Abstract:
A pressure seal assembly for sealing a bonnet assembly that is removably received by an access port defined by a body portion of a high pressure control device. The seal assembly inhibits leakage between the body portion and the bonnet assembly and includes an annular graphite gasket having a tapered portion defining an angled surface engageable with a complementally-formed surface on the bonnet assembly. A first anti-extrusion ring is urged into sealing engagement with a surface defined by the access port. A pair of inner and outer anti-extrusion rings are urged into sealing contact with an access port surface and bonnet surface. The bonnet assembly includes a reduced diameter section which defines a seal assembly receiving cavity. The anti-extrusion rings include overlapped ends, which allow expansion and contraction while maintaining overlapping contact. Camming surfaces may include retaining segments which maintain the inner and outer extrusion rings in an assembled relation.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application Ser. No. 61/476,467, filed Apr. 18, 2011; and U.S. Provisional Application Ser. No. 61/552,103, filed Oct. 27, 2011, the subject matter of which is incorporated herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to sealing and, in particular, to a seal assembly for use in a high pressure control device such as a pressure seal valve. 
       BACKGROUND 
       [0003]    Pressure seal valves are commonly used in high pressure applications such as steam generation. These valves not only must operate at high pressures, but also at high temperatures. Special high pressure seals are often used to inhibit working fluid leakage past a bonnet assembly that usually forms part of these types of devices. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a new and improved seal assembly and method of sealing for use in high pressure control devices such as pressure seal valves of the type that are often used in steam generation. 
         [0005]    According to one embodiment of the invention, a pressure seal assembly for use in a high pressure control device is disclosed. The seal assembly includes an annular graphite gasket having an angled seal surface that is sealingly engageable with a complementally-formed first sealing surface defined on a bonnet assembly that forms part of the control device. A pair of anti-extrusion rings are spaced from the sealing surface. One of the pair of anti-extrusion rings is sealingly engageable with a second surface on the bonnet assembly, the other of the anti-extrusion rings is sealingly engageable with a surface defined by an access bore forming part of the control device. The anti-extrusion rings inhibit the flow of gasket material past the rings. A third anti-extrusion ring is located in the vicinity of the bonnet sealing surface and is sealingly engageable with the access bore surface. The engagement of the bonnet sealing surface by the third anti-extrusion ring inhibits the flow of graphite material past the third ring. 
         [0006]    In one illustrated embodiment, camming surfaces are used for urging at least some of the anti-extrusion rings into sealing contact with their associated surfaces. In a more preferred embodiment, the camming surfaces are formed in the annular graphite gasket. In an alternate embodiment, the camming surfaces are formed on a thrust member that abutably engages the annular graphite gasket. 
         [0007]    In still another embodiment, the camming surfaces are formed on the annular graphite gasket and includes segments that abutably engage the pair of anti-extrusion rings prior to installation of the seal assembly. These segments maintain the assembled relationship of the pair of anti-extrusion rings and the annular graphite gasket, which facilitates installation. 
         [0008]    According to a feature of the invention, the seal assembly comprises an annular graphite gasket defining an angled seal surface sealingly engageable with a complementally-shaped sealing surface formed on a bonnet that forms part of the control device and which is used to cap or close off a bore in the device. A pair of radially spaced apart anti-extrusion wire rings inhibit graphite migration out of a seal region. Angled surfaces on the graphite gasket urge these wire rings into sealing engagement with the bonnet and bore structure. Another anti-extrusion wire ring, preferably larger in diameter than the aforementioned wire rings, inhibits graphite migration into a working fluid (i.e. steam) region of the control device, the region that is sealed off by the bonnet. 
         [0009]    According to the invention, when compression forces are applied to the seal assembly of the present invention, the anti-extrusion rings are urged into “sealing” engagement with associated surfaces and the graphite material itself is deformed or “flows” plastically to fill voids in the sealing cavity. The gasket material does not otherwise gall or damage the bonnet or bore surfaces and, hence, disassembly for service is greatly facilitated. 
         [0010]    According to another aspect of the invention, a high pressure control assembly is disclosed that includes a body portion defining an access port. A bonnet assembly is removably received by the body for at least partially closing off the access port. A seal assembly inhibits leakage between the body portion and the bonnet assembly includes an annular graphite gasket. The annular gasket includes a tapered portion defining an angled surface engageable with a complementally-formed angled surface on the bonnet assembly. A first anti-extrusion ring associated with the tapered portion of the annular graphite seal is urged into sealing engagement with a surface defined by the access port, when a clamping pressure is applied to the annular graphite gasket. A pair of inner and outer anti-extrusion rings, one of which being associated with an outer diameter of the annular graphite gasket, the other of which being associated with an inner diameter of the annular graphite gasket, are urged into sealing contact with the access port surface and the associated bonnet assembly surface, respectively. These rings are urged into sealing engagement with their respective surfaces when a clamping force is applied to the annular graphite gasket. in one illustrated embodiment, the bonnet assembly includes a reduced diameter section which at least partially defines a seal assembly receiving cavity between the bonnet assembly and the body portion. 
         [0011]    The term “sealing” used in connection with the anti-extrusion rings describes contact between the rings and associated surfaces that is sufficient to inhibit leakage of graphite material past the rings. The “sealing” engagement of the rings is not intended to necessarily prevent leakage of fluid (i.e., steam) out of the working fluid region. The graphite based gasket provides this sealing function. 
         [0012]    According to a feature of the invention, the ant-extrusion wire rings are split, the ends of which are joined using a lap joint such as a shiplap joint. The joint allows a given wire ring to expand or contract radially but inhibits leakage of graphite gasket material between the joined ends. 
         [0013]    Additional features of the invention will become apparent and a fuller understand obtained by reading the following detailed description made in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a fragmentary sectional view of a portion of a high pressure control assembly that utilizes a high pressure seal constructed in accordance with a preferred embodiment of the invention; 
           [0015]      FIG. 2  is a fragmentary sectional view showing the seal of the present invention in a relaxed position; 
           [0016]      FIG. 3  is a fragmentary sectional view showing the seal of the present invention in a fully installed position 
           [0017]      FIG. 4  illustrates the configuration of a lap joint that forms part of anti-extrusion wire rings constructed in accordance with a preferred embodiment of the invention; 
           [0018]      FIG. 5  is a fragmentary sectional view showing an alternate embodiment of the seal of the present invention in a relaxed position; and 
           [0019]      FIG. 6  is a fragmentary sectional view showing another alternative embodiment of the seal of the present invention, shown in a relaxed position. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  is, a fragmentary sectional view showing a method and apparatus for sealing a bonnet assembly in a control device  10  that is used in high pressure fluid applications such as steam generation. The device  10  to which this invention pertains may be what is termed a pressure seal valve, of which there are several varieties including a globe valve, a gate valve or a tilt disc valve. The structure shown in  FIG. 1  may be used as a port for gaining access to valve components, or, alternatively, the structure may operatively mount a valve actuating mechanism by which a valving element (not shown) is opened or closed. Those skilled in the art will recognize that the structure shown in  FIG. 1  may be used to sealingly support, for example, a valve operating stem which would extend along the centerline  12  and which would be attached to an operating member at its upper end (as viewed in  FIG. 1 ) and a valving component at its lower end. 
         [0021]    To facilitate the explanation, the structure in  FIG. 1  will be described as an access port, which includes a port or valve body  20 . As should be apparent, the body  20  extends downwardly and includes a housing that encloses valving or other components. The portion of the body shown in  FIG. 1  provides access to the components when a bonnet assembly indicated generally by the reference character  24  is removed. In general, the region below the bonnet assembly indicated by the reference character  26  is generally at extremely high pressure and may also be at high temperature. A typical application for the device shown in  FIG. 1  is in a steam generation facility. 
         [0022]    The device body  20  may include a stepped bore  30 , which defines a lower step  32  and an annular recess  34 . A bonnet  28  acts as a closure for the bore  30 . A sealing arrangement indicated by the reference character  40  seals the bonnet  24   a  to the bore  30  and inhibits leakage of high pressure fluid from the region  26  to the outside ambient. 
         [0023]    As is conventional the bonnet  28  defines a reduced diameter section  28   a , which defines a gap between the bonnet,  28   a  and the body bore  30 ; the gap receives the sealing arrangement  40 . A conventional segmented ring  42  is captured between the reduced diameter section  28   a , of the bonnet  28  and the annular recess  34  defined by the body  20 . The segmented ring  42  acts a retainer for the bonnet  28  and maintains its position within the body bore  30 . 
         [0024]    In the illustrated construction, a backing ring  46  is located below the segmented ring  42  and is used to apply compression forces to a seal assembly  50  constructed in accordance with a preferred embodiment of the invention. The upper end of the body  20  receives a retaining cap  54 , that includes a reduced diameter section  54   a , which is receivable by the body bore  30 . A plurality of bonnet clamping studs  56  have lower ends  56   a  threadedly received by the bonnet  28  and a threaded upper end which extends through bores  54   b  formed in the retaining cap and which threadedly receive fasteners such as nuts  58 . The nuts  58  apply tension forces to the retaining studs  56 . As should be apparent from  FIG. 1 , the bonnet  28  and associated seal components are placed in the body bore and the bonnet is lowered until it sits atop the body step  32 . In this position, sufficient clearance is provided to insert the segmented ring  42 . The clamping studs  56  are then threaded into the bonnet  28 . The retaining cap  54  is then aligned with and then placed on the upper end of a valve body  20  allowing the studs  56  to extend through the bores  54   b . The nuts  58  are then threaded onto the upper ends  56   b  of the studs  56  and are tightened in order to pull the bonnet assembly upwardly until the backing ring  46  contacts the underside of the segment ring  34  thereby applying compression forces to the seal assembly  50 . 
         [0025]    Turning also to  FIGS. 2 and 3 , the construction and operation of the seal assembly  50  will now be described. According to the invention, the seal assembly  50  includes a shaped graphite ring or gasket  60  that, in the preferred embodiment, includes a lower angled surface  60   a . The surface is configured to conform to an angled seal surface  28   b  formed on the bonnet  28 . In the preferred and illustrated embodiment, the graphite gasket  60  has a density of 0.8 grams of graphite per cubic centimeter or higher. 
         [0026]    The seal assembly includes a pair of upper anti-extrusion wire rings  70 ,  72  and a lower anti-extrusion wire ring  74  which inhibit the flow or migration of graphite when under pressure, out of the seal cavity defined between the backing ring  46  and the bonnet sealing surfaces. In the preferred and illustrated embodiment, the lower anti-extrusion ring is of a larger wire diameter than the upper rings. In the preferred and illustrated embodiment, the seal assembly also includes an annular thrust plate  64  that sits atop the two upper anti-extrusion wire rings  70 ,  72 . 
         [0027]      FIG. 2  illustrates the configuration and shape of the graphite seal element  60  prior to the application of compression forces by the clamping studs  56 . As seen in  FIG. 2 , the upper portion of the graphite seal  60  includes angled surfaces  60   b ,  60   c  that are contacted by associated anti-extrusion rings. During compression, this angled surface  60   a  urges the inner anti-extrusion ring  70  radially inwardly and the outer anti-extrusion wire ring  72  radially outwardly. Thus, the inner anti-extrusion ring  70  is urged into sealing engagement with the reduced diameter section  28   a  of the bonnet  28  and the underside of the thrust ring  64 . The outer anti-extrusion wire ring  72  is urged into sealing contact with the body bore  30  and the underside of the thrust plate  64 . 
         [0028]    During compression the lower anti-extrusion wire ring  74  is urged radially outwardly and thus sealingly engages the body bore  30  and the angled bonnet sealing surface  28   b  and thus inhibits the flow or migration of graphite out of the sealing region and into the interface between the bonnet  28  and the body bore  30 .  FIG. 3  illustrates the configuration of the graphite seal and the position of the anti-extrusion wire rings after the predetermined and desired clamping force is applied to the bonnet  28  by the clamping studs  56 . 
         [0029]    In the preferred and illustrated embodiment, each extrusion ring is slit to enable the rings to expand and contract radially during installation then compression of the graphite seal. Referring to  FIG. 4 , this feature is achieved by lapping the ends of each extrusion ring as shown to form a joint  80  that allows the ends of the wire ring to slide relative to each other as the ring contacts or expands radially. The illustrated “shiplap” joint  80  inhibits the flow or extrusion of graphite between the ends of an extrusion ring while allowing relative movement between the ends. Other types of overlapping joints for the wire ring ends can be used and are contemplated by the present invention. 
         [0030]      FIG. 5  illustrates an alternate embodiment for the seal assembly in which the camming or angled surfaces for urging the upper anti-extrusion rings (as viewed in  FIG. 5 ) into sealing engagement with associated surfaces is provided by a thrust ring  64 ′. In particular, the thrust ring  64 ′ includes annular, angled or camming surfaces  60   b ′ and  60   c ′. The surface  60   b ′ urges the associated anti-extrusion ring  70 ′ radially inwardly into graphite sealing contact with the reduced diameter surface  28   a  of the bonnet  28  and the camming surface  60   c ′ urges the associated anti-extrusion ring  72 ′ radially outwardly into sealing contact with the bore surface  30 . The surfaces  60   b ′ and  60   c ′ formed on the thrust ring  64 ′ provide the same function as the annular angled surfaces  60   b ,  60   c  formed on the graphite gasket  60  (shown in  FIG. 2 ). 
         [0031]      FIG. 6  shows still another embodiment of the invention. In this embodiment, a graphite ring  60 ″ includes angled or camming surfaces  60   b ″,  60   c ″ for urging respective anti-extrusion rings  70 ″,  72 ″ into sealing contact with a reduced diameter section  28   a  of the bonnet  28  (shown in  FIG. 1 ) and with the body bore  30  (also shown in  FIG. 1 ), respectively. Unlike the  FIG. 2  embodiment, the angled or surfaces  60   b ″,  60   c ″ terminate at their upper ends (as viewed in  FIG. 6 ) in respective vertical segments  90 ,  92 . Together, the vertical segments  90 ,  92  define a section  96  of the seal  60 ″ having a uniform cross-section that is immediately adjacent a portion  98  having a tapered cross-section as defined by the angled surfaces  60   b ″,  60   e ″. The vertical segments  90 ,  92  aid in the retention of the anti-extrusion rings  70 ″,  72 ″, respectively, during assembly, transport and seal installation. As seen in  FIG. 6 , the vertical segments  90 ,  92  have sufficient heights such that an upper edge of each vertical segment contacts its respective anti-extrusion ring at or above its midpoint. 
         [0032]    With the disclosed embodiment, the angled surfaces  60   b ″,  60   c ″ urge the respective anti-extrusion rings radially inwardly and radially outwardly, respectively as compression forces are exerted by the clamping studs  56  (see  FIG. 1 ). After a predetermined and desired clamping force is applied to the bonnet  28 , the graphite seal and the anti-extrusion wire rings  70 ″,  72 ″ assume a configuration and positions substantially similar to that shown in  FIG. 3 . Plastic flow occurs in the graphite seal  60 ″ such that it fills the void in the seal region just as the seal element  60  shown in  FIG. 3 . 
         [0033]    With the disclosed alternate embodiment, assembly, shipping and installation of the graphite seal with associated anti-extrusion rings is greatly facilitated. In addition, the alternate embodiment of the invention permits the construction of seal elements  60 ″ with smaller cross sections. In other words, the seal construction of the alternate embodiment shown in  FIG. 6  contains all of the advantages of the construction of the seal shown in  FIG. 2 , with several additional advantages. 
         [0034]    With the present invention, an extremely effective seal between the bonnet  28  and a high pressure device body can be achieved while allowing easy disassembly when repair or service of the device is needed. Unlike prior art metal gaskets, the seal of the present invention does not gall or damage the sealing surfaces, which, in prior art devices, makes disassembly very difficult. 
         [0035]    Although the invention has been described with a certain degree of particularity, those skilled in the art will recognize that various changes can be made to it without departing from the spirit or scope of the invention as hereinafter claimed.