Abstract:
A valve seat assembly includes a valve body defining a cavity having an axis, the cavity having a threaded receiving portion and a conical contact surface substantially concentric with the threaded receiving portion. The valve seat assembly further includes a seat ring formed about a center axis having a threaded portion to engage the valve body threaded receiving portion. The seat ring also having an outer surface including a curved contact surface sized to sealingly engage the conical contact surface wherein the curved contact surface is slidable over the conical contact surface to position the seat ring so that the seat ring center axis is substantially aligned with the cavity axis.

Description:
RELATED APPLICATIONS 
   This is a continuation-in-part of U.S. patent application Ser. No. 10/336,476 entitled “Improved Seat Ring for Valves” which was filed on Jan. 3, 2003, now U.S. Pat. No. 6,789,784, and which is hereby expressly incorporated by reference herein. 

   FIELD OF THE DISCLOSURE 
   The present invention relates generally to seat rings and, more specifically, to an improved seat ring and valve assembly having improved alignment features, reduced torque requirements, and reduced leakage. 
   BACKGROUND OF THE DISCLOSURE 
   Conventional globe style control valves and other types of valves commonly include screwed-in or clamped-in seat rings. Seat rings are typically inserted into a cylindrical cavity formed within a valve body of a valve and include an annular thrust-bearing rim against which a valve plug may be applied to close the valve. The seat ring is subject to wear and, therefore, is typically formed as a removable part to facilitate replacement. Several economical materials can be used to construct seat rings, including steel, stainless steel, and hardened materials such as stellited, ceramic, and Tungsten carbide. The clamped-in seat ring typically requires a compressed gasket to seal the seat ring and prevent fluid leakage. The clamped construction typically requires additional parts and complexity due to the load that is needed in the valve assembly to compress the gasket. Screwed-in seat rings, on the other hand, generally do not require a gasket, but rely on a metal-to-metal sealing formed by complementary surfaces formed in the seat ring and the valve body. However, conventional screwed-in seat rings suffer certain alignment disadvantages, concentricity disadvantages, and seal limitations as described below. 
     FIG. 1  is a cross-sectional view of prior art seat ring valve assembly  10 , which includes seat ring  100  screwed into an interior cylindrical sidewall surface  122  of valve body  120 . Seat ring  100  includes annular thrust-bearing rim  102 , which engages a valve plug  160  to close the valve. When in the closed position, the valve plug  160  is compressed against the seat ring  100  and prevents fluid from flowing through a passage  152  created when the valve plug is lifted from the seat ring. The seat ring  100  further includes an exterior cylindrical sidewall  112 , which is generally formed at a 90-degree angle to an upper flange  113  and faces an interior cylindrical surface  138  of the valve body  120  (described in more detail below). Just below the cylindrical sidewall  112 , a tapered exterior surface  132  extends generally downward and toward a center of the seat ring  100 . The cylindrical sidewall  112  is sloped to complement a tapered valve body surface  136  of the valve body  120 . Both tapered surfaces,  132  and  136 , are typically manufactured to be oriented at approximately 45 degrees with respect to a center axis  137  along which the plug actuates. Engagement of the tapered surfaces,  132  and  136 , forms a primary seal  106  between the seat ring  100  and the valve body  120 . Below this sealing surface, a first threaded portion  104  of the seat ring  100  mates with a second threaded portion  110  of the valve body  120 , which helps secure and guide the seat ring  100  into the cavity  130 . The threaded engagement also provides a secondary seal. 
   The effectiveness of primary seal  106 , however, is highly dependent on the alignment (meaning straightness and centering) of the seat ring  100  within the cavity  130 . If the seat ring  100  is misaligned, a tight uniform circular seal will not occur because certain sections of the primary seal  106  will have gaps, particularly where there are minute surface flaws in the metal, resulting in unacceptable leakage between the seat ring  100  and the valve body  120 . Additionally, the effectiveness of the seal formed by the interface of the thrust-bearing rim  102  and the valve plug  160  is also highly dependent on the alignment of the seat ring  100  within the cavity  130 . In this case, if the seat ring  100  is misaligned with respect to the center axis  137 , then the thrust-bearing rim  102  is correspondingly misaligned with respect to the center axis. Therefore, as the valve plug  160  closes, it will fail to form a tight circular seal with the thrust-bearing rim  102 . Unfortunately, conventional screwed-in seat ring assemblies of this type are particularly susceptible to misalignment resulting in leakage for at least three reasons. First, conventional screwed-in seat rings rely on their mated threads to provide alignment and a secondary seal. However, industry standard threads typically include relatively loose tolerance requirements, resulting in excessive clearance or play between the mating threaded parts. Conventional valve seat assemblies use the threaded connection between the seat ring  100  and valve body  120  to position the seat ring, and therefore the ultimate location of the thrust-bearing rim  102  may vary. The valve plug  160  is movably positioned with respect to the valve body  120 , and therefore the imprecise location of the seat ring  100  within the valve body cavity  130  increases the uncertainty that the seat ring  100  and plug  160  will be concentric. As used herein, the terms “concentric” and “concentricity” mean that a center axis of the seat ring  100  and the center axis  137  are substantially aligned. Both the valve plug  160  and the valve body  120  are concentric with respect to the center axis, thus a lack of thrust-bearing rim  102  concentricity may result in misalignment of the valve plug  160  causing leakage. Second, the area in which the sidewall  112  joins the tapered exterior surface  132  of the seat ring  100  is formed as a sharp edge that may contact the valve body surface  136  during assembly. As such, minute surface imperfections or irregularities in this edge area may result in significant misalignment, therefore small machining tolerances are required. Third, the relatively shallow angle (45 degrees) of the valve body surface  136  does not sufficiently direct the seat ring  100  toward the center of the cavity  130 , causing the seat ring  100  to be susceptible to misalignment. In other words, a conventional seat ring is particularly susceptible to leakage because it is either not properly centered, misaligned, or both, causing at least a portion of the seal  106  to be susceptible to leakage where there is insufficient contact between mating surfaces. 
   The effectiveness of the primary seal  106  is also highly dependent on tolerances associated with the tapered surfaces,  132  and  136 . Normal industry standards introduce tolerance differences between the tapered valve body surface  136  which prevent true parallel sealing surfaces. Such tolerance differences prevent a tight uniform seal resulting in unacceptable leakage between the seat ring  100  and the valve body  120 . Additionally, the aforementioned problems caused by thread clearance are further exacerbated by such tolerance differences. 
   Alternatively, in light of the difficulty of controlling tolerances when manufacturing parallel surfaces, intentional angular deviations between the sealing surfaces present similar problems. In such a configuration, the sharp edge of the sidewall surface  112  tends to engage and impinge (i.e., dig into) to the tapered surface  136 , which may also cause misalignment between the thrust-bearing rim  102  and the valve plug  160 . To compensate for these limitations and to reduce leakage to an acceptable level, the assembler must apply an unacceptably high level of torque to the seat ring  100  to excessively compress it within the cavity  130 . The required excessive compression creates a complicated assembly process, causes torque stress on the assembly parts, and contributes to a high failure rate of the primary seal  106 . 
   What is needed is an improved seat ring valve assembly having self-aligning characteristics for creating a stronger seal with reduced leakage without requiring an unacceptably high level of torque during assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a prior art seat ring valve assembly; 
       FIG. 2  is a cross-sectional view of an improved seat ring valve assembly in accordance with a first embodiment of the present disclosure; 
       FIG. 2A  is an exploded view detail of  FIG. 2  in accordance with the first embodiment of the present disclosure; 
       FIG. 3  is a cross-sectional view of an improved seat ring valve assembly in accordance with a second embodiment of the present disclosure; and 
       FIG. 3A  is an exploded view detail of  FIG. 3  in accordance with the second embodiment of the present disclosure. 
   

   While the disclosure is susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof are shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific embodiments disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the disclosure as defined by the appended claims. 
   DETAILED DESCRIPTION 
   Referring now to  FIG. 2 , a cross-sectional view of a substantially cylindrical seat ring valve assembly  200  is shown. Valve body  205  includes a valve body cavity  210  and valve body threads  215  for receiving seat ring threads  220  of seat ring  225 . The seat ring  225  further includes a lug portion  230  used to threadably secure the seat ring  225  to the valve body  205  as shown. A special tool (not shown) interfaces with the lug portion  230  to threadably rotate the seat ring  225  into or out of the valve body  205 . For convenience, the combination of seat ring  225  and valve body  205  is referred to as a seat ring valve assembly  200 . Seat ring  225  has an annular thrust-bearing rim  235  sized to engage a valve plug  236 , thereby to close the seat ring valve assembly. The valve plug  236  may be actuated to a position spaced away from the rim  235 , known as the open position, to allow fluid to flow through a seat ring cavity  240 . 
   In accordance with the present disclosure, rather than parallel tapered surfaces (surfaces  132  and  136  of  FIG. 1 ) or a sharp edge (such as the edge formed between the upper flange  113  and angled side wall  112  of  FIG. 1 ) to form a primary seal, a curved contact surface  245  is formed between the seat ring  225  and the valve body  205 . As shown, the lug portion  230  of the seat ring includes a substantially cylindrical exterior sidewall surface  250 , which generally extends at a 90-degree angle from a top surface of the lug portion  230 , a portion of the exterior sidewall surface  250  faces an interior surface  255  of the valve body  205 . Below the lug portion  230 , the seat ring  225  has a tapered surface  260  that is oriented at an angle β with respect to a center axis  237  as illustrated in  FIG. 2 . The angle β is shown in  FIG. 2  having an orientation of approximately 45 degrees, although alternate angles may be used, as discussed and shown later. 
   The curved contact surface  245  joins the sidewall surface  250  and the tapered surface  260 , the contact surface  245  operating as the primary seal between the seat ring  225  and a conical contact surface  265  of the valve body  205 . The curved contact surface  245  is an arcuate, curved, or radiused surface that extends between the sidewall surface  250  and the tapered surface  260 . Preferably, the curved contact surface  245  is formed to have a radius of approximately 0.03 inches, however other geometries and/or radii may be used. The conical contact surface  265  of the valve body  205  is preferably formed at an angle (α) that is less than the angle β of the tapered surface  260 . In this illustrated embodiment, the angle α is approximately 30 degrees, however other angles preferably in the range of 10 to 40 degrees could be used. 
   The relatively small angle a creates a relatively steep conical contact surface  265  with respect to the tapered surface  260  when the valve is oriented as shown in the Figures. The steep angle ensures that the curved contact surface  245  will engage the surface  265 . The conical surface  265 , therefore, directs the seat ring  225  toward the center of the valve body cavity  210 , thereby more reliably aligning the center axes of the thrust-bearing rim  235  of the seat ring  225  with the valve plug  236 . A wedging and sliding effect of the conical contact surface  265  additionally prevents tilting of the plane perpendicular to the center axis  237  and uniformly guides the seat ring  225  to a low center point within the valve body cavity  210 . Once in place, the valve body threads  215  and seat ring threads  220  may secure the valve body  205  and seat ring  225  together without concern that thread tolerance, and resulting clearance, will affect their concentricity. Because the area of the seat ring that contacts the conical contact surface  265  is curved (i.e., the curved contact surface  245 ), the seat ring  225  more easily slides along the conical surface  265  rather than engaging and digging into the surface as seen with conventional seat rings. 
     FIG. 2A  is an exploded view detail of  FIG. 2  showing the tapered surface  260 , the curved contact surface  245 , and the conical contact surface  265  in greater detail. 
   Referring now to  FIG. 3  (having similar numerical identification numbers of  FIG. 2 , offset by 100), a cross-sectional view of a second embodiment of a seat ring valve assembly  300  is shown having a substantially cylindrical seat ring  325  and valve body  305 . The valve assembly  300  of  FIG. 3  shows alternate geometries of a tapered surface  360  and a curved contact surface  345  than the embodiment of  FIG. 2 . As with the previous embodiment, a conical contact surface  365  centers and aligns a thrust-bearing rim  335  of the seat ring  325 , rather than the threaded engagement of the receiving seat ring threads  320  and valve body threads  315 . During the axial alignment between the seat ring  325  and the valve body  305 , the curved contact surface  345  easily slides along the conical contact surface  365  (i.e., does not “dig-in” to the surface) to facilitate movement of the seat ring  325  toward the proper position. With the seat ring  325  so aligned, it may easily be threaded without requiring extremely high torque to sealingly engage the curved contact surface  345  with the conical contact surface  365 . Furthermore, the seat ring  325  concentricity ensures that a valve plug  336  seats properly on the thrust-bearing rim  335 , thereby preventing leakage. 
     FIG. 3A  is an exploded view detail of  FIG. 3  showing the tapered surface  360 , the curved contact surface  345 , and the conical contact surface  365  in greater detail. As can be seen with reference to  FIG. 3A , the tapered surface  360  is substantially perpendicular to a center axis  337  so that the curved contact surface  345  defines an arc having a central angle of approximately 90 degrees. 
   The curved contact surface described above significantly reduces the torque requirements during assembly and minimizes the effects of minute flaws in the metal contact surface. Also, the curved contact surface described above has another advantage of self-aligning a seat ring within a cavity during installation by reducing the area needed for alignment to form the seal. 
   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and as described above. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed. For example, as shown in  FIGS. 3 and 3A , the tapered portion  260  could be other angles or geometries to form the transition from the curved contact surface  245  to the threaded portion  220 . In addition, while the illustrated embodiments show curved contact surfaces having substantially uniform radii, the contact surface may be simply curved or arcuate with a non-uniform radius. Further, the shape and style of the annular thrust-bearing to accommodate a valve plug may vary depending on the type of valve plug used. Also, in view of the detailed discussion above, one skilled in the art should appreciate that the seat ring valve assembly described below may be incorporated into any conventional valve, such as an on/off valve or globe style control valve. Other modifications should also become apparent in view of the above disclosure without departing from the spirit and scope of the claimed invention.