Abstract:
A backpressure-protected valve suitable for subsea well treatment chemical injection applications includes an improved gate valve, a check valve and a pressure compensator responsive to ambient hydrostatic pressure. The check valve may comprise a poppet valve in a detachable outlet body. The pressure compensator comprises a free piston in a cylinder. The portion of the cylinder on one side of the piston is exposed to seawater whereas the portion of the cylinder on the opposing side of the piston is filled with a selected hydraulic fluid. In this way the valve actuator is not exposed to the corrosive and/or contaminating effects of seawater.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   None 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   None 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to valves. More particularly, it relates to pressure-compensated valves adapted for use in the subsea environment. 
   2. Description of the Related Art 
   U.S. Pat. No. 4,456,028 discloses a relief gate valve that comprises a hollow body having an inlet and an outlet, a seat ring in the outlet having a face adapted to engage a gate, a closure mounted for movement relative to the seat ring and including a carrier having a passage therethrough in which are slidably disposed a lipped gate disc adapted to engage the seat face and block the outlet when the valve is in closed position, a pressure spring pressing the gate disc against the seat face when the valve is in closed position, and a ball transferring the reaction of the spring to the body. The closure further includes a stem extending into a port through the body, a plate engaging the tip of the stem, a helical spring engaging the plate, and a screw cap encompassing the length of the spring and engaging a threaded cuff extending from the body. The cap bears on the spring at its end opposite from that which engages the plate, the helical spring urging the valve closure to a closed position in which a shoulder on the closure engages the seat ring, providing a travel limit stop, and retaining the closure in the body. An outlet part secured to the body over the outlet retains the seat ring in the body. 
   The valve described in U.S. Pat. No. 4,456,028 is not suitable for use in the subsea environment. It requires manual adjustment for setting the opening pressure and does not compensate for the hydrostatic pressure. Moreover, it has no means for preventing backflow through the valve if, for any reason, the outlet pressure exceeds the inlet pressure. The present invention solves these problems. 
   BRIEF SUMMARY OF THE INVENTION 
   A relief gate valve includes a body having an axial passage, one end of which forms the valve inlet, and a lateral passage transecting the axial passage, the outer end of the lateral passage forming the valve outlet. A check valve is positioned within the lateral passage for preventing backflow of fluid through the valve. In one preferred embodiment, the check valve comprise a poppet valve. The inner end of the lateral passage may be provided with a replaceable valve seat. 
   A valve actuator moves axially in the axial passage parallel to the face of the valve seat. The actuator includes a carrier peripherally spaced from the walls of the axial passage and having an axial stem passing through a stem port in the body at the end of the axial passage opposite from the inlet. The stem makes a sliding fit with the port to guide the actuator as it moves axially. 
   The stem extends from the main body part into a pressure compensator which engages a threaded stub extending from the main valve body. 
   One side of a bearing plate rests on the outer end of the stem and the other side of the bearing plate may be engaged by one or more elastic members which, in one preferred embodiment, are helical compression load springs. A threaded cap retains the other end of the load springs and additionally comprises an axial cylinder with a free piston. One side of the piston is in fluid communication with the environment via an orifice and is therefore subject to ambient pressure. The other side of the piston is exposed to a fluid-filled cavity within the body of the pressure compensator. The stem of the valve actuator is also in fluid communication with this cavity such that the ambient pressure may act on the cross-sectional area of the stem, urging it towards the closed position. Thus, both hydraulic and mechanical forces are brought to bear on the stem. 
   The carrier may be provided with a transverse, generally cylindrical cavity within which may be disposed an elastic member such as a helical pressure spring that is coaxial with the cavity. The spring bears at one end against the terminus of the cavity and, at the other end, presses against a seal disc or gate which engages the valve seat in a sliding arrangement. 
   The valve seat protrudes into the axial passage and engages a shoulder on the carrier to limit axial travel of the carrier in the direction of extension of the load spring(s), whereby the seal disc may be normally positioned over the valve seat to prevent flow through the lateral passage. Sufficient fluid pressure at the valve inlet, however, will cause the carrier to move in a direction which compresses the load spring(s), thereby unseating the seal disc and opening the valve. 
   To facilitate manufacture and to enable easy replacement of the valve seat, the body may be made in three parts including a main part, an outlet part and a pressure compensation part, the parts being held together by threaded engagement. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
       FIG. 1  is a cross-sectional view of a valve according to one embodiment of the invention. 
       FIG. 2  is an enlarged view of the gate portion of the valve illustrated in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings and more particularly to  FIG. 1 , there is shown a valve  10  comprising a main body part  12 , an outlet body part  14  and a pressure compensator  76 . The three body parts may be generally rectangular parallelepipeds secured together by threaded engagement. Seal  44  which may be an O-ring seal received in an annular groove  45  in one side of body part  14  seals with the adjacent side of body part  12 . 
   A cylindrical bore provides an axial passage  16  in body part  12 . The passage has an inlet  20  at one end which may be adapted to make a connection with a pipe or other conduit leading to a fluid reservoir or line whose pressure may be limited by valve  10 . 
   A cylindrical bore  22  in the outlet part  14  of valve  10  provides a lateral passage in the body. This passage at its outer end provides an outlet  18  which, by way of example, may be adapted to make a fluid connection with a conduit leading to a subsea well. The passage further includes transverse cylindrical bore  64  in main body part  12 . A cylindrical seat ring  46  received in bore  64  provides a valve seat. Seat ring  46  has a passage  48  extending axially therethrough. Bore  64  may be of larger diameter than bore  22 . Seat ring  46  abuts against the shoulder  51  formed at the juncture of bores  22  and  64 . When it is desired to replace seat ring  46 , body part  14  may be removed and seat ring  46  may be pulled out. By the reverse procedure a new seat ring can be installed. A seal  50  which may be an elastomeric O-ring received in an annular groove around the seat ring seals between and among seat ring  46 , bore  64  and the adjoining face of check valve body  14 . This seal arrangement is an improvement over that employed in the valve disclosed in U.S. Pat. No. 4,456,028 which provides a seal only between the seat ring and the bore which contains the seat ring. A seal according to the present invention presents less opportunity for leakage of fluid from the valve to the environment. 
   The inner end of seat ring  46  protrudes into axial bore  16  in the main body part and its upper edge  66  provides a mechanical stop for positioning valve actuator  68 . The inner face  62  of seat ring  46  may be planar and adapted to seal with gate or seal disc  58  of the valve actuator. 
   Valve actuator  68  may be generally cylindrical but of smaller diameter than bore  16  thereby providing annulus  70  ( FIG. 2 ) for the flow of fluid from inlet  20  through passage  48  and poppet  26  to outlet  18 . A blind hole or cavity  56  is provided in one side of actuator  68 . Hole or cavity  56  may be generally cylindrical. In the illustrated embodiment, helical pressure spring  60 , disposed within cavity  56  bears at one end against the closed end of cavity  56  and at the other end against gate or seal disc  58  and may be slightly compressed when assembled in the valve. The pressure of the axially compressed spring presses disc  58  against face  62  of seat ring  46 . The gate may be a cylindrical disc which extends around and overlaps the periphery of passage  48  in the seat ring, sealing off the passage as long as the disc and seat ring are coaxial. This design is an improvement over the ball-and-spring mechanism used in the valve disclosed in U.S. Pat. No. 4,456,028. The parts count is reduced and a stronger and more reliable shear seal is provided. The elimination of the ball element renders a valve according to the present invention less susceptible to a malfunction due to fluid contamination or the scoring, pitting or corrosion of the wall of bore  16 . 
   The outer diameter of disc  58  may be slightly smaller than the inner diameter of blind hole  56  so that the disc  58  can slide freely in the cavity and can cant slightly to ensure that it is coplanar with seat ring face  62 . The force of pressure spring  60  may be very small, being of the order of magnitude of one ounce in a preferred embodiment, so that the gate-seat friction due to the force of the pressure spring  60  may be negligible compared to that generated by the pressure of fluid against the gate. In certain chemical injection applications the fluid pressure may be as high as 30,000 psi. In those situations, inlet fluid pressure acting over the cross sectional area of passage  48 , may exert a force of up to several thousand pounds on the gate of valve  10 . 
   In the illustrated embodiment, outlet body  14  has threaded nose section  42  for engaging a corresponding socket in the side of main body  12 . The face of nose section  42  may be provided with annular groove  45  for retaining seal  44  which may be an elastomeric O-ring in certain embodiments. Outlet body  14  comprises central axial bore  22  the distal portion of which is fluid outlet  18 . The opposing end of bore  22  has a reduced diameter and the transition between the two regions is in the form of frusto-conical section  24 . Poppet valve  26  fits within bore  22  in sealing engagement with conical seat  24 . Poppet valve  26  comprises end piece  36  having central passage  40 , nose piece  28  having seal  30  having openings  32  and spring  34  which bears against end piece  36  and urges nose piece  28  into sealing engagement with surface  24 . End piece  36  is held in bore  22  by retainer  38  which may be a snap ring or the like. 
   This design provides for easy replacement, refurbishment or repair of the check valve. Outlet body  14  may simply be unscrewed from main body  12  thereby providing access to poppet valve  26  including seal  30  which, in certain embodiments, may be a replaceable seal. 
   If the fluid pressure at inlet  20  exceeds that at outlet  18 , when valve  10  opens, fluid pressure acts against nose piece  28 , compressing spring  34  which moves seal  30  away from surface  24 . Fluid may then flow through openings  32  in nose piece  28 , through the axial channel surrounded by spring  34  and out through passage  40 . However, if the fluid pressure at outlet  18  exceeds the fluid pressure at inlet  20 , both spring  34  and the fluid pressure act to urge seal  30  against surface  24 , preventing the backflow of fluid through valve  10 . In this way poppet  26  acts as a check valve and ensures that fluid only flows through valve  10  in the intended direction. Spring  34  may be selected so as to provide a minimum pressure differential between inlet  20  and outlet  18  that will act to open poppet valve  26 . 
   A valve stem  52  extends coaxially from actuator  68  away from inlet  20  into reduced diameter portion  17  of cylindrical bore  16 . An O-ring  74  in a circumferential groove  75  seals the stem to the body of bore  16 . The stem may be of larger diameter than seat ring passage  48 , i.e., the cross section of stem  52  may be larger than the area of seal disc  58  exposed to the differential of inlet and outlet pressure. Therefore, the force of the fluid pressure tending to move actuator  68  toward stem bore  16  may be greater than the fluid force tending to hold the seal disc against its seat. Since the frictional force created between the gate and seat by the normal force therebetween may be only a small percentage of the normal force, it will be appreciated that variations in the frictional force due to variation in outlet pressure will be reduced compared with the situation if the valve stem diameter were smaller than the diameter of bore  48 . 
   Generally cylindrical stub  13  which may have external straight threads projects from main body  12 . Pressure compensator  76  may have skirt  80  with corresponding interior threads such that it may be screwed onto stub  13 . Seal  117 , which may be an elastomeric seal, prevents leakage of hydraulic fluid from the interior of pressure compensator  76  from the threaded connection to main body  12  at stub  13 . 
   In the illustrated embodiment, pressure compensator  76  comprises a central axial chamber within which is disposed concentric helical compression load springs  102  and  104 . The upper ends of the load springs bear against threaded cap  84  which is retained in the central bore of pressure compensator  76  by engagement with threaded portion  82 . Tool receiver sockets  108  may be provided in the exposed end surface of cap  84  for engaging a spanner. Seal  116  provides a fluid-tight seal between cap  84  and body  78 . 
   The lower ends of load springs  102  and  104  bear against generally circular bearing plate  96  having inner and outer shoulders  98  and  100 , respectively. Bearing  96  may have a conical socket  106  on its distal face for contacting hemispherical end  54  of stem  52 . 
   Cap  84  comprises cylinder  86  which may be formed by a central, axial bore. Piston  88  partitions cylinder  86  into chambers  118  and  120 . Plug  110  may be pressed or threaded into the exposed opening of chamber  118 . Plug  110  may comprise orifice  114  in fluid communication with central axial passage  112  and chamber  118  such that one side of free-floating piston  88  in cylinder  86  may be normally exposed to ambient pressure. Piston  88  comprises ring seal  90  which seals against the walls of cylinder  86 . 
   Load springs  102  and  104  act to push stem  52  into main body part  12  (downward in  FIG. 1 ) until shoulder  72  on actuator  68  rests on upper edge  66  of the inner end of seat ring  46  which protrudes into passage  16 . In this position seal disc  58  may be coaxial with seat ring passage  48  and the valve is closed. If the inlet pressure rises sufficiently to overcome the combined force of springs  102  and  104 , the ambient pressure and the gate-seat friction, the actuator  68  moves such that stem  52  extends farther out of main body part  12 . This action compresses the load spring(s), expels fluid in chamber  94  through opening  92  into chamber  120  (displacing piston  88 ), and the gate seat disc  58  moves away from coaxial alignment with seat ring passage  48  thereby opening the valve. 
   A reduction in inlet pressure will allow the load springs and the ambient pressure due to the hydrostatic head acting on stem  52  to return the valve to the closed position wherein passage  48  is blocked by seal disk  58 . 
   The valve construction may be such that the carrier retains the spring and seal disc as long as the carrier is in the body. The actuator may be inserted in the body before the load spring is compressed and before the outlet part of the body and seat ring are assembled. Once the seat ring is in place and the outlet part of the body is secured to the main part, the carrier may be locked in the body by the seat ring and the seat ring may be captured between the carrier and the outlet part of the body. 
   Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope of the invention as described and defined in the following claims.