Patent Publication Number: US-2010115772-A1

Title: Method for lapping, grinding, honing and polishing seal surfaces on inner diameter of semi-blind cavity in valve body

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application 61/113,326, filed Nov. 11, 2008. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to fabricating valve bodies and, in particular, to an improved system, method and apparatus for lapping, grinding, honing and polishing seal surfaces on inner diameters of semi-blind cavities in valves bodies. 
     BACKGROUND OF THE INVENTION 
     A valve is a device that generally regulates the flow of a material by opening, closing, or partially obstructing a path through the valve. The material may be a gas, a liquid, a fluidized solid, or slurry. A variety of different types of valves exist, such as gate valves, and globe valves. 
     A gate valve typically consists of a valve body, a bonnet, a valve member, and a valve seat. The valve body and the bonnet form the area in the valve that contains and directs material through the valve. The valve body, for example, may have a bore that extends through the valve body. The valve member interacts with the valve body to control the flow of material passing through the valve by being positioned to close or restrict flow. For example, a sliding gate with an opening can function as a valve member such that the opening aligns with the passage in the valve body to allow flow. Alternatively, a solid portion of the sliding gate can be aligned partially or fully with the passage to restrict or block flow. The bonnet is attached to the valve to hold other valve components, such as the valve member, in place and can be removed to provide access to the internal parts during maintenance. 
     In a gate valve, the valve seat is the interior surface in the valve body that contacts the gate to form a seal. The gate comes into contact with the seat when the valve is closed. The body and the seat could both come in a single piece of solid material or, alternatively, the seat could be a separate valve part that is attached or fixed to a seat pocket on the inside of the valve body. 
     When the seat is a separate valve part, the dimensions of the seat and seat pocket must correspond or the seat will not sit properly within the seat pocket of the valve body and leakage may occur. Thus, it is important that the dimensions of the seat pocket are precisely machined to prevent form errors and surface finish errors. The finishing tools used to correct or prevent such errors can thus be very expensive. Therefore, a more cost-effective technique for lapping, grinding, honing, and polishing seal surfaces on the seat pockets of a valve is desired. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present technique, a tool assembly comprising a cylindrical member, wheels of varying diameter and abrasive surface, and a rod for applying torque, are provided. A tool assembly allows for lapping, grinding, honing and polishing seal surfaces on inner diameters of semi-blind cavities in valve bodies. The tool can bring seat surfaces up to specification with the abrasive wheels having varying grit sizes. The wheels are provided in an array of sizes to form a set of tools for a range of diameters, abrasion and surface finish capacity. This design allows the first (i.e., smallest) tool to enter a cavity in a valve body to begin cutting. The wheel is attached to an end of the cylindrical member and rotated by applying torque through the rod that may traverse the opposite end of the cylindrical member. Once the small diameter wheel has completed its cutting operation, it can be replaced with the larger size wheels to increase the diameter of the cavity, or valve seat, being machined until the desired size is achieved. Wheels with finer abrasive can then used to improve the surface finish of the cavity while minimizing the material removal. When the desired diameter and/or surface finish has been achieved, a felt wheel may be used with a fine abrasive paste to provide the cavity with the final surface finish specification. 
     The use of the tool assembly to obtain a desired diameter and finish for valve seat sealing surfaces is cost-effective. In the past, expensive and specialized machining tools were required to obtain the desired diameter and finish. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the present invention are attained and can be understood in more detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings. However, the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a sectional isometric view of one embodiment of a gate valve constructed in accordance with the invention; 
         FIG. 2  is an enlarged sectional isometric view of one embodiment of a portion of a seat and seat pocket of the gate valve of  FIG. 1 , and is constructed in accordance with the invention; 
         FIG. 3  is a schematic side view of one embodiment of a surface finishing tool for gate valves and is constructed in accordance with the invention; 
         FIG. 4  is an isometric view of one embodiment of a set of tooling for the surface finishing tool of  FIG. 3 ; and 
         FIG. 5  is a schematic side view of another embodiment of a surface finishing tool for gate valves and is constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-5  depict embodiments of an improved system, method and apparatus for lapping, grinding, honing and polishing seal surfaces on inner diameters of semi-blind cavities in valves bodies. With respect to  FIG. 1 , the present invention is described as it may be applied in conjunction with an exemplary technique, in this case a technique for finishing a surface within the bore of a gate valve assembly for controlling the flow of a fluid, such as oil and/or gas, and is represented generally by reference numeral  20 . In addition, the illustrated embodiment of the gate valve  20  is a grease-less valve. However, the present technique may be used in valves other than gate valves and other than grease-less valves. 
     In the illustrated embodiment, the gate valve assembly  20  comprises a valve body  22  having a bore  24  extending though the valve body  22 . The bore  24  has a first opening  26  and a second opening  28 . In this embodiment, the gate valve  20  is a bi-directional valve. Therefore, the first opening  26  may be used as an inlet to the bore  24  in one configuration and as an outlet in another configuration, as can the second opening  28 . 
     In addition, this embodiment of the valve  20  has a valve cavity  30  that is covered by a bonnet  32 . A pair of seats  34  extends into the cavity  30  from seat pockets  36  formed on opposite sides of the cavity  30  in the bore  24  through the valve body  22 . A gate  38  is housed in the cavity  30  between the seats  34 . The gate  38  has an opening  40  and a solid portion  42  that are positioned to control flow through the gate valve  20 . When the gate  38  is positioned with the opening  40  aligned with the bore  24 , the valve  20  is open and fluids are able to pass through the bore  24  via the opening  40  in the gate  38 . When the gate  38  is positioned with the solid portion  42  aligned with the bore  24 , the valve  20  is closed and fluids are blocked from flowing through the bore  24  by the solid portion  42  of the gate  38 . 
     The valve  20  also has a valve stem  44  that extends through the bonnet  32  to enable a user to position the gate  38  in either the open or closed configuration. A hand wheel (not shown) or some other actuator may be used to position the valve stem  44 . For example, a hydraulic actuator may be used to control the position of the gate  38 . An electrical or pneumatic actuator may be used, as well. 
     Referring generally to  FIG. 2 , the seat  34  has a seat seal  46  that is used to form a seal between the seat  34  and the seat pocket  36 . The seat seal  46  prevents flow from leaking from the bore  24  via the seat  34 . In this embodiment, the seat seal  46  has a U-shaped portion  48  with a pair of sealing surfaces  50  that contact the seat pocket  36  on one side and the seat  34  on the opposite side of the seat seal  46 . A standoff ring  52  is provided to extend between the back face  54  of the seat pocket  36  and the U-shaped sealing portion  48  of the seat seal  46 . When installed in the seat pocket  36 , the standoff ring  52  abuts the seat pocket back face  54  and urges the U-shaped sealing portion  48  of the seat seal  46  outward so that the sealing surfaces  50  of the seat seal  46  make contact with the seat pocket  36  and seat  34 , respectively. In addition, the seat  34  has a seat spring  56  that urges the seat  34  against the gate  38 . 
     The correspondence between the seat seal  46  and the seat pocket  36  enables a seal to be maintained without the use of grease. However, if the surface profile of the seat pocket  36  is rough or is not round, the sealing surfaces  50  of the seat seal  46  may not maintain a seal. Therefore, leakage from the bore  24  may occur. The surface profile of the seat pocket  36  may be too rough for a proper seal due to chatter from the machining operations used to form the seat pocket  36 , incidental damage to the surface finish, or some other cause. Similarly, the surface profile of the seat pocket  36  may have been machined somewhat oval rather than round. The surface profile of the seat pocket  36  may be repaired if it is not sufficiently smooth and/or round. For example, the seat pocket  36  may be machined to re-bore the seat pocket  36 . 
     Referring now to  FIGS. 3 and 4 , embodiments of an improved system, method and apparatus for lapping, grinding, honing and polishing seal surfaces on the inner diameters of semi-blind cavities in valves bodies are shown. In one embodiment, a round, fixed abrasive tool  61  has an abrasive wheel  63  fixed to the distal end  64  of an outside diameter of a central cylindrical shaft  65 . In one embodiment ( FIG. 5 ), the shaft  65  may be supported on or adjacent its ends  64 ,  66  by bushings  70 . The grit sizes of the abrasive on the wheels  63  (see, e.g.,  FIG. 4 ) may be provided in the range of 325 to 1200 mesh, for example. The wheels  63  are provided in an array of sizes, as shown, to form a set of tools for a range of diameters, abrasion and surface finish capacity. This design allows the first (i.e., smallest) tool to enter the cavity defined within the cylindrical surface  36  and begin cutting. Additional larger sizes of the wheels  63  increase the diameter of the cavity being machined until the desired size is achieved. 
     The shaft  65  is inserted into the bore  24  ( FIG. 3 ) and the cylindrical tool shape may be used to radially constrain the device  61 . The bushings  70  ( FIG. 5 ) are located in the bore  24  for precise, additional support of the shaft  65 . Both pockets  36  are machined with this process. In one embodiment, this tool  61  is not designed to machine axial surfaces  54 , but only radial or cylindrical surfaces  36 . Axial surface  64  is normal to the axis of bore  24 . The abrasive tool  61  is pushed forward (i.e., to the left in  FIG. 3 ) to machine the front radial pocket (i.e., left side  36  in  FIG. 3 ), and the abrasive tool  61  is pulled rearward (i.e., to the right) to machine the rear radial pocket  36  (i.e., right side  36 ). 
     The assembled tooling  61  may be rotated by hand or machine-driven. For example, in  FIG. 3 , a handle  67  is mounted to the proximal end  66  of shaft  65 . Without one of the wheels  63  (and, e.g., one of the bushings  70 ) attached, the distal end  64  of the shaft  65  is axially positioned within valve cavity  30 . A selected one of the wheels  63  is then attached to distal end  64  and secured thereto while in cavity  30 . While rotating the tool  61  by hand, the wheel  63  has sufficient taper to allow its leading edges to enter the cavities  36  and allow the abrasive to engage the inside diameter of the cavity wall  36 . The tool  61  is continually rotated and slowly fed into the cavity  36  until the wheel  63  is fully inserted into the cavity to treat the surfaces  36 . 
     After sufficient lapping, grinding, honing and/or polishing has occurred with a selected one of the wheels, the smaller diameter wheel is replaced by a next larger diameter wheel, and the process is repeated as needed. When the diameter of the cavity has been enlarged to the lower end of the desired range of diameters, the wheels with finer abrasive are then used to improve the surface finish of the cavity while minimizing the material removal. When the desired diameter and/or surface finish has been achieved, a felt wheel may be used with a fine abrasive paste to provide the cavity with the final surface finish specification. 
     Form errors and surface finish errors contributing to leakage in high pressure valves were corrected to achieve superior sealing in extreme subsea applications. In the machining of super alloys and super alloy coated parts, there is difficulty in the machining of the super alloy. Special tooling and expensive machinery are required to perform these processes. The invention machines the surfaces to allow an existing machining process to be used to rough machine a part at a low cost and with precision. The part is then finished machined using hand operated tooling that requires no high cost equipment. The valves operate with better sealing and without the need of new equipment to perform the operation. The industry standard is a machining practice and the new method had not been applied to this application. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.