Abstract:
In an improved floating ball check valve including a body having an outlet port, a cap, having an inlet port, joined with the body, a ball guide slidably received within the body and having a ball pocket for receiving a sealing ball, a biasing spring for biasing the ball toward the inlet port, an annular seat retainer in the cap and a generally ring-shaped seat, of non-resilient plastic material of preferably thermoplastic or fluoroplastic composition, substantially received within a conical bore within the cap and retained therein via the seat retainer, with the spherical surface of the ball being urged into a sealing engagement with an arcuate/spheric sealing surface of the seat having a radius slightly larger than the radius of the sealing ball, with the noted seat materials and geometries permitting seat self-compensation for pressure and temperature. An improved manufacturing method is also set forth.

Description:
CROSS-REFERENCE TO RELATED CASES 
   The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/557,652, filed Mar. 30, 2004, the full disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to check valves, particularly to floating ball check valves with replaceable thermoplastic valve seats having specific curved seat surfaces that mate with spherical radii of associated sealing balls, thus permitting self-compensation for pressure and temperature. 
   BACKGROUND OF THE INVENTION 
   Check valves are routinely used to permit fluid flow in but one direction, with two common types thereof being the ball check valve and the poppet valve. Both types may include a spring to bias the sealing member, either the ball or the poppet, against the valve seat to maintain a seal until the upstream fluid pressure, acting against the sealing member, exceeds the spring force to unseat the sealing member to allow fluid flow above a predetermined fluid or crack pressure. 
   Elastomeric material seats are commonly used in both of the noted types of check valves, as this material provides for an excellent and reliable seal at both low and high fluid pressures. Since elastomeric materials are resilient, the nature of elastomeric seals is dynamic, which only requires a minimum force to bias the ball or poppet against the valve seat to produce a seal at either low or high fluid operating pressures. Many check valve designs limit the bias force of the ball or poppet against the seat since excessive force can damage the elastomeric material and cause the valves to leak. However, elastomeric seals are limited in application, typically by either degradation, caused by chemical attack, or extreme low or high operating temperatures. 
   Thermoplastic materials are often used as replacements for elastomeric materials when these noted conditions apply. However, thermoplastic materials, unlike elastomeric materials, are not resilient and therefore are not well suited to provide for reliable dynamic seals. Thermoplastic materials provide the most effective seals under static loading conditions, requiring compressive forces that must increase with increasing fluid pressures to maintain their seals. However, thermoplastic materials also have high coefficients of thermal expansion and low compressive yield strengths that decrease rapidly with increasing operating temperatures. They also deform permanently, over time, at stress levels well below their yield strengths, and deform in greatly increasing amounts at higher operating temperatures, all of which can create difficulties when thermoplastic materials are used for seat materials in check valves. 
   In the existing art, some check valve designs offer thermoplastic seats as an option for elastomeric material seats, with either identical or closely similar valve seat geometries. However, check valves designed for resilient dynamic seals will not perform well with non-resilient seat materials. This is reflected in published limitations on seal tightness as well as temperature and pressure limits which fall well below the full capabilities of thermoplastic materials. 
   Other types of valve designs that commonly use thermoplastic materials for valve seats, such as quarter turn ball valves, offer seal tightness performance at pressure and temperature operating ranges that typically exceed those for existing check valves. It is therefore desirable and possible to improve upon the design of existing art ball check valves with thermoplastic seat materials such that a ball check valve will provide an equal or superior level of seal performance to that of a quarter turn ball valve. 
   The design and construction of the present invention is focused on easily replaceable, generally soft and somewhat flexible non-resilient plastic material, such, as for example, fluoroplastic material, valve seats. Such seats are machined or molded to final shape and separate coining is not required. These seats do have arcuate/spheric seating surfaces, but the seat radii thereof are slightly larger, by design, than the radii of the mating balls, thus permitting self-compensation for pressure and temperature. In addition, the amount of seat seal area can be designed specifically for a particular seat material, based on its mechanical properties and desired temperature and pressure operating conditions, all of which will be explained in more detail hereinafter. 
   The patent literature includes a large number of ball check valve constructions, with FIG. 4 of U.S. Pat. No. 5,107,890, to Gute, which pertains to a ball check valve, showing a curved seating surface matched to the ball. However, it is stated therein that the coined arcuate (curved) seating surface is complimentary to the ball surface to reduce leakage and the claimed method of manufacturing the seat requires a coining operation to “form a desired arcuate configuration seating surface”. In addition, the seat is generally manufactured from a brass material and that the seat is press-fit or friction fit into the valve body. As previously noted, the seat of the present invention is formed of a non-resilient plastic material and has an arcuate/spherical seating surface having a radius slightly larger than the radius of the ball, thus permitting self-compensation for pressure and temperature. 
   U.S. Pat. No. 4,197,875, to Schieferstein et al., and U.S. Pat. No. 5,749,394, to Boehmer et al., both pertaining to check valves, set forth that the seats are constructed of elastomeric sealing material, not the thermoplastic seat material of the present invention, and these constructions also use a conical seat which is a of a completely different geometry than the spherical design and construction of the seat of the present invention. Further prior art patents, relating to ball check valves additionally include: U.S. Pat. No. 3,040,771 to Droitcour et al.; U.S. Pat. No. 4,084,304 to Myers; U.S. Pat. No. 4,541,412 to Bagshaw et al.; U.S. Pat. No. 4,613,738 to Saville; U.S. Pat. No. 4,736,083 to Saville; U.S. Pat. No. 5,251,664 to Arvidsson et al.; and U.S. Pat. No. 6,250,336 B1 to Murphy et al. However, none of these prior art structures disclose a spherical ball valve received within a spherical seat seal, where the seat seal is preferably formed of a fluoroplastic-type material and has a radius slightly larger than the radius of the mating ball. 
   SUMMARY OF THE INVENTION 
   Accordingly, in order to overcome the deficiencies of the prior art devices, the present invention provides an improved floating ball check valve that utilizes a generally ring-shaped seat of a non-resilient plastic material, such as a thermoplastic or fluoroplastic material, for example that can be molded or machined without requiring coining thereof. These seats have an arcuate/spheric seating surface that mates with the spherical surface of the sealing ball with the seat radius being slightly larger than the radius of the ball thus permitting self-compensation of the seat for operating pressure and temperature. In addition, the area of the seat seal can be tailored to the use of a specific material&#39;s mechanical properties. 
   At low fluid back pressure, the seal area is small thus generating high surface contact pressure that improves the low pressure seal. As back pressure increases, the ball is forced further into the seat, thus causing the seal to flex which increases the seal surface area in contact with the ball and helps to maintain stress in the seat at an acceptable level to prevent permanent deformation or damage to the plastic seat. 
   However, the seat can also compensate for high back pressures that result in seat stress into the plastic deformation range thereof. In this case, as the ball pushes deeper into the seat, the seal contact area continues to increase until the stress is redistributed and reduced to levels below the material&#39;s plastic deformation limit. The result is a permanent deformation of the seat which provides for an increase in sealing area that can, in turn, provide higher pressure loading. 
   While PTFE-type thermoplastics have high coefficients of thermal expansion, rapidly lose strength and are prone to extrusion (commonly called hot-flow) at higher temperatures, the seat of the present invention is designed to compensate for these material characteristics in two ways. The first way is substantially similar to the high pressure compensation mechanism already described previously, namely that the ball is forced into the seat, thereby increasing the seal area supporting the ball load until the stress is stabilized below the creep range at the particular operating temperature. 
   If the temperature becomes high enough, then the plastic material will expand and extrude a substantial amount. The present invention takes advantage of this phenomenon by directing material flow into an area in front of the ball and through the aperture in the seat retaining washer. This adds material to the seat area which reduces stress in the seat. The increase in seal area continues until seat stress is reduced below the material&#39;s creep limit. During testing, increases in the seal area of over 100%, at maximum operating temperature and back pressure, have been observed. The benefit of this seal enlargement is that it permits higher pressure and temperature operation while still maintaining acceptable seat stress levels. 
   Specifically, the structure and function, of this invention, in a floating ball check valve, comprise in combination: a. a body having inner and outer axially adjacent portions, with each non-adjacent end thereof including a coupling member, the body including a through bore, with the body inner portion having first and second concentric cylindrical bores and an outlet port; b. a cap having inner and outer axially adjacent cap portions, with each non-adjacent end thereof including a coupling member, the cap including a through bore, with the cap inner portion having a plurality of concentric cylindrical bores, the cap outer portion having an inlet port and a concentric conical surface, the body inner portion being adapted to be inserted into a first one of the plurality of cylindrical bores of the cap inner portion and coupled with the cap inner portion; c. a generally cylindrical ball guide having a through bore and axially spaced internal bore portions, the ball guide being adapted to be slidably inserted into the first bore of the body inner portion, the ball guide also including a ball pocket portion for receiving and axially centering a sealing ball; d. a biasing spring adapted to be inserted into the body through bore and confined between the body inner portion second bore and an opposing internal bore portion of the ball guide, the spring serving to bias the sealing ball in the direction of the inlet port; e. a generally annular seat retainer located adjacent to the conical bore of the cap outer portion, with a radial outer annular portion of the seat retainer being biased against a radial outer shoulder portion of the conical bore portion by an end surface of the body inner portion; and f. a generally ring shaped seat, of non-resilient plastic material, substantially received within the conical surface of the cap outer portion and retained therein via a radial inner annular portion of the seat retainer, with the spherical surface of the sealing ball being urged into a sharp corner line contact sealing engagement with an adjoining seal surface of the seat, wherein the adjoining seal surface takes the form of a curved seal surface having sufficient seal surface contact with the sealing ball to prevent excessive yielding of the non-resilient plastic seat material at predetermined operating temperatures and back pressures. 
   In one version thereof the curved seal surface takes the form of a spherical surface, wherein the spherical seal surface has a radius slightly larger than the spherical radius of the sealing ball, with the spherical seat surface radius being slightly larger, in the range of about 0.002 to 0.010 inches, than the spherical radius of the sealing ball. 
   In another version, the spherical radius of the sealing ball is slightly smaller than the radius of the spherical seal surface, with the spherical radius of the sealing ball being slightly smaller, in the range of about 0.002 to 0.010 inches, than the radius of the spherical seal surface. 
   In a further version, wherein the ring-shaped seat further includes a concentric through bore and a flat annular end surface, the seal spherical surface is bounded, on one end, by a radial inner end of the annular end face and, on another end, by one end of the concentric through bore, with a first intersection of the sealing ball spherical surface with the seal spherical surface being bounded by the radial inner end of the seat annular end face and a second such intersection of the sealing ball spherical surface with the one end of the seal spherical surface being bounded by the one end of the seat concentric through bore. The first intersection of the sealing ball spherical surface with the annular end face radial inner end, when viewed in cross section, is defined by a first angle bounded by two line segments extending from the center of the sealing ball to the first intersection, while the second intersection of the sealing ball spherical surface with the one end of the seat concentric through bore, when viewed in cross section, is defined by a second angle bounded by two additional line segments extending from the center of the sealing ball to the second intersection. In addition, the first angle is of a greater angular extent than the second angle, with the angular extent of the first angle ranging from about 110 to about 160 degrees and the angular extent of the second angle ranging from about 80 to about 130 degrees. 
   In an additional version, the non-resilient plastic material of the seat is a fluoroplastic type of material or one of a thermoplastic and fluoroplastic material and preferably selected from the group consisting of PTFE of filled, unfilled and advanced copolymer grades thereof. 
   In yet a differing version, the non-resilient plastic materials are selected from the group consisting of acetal, ultra high molecular weight polyethylene, filled and unfilled polymide as well as filled and unfilled polyetheretherketone materials. 
   Furthermore, in a method of manufacturing a ball check valve, comprising: a. providing a body having inner and outer axially adjacent portions, with each non-adjacent end thereof including a coupling member, the body including a through bore, with the body inner portion having first and second concentric cylindrical bores and an outlet port; a cap having inner and outer axially adjacent cap portions, with each non-adjacent end thereof including a coupling member, the cap including a through bore, with the cap inner portion having a plurality of concentric cylindrical bores, the cap outer portion having an inlet port and concentric conical surface; a generally cylindrical ball guide having a through bore and axially spaced internal bore portions, the ball guide also including a ball pocket portion for receiving and axially centering a sealing ball; b. inserting the body inner portion into a first one of the plurality of cylindrical bores of the cap inner portion and coupling same together; c. slidably inserting the ball guide into the first bore of the body inner portion member inner portion; d. inserting a biasing spring into the body through bore and confining same between the body inner portion second bore and an opposing internal bore portion of the ball guide, the spring serving to bias the sealing ball in the direction of the inlet port; e. locating a generally annular seat retainer adjacent the conical surface of the cap outer potion and biasing, via an end surface of the body inner portion, a radial outer annular portion of the seat retainer against a radial outer shoulder portion of the conical bore portion; and f. locating a generally ring-shaped seat, of non-resilient plastic material, substantially within the conical surface of the cap outer portion and retaining same therein via a radial inner annular portion of the seat retainer and urging the spherical surface of the sealing ball into a sharp corner line contact sealing engagement with an adjoining conical seal surface of the seal, the improvement comprising: g. modifying the adjoining conical seal surface to the form of a curved seal surface having sufficient seal surface contact with the sealing ball to prevent excessive yielding of the non-resilient plastic material at predetermined operating temperatures and back pressures. 
   The improved method of manufacturing further includes modifying the adjoining conical seal surface to the form of a spherical seat surface. 
   Another version further of the improved method includes additionally modifying the adjoining conical seal surface to have a radius slightly larger than the spherical radius of the sealing ball, by increasing the spherical seal surface radius to be slightly larger, in the range of about 0.002 to about 0.010 inches, than the spherical radius of the sealing ball. 
   In a differing version, the improved method further includes: selecting the non-resilient plastic material from the group consisting of thermoplastic and fluoroplastic materials. 
   The previously-described advantages and features, as well as other advantages and features, will, become readily apparent from the detailed description of the preferred embodiments that follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view, partially in section, of the floating ball check valve in accordance with this invention, illustrated in a closed position; 
       FIG. 2  is a view, similar to that of  FIG. 1 , illustrating the check valve in an open position; 
       FIG. 3  is an enlarged cross sectional side of the valve seat sealing area, illustrating the principle of the method of sealing at low back pressure and operating temperature; 
       FIG. 4  is a view, similar to that of  FIG. 3 , illustrating the principle of the method of sealing at moderate back pressure and operating temperature; 
       FIG. 5  is a view, also similar to that of  FIG. 3 , illustrating the principle of the method of sealing at high back pressure and operating temperature; 
       FIG. 6  is a view, again similar to that of  FIG. 3 , illustrating the principle of the method of seat extrusion at high temperature; 
       FIG. 7  is a longitudinal cross sectional side view of the valve body of this invention; 
       FIG. 8  is a longitudinal cross sectional side view of the valve cap of this invention; 
       FIG. 9  is a longitudinal cross sectional side view of the ball guide of this invention; 
       FIG. 10  is a longitudinal cross sectional side view of the thermoplastic seal of this invention; 
       FIG. 11  is a longitudinal cross sectional side view of the seat retainer of this invention; and 
       FIG. 12  is a view, similar to that of  FIG. 10 , with the addition of a mating ball, illustrating the sealing relationship therebetween. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the several drawings, illustrated in  FIG. 1  is a partial longitudinal cross section of the floating ball check valve, generally indicated at  20 , showing check valve  20  in the closed position, i.e., showing the locations of the axially movable components when the biasing force exceeds the upstream pressure, thereby preventing pressurized fluid flow through check valve  20 .  FIG. 2  is a view, similar to that of  FIG. 1 , but showing check valve  20  in the open position, i.e., when the upstream fluid pressure exceeds the biasing force, thereby allowing pressurized fluid to flow through check valve  20 . As best illustrated in  FIGS. 1 and 2 , ball check valve  20  includes a valve body  22 , a valve cap  24 , a bias spring  26 , a ball guide  28 , a sealing ball  30 , a seat retainer  32  and a seat  34 . In addition, check valve  20  is provided with an inlet port  38  and an outlet port  40 , with the flow through check valve  20  being unidirectional from inlet port  38  to outlet port  40 . 
   Before proceeding with a detailed description of the assembly and function of check valve  20 , a detailed description of the above-noted main component parts will now follow. As best shown in  FIG. 7 , valve body  22 , which is preferably fabricated of metal, is generally cylindrical in shape and includes an externally threaded portion  42  for operative engagement with valve cap  24 . Body  22  further includes an external tool receiving portion  44 , preferably of hexagonal shape, a cylindrical internal bore  46  which serves as a fluid flow passage and as a precise axial guiding bore for axially movable ball guide  28 . The inner end of bore  46  is provided a flat annular surface  48  for limiting the axial travel of ball guide  28 , which then limits the compression of bias spring  26  and prevent the over-compression thereof. Flat annular surface  48 , in turn, merges into a counterbore  50  that functions to retain and center bias spring  26 . Outlet port  40  provides for a fluid connection with internal bore  46 . The upstream end of valve body  22  includes a cylindrical section  52  having an external diameter closely matched to the internal diameter of cap counterbore  76  ( FIG. 8 ) such that when cap  24 . is threadably connected with body  22 , the center axes of cap  24  and body  22  are coaxial, which assures the close axial alignment of all internal valve components. A flat end  54  of body  22  functions as a gasket surface which, when cap  24  is threadably engaged onto body  22 , forms a compression seal against seat retainer  32  ( FIG. 1 ) to prevent external leakage. Preferably a surface coating (not shown) is applied, if at all, only to valve body internal bore  46  to reduce friction, prevent galling and/or to prevent undesired deposits, and can be beneficial because it will prevent ball guide  28  from getting stuck inside bore  46 . 
   Bias spring  26 , as best shown in  FIGS. 1 and 2 , which is preferably made of metal, is generally cylindrical with parallel ground ends. The diameter of spring  26  is such that it will fit into valve body counterbore  50  and the internal bore  64  ( FIG. 9 ) of ball guide  28  with minimal clearance to align the axes of spring  26  and counterbore  50  while also permitting sufficient clearance for the increase in spring diameter when same is compressed. The diameter of the spring wire and the number of coils will be determined by the preload force required to define the upstream pressure needed to open or crack check valve  20  to allow fluid to flow. 
   Turning now to  FIG. 9 , illustrated there is ball guide  28 , preferably made of metal, which is generally cylindrical and has a diameter closely matched with that of valve body cylindrical bore  46  for clearance and alignment purposes. A radial relief groove  58  on outside cylindrical surface  60  reduces contact area, minimizes friction and reduces crack pressure variation. Lead-in chamfers  62   a ,  62   b  on the outside ends of cylindrical surface  60  serve to prevent ball guide  28  from gouging valve body internal bore  46  during axial movements of the former. The downstream end of ball guide  28  contains an internal bore  64  for housing spring  26  and to provide a fluid flow passage through ball guide  28 . A flat annular end surface  66 , at chamfer  62   b , provides a surface which contacts flat surface  48  of valve body  22  to limit the axial travel of ball guide  28 , which then limits the compression of bias spring  26 . The upstream end of ball guide  28  contains a cylindrical orifice bore  68  which provides for controlled fluid flow passage, the diameter of which can be sized appropriately for precise fluid flow. A plurality of peripheral metering slots  70  provide a fluid flow passage around sealing ball  30  and by the size, number and length of slots  70  can also control metering flow. A coaxial conical bore  72  provides a pocket for receiving and axially centering sealing ball  30 . 
   Continuing now with  FIG. 8 , illustrated there is valve cap  24 , preferably made of metal, which is generally cylindrical in shape, includes inlet port  38 , for fluid passage, an internally-threaded portion  74  for threadably engaging valve body  22 , and an external tool-receiving portion  75 , preferably of hexagonal shape. An internal counterbore  76  has a diameter closely matched to the diameter of valve body cylindrical section  52  such that when cap  24  is threadably engaged into valve body  22 , the center axes of valve cap  24  and body  22  are preferably coaxially aligned thus assuring the close axial alignment of all internal components. An annular flat end surface  78  of counterbore  76  acts as a gasket surface which, when valve cap  24  is threadably engaged onto body  22 , forms a compression seal against seat retainer  32  ( FIGS. 1 ,  2 ) to prevent external leakage. As best seen in  FIGS. 1 and 2 , thermoplastic valve seat  34  is contained within valve cap circular counterbore  80  at the inlet end of valve cap  24 . A conical surface  108  ( FIG. 10 ) of the back side of seat  34  matches conical surface  82  of the bottom of valve cap circular counterbore  80 , which serves to precisely center valve seat  34  to the central longitudinal axis of valve cap  24 . The length of conical surface  82 , truncated by radius  84  which, in turn, blends tangentially into a conical bore  86  in valve cap inlet port  38 , is by design, shorter than the length or annular extent  108   a  ( FIG. 10 ) of a conical surface  108  on the back side of seat  34  such that the radial inner portion of conical surface  108  remains unsupported by conical surface  82 . A fulcrum line  88  relative to which valve seat  34  flexes, under the load applied by sealing ball  30 , is established by diameter  85  of radius  84 . The size of diameter  85  will affect the amount of flexure of valve seat  34 , with the amount of flexure increasing as diameter  85  increases. Fulcrum line  88  is of importance in the design of seat  34  as it directly impacts the development of the seat stress required for seat flexing. A conical bore  86 , in valve cap inlet port  38 , is designed to direct and/or control seat material extrusion in front of sealing ball  30  so as to create an additional seal contact area  120   d  ( FIG. 6 ) of the spherical surface  110  ( FIG. 10 ). The extruded seat material  34   a  ( FIG. 6 ) is rigidly supported by conical bore  86 , which then, in turn, restricts the forward movement of sealing ball  30  and prevents same from pushing completely through seat  34  and contacting the surface of conical bore  86 . However, should the operating temperature increase to the point of destroying thermoplastic seat  34 , sealing ball  30  will seat against the surface of conical bore  86  and effect a seal, although this seal will permit a certain amount of through leakage. 
   Focusing now on  FIG. 11 , illustrated therein is seat retainer  32 , preferably constructed of metal and spring-tempered to improve seat clamping force, with retainer  32  being generally round, having a concentric through bore  90 , outside diameter  92 , inside diameter  94 , opposed annular end faces  96 ,  98 , and a substantially rectangular cross section. Seat retainer  32  performs a dual function, first to retain valve seat  34  within valve cap counterbore  80  and secondly, to form an external seal between valve body  22  and cap  24 . This external seal is achieved when valve body  22  is threadably engaged into cap  24  with a specified amount of torque. The respective front and rear surfaces  96 ,  98  of seat retainer  32  are compressed between valve body flat end  54  and valve cap flat shoulder  78 , thus effecting a seal. The surfaces of seat retainer  32  may be coated, such as at  102 , with a material such as PTFE to improve the integrity and tightness of the external seal, or separate gaskets (not shown), compressed between valve body end  54 , seat retainer  32  and valve cap counterbore shoulder  78 , may be employed as an alternative. Seat retainer  32  retains valve seat  34  in valve cap counterbore  80  under a spring-like compression force. The thickness or axial extent of seat  34  is slightly greater than the depth or axial extent of cap counterbore  80  so that, when valve body  22  is threadably engaged into cap  24  with a specified amount of torque and seat retainer  32  is compressed between valve body flat end  54  and valve cap flat shoulder  78 , an area  100  (best shown in  FIG. 1 ) of seat retainer  32 , unsupported by valve body flat end  54 , flexes elastically, thereby generating a clamping force against seat  34 . 
   As illustrated in  FIG. 2 , sealing ball  30 , preferably made of metal, has a spherical surface  36 . 
     FIG. 10  illustrates seat  34  as being generally cylindrical, having a concentric through bore  106  and an, adjoining conical counterbore  108  which has an internal angle substantially similar to cap conical bottom surface  82 , with the substantial matching of theses conical surfaces assuring the precise axial centering of seat  34  relative to cap  24 . Seat  34  further includes spherical seal surface  110 , inner peripheral surface  112 , outer peripheral surface  114 , radius portion  116  and flat annular end face  118 . The radius of spherical seal surface  110  is slightly larger, generally in the range of 0.002 to 0.010 inches, than spherical radius  36  of sealing ball  30 . The amount of surface area contained in spherical surface  110  can be adjusted, by design, to provide for an optimum level of contact stress for low pressure seal tightness and/or high back pressure load capacity and/or for a particular type of thermoplastic seat material and/or for a specific application requirement while still maintaining the modular design of the valve and utilizing fully interchangeable valve seats. Seat  34  is generally constructed of thermoplastic or fluoroplastic materials, examples of which include PTFE (filled or unfilled and advanced copolymer grades), acetal, ultra high molecular weight polyethylene, polymide (filled or unfilled) and polyetheretherketone (filled or unfilled) materials. 
   Turning now to a detailed description of the assembly and function of check valve  20 , check valve  20  includes sealing ball  30  that is axially movable within valve body cylindrical bore  46 . Ball  30  is guided during axial movement by axially movable cylindrical ball guide  28  which also provides for the precise centering of ball  30  relative to the axis of valve body cylindrical bore  46 . However, since ball  30  is not physically attached to ball guide  28 , ball  30  may still align itself to mating spherical seal surface  110  of valve sear  34  if ball guide  28  should become slightly misaligned relative to bore  46  as a result of the necessary clearance between ball guide  28  and valve body cylindrical bore  46 . Ball guide  28  also includes internal bore portion  64  for accepting spring  26  which provides the bias force, for ball guide  28 , that is subsequently transferred to ball  30 . 
   Thermoplastic valve seat  34  is contained within valve cap circular counterbore  80  in the vicinity of valve cap inlet port  38 . Conical surface  108 , on the back side or surface of seat  34 , substantially matches conical surface  82  of the bottom of valve cap circular counterbore  80  which precisely centers seat  34  to the center axis of cap  24 . Seat  34  is retained within counterbore  80  by circular retaining washer or seat retainer  32  which fits into valve cap counterbore  76 . Retaining washer  32  also exerts and maintains a spring clamping force on seat  34  to establish a seal between seat surface  108  and conical bottom surface  82  of cap counterbore  80 . Retaining washer  32 , having concentric through bore  90 , has its inside diameter  94  dimensioned large enough to expose spherical seal surface  110 , molded or machined on the front side of seat  34  and with which axially movable ball  30  engages, to form a seal when the bias force, exerted by spring  26  and combined with the force exerted by fluid pressure at valve outlet port  40 , exceeds the force exerted by the fluid pressure at valve inlet port  38 . 
   The seal, between ball  30  and seat  34 , is achieved by a compressive load applied to seat spherical seal surface  110  by ball spherical surface  36 . This compressive load produces a contact surface stress between ball surface  36  and seal surface  110  of a magnitude dependent upon the contact area according to the formula of applied force divided by contact area. Generally, with a compressive type seal, the contact stress must reach a certain minimum level before a seal can be achieved. This magnitude of the minimum required contact stress is affected by the manufacturing precision of the sealing members, seal material properties such as hardness, and the type of fluid or gas to be sealed. Seat spherical seal surface  110  is configured so as to provide a variable portion of this seal area to be in contact with ball  30  in order to maintain the contact stress above the required minimum level to provide for consistent seal tightness at low fluid pressure while also providing for an increased seal contact area in order to reduce seat stress and minimize plastic deformation at higher fluid pressures. These objectives are achieved by machining or molding spherical surface  110  into seat  34  with a radius that is slightly larger, preferably in the range of 0.002 to 0.010 inches, than the radius of ball  30  and by providing partial support to seat back surface  108  such that a bending moment will develop in seat  34  due to the force applied by ball  30 , thus causing seat  34  to flex. 
   Referring now to  FIG. 3 , at low fluid back pressure, only a small area  120   a  of seat spherical seal surface  110  is in contact with ball spherical surface  36 , resulting in high contact stress which improves low pressure sealing capabilities. As fluid back pressure increases, the force of ball  30  pushing into seal surface  110  increases, thereby developing a bending moment in seat  34 . Continuing to  FIG. 4 , this bending moment results in a flexure in seat  34 , which exposes an increased area  120   b  of seal surface  110  into contact with ball surface  36 . This increase in contact area limits the increase in seat stress as the fluid pressure increases, thus preventing significant permanent deformation to thermoplastic seat  34  over an extended range of fluid pressure while still achieving an amount of contact stress above the required minimum to maintain a satisfactory seal. 
   Seat  34  can also compensate for high fluid back pressures that result in stress that is above the plastic deformation range of the thermoplastic material from which seat  34  is formed. Turning specifically to  FIG. 5 , as ball  30  pushes deeper onto seal surface  110 , the area of seal surface  110  is enlarged by plastic deformation until the stress is redistributed and reduced to a level below that of the plastic deformation range of the thermoplastic seat material. This results in a permanent deformation (via enlargement) of seal surface  110  which in turn provides for an increased seal contact area  120   c  that can support the higher fluid pressure loading. It should be understood that the plastic deformation of thermoplastic seat  34  will not compromise low pressure seal performance. As seal surface  110  is deformed, the precision of its spherical surface is improved so as to more closely match that of ball surface  36 . In addition, any surface irregularities in spherical surface  110  that result from the original formation thereof are reduced, thus producing an improved surface finish that lowers the minimum level of surface contact stress required to achieve a seal. At elevated operating temperatures, the thermoplastic material of seat  34  expands greatly in volume and also softens, which, when under a compressive load, can result in significant plastic deformation, commonly referred to as hot flow. Seat  34  is designed to compensate for this condition in two ways, the first of which is identical to the high fluid pressure compensation, as already previously described, wherein ball  30  is forced deeper into seal surface  110  thus enlarging contact area  120   c  until the stress is stabilized below the creep range of the seat material. 
   Now referring to  FIG. 6 , in the second compensating way, as ball  30  is forced deeper into seat  34 , the plastic material of seat  34  is extruded outwardly around ball  30  in opposing directions. One portion  34   a  of the material of seat  34  extrudes in front of ball  30  into conical bore  86  in valve cap inlet port  38 , thereby producing an additional seal contact area  120   d  of spherical seal surface  110 . This extruded material is rigidly supported by conical bore  86  in valve cap inlet port  38  which restricts the forward movement of ball  30  and prevents ball  30  from pushing completely through seat  34  and contacting the surface of conical bore  86 . Another portion  34   b  of the seat material extrudes between ball  30  and seat retainer through bore  90 , with this extruded material being rigidly supported by the cylindrical wall of bore  90 , thereby producing a yet additional seal contact area  120   e  of spherical seal surface  110 . The enlargement of the seal contact area of seal surface  110 , resulting from noted seat material extrusions  34   a ,  34   b , continues until the seat stress stabilizes below the creep limit of the thermoplastic seat material. 
   The desired result of the extrusions  34   a ,  34   b , of the thermoplastic material of seat  34  is the total enlarged seal contact area  120   f  of spherical seal surface  110 , with contact area  120   f  being the combination of additional contact areas  120   d  and  120   e  with original contact area  120   g . The acceptable amount of load for seat  34  is limited by the yield strength and resistance to creep (hot flow) of the seat material at a specific operating temperature. Stress being defined as unit load per area, an increase in unit area will permit a related increase in unit load while still maintaining identical stress in the seat material. 
   Turning now to  FIG. 12 , it illustrates seat  34  in combination with sealing ball  30 . The relationship between the size of the outside diameter  31  of ball  30  and the spherical sealing surface  110  of seat  34  can be varied or adjusted to affect the sealing performance to best suit the material or material characteristics of seat  34  and the operating pressure and temperature conditions encountered during use. This relationship is best illustrated by the two shown included angles  135  and  136  that are defined by the intersection of ball spherical surface  36  and the spherical sealing surface  110  of seat  34 . The smaller or interior angle  136  is defined by two line segments  138   a ,  138   b , drawn from the center point of ball  30  to the intersection points of spherical surface  36  and spherical sealing surface  110  at a point  142  at the end of through bore  106  of seat  34 . The larger or exterior angle  135  is defined by two line segments  140   a ,  140   b , drawn from the center point of ball  30  to the intersection points of spherical surface  110  at a point  144  at the radial inner end of flat annular end face  118  of seat  34 . 
   The size or angular extent of interior angle  136  is important and measurably affects both the load capacity and the low pressure seal ability of seat  34 . Reducing the extent of interior angle  136 , which can be accomplished by reducing the size of concentric through bore  106  of seat  34  relative to the diameter of ball  30 , adds surface area to spherical sealing surface  110 , thereby increasing the load capacity of seat  34 . However, at the same time, the ability of seat  34  to seal at low pressure is reduced correspondingly as the extent of interior angle  136  is reduced. 
   The reduction of low pressure seal ability, as the extent of interior angle  136  is reduced, is due to a reduction of the mechanical advantage that consequently reduces the magnitude of the force that generates the contact stress necessary to effect a seal in the small area  120   a  ( FIG. 3 ) of seat spherical seal surface  110 . In effect, ball  130  acts as a wedge driven into seat  34  at low pressure, generating an outwardly radial load against small area  120   a . Similar to a simple wedge, the magnitude of the outwardly radial force can be many times greater than the end force and is commonly referred to as “mechanical advantage”. The mechanical advantage of a simple wedge is a direct function of the wedge angle, with greater advantage being achieved as the extent of the wedge angle is reduced. A similar effect is observed in the present invention, wherein an increase in the extent of interior angle  136  creates an increase in mechanical advantage in a manner similar to decreasing the extent of a simple wedge angle. 
   Thus, an improved low pressure seal is achieved by increasing the extent of the interior angle  136 , which for a given load on ball  30 , due to the applied pressure, increases the force applied by ball  30  on small area  120   a , which in turn results in increased contact stress between ball  30  and small area  120   a , thus extending the lower range of pressure where a minimum contact stress, necessary to effect a seal in small area  120   a , can be achieved. 
   Based on the above discussion, there exist optimal geometries of interior angle  136  and exterior angle  135 , irrespective of the actual size of these components, that provide for optimal sealing performance of floating ball check valve  20 , dependent upon: the material composition of seat  34 ; the desired range of sealing pressures; the operating temperature of the application; and the type of fluid to be sealed. Preferably the extent of interior angle  136  ranges or varies between 80 degrees, for higher pressure applications, and 130 degrees for lower pressure applications. The extent of exterior angle  135  preferably ranges or varies between 110 degrees and 160 degrees, and is established so as to provide a sufficient amount of spherical surface area  110  of seat  34  for a particular material composition of seat  34  so as to support the loading of ball  30  at the maximum pressure and temperature of the specific application. 
   Thus, as previously described, the present invention provides a ball check valve with a thermoplastic seat that has the unique ability to self-adjust its seal surface area and geometry to provide for optimum seal performance, for the specific operating temperature and pressure of a particular application, within an expanded overall allowable range of fluid pressures and temperatures for a specific type of thermoplastic seat material. 
   It is deemed that one of ordinary skill in the art will readily recognize that the present invention fills remaining needs in this art and will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as described herein. While the present invention has been described with reference to but one type of a floating ball check valve, this invention is deemed to be readily applicable to all types of such valves. Thus, it is deemed that the protection granted hereon be limited only by the scope of the appended claims and their equivalents.