Patent Publication Number: US-11655900-B2

Title: Valve with pressure differential seating

Description:
FIELD OF THE INVENTION 
     The invention relates to an improved design for a valve that uses asymmetric pressure applied to a bushing and sealing mechanism to allow for improved sealing performance and reduced maintenance requirements. Although the valve is primarily described in reference to a plug valve, it could be equally applicable to other types of valves, including but not limited to a ball valve or gate valve. 
     BACKGROUND OF THE INVENTION 
     Valves generally comprise a valve body with an interior bore for the passage of fluid, and a means of sealing off the interior bore to stop the flow of fluid. Certain types of valves, such as plug valves or ball valves, include a plug or ball that is capable of rotating between an open position, in which fluid is allowed to flow through the interior bore, and a closed position, in which the plug or ball blocks the flow of fluid through the interior bore. Other types of valves, such as gate valves, include a gate that is vertically lowered to block the flow of fluid through the interior bore. All of these types of valves are often used in connection with the production of hydrocarbons such as crude oil or natural gas. 
     The valve of the present invention will be primarily described in the context of an embodiment using a plug valve, but it could also be used in ball valves, gate valves, or other types of valves. In some applications, it might be preferable to use a ball valve, rather than a plug valve, which allows for more even distribution of the contact pressure around the seat. In any event, the particular type of valve is not critical to the operation of the invention and the claims of the present application should not be interpreted as limited to any specific type of flow barrier used in the valve. It will be readily apparent to one of ordinary skill in the art how to implement the present invention in a type of valve other than a plug valve. 
     Plug valves require a sealing interface so that, when in the closed position, the plug will contain the pressure of the fluid within the interior bore of the valve. In many applications, such as the production of hydrocarbons, interior pressures can be extremely high, on the order of 15,000 pounds per square inch or higher. In addition, the fluid within the interior bore may be corrosive or otherwise potentially damaging to the seals. Accordingly, the integrity and reliability of the sealing interface is of utmost importance in the design of a plug valve. 
     One of the primary failure modes of most valves is damaged sealing surfaces. One of the reasons for this is the common use of elastomeric or rubberized seals in hazardous environments like those encountered in the production of hydrocarbons such as crude oil or natural gas. The use of elastomers or rubberized components can create increased risks for degradation and failure within the valve and create increased maintenance costs due to the location of the damaged seals or valve components and lead to production down time. 
     Another problem with existing plug valve designs is that they traditionally seal only on one side of the valve, generally the downstream side, when considering the typical direction of the fluid flow through the valve. This design is prone to failure from contamination of the sealing surfaces because the sealing surfaces are only engaged when the valve is closed. When the valve is open, there is a gap between the sealing surfaces. The lack of constant engagement allows chemicals and/or particulates in the fluid stream to degrade the sealing surfaces to the point that they no longer effectuate a seal. For example, sand or other particulate matter may cause abrasion of the sealing surface, particularly if the seal is formed from an elastomeric material. Separate from the risk of abrasion, particulate matter such as sand may remain in the gap between sealing surfaces when an operator is attempting to open or close the valve and may physically interfere with the formation of a solid seal and/or may increase the difficulty of rotating the valve to or from an open or closed position. 
     The gap between sealing surfaces in a typical plug valve is also problematic because valves generally require grease to function; without grease or some other lubricant in the valve body, the plug or ball cannot rotate to a closed position. A gap between sealing surfaces typically allows grease to move from the interior of the valve body to the fluid stream. This migration of grease creates a loss of lubrication which can result in the plug being unable to rotate to the open (or closed) position. 
     Although there are other valve designs with double seals, like that found in U.S. Pat. No. 5,624,101, those designs generally rely on double energization of the seals in order to create a double sealing mechanism and reliance on a block and bleed function to normalize pressure on the seals. This block and bleed function can lead to similar seal issues as described above. 
     Another problem with certain prior art plug valves is that when in the closed position, the plug and the valve body may seize under high pressures. When high working pressures exist in fluid either downstream or upstream of the plug valve, the plug cannot move from its sealed position due to the high pressure forces exerted on the valve and gets stuck in place. The likelihood of such an occurrence is higher when the valve body has lost grease, a problem already discussed above. These high pressure environments can be hazardous and create issues with maintenance of the plug valve as well as potential failure mechanisms for the plug valve itself when operated against such high pressures. At the same time, the standard design can also be prone to leaking at low pressures because the design is meant to be at a high pressure to engage the sealing surfaces when the valve is closed. The aforementioned problem with grease loss can also exacerbate the problem with leaking at low pressure, as grease often serves as the low pressure seal in existing valve designs. 
     For the above reasons and others, standard existing valve designs are often unreliable. The unreliability of these valves frequently prompts users to stack multiple valves together to ensure they are able to stop the flow of fluid. 
     The present invention addresses the unmet need for a valve that can be seated in hazardous environments, high pressure environments, with more easily replaceable parts, and/or creates a pressure differential at the seats automatically based on the geometry of the components used to provide a sealing surface against the plug. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to create a valve with a seat and seat bushing configuration such that the seat is maintained in sealing contact with the plug body regardless of whether the valve is in the opened or closed position. 
     In an exemplary embodiment, the seat and seat bushing are both located in a recess of the valve body and configured such that, when the valve is in the open position, the seat is maintained in sealing engagement with the flow barrier on both the upstream and downstream sides of the valve. 
     The seat is generally annular in shape with two radial surface areas. When the valve is in the open condition, the fluid in the interior bore exerts pressure on both surface areas of the seat but, due to a differential in the two surface areas, a net positive force tends to urge the seat into sealing engagement with the flow barrier. In addition, the fluid also exerts pressure on the radial surface area of the seat bushing closest to the flow barrier, tending to push the seat bushing away from the flow barrier. However, the opposite side of the seat bushing, the radial surface farthest from the flow barrier, engages with a shoulder of the valve body, rather than the seat. Accordingly, the pressure exerted on the seat bushing does not interfere with the seal between the seat and the flow barrier. 
     When the valve is in the closed position, a primary seal is maintained on the upstream side similar to when the valve is in the open position, while a secondary seal is also maintained on the downstream side of the valve. 
     In an exemplary embodiment, in addition to an improved sealing mechanism, the seat and seat bushing are formed from stainless steel or another metal, rather than the rubber or elastomeric seals generally found in prior art plug valves. This provides for increased durability, longer life between required maintenance, and a more robust metal-to-metal seal. 
     In an exemplary embodiment, in addition to an improved sealing mechanism, the seat bushing and seat each comprise corresponding keyed portions that allow for easy removal of the seat for maintenance purposes during down time or for inspection. Rotating the seat bushing relative to the seat can engage the keyed portions to allow the seat bushing to assist with the removal of the seat from the valve body, or can disengage the keyed portions to allow the seat bushing to be separated from the seat. This provides for reduced maintenance time and reduced cost of maintenance. 
     References throughout the description to “upstream” and “downstream” should not be interpreted as limiting which term could be used to refer to which particular portion of the invention. Those of skill in the art will understand that which portion of the valve is upstream or downstream depends on which direction fluid is flowing, and is therefore unrelated to the structure of the device itself. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the invention are described below with reference to the figures accompanying this application. The scope of the invention is not limited to the figures. 
         FIG.  1    depicts a perspective view of an embodiment of the plug valve when it is manufactured with flanges ready to be installed. 
         FIG.  2    depicts a side view of the interior of the embodiment shown in  FIG.  1   . 
         FIG.  3    depicts a close up view of the seat and seat bushing in relation to the plug and valve body when the embodiment of the valve shown in  FIG.  1    is in the open position. 
         FIG.  4    depicts the same view as  FIG.  3   , with annotations indicating the pressure exerted by fluid when the valve is in the open position. 
         FIG.  5    depicts the same view as  FIG.  4   , when the valve is in the closed position. 
         FIG.  6    depicts a side view of the interior of an embodiment of the valve body for an alternative gate valve embodiment. 
         FIG.  7    depicts a close-up view of the seat and seat bushing in relation to the gate and valve body when the alternative gate valve embodiment shown in  FIG.  7    is in the closed position. 
         FIG.  8    depicts a close-up side view of the alternative gate valve embodiment shown in  FIG.  7   , with the valve in the open position and annotations indicating the pressure exerted by fluid when the alternative gate valve embodiment is in this position. 
         FIG.  9    depicts the same view as  FIG.  8   , when the valve is in the closed position. 
         FIGS.  9 A- 9 B  depict close-up side views of the seat and seat bushing in relation to the gate and valve body of additional alternative gate valve embodiments including a biasing member. 
         FIG.  10    depicts a side view of the interior of the body of an alternative embodiment of a gate valve comprising a body bushing. 
         FIG.  11    depicts a close-up view of the seat, body bushing, and seat bushing in relation to the gate and valve body when the embodiment of the valve shown in  FIG.  10    is in the open position. 
         FIG.  12    depicts the same view as  FIG.  11   , with annotations indicating the pressure exerted by fluid when the valve is in the open position. 
         FIGS.  12 A- 12 B  depict close-up side views of the seat, body bushing, and seat bushing in relation to the gate and valve body of additional alternative gate valve embodiments including a biasing member. 
         FIG.  13    depicts the same view as  FIG.  12   , when the valve is in the closed position. 
         FIG.  14    depicts the keyed portions of the seat bushing and seat of an alternative embodiment of the valve. 
         FIG.  15    depicts the seat bushing being displaced relative to the valve body to engage the seat bushing&#39;s keyed portions with the seat&#39;s keyed portions for more easily removing the seat from the body of the valve. 
         FIG.  15 A  depicts the seat shown in  FIG.  15    being removed from the valve body using the engagement of the keyed portions of the seat and seat bushing. 
         FIG.  16    depicts the keyed portions of the seat bushing and seat disengaged to allow them to be separated from one another. 
         FIG.  17 A  depicts an alternative embodiment of a gate valve in which a removable bore-end connection is used to maintain the seat in position in relation to the flow barrier and a biasing member disposed between the valve body and a support attached to the seat is used to aid in sealing between the seat and the flow barrier under low-pressure operating conditions. 
         FIG.  17 B  depicts a close-up view of the seat and removable bore-end connection shown in  FIG.  17 A . 
         FIG.  18 A  depicts an alternative embodiment of a gate valve in which a removable bore-end connection is used to maintain the seat in position in relation to the flow barrier and a biasing member disposed between the removable bore-end connection and the downstream surface of the seat is used to aid in sealing between the seat and the flow barrier under low-pressure operating conditions. 
         FIG.  18 B  depicts a close-up view of the seat and removable bore-end connection shown in  FIG.  18 A . 
         FIG.  19 A  depicts an alternative embodiment of a gate valve in which a seat and seat bushing are partially disposed within a recess formed in the valve body and a biasing member is disposed between the seat bushing and a support attached to the seat to aid in sealing between the seat and the flow barrier under low-pressure operating conditions. 
         FIG.  19 B  depicts a close-up view of the seat and seat bushing of the embodiment shown in  FIG.  19 A . 
         FIG.  20 A  depicts an alternative embodiment of a gate valve in which a seat and seat bushing are partially disposed within a recess formed in the valve body and a biasing member is disposed between the valve body and the downstream surface of the seat to aid in sealing between the seat and the flow barrier under low-pressure operating conditions. 
         FIG.  20 B  depicts a close-up view of the seat and seat bushing of the embodiment shown in  FIG.  20 A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG.  1   , the valve  100  includes a flanged connection to be installed in an oil and gas production area or similar application. The fluid enters into the valve at the upstream flanged connection  10  and is allowed to flow through the valve body  20  and exits the downstream flanged connection  30 . The valve is operable by a valve stem that connects to the plug and is operable to rotate the plug from the open to closed position. The operation of the valve may be controlled by hydraulic actuator  40 . Other types of actuators, including electronic, could also be used. 
     Referring to  FIG.  2   , an exemplary embodiment of the valve  100  is shown. Extending between upstream flanged connection  10  and downstream flanged connection  30  is interior bore  110 . 
     Within valve body  20  is disposed plug  120 , seat  130  and seat bushing  140 . Seat  130  and seat bushing  140  are generally annular in shape and both located within recess  150  formed in the valve body. Both seat  130  and seat bushing  140  may be formed of metal, such as stainless steel. Cavity  106  is formed within valve body  20  and plug  120  rotates within cavity  106 . Fluid may flow through interior bore  110  in the direction indicated by arrow F but, as noted above, fluid may also flow in the opposite direction and the valve will still function as described below. 
     Referring to  FIG.  3   , the downstream side of seat  130  comprises surface  200 . Surface  200  is adjacent to valve body  20  at interface  300 . The upstream side of seat  130  comprises surface  210 . Surface  210  is adjacent to plug  120  at interface  310 . As shown, seat  130  may have a generally “L-shaped” configuration, such that surface  200  is larger than surface  210 . In addition, there is a radially projecting shoulder  215  formed in the intermediate portion of seat  130 . Thus, the outer surface of seat  130  comprises two distinct portions, surface  212  on the upstream side and surface  214  on the downstream side. 
     The downstream side of seat bushing  140  comprises surface  220 . As shown in  FIG.  3   , recess  150  has a stepped configuration which forms radially projecting shoulder  230 . At interface  320 , shoulder  230  contacts a portion of surface  220  on seat bushing  140 . The remainder of surface  220  on seat bushing  140  does not make contact with any other portion of valve  100 . Instead, there is a chamber  340  formed by portions of surface  220 , shoulder  215 , surface  212 , and shoulder  230 . Chamber  340  will generally enclose an area of relatively low pressure, compared to the other portions of valve  100 . The upstream side of seat bushing  140  comprises surface  240 . Surface  240  does not contact any other portion of valve  100 . Seat bushing  140  also comprises bottom surface  250 , which contacts surface  212 . Seat  130  and seat bushing  140  make contact with each other at the interface formed between surface  250  and surface  212 . 
     In operation, when valve  100  is in the open position, the fluid within the interior bore  110  and cavity  106  will generally be at the same pressure. The fluid will generally exert pressure P 1  on surface  200  of seat  130  at interface  300 . This pressure will be exerted in an axial direction, as shown by the arrows in  FIG.  4   . Pressure P 2  will also be exerted in the opposite axial direction on surface  210  of seat  130  at interface  310 . Pressure P 3  will also be exerted, in the same axial direction as P 2 , on surface  240  of seat bushing  140 . 
     Due to the difference in surface area between surface  200  and surface  210 , the total force (pressure times surface area) exerted by pressure P 1  is greater than the total force exerted by pressure P 2 . This differential in force tends to urge seat  130  into sealing engagement with plug  120  at interface  310 . In addition, although pressure P 3  is exerted in the opposite direction of P 1 , it does not interfere with the sealing engagement of seat  130  because the combination of shoulder  230  and chamber  340  prevents surface  220  of seat bushing  140  from coming into contact with seat  130 . Instead, pressure P 3  is countered by a reaction force at shoulder  230 . Accordingly, the differential in force resulting from pressure P 1  as compared to P 2  is sufficient to ensure a robust metal-to-metal seal at interface  310 . In addition, as noted above, as the pressure within interior bore  110  increases, the difference in force exerted by P 1  and P 2  will also increase and so the performance of the seal, and thus the valve, will improve as the interior pressure increases. The foregoing description of the operation of valve  100  in the open position applies equally to the upstream and downstream side of plug  120 . 
     In certain situations, the fluid pressure in cavity  106  may be higher than the fluid pressure in bore  110 . One point at which this scenario may occur is after pressure has been drained completely from bore  110 , and the previous operating pressure, sometimes as high as  15 , 000  psi, may be contained in cavity  106 . Such a pressure differential can be dangerous for personnel working in proximity to the valve, including for example maintenance personnel who attempt to service the valve while high pressure is trapped in cavity  106 . To address such a situation, seat  130  may include a surface  216  at a smaller diameter than surface  212  to serve as a pressure-relieving feature for cavity  106 . As shown in  FIG.  4   , surface  212  may take the form of a beveled corner. In this embodiment, the pressure in cavity  106  will cause a force P 5  to be exerted on surface  216  with some component of the force acting in the axial direction away from plug  120 . When the pressure in bore  110  is small enough such that the force P 1  is smaller than the force P 5 , seat  130  will move away from plug  100 , which will allow pressure in cavity  106  to drain into bore  110  across surface  210 . 
     When valve  100  is in the closed position, the operation of valve body  20 , plug  120 , seat  130 , and seat bushing  140  on the upstream side of plug  120  is essentially the same as that described above. Thus, the operation on the upstream side is independent of whether the valve is in the open or closed position. 
     When valve  100  is in the closed position, a seal is maintained on the downstream side of plug  120 , but potentially via a different mechanism. If pressure is equalized, such that there remains approximately equal pressure on both the upstream and downstream sides of plug  120 , then the sealing mechanism will be essentially the same as that described above when valve  100  is in the open position. However, if pressure is not equalized, such that upstream pressure exceeds downstream pressure, as shown in  FIG.  5   , pressure P 4  is exerted by plug  120  in an axial direction but there is no (or lesser) pressure acting in the opposite direction of pressure P 4 . Accordingly, pressure P 4  will tend to force plug  120  into seat  130  at interface  310 . In this way, when valve  100  is in the closed position, a seal is maintained on both the upstream and downstream sides of plug  120 , regardless of the relative pressure on either side of the plug. 
     As also shown in  FIGS.  3  and  4   , additional seals may be disposed at the interfaces between surface  250  of seat bushing  140  and surface  212  of seat  130 , the interface between surface  214  of seat  130  and valve body  20 , and/or the interface between the top surface of seat bushing  140  and valve body  20 . Such seals may be elastomeric such as, for example, o-rings. 
     Referring to  FIG.  6   , an alternative embodiment is shown using a gate valve  400 , rather than a valve that rotates, such as a plug or ball valve. Although the orientation of the components differs from the embodiment shown in  FIGS.  1 - 5   , the basic concept is the same. Extending between upstream flanged connection  410  and downstream flanged connection  430  is interior bore  510 . 
     Within valve body  420  is disposed gate  520 , seat  530  and seat bushing  540 . Seat  530  and seat bushing  540  are generally annular in shape and both located within recess  550  formed in the valve body. Cavity  406  is formed within valve body  420  and gate  520  moves within cavity  406 . Referring to  FIG.  7   , the downstream side of seat  530  comprises surface  600 . Surface  600  is adjacent to valve body  420  at interface  700 . The upstream side of seat  530  comprises surface  610 . Surface  610  is adjacent to gate  520  at interface  710 . As shown, seat  530  may have a generally “L-shaped” configuration, such that surface  600  is larger than surface  610 . In addition, there is a radially projecting shoulder  615  formed in the intermediate portion of seat  530 . Thus, the outer surface of seat  530  comprises two distinct portions, surface  612  on the upstream side and surface  614  on the downstream side. 
     The downstream side of seat bushing  540  comprises surface  620 . As shown in  FIG.  7   , recess  550  has a stepped configuration which forms radially projecting shoulder  630 . At interface  720 , shoulder  630  contacts a portion of surface  620  on seat bushing  540 . The remainder of surface  620  on seat bushing  540  does not make contact with any other portion of valve  400 . Instead, there is a chamber  740  formed by portions of surface  620 , shoulder  615 , surface  612 , and shoulder  630 . Chamber  740  will generally enclose an area of relatively low pressure, compared to the other portions of valve  400 . The upstream side of seat bushing  540  comprises surface  640 . Surface  640  does not contact any other portion of valve  400 . Seat bushing  540  also comprises bottom surface  650 , which contacts surface  612 . Seat  530  and seat bushing  540  make contact with each other at the interface formed between surface  650  and surface  612 . 
     In operation, when valve  400  is in the open position, the fluid within the interior bore  510  will generally exert pressure P 5  on surface  600  of seat  530  at interface  700 . This pressure will be exerted in an axial direction, as shown by the arrows in  FIG.  8   . Pressure P 6  will also be exerted in the opposite axial direction on surface  610  of seat  530  at interface  710 . Pressure P 7  will also be exerted, in the same axial direction as P 6 , on surface  640  of seat bushing  540 . 
     Due to the difference in surface area between surface  600  and surface  610 , the total force (pressure times surface area) exerted by pressure P 5  is greater than the total force exerted by pressure P 6 . This differential in force tends to urge seat  530  into sealing engagement with gate  520  at interface  710 . In addition, although pressure P 7  is exerted in the opposite direction of P 5 , it does not interfere with the sealing engagement of seat  530  because the combination of shoulder  630  and chamber  740  prevents surface  620  of seat bushing  540  from coming into contact with seat  530 . Instead, pressure P 7  is countered by a reaction force at shoulder  630 . Accordingly, the differential in force resulting from pressure P 5  as compared to P 6  is sufficient to ensure a robust metal-to-metal seal at interface  710 . In addition, as noted above, as the pressure within interior bore  510  increases, the difference in force exerted by P 5  and P 6  will also increase and so the performance of the seal, and thus the valve, will improve as the interior pressure increases. The foregoing description of the operation of valve  400  in the open position applies equally to the upstream and downstream side of gate  520 . 
     When valve  400  is in the closed position, the operation of valve body  420 , gate  520 , seat  530 , and seat bushing  540  on the upstream side of gate  520  is essentially the same as that described above. Thus, the operation on the upstream side is independent of whether the valve is in the open or closed position. 
     It will be understood by those of skill in the art that seat  530  may include a pressure relief feature similar to that described above in connection with seat  130 , such that valve  400  will not experience extreme pressure differentials between cavity  406  and bore  510 . 
     When valve  400  is in the closed position, a seal is maintained on the downstream side of gate  520 , but potentially via a different mechanism. If pressure is equalized, such that there remains approximately equal pressure on both the upstream and downstream sides of gate  520 , then the sealing mechanism will be essentially the same as that described above when valve  400  is in the open position. However, if pressure is not equalized, such that upstream pressure exceeds downstream pressure, as shown in  FIG.  9   , pressure P 8  is exerted by gate  520  in an axial direction but there is no (or lesser) pressure acting in the opposite direction of pressure P 8 . Accordingly, pressure P 8  will tend to force gate  520  into seat  530  at interface  710 . In this way, when valve  400  is in the closed position, a seal is maintained on both the upstream and downstream sides of gate  520 , regardless of the relative pressure on either side of the gate. 
     Referring to  FIG.  9 A , an alternative embodiment of valve  400  is shown. Support  760  may be attached to seat  530  and extending in a generally radial direction, with biasing member  750  extending axially between support  760  and valve body  420 . Biasing member  750  may be a spring, a Belleville washer, or any other suitable device that is biased to exert axial pressure on support  760  in the direction of gate  520 . Support  760  may be a post, arm, spoke, or any radially extending structure configured to transmit the axial force exerted by biasing member  750 . As a result of the attachment between seat  530  and support  760 , the axial force exerted by biasing member  750  assists in maintaining a seal between seat  530  and gate  520 , particularly under low-pressure operating conditions. As shown in  FIG.  9 B , biasing member  750  may instead extend axially between support  760  and seat bushing  540 . 
     Referring to  FIG.  10   , an alternative embodiment of a valve  800  is shown. Similar to valve  100  shown in  FIG.  2   , extending between upstream flanged connection  810  and downstream flanged connection  830  is interior bore  805 . 
     Within valve body  820  is disposed gate  920 , seat  930 , seat bushing  940 , and body bushing  945 . Seat  930 , seat bushing  940 , and body bushing  945  are generally annular in shape and both located within recess  950  formed in the valve body. Seat  930 , seat bushing  940 , and body bushing  945  may be formed of metal, such as stainless steel. Alternatively, seat  930  may be formed of a material different from seat bushing  940  and/or body bushing  945 , in order to be more resistant to the forces exerted on seat  930  as a result of its sealing engagement with gate  920 . Cavity  806  is formed within valve body  820  and gate  920  moves within cavity  806 . Fluid may flow through interior bore  805  in the direction indicated by arrow F but, as noted above in connection with the other disclosed embodiments, fluid may also flow in the opposite direction and the valve will still function as described below. 
     Referring to  FIG.  11   , the downstream side of body bushing  945  comprises surface  1000 . Surface  1000  is adjacent to valve body  820  at interface  1100 . The upstream side of body bushing  945  comprises surface  1120 . The downstream side of seat  930  comprises surface  1130 . Surface  1120  of body bushing  945  is adjacent to surface  1130  of seat  930  at interface  1140 . As shown in  FIG.  11   , the area of surface  1120  and the area of surface  1130  are preferably substantially equivalent. 
     The upstream side of seat  930  comprises surface  1010 . Surface  1010  is adjacent to gate  920  at interface  1110 . As shown, seat  930  may have a generally “L-shaped” configuration, such that surface  1010  is smaller than surface  1130 . Similarly, surface  1000  of body bushing  945  may be smaller than surface  1120 . In addition, there is a radially projecting shoulder  1015  formed in the intermediate portion of seat  930 . Thus, the outer surface of seat  930  comprises two distinct portions, surface  1012  on the upstream side and surface  1014  on the downstream side. 
     The downstream side of seat bushing  940  comprises surface  1020 . As shown in  FIG.  11   , body bushing  945  has a stepped configuration which forms radially projecting shoulder  1030 . At interface  1025 , shoulder  1030  of body bushing  945  contacts a portion of surface  1020  on seat bushing  940 . The remainder of surface  1020  on seat bushing  940  does not make contact with any other portion of valve  800 . Instead, there is a chamber  1170  formed by portions of surface  1020 , shoulder  1015 , surface  1012 , and shoulder  1030 . Chamber  1170  will generally enclose an area of relatively low pressure, compared to the other portions of valve  800 . The upstream side of seat bushing  940  comprises surface  1040 . Surface  1040  does not contact any other portion of valve  800 . Seat bushing  940  also comprises bottom surface  1050 , which contacts surface  1012 . Seat  930  and seat bushing  940  make contact with each other at the interface formed between surface  1050  and surface  1012 . 
     In operation, when valve  800  is in the open position, the fluid within the interior bore  805  and cavity  806  will generally be the same pressure. The fluid will generally exert pressure P 10  on surface  1130  of seat  930  at interface  1140 . This pressure will be exerted in an axial direction, as shown by the arrows in  FIG.  12   . 
     Pressure P 11  will also be exerted in the opposite axial direction on surface  1010  of seat  930  at interface  1110 . Pressure P 12  will also be exerted, in the same axial direction as P 11 , on surface  1040  of seat bushing  940 . 
     Due to the difference in surface area between surface  1130  and surface  1010 , the total force (pressure times surface area) exerted by pressure P 10  is greater than the total force exerted by pressure P 11 . This differential in force tends to urge seat  930  into sealing engagement with gate  920  at interface  1110 . In addition, although pressure P 12  is exerted in the opposite direction of P 10 , it does not interfere with the sealing engagement of seat  930  because the combination of shoulder  1030  of body bushing  945  and chamber  1170  prevents surface  1020  of seat bushing  940  from coming into contact with seat  930 . Instead, pressure P 12  transfers to body bushing  945  by a reaction force P 13  at shoulder  1030 , causing body bushing  945  to axially engage valve body  820  at interface  1100 . Accordingly, the differential in force resulting from pressure P 10  as compared to P 11  is sufficient to ensure a robust metal-to-metal seal at interface  1110 . In addition, as noted above, as the pressure within interior bore  805  increases, the difference in force exerted by P 10  and P 11  will also increase and so the performance of the seal, and thus the valve, will improve as the interior pressure increases. 
     It will be understood by those of skill in the art that seat  930  may include a pressure relief feature similar to that described above in connection with seat  130 , such that valve  800  will not experience extreme pressure differentials between cavity  806  and bore  805 . 
     The foregoing description of the operation of valve  800  in the open position applies equally to the upstream and downstream side of gate  920 . When valve  800  is in the closed position, the operation of valve body  820 , gate  920 , seat  930 , seat bushing  940  and body bushing  945  on the upstream side of gate  920  is essentially the same as that described above. Thus, the operation on the upstream side is independent of whether the valve is in the open or closed position. 
     When valve  800  is in the closed position, a seal is maintained on the downstream side of gate  920 , but potentially via a different mechanism. If pressure is equalized, such that there remains approximately equal pressure on both the upstream and downstream sides of gate  920 , then the sealing mechanism will be essentially the same as that described above when valve  800  is in the open position. However, if pressure is not equalized, such that upstream pressure exceeds downstream pressure, as shown in  FIG.  13   , pressure P 14  is exerted by gate  920  in an axial direction but there is no (or lesser) pressure acting in the opposite direction of pressure P 14 . Accordingly, pressure P 14  will tend to force gate  920  into seat  930  at interface  1110 . Seat  930  will exert pressure P 15  on seat bushing  945  by virtue of the contact between surface  1120  of body bushing  945  and surface  1130  of seat  930  at interface  1140 . In this way, when valve  800  is in the closed position, a seal is maintained on both the upstream and downstream sides of gate  920 , regardless of the relative pressure on either side of the plug. 
     As also shown in  FIGS.  10 - 13   , additional seals may be disposed at the various interfaces between the surfaces of seat  930 , seat bushing  940 , and body bushing  945 . Although these seals may be elastomeric, similar to those described above in connection with the other embodiments, the embodiment shown in  FIGS.  10 - 13    provides at least one additional advantage. Because seat  930  does not directly contact valve body  820 , there is no need for any of the seals to be located in a recess formed by removing material from either seat  930  or valve body  820 . As shown in  FIGS.  10 - 13   , all seals may be located in grooves formed in seat bushing  940  or body bushing  945 , which aids in manufacturing and durability of the overall design of valve  800 . 
     As also shown in  FIGS.  10 - 13   , biasing member  1150  may be located between annular shoulder  1160  of body bushing  945  and valve body  820 . Biasing member  1150  may be a spring, a Belleville washer, or any other suitable device that is biased to exert axial pressure on annular shoulder  1160  in the direction of gate  920 . As a result of the contact between surface  1120  of body bushing  945  and surface  1130  of seat  930  at interface  1140 , the axial force exerted by biasing member  1150  assists in maintaining a seal between seat  930  and gate  920 , particularly under low-pressure operating conditions. 
     As also shown in  FIGS.  10 - 13   , seat bushing  940  and body bushing  945  may be connected through the use of attachment member  1180 . Attachment member  1180  may be a screw, pin, or any other suitable device to fixedly connect seat bushing  940  and body bushing  945 , ensuring that surface  1040  of seat bushing  940  does not contact gate  920 . 
     The addition of body bushing  945  has several potential benefits in comparison to the embodiment shown in  FIGS.  1 - 9   . The use of body bushing  945  allows for the use of a seat  930  that is significantly smaller than seat  130 . The seat is generally the component within this type of valve that must be replaced the most frequently, and it is often formed of materials that are more expensive than those used to form the other components. Accordingly, using a smaller seat makes the overall design of the valve more economical. In addition, as noted above, the use of body bushing  945  avoids potential problems associated with locating sealing elements within grooves formed in either seat  930  or valve body  820 . In addition, the use of body bushing  945  facilitates the use of biasing member  1150  to aid in low-pressure sealing. 
     Referring to  FIG.  12 A , an alternative embodiment of valve  800  is shown. Support  1170  may be attached to seat  930  and extending in a generally radial direction, with biasing member  1150  extending axially between support  1170  and body bushing  945 . Biasing member  1150  may be a spring, a Belleville washer, or any other suitable device that is biased to exert axial pressure on support  1170  in the direction of gate  920 . Support  1170  may be a post, arm, spoke, or any radially extending structure configured to transmit the axial force exerted by biasing member  1150 . As a result of the attachment between seat  930  and support  1170 , the axial force exerted by biasing member  1150  assists in maintaining a seal between seat  930  and gate  920 , particularly under low-pressure operating conditions. As shown in  FIG.  12 B , biasing member  1150  may instead extend axially between support  1170  and seat bushing  940 . 
     Referring to  FIG.  14   , an alternative embodiment of a valve  400  is shown. This embodiment shows the potential for use of a keyed seat bushing  940  and seat  931  relative to valve body  420  to facilitate removal of seat  931  from valve body  420 . Seat  931  may have a keyed portion at  1141  and seat bushing  940  may have a keyed portion at  1140 .  FIG.  14    shows the keyed portions when seat bushing  940  and seat  931  are installed in valve body  420  during standard operation of valve  400 .  FIG.  15    shows seat bushing  940  partially removed from valve body  420  such that keyed portion  1140  of seat bushing  940  is engaged with keyed portion  1141  of seat  931  during disassembly of valve  400 .  FIG.  15 A  shows seat bushing  940  removing seat  931  from valve body  420  via keyed portions  1140  and  1141 .  FIG.  16    shows the disengaged arrangement of keyed portions  1140  and  1141  to allow seat bushing  940  and seat  931  to be separated from each other when one of them is rotated. Thus, the operation of the valve would not be diminished through the use of the keyed seat  931  and seat bushing  940 , but rather maintenance cost and down time would be reduced because of the ability to more quickly change out a worn seat  931  in the valve  400 . 
     Referring to  FIG.  17 A , an alternative embodiment is shown in which valve  2100  comprises a removable bore-end connection to maintain the seat in position in relation to the flow barrier. Similar to the other embodiments described above, extending between upstream flanged connection  2101  and downstream flanged connection  2102  is interior bore  2103 . Upstream flanged connection  2101  comprises a portion of removable bore-end connection  1946 . Similarly, downstream flanged connection  2102  comprises a portion of removable bore-end connection  1945 . Removable bore-end connections  1945  and  1946  connect to valve body  1820  as described in further detail below. 
     Within valve body  1820  is disposed gate  1920 , and seat  1930 . Seat  1930  is generally annular in shape and located within recess  1950  formed in the valve body. Seat  1930  may be formed of metal, such as stainless steel. Cavity  1806  is formed within valve body  1820  and gate  1920  moves within cavity  1806 . Fluid may flow through interior bore  2103  in the direction indicated by arrow F but, as noted above, fluid may also flow in the opposite direction and the valve will still function as described below. 
     Referring to  FIG.  17 B , the downstream side of seat  1930  comprises surface  2120 . Surface  2120  is adjacent to removable bore-end connection  1945  at interface  2140 . The upstream side of seat  1930  comprises surface  2010 . Surface  2010  is adjacent to gate  1920  at interface  2110 . As shown, seat  1930  may have a generally “L-shaped” configuration, such that surface  2120  is larger than surface  2010 . In addition, there is a radially projecting shoulder  2015  formed in the intermediate portion of seat  1930 . Thus, the outer surface of seat  1930  comprises two distinct portions, surface  2012  on the upstream side and surface  2014  on the downstream side. 
     Recess  1950  is formed such that valve body  1820  comprises radially projecting shoulder  1955 . The downstream side of radially projecting shoulder  1955  comprises surface  2020 , while the upstream side comprises surface  2025 . Radially projecting shoulder  2015  of seat  1930  does not contact any portion of surface  2020 . Instead, there is a chamber  2170  formed by portions of surface  2020 , valve body  1820 , surface  2012 , and radially projecting shoulder  2015 . Chamber  2170  will generally enclose an area of relatively low pressure, compared to other portions of valve  2100 . 
     Radially projecting shoulder  1955  comprises bottom surface  2050 , which contacts surface  2012 . Valve body  1820  and seat  1930  make contact with each other at the interface formed between surface  2050  and surface  2012 . 
     Removable bore-end connection  1945  may be connected to valve body  1820  using threaded connection  1845 . Any suitable form of threaded connection may be used to connect the removable bore-end connection and the valve body. Alternatively, any other form of removable connection may be used to attach removable bore-end connection  1945  to valve body  1820 , including, for example, locking dogs, pins, lugs, a rotating collar, magnets, or a snap-fit connection. 
     In operation, when valve  2100  is in the open position, the fluid within the interior bore  2103  and cavity  1806  will generally be at the same pressure. The fluid will generally exert pressure P 21  on surface  2120  of seat  1930  at interface  2140 . This pressure will be exerted in an axial direction, as shown by the arrows in  FIG.  17 B . 
     Pressure P 22  will also be exerted in the opposite axial direction on surface  2010  of seat  1930  at interface  2110 . Due to the difference in surface area between surface  2120  and surface  2010 , the total force (pressure times surface area) exerted by pressure P 21  is greater than the total force exerted by pressure P 22 . This differential in force tends to urge seat  1930  into sealing engagement with gate  1920  at interface  2110 . 
     In addition, although pressure within cavity  1806  is exerted on the upstream side of valve body  1820 , it does not interfere with the sealing engagement of seat  1930  because the combination of radially projecting shoulder  1955  and chamber  2170  prevents surface  2020  of valve body  1820  from coming into contact with seat  1930 . Accordingly, the differential in force resulting from pressure P 21  as compared to P 22  is sufficient to ensure a robust metal-to-metal seal at interface  2110 . In addition, as noted above, as the pressure within interior bore  2103  increases, the difference in force exerted by P 21  and P 22  will also increase and so the performance of the seal, and thus the valve, will improve as the interior pressure increases. The foregoing description of the operation of valve  2100  in the open position applies equally to the upstream and downstream side of gate  1920 . 
     Additional seals  1990  may be disposed at the interfaces between surface  2050  of radially projecting shoulder  1955  of valve body  1820  and surface  2012  of seat  1930 , the interface between surface  2014  of seat  1930  and valve body  1820 , and/or the interface between the top surface of removable bore-end connection  1945  and valve body  1820 . Such seals may be elastomeric such as, for example, o-rings. 
     When valve  2100  is in the closed position, a seal is maintained on the downstream side of gate  1920  similar to the mechanism described above in connection with the various other embodiments. 
     Optionally, valve  2100  may comprise support  1940  attached to seat  1930  and extending in a generally radial direction, with biasing member  1960  extending axially between support  1940  and valve body  1820 . Biasing member  1960  may be a spring, a Belleville washer, or any other suitable device that is biased to exert axial pressure on support  1940  in the direction of gate  1920 . Support  1940  may be a post, arm, spoke, or any radially extending structure configured to transmit the axial force exerted by biasing member  1960 . As a result of the attachment between seat  1930  and support  1940 , the axial force exerted by biasing member  1960  assists in maintaining a seal between seat  1930  and gate  1920 , particularly under low-pressure operating conditions. 
     As shown in  FIG.  18 B , biasing member  1960  may instead be located within a cavity  1970  formed within removable bore-end connection  1945 . In this configuration, biasing member  1960  exerts an axial force on surface  2120  of seat  1930 . This axial force tends to urge seat  1930  in the direction of gate  1920 , which assists in maintaining a seal, particularly under low-pressure operating conditions. 
     Referring to  FIG.  19 A , an alternative embodiment is shown in which valve  3100  comprises a seat bushing partially disposed within a cavity formed in the valve body. Similar to the other embodiments described above, extending between upstream flanged connection  3101  and downstream flanged connection  3102  is interior bore  3103 . 
     Within valve body  2820  is disposed gate  2920 , and seat  2930 . Seat  2930  is generally annular in shape and located within cavity  2806  formed within valve body  2820 . In one embodiment, as shown in  FIG.  19 A , a portion of seat  2930  may optionally be disposed within a recess formed in valve body  2820 . Gate  2920  also moves within cavity  2806 . Seat  2930  may be formed of metal, such as stainless steel. Fluid may flow through interior bore  3103  in the direction indicated by arrow F but, as noted above, fluid may also flow in the opposite direction and the valve will still function as described below. 
     Referring to  FIG.  19 B , the downstream side of seat  2930  comprises surface  3120 . Surface  3120  is adjacent to valve body  2820  at interface  3140 . Interface  3140  may optionally be located within a recess formed in valve body  2820 , as shown in  FIG.  19 A . The upstream side of seat  2930  comprises surface  3010 . Surface  3010  is adjacent to gate  2920  at interface  3110 . As shown, seat  2930  may have a generally “L-shaped” configuration, such that surface  3120  is larger than surface  3010 . In addition, there is a radially projecting shoulder  3015  formed towards the downstream portion of seat  2930 . Thus, the outer surface of seat  2930  comprises two distinct portions, surface  3012  on the upstream side and surface  3014  on the downstream side. 
     Valve  3100  also includes seat bushing  2935 , which comprises upstream portion  3015  and downstream portion  3020 . Upstream portion  3015 , which comprises radially projecting shoulder  3055  and surface  3016 , is disposed within cavity  2806 . Downstream portion  3020  is disposed within cavity  2950  formed in valve body  2820 . 
     The downstream side of radially projecting shoulder  3055  comprises surface  3020 , while the upstream side comprises surface  3025 . Radially projecting shoulder  3015  of seat  2930  does not contact any portion of surface  3020 . Instead, there is a chamber  3170  formed by portions of surface  3020 , surface  3016 , surface  3012 , and radially projecting shoulder  3015 . Chamber  3170  will generally enclose an area of relatively low pressure, compared to other portions of valve  3100 . 
     Radially projecting shoulder  3055  of seat bushing  2935  comprises bottom surface  3050 , which contacts surface  3012 . Seat bushing  2935  and seat  2930  make contact with each other at the interface formed between surface  3050  and surface  3012 . 
     In operation, when valve  3100  is in the open position, the fluid within the interior bore  3103  and cavity  2806  will generally be at the same pressure. The fluid will generally exert pressure P 31  on surface  3120  of seat  2930  at interface  3140 . This pressure will be exerted in an axial direction, as shown by the arrows in  FIG.  19 B . 
     Pressure P 32  will also be exerted in the opposite axial direction on surface  3010  of seat  2930  at interface  3110 . Due to the difference in surface area between surface  3120  and surface  3010 , the total force (pressure times surface area) exerted by pressure P 31  is greater than the total force exerted by pressure P 32 . This differential in force tends to urge seat  2930  into sealing engagement with gate  2920  at interface  3110 . 
     In addition, although pressure within cavity  2806  is exerted on the upstream portion  3015  of seat bushing  2935 , it does not interfere with the sealing engagement of seat  2930  because the combination of radially projecting shoulder  3055  and chamber  3170  prevents surface  3020  of seat bushing  2935  from coming into contact with seat  2930 . Accordingly, the differential in force resulting from pressure P 31  as compared to P 32  is sufficient to ensure a robust metal-to-metal seal at interface  3110 . In addition, as noted above, as the pressure within interior bore  3103  increases, the difference in force exerted by P 31  and P 32  will also increase and so the performance of the seal, and thus the valve, will improve as the interior pressure increases. The foregoing description of the operation of valve  3100  in the open position applies equally to the upstream and downstream side of gate  2920 . 
     Additional seals  2990  may be disposed at the interfaces between surface  3050  of radially projecting shoulder  3055  of seat bushing  2935  and surface  3012  of seat  2930 , the interface between surface  3014  of seat  2930  and surface  3016  of the upstream portion  3015  of seat bushing  2935 , and/or the interface between the lower surface of the downstream portion  3020  of seat bushing  2935  and valve body  2820 . Such seals may be elastomeric such as, for example, o-rings. 
     When valve  3100  is in the closed position, a seal is maintained on the downstream side of gate  2920  similar to the mechanism described above in connection with the various other embodiments. 
     Optionally, valve  3100  may comprise support  2940  attached to seat  2930  and extending in a generally radial direction, with biasing member  2960  extending axially between support  2940  and seat bushing  2935 . Biasing member  2960  may be a spring, a Belleville washer, or any other suitable device that is biased to exert axial pressure on support  2940  in the direction of gate  2920 . Support  2940  may be a post, arm, spoke, or any radially extending structure configured to transmit the axial force exerted by biasing member  2960 . As a result of the attachment between seat  2930  and support  2940 , the axial force exerted by biasing member  2960  assists in maintaining a seal between seat  2930  and gate  2920 , particularly under low-pressure operating conditions. 
     As shown in  FIG.  20 B , biasing member  2960  may instead be located within a cavity  2970  formed within valve body  2820 . In this configuration, biasing member  2960  exerts an axial force on surface  3120  of seat  2930 . This axial force tends to urge seat  2930  in the direction of gate  2920 , which assists in maintaining a seal, particularly under low-pressure operating conditions. 
     It is understood that variations may be made in the foregoing without departing from the scope of the present disclosure. In several exemplary embodiments, the elements and teachings of the various illustrative exemplary embodiments may be combined in whole or in part in some or all of the illustrative exemplary embodiments. In addition, one or more of the elements and teachings of the various illustrative exemplary embodiments may be omitted, at least in part, and/or combined, at least in part, with one or more of the other elements and teachings of the various illustrative embodiments. 
     Any spatial references, such as, for example, “upper,” “lower,” “above,” “below,” “between,” “bottom,” “vertical,” “horizontal,” “angular,” “upwards,” “downwards,” “side-to-side,” “left-to-right,” “right-to-left,” “top-to-bottom,” “bottom-to-top,” “top,” “bottom,” “bottom-up,” “top-down,” etc., are for the purpose of illustration only and do not limit the specific orientation or location of the structure described above. 
     In several exemplary embodiments, while different steps, processes, and procedures are described as appearing as distinct acts, one or more of the steps, one or more of the processes, and/or one or more of the procedures may also be performed in different orders, simultaneously and/or sequentially. In several exemplary embodiments, the steps, processes, and/or procedures may be merged into one or more steps, processes and/or procedures. 
     In several exemplary embodiments, one or more of the operational steps in each embodiment may be omitted. Moreover, in some instances, some features of the present disclosure may be employed without a corresponding use of the other features. Moreover, one or more of the above-described embodiments and/or variations may be combined in whole or in part with any one or more of the other above-described embodiments and/or variations. 
     Although several exemplary embodiments have been described in detail above, the embodiments described are exemplary only and are not limiting, and those skilled in the art will readily appreciate that many other modifications, changes and/or substitutions are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications, changes, and/or substitutions are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, any means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Moreover, it is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the word “means” together with an associated function.