Abstract:
A three-way poppet valve that can restrict or eliminate backflow without the use of an external check valve. Unlike conventional three-way poppet valves wherein both inlets can be open to the outlet at the same time during movement of the poppet, the valve according to the present invention can be configured to close both inlets prior to opening one of the inlets to the outlet. The valve features a poppet valve assembly that is operable to prevent both inlets from being open to the outlet at the same time. The valve assembly also reduces the force needed to close the valve against at least one of the inlets thereby allowing the use of a smaller actuator. A unique seal including a valve seat that is configured to conform to a valve member under sufficient pressure.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/871,465, filed Oct. 12, 2007, which claims the benefit of U.S. Provisional Application No. 60/829,423 filed Oct. 13, 2006, both of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to valves. More particularly, the invention relates to poppet valves. 
       BACKGROUND OF THE INVENTION 
       [0003]    Three-way poppet valves are common in the existing art and are used in many applications for controlling and directing the flow of liquids or gases from either one of two discrete inlet ports to a single common outlet port. They are commonly constructed with an axially movable poppet located inside a valve body with a cylindrical bore. A biasing shaft is connected to the poppet to transmit a bias force to the poppet. Common methods for applying the bias force to the shaft include springs, pneumatic or hydraulic cylinders, and solenoid coils. 
         [0004]    In some constructions, the discrete valve inlets and seats are located at each end of the cylindrical bore of the valve body with the common outlet located between the two inlets. The valve poppet, with valve members on both ends, is biased against one seat at one end of the cylindrical bore to stop flow from that inlet while at the same time the poppet bias pulls the poppet sealing member away from the seat at the opposite end of the cylindrical bore allowing flow to commence from the one open discrete inlet to the common outlet. Reversing the bias of the poppet closes the open inlet and opens the closed inlet, thus allowing flow from the formally closed discrete inlet to the common outlet. 
         [0005]    Elastomeric seats are commonly used in three-way poppet valves as the material provides for an excellent and reliable dynamic seal at both low and high fluid pressures. Because elastomeric materials are resilient, such seals generally require a minimal force to create a seal at either low or high fluid pressure. Many designs employ o-rings constrained in a groove on the poppet that seal either laterally against a cylindrical bore or as a face seal against a flat surface. 
         [0006]    A distinct disadvantage typical of poppet valves of the construction just described becomes evident during the axial movement of the poppet when both inlets and the outlet can be in fluidic communication. Because the direction of fluid flow will always be from the high pressure port to the low pressure port, unintentional and undesirable backflow can occur through the lowest pressure inlet until the poppet completes its axial movement and pressure at the common outlet drops below the pressure at the open inlet. The typical solution for preventing backflow is to use one or more external check valves in the inflow lines. 
         [0007]    Further, while elastomeric valve seats are adequate for many applications, they have limitations. For example, elastomers typically are not very resistant to abrasion and thus are prone to damage from contaminants in the fluids and wear from repeated valve cycling. Elastomers can also suffer from degradation caused by chemical attack or extreme low or high temperatures. Thermoplastic materials have sometimes been used as a replacement for elastomers when such conditions apply. However, thermoplastic materials, unlike elastomeric materials, are typically not resilient and therefore are not well suited to provide for a reliable dynamic seal. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a three-way poppet valve that can restrict or eliminate backflow without the use of an external check valve. Unlike conventional three-way poppet valves wherein both inlets can be open to the outlet at the same time during movement of the poppet, the valve according to the present invention can be configured to close both inlets prior to reopening one of the inlets to the outlet. Accordingly, the valve features a poppet valve assembly that is operable to prevent both inlets from being open to the outlet at the same time. The valve assembly also reduces the force needed to close the valve against at least one of the inlets thereby allowing a reduction in the size of the actuator. A unique seal including a valve seat that is configured to conform to a valve member under sufficient pressure is also provided. 
         [0009]    According to one aspect of the invention, a three-way poppet valve comprises a valve body, and a valve assembly movable in a chamber of the valve body for controlling communication between a high pressure passage, a low pressure passage and an outlet passage. The valve assembly has a first valve member movable between an open and closed position to respectively permit or block flow through the high pressure passage, and a second valve member movable between an open and closed position to respectively permit or block flow through the low pressure passage. The second valve member is moved by the first valve member to the closed position when the first valve member is moved to the open position. When the first valve member is moved to the closed position, the second valve member remains in the closed position until a pressure differential between the chamber and the low pressure inlet reaches a prescribed criteria. 
         [0010]    More particularly, the first and second valve members are supported on a valve stem connected to an actuator. The first valve member is supported for movement with the valve stem while the second valve member is supported on the valve stem for axial movement relative thereto. 
         [0011]    The valve can be arranged in a plurality of configurations. For example, the first valve member can be biased towards its closed position, and the second valve member can be configured to open when the first valve member is in its closed position and the pressure in the chamber is less than the pressure at the low pressure inlet. In another configuration, the second valve member can be biased towards its open position such that the second valve member will open when the first valve member is in its closed position and the pressure level in the chamber is a prescribed amount greater than the pressure level at the low pressure inlet. In another configuration, the second valve member is biased towards its closed position such that the second valve member will open when the first valve member is in its closed position and the pressure level in the chamber is a prescribed amount less than the pressure level at the low pressure inlet. 
         [0012]    Due to the second valve member moving independent of the first valve member, the force required to shift the first valve member from its open position to its closed position against the pressure in the high pressure inlet is a function of the cross-sectional area of the valve stem. 
         [0013]    Both the low pressure inlet and the high pressure inlet can include a generally annular valve seat having a spherical surface against which a spherical surface on a respective valve element engages. The radius of curvature of the valve seat spherical surfaces can be greater than the radius of curvature of the valve member spherical surfaces. The valve seats can be formed of a thermoplastic material, for example. 
         [0014]    In accordance with an aspect of the invention, a poppet valve comprises a valve body having a passage, a valve seat element supported by the valve body and having a thermoplastic radially inner spherical sealing surface, and a valve element supported for axial movement within the passage and having a spherical sealing surface for engaging the sealing surface of the valve seat. The spherical sealing surface of the seat element has a larger spherical diameter than the spherical sealing surface of the valve element. 
         [0015]    More particularly, the seat spherical seal surface can be configured to deform under sufficient pressure applied thereto by the valve element to provide for a variable amount of seal area to be in contact with the valve element in order to maintain a contact stress above a minimum level required to provide for consistent seal tightness at low pressure while also providing for increased seal contact area in order to reduce seat stress and minimize plastic deformation of the valve element sealing surface at higher pressure. The valve seat element can be supported by the valve body such that upon application of sufficient pressure by the valve element to the valve seat sealing surface, at least a portion of the valve seat will extrude into a space between the valve body and the valve element providing for further increased seal contact area thereby to reduce seat stress and minimize plastic deformation of the valve element sealing surface at higher pressures. The valve seat element can be made of a non-resilient plastic material, such as a thermoplastic or a flouroplastic material like PTFE of filled, unfilled or advanced copolymer grades, for example. 
         [0016]    In accordance with another aspect of the invention a seal assembly for sealing a passageway in a valve comprises a plastic valve seat element having a radially inner spherical sealing surface and a valve element having a spherical sealing surface for engaging the sealing surface of the valve seat. The spherical sealing surface of the seat element has a larger spherical diameter than the spherical sealing surface of the valve element. The seat spherical seal surface can be configured to deform under sufficient pressure applied thereto by the valve element to provide for a variable amount of contact area with the valve element in order to maintain a contact stress above a minimum level required to provide for consistent seal tightness at low valve pressures while also providing for increased seal contact area in order to reduce seat stress and minimize plastic deformation of the valve element sealing surface at high valve pressures. The valve seat element can also be configured to extrude into a space between a valve body in which the valve seat is supported and the valve element providing for increased seal contact area thereby to reduce seat stress and minimize plastic deformation of the valve element sealing surface at higher valve pressures. The valve seat element can be made of a non-resilient plastic material, such as a thermoplastic or a flouroplastic material like PTFE of filled, unfilled or advanced copolymer grades, for example. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  an exploded partial cross-sectional view of an exemplary poppet valve assembly in accordance with the invention 
           [0018]      FIG. 2  is a partial cross-sectional view of the poppet valve assembly in a first state. 
           [0019]      FIG. 3  is an enlarged portion of  FIG. 2 . 
           [0020]      FIG. 4  is a partial cross-sectional view of the poppet valve assembly in a second state. 
           [0021]      FIG. 5  is an enlarged portion of  FIG. 4 . 
           [0022]      FIG. 6  is a partial cross-sectional view of the poppet valve assembly in a third state. 
           [0023]      FIG. 7  is an enlarged portion of  FIG. 6 . 
           [0024]      FIG. 8  is an enlarged portion of  FIG. 2  showing the sliding poppet. 
           [0025]      FIG. 9  is a cross-sectional view of a valve body of the poppet valve. 
           [0026]      FIG. 10  is a cross-sectional view of an actuator housing of the poppet valve assembly. 
           [0027]      FIG. 11  is a cross-sectional view of an inlet port adaptor of the poppet valve assembly. 
           [0028]      FIG. 12  is a cross-sectional view of a lower bonnet of the poppet valve assembly. 
           [0029]      FIG. 13  is a cross-sectional view of an inner bonnet of the poppet valve assembly. 
           [0030]      FIG. 14  is a cross-sectional view of an upper bonnet of the poppet valve assembly. 
           [0031]      FIG. 15  is a cross-sectional view of a lower poppet of the poppet valve assembly. 
           [0032]      FIG. 16  is a cross-sectional view of a sliding poppet of the poppet valve assembly. 
           [0033]      FIG. 17  is a cross-sectional view of a piston of the poppet valve assembly. 
           [0034]      FIG. 18  is an enlarged portion of  FIG. 2  showing a seal of the poppet valve assembly in a low pressure seal state. 
           [0035]      FIG. 19  is an enlarged portion of  FIG. 2  showing a seal of the poppet valve assembly in a high pressure seal state. 
           [0036]      FIG. 20  is another showing a seal of the poppet valve assembly in a high pressure seal state. 
           [0037]      FIG. 21  is another showing a seal of the poppet valve assembly in a high temperature seal state. 
           [0038]      FIG. 22  illustrates the seat geometry of the seal. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Referring now to the drawings in detail, and initially to  FIG. 1 , the main components of the three-way poppet valve assembly  5  include a valve body assembly  10 , a poppet assembly  180  for controlling fluid flow from inlet ports  13  and  14  through the valve body assembly  10  to common outlet port  12 , a bonnet assembly  230  that seals the top of the valve body assembly  10  and provides a passage for fluid to flow from inlet port  14  to the outlet port  12 , and an actuator assembly  380  for applying a biasing force to control the axial movement of the poppet assembly  180 . 
         [0040]    Now referring to  FIGS. 2-8 , the valve body assembly  10  includes a valve body  11  (See  FIG. 9 ) having three fluid passages: common outlet port  12 , inlet port  13 , and inlet port  14 . Inlet port  13  is relatively higher pressure inlet port and inlet port  14  is a relatively lower pressure inlet port (e.g., the pressure at inlet port  13  is higher that the pressure at inlet port  14 ). The common outlet port  12  and lower pressure inlet port  14  are configured to threadably engage respective port adapters  15  and  17  to allow connection to external piping using any preferred type of connection. Both port adapters  15  and  17  are sealed against internal pressure to the valve body  11  via an o-ring  40 . The common outlet port  12  is directly fluidly connected to an innermost valve cavity  18 . A vertical passage  25  directs flow from the low pressure inlet port  14  into an outermost cylindrical cavity  28  of the valve body  11 . 
         [0041]    The high pressure inlet port  13  is directly fluidly connected to the innermost valve cavity  18  and is also configured to threadably engage a port adapter  16  (See  FIG. 11 ) in a similar fashion as the other two ports  12  and  14 . Inlet port  13  further includes a cylindrical counterbore  22  (see  FIG. 9 ) which closely receives a lower seat  70  (see  FIG. 3 ). The port adapter  16  (see  FIG. 11 ) has an extended cylindrical portion  30  with a unique end facing, designed to allow the seat to flex under load applied by the poppet, that is closely received within the port counterbore  22  and which then forms the bottom of a wedge-shape cavity in which the lower seat  70  is contained. The length of the extended cylindrical portion  30  of the adapter  16  and the depth of the cylindrical counterbore  22  of the inlet port  13  are adjusted such that the lower seat  70  is slightly compressed in order to firmly clamp the seat  70  into position and establish a fluid tight seal between the seat  70 , inlet port counterbore  22 , and end face of the adapter  16 . 
         [0042]    The poppet assembly  180  consists of a lower poppet  130 , a sliding poppet  150  and a poppet spring  125 . Referring to  FIG. 15 , the lower poppet  130  has at one end an enlarged cylindrical portion  138  with a spherical head  135 . Extending from the base of the cylindrical portion  138  is an elongated cylindrical stem  133  with a threaded end that threadably engages an actuator piston  340  as will be described such that the piston  340  and the poppet assembly  180  move axially in conjunction with each other. With reference to  FIGS. 8 and 16 , the sliding poppet  150  has an internal bore  159  closely received to the lower poppet stem  133  such that it is free to independently move axially along the stem  133 . A series of O-rings  161  or other packing is provided in a counterbore  155  to provide a fluid tight seal between the stem  133  and the sliding poppet  150 . The series of O-rings  161  or other packing is contained inside the counterbore  155  with a bushing  162  and snap ring  163 . The poppet spring  125  is positioned axially over the lower poppet stem  133  with one end abutted against a spherical tip  157  of the sliding poppet  150  and the opposite end abutted against a bottom surface of a counterbore  258  of an inner bonnet  240 . In this configuration, the poppet spring  125  exerts a biasing force against the sliding poppet  150  to push it towards the cylindrical head  138  of the lower poppet  130 . 
         [0043]    The bonnet assembly  230  includes a lower bonnet  210 , an inner bonnet  240 , and an upper bonnet  280 . The lower bonnet  210  (see  FIG. 12 ) has a cylindrical portion  219  closely received in a valve body bore  21  and a larger cylindrical portion  223  with an extending flat bottom portion  227  that mates against a flat bottom  20  of the outermost bore  28  of the valve body  11 . An O-ring seal  275  is received within a groove in the lower bonnet  210  and provides a fluid tight seal between the lower bonnet  210  and the valve body  11 . A counterbore  225  in the smaller cylindrical portion  219  receives the sliding poppet  150  with a defined clearance and depth which is discussed below. Opposite the counterbore  225  is a second counterbore  212  which closely receives an upper seat  80 . 
         [0044]    The inner bonnet  240  (see  FIG. 13 ) has a cylindrical portion  247  closely received within the seat counterbore  212  of the lower bonnet  210  and a flat face  243  that mates against a top flat face  216  of the lower bonnet  210 . The axial length of the cylindrical portion  247  is adjusted to form a cavity  28  (see  FIG. 3 ) for the upper seat  80  such that the upper seat  80  is slightly compressed in order to firmly clamp the seat  80  into position and establish a fluid tight seal between the seat  80 , the seat counterbore  212  and the unique end face of the inner bonnet  240 , which will be discussed later. An o-ring backup seal  273  is received within a groove in the flat face  216  of the lower bonnet  210  to provide a secondary fluid tight seal between the lower bonnet  210  and the inner bonnet  240 . Passages  256  extending radially from a central bore  258  to an outer diameter  248  provide a flow path for fluids from the low pressure inlet port  14  into the vertical passage  25  of the valve body  11  and into the flow cavity  28 . 
         [0045]    The inner bonnet  240  is closely received in a counterbore  299  of the upper bonnet  280  (See  FIG. 14 ). A flat bottom  293  of the counterbore  299  mates against a flat top surface  249  of the inner bonnet  240 . The upper bonnet  280  is closely received in the outermost bore  28  of the valve body  11 . An O-ring seal  270  is received within a groove in the upper bonnet  280  and provides a fluid tight seal against internal valve pressure between the upper bonnet  280  and the valve body  11 . A series of O-rings  120  or other packing is provided in a counterbore  291  to provide a fluid tight seal against internal valve pressure between the lower poppet stem  133  and upper bonnet  280 . The series of O-rings  120  or other packing is contained inside the counterbore  291  by the top face  249  of the inner bonnet  240 . A thru bore  294  of the upper bonnet  280  closely receives the lower poppet stem  133  and serves to center and guide the lower poppet stem  133  during axial movement. Passages  297  extending radially from the central through bore  294  into a groove around a outer diameter  287  provide a flow passage to a bleed port  392  in a housing  385  of the actuator assembly  380  for any fluid that should leak past a valve stem packing  120 . The purpose of the bleed port  392  is to give an external indication of the valve stem packing  120  leakage and also to prevent high pressure fluid contained within the valve body cavities  18  and  28  from entering the actuator cavity  387  in the event that an actuator stem packing  320  is also compromised. 
         [0046]    The upper bonnet  280  is closely received in a counterbore  398  of the actuator housing  385  (See  FIG. 10 ) with a flat top surface  295  of the upper bonnet  280  mating with a flat bottom surface  388  of the actuator housing counterbore  398 . A series of O-rings  320  or other packing is provided in a counterbore  395  to provide a fluid tight seal against internal actuator pressure between the lower poppet stem  133  and the actuator housing  385 . The series of O-rings  320  or other packing is contained inside the counterbore  395  by a top face  295  of the upper bonnet  280 . A thru bore  397  of the actuator housing  385  closely receives the lower poppet stem  133  and serves to center and guide the lower poppet stem  133  during axial movement. 
         [0047]    Referring back to  FIG. 2 , a cylindrical base  338  of the piston  340  is closely received in an actuator housing bore  386 . An O-ring seal  342  is received within a groove and provides a fluid tight seal against internal actuator pressure between the piston  340  and the actuator housing bore  386 . Return springs  351  and  354  are seated in grooves on a top face  336  of the piston base  338  and retained within the actuator housing  385  by an actuator cap  370 . Four bolts  400  securely retain the actuator assembly  380  and the bonnet assembly  230  to the valve body assembly  10 . 
         [0048]    The piston  340  is axially moveable within the actuator housing bore  386 . The direction of piston movement is effected by a net bias force composed of the bias force applied by the fluid pressure in an actuation cavity  387  and the bias force applied by the return springs  351  and  354 . The poppet assembly  180 , being threadably engaged to the piston  340 , moves axially in conjunction with the piston  340 . As shown in this embodiment, the return springs  351  and  354  urge the piston  340  downwardly toward the valve body assembly  10 . However, the orientation of the piston  340  can be reversed such that the return springs  351  and  354  will urge the piston  340  upwardly toward the bonnet assembly  230 . 
         [0049]    The described poppet valve minimizes or eliminates undesirable backflow from the high pressure inlet port  13  into the lower pressure inlet port  14  during the axial movement of the poppet assembly  180  when both inlet ports  13  and  14  and the common outlet port  12  are in fluidic communication. In general, a specific pressure arrangement should be applied for the valve to function as desired. The higher pressure fluid supply should access the valve through the bottom inlet port  13 . The lower pressure fluid supply should access the valve through the side inlet port  14 . The common outlet port  12  typically should continuously drain fluid pressure such that when the lower poppet  130  closes against the high pressure seat  70 , the internal valve cavity  18  pressure will continue to decrease until the pressure is slightly above or below the fluid pressure in the low pressure inlet port  14 . The reason for such pressure arrangement should be evident in the following discussion of valve function. 
         [0050]    Referring now back to  FIGS. 2 and 3 , the valve assembly  5  is shown in a first static position where the high pressure inlet port  13  is in direct fluidic communication with the common outlet port  12  while flow from the low pressure inlet port  14  is prevented. The fluid flow path in this first static position is from the inlet port  13  into the inner valve cavity  18  and out through the common outlet port  12 . This first valve state is achieved in the preferred embodiment when a pressurized fluid is supplied to the actuation cavity  387  via a supply port  391  in the actuator housing  385  such that the bias force of the pressurized fluid exceeds the bias force of the return springs  351  and  354  and axially moves the piston  340  and also the poppet assembly  180  upwardly until the spherical surface  164  of the sliding poppet  150  mates with a spherical sealing surface  81  of the upper seat  80 , effecting a fluid tight seal that prevents flow into the low pressure inlet port  14 . 
         [0051]    Referring now to  FIGS. 4 and 5 , the valve assembly  5  is shown in a second static position where neither inlet port  13  nor  14  is in fluidic communication with the common outlet port  12 . This second valve state is achieved in the illustrated embodiment when the pressurized fluid is vented from the actuation cavity  387  and the bias force exerted by the return springs  351  and  354  exceeds the bias force induced by the pressure contained within the internal valve cavity  18  exerted on the lower poppet  130 . A further condition for this second valve state is that the internal pressure of the valve cavity  18  is greater than the pressure of the low pressure inlet port  14 . The valve cavity  18  pressure, acting on the net area of the sliding poppet  150  exerts an upwardly acting bias force. This bias force must be greater than the downwardly acting bias force on the opposing side of the sliding poppet  150 . This downward bias force is a combination of two separate forces, the first resulting from the compressed poppet spring  125  and the second a pressure induced force, generated by the lower pressure of the inlet port  14  acting on the net area of the sliding poppet  150 . 
         [0052]    In this second state, a spherical surface  132  of the lower poppet head  135  mates with a spherical sealing surface  71  of the lower seat  70 , effecting a fluid tight seal that prevents flow from the high pressure inlet port  13  into the inner valve cavity  18 . Simultaneously, the spherical surface  164  of the sliding poppet  150  remains mated with the spherical sealing surface  81  of the upper seat  80 , effecting a fluid tight seal that prevents flow into the low pressure inlet port  14  in conjunction with the poppet stem seal  161  that provides a fluid tight seal between the lower poppet stem  133  and the counterbore  155  of the sliding poppet  150 . 
         [0053]    The uniqueness and advantages offered by the new invention are further illustrated when examining the pressure induced biasing forces acting on the valve during this second state. As stated previously, when the actuation cavity  387  is vented of fluid pressure, the bias force of the return springs  351  and  354  must at a minimum exceed the pressure induced upwardly bias force exerted on the lower stem  130  to initiate the axial movement of the piston  340 . The magnitude of the upwardly bias force in units of force is the product of pressure, in this case the internal valve cavity  18  pressure, multiplied by the area in which the pressure is acting against. In this case, because the lower poppet  130  is free to move axially independently of the sliding poppet  150 , the area only includes the cross section area of the lower poppet stem  133 . This area is substantially less than the entire area of the pressure boundary of an inside bore  84  of the seat  80  resulting in a substantially reduced force. The force is reduced by a factor of the square of the radius difference between the lower poppet stem  133  diameter and the radius of the inside bore  84  of the seat  80  as illustrated by the formula for the surface area of a ring H(R 1   2 -R 2   2 ). This force reduction permits the use of smaller and less costly actuator components than would otherwise be required in a valve without a sliding poppet  150 . 
         [0054]    Now referring to  FIGS. 6 and 7 , the valve is shown in a third static position where the low pressure inlet port  14  is in direct fluidic communication with the common outlet port  12 , while flow is prevented from the high pressure inlet port  13 . The fluid flow path in this third static position is from the inlet port  14  into the vertical passage  25  in the valve body  11 , then into the cavity formed between the outermost bore  28  in the valve body  11  and the outside diameters of the lower bonnet  210  and inner bonnet  240 , then into the radial passages  256  of the inner bonnet  240 , then into the counterbore  258  of the inner bonnet  240 , then into the inner valve cavity  18  and out through the common outlet port  12 . 
         [0055]    This third valve state is achieved when the pressure in the internal valve cavity  18  is reduced to a predetermined level via outward flow through the common outlet port  12 . As the pressure drops in the internal valve cavity  18 , the upwardly bias force acting against the sliding poppet  150  is reduced accordingly. The sliding poppet  150  will remain seated against the upper seat  80  until the downwardly bias force exerted on the sliding poppet  150  exceeds the upwardly bias force. Because the pressure induced biasing forces exerted on either side of the pressure boundary across the sliding poppet  150  are applied to equal areas, the sliding poppet  150  will be forced off the upper seat  80  by the additional downwardly bias force exerted on the sliding poppet  150  by the poppet spring  125  while the pressure in the internal valve cavity  18  is higher than the pressure in the low pressure inlet port  14 . 
         [0056]    The pressure differential at which the sliding poppet  150  will unseat can be established by the design of poppet spring characteristics such as the number of coils, wire diameter, compressed height, etc. to adjust the bias force it applies to the sliding poppet  150  while in the compressed state when the sliding poppet  150  is closed. Because the pressure in the internal valve cavity  18  is higher than the low pressure inlet port  14 , there will be an initial amount of back flow towards the low pressure inlet port  14  until pressure is stabilized and forward flow through the valve commences. The amount of back flow depends on the rate of pressure sinkage at the common outlet port  12  and the selected pressure differential where the bias force exerted by the poppet spring  125  will force the sliding poppet  150  off of the upper seat  80 . By proper design, the pressure differential can be kept very low, greatly reducing the amount of backflow that would be experienced in a similar three-way poppet valve without the internal check feature of the present invention. 
         [0057]    The illustrated embodiment discussed previously is most applicable to those applications where the available pressure drop from inlet to outlet is relatively low and assurance is desired, in the form of an additional downwardly bias force supplied by the poppet spring  125 , that the poppet  150  will fully open. Another configuration of the valve is available that can eliminate backflow to the low pressure inlet port  14  and is applicable where the available pressure drop from inlet to outlet is substantially greater. In this configuration, the position of the poppet spring  125  is reversed to the other side of the sliding poppet  150 . With the poppet spring  125  in this position, the sliding poppet  150  functions in an identical manner as a typical check valve, providing an upwardly bias force to keep the sliding poppet  150  seated in the upper seat  80  until the low pressure inlet port  14  pressure is higher than the internal valve cavity  18  pressure to a predetermined level at which the pressure induced downwardly bias force exceeds the upwardly bias force from internal valve cavity  18  pressure and the poppet spring  125 , forcing the sliding poppet  150  to unseat and flow through the valve to commence. Because the sliding poppet  150  will not unseat until the low pressure inlet port  14  pressure is greater than the internal valve cavity  18  pressure, no backflow will occur. A drop in pressure differential between the inlet port  14  and the internal valve cavity  18  to a level below the unseating differential pressure crack pressure will result in the sliding poppet  150  reseating in the upper seat  80 . 
         [0058]    In a third configuration, the poppet spring  125  is removed. In this configuration, the check poppet  150  will unseat when the pressure in the inlet port  14  is approximately equal to the pressure in the internal valve cavity  18 . Minimal backflow will occur in this configuration. 
         [0059]    Turning now to  FIGS. 18-22 , the sealing mechanisms for sealing the lower poppet  130  with seat  70  and sliding poppet  150  with seat  80  will be described. The following description refers to the seal between the lower poppet  130  and seat  70 , but it will be appreciated that the seal between the sliding poppet  150  and seat  80  functions in a similar manner. 
         [0060]    The seal between the lower poppet  130  and seat  70  is achieved by a compressive load applied to a spherical seal surfaces  71  of seat  70  by the spherical surface  132  of the lower poppet  130 . The compressive load creates a contact surface stress between the poppet spherical surface  132  and the seat spherical seal surface  71  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. The magnitude of the minimum contact stress required is typically 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. 
         [0061]    The seat spherical seal surface  71  is configured to provide for a variable amount of seal area to be in contact with the poppets  130  in order to maintain the contact stress above the minimum required level to provide for consistent seal tightness at low pressure while also providing for increased seal contact area in order to reduce seat stress and minimize plastic deformation at higher pressure. This is achieved by machining or molding the spherical seal surface  71  of the seat  70  with a radius slightly larger than the spherical radius of the poppet  130  and by providing partial support to a back surface  73  of the seat  70  such that a bending moment will develop in the seat  70  due to the force applied by the poppet  130 , thereby causing the seat  70  to flex. 
         [0062]    Referring to  FIG. 18 , at low back pressure, only a small area  79   a  of the seat spherical seal surface  71  is in contact with the poppet spherical surface  132  resulting in high contact stress which improves the low pressure seal. As back pressure increases, the force of the poppet ball  135  pushing into the seat spherical seal surface  71  increases, developing a bending moment in the seat  70 . Now referring to  FIG. 19 , the bending moment results in seat flexure, which exposes an increased area  79   b  of the seat spherical seal surface  71  into contact with the poppet spherical surface  132 . The increased contact area  79   b  limits the increase in seat stress as pressure increases, preventing significant permanent deformation to the thermoplastic seat  70  over an extended range of pressure while still achieving contact stress above the minimum required to maintain a seal. 
         [0063]    The seat  70  can also compensate for high back pressures that result in stress above the plastic deformation range of the thermoplastic material. Now referring also to  FIG. 20 , as the poppet  130  pushes deeper into the seat spherical seal surface  71 , the surface area of the spherical seal surface  71  is enlarged by plastic deformation until stress is redistributed and reduced to levels below the plastic deformation range. The result is a permanent deformation enlargement of the spherical seal surface  71  which provides for an increased seal contact area  79   c  that can support higher pressure loading. 
         [0064]    The plastic deformation of the thermoplastic seat  70  generally will not compromise low pressure seal performance. As the spherical seal surface  71  is deformed, the precision of the spherical surface  71  is improved to more closely match the spherical poppet surface  132 . In addition, surface irregularities in the spherical seal surface  71  that result from the original machining or molding process used to manufacture the seat  70  are reduced, producing an improved surface finish that lowers the minimum level of surface contact stress required to achieve a seal. 
         [0065]    At elevated temperatures, the thermoplastic seat  70  material expands greatly in volume and softens, which, when under compressive load, can result in significant plastic deformation, commonly referred to as hot flow. The seat  70  is designed to compensate for this condition in two ways. The first is identical to the high pressure compensation described above. The spherical poppet head  135  is forced deeper into the spherical seal surface  71 , enlarging the seal contact area  79   c  until stress is stabilized below the creep range. 
         [0066]    Now referring also to  FIG. 21 , a portion  136   a  of the seat  70  extrudes in front of the spherical poppet head  135  into a conical bore  34  on the end face of the inlet port adapter  16 , creating an additional seal contact area  79   d  of the spherical seal surface  71 . The extruded portion  136   a  is rigidly supported by the conical bore  34  in the end face of the inlet port adapter  16 , which restricts forward spherical poppet head  135  movement and prevents the poppet head  135  from pushing completely through the seat  70  and contacting the surface of the conical bore  34  in the face of the inlet port adapter  16 . Another portion  136   b  of the seat extrudes in the opposite direction between the spherical poppet head  135  and a through bore  23  of the valve body  11 . The extruded portion  136   b  is rigidly supported by a cylindrical wall  24  of the through bore  23  of the valve body  11 , creating an additional seal contact area  79   e  of the spherical seal surface  71 . The enlargement of the seal contact area of the seat spherical seal surface  71  resulting from material extrusion continues until seat stress stabilizes below the creep limit. 
         [0067]    The desired result of the extrusion of the seat  70  is a total enlarged seal contact area  79   f  of the seat spherical seal surface  71 . The benefit this provides is an increased load capacity of the seat  70 , the load being defined as force due to back pressure. The acceptable amount of load for the seat  70  is limited by the yield strength and resistance to creep hot flow of the seat material at temperature a given temperature. Stress being defined as unit load per unit area, therefore an increase in unit area will permit a related increase in unit load while still maintaining identical stress in the material. 
         [0068]    As with permanent spherical seal surface  71  deformation due to high back pressure as described above, the plastic deformation of the thermoplastic seat  70  due to high temperature will not compromise low pressure seal performance. As the spherical seal surface  71  deforms, the precision of the surface is improved to more closely match the spherical surface  132  of the poppet head  135 . In addition, surface irregularities in the spherical seal surface  71  that result from the original machining or molding process used to manufacture the seat  70  are reduced, producing an improved surface finish that lowers the minimum level of surface contact stress required to achieve a seal. 
         [0069]    Referring now to  FIG. 22 , the relationship between the size of the spherical diameter of the spherical poppet head  135  to the spherical sealing surface  71  of seat  70  can be adjusted to affect sealing performance to best suit the seat material and pressure and temperature conditions. This relationship is best illustrated by two included angles,  170  and  171 , that can be defined by the intersection of the poppet head spherical surface  132  and the spherical sealing surface  71  of the seat  70 . The interior angle  170  is defined by two line segments  138   a  and  138   b  drawn from the center point of the spherical diameter of the poppet head  135  to the intersection points  142  of the spherical sealing surface  71  and a through bore  74  of the seat  70 . The exterior angle  171  is defined by two line segments  140   a  and  140   b  drawn from the center point of the spherical diameter of the poppet head  135  to the intersection points  144  of the spherical sealing surface  71  and the flat annular end face  72  of the seat  70 . 
         [0070]    Interior angle  170  is most important and greatly affects both load capacity and low pressure seal ability of seat  70 . Reducing the interior angle  170 , accomplished by reducing the size of the through bore  74  relative to the diameter of the spherical poppet head  135 , adds surface area to spherical sealing surface  71 , increasing the load capacity of the seat  70 . However, at the same time, the ability of seat  70  to seal at low pressure is reduced correspondingly as the interior angle  170  is reduced. 
         [0071]    The loss of low pressure seal ability as the interior angle  170  is reduced is due to a reduction of mechanical advantage that consequently reduces the magnitude of the force that generates contact stress necessary to effect a seal in the small area  79   a  of the seat spherical seal surface  71 . In effect, the spherical head of the poppet  135  acts as a wedge driven into the seat  70  at low pressure, generating an outwardly radial load against the small area  79   a  of seat spherical seal surface  71 . As identical 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 achieved as the wedge angle is reduced. This same effect is observed in the invention, with decreased simple wedge angle that increases mechanical advantage corresponding to increased interior angle  170 . 
         [0072]    Thus, an improved low pressure seal can be achieved by increasing the interior angle  171 , which for a given load due to pressure, increases the force applied by the spherical poppet head  135  on the small area  79   a  of seat spherical seal surface  71 , which in turn results in increased contact stress between the spherical surface  132  of the poppet head  135  and small area  79   a  of the seat spherical seal surface  71 , which in turn extends the lower range of pressure where minimum contact stress necessary to effect a seal in the small area  79   a  of the seat spherical seal surface  71  can be achieved. 
         [0073]    It is therefore reasonable to conclude from the above discussion that there should exist an optimal geometry of interior angle  170  and exterior angle  171 , irrespective of the actual size of components, that would provide for optimal sealing performance dependent upon the seat material, desired range of sealing pressure, temperature of application and type of fluid to be sealed. Generally, interior angle  170  varies between 80 degrees for higher pressure applications and 130 degrees for lower pressure applications. Exterior angle  171  generally varies between 110 degrees to 175 degrees, and is established so as to provide an adequate amount of spherical seal surface  71  for a particular seat material to support poppet loading at the maximum pressure and temperature of the application. 
         [0074]    An additional advantage of the first preferred embodiment, in which the poppet spring  125  is located above the sliding poppet  150  is made apparent in the detailed description of the unique seal design. As discussed previously, the spherical poppet head  135  will be pushed further into the seat  70  at higher temperatures and pressures resulting in an enlargement of the spherical seal surface  71  of the seat  70 . The deformation of the seat  70  is a result of pressure loading that creates stress in the seat  70  that is above the creep range of the seat material. In effect, the entire bearing load of the poppet head  135  is resisted by the seat  70  alone. 
         [0075]    However, in the first embodiment the poppet spring  125  does provide additional support to resist the bearing load applied by the sliding poppet  150  on the upper seat  80 . The bearing support provided by the poppet spring  125  increases as the deformation of the seat  80  increases because the poppet spring  125  is further compressed by the increased travel of the sliding poppet  150  into the seat  80 . It is therefore possible and desirable to design the poppet spring  125  such that it will provide additional pressure and temperature capacity to the valve by effectively limiting excessive deformation of the seat  80  that would otherwise occur if the seat  80  alone was supporting the full bearing load of the sliding poppet  150 .