Abstract:
A valve body and seal assembly comprising a valve body having one or more peripheral integral seal retention grooves and one or more peripheral single-durometer and/or dual-durometer elastomeric seals, each seal being cast-in-place in an integral seal retention groove having adhesion-inhibiting surfaces and offset circular serrations of predetermined height on opposing groove walls. Adhesion-inhibiting surfaces reduce or eliminate internal elastomer stress due to curing of cast-in-place elastomeric seals. Serrations on each of the opposing groove walls are offset to accommodate differing stress concentrations in the walls. Opposing groove walls diverge peripherally. Offset serrations result in decreased metal fatigue from impact and bending stress. Offset serrations also improve valve sealing and increase valve service life by maintaining correct seal position on the valve body while minimizing background stress in the seal elastomer(s). Valve service life is also increased by appropriate choice of differing elastomer compositions for dual-durometer seals.

Description:
This is a continuation-in-part (CIP) patent application of U.S. Ser. No. 10/179,804, which was filed Jun. 25, 2002 now U.S. Pat. No. 6,955,181. 

   FIELD OF THE INVENTION 
   The present invention relates generally to valves suitable for rapid open-close cycling with abrasive fluids, such as drilling mud, under high pressures. More specifically, the invention relates to valves comprising an elastomeric seal in a peripheral seal retention groove on the moving element of the valve. 
   BACKGROUND OF THE INVENTION 
   Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the moving element, whereas in other applications, the term “valve” includes the moving element, the valve seat, and the housing that contains the moving element and the valve seat. To clarify the following description of the present invention, a valve suitable for abrasive fluids, such as oil field drilling mud, comprises a valve body (the moving element) and a corresponding valve seat. 
   The valve body incorporates an elastomeric seal within a peripheral seal retention groove. These valves are usually mounted in the fluid end of a high-speed pump incorporating positive displacement pistons or plungers in multiple cylinders. Such valves are frequently web-seat, stem-guided designs or full open seat designs adapted for high pressures and repetitive impact loading of the valve body and valve seat. Both leakage and premature failure due to metal fatigue must be overcome in designing these valves, and special attention is focused on the moving element (or valve body). 
   The valve body element typically comprises at least one groove for a peripheral elastomeric seal. If preformed seals are to be used, such a groove requires finish machining to closely match the dimensions of seals like the “snap-on” type or seals secured with a removable seal retention plate. This finish machining may be reduced or eliminated if elastomeric seals are cast and cured in place (herein “cast-in-place”) in their groove. Further, cast-in-place seals may be mechanically locked to a valve element by forming them over interengaging or interlocking (herein “interdigitating”) projecting-receiving formations on the element. Such interdigitating of valve element and seal has become a recommended structural feature of cast-in-place seals that was evidently difficult or impossible to achieve with “snap-on” type seals. See, for example, the entire U.S. Pat. No. 4,860,995 (incorporated herein by reference), particularly col. 7, lines 17-57. 
   But manufacture of valve elements with interdigitating cast-in-place seals, such as those described in the &#39;995 patent has historically involved added costs. These added costs arose because, as described in the &#39;995 patent, the seals are preferably bonded to a valve element to increase its overall integrity. See the &#39;995 patent, col 7, lines 47-50. 
   Even though the manufacturing cost of valve bodies for bonded cast-in-place seals is almost identical to the analogous cost of valve bodies for “snap-on” seals, the added cost of preparing the valve body for bonding increases the cost of the valve to the point that these valves have not been competitive on price. The added costs of bonding include cleaning the valve groove of all oil and contaminants, applying a bonding adhesive, and storing the valves in a low-humidity, dust-free environment while the valves await casting, bonding, and curing of the urethane on the valve body. 
   Despite their added cost and, as noted in the Detailed Description herein, their unexpectedly short service life, bonded cast-in-place seals have achieved limited acceptance on valve bodies having circular “Channel-Beam” sections or other one-piece valve bodies. Channel-Beam valve bodies are characterized by a forged bowl shape as seen, for example, in FIG. 1 of U.S. Pat. No. 5,249,600 (the entire &#39;600 patent is incorporated herein by reference). The acceptance of bonded cast-in-place seals in these applications, notwithstanding their relatively high cost and shortened service life, may be attributed in part to the benefits of the Channel-Beam design itself and in part to the problems of using “snap-on” type seals on this type of valve body. 
   Forged Channel-Beam valve bodies such as those in the &#39;600 patent have exceptional stiffness, strength, and resistance to fatigue failure. But, in addition to the above problems with bonded cast-in-place seals, they also have several disadvantages when used with “snap-on seals. First, rough valve body forgings in the Channel-Beam shape require substantial material removal for finish machining of the integral seal retention groove if a “snap-on” seal is to fit properly. Second, even if an accurately preformed elastomeric seal in the shape of a toroidal ring is snapped into a carefully machined seal retention groove, it may not fully seat within the groove due to stack-up (i.e., additive effects) of manufacturing tolerances. The resulting out-of-round seal condition can cause early valve failure. Similar disadvantages are also seen in other Channel-Beam designs, such as those described in U.S. Pat. Nos. 3,191,617; 3,202,178; 3,742,976; 4,180,097; 5,345,965; and 5,431,186, all incorporated herein by reference. 
   Thus, notwithstanding their relatively high cost and/or problems with limited service life, valve bodies having “snap-on” or cast-in-place seals in a seal retention groove analogous to the one-piece Channel-Beam design have gained limited industry acceptance. Although the seals have been problematical, the strength and stiffness of the Channel-Beam shape tend to reduce valve body distortion about one or more radial axes (i.e., axes radiating perpendicularly from the valve body&#39;s longitudinal axis of symmetry). This type of distortion is associated with both leaks and fatigue failures, and it is particularly likely to occur on valve bodies mating with web seats. 
   Metal fatigue is induced when cyclical high pressure is applied to a valve body sealed against, and being distorted by, a web seat. The pressure tends to repeatedly force the disc-shaped area of the valve body (i.e., the flange) into the spaces between the seat webs. This distortion, plus impact loads secondary to valve closure and bending moments caused by pressure on the valve seal, causes non-uniform stresses in the flange, particularly in the two opposing walls of the flange&#39;s peripheral seal retention groove. 
   For example, the groove wall closest to the valve seat typically sustains relatively high impact load stress transmitted through a peripheral metal sealing surface (i.e., a valve body impact area) for contacting the seat when the valve closes. This area of the flange, then, tends to be particularly susceptible to peripheral metal fatigue failures. In contrast, the flange area comprising the opposite groove wall tends to be bent longitudinally through force exerted by the seal within the groove when the seal is displaced as the valve fully closes. This bending stress is minimal peripherally and greatest in that part of the wall nearest the valve body&#39;s longitudinal axis of symmetry, that is, the central part of the wall (i.e., that part of the wall nearest the base of the groove). 
   Peripheral flange stresses (and the associated fatigue failures) may be reduced to a limited extent, and valve sealing improved, by a properly-placed peripheral elastomeric seal which contacts the valve seat on closure. Improper placement of this seal, however, leads to an out-of-round condition that may actually increase leaks and hasten valve failure. Leaks may occur, for example, past an out-of-round “snap-on” seal or around a cast-in-place seal that shifts during use within its seal retention groove. 
   The potential advantages of an elastomeric seal insert (including extended valve service life and improved valve performance) are commonly reduced or lost entirely if the seal is poorly fitted to the valve body or if the seal shifts in its retention groove during use. Both of these conditions allow leaking high-pressure fluid to jet through one or more leak paths. If the valve remains in service, each jet of high-pressure fluid will literally wash away a portion of the hardened steel of the valve body and/or seat. Multiple and near-simultaneous failures of this kind may give a valve body flange the appearance of a wrinkled cupcake paper. 
   Further, leaks due to displacement of cast-in-place elastomeric seals often occur secondary to failure of the elastomer adjacent to the special adhesive that bonds the seal to the groove. If a portion of the seal is tightly bonded to the groove wall, background stress within the seal elastomer will increase as the elastomer cures because the seal as a whole tends to shrink away from the walls to which it is bonded. Deleterious effects of this background elastomer stress (i.e., elastomer stress that is present even when the seal is not in contact with a valve seat) may be significantly aggravated when a valve body closes against its seat. The resulting elevated internal elastomer stress tends to reduce the service life of the seal by predisposing it to cracking, tearing and/or extrusion.  FIG. 1A  schematically illustrates an example of such stress-induced tearing in an elastomeric seal on a valve body shaped similarly to that in FIG. 2 of the &#39;995 patent. 
   By including a schematic representation of a valve seat,  FIG. 1A  also shows the proximity of a stress-induced tear with the portion of an elastomeric seal that would be subject to extrusion stress as the valve body mates with the valve seat. The requirement that a valve seal make contact with the valve seat prior to metal-to-metal contact between the valve body and seat means that the seal elastomer is subjected to strong compressive forces as the valve is closing. Since movement of the seal against the valve seat is restricted by friction, by the metal portion of the valve body, and by the valve seat itself, a portion of the seal elastomer tends to be extruded into the (schematically shown) extrusion gap. Thus a portion of the seal tends to be repeatedly deformed by this extrusion process each time the valve closes. Elastomer stress due to this deformation can thereby add to the background elastomer stress noted above to precipitate premature failures like the illustrated tear. On the other hand, reductions in background elastomer stress can reduce overall elastomer stress during valve closing, thereby lengthening the service life of the seal. 
   To avoid the extra cost of adhesive and the induced background elastomer stress associated with curing of cast-in-place adhesively-bonded seals, a deformable seal may be stabilized in a groove by threaded serrations directly opposing each other on the sides of a seal retention groove. Stabilization of this general type is termed mechanical locking in the &#39;995 patent. Serrations for this purpose are illustrated in U.S. Pat. Re. Nos. 29,299 and 4,676,481 (both patents incorporated herein by reference). While effective for limiting seal movement within the groove, serrations arranged in this manner can themselves raise stress in both the groove walls and the elastomer. And any unnecessary increases in stress will predispose the elastomer seal to premature failure. 
   Premature stress-induced failures, however, would be relatively rare in the valves illustrated and described in the &#39;299 and &#39;481 patents. These are pipeline valves that are intended for applications with a maximum total valve service life of about 1000 open-close cycles. In contrast, stem-guided valves of web-seat or open seat design that are suitable for drilling mud commonly operate at relatively high pressures and complete two open-close cycles per second. Service life for these valves is measured in millions of cycles (commonly about 1 to 10 million open-close cycles). Incipient valve body fatigue failures that are not manifest after only 1000 open-close cycles become a significant maintenance problem in a valve experiencing millions of cycles during its expected service life. 
   For the latter valves, a more robust seal assembly is needed, comprising an elastomeric seal that is cast-in-place on a valve body having an integral seal retention groove. Use of a separate adhesive for bonding the seal within the groove, although taught in the &#39;995 patent, should in fact be avoided to reduce background stress in the cured seal elastomer. Further, serrations to retain the seal in the groove should be placed to minimize their effects as stress raisers for the valve body, with consideration being given to the different stress distributions in the opposing groove walls. 
   Such serration placement would minimize valve body fatigue failures due to impact loads and bending stress. Further, serrations should be designed so the seal elastomer can retain effective contact with the serrations on the groove walls while it is being cured. Such continued effective contact (i.e., interdigitation) would ensure that, notwithstanding shrinkage of the elastomer during curing, the seal could be retained in its groove without suffering displacement that would materially reduce its service life. 
   SUMMARY OF THE INVENTION 
   The present invention relates to elastomeric seals, each seal being cast-in-place in a mold comprising a peripheral seal retention groove of a valve body that comprises at least one such groove. Unlike earlier cast-in-place seals, however, seals of the present invention exhibit unexpectedly long service life because of their unique structure and because they avoid the errors of earlier teachings such as those related to bonding of seal materials to valve elements in the &#39;995 patent. 
   An illustrated embodiment of the present invention is a valve body and seal assembly having a longitudinal axis of symmetry and comprising a peripheral seal retention groove spaced apart from the longitudinal axis. The seal retention groove has first and second opposing sides (i.e., groove walls). A valve body impact area for contacting a valve seat is peripherally located on the first opposing groove wall of the peripheral seal retention groove, and circular serrations are located on the first and second opposing groove walls of the peripheral seal retention groove. 
   An elastomeric seal is cast-in-place in the peripheral seal retention groove, the elastomeric seal comprising a first seal section and a second seal section. The first seal section comprises a peripheral frusto-conical surface for contacting a valve seat, the frusto-conical surface being adjacent to the valve body impact area. And the first seal section comprises relatively high modulus polyurethane. The second seal section comprises a peripheral surface adjacent to the peripheral frusto-conical surface, at least a portion of the second seal section comprising polyurethane having a lower modulus than the relatively high modulus polyurethane. 
   Thus, certain embodiments of the invention include cast-in-place dual-durometer elastomeric seals wherein the first seal section is relatively harder (typically with a Shore A durometer rating of about 95) than the second seal section (typically with a Shore A durometer rating of about 75 to about 85). Since its peripheral frusto-conical surface is subject to extrusion as it contacts a valve seat, the first seal section is made more extrusion-resistant than the second seal section. This is required because of the importance of controlling or limiting extrusion damage for extending the service life of such peripheral valve body seals. In these applications, MDI polyester thermoset urethanes (comprising a monodiphenylethane polymer and about 14-16% of a diisocyanate curative) have been shown to have excellent extrusion resistance, as well as excellent abrasion resistance, at a Shore A durometer hardness of 95. 
   On the other hand, since the relatively harder elastomers suitable for the first seal section typically exhibit relatively little shape memory to accommodate minor irregularities in a valve seat, elastomer for the second seal section is chosen to exhibit relatively greater shape memory. The resulting cast-in-place dual-durometer seal has a combination of desirable properties that would be difficult to achieve in a single snap-on seal, in part because the distortion associated with installation of a snap-on seal would tend to separate the two seal sections and would predispose these two sections to misalignment and/or an out-of-round condition of the installed seal. 
   Mold surfaces coming into contact with the elastomer(s) as a seal of the present invention is cast-in-place are chosen and/or prepared so as to inhibit adhesion of the elastomer to each such surface, rather than to encourage the bonding taught in the &#39;995 patent. Adhesion-inhibiting properties of a mold-elastomer interface may be obtained by appropriate choice of materials for the mold and the elastomer, and/or by subsequent treatment of these materials. For example, if a valve body is quenched in oil after heat-treatment (e.g., carburization), a thin layer of the quenching oil may be retained on the seal retention groove after quenching is completed for treating the groove to increase its adhesion-inhibiting properties. An adhesion-inhibiting seal retention groove may also be prepared, for example, by polishing the groove and/or by the applying to the groove one or more layers of non-stick materials such as oils, greases, waxes or plastics having properties similar to Teflon®. In each case, according to the present invention, an elastomeric seal cast-in-place in contact with adhesion-inhibiting surfaces preferably exhibits minimal or no adherence to such surfaces during curing. That is, forces due to adhesion of the elastomer to mold surfaces, if present, will not materially add to the elastomer&#39;s background stress and thus will not materially reduce service life through a mechanism similar to that schematically illustrated in  FIG. 1A . 
   Embodiments of valve bodies having one or more peripheral seal retention grooves (as, for example, a stem-guided valve body for use with a web seat or open seat) are symmetrical about a longitudinal axis. They are relatively stiff for their weight, and do not employ any means such as a seal retention plate for applying longitudinal compressive forces to a seal in the groove. They resist distortion about radial axes better than valve bodies of equal weight having seal retention plates, and they avoid imposition of background stress on an elastomeric seal in the groove that could result from longitudinal compressive forces applied to the seal by a seal retention plate. 
   Valve bodies of the present invention comprise offset circular serrations of predetermined height on first and second opposing walls of an integral seal retention groove that is spaced apart from the valve body longitudinal axis. Offset serrations (or groups of serrations) on the first and second opposing walls are located at different distances from the longitudinal axis. Where a plurality of serrations are grouped on one or both of the opposing groove walls, distance from the longitudinal axis attributed to a serration group is the mean distance from the longitudinal axis for serrations within that group. 
   First opposing walls of the insert seal retention grooves on valve bodies of the present invention are closer than second opposing walls to an area of high impact loads because they are closer to a peripheral metal sealing surface (valve body impact area) on the valve body flange. Thus, the periphery of first opposing walls is subject to relatively higher impact stress than other parts of the opposing groove walls when the valve body strikes the valve seat. 
   Second opposing walls, while more distant from the site of maximum metal-to-metal impact stress than first opposing walls, nevertheless experience significant bending stress due to forces transmitted to them through the seal. This bending stress on the second opposing groove wall is lowest peripherally and highest centrally (i.e., highest in that part of the wall that is closest to the longitudinal axis of the valve body). The area of maximum bending stress on second opposing groove walls is more central than the area of maximum metal-to-metal impact stress on first opposing walls because second opposing walls are effectively cantilevered peripherally from the most centrally located area of the seal retention groove. 
   Because the periphery of first opposing walls sustains relatively high impact stress, serrations on first opposing walls are preferably located as far centrally on such groove walls as practicable to maximize the distance from the impact area and thus minimize stress build-up near the serrations (which act as stress raisers). In a stem-guided valve body having a longitudinal axis of symmetry, for example, this means that serrations on first opposing walls are located as close as practicable to the longitudinal axis of symmetry (i.e., relatively centrally in the seal retention groove). 
   On the other hand, the bending stress acting on the second opposing wall of an integral seal retention groove on a stem-guided valve body will typically be greatest near the portion of the second opposing wall nearest the longitudinal axis of symmetry. Hence, for the stem-guided valve bodies of the present invention, serrations on second opposing walls are located as far as practicable from the longitudinal axis of symmetry (i.e., peripherally, relative to the centrally-placed serrations on the first opposing walls as noted above). 
   Serrations (or serration groups) on first and second groove walls are therefore offset. Serration offset, in turn, minimizes the detrimental stress-raising effects of the serrations on their respective valve body flange areas. Additionally, use of offset serrations means that retaining forces exerted on an elastomeric seal in a groove by serrations on the opposing groove walls are further spaced apart as a function of the offset distance. This further spacing apart adds to the separation of forces acting on the seal and thus reduces stress concentration in the seal elastomer that would tend to tear the seal apart. Reduced stress concentration, in turn, results in reduced elastomer stress for seals in the present invention compared with seals illustrated in the &#39;299 and &#39;481 patents (where serrations are not offset but instead are directly opposite each other). 
   Thus, the offset serrations in a seal retention groove of the present invention act to reduce the likelihood of fatigue failures in valve body flanges, and they simultaneously reduce stress concentration in the elastomer of a seal in the groove. The latter benefit may be further increased if spacing between the respective flange areas that form the opposing walls of a peripheral seal retention groove increases as a function of increasing distance from the valve body longitudinal axis. Spacing between seal retention groove walls is determined in part by industry standards but may preferably be increased in valve bodies of the present invention to obtain additional elastomer stress relief. For this reason, the parallel seal retention groove walls illustrated in the &#39;299 and &#39;481 patents are less preferred than groove walls which diverge peripherally. 
   For many embodiments of the present invention, the desired peripheral divergence of seal retention groove walls is already inherent in industry standards. For example, valve bodies intended to be used as replacements for another valve body previously used with a TRW Mission 4-web seat must match analogous dimensions on the previously-used valve body. On the other hand, internal dimensions such as the degree of divergence of the opposing groove walls (reflecting the groove width at each point) can to some extent be uniquely specified. 
   In preferred embodiments of the present invention, seal retention groove width increases as a function of increasing distance from the valve body longitudinal axis. The amount of groove width increase at each point is sufficient to allow escape for air bubbles that may be present during casting of an elastomeric seal in a mold comprising the groove when the longitudinal axis is in the preferred (vertical) orientation. A desirable amount of groove width increase with increasing distance from the valve body longitudinal axis may be specified in terms of the included angle measured between straight (or nearly straight) portions of seal retention groove walls. The preferred range of these included angles for the present invention is about 10 to 60 degrees, depending on the particular valve body design. 
   Note that curvature of groove walls toward the center of the groove is allowable if the resulting curved wall would not trap air bubbles in liquid elastomer when the valve body is in position for casting of a seal in the groove. Included angle measurements for any curved portion of the groove wall are preferably made using the chord of any such curved portion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  schematically illustrates areas of typical failure modes of cast-in-place elastomeric seals that are bonded on a valve body. 
       FIG. 1B  schematically illustrates a partial cross-section of a web-seat, stem-guided valve comprising a valve seat with a valve body and seal assembly. 
       FIG. 1C  schematically illustrates a partial cross-section of an open-seat, stem-guided valve comprising a valve seat with a valve body and seal assembly. 
       FIG. 2  schematically illustrates an enlarged partial cross-section of the valve body of  FIG. 1B  comprising a peripheral integral seal retention groove having offset serrations and adhesion-inhibiting surfaces. 
       FIG. 3  schematically illustrates an enlarged view of a peripheral integral seal retention groove section similar to that shown in  FIGS. 1B and 2 , indicating areas of maximum impact stress and maximum bending stress. 
       FIG. 4  schematically illustrates a cross-section showing casting-in-place of a single-durometer elastomeric seal in a mold comprising a mold shell mated with a peripheral integral seal retention groove of a valve body. 
       FIG. 5  schematically illustrates a cross-section showing a cast-in-place first section of a dual-durometer elastomeric seal in a mold analogous to the mold shown in  FIG. 4 ; a second liquid elastomer is shown being pored for casting-in-place a second section of a dual-durometer elastomeric seal. 
       FIG. 6  schematically illustrates a cross-section showing, after removal of the mold shell shown in  FIG. 5 , a cured dual-durometer elastomeric seal that has been cast-in-place on a valve body comprising a peripheral seal retention groove. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1A  indicates typical areas of high elastomer stress and associated premature seal failure expected in bonded seals on valve bodies like that in FIG. 2 of the &#39;995 patent. Note that the &#39;995 patent does not discuss seal failure due to high elastomer stress at all. On the contrary, by describing increased overall valve element integrity associated with the bonding of valve seal inserts to a valve body, the &#39;995 patent teaches away from the adhesion-inhibiting structures and functions of the present invention. 
   In so teaching, the &#39;995 patent simply reinforces the past failure of valve manufacturers to appreciate the important effects of seal elastomer background stress on valve durability. Until the present invention, the problem of elevated background elastomer stress in bonded seals was neither recognized nor effectively addressed. Indeed, the problem was actually compounded by the widespread industry practices reflected in the teachings of the &#39;995 patent. 
   In contrast, the present invention provides means to reduce background elastomer stress and reduce its deleterious effects. As described in the Background above, internal tears in elastomeric valve seal inserts like those schematically illustrated in  FIG. 1A  are precipitated by the repeated stress of valve closing and opening on the seal. In the past, bonding of cast-in-place seals unexpectedly hastened the occurrence of the tears. Valve service life was thereby shortened, but the seal failure mechanism (i.e., the presence of unnecessarily high background elastomer stress) was not understood. Indeed, although the invention of the &#39;995 patent reflects about fifteen years of accumulated industry experience with bonded urethane seal valves, the deleterious effects of background elastomer stress due to bonding is neither recognized nor addressed in the &#39;995 patent. Thus the long-felt need for improved durability of valves having cast-in-place elastomeric seals remained unmet after issuance of the &#39;995 patent. And this need remained unmet until the present invention refuted the teaching of the &#39;995 patent that encouraged bonding of cast-in-place seals to a valve body. The unexpected teaching of the present invention is that such bonding should be inhibited rather than encouraged, and means are described herein to achieve the desired adhesion inhibition. 
   The provision of adhesion-inhibiting surfaces in molds for cast-in-place elastomeric valve seals is therefore a distinguishing feature of the present invention that results in materially improved seal performance with conventional seal elastomers. For example, although the MDI polyester thermoset urethanes are well known seal materials, the present invention includes new and non-obvious ways to use these materials. Elastomeric valve seals cast-in-place on valve bodies according to the present invention differ materially from prior cast-in-place valve seals due to the inhibition of bonding or adhesion between the elastomer and the valve body, resulting in materially lower levels of background stress in the cured elastomer seal. Hence, valve seal assemblies comprising valve bodies and cast-in-place seals made by the new methods are also part of the present invention. 
     FIG. 1B  schematically illustrates a web-seat, stem-guided valve  10  comprising a valve seat  30  with a valve body and seal assembly  101 . Valve seat  30  comprises sealing surface  33  connected to stem guide  32  by a plurality of webs  31 . Valve body and seal assembly  101  comprises a stem-guided valve body  20 , and an elastomeric seal  40  in a peripheral integral seal retention groove  26 . Valve body  20  comprises guide stem  21  and guide stem  22 , guide stem  22  lying within stem guide  32  of valve seat  30 . Valve body  20  also comprises flange  27 , which in turn comprises first and second groove walls  28  and  29  respectively of peripheral integral seal retention groove  26 . First groove wall  28  is near peripheral metal sealing surface  23  (valve body impact area) of valve body  20 . When valve  10  closes, sealing surface  23  of valve body  20  strikes sealing surface  33  of web seat  30 , causing relatively high impact stress in sealing surface  23  and adjacent areas of the periphery of first seal retention groove wall  28 . 
     FIG. 1C  schematically illustrates an open-seat, stem-guided valve  10 ′ analogous to the web-seat, stem-guided valve  10  of  FIG. 1B . Valve  10 ′ in  FIG. 1C  comprises several structural features (and groups of structural features) labeled with primed numerals because they are analogous to structural features having the same numeral label in  FIG. 1B . For example, valve  10 ′ comprises a valve seat  30 ′ with a valve body and seal assembly  101 ′. Valve seat  30 ′ comprises sealing surface  33 ′. Valve body and seal assembly  101 ′ comprises a stem-guided valve body  20 ′ having a characteristic Channel-Beam shape and an elastomeric seal  40 ′ in a peripheral integral seal retention groove  26 ′. Valve body  20 ′ comprises guide stem  21 ′ and a plurality of valve guide feet  18  lying within open valve seat  30 ′, guide feet  18  being guided by cylindrical sidewall  32 ′ of valve seat  30 ′. Valve body  20 ′ also comprises Channel-Beam  27 ′ (analogous to flange  27  in  FIG. 1B ), which in turn comprises first and second groove walls  28 ′ and  29 ′ respectively of peripheral integral seal retention groove  26 ′. First groove wall  28 ′ is near peripheral metal sealing surface  23 ′ (valve body impact area) of valve body  20 ′. When valve  10 ′ closes, sealing surface  23 ′ of valve body  20 ′ strikes sealing surface  33 ′ of open seat  30 ′, causing relatively high impact stress in sealing surface  23 ′ and adjacent areas of the periphery of first seal retention groove wall  28 ′. 
     FIG. 2  schematically illustrates an enlarged partial cross-section of valve body  20  as shown in  FIG. 1B  but without seal  40 . Serration group  25  is shown offset peripherally a distance D from serration group  24 . Serration group  24  on first groove wall  28  of peripheral integral seal retention groove  26  is located as far centrally as practicable (i.e., as close to the longitudinal axis of symmetry x-x of valve body  20  as practicable). Conversely, serration group  25  on second groove wall  29  is located as far peripheral as practicable (i.e., as far from the longitudinal axis of symmetry x-x of valve body  20  as practicable). Distance D represents the difference in the mean of distances measured from the longitudinal axis x-x to serrations in serration group  25 , minus the mean of distances measured from the longitudinal axis x-x to serrations in serration group  24 . 
     FIG. 3  schematically illustrates an enlargement of the partial cross-section of valve body  20  shown in  FIG. 1B . In addition to showing serration group  25  offset peripherally a distance D from serration group  24 ,  FIG. 3  shows areas of peripheral integral seal retention groove  26  which are subjected to high bending stress (area B), as well as areas of groove  26  which are subjected to high impact stress (area I). 
   Note that placement of serration group  24  centrally is limited because the serrations must resist movement of seal  40  out of groove  26 . This is best accomplished if serration group  24  is located on the relatively straight part of groove wall  28  as far central as this straight part extends. Similarly, serration group  25  is preferably located peripherally on the relatively straight part of groove wall  29 . Any offset greater than zero between serration groups  24  and  25  confers a benefit by reducing bending and/or impact stress in flange  27  relative to earlier cited valve designs. 
   The depth of serrations in groups  24  and  25  is predetermined in light of shrinkage anticipated during curing of elastomeric seal  40  so that after maximal shrinkage, seal  40  will remain effectively interdigitated with serration groups  24  and  25 . Effective interdigitation is the minimum interdigitation required to prevent movement of seal  40  within groove  28  that would lead to premature failure of the valve. As a general guide, assuming elastomer shrinkage of about 2% during curing, the height of a given serration is preferably 5% to 15% of the shortest distance between groove walls  28  and  29  as measured at the given serration. 
   Note that a serration height equal to 5% of the distance between groove walls, when combined with 2% elastomer shrinkage on curing, means there is a minimum overlap of serrations with elastomer of approximately 3% of the distance between groove walls. Further, the cured elastomeric seal, although shrunk, remains interdigitated with the serrations for greater than half of their predetermined height. For relatively less severe service conditions, this overlap may constitute sufficient interdigitation of serrations and elastomer to retain the seal in proper position in the groove. For more severe service conditions, overlap of approximately 13% (obtained with a serration height of approximately 15% of the distance between groove walls) may be needed to retain the seal in proper position. In this latter case, the cured elastomeric seal remains interdigitated with said serrations for greater than 86% of the predetermined height. But note also that as serration height increases, internal stress concentration in the seal elastomer also increases. Thus, service conditions must be considered in light of elastomer properties to determine the serration height for greatest service life or least life-cycle cost in any particular application. 
     FIG. 4  schematically illustrates a mold  202  comprising a peripheral integral seal retention groove  26  of a valve body  20  (see also  FIG. 1B ). Mold  202  also comprises mold shell  60 , which mates with valve body  20 . In use, mold shell  60  is temporarily mated with valve body  20  to form mold  202 . Liquid elastomer (liquid urethane for example) is poured into mold  202  and cured therein to form a single-durometer cast-in-place elastomeric seal  40 . During curing of seal  40 , normal shrinkage of the elastomer takes place without appreciable adhesion of seal  40  to surfaces of mold  202  because these are adhesion-inhibiting surfaces. After seal  40  is cured, mold shell  60  will be removed, leaving a single-durometer valve body and seal assembly  101 . Only minor machining of seal  40  may be required after removal of mold shell  60 . 
     FIG. 5  schematically illustrates a cross-section showing a cast-in-place first section  41  of a dual-durometer elastomeric seal  45  (see  FIG. 6 ) in a mold  202  (analogous to mold  202  shown in  FIG. 4 ). After casting-in-place second seal section  42  in a manner analogous to that illustrated in  FIG. 5 , followed by removal of the mold shell  60 , a dual-durometer valve body and seal assembly  105  of the present invention is produced as schematically illustrated in  FIG. 6 . 
   Specifically,  FIG. 6  schematically illustrates a (dual-durometer) valve body and seal assembly  105  having a longitudinal axis of symmetry and comprising a peripheral seal retention groove  26  spaced apart from the longitudinal axis. Seal retention groove  26  has first and second opposing groove walls  28  and  29  respectively. A valve body impact area  23  for contacting a valve seat is peripherally located on first opposing groove wall  28  of peripheral seal retention groove  26 , and circular serrations  24  and  25  are located on the first and second opposing groove walls  28  and  29  respectively of peripheral seal retention groove  26 . An elastomeric seal  45  is cast-in-place in peripheral seal retention groove  26 , and elastomeric seal  45  comprises a first seal section  41  and a second seal section  42 . First seal section  41  comprises a peripheral frusto-conical surface  63  for contacting a valve seat, frusto-conical surface  63  being adjacent to the valve body impact area  23 . And first seal section  41  comprises relatively high modulus polyurethane  141 . Second seal section  42  comprises a peripheral surface  83  adjacent to peripheral frusto-conical surface  63 , at least a portion of second seal section  42  comprising polyurethane  142  having a lower modulus than the relatively high modulus polyurethane  141  of first seal section  41 . 
   The present invention includes a method of making a (dual-durometer) valve body and seal assembly  105  comprising a seal  45  having first and second seal sections  41  and  42  respectively. The method comprises providing first and second liquid elastomers  141  and  142  respectively of different composition for casting first and second seal sections  41  and  42  respectively. A valve body  20  having a longitudinal axis is provided, valve body  20  comprising a peripheral seal retention groove  26  spaced apart from the longitudinal axis. Seal retention groove  26  has first and second opposing groove walls  28  and  29  and an impact area  23  for contacting a valve seat, impact area  23  being peripherally located on first opposing groove wall  28  of peripheral seal retention groove  26 . Circular serrations  24  and  25  are located on first and second opposing groove walls  28  and  29  respectively of peripheral seal retention groove  26 . A mold  202  is provided for containing liquid elastomers  141  and  142 , mold  202  comprising a mold shell  60  mated with valve body  20 . Mold shell  60  comprises a frusto-conical portion  73  for forming a frusto-conical surface  63  on first seal section  41  for contacting a valve seat, the frusto-conical portion  73  of mold shell  60  being adjacent to impact area  23  of valve body  20 . The first liquid elastomer  141  is poured into mold  202  to form first seal section  41  comprising frusto-conical surface  63 , and the second liquid elastomer  142  is then poured into mold  202  to form second seal section  42 . The poured first and second elastomers  141  and  142  respectively are cured in mold  202 , and mold shell  60  is then removed to make a (dual-durometer) valve body and seal assembly  105 .