Abstract:
A valve ( 120 ) for controlling fluid flow therethrough in downhole applications is disclosed. The valve ( 120 ) comprises a valve housing having a valve closure mechanism ( 122 ) and a valve seat ( 124 ) disposed therein. The valve closure mechanism ( 122 ) has sealing surface ( 128 ) and a secondary load bearing surface ( 142 ). The valve seat ( 124 ) has a valve seat sealing surface ( 126 ) that mates with the sealing surface ( 128 ) of the valve closure mechanism ( 122 ). The secondary load bearing surface ( 142 ) of the valve closure mechanism ( 122 ) mates with a valve secondary load bearing surface ( 134 ) which may be supported by the valve seat ( 124 ).

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 09/309,716 filed on May 11, 1999 now U.S. Pat. No. 6,196,261. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to subsurface safety valves and, in particular, to a subsurface safety valve that includes a valve sealing surface and a secondary load bearing surface. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, the background will describe surface controlled, subsurface safety valves, as an example. 
     Surface controlled, subsurface safety valves are commonly used to shut in oil and gas wells in the event of a failure or hazardous condition at the well surface. Such safety valves are typically fitted into the production tubing and operate to block the flow of formation fluid upwardly therethrough. The subsurface safety valve provides automatic shutoff of production flow in response to a variety of out of range safety conditions that can be sensed or indicated at the surface. For example, the safety conditions include a fire on the platform, a high or low flow line temperature or pressure condition or operator override. 
     During production, the subsurface safety valve is typically held open by the application of hydraulic fluid pressure conducted to the subsurface safety valve through an auxiliary control conduit which extends along the tubing string within the annulus between the tubing and the well casing. Flapper type subsurface safety valves utilize a closure plate which is actuated by longitudinal movement of a hydraulically actuated, tubular piston. The flapper valve closure plate is maintained in the valve open position by an operator tube which is extended by the application of hydraulic pressure onto the piston. A pump at the surface pressurizes a reservoir which delivers regulated hydraulic control pressure through the control conduit. Hydraulic fluid is pumped into a variable volume pressure chamber and acts against the crown of the piston. When, for example, the production fluid pressure rises above or falls below a preset level, the control pressure is relieved such that the piston and operator tube are retracted to the valve closed position by a return spring. The flapper plate is then rotated to the valve closed position by a torsion spring or tension member. 
     In conventional subsurface safety valves of the type utilizing an upwardly closing flapper plate, the flapper plate is seated against an annular sealing face, either in metal-to-metal contact or metal against an annular elastomeric seal. In one design, the flapper closure plate has a flat, annular sealing face which is engageable against a flat, annular valve seat ring, with sealing engagement being enhanced by an elastomeric seal ring which is mounted on the valve seat. In another design, the valve seat includes a downwardly facing, conical segment having a sloping sealing surface and the flapper closure plate has a complementary, sloping annular sealing surface which is adapted for surface-to-surface engagement against the conical valve seat surface. 
     Typically, the flapper closure plate is supported for rotational movement by a hinge assembly which includes a hinge pin and a torsion spring or tension member. It will be appreciated that structural distortion of the flapper valve closure plate, or damage to the hinge assembly which supports the flapper closure plate, can cause misalignment of the respective sealing surfaces, thereby producing a leakage path through the safety valve. 
     Such misalignment will prevent correct seating and sealing of the flapper closure plate, and formation fluid may escape through the damaged valve, causing waste and pollution. During situations involving damage to the wellhead, the well flow must be shut off completely before repairs can be made and production resumed. Even a small leak through the flapper safety valve in a gas well can cause catastrophic damage. 
     Attempts have been made to overcome this misalignment problem. For example, one design involves the use of a valve seat and an upwardly closing flapper plate each having a sealing surface with a matched spherical radius of curvature. That is, the valve seat is a concave spherical segment and the sealing surface of the flapper plate is a convex spherical segment. In this arrangement, the spherical radius of curvature of the concave valve seat spherical segment is matched with the spherical radius of curvature of the convex spherical segment which defines the sealing surface on the flapper plate. The matching spherical surfaces are lapped together to provide a metal-to-metal seal along the interface between the nested convex and concave sealing surfaces. 
     As such, the convex spherical sealing segment of the flapper plate is received in nesting engagement within the concave spherical segment surface of the valve seat, which allows some angular displacement of the flapper plate relative to the valve seat without interrupting surface-to-surface engagement therebetween. Thus, the concave spherical seating surface of the safety valve seat will tolerate a limited amount of misalignment of the flapper plate which might be caused by structural distortion of the closure plate or warping of the hinge assembly. 
     It has been found, however, the even when using spherical sealing surfaces leakage may occur. Specifically, applications using large diameter tubing and having a low ratio between the outer diameter and the inner diameter of the sealing surfaces, distortion of the flapper closure plate caused by increased loads on the flapper closure plate may result in a loss of the seal. These increased loads are developed as a consequence of using larger safety valves having larger flapper closure plates in larger tubing. 
     Therefore, a need has arisen for a flapper valve that maintains a seal in a well requiring a large diameter flapper valve having a low ratio between the outer diameter and the inner diameter of the sealing surfaces. A need has also arisen for such a flapper valve that does not experience a loss of the seal in response to distortion of the flapper closure plate caused by the increased loads associated with such designs. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein is a valve comprising a valve closure mechanism that mates with a valve seat, where the valve has enhanced load-bearing capability. The valve of the present invention has separate sealing and load bearing surfaces, and can thus be deployed in a well requiring a large diameter valve having a low ratio between the outer diameter and the inner diameter of the sealing surfaces. The enhanced load-bearing capability of the valve of the present invention is particularly applicable in high pressure situations. Furthermore, the valve of the present invention does not experience a loss of the seal in response to distortion of the valve closure mechanism due to the increased loads on the valve that are associated with such applications. 
     The valve of the present invention comprises a valve housing, a valve closure member having a sealing surface and a secondary load bearing surface, a valve seat having a valve seat sealing surface, and a secondary load bearing surface that is located on either the valve seat or as part of the valve housing or on both the valve seat and the valve housing. The valve closure mechanism includes a secondary load bearing surface that may be located anywhere on, or formed as an integral part of, the valve closure mechanism. The valve closure mechanism secondary load bearing surface may be either an internal or external shoulder, or one or more internal or external support members, or any combination thereof. The load bearing surface of the valve closure mechanism will mate or engage with a load bearing surface found either on the valve seat or the valve housing, or both. 
     Should the valve seat include a secondary load bearing surface, the secondary load bearing surface may be either an internal load bearing surface of the valve seat or an external load bearing surface of the valve seat. The secondary load bearing surface of the valve seat may be either an internal or external shoulder, or one or more internal support members, or any combination thereof. A secondary load bearing surface may alternatively be coupled to the valve housing or integrally formed thereon. Should the valve housing include a secondary load bearing surface, the secondary load bearing surface of the valve housing may, for example, be an internal shoulder or one or more internal support members, or any combination thereof. 
     The valve of the present invention may be a flapper valve. Alternatively, the valve of the present invention may be a gate valve, a ball valve, a poppit, a valve having sliding members, a valve having sleeves, and any other types of valves known in the art. Accordingly, the valve closure member of the valve may be a flapper closure plate, a gate, a ball, a sleeve, a sliding member, or any other structure that forms a seal when mated to or engaged with a corresponding valve seat. Furthermore, a flapper closure plate may be flat or contoured. 
     In one embodiment, the valve includes a tubular valve housing having a valve chamber. A valve seat is mounted within a housing. The valve seat has a sealing surface and a secondary load bearing surface. A valve closure mechanism is provided as a flapper closure plate having a sealing surface and a secondary load bearing surface. The flapper closure plate is disposed within the valve chamber and rotates between a valve open position, in which the flapper closure plate is removed from the valve seat, and a valve closed position, in which the sealing surface of the flapper closure plate sealingly engages the valve seat sealing surface for preventing flow therethrough. When the flapper closure plate is in the valve closed position, the secondary load bearing surface of the valve seat defines the maximum travel of the flapper closure plate. 
     In one embodiment of the present invention, the secondary load bearing surface of a valve seat is an internal load bearing shoulder that may be machined as an integral part of the valve seat. In another embodiment, the valve seat may include a seal ring insert that comprises a material having a hardness greater than that of the valve seat. The seal ring insert may be a solid ring. Alternatively, the seal ring may be a machined weld bead. In either case, the seal ring insert forms a portion of the valve seat sealing surface and may serve as an internal load bearing shoulder. 
     In another embodiment, the secondary load bearing surface of the valve seat is an external load bearing shoulder. In this embodiment, the flapper closure plate includes a ballast member extending from the end of the flapper closure plate opposite the pivot pin, such that the external load bearing shoulder of the valve seat and the ballast member of the flapper closure plate define the maximum travel of the flapper closure plate. The external load bearing shoulder may be used alone or in combination with an internal load bearing shoulder or internal support members, each serving as secondary load bearing surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic illustration of an offshore oil or gas production platform operating a subsurface safety valve of the present invention; 
     FIGS. 2A-2B are half sectional views of a subsurface safety valve of the present invention in the valve open position; 
     FIGS. 3A-3B are half sectional views of a subsurface safety valve of the present invention in the valve closed position; 
     FIG. 4 is a cross sectional view of a valve of the present invention in the valve closed position; 
     FIG. 5 is a perspective view of a flapper closure plate of a valve of the present invention; 
     FIG. 6 is a cross sectional view of a valve of the present invention in the valve closed position under typical load conditions; 
     FIG. 7 is a cross sectional view of a valve of the present invention in the valve closed position under high load conditions; 
     FIG. 8 is a cross sectional view of a valve of the present invention in the valve closed position under typical load conditions; 
     FIG. 9 is a cross sectional view of a valve of the present invention in the valve closed position; 
     FIG. 10 is a perspective view of a flapper closure plate of a valve of the present invention; 
     FIG. 11 is a cross sectional view of a valve of the present invention in the valve closed position; 
     FIG. 12 is a perspective view of a flapper closure plate positioned against a support member of a valve of the present invention; 
     FIG. 13 is a cross sectional view of a valve of the present invention in the valve closed position; and 
     FIG. 14 is a perspective view of a flapper closure plate positioned against a pair of support members of a valve of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention is discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     Referring to FIG. 1, a subsurface safety valve in use with an offshore oil and gas production platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . Wellhead  18  is located on deck  20  of platform  12 . Well  22  extends through the sea  24  and penetrates the various earth strata including formation  14  to form wellbore  26 . Disposed within wellbore  26  is casing  28 . Disposed within casing  28  and extending from wellhead  18  is production tubing  30 . A pair of seal assemblies  32 ,  34  provide a seal between tubing  30  and casing  28  to prevent the flow of production fluids therebetween. During production, formation fluids enter wellbore  26  through perforations  36  of casing  28  and travel into tubing  30  through sand control device  38  to wellhead  18 . Subsurface safety valve  40  is located within the production tubing  30  and seals the wellhead  18  from the well formation  14  in the event of abnormal conditions. Subsurface safety valve  40  includes a valve closure mechanism that, during production from formation  14 , is maintained in the valve open position by hydraulic control pressure received from a surface control system  42  through a control conduit  44 . 
     Referring now to FIGS. 2A,  2 B,  3 A and  3 B, a subsurface safety valve  50  is illustrated. Safety valve  50  has a relatively larger production bore and is, therefore, intended for use in high flow rate wells. Safety valve  50  is connected directly in series with production tubing  30 . Hydraulic control pressure is conducted in communication with a longitudinal bore  52  formed in the sidewall of the top connector sub  54 . Pressurized hydraulic fluid is delivered through the longitudinal bore  52  into an annular chamber  56  defined by a counterbore  58  which is in communication with an annular undercut  60  formed in the sidewall of the top connector sub  54 . An inner housing mandrel  62  is slidably coupled and sealed to the top connector sub  54  by a slip union  64  and seal  66 , with the undercut  60  defining an annulus between inner mandrel  62  and the sidewall of top connector sub  54 . 
     A piston  68  is received in slidable, sealed engagement against the internal bore of inner mandrel  62 . The undercut annulus  60  opens into a piston chamber  70  in the annulus between the internal bore of a connector sub  72  and the external surface of piston  68 . The external radius of an upper sidewall piston section  74  is machined and reduced to define a radial clearance between piston  68  and connector sub  72 . An annular sloping surface  76  of piston  68  is acted against by the pressurized hydraulic fluid delivered through control conduit  44 . In FIGS. 2A-2B, piston  68  is fully extended with the piston shoulder  78  engaging the top annular face  80  of an operator tube  82 . In this valve open position, return spring  84  is fully compressed. 
     In the illustrated embodiment, a flapper plate  86  is pivotally mounted onto a hinge sub  88  which is threadably connected to the lower end of spring housing  90 . A valve seat  92  is confined within a counterbore formed on hinge sub  88 . The lower end of safety valve  50  is connected to production tubing  30  by a bottom sub connector  94 . The bottom sub connector  94  has a counterbore  96  which defines a valve chamber  98 . Thus, the bottom sub connector  94  forms a part of the valve housing enclosure. Flapper plate  86  pivots about pivot pin  100  and is biased to the valve closed position as shown in FIGS. 3A-3B by coil spring  102 . In the valve open position as shown in FIGS. 2A-2B, the spring bias force is overcome and flapper plate  86  is retained in the valve open position by operator tube  82  to permit formation fluid flow up through tubing  30 . 
     When an out of range condition occurs and subsurface safety valve  50  must be operated from the valve open position to the valve closed position, hydraulic pressure is released from conduit  44  such that return spring  84  acts on the lower end of piston  68  which retracts operator tube  82  longitudinally through valve chamber  98 . Flapper closure plate  86  will then rotate through chamber  98 . As flapper closure plate  86  nears the valve closed position within valve chamber  98  where significant throttling of fluid flow occurs, the high magnitude reaction forces may distort the operator tube  82 , flapper closure plate  86  or pivot pin  100 . Moreover, the alignment of flapper plate  86  relative to valve seat  92  may be disturbed in response to slamming impact of flapper closure plate  86  against valve seat  92 . 
     Referring now to FIG. 4, a valve is depicted and generally designated  120 . Valve  120  includes a valve closure mechanism which is depicted as flapper closure plate  122 . Valve  120  also includes a valve seat  124 . In the illustrated embodiment, the sealing surfaces of flapper closure plate  122  and valve seat  124  have mating segments which are matched in curvature to provide a metal-to-metal seal. Sealing surface  126  of valve seat  124  is a concave spherical segment. Sealing surface  128  of flapper closure plate  122  is a convex spherical segment. Convex flapper closure plate sealing surface  128  and concave valve seat sealing surface  126  are both generally a surface of revolution produced by revolving a semi-circular arc having an arc length  130  and radius of curvature  132 . As shown in FIG. 4, the radius of curvature of convex flapper closure plate sealing surface  128  is substantially equal to the radius of curvature of concave valve seat sealing surface  126 . 
     Specifically, the spherical radius of curvature of the concave valve seat sealing surface  126  is matched with the spherical radius of curvature of the convex flapper closure plate sealing surface  128 . As used herein, “matched radius of curvature” means that the radius of curvature of the flapper plate convex sealing surface  128  is substantially the same as, but not greater than, the radius of curvature of the concave valve seat sealing surface  126 . Preferably, the convex and concave surfaces are matched in curvature to provide smooth, non-binding surface engagement of convex flapper closure plate sealing surface  128  against concave valve seat sealing surface  126 . The matching convex and concave spherical surfaces  128 ,  126  are lapped together to permit close nesting engagement of flapper closure plate  122  within valve seat  124 . This arrangement permits smooth angular displacement of flapper closure plate  122  relative to valve seat  124  without interrupting surface-to-surface engagement therebetween. 
     Valve seat  124  includes a secondary load bearing surface which, in the illustrated embodiment, is an internal load bearing shoulder  134  extending generally radially inwardly from concave valve seat sealing surface  126 . As explained in more detail below, internal load bearing shoulder  134  defines the maximum travel of flapper closure plate  122  relative to valve seat  124 . 
     Referring now to FIG. 5, flapper closure plate  122  has a convex spherical sealing surface  128  and a semi-cylindrical channel  136  across the top of flapper closure plate  122  in alignment with its longitudinal axis  138 . The radial projection of flapper closure plate  122  is minimized, so that in the valve open position as shown in FIGS. 2A-2B, operator tube  82  is received within semi-cylindrical channel  136 , with convex spherical sealing surface  128  projecting into the annulus between operator tube  82  and bottom sub connector  94 . Flapper closure plate  122  has a secondary load bearing surface depicted as shoulders  142 . 
     Referring now to FIGS. 6 and 7, valve  120  is depicted in a view that is rotated 90 degrees from that in FIG.  4 . Valve  120  includes flapper closure plate  122  and valve seat  124 . As explained above with reference to FIG. 4, sealing surface  126  of valve seat  124  is a concave spherical segment and sealing surface  128  of flapper closure plate  122  is a convex spherical segment. Concave sealing surface  126  of valve seat  124  has a radius of curvature that is substantially equal to that of convex flapper closure plate sealing surface  128 . Valve seat  124  includes an internal load bearing shoulder  134  extending generally radially inwardly from concave valve seat sealing surface  126  which defines the maximum travel of flapper closure plate  122  relative to valve seat  124 . 
     Under typical flow rate regimes, the matching convex and concave spherical surfaces  128 ,  126  are lapped together to permit close nesting engagement of flapper closure plate  122  within valve seat  124  as shown in FIG. 6 wherein a gap  140  exists between shoulders  142  of flapper closure plate  122  and internal load bearing shoulder  134  of valve seat  124 . In applications where large diameter tubing and large diameter flapper closure plates are necessary and where the ratio of the outer and inner diameters of the sealing surfaces are low, the loads on flapper closure plate  122  tend to deform flapper closure plate  122  about axis  138  which may result in a loss of seal. Specifically, as flapper closure plate  122  deforms about axis  138 , the seal area between flapper closure plate  122  and valve seat  124  could be reduced. As best seen in FIG. 7, internal load bearing shoulder  134  of valve seat  124  defines the maximum travel of flapper closure plate  122  such that any deformation of flapper closure plate  122  about axis  138  that closes gap  140  between shoulders  142  of flapper closure plate and internal load bearing shoulder  134  of valve seat  124  will not reduce the seal area between flapper closure plate  122  and valve seat  124  and will not interrupt surface-to-surface engagement between the nested spherical segments, but will merely shift the region of overlapping engagement. Consequently, a continuous, positive metal-to-metal seal is maintained completely around the spherical segment interface. 
     Referring next to FIG. 8, therein is depicted another embodiment of a valve of the present invention that is generally designated  150 . Valve  150  has valve closure member shown as a flapper closure plate  122  and valve seat  152 . As with valve  120  of FIGS. 6 and 7, valve seat  152  has concave valve seat sealing surface  126  and flapper closure plate  122  has a convex flapper closure plate sealing surface  128 . Concave sealing surface  126  of valve seat  152  has a radius of curvature that is substantially equal to that of convex flapper closure plate sealing surface  128 . 
     Valve seat  152  includes a seal ring insert  154 . Seal ring insert  154  forms a portion of concave sealing surface  126  and forms the secondary load bearing surface illustrated as internal load bearing shoulder  134  that extends generally radially inwardly from concave valve seat sealing surface  126 . Internal load bearing shoulder  134  defines the maximum travel of flapper closure plate  122  relative to valve seat  152 . Preferably, seal ring insert  154  comprises a material that has a higher hardness than valve seat  152 . As seal ring insert  154  must withstand extreme loads exerted by shoulders  142  of flapper closure plate  122 , the hardness of seal ring insert  154  is an important feature of the present invention. For example, seal ring insert  154  may be formed by machining out a section of valve seat  152  and laying a weld bead therein. The weld bead is then machined smooth to form a portion of concave sealing surface  126  and internal load bearing shoulder  134 . Alternatively, seal ring insert  154  may be a solid ring that is welded in place within valve seat  152  then machined smooth to form a portion of concave sealing surface  126  and internal load bearing shoulder  134 . 
     Referring now to FIG. 9, a valve is depicted and generally designated  160 . Valve  160  includes a valve closure member shown as a flapper closure plate  162  and a valve seat  164 . In the illustrated embodiment, the sealing surfaces of flapper closure plate  162  and valve seat  164  have mating segments which are matched in curvature to provide a metal-to-metal seal. Sealing surface  166  of valve seat  164  is a concave spherical segment. Sealing surface  168  of flapper closure plate  162  is a convex spherical segment. The radius of curvature  170  of convex flapper closure plate sealing surface  168  is substantially equal to the radius of curvature of concave valve seat sealing surface  166 . 
     Specifically, the radius of curvature of the flapper plate convex sealing surface  168  is substantially the same as, but not greater than, the radius of curvature of the concave valve seat sealing surface  166 . Preferably, the convex and concave surfaces are matched in curvature to provide smooth, non-binding surface engagement of convex flapper closure plate sealing surface  168  against concave valve seat sealing surface  166 . The matching convex and concave spherical surfaces  168 ,  166  are lapped together to permit close nesting engagement of flapper closure plate  162  within valve seat  164 . This arrangement permits smooth angular displacement of flapper closure plate  162  relative to valve seat  164  without interrupting surface-to-surface engagement therebetween. 
     Valve seat  164  includes two secondary load bearing surfaces, specifically an internal load bearing shoulder  172  extending generally radially inwardly from concave valve seat sealing surface  166  and an external load bearing surface or shoulder  174  extending generally radially outwardly from concave valve seat sealing surface  166 . Flapper closure plate  162  also includes two secondary load bearing surfaces depicted as shoulders  188  and ballast member  176 . External load bearing shoulder  174  is axially aligned with ballast member  176  of flapper closure plate  162 . Ballast member  176  is integral with flapper closure plate  162  and is disposed opposite of pivot pin support member  178 . Together, these secondary load bearing surfaces, internal load bearing shoulder  172  and external load bearing shoulder  174 , define the maximum travel of flapper closure plate  162  relative to valve seat  164 . It should be noted by those skilled in the art that even though ballast member  176  is depicted as integral with flapper closure plate  162 , a ballast member could be attached to flapper closure plate  162  using a variety of methods including, but not limited to, welding or bolting. 
     In application where large diameter tubing and large diameter flapper closure plates are necessary and wherein the ratio between the outer and inner diameters of the sealing surfaces is low, the loads on flapper closure plate  162  tend to deform flapper closure plate  162  about both axis  180  and axis  182 , as best seen in FIG.  10 . As flapper closure plate  162  deforms about axis  180  and gap  184  is closed, internal load bearing shoulder  172  of valve seat  164  defines the maximum travel of shoulders  188  of flapper closure plate  162 . Likewise, as flapper closure plate  162  deforms about axis  182  and gap  186  is closed, external load bearing shoulder  174  of valve seat  162  defines the maximum travel of ballast member  176  of flapper closure plate  162 . As such, any deformation of flapper closure plate  162  about axis  180  or axis  182  will not reduce the seal area between flapper closure plate  162  and valve seat  164  and will not interrupt surface-to-surface engagement between the nested spherical segments, but will merely shift the region of overlapping engagement. Consequently, a continuous, positive metal-to-metal seal is maintained completely around the spherical segment interface. 
     Even though FIG. 9 depicts two secondary loads bearings surfaces, internal load bearing shoulder  172  and external load bearing shoulder  174 , it should be understood that by those skilled in the art that a single secondary load bearing surface may alternatively be utilized such as internal load bearing shoulder  172 , as explained above with reference to FIGS. 4-8, or external load bearing shoulder  174 . 
     Referring now to FIG. 11, a valve is depicted and generally designated  200 . Valve  200  includes a valve closure mechanism depicted as a flapper closure plate  202  and a valve seat  204 . In the illustrated embodiment, the sealing surfaces of flapper closure plate  202  and valve seat  204  have mating segments which are matched in curvature to provide a metal-to-metal seal. Sealing surface  206  of valve seat  204  is a concave spherical segment. Sealing surface  208  of flapper closure plate  202  is a convex spherical segment. Convex flapper closure plate sealing surface  208  and concave valve seat sealing surface  206  are both generally a surface of revolution produced by revolving a semi-circular arc having an arc length  210  and radius of curvature  212 . As shown in FIG. 11, the radius of curvature of convex flapper closure plate sealing surface  208  is substantially equal to the radius of curvature of concave valve seat sealing surface  206 . 
     Preferably, the convex and concave surfaces are matched in curvature to provide smooth, non-binding surface engagement of convex flapper closure plate sealing surface  208  against concave valve seat sealing surface  206 . The matching convex and concave spherical surfaces  208 ,  206  are lapped together to permit close nesting engagement of flapper closure plate  202  within valve seat  204 . This arrangement permits smooth angular displacement of flapper closure plate  202  relative to valve seat  204  without interrupting surface-to-surface engagement therebetween. 
     Valve seat  204  includes a secondary load bearing surface depicted as internal support member  214  extending generally radially inwardly about a portion of the circumference of concave valve seat sealing surface  206  on the side opposite hinge  216 . Internal support member  214  is positioned within a pocket  218  cut in concave valve sealing surface  206  of valve seat  204 . Internal support member  214  is securably attached within pocket  218  using suitable means of such a one or more bolts  220 . Internal support member  214  is properly aligned within pocket  218  using pin  222  that extends into hole  224  of internal support member  214  and hole  226  of valve seat  204 . Internal support member  214  defines the maximum travel of flapper closure plate  202  relative to valve seat  204 . 
     Under typical flow rate regimes, the matching convex and concave spherical surfaces  208 ,  206  are lapped together to permit close nesting engagement of flapper closure plate  202  within valve seat  204  as shown in FIG. 11 wherein a gap  228  exists between a secondary load bearing surface  230  of flapper closure plate  202  and surface  232  of internal support member  214 . In applications where large diameter tubing and large diameter flapper closure plates are necessary and where the ratio of the outer and inner diameters of the sealing surfaces are low, the loads on flapper closure plate  202  tend to deform flapper closure plate  202  about both axis  234  and axis  236 , as best seen in FIG. 12, which may result in a loss of seal. Specifically, as flapper closure plate  202  deforms about axes  234 ,  236 , the seal area between flapper closure plate  202  and valve seat  204  could be reduced. Internal support member  214  defines the maximum travel of flapper closure plate  202  such that any deformation of flapper closure plate  202  closes gap  228  but will not reduce the seal area between flapper closure plate  202  and valve seat  204  and will not interrupt surface-to-surface engagement between the nested spherical segments, merely shifting the region of overlapping engagement. Consequently, a continuous, positive metal-to-metal seal is maintained completely around the spherical segment interface. 
     While FIG. 11 has been described with reference to a single secondary load bearing surface, i.e., support member  214 , it should be understood by those skilled in the art that support member  214  may be used in conjunction with an internal load bearing shoulder  134  as described above with reference to FIGS. 4-7 or an external load bearing shoulder  174  as described above with reference to FIGS. 9-10 or both. 
     Alternatively, it should be noted that internal support member  214  may be secured to flapper closure plate  202  such that when flapper closure plate  214  is in the closed position, internal support member  214  is received within pocket  218  which serves as the secondary load bearing surface of valve seat  204 . In another alternative, internal support member  214  may be received within or against a secondary load bearing surface of the valve housing as opposed to the valve seat  214 . 
     Referring now to FIG. 13, a valve is depicted and generally designated  240 . Valve  240  includes a valve closure member depicted as a flapper closure plate  242  and a valve seat  244 . In the illustrated embodiment, the sealing surfaces of flapper closure plate  242  and valve seat  244  have mating segments which are matched in curvature to provide a metal-to-metal seal. Sealing surface  246  of valve seat  244  is a concave spherical segment. Sealing surface  248  of flapper closure plate  242  is a convex spherical segment. Preferably, the convex and concave surfaces are matched in curvature to provide smooth, non-binding surface engagement of convex flapper closure plate sealing surface  248  against concave valve seat sealing surface  246 . The matching convex and concave spherical surfaces  248 ,  246  are lapped together to permit close nesting engagement of flapper closure plate  242  within valve seat  244 . This arrangement permits smooth angular displacement of flapper closure plate  242  relative to valve seat  244  without interrupting surface-to-surface engagement therebetween. 
     Valve seat  244  includes a secondary load bearing surface depicted as a pair of internal support members  250 ,  252  extending generally radially inwardly about portions of the circumference of concave valve seat sealing surface  246 . Internal support members  250 ,  252  are positioned within pockets  254 ,  256  cut in concave valve sealing surface  246  of valve seat  244 . Internal support members  250 ,  252  are secured within pockets  254 ,  256  using by suitable means such as one or more bolts  258 . Internal support member  250  is aligned within pocket  254  using a pin  260  that extends between hole  262  of valve seat  244  and hole  264  at internal support member  250 . Internal support member  252  is aligned within pocket  256  using a pin  266  that extends between hole  268  of valve seat  244  and hole  270  of internal support member  252 . 
     Internal support members  250 ,  252  define the maximum travel of flapper closure plate  242  relative to valve seat  244 . Under typical flow rate regimes, the matching convex and concave spherical surfaces  248 ,  246  are lapped together to permit close nesting engagement of flapper closure plate  242  within valve seat  244 , as shown in FIG. 13, wherein gaps  272 ,  274  exists between secondary load bearing surface  280  of flapper closure plate  242  and internal support members  250 ,  252 . In applications where large diameter tubing and large diameter flapper closure plates are necessary and where the ratio of the outer and inner diameters of the sealing surfaces are low, the loads on flapper closure plate  242  tend to deform flapper closure plate  242  about both axis  276  and axis  278 , as best seen in FIG. 14, which may result in a loss of seal. Specifically, as flapper closure plate  242  deforms about axes  276 ,  278 , the seal area between flapper closure plate  242  and valve seat  244  could be reduced. Internal support members  250 ,  252  defines the maximum travel of flapper closure plate  242  such that any deformation of flapper closure plate  242  closes gaps  272 ,  274  but will not reduce the seal area between flapper closure plate  242  and valve seat  244  and will not interrupt surface-to-surface engagement between the nested spherical segments, merely shifting the region of overlapping engagement. Consequently, a continuous, positive metal-to-metal seal is maintained completely around the spherical segment interface. 
     Even though FIG. 13 has depicted the secondary load bearing surface as consisting of a pair of internal support members  250 ,  252 , it should be understood by those skilled in the art that these secondary load bearing surfaces may be used in conjunction with the other secondary load bearing surfaces described above including internal load bearing shoulder  134  of FIG.  4  and external load bearing shoulder  174  of FIG.  9 . 
     Alternatively, it should be noted that internal support members  250 ,  252  may be secured to flapper closure plate  242  such that when flapper closure plate  242  is in the closed position, internal support members  250 ,  252  are received within pockets  254 ,  256  which serve as the secondary load bearing surface of valve seat  244 . In another alternative, internal support members  250 ,  252  may be received within or against a secondary load bearing surface of the valve housing as opposed to the valve seat  244 . 
     While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.