Abstract:
A gate valve is disclosed which comprises a valve body which includes a flow bore that extends completely therethrough and a gate cavity that extends partially therethrough and intersects the flow bore, a bonnet which is connected to the valve body over the gate cavity, a gate which is disposed in the gate cavity across the flow bore and includes a transverse opening that extends completely therethrough, a valve stem which includes a first portion that is connected to the gate and a second portion that extends through the gate cavity, a handwheel, and a roller screw assembly which is connected between the handwheel and the second portion of the valve stem and which converts rotation of the handwheel into translation of the valve stem. In this manner, translation of the valve stem raises or lowers the gate to bring the opening into or out of alignment with the flow bore to either open or close the gate valve, respectively.

Description:
This application is based on U.S. Provisional Patent Application No. 60/314,985, which was filed on Aug. 24, 2001. 

   BACKGROUND OF THE INVENTION 
   The present invention is directed to a gate valve. More particularly, the invention is directed to a manually actuated, rising stem gate valve which includes a roller screw assembly to efficiently convert rotation of the handwheel into translation of the valve stem. 
   Gate valves are used in a variety of industries to control the flow of fluids. In particular, gate valves are used extensively in the oil and gas industry to control the flow of produced fluids at various stages of production. Most gate valves used in this industry comprise a valve body having a longitudinal flow bore and a transverse gate cavity that intersects the flow bore. A gate having a gate opening extending transversely therethrough is disposed in the gate cavity. A stem is provided for moving the gate between an open position, in which the gate opening is aligned with the flow bore, and a closed position, in which the gate opening is offset from the flow bore. The gate is usually positioned between a pair of seats, each of which seals against the gate under pressure to prevent fluid from passing through the flow bore when the gate is in the closed position. 
   The gate cavity is normally covered by a bonnet having an axial through bore. The stem passes through the through bore and is sealed to the bonnet by a stem packing to contain the fluid pressure within the gate cavity. Many gate valves are also provided with a backseat mechanism, that is, cooperating sealing surfaces on the stem and the bonnet which are located below the stem packing. Often a desire exists to perform maintenance or repair on the gate valve, such as replacing the stem packing, without removing the gate valve from the conduit system to which it is connected. In such instances, the stem is moved upwardly until the backseat sealing surfaces on the stem and the bonnet engage and form a metal-to-metal seal. This backseating procedure thus isolates the stem from the gate cavity and allows the desired maintenance to be performed without having to remove the gate valve from the conduit system. For safety reasons, the pressure in the gate cavity is bled down to ambient pressure before any maintenance is performed. In addition, any residual pressure between the stem packing and the backseat is usually bled off through a bleeder plug provided in the bonnet. 
   Gate valves are provided with means for manipulating the stem to raise and lower of the gate. In this respect, gate valves may be divided into two groups: (a) rising stem gate valves and (b) non-rising stem gate valves. In a non-rising (or rotating) stem gate valve, the stem is threadedly connected to the gate such that rotation of the stem causes the gate to move up and down. An actuation mechanism is provided for selectively rotating the stem clockwise or counterclockwise in order to open or close the gate valve. On this type of gate valve, the backseat is set by driving the gate down until it bottoms out on the valve body, and then allowing the stem to move upward until it backseats against the bonnet. Such valves may be automatically or remotely actuated, such as by an electric motor. Alternatively, these gate valves may be manually actuated, such as by a handwheel adapted to rotate the stem directly. An example of such a manual gate valve is shown in U.S. Pat. No. 5,762,320 to Williams et al. 
   In a rising stem gate valve, the stem is attached to the gate in a manner which prevents axial movement of the stem relative to the gate. A mechanism is then provided for selectively driving the stem up and down in order to open and close the valve. On this type of gate valve, the backseat is set by moving the stem and the gate upwards until the stem backseats against the bonnet. Such valves may be automatically or remotely actuated, such as by a hydraulic cylinder. Alternatively, these valves may be manually actuated by providing a transmission means to convert the rotational motion of a handwheel into axial motion of the stem. 
   One such transmission means is a direct threaded connection between the handwheel and the stem. Unfortunately, for many large or high pressure valves which require large actuating forces, this method requires more torque to be applied to the handwheel than is practical to exert by hand. When the valve is closed, the entire upstream side of the gate is exposed to the full working pressure of the fluid while a portion of the downstream side of the gate is often at ambient pressure. This pressure differential results in very high forces which push the gate against the downstream seat. This engagement between the gate and the downstream seat in turn creates large gate-to-seat drag forces which must be overcome when gate is moved from the closed position to the open position. Another force which must be overcome is the drag which the stem packing exerts on the stem. 
   Rising stem gate valves can be further divided into two types: (a) balanced stem gate valves and (b) un-balanced stem gate valves. In a balanced stem gate valve, a second stem is attached to the gate at the end opposite the first stem. An example of this type of gate valve is shown in U.S. Pat. No. 4,230,299 to Pierce, Jr. It will be appreciated that when pressurized fluid is present in the gate cavity, a force is exerted on each stem which is equal to the product of the pressure and the cross-sectional area of the stem where it passes through the stem packing. In a balanced stem gate valve, the forces acting on the two stems will cancel each other out, resulting in substantially zero (or a balanced) net force to overcome when moving the gate. The disadvantages of balanced stem gate valves include increased cost and complexity and the creation of an additional potential leak path between the second stem and its corresponding stem packing. 
   An example of an unbalanced stem gate valve is disclosed in U.S. Pat. No. 4,569,503 to Karr, Jr. Although in this type of gate valve the unbalanced stem forces must be overcome when moving the gate, it will be appreciated that this design is simpler than the balanced stem gate valve. In the valve shown in Karr, Jr., the gate opening is disposed in the upper part of the gate, such that the valve is open when the gate is in its lowered position and closed when the gate is in its raised position. The disadvantage of this configuration is that when the valve is moved from the closed position to the open position, both the unbalanced stem force and the maximum gate-to-seat drag forces must be overcome simultaneously. 
   In order to overcome these combined forces and still maintain the required handwheel torque at an acceptable level, a transmission means which provides a substantial mechanical advantage must usually be utilized. Karr, Jr. provides a ball screw device for raising and lowering the stem. Other valves utilize bevel or worm gear reduction boxes. One disadvantage of these devices is that, in order to sufficiently reduce the required torque on the handwheel, the gear ratio must be very high. Consequently, a large number of turns is required to open or close the valve. Moreover, since the rate at which an operator can turn the handwheel is limited, the gate necessarily traverses very slowly from one position to the other. 
   This relatively slow traverse is especially troublesome when moving the gate from the closed position to the open position. As soon as the gate opening intersects the flow bore in the downstream seat, the gate-to-seat seal is broken and a high velocity jet of fluid is forced through the intersection area. In many cases, the fluid may contain abrasive particles which tend to erode the valve components during high velocity flow. The longer the intersection area remains small, the longer it takes for pressure to equalize on the opposites sides of the gate. Thus, the slower the gate moves to the open position, the greater the amount of erosion. 
   A further disadvantage of the gate valves shown in the Williams et al., Pierce, Jr. and Karr, Jr. patents is that these valves must be in the closed position in order to backseat the stem against the bonnet. Consequently, multiple actuations of the valve are required to ensure that both the gate cavity and the bonnet are at ambient pressure. Typically, the valve must first be actuated to the open position in order to bleed down the system pressure on both sides of the valve. Then the valve must be actuated to the closed position in order to backseat the stem against the bonnet. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, these and other disadvantages in the prior art are overcome by providing a gate valve which comprises a valve body which includes a flow bore that extends completely therethrough and a gate cavity that extends partially therethrough and intersects the flow bore, a bonnet which is connected to the valve body over the gate cavity, a gate which is disposed in the gate cavity across the flow bore and includes a transverse opening that extends completely therethrough, a valve stem which includes a first portion that is connected to the gate and a second portion that extends through the gate cavity, a handwheel, and a roller screw assembly which is connected between the handwheel and the second portion of the valve stem and which converts rotation of the handwheel into translation of the valve stem. In this manner, translation of the valve stem raises or lowers the gate to bring the opening into or out of alignment with the flow bore to either open or close the gate valve, respectively. 
   In comparison to prior art gate valves, the roller screw assembly requires substantially fewer turns of the handwheel in order to raise and lower the gate. Thus, assuming the operator turns the handwheel at a constant rate, the gate in the gate valve of the present invention will rise substantially faster than the gates in prior art gate valves. Consequently, any erosion of the gate which may occur when the opening first intersects the flow bore will be minimized. 
   These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of the gate valve of the present invention shown in the closed position; 
       FIG. 2  is a cross sectional view of the lower portion of the gate valve shown in  FIG. 1 ; and 
       FIG. 3  is a cross sectional view of the upper portion of the gate valve shown in FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , the gate valve of the present invention, which is indicated generally by reference number  10 , is shown to comprise a valve body  12  which comprises a flow bore  14  that extends longitudinally through the valve body between a first port  16  and a second port  18  and a gate cavity  20  that extends partially through the valve body generally transverse to the flow bore. The gate valve  10  also comprises a pair of seats  22   a ,  22   b , each of which is positioned at least partially in a corresponding seat pocket that is formed at the intersection of the flow bore  14  and the gate cavity  20 . Each seat  22   a ,  22   b  includes a transverse bore  24   a ,  24   b  that is aligned with the flow bore  14  to thereby define a flow passage through the valve body  12 . 
   Referring also to  FIG. 2 , the gate valve  10  further includes a gate  26  which is slidably disposed between the seats  22   a ,  22   b , each of which is preferably urged into contact with the gate by a respective Belleville spring  28   a ,  28   b . The gate  26 , which in an exemplary embodiment of the invention comprises a generally rectangular configuration, includes an upper end  30 , a lower end  32  and a transverse opening  34  that extends completely through the gate proximate the lower end. The gate  26  is secured to a stem  36  by a lift nut  38 , which is mounted in a corresponding recess in the upper end  30  of the gate and threaded to the lower end of the stem. In addition, the stem  36  is secured against rotation relative to the lift nut  38  and the gate  26  by a pin  40 . Thus, any axial translation of the stem  36  will result in a corresponding translation of the gate  26 . 
   The gate  26  is adapted for reciprocal motion between an upper or open position and a lower or closed position, the latter of which is shown in the Figures. In the open position, the opening  34  in the gate is substantially aligned with the bores  24   a ,  24   b  in the seats  22   a ,  22   b , and fluid is permitted to flow though the flow bore  14  between the first and second ports  16 ,  18 . In the closed position, the opening  34  is offset from the bores  24   a ,  24   b  and the gate  26  thus blocks the flow of fluid through the flow bore  14 . Furthermore, since the gate  26  is secured to the stem  36 , the gate valve  10  may be selectively opened or closed by raising or lowering the stem. 
   Referring again to  FIG. 1 , the gate valve  10  also includes a bonnet  42  which is secured to the valve body  12  over the gate cavity  20  by suitable means, such as a number of bolts  44  and nuts  46 . The bonnet  42  is preferably sealed to the valve body  12  via a bonnet gasket  48 . The stem  36  extends upwardly through the bonnet  42  and is sealed thereto by a stem packing  50 . The stem packing  50  is retained within the bonnet  42  by a packing nut  52 , which is threadedly connected to the bonnet generally at  54 . 
   Referring also to  FIG. 3 , the gate valve  10  further includes a generally cylindrical bonnet cap  56 , which is secured to the bonnet  42  such as by threads  58 . One or more set screws  60  may be provided to prevent relative rotation between the bonnet cap  56  and the bonnet  42 . In addition, an O-ring  62  is ideally positioned between the bonnet cap  56  and the bonnet  42  to provide a seal therebetween. 
   Referring still to  FIG. 3 , the gate valve  10  further comprises a roller screw assembly  64  which is positioned in the bonnet cap  56  immediately above a thrust bushing  66 , which in turn is mounted in the bonnet cap immediately above the bonnet  42 . The roller screw assembly  64  includes a roller screw nut  68  which is rotatably connected to a roller screw shaft  70 . In a preferred embodiment of the invention, the roller screw nut  68  and the roller screw shaft  70  comprise a planetary roller screw assembly such as is shown and described in the “SKF® roller screws” leaflet published by the SKF Group (Catalogue No. 4351/5E, June 1999, France), which is hereby incorporated herein by reference. In this type of roller screw assembly, the roller screw nut  68  comprises a central bore having internal threads, and the roller screw shaft  70  comprises an outer diameter having corresponding external threads. In addition, disposed between and engaging the internal and external threads are a plurality of threaded rollers (not shown in the Figures), which serve to convert the rotational motion of the roller screw nut  68  into axial translation of the roller screw shaft  70 . 
   The lower end of the roller screw shaft  70  is connected to the upper end of the stem  36  such as by threads  72 . The roller screw shaft  70  and the stem  36  are ideally also pinned together to prevent relative rotation therebetween. Thus, it can be appreciated that the roller screw shaft  70 , the stem  36 , the lift nut  38  and the gate  26  will all translate axially as a unit, but will not rotate relative to each other. 
   The roller screw nut  68  comprises a radially outwardly extending flange  74  which is disposed between a roller thrust bearing  76  and a roller adapter shaft  78 . In addition, the flange  74  is preferably pinned to the roller thrust bearing  76  and the roller adapter shaft  78  via one or more pins  80  so that the roller screw nut  68 , the roller thrust bearing and the roller adapter shaft will rotate as a unit. This unit is supported on a lower roller bearing  82  which is mounted between the roller thrust bearing  76  and the bonnet cap  56  and which in turn is supported on the thrust bushing  66 . Furthermore, an upper roller bearing  84  is mounted between the roller adapter shaft  78  and the bonnet cap  56 . In a manner well known in the mechanical arts, the roller bearings  82 ,  84  serve to guide the roller thrust bearing  76  and the roller adapter shaft  78  as they rotate, while reducing rotational drag on these components. Roller bearings  82 ,  84  further serve to transmit axial and radial loads from the roller thrust bearing  76  and the roller adapter shaft  78  to the thrust bushing  66  and the bonnet cap  56 . One or more grease fittings  86  and  88  may be provided on the bonnet cap  56  and the roller adapter shaft  78 , respectively, for providing lubrication to the roller bearings  82 ,  84  and the roller screw assembly  64 . 
   The upper end of the roller adapter shaft  78  extends beyond the bonnet cap  56  and terminates in a handwheel adapter portion  90 . As shown in  FIG. 1 , a Handwheel  92  is attached to the adapter portion  90  by suitable means, such as a pin  94 . 
   Referring again to  FIG. 3 , the gate valve  10  also includes a bonnet cap adapter  96 , which is preferably threadedly connected to the bonnet cap  56  generally at  98 . In addition, one or more set screws  100  may be provided to prevent rotation of the bonnet cap adapter  96  relative to the bonnet cap  56 . The bonnet cap adapter  96  serves to retain a packing  102  which is disposed between the roller adapter shaft  78  and an upper portion of the bonnet cap  56 . A wiper ring  104  is ideally positioned between bonnet cap adapter  96  and the roller adapter shaft  78  to prevent contamination of the packing  102 . 
   A primary purpose of the packing  102  is to impart rotational drag to the roller adapter shaft  78 . Because the gate valve  10  is an unbalanced stem gate valve, fluid pressure in the gate cavity  20  will impart an upward force on the stem  36 . Since the roller screw assembly  64  is somewhat susceptible to backdrive, this upward force could move the gate  26  upwards toward the open position. Therefore, the size, design, material and preload of packing  102  should be selected to provide an optimal amount of drag on the roller adapter shaft  78  to prevent this backdrive. 
   The normal operation of the gate valve  10  will now be described. In order to open the gate valve  10 , an operator applies a torque to the handwheel  92 . This torque is transmitted to the roller adapter shaft  78  via the pin  94 , and then to the roller screw nut  68  and the roller thrust bearing  76  via the pins  80 . As described above, the roller screw assembly  64  serves to convert the torque on the roller screw nut  68  into an upward axial force on the roller screw shaft  70 . The resulting downward axial reaction force on the roller screw nut  68  is transmitted through the roller thrust bearing  76 , the lower roller bearing  82 , the thrust bushing  66  and the bonnet  42  to the valve body  12 . It should be noted that the roller screw assembly  64  provides an improved mechanical advantage over the ball screw devices utilized in the prior art. Consequently, less applied torque is required at the handwheel  92  in order to actuate the gate valve  10 . 
   As the handwheel  92  is actuated to open the gate valve  10 , the resulting upward axial force on the roller screw shaft  70  is transmitted through the stem  36 , the pin  40  and the lift nut  38  to the gate  26 . As the torque applied to handwheel  92  is increased, the upward axial force on the gate  26 , the stem  36  and the roller screw shaft  70  increases accordingly. When this upward force is sufficient to overcome the sum of the gate-to-seat drag between the gate  26  and the seats  22   a ,  22   b  and the stem-to-packing drag between the stem  36  and the packing  50 , the gate, the stem and the roller screw shaft  70  will begin to rise towards the open position. At this point, the advantage of using a standard gate  26 , with the gate opening  34  proximate the lower end  32 , will become apparent. In most gate valves, greater force is required to open the valve than to close the valve. This is because in the closed position, the full differential pressure across the gate creates the greatest gate-to-seat drag. By using a standard gate  26  in the gate valve of the present invention, the upward force acting on the unbalanced stem will help to move the gate into the open position. 
   As the operator continues to apply torque to the handwheel  92 , the handwheel will rotate and the gate  26  will rise. For simplicity sake, let us assume that the high pressure side of the gate valve  10  is to the left of the gate  26  (as viewed in the Figures). Thus, when the gate valve  10  is open, flow will proceed through the flow bore  14  from the first port  16  to the second port  18 . As discussed above, when the upper edge of gate opening  34  reaches the bore  24   b  in the downstream seat  22   b , a high velocity jet of fluid will be forced through the intersection of the gate opening and the bore. Therefore, it is desirable to raise the gate  26  as quickly as possible in order to minimize any erosion which may be caused by this jet of fluid. 
   At this point an additional advantage of the roller screw assembly  64  over prior art ball screws will be made apparent. The roller screw assembly  64  requires substantially fewer turns on the handwheel  92  in order to raise the roller screw shaft  70  the distance required to fully open the gate  26 . Thus, assuming the operator turns the handwheel  92  at a constant rate, the gate  26  will rise substantially faster in the gate valve  10  than in prior art gate valves which utilize ball screws. Consequently, erosion of the gate opening  34  and the bore  24   b  of the downstream seat  22   b  will be minimized. 
   As the gate  26  is raised still further, the gate opening  34  will eventually become substantially aligned with bores  24   a ,  24   b  in the seats  22   a ,  22   b . In this configuration, the gate valve  10  is fully open and fluid may flow freely through the flow bore  14 . As the gate  26  reaches its fully open position, a back seat surface  106  on the stem  36  will engage a corresponding backseat surface  108  on the bonnet  42  and create a metal-to-metal backseat seal. At this point, an additional advantage to using a standard gate  26  will become apparent. With the gate valve  10  thus actuated to the open position and simultaneously backseated, pressure may be bled off from both sides of the gate  26  and from the bonnet  42  without having to actuate the gate valve a second time. Thus, fewer steps are required to ensure that no gage pressure exists in the gate cavity  20  and the bonnet  42 . Consequently, maintenance and repair functions, such as replacing the stem packing  50 , may be safely performed relatively easily and inexpensively. 
   When it is desired to close the gate valve  10 , the operator rotates the handwheel  92  in the opposite direction, and the roller screw shaft  70 , the stem  36  and the gate  26  are driven downward toward the closed position. The unbalanced stem force now opposes the motion of the gate  26 . However, this opposing force is offset by the fact that essentially no gate-to-seat drag exists when the gate valve  10  is open since no differential pressure exists across the gate  26 . Moreover, the upward reaction force exerted on the roller screw nut  68  is transmitted through the roller adapter shaft  78 , the upper roller bearing  84 , the bonnet cap  56  and the bonnet  42  to the valve body  12 . 
   It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.