Patent Publication Number: US-10774933-B2

Title: Valve assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is continuation application of and claims priority to and all benefits of U.S. patent application Ser. No. 14/085,417, filed on Nov. 20, 2013, which is a divisional application of and claims priority to and all benefits of U.S. patent application Ser. No. 12/570,763, filed on Sep. 30, 2009, which claims priority to and all the benefits of U.S. Provisional Patent Application No. 61/101,804 filed on Oct. 1, 2008, and U.S. Provisional Patent Application No. 61/166,088 filed on Apr. 2, 2009, the entire contents of which are expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a valve assembly, and more specifically, a valve assembly for controlling pressures on opposing sides of the valve assembly prior to opening the valve assembly. 
     2. Description of the Related Art 
     Valve assemblies are used for regulating flow in a fluid pipeline system. These valve assemblies comprise a housing having a first chamber, a second chamber, and a valve seat disposed between the first and second chambers. A valve head is disposed in the housing and is moveable relative to the housing between an open position spaced from the valve seat and a closed position engaged with the valve seat. 
     Valve assemblies are commonly used in a fluid pipeline system having a bidirectional flow. When the valve head is in the closed position, the fluid can exert a pressure against a front portion and/or a back portion of the valve head to create a pressure differential between the front and back portions. In one instance, when a pressure exerted on the back portion is greater than that on the front portion, the pressure on the back portion presses the valve head toward the closed position and impedes the movement of the valve head to the open position. Additionally, the pressure on the back portion may become great enough to cause damage to one or more elements of valve assembly and/or the fluid pipeline system. It would be desirable to have a valve capable of equalizing the pressure between the front and back portions of the valve head or otherwise relieving pressure build-up at the valve head. 
     In addition, valve assemblies of the prior art have internal components or structure that cause flow restrictions that interfere with the fluid flow. This interference can produce energy loss as fluid flows from an inlet to outlet of the valve assembly thereby decreasing fluid flow performance. It would be desirable to reduce flow restrictions to increase the flow through the valve assembly. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     A valve assembly comprises a housing having a first chamber, a second chamber, and a valve seat disposed between the first and second chambers. A valve head is disposed in the housing and is moveable relative to the housing between an open position spaced from the valve seat and a closed position engaged with the valve seat. The valve head defines a bore therethrough along an axis. A valve stem is coupled to the valve head for moving the valve head between the open and closed positions. The valve stem is disposed in the bore and is moveable relative to the valve head from a sealed position preventing fluid communication through the bore to an unsealed position allowing fluid communication through the bore for relieving pressure differences between the first chamber and the second chamber when the valve head is in the closed position. An intermediate member is separate from and coupled to the valve seat and the valve head in the bore. The intermediate member is axially fixed to one of the valve stem and the valve head. The other of the valve stem and the valve head defines a travel space with the intermediate member retained in the travel space between the valve stem and the valve head. The travel space is sized larger than the intermediate member along the axis for allowing selective movement of the valve stem relative to the valve head between the sealed and unsealed positions. 
     The valve assembly advantageously allows for the pressure between the first and second chambers to approach equilibrium before the valve head is moved to the open position. This is beneficial when pressure in the second chamber is greater than that in the first chamber such that the pressure in the second chamber urges the valve head toward the closed position. The valve stem can be moved to the unsealed position to allow the pressures between the first and second chambers to equilibrate prior to moving the valve head to the open position. Because the pressures between the first and second chambers are equilibrated, the force required to move the valve head from the closed position to the open position is reduced. 
     The present invention also includes a valve assembly comprising a first chamber, a second chamber, and a valve seat disposed between the first and second chambers. A valve head is disposed in the housing and is moveable relative to the housing between an open position spaced from the valve seat and a closed position engaging the valve seat. A valve stem is coupled to the valve head and the housing for moving the valve head relative to the housing between the open and closed positions. The valve head defines a bore extending through the valve head from the first chamber to the second chamber when the valve head is in the closed position. A check valve is disposed in the bore and is configured to move between a sealed position preventing flow through the bore when a pressure difference between the second chamber and the first chamber is below a set value and an unsealed position allowing flow through the bore when the pressure difference is greater than the set value for relieving the pressure difference toward the set value. The valve stem extends into the bore and is engaged with the valve head in the bore with a space defined between the valve stem and the valve head in the bore for allowing flow through the bore between the valve stem and the valve head when the check valve is in the unsealed position. 
     The check valve advantageously relieves the pressure difference between the second and first chambers. This pressure relief prevents damage to components of the valve assembly and the system to which the valve assembly is connected. This check valve is also beneficial when the pressure in the second chamber urges the valve head toward the closed position. The check valve relieves the pressure difference such that the force required to move the valve head from the open position to the closed position is reduced. 
     The present invention also includes a valve assembly comprising a housing defining a chamber having an inlet and an outlet spaced from each other along a first axis. The inlet has an inlet diameter and the outlet has an outlet diameter greater than or equal to the inlet diameter. A valve stem is moveably engaged with the housing and extends along a second axis into the chamber. A valve head is disposed in and moveable relative to the housing between an open position and a closed position. The valve head defines a bore extending therethrough along the second axis with the valve stem coupled to the valve head in the bore for moving the valve head along the second axis between the open and closed positions. The housing includes a valve seat disposed in the chamber between the inlet and the outlet with the valve seat extending perpendicular to the second axis. The valve head engages the valve seat in the closed position and is spaced from the valve seat in the open position. The second axis extends at an acute angle relative to the first axis and the housing defines a pocket extending along the second axis for receiving the valve head in the open position to reduce flow restrictions between the inlet and the outlet. 
     Such a configuration reduces flow restrictions to increase the flow through the valve assembly. Specifically, outlet diameter being greater than or equal to the inlet diameter and the valve head being recessed into the pocket reduces flow restrictions between the inlet and the outlet. The valve seat being perpendicular to the second axis and the second axis extending at an acute angle relative to the first axis also reduces flow restrictions. This configuration ensures that the fluid does not pass through any space in which the cross-sectional area, normal to the flow, is less than the cross-sectional area of the inlet and outlet, which advantageously increases the flow through the valve assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is side view of a valve assembly; 
         FIG. 2  is a partially exploded view of the valve assembly; 
         FIG. 3  is a cross-sectional view of a first embodiment of the valve assembly including a valve head in a closed position; 
         FIG. 4  is a cross-sectional view of the first embodiment of the valve assembly with the valve head in an open position; 
         FIG. 5  is a cross-sectional view of the first embodiment with an rivet retaining a seal on the valve head; 
         FIG. 6  is a partial cross-sectional view of a second embodiment of the valve assembly with the valve head in an open position; 
         FIG. 7A  is a partial cross-sectional view of the second embodiment with the valve head in the closed position and a valve stem in an unsealed position; 
         FIG. 7B  is a partial cross-sectional view of the second embodiment with the valve head in the closed position and the valve stem in the sealed position; 
         FIG. 8A  is a magnified view of a portion of  FIG. 7A ; 
         FIG. 8B  is a magnified view of a portion of  FIG. 7B ; 
         FIG. 9  is a partial cross-sectional view of a third embodiment of the valve assembly with the valve head in the open position; 
         FIG. 10A  is a partial cross-sectional view of the third embodiment with the valve head in the closed position and a check valve in a sealed position; 
         FIG. 10B  is a partial cross-sectional view of the third embodiment with the valve head in the closed position and the check valve in an unsealed position; and 
         FIG. 11  is an exploded view of the valve head and check valve of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, a valve assembly is shown generally at  20 . The valve assembly  20  is of the type referred to in industry as a globe valve. As shown in  FIG. 1 , the valve assembly  20  is typically coupled to pipes  22  of a fluid pipe system (not shown) for providing fluid communication between the pipes  22 . The fluid pipe system can, for example, handle propane, compressed nitrogen, anhydrous ammonia, petrochemicals, or other types of chemicals. The valve assembly  20  can be used in, for example, bobtail, transport, and bulk plant applications. The pipes  22  to which the valve assembly is coupled can be, for example, 1¼″ or larger diameter. 
       FIG. 2  depicts a partially exploded view of the valve assembly  20 . A first embodiment of the valve assembly  20  is shown in  FIGS. 3-5 ; a second embodiment of the valve assembly  20  is shown in  FIGS. 6-8 ; and a third embodiment of the valve assembly  20  is shown in  FIGS. 9-11 . Common features among the three embodiments are identified with common numerals. The valve assemblies  20  shown in  FIGS. 1-11  can be referred to in industry as globe valves; however, it is to be appreciated that the valve assembly could also be an angle valve that changes the direction of the flow, e.g., by 90°. It is also to be appreciated that selected tolerances shown in  FIGS. 1-11  are exaggerated for illustrative purposes only. 
     Referring to  FIGS. 3, 6, and 9 , the valve assembly  20  includes a housing  24 . The housing  24  defines a chamber having an inlet  26  and an outlet  28 . More specifically, the chamber is divided into a first chamber  30  and a second chamber  32  with the first chamber  30  defining the inlet  26  and the second chamber  32  defining the outlet  28 . The valve assembly  20  is coupled to the pipes  22  at the inlet  26  and the outlet  28 . A valve seat  34  is disposed between the first  30  and second  32  chambers. 
     The valve assembly  20  includes a valve head  36  disposed in the housing  24  and a valve stem  38  coupled to the valve head  36 . The valve head  36  is moveable relative to the housing  24  between an open position and a closed position. The valve stem  38  extends along a stem axis S and the valve stem  38  is moveably coupled to the housing  24  for moving the valve head  36  along the stem axis S between the open and closed positions, as set forth further below. As shown in  FIGS. 4-6 and 9 , when the valve head  36  is in the open position the valve head  36  is spaced from the valve seat  34  such that the first  30  and second  32  chambers are in fluid communication with each other. As shown in  FIGS. 3, 7A -B, and  10 A-B, when the valve head  36  is in the closed position, the valve head  36  is engaged with the valve seat  34  to prevent fluid communication between the first  30  and second  32  chambers. The valve stem  38  and the valve head  36  are typically formed of metal such as stainless steel. 
     The inlet  26  and the outlet  28  are both generally circular in shape and are spaced from each other along a pipe axis P. Each of the inlet  26  and outlet  28  are adapted for coupling to the pipes  22  for fluid communication in the fluid pipe system, e.g., via threaded fitting, press fit, etc. The diameter of the outlet  28  is greater than or equal to the inlet  26  to reduce restrictions of flow from the inlet  26  to the outlet  28 . Typically, the diameters of the inlet  26  and the outlet  28  are the same. 
     As shown in  FIGS. 3-6 and 9 , the valve seat  34  is disposed centrally about the stem axis S between the first chamber  30  and the second chamber  32 . The valve seat  34  defines an opening  40  between the first  30  and second  32  chambers. The opening  40  is defined along both the stem axis S and the pipe axis P. In the closed position, the valve head  36  is seated on and sealed to the valve seat  34  to prevent flow through the opening  40 . In the open position, the valve head  36  is spaced from the valve seat  34  to allow flow through the valve seat  34 . The valve seat  34  is generally circular in shape, although other suitable shapes may be used such that the valve head  36  can seat against and seal to the valve seat  34 . 
     The opening  40  of the valve seat  34  has a diameter to so as to minimize restriction of flow through the housing  24 . Preferably, the opening  40  has a diameter that is at least 75% of the diameter of the inlet  26  and outlet  28 . More preferably, the opening  40  has a diameter that is at least 95% of the diameter of the inlet  26  and outlet  28 . Most preferably, the opening  40  has the same or larger diameter than the diameter of the inlet  26  and outlet  28  to maximize flow. The housing  24  is constructed so that the fluid does not pass through any space in which the cross-sectional area, normal to the flow, is less than the cross-sectional area of the inlet  26  and outlet  28 . 
     The pipe axis P intersects the stem axis S and is positioned at an acute angle α from the stem axis S. The acute angle α can be from about 10 degrees to about 70 degrees, more preferably from about 20 degrees to about 50 degrees, and most preferably from about 30 degrees to about 40 degrees. In one configuration, the acute angle α is about 35 degrees. The acute angle α is preferably optimized to maximize the flow rate of fluid through the valve assembly  20  by reducing the twists and turns and other flow restrictions that may otherwise impede flow through the valve assembly  20  between the inlet  26  and outlet  28 . 
     With reference to  FIG. 2 , the housing  24  includes a housing body  42  and a bonnet  44  attached to the housing body  42 . The housing body  42  includes a flange  46 . A plurality of bolt fasteners  48  couples a lower flange  50  of the bonnet  44  to the flange  46  of the housing body  42 . It is to be appreciated that any suitable fasteners may be used. A bonnet gasket  52  is disposed between the flange  46  of the housing body  42  and the lower flange  50  of the bonnet  44  to seal between the housing body  42  and the bonnet  44 . The housing body  42  and the bonnet  44  are typically formed of metal such as ductile iron with a powder coat finish. 
     As shown in  FIGS. 3-6 and 9 , the valve stem  38  is threadedly coupled to the bonnet  44  of the housing  24  such that rotation of the valve stem  38  relative to the housing  24  moves the valve stem  38  relative to the housing  24  along the stem axis S. Specifically, the bonnet  44  defines a throughbore  54 . The valve stem  38  is disposed in the throughbore  54  and is moveably coupled to the throughbore  54 . More specifically, the throughbore  54  and the valve stem  38  are threadedly engaged. 
     A hand wheel  56  is coupled to the valve stem  38  to rotate the valve stem  38  and move the valve head  36  between the open position and the closed position. More specifically, the hand wheel  56  is typically rotated clockwise to move the valve head  36  to the closed position and the hand wheel  56  is rotated counterclockwise to move the valve head  36  to the open position. 
     A bushing  58  is retained to the upper end of the bonnet  44  by, for example, threaded engagement or press fit engagement. The valve stem  38  extends through the bushing  58 . The valve stem  38  is sealed to the bushing  58 . 
     As best shown in  FIGS. 3-5 , typically, for example, one or more bushing seals  60  seal between the valve stem  38  and the bushing  58  to prevent the escape of fluid therebetween. The bushing seals  60  are generally annular u-shaped cups, however, any other suitable configuration may be used without departing from the nature of the present invention. 
     A seal expander  62  is disposed between the bushing  58  and the valve stem  38 . An expander spring  64  is disposed in the throughbore  54  and biases the seal expander  62  toward the upper end of the bonnet  44 , thereby biasing the bushing seals  60  toward the upper end of the bonnet  44 . An end of the seal expander  62  is generally tapered to press against the bushing seals  60  due to the bias of the expander spring  64  such that the seal expander spreads the bushing seals  60  into simultaneous contact with the bushing  58  and the valve stem  38 . The bushing seals  60  are stacked so that each bushing  58  seal forces the adjacent bushing  58  seal to spread and contact the bushing  58  and the valve stem  38 . 
     The valve head  36  includes a back portion  66  having a first diameter and a front portion  68 , having a second diameter larger than the first diameter. As shown in  FIGS. 3 and 4 , a rear seal  70  can be disposed on a rear surface of the back portion  66  to prevent fluid flow through the throughbore  54  of the bonnet  44 . The lower flange  50  of the bonnet  44  has a sealing surface  72  for sealing against the valve head  36  when the valve head  36  is in the open position. When the valve head  36  is in the open position, the rear seal  70  abuts and seals against the sealing surface  72  to prevent fluid from flowing into the throughbore  54 . The rear seal  70  may be formed of a suitable sealing material such as nitrile, PTFE (polytetrafluoroethylene), or Viton® and can be attached to the back portion by any suitable adhesive, being press fit in a channel, etc. 
     As shown in  FIGS. 3-6 and 9 , the bonnet  44  defines a rear pocket  75 . When moved to the open position, the valve head  36  is seated within the rear pocket  75  and is recessed away from the flow path of fluid through the housing body  42  to reduce flow restrictions. Recall that the fluid preferably does not pass through any space in which the cross-sectional area, normal to the flow, is less than the cross-sectional area of the inlet  26  and outlet  28 . The movement of the valve head  36  into the rear pocket  75  in the open position aids in accomplishing such a configuration by further recessing the valve head  36  out of the flow path along the pipe axis P. 
     A valve seal  74  can be disposed in an annular recess  77  (see  FIG. 11 ) defined in the front portion  68  of the valve head  36  to seal against the valve seat  34  when the valve head  36  is in the closed position. When the valve head  36  is in the closed position, the valve seal  74  abuts the valve seat  34  to prevent fluid flow from the first chamber  30  into the second chamber  32 . The valve seal  74  may be formed of a suitable sealing material such as, for example, nitrile, Teflon®, or Viton®. 
     A retainer plate  76  is coupled to the valve head  36  to retain the valve seal  74  in position. For example, as shown in  FIG. 4 , a threaded fastener  78  extends through the retainer plate  76  into the valve head  36  to secure the valve seal  74  to the valve head  36 . Alternatively, as shown in  FIG. 5 , the valve head  36  includes an integrally formed rivet  80  that secures the retainer plate  76  to the valve head  36 , thereby retaining the valve seal  74 . In other words, during manufacturing the rivet  80  is simply a cylindrical shaped protrusion (see  FIG. 11 ) that is flattened at its distal end once the retainer plate  76  is positioned over the rivet  80  to form an enlarged head  128 , i.e., the rivet  80  includes a post  126  extending from the front portion  68  of the valve head  36  and the enlarged head  128  is spaced from the front portion  68 , as shown in  FIGS. 9-10B . The valve seal  74  includes a hole  130  that receives the post  126  between the enlarged head  128  and the front portion  68 , as shown in  FIGS. 9-11 . For example, the rivet  80  can be formed by using an orbital forming process such that the integrally formed rivet cooperates with the retainer plate  76  to secure the retainer plate  76 . It is to be appreciated that the term orbital forming refers to a technique for deforming a material to a desired shape. Additionally, it is to be appreciated that the retainer plate  76  may be secured to the valve head  36  using other methods known to one skilled in the art without deviating from the nature of the present invention. 
     As shown in  FIGS. 3-10B , the valve head  36  defines a bore  82  that receives the valve stem  38  with the valve stem  38  coupled to the valve head  36  in the bore  82 . Specifically, the valve stem  38  extends into the bore  82  and intermediate members  84  for example ball bearings, are disposed between the valve stem  38  and the valve head  36  in the bore  82 . The intermediate members  84  are separate from and coupled to the valve head  36  and the valve stem  38 , i.e., the intermediate members  84  are formed separately from the valve head  36  and the valve stem  38  and are introduced between the valve head  36  and the valve stem  38  as separate units. The intermediate members  84  engage the valve stem  38  and the valve head  36  in the bore  82  to retain the valve head  36  in the bore  82 . 
     The intermediate members  84  roll relative to the valve stem  38  and/or the valve head  36  such that the valve head  36  can typically swivel 360° about the valve stem  38 . When the valve head  36  contacts the valve seat  34  as the valve stem  38  is rotated toward the closed position, the valve head  36  stops rotating and the valve stem  38  is further tightened toward the closed position to ensure proper sealing between the valve head  36  and the valve seat  34 . This configuration eliminates scouring of the valve seat  34  by the valve head  36  when the valve head  36  is moved to the closed position thereby increasing the reliability and durability of the valve assembly  20 . 
     The intermediate members  84  are typically spherical; however, it is to be appreciated that the intermediate members  84  can be any shape such that the intermediate members  84  retain the valve head  36  and the valve stem  38  together in a swiveling configuration. The intermediate members  84  are typically formed of metal; however, it is to be appreciated that the intermediate members  84  can be formed of any type of rigid material that maintains shape during swiveling of the valve head  36  relative to the valve stem  38 . 
     The valve head  36  defines an aperture  86 , as shown in  FIGS. 3 and 4 , for introduction of the intermediate members  84  to between the valve head  36  and the valve stem  38 . The aperture  86  is sized to receive the intermediate members  84 . A fastener, such as a set screw  88 , retains the intermediate members  84  between the valve head  36  and the valve stem  38 . With the set screw  88  removed from the aperture  86 , the intermediate members  84  are introduced between the valve head  36  and the valve stem  38  through the aperture  86 . The set screw  88  is subsequently inserted into the aperture  86 . The set screw  88  is typically threadedly engaged with the valve head  36  in the aperture  86 . 
     In the first embodiment, as shown in  FIGS. 3-5 , the valve stem  38  and the valve head  36  define corresponding races  90 . The corresponding races  90  have a shape and size corresponding to the intermediate members  84 . The intermediate members  84  are retained between the valve stem  38  and the valve head  36  in the corresponding races  90  for engaging the valve stem  38  to the valve head  36 . Typically, the corresponding races  90  are each continuous around the circumference of the bore  82 ; however, it is to be appreciated that one of the corresponding races  90  can be discontinuous, i.e., a plurality of discontinuous races spaced from each other about the circumference of the bore. 
     The intermediate members  84  in the corresponding races  90  retain the valve head  36  in position along the stem axis S relative to the valve stem  38 . The intermediate members  84  roll along the corresponding races  90  such that the valve head  36  swivels about the valve stem  38 . This configuration with the valve stem  38  extending into the bore  82  and engaging the valve head  36  in the bore  82  allows for the valve head  36  to be recessed further into the rear pocket  75 . This compact construction advantageously decreases the restriction of flow through the housing body  42  thereby increasing the flow through the housing body  42 . 
     The second embodiment is described in the following paragraphs. In the second embodiment, as shown in  FIGS. 6-7B , the bore  82  extends along the stem axis S through the valve head  36  such that, when the valve head  36  is in the closed position, the bore  82  extends from the first chamber  30  to the second chamber  32 . 
     The valve stem  38  selectively allows fluid communication between the first  30  and second  32  chambers through the bore  82  to relieve pressure differences between the second  32  and first chambers. Specifically, the valve stem  38  is disposed in the bore  82  and is moveable along the stem axis S relative to the valve head  36  from a sealed position to an unsealed position. In the sealed position, as shown in  FIGS. 7B and 8B , the valve stem  38  seals to the valve head  36  and blocks the bore  82  to prevent fluid communication through the bore  82 . When the valve stem  38  is in the unsealed position, as shown in  FIGS. 7A and 8A , the bore  82  is open to allow fluid communication through the bore  82 , i.e., the first  30  and second  32  chamber  32  are in fluid communication with each other through the bore  82 . A space  92  is defined between the valve head  36  and the valve stem  38  to allow fluid communication in the unsealed position. 
     With reference to  FIGS. 6-8B , the valve head  36  defines a ledge  94  in the bore  82 . A bore seal  98  is disposed in the bore  82  between the ledge  94  and the valve stem  38 . As set forth further below, an end  96  of the valve stem  38  contacts the bore seal  98  and seals against the bore seal  98  when the valve stem  38  is in the sealed position and is spaced from the bore seal  98  when the valve stem  38  is in the unsealed position. The bore seal  98  is formed of a suitable sealing material such as, for example, nitrile, Teflon®, Viton®, or the like. The end  96  of the valve stem  38  has a chamfered configuration for cooperating with the bore seal  98 . 
     The intermediate members  84  are fixed along the stem axis S relative to one of the valve stem  38  and the valve head  36  with the other of the valve stem  38  and the valve head  36  defining a travel space  100 . The intermediate members  84  are retained in the travel space  100  between the valve stem  38  and the valve head  36 . The travel space  100  typically is continuous about the circumference of the valve stem  38 , i.e., an elongated groove. Alternatively, the travel space  100  can be discontinuous, i.e., the valve stem  38  can define a plurality of travel spaces spaced from each other about the circumference of the valve stem  38 . 
     In the embodiment shown, the intermediate members  84  are fixed relative to the valve head  36  and the valve stem  38  defines the travel space  100 . The travel space  100  is spaced from the end  96  of the valve stem  38 . The travel space  100  extends circumferentially about the valve stem  38 . 
     The valve head  36  defines a depression  102  with the intermediate members  84  fixed along the stem axis S in the depression  102 . The depression  102  can be, for example, an opposing race  104  opposing the travel space  100 . However, it is to be appreciated that the depression  102  can be any sort of depression capable of fixing the intermediate members  84  along the stem axis S relative to the travel space  100 . 
     Typically, the diameter of the intermediate members  84  generally approximates the size of the opposing race  104  such that the intermediate members  84  are fixed along the stem axis S relative to the opposing race  104 . However; it is to be appreciated that the opposing race  104  may be slightly larger than the diameter of the intermediate members  84  as long as the width of the travel space  100  along the stem axis S is greater than the width of the opposing race  104  along the stem axis S such that the intermediate members  84  are fixed relative to the opposing race  104  as the intermediate members  84  move along the travel space  100 , as set forth further below. 
     The travel space  100  is longer along the stem axis S than the opposing race  104 . Preferably, the width of the travel space  100  along the stem axis S is at least 1% greater, more preferably at least 10% greater than the width of the opposing race  104  along the stem axis S. For example, the width of the travel space  100  along the stem axis S is 0.08 inches larger than the width of the opposing race  104  along the stem axis S. 
     The travel space  100  is sized larger than the intermediate members  84  along the stem axis S for allowing selective movement of the valve stem  38  between the sealed and unsealed positions. In other words, the intermediate members  84  move relative to the travel space  100  along the stem axis S when the valve stem  38  is moved along the stem axis S between the sealed and unsealed positions. 
     Specifically, a first wall  106  and a second wall  108  are disposed along the travel space  100 . The first  106  and second  108  walls are spaced from each other along the stem axis S and oppose each other about the travel space  100 . The first wall  106  engages the intermediate members  84  when the end  96  contacts the bore seal  98  for preventing excessive compression of the bore seal  98  by the end  96 . The second wall  108  engages the intermediate members  84  when the valve stem  38  moves toward the unsealed position for moving the valve head  36  to the open position. The intermediate members  84  are spaced from the second wall  108  when the valve stem  38  is in the sealed position and is spaced from the first wall  106  when the valve stem  38  is in the unsealed position. Typically, the first  106  and second  108  walls correspond in shape to the intermediate members  84 . 
     The valve assembly  20  of the second embodiment operates as follows. Rotation of the valve stem  38  relative to the housing  24  moves the valve stem  38  between the sealed and unsealed positions and moves the valve head  36  between the open and closed positions. When the valve head  36  is in the closed position, the first wall  106  engages the intermediate members  84  such that the valve stem  38  is in the sealed position with the bore seal  98  disposed between and sealing to the end  96  of the valve stem  38  and the ledge  94  of valve head  36 , as shown in  FIGS. 7B and 8B . 
     To move the valve head  36  to the open position, the hand wheel  56  is rotated counter-clockwise thereby traversing the valve stem  38  back up through the bonnet  44 . Because the travel space  100  is larger than the intermediate members  84  along the stem axis S, the valve stem  38  moves from the sealed position to the unsealed position, i.e., the end  96  of the valve stem  38  separates from the bore seal  98 , before the second wall  108  engages the intermediate members  84 . As such, the end  96  of the valve stem  38  separates from the bore seal  98  before the valve head  36  is moved away from the valve seat  34 , as shown in  FIGS. 7A and 8A . As a result, a pressure difference that may exist between the second chamber  32  and the first chamber  30  equalizes as the valve stem  38  separates from the bore seal  98  before the valve head  36  is moved away from the valve seat  34 . Arrows in  FIGS. 7A and 8A  illustrate the flow of fluid from the second chamber  32  to the first chamber  30  to reduce the pressure differential therebetween. Typically the bore seal  98  is uncompressed in less than one full rotation of the hand wheel  56 . One benefit of reducing or eliminating the pressure differential between the second  32  and first  30  chambers is that a force required to rotate the hand wheel  56  is reduced since the pressures in the second  32  and first  30  chambers approach one another or equalize. This is particularly beneficial when the pressure of the second chamber  32  is substantially greater than the pressure of the first chamber  30 . 
     As the valve stem  38  is further rotated counter-clockwise, the valve stem  38  continues to move along the stem axis S relative to the valve head  36  until the intermediate members  84  engage the second wall  108 . Once the intermediate members  84  engage the second wall  108 , further counter-clockwise rotation of the valve stem  38  results in movement of the valve head  36  from the closed position toward the open position, i.e., the valve head  36  separates from the valve seat  34 . The valve stem  38  is rotated counter-clockwise further to move the valve head  36  to the open position recessed in the rear pocket  75  of the bonnet  44 , as shown in  FIG. 6 . 
     To move the valve head  36  back to the closed position, the hand wheel  56  is rotated clockwise thereby traversing the valve stem  38  and the valve head  36  toward the valve seat  34 . As the valve head  36  contacts the valve seat  34 , the valve head  36  ceases to rotate and the hand wheel  56  is rotated clockwise further to tightly seal the valve head  36  against the valve seat  34  in the closed position. In the closes position, the intermediate members  84  engage the first wall  106 . As set forth above, the distance between the end  96  of the valve stem  38  and the first wall  106  is sized such that the end  96  properly seals against the bore seal  98  without damaging the bore seal  98  by overcompression. 
     The third embodiment is described in the following paragraphs. In the third embodiment, as shown in  FIGS. 9-10B , the bore  82  extends through the valve head  36  from the inlet  26  to the outlet  28  when the valve head  36  is in the closed position. An integrated back check valve, hereinafter referred to as check valve  110 , is disposed in the bore  82  and is configured to move between a sealed position and an unsealed position. In the sealed position, as shown in  FIG. 10A , the check valve  110  prevents flow through the bore  82  when a pressure difference between the inlet  26  and the outlet  28  is below a set value. In the unsealed position, as shown in  10 B, the check valve  110  allows flow through the bore  82  when the pressure difference is greater than the set value for relieving the pressure difference. A space  112  is defined between the valve stem  38  and the valve head  36  and also between the intermediate members  84  in the bore  82  for allowing flow through the bore  82  between the valve stem  38  and the valve head  36  when the check valve  110  is in the unsealed position. This configuration with the valve stem  38  engaging the valve head  36  in the bore and with the check valve  110  disposed in the bore is an advantageously compact construction thereby reducing flow restriction when the valve head  36  is in the open position. 
     The intermediate members  84  are disposed in the bore  82  between the valve stem  38  and the valve head  36  with the intermediate members  84  fixed relative to the valve stem  38  and the valve head  36  along the stem axis S. The valve stem  38  and the valve head  36  define corresponding races  114  in the bore  82  with the intermediate members  84  retained in the corresponding races  114  between the valve stem  38  and the valve head  36  such that the valve head  36  swivels relative to the valve stem  38 . The corresponding races  114  extend circumferentially about the valve stem  38 . A diameter of each of the intermediate members  84  generally approximates the diameter defined by the corresponding races  114  but still allowing each of the intermediate members  84  to freely traverse in the corresponding races  114 . 
     An exploded view of the check valve  110  is shown in  FIG. 11 . The check valve  110  includes a plug  116  sealing to the valve head  36  in the bore  82  in the sealed position, as shown in  FIG. 10A , and spaced from the valve head  36  in the unsealed position, as shown in  FIG. 10B . The check valve  110  includes a plug holder  118  defining a cavity for receiving the plug  116 . A spring  120  is coupled to the plug  116  by abutting the plug holder  118  to urge the plug  116  toward the sealed position, as set forth further below. The plug  116  is preferably formed of a suitable sealing material such as nitrile, Teflon®, Viton®, or the like. 
     The bore  82  changes diameter to define a check valve seat  122 . The check valve  110  is disposed in the bore  82  adjacent the check valve seat  122 . A circumference of the plug holder  118  has a polygon configuration and the bore has a cylindrical shape for defining flow paths between the plug holder  118  and the valve head  36  for providing fluid communication through the bore  82  when the check valve  110  is in the unsealed position. 
     A spring retainer  124  is disposed in the bore  82  with a spring  120  disposed between the spring retainer  124  and the plug holder  118 . The spring  120  is typically a coil spring  120  formed of metal such as stainless steel. The spring retainer  124  engages the valve head  36  in the bore  82  such that the spring retainer  124  retains the spring  120  in the bore  82  and supports the spring  120 . The spring retainer  124  may be formed of metal or non-metal materials. The spring retainer  124  can be, for example, press fit, welded or otherwise fixed to the valve head  36 . 
     The check valve  110  operates as follows. When the check valve  110  is in the sealed position and the pressure difference between the second chamber  32  and the first chamber  30  exceeds the set value, the pressure on the plug  116  overcomes opposing force providing by the spring  120  in combination with any pressure in the first chamber  30  such that the spring  120  compresses and the plug  116  moves away from the check valve seat  122  to allow communication between the first  30  and second  32  chambers through the bore  82 . As a result, the pressures of the first  30  and second  32  chambers begin to equilibrate until the pressure difference reaches the set value. At that time, the spring  120  overcomes the pressure on the plug  116  to bias the relief plug  116  back against the check valve seat  122 . Preferably, the set value is greater than 75 pounds per square inch (psi), more preferably between 75 and 100 psi. It is to be appreciated that the “set value” is a predetermined value that be changed by changing components of the check valve  110  such as, for example, altering the spring constant of the spring  120 . 
     Such a configuration advantageously alleviates pressure in a downstream fluid pipe  22  that has a closed downstream valve (not shown). For instance, the pipe  22  may have a main relief valve set to open at a relief pressure, e.g., 400 psi or greater. When the valve assembly  20  is closed and the downstream valve is closed, a closed spaced is defined between the check valve  110  and the downstream valve. If this space is heated, the fluid pressure increases. Without the check valve  110 , if the pressure in the downstream pipe  22  increases beyond the relief pressure, fluid in the downstream pipe is lost through the main relief valve to the atmosphere. The check valve  110  of the present invention can be incorporated into such a system to prevent such loss of fluid to the atmosphere by relieving the excess pressure in the closed space to the upstream pipe or container. 
     The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings, and the invention may be practiced otherwise than as specifically described.