mounts multiple chevron seals  63  in a counterbore on the upper side of the through hole. The chevron seals  63  are oriented to prevent the escape of internal pressure in the annular gap between gate guide  55  and stem  45 . Stem  45  is journaled with a close fit in the central through hole  57  of guide  55 . Near the upper end of gate guide  55  are located a pair of diametrically opposed drilled and tapped holes which contain inwardly projecting half dog set screws  64 .  
         [0042]     The rear section  66  of the operator nose is a heavy walled cylindrical tube with a thick transverse diaphragm having a through hole on its lower end. The inner diameter of the operator nose&#39;s rear section through hole provides a close slip fit to the stem  45 . At the lower end of the rear section  66 , the through hole has a short cylindrical counterbore  68 . The outer diameter of rear section  66  is reduced at a transverse shoulder near its lower end so that the lower end of the rear section  66  can enter the upper section of the bore of the gate guide  55  and the external shoulder abut the upper transverse end of the gate guide. Diametrically opposed detent holes  69  are match drilled through the drilled and tapped set screw holes of the gate guide  55  at assembly of the gate guide and the rear section  66  so that set screws  64  can retain the pieces in their desired abutted position. The upper transverse end of the rear section  66  projects slightly above the upper end of neck  17  of choke body  12 . The upper interior bore of the rear section  66  has female thread  67  for connection with the actuator mounting hub  104  of actuator  100 . A radial set screw hole  70  penetrates the wall of the rear section  66  at approximately midlength so that the threaded connection of the rear section and the actuator mounting hub  104  can be locked with set screw  71 .  
         [0043]     A short annular right circular cylindrical ring with a reduced outer diameter tip on its downward side serves as a seal contactor  72  for the seals  63 . The reduced outer diameter of seal contactor  72  is a close slip fit to the seal housing counterbore of gate guide  55 . The bore of seal contactor  72  is a close fit to the stem  45 , and the transverse lower tip of the seal contactor bears on the heel of the uppermost of the stack of seals  63 . Multiple through vent holes offset from and parallel to the axis of the seal contactor  72  aid in the avoidance of fluid lock in the seal cavity.  
         [0044]     Shear ring keeper nut  76  is a thick walled right circular cylindrical annular ring with a transverse diaphragm  77  having a coaxial through hole at its lower end. The through hole has a slip fit with the shank  46  of stem  45 . The upper bore of shear ring keeper nut  76  is threaded for connecting with the reciprocable actuator screw  130  of the actuator  100 . Adjacent its upper end, shear ring keeper nut  76  has a pair of diametrically opposed drilled and tapped holes which mount set screws  78  for locking the threaded connection of nut  76  to the screw  130 . Closer to the diaphragm  77  at the lower end of nut  76 , a pair of diametrically opposed holes is aligned with the axis of shear pin hole  48  of stem  45  when the shearable actuator assembly is made up. This pair of holes provides access to the shear pin  49 .  
         [0045]     Hollow keeper nut  97  is threadedly attached to the male threads  18  of the externally threaded actuator mounting neck  17  of the choke body  12  and serves to retain the internal components of the choke, which include the choke gate  40  and the stem  45 , gate guide  55 , rear section  66 , seal contactor  72 , and seals  63 . The keeper nut  97  has a heavy walled right circular cylindrical annular body open at its lower end and with a female thread  99  threadedly comated to the male thread  18  on the upper mounting neck  17  of choke body  12  on its interior. At the upper end of keeper nut  97  is a transverse diaphragm  98  with a central through hole which provides a shoulder for engaging the upper transverse end of the rear section  66 .  
         [0046]     The actuator  100  is not described in detail, since such actuators are in very broad use and are well known to those skilled in the art. Only a general description of one type of actuator is given here. The actuator  100  may be manual or either electrically, hydraulically, or pneumatically operated. In most cases, the actuator  100  will be powered and also provided with a separate manual override, as is shown in  FIGS. 1 and 2 . Referring to  FIG. 1 , the actuator box  101  of the actuator is a rectangular prismatic hollow box with a removable lid and exterior mounting bosses to which the actuator drive  102  and the actuator mounting hub  104  are mounted by screws. Mounting hub  104  is a transverse circular flange having a bolt hole circle for connection to the box  101  and with a coaxial right circular cylindrical neck extending downwardly and a coaxial through hole. Male mounting hub neck threads  105  are on the lower exterior end of the neck of mounting hub  104  and serve to attach the actuator to the choke  11  by being threadedly engaged into the female threads  67  of the rear section  66 . Set screw  71  restrains that connection.  
         [0047]     The actuator drive  102  is a rotary device powered by an electric power line or hydraulic or pneumatic hoses (not shown). Coaxial with and on the opposed side of the box  101  from the actuator drive  102  is a selectably manually engagable handwheel  103  which is normally declutched, but can be used to operate the internal worm gear drive of the actuator  100  if the actuator drive malfunctions. The handwheel shaft  108  is supported in a bearing (not shown) in the external boss projecting from the actuator body  101  on the handwheel side. Internal to the body  101  is the worm gear set mount  112  in which are a mounted worm  111  and a screw drive worm gear  117  driven by the driven shaft  110  common to the drive shaft  106  and the handwheel shaft  108 . Shaft  110  is directly attached to the worm  111 . Both gears are supported on bearings in the mount  112  and prevented from shifting axially by their mountings therein. Worm gear  117  is driven on its outer periphery by worm  111 . Coaxial with the worm gear peripheral gear and its journaling ends is the interior bore of the worm gear  117 , wherein female acme drive thread  118  is located. The worm gear drive is used in order to provide a torque multiplication and speed reduction for the drive and also to resist backdriving of the actuator by thrusts on the actuator shaft. An antirotation flange  115  is mounted on top of the worm gear set mount  112  in a position coaxial with the worm gear  117 . Antirotation flange  115  consists of a transverse flange with mounting holes and a coaxial upwardly projecting right circular cylindrical tubular neck which has an internal integral rectangular section antirotation key  116  extending radially into the through bore of the antirotation flange.  
         [0048]     Axially reciprocable actuator screw  130  provides the output for the actuator. Actuator screw  130  has a male acme thread  131  in its midsection and a reduced diameter coaxial lower shank  132  extending downwardly. The region of the lower shank  132  adjacent to the acme thread  131  of the actuator screw has a male thread  133  that may be threadedly engaged with the female upwardly-looking threads of the shear ring keeper  76 . Lower shank  132  fits within the bore of the shear ring keeper  76 . The lower shank  132  has a downwardly opening coaxial bore lower shank socket  134  which is a close sliding fit to the shank  46  of stem  45 . A transverse shoulder connects the main bore of socket  134  with a short enlarged counterbore which is a close fit to the outside of the shear ring halves  52  when they are mounted in the shear ring groove  47  of the stem  45 . A diametrical shear pin hole  135  sized to accommodate shear pin  49  extends through lower shank  132  of actuator screw  130  in a position that is coaxial with the corresponding shear pin hole  48  of stem  45  when the shearable actuator subassembly  44  is assembled together. Reduced diameter cylindrical upper shank  136  is located on the upper end adjoining acme thread  131  of actuator screw thread  130 . An upwardly opening longitudinal antirotation keyway  137  is cut into the length of upper shank  136 . Keyway  137  is sized to slidingly engage the antirotation key  116  of the antirotation flange  115  so that actuator screw  130  cannot be rotated.  
       OPERATION OF THE INVENTION  
       [0049]     The choke  11  of the hydraulic choke valve assembly  10  shown in the present invention is operated by nonrotating linear up and down stroking of the actuator screw  130  of the actuator  100  shown herein. The choke gate  40 , supported and operated by the shearable actuator subassembly  44 , is guided in its reciprocation by the gate guide  55 . The chevron seals  63  are held in place so that they can seal to the stem  45  by the rear section  66  and the seal contactor  72 , which are themselves held in place by keeper nut  97  engaging the threads  18  of the actuator mounting neck  17 . Likewise, the gate guide  55  is held in place axially by being abutted by both the guide stop shoulder  26  and the operator nose&#39;s rear section  66 . The actuator  100  is rigidly mounted to the choke  11  by the threads  105  of the actuator mounting hub  104  engaging the female threads  67  of the rear section  66 .  
         [0050]     The antirotation flange  115  of the actuator restrains the axially reciprocable actuator screw  130  against rotation when the drive shaft  110  and its attached worm  111  rotate worm gear  117 . The worm gear  117  is itself held against axial translation by its fixity against linear motion by its mounting in the worm gear set mount  112  of the actuator  100 . Accordingly, the actuator screw  130  is caused to reciprocate by appropriate rotation of the worm gear  117 .  
         [0051]     The choke valve  11  has its choke gate  40  pressure balanced because of the communication of fluid pressure from one end of the choke gate to the other through the internal flow passages  43  of the gate. This pressure balancing of the gate permits the pressure on the stem  45  of the choke to be reduced and, accordingly, the pressure loads typically expected on the actuator through the stem will be correspondingly reduced. This is because the pressure in the outlet of the choke acts only on the cross-sectional area of the shank  46  of the stem  45 ; the pressure load on the actuator is the product of the outlet pressure and the shank area. The consequence of this is that smaller actuators can be used to control a given flow condition, when compared to the conventional unbalanced chokes.  
         [0052]     Normally, pressures in the outlet branch of the choke  11  (i.e., through the bores of the choke seat  32  and the outlet liner  34 ) are much lower than in the inlet port  19 . When the choke is in good condition, it will reliably seal when the sealing face  42  of the gate  40  is pressed against the seat  32 . Since the outlet side of the choke is typically vented, the pressure on the outlet line would thus be very low in such a case. Even when the choke is opened and exposed to a high inlet pressure, it is typically operated in a manner such that a very, high pressure drop is taken across the flow orifice opened between the gate  40  and the seat  32 , with the result that the outlet pressure still would be low. Thus, in the normal situation, the axial loads transmitted to the stem  45  and hence to the actuator  100  through the connection of the stem to the actuator screw  130  are low.  
         [0053]     In the event of a stoppage in the outlet line or some other flow upset, such as a downstream water hammer or the opening of a valve at the wrong time, very high pressures can be produced in the outlet line of the choke  11 . In such an instance, a high pressure induced axial compression load is translated to the stem  45  of the choke. This high load has the potential to damage the stem  45 , the actuator  100 , or both the stem and actuator. However, referring to  FIGS. 5 through 6 , it is apparent that the compressive reaction load path from the stem  45  to the actuator screw  130  has to pass through the split shear rings  52  and the shear pin  49 . Thus, the shear areas and shear strengths of the shear pin  49  and the split shear rings  52  are preselected to structurally fail when exposed to a predetermined, safe axial load through shear. The resultant shear failure will cause the shear pin  49  and the split ring halves  52  to separate on the cylindrical interface between the shank  46  of the stem  45  and the lower shank socket  134  of the actuator screw  130 , as shown in  FIG. 7 . The shear pin will separate into residual segments  49   a  and  49   b , while the split shear rings will separate into segments  52   a  and  52   b , as seen in  FIG. 7 . After the failure of these weak link members, the choke gate  40  and the attached stem  45  both will shift upwardly toward the actuator  100  by telescoping of the stem into the lower shank socket  134  under the action of the pressure force on the stem. When this has happened, the orifice between the choke gate  40  and the seat  32  will be fully opened, as shown in  FIG. 8 , thereby permitting the outlet side pressure to vent upstream if the pressure comes from downstream. The venting in such a case, due to careful selection of the failure properties of the shearing members  49  and  52 , should prevent excessive loads from occurring to either the actuator or the stem. If the pressure on the inlet side of the choke  11  is already high when the backpressure becomes elevated, the stem  45  is still protected by virtue of its effective unsupported length being shortened due to telescoping into the actuator screw  130 . The buckling tendencies of an axially compressed member generally are much reduced when its unsupported length is reduced. The actuator  100  and the upper group of choke internal parts  40  through  97  readily can be removed from the choke body  12  by disconnecting keeper nut  97 . This permits easy access to the choke internal parts so that the shear pin  49  and the split shear rings  52  simply can be replaced and the choke reassembled after the backpressure condition is eliminated.  
         [0054]     The present invention permits the use of a smaller, less expensive actuator for a choke while at the same time greatly reducing the likelihood of failure of the choke stem or the actuator due to an incident of high backpressure on the choke outlet line. Two separate modes of equipment risk reduction result from the use of a shearable, telescoping link between the actuator and the stem supporting the pressure balanced choke gate. The first is the reduction of the unsupported length of the stem, whereby its tendency to buckle and overstress is greatly reduced. The second advantage is the venting of pressure upstream so that the high backpressure is released prior to damaging the choke assembly. This second advantage requires that the upstream pressure be relatively low when the high backpressure occurs. This and other advantages will be readily apparent to those familiar with the art.  
         [0055]     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.