Abstract:
A pressure relief valve has a housing fluidly connected to a pressurized system line carrying a system fluid. The housing carries a moveable piston and a valve member that both move between closed and open positions. The piston defines an upper chamber and a lower chamber within the housing. A pressurized fluid is supplied to the upper chamber to bias the piston toward the closed position. After the relief valve opens from excessive system pressure, the pressurized fluid is injected into the lower chamber to delay the return of the piston to the closed position. Initially, the fluid in the lower chamber flows through a check valve while the piston returns to the closed position. Then the fluid vent in the lower chamber vents from the lower chamber as the piston and valve member approach the closed position.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates in general to valves and in particular to an improved pressure relief valve that relieves a pressure of a system fluid by entering an open position and then by returning to the normally-closed position following a delay period. 
   2. Description of Related Art 
   In a fluid transport system having a system fluid with a pressure that varies, it is often desirable to relieve or lower the pressure of the system fluid if the pressure reaches an unacceptably high level. The most common way of relieving these high pressures is by use of a pressure relief valve. The pressure relief valve, which is in fluid communication with the system fluid, is designed to detect an unacceptable pressure level and relieve the pressure by opening the valve and allowing the system fluid to escape the system line. 
   A problem sometimes develops in the use of these valves in systems that experience rapidly varying pressures. For example, in certain oil and gas well treatments, high pressure liquid is pumped down the well to fracture the earth formation. Large high pressure reciprocating pumps at the surface pump the liquid through flow lines leading into the well. In these systems, the pressure in the system lines can become cyclical, with the pressure of the system fluid exceeding and dropping below an acceptable level. 
   A typical pressure relief valve for use in one of these systems includes a housing having an inlet port and an outlet port. The inlet port is connected to the system line. A valve member is located within the housing that is capable of moving between an open position and a closed position. In the open position, fluid communication is allowed between the inlet port and the outlet port. In the closed position, the valve member sealingly engages a portion of the housing, thereby preventing fluid communication between the inlet port and the outlet port. The valve member is biased such that it remains in the closed position when the pressure of the system fluid is at an acceptable level. As the pressure rises above an acceptable level, the valve member moves to the open position, thereby relieving the pressure by allowing the fluid to flow out of the outlet port of the valve. 
   A standard pressure relief valve includes a spring which provides a force to bias the valve member into the closed position. The size and type of spring is chosen based on the desired acceptable level of pressure of the system fluid. As the pressure of the system fluid rises above the acceptable level, the force exerted on the valve member by the fluid exceeds the force exerted by the spring, causing the valve member to move to an open position. As soon as the pressure of the system fluid returns to an acceptable level, the force exerted by the fluid becomes less than that exerted by the spring, and the valve member immediately returns to the closed position. 
   The spring may be mechanical or it may be a compressed gas chamber. A gas cushion spring includes a piston that is connected to one end of the valve member. The piston is disposed within a pressure chamber in the housing, and the piston and the valve member are adapted to move together from the open position to the closed position. A bias or control fluid, which is usually nitrogen gas, is introduced into the pressure chamber above the piston. The pressure of the control fluid exerts a biasing force on the piston, which pushes the piston and the valve member into the closed position. The valve member and piston move to the open position when the force exerted on the valve member by the system fluid exceeds the force exerted on the piston by the control fluid. 
   Both the mechanical spring and gas spring valves described above provide adequate ventilation of the system fluid when it reaches an unacceptably high pressure. However, both of these valves return immediately to the closed position when the pressure of the system fluid returns to an acceptable level. This method of operation is undesirable when the pressure of the system fluid varies rapidly. A rapid variation of the system fluid pressure causes these standard valves to “chatter,” as they rapidly open and close. The rapid movement of the valve member within the housing causes excessive valve wear and excessive heat to be generated, both of which are undesirable features. 
   One solution to the “chatter” problem is currently employed in some pressure relief valves. These valves incorporate a manual reset feature that requires an operator to reset the valve once the valve has moved to an open position. Valves of this type typically use a valve member which is biased into the closed position by a mechanical spring. As the pressure of the system fluid rises to an unacceptable level, the valve member moves to an open position. Once it reaches the open position, the valve member is locked until an operator manually resets the valve, allowing the valve member to return the closed position. The problem with this type of valve is that it requires extensive operator monitoring and involvement when the pressure of the system fluid varies rapidly. Additionally, because the valve will not return to a closed position until manually reset, once the valve is opened the system fluid will be expelled from the valve even if the pressure returns to an acceptable level. 
   U.S. Pat. No. 6,209,561 solved the chatter problem by introducing a pressurized fluid, or delay fluid, beneath the piston to slow the return of the piston and valve member to the closed position. A one-way check valve extends through the piston from the lower portion to the upper portion of the pressure chamber. While returning to the closed position, the fluid in the lower chamber would flow through to the check valve to the upper chamber. While solving the “chatter” problem due to quick returns to the closed position, sometimes pressurized gas would remain in the lower portion of the pressure chamber and prevent the piston and check valve from fully returning to the closed position. 
   BRIEF SUMMARY OF THE INVENTION 
   The pressure relief valve according to the present invention solves the problems associated with the prior art. The pressure relief valve of the present invention includes a piston that is connected to one end of the valve member. The piston is disposed within a pressure chamber in the housing, the piston defining an upper chamber and a lower chamber within the pressure chamber. The piston and the valve member are adapted to move together from the open position to the closed position. Like the gas spring valve described above, the valve according to the present invention uses a control fluid such as nitrogen gas, which is introduced into the upper chamber. The pressure of the control fluid exerts a biasing force on the piston, which pushes the piston and the valve member into the closed position. The valve member and piston move to the open position when the force exerted on the valve member by the system fluid exceeds the force exerted on the piston by the control fluid. 
   The pressure relief valve according to the present invention uses a delay fluid to delay a return of the valve member from the open position to the closed position. As the valve is exposed to a system fluid with an unacceptably high pressure, the valve moves from the closed position to the open position. Between the closed and open positions is an intermediate position, at which point a control fluid is introduced into the chamber beneath the piston. A fluid injector stab injects pressurized fluid into the portion of the chamber beneath the piston after the valve moves from the closed position to the open position. The pressure of the control fluid beneath the piston provides a delay force to the piston which is opposite in direction to the biasing force provided by the control fluid above the piston. As the pressure of the system fluid returns to an acceptable level, the fluid beneath the piston acts against the piston to delay the return of the piston to the closed position. 
   As the piston and valve member initially return to the closed position, the fluid flows from the portion of the chamber below the piston. Preferably, the fluid beneath the piston travels from below the piston into the chamber above the piston by way of a check valve carried by the piston. As the piston and valve member continue to return to the closed position, the fluid below the piston is exposed to a port that allows the fluid to vent from the area below the piston. Venting the fluid helps the piston and valve member fully return or land in a valve seat of the pressure relief valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a valve according to the present invention, the valve being shown in a closed position. 
       FIG. 2  is an enlarged view of a portion of the valve of  FIG. 1 , showing a seating area of the valve of  FIG. 1 . 
       FIG. 3  is a cross-sectional view of the valve of  FIG. 1 , the valve being shown in an intermediate position. 
       FIG. 4  is a cross-sectional view of the valve of  FIG. 1 , the valve being shown in an open position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1–4  in the drawings, the preferred embodiment of a pressure relief valve  11  according to the present invention is illustrated. Valve  11  includes a housing  13  having a wall and a relief passage with an inlet  15  and an outlet  17 . Inlet  15  is situated such that its longitudinal axis intersects and forms a right angle to the longitudinal axis of outlet  17 . A system line  19  is fluidly connected to inlet  15 , system line  19  carrying a system fluid  21  which is introduced to valve  11  through inlet  15 . A valve seat  23  is disposed within housing  13  between inlet  15  and outlet  17 . 
   A valve member  25  having an upper end and a lower end is slidingly disposed within housing  13  so that the longitudinal axis of valve member  25  is coaxial to the axis of inlet  15 . Valve member  25  passes through a partition  27 . A seal  29  provides a sealing engagement between partition  27  and valve member  25 . Toward its upper end, valve member  25  is engaged by a guide  31  which is disposed within a counterbore  33 . Guide  31  is used to control the translational movement of valve member  25  within housing  13  so that it moves in a direction parallel to the longitudinal axis of valve member  25 . A sealing area  35  is located at the lower end of valve member  25 . 
   Valve member  25  is adapted to move within housing  13  between a closed position shown in  FIG. 1  and an open position shown in  FIG. 4 . In the closed position, sealing area  35  fully engages valve seat  23  to block flow of system fluid  21  through the relief passage. In the open position, sealing area  35  no longer engages valve seat  23 , thereby allowing flow of system fluid  21  through the relief passage. 
   Referring now to  FIG. 2  in the drawings, the components associated with valve seat  23  and sealing area  35  are illustrated. The components of valve seat  23  work cooperatively with the components of sealing area  35  to block flow of system fluid  21  through the relief passage when valve member  25  is in the closed position. Valve seat  23  includes a seal  37  secured by an upper and a lower retainer  39 . Retainers  39  sealingly engage an interior wall of housing  13  to prevent leakage of system fluid  21  when valve member  25  is in the closed position. 
   The primary component associated with sealing area  35  is a bushing  41  which is releasably connected to valve member  25 . Bushing  41  is sealingly disposed in a circumferential depression milled in the lower end of valve member  25 . Bushing  41  forms a sealing engagement with seal  37  when valve member  25  is in the closed position. The sealing engagement between seal  37  and bushing  41  provides the primary method of sealing the relief passage when valve member  25  is in the closed position. 
   Referring again to  FIGS. 1 ,  3 , and  4 , a pressure chamber  43  is formed within housing  13 , the pressure chamber being annular in shape with an inner annular surface. A pressure barrier or piston  45  is connected to the upper end of valve member  25 . Piston  45  is annular in shape and has an upper, or first side and a lower, or second side. Piston  45  sealingly and slidingly engages the inner annular surface of pressure chamber  43 . Piston  45  moves within chamber  43  in conjunction with valve member  25  between the open and closed positions. 
   Piston  45  divides pressure chamber  43  into an upper portion  47  and a lower portion  49 . A delay passage  51  extends through piston  45  and carries a check valve  53 . Together, delay passage  51  and check valve  53  make up a delay fluid outlet port, providing unidirectional fluid communication between lower portion  49  below piston  45  and upper portion  47  above piston  45 . In the preferred embodiment, fluid communication through check valve  53  is possible only when fluid flow is from lower portion  49  to upper portion  47 . Check valve  53  prevents fluid flow from upper portion  47  to lower portion  49 . 
   Referring to  FIGS. 1 ,  3 , and  4 , an axial bore  55  is formed in an upper portion of valve member  25 . A fluid injector stab  57  extends through a side of pressure chamber  43  and stabs into bore  55 . Fluid injector stab  57  remains stationary when piston  45  and valve member  25  move between the open and closed positions. A seal  59  located toward the end of injector stab extending into bore  55  sealingly engages the interior surface of valve member  25 . Fluid injector stab  57  is preferably tubular, and is connected to a pressurized fluid source  61  through a fluid line  63 . In the preferred embodiment, a lower stab port  65  is located adjacent seal  59 , and extends from the interior of tubular injector stab  57  to an annulus  67  defined by bore  55  and injector stab  57 . Annulus  67  extends away from seal  59  to upper portion  47  of pressure chamber  43 . An upper stab port  69  located on a portion of injector stab  57  that is not received within bore  55  extends through a sidewall of injector stab  57  into upper portion  47  of pressure chamber  43 . A control fluid  71 , preferably nitrogen gas, communicates from fluid source  61 , through fluid line  63  and the interior of injector stab  57  to ports  65 ,  69 . When valve member  25  and piston  45  are in the closed position, as shown in  FIG. 1 , both ports  65 ,  69  transmit control fluid  71  into upper portion  47 . 
   A valve member port  73  extends from bore  55  through a sidewall of valve member  25 . In the preferred embodiment, valve member port  73  is positioned so that seal  59  sealingly engages bore  55  between valve member port  73  and lower stab port  65  when piston  45  and valve member  25  are in the closed or lower position. Valve member port  73  is in fluid communication with lower portion  49  of pressure chamber  43 . Valve member port  73  moves relative to seal  59  and lower stab port  65  when piston  45  and valve member  25  are in the intermediate position ( FIG. 3 ) or the open position ( FIG. 4 ). In both the intermediate and open positions, lower stab port  65  is in fluid communication with valve member port  73 , thereby allowing fluid source  61  to transmit control fluid  71  into lower portion  49  of pressure chamber  43 . Control fluid  71  continues to communicate from fluid source  61  to upper portion  47  of pressure chamber through upper stab port  69 . 
   A venting port  75  extends from bore  55  through a sidewall of valve member  25 . Venting port  75  transmits control fluid  71  within bore  55  below seal  59  to either atmosphere or to a control fluid collection assembly (not shown). Venting port  75  does not communicate with lower portion  49  of pressure chamber when valve member port  73  is above seal  59 , as shown in  FIGS. 3 and 4  of the intermediate and open positions. Therefore, seal  59  helps to prevent control fluid  71  from exiting lower portion  49  of pressure chamber  43  while valve member port  73  is above seal  59 . Control fluid  71  flows through check valve  53  of piston  45  and valve member  25  moves from the open position shown in  FIG. 4  to the intermediate position shown in  FIG. 3  while valve member port  73  is substantially at or above seal  59 , thereby delaying the return of piston  45  and valve member  25  to the closed position shown in  FIG. 1 . Control fluid  71  vents through valve chamber port  73  and venting port  75  when valve chamber port  73  is below seal  59 , thereby allowing the full return to the closed position. Control fluid  71  typically vents from lower portion  49  of pressure chamber  43  while piston  45  and valve member  25  are returning to the closed position shown in  FIG. 1 . 
   The upper side of piston  45  has a pressure area proportional to the squared value of the diameter of pressure chamber  43 . The lower side of piston  45  has a pressure area proportional to the squared value of the diameter of chamber  43  minus the diameter of guide  31 . The result is that the upper pressure area of piston  45  is greater than the lower pressure area. If the pressure on both upper and lower sides of piston  45  is the same, the net pressure force is downward. The significance of the difference in the pressure areas is explained below in relation to the operation of valve  11 . 
   Referring now primarily to  FIGS. 1 ,  3 , and  4 , the operation of valve  11  is illustrated. Relief passage inlet  15  is connected to system line  19  to regulate the pressure of system fluid  21 . During a low pressure operation, when the pressure of system fluid  21  is below or at an acceptable level, valve member  25  remains in the closed position (see  FIG. 1 ). During a high pressure operation, when the pressure of system fluid  21  is above the acceptable level, valve member  25  moves to the open position (see  FIG. 4 ). In the open position, system fluid  21  is allowed to flow through the relief passage, exiting the valve through outlet  17 . 
   In the preferred embodiment, the acceptable level of pressure of system fluid  21  is determined and set by the pressure of control fluid  71 . During normal operations, when in the closed position of  FIG. 1 , control fluid is introduced into upper portion  47  of pressure chamber  43  through stab ports  65 ,  69  on injector stab  57 . Control fluid flows directly into upper portion  47  from stab port  69 . Control fluid communicates through annulus  67 , above seal  59 , from stab port  65 . The presence of pressurized control fluid  71  in upper portion  47  causes a biasing force to be exerted on the upper side of piston  45 . The biasing force pushes piston  45  and valve member  25  toward into the closed position (see  FIG. 1 ). Piston  45  and valve member  25  remain in the closed position while system fluid  21  is below the maximum pressure level. When the pressure of system fluid  21  exceeds the acceptable level, the force exerted by system fluid  21  on the lower end of valve member  25  exceeds the biasing force exerted on the upper side of piston  45 , thereby causing valve member  25  and piston  45  to move into the open position (see  FIG. 4 ). Valve member  25  will stay in the open position during high pressure operation of valve  11 . System fluid  21  flows through relief passage outlet  17 . 
   As the pressure of system fluid  21  exceeds the predetermined level necessary to overcome the biasing force due to control fluid  71  in upper portion  47  of chamber  43 , piston  45  and valve member  25  move through the intermediate position shown in  FIG. 3  to the open position shown in  FIG. 4 . Stab port  65  begins injecting control fluid  71  into lower portion  49  of pressure chamber  43  after valve member port  73  moves passed seal  59 . Control fluid  71  applies the same pressure on both sides of piston  45 . Therefore, control fluid  71  in lower portion  49  of chamber  43  creates a force on the lower side of piston  45  that delays a quick return of piston  45  and valve member  25  to the closed position shown in  FIG. 1 . 
   As mentioned above, because the surface area of the upper side of piston  45  is larger than the surface area of the lower side of piston  45 , the biasing force from control fluid  71  in upper portion  47  is larger than the delay force from control fluid in lower chamber  49  even though the pressure is the same. Therefore, piston  45  and valve member  25  begin returning to the closed position of  FIG. 1  when the pressure of system fluid  21  returns to below the predetermined level. Control fluid  71  passes through check valve  53  from lower portion  49  to upper portion  47  of pressure chamber  43  while valve member port  73  is on the same side of seal  59  as stab port  65 . After valve member  25  returns toward the closed position far enough for valve member port  73  to be on the opposite side of seal  59  from stab port  65 , control fluid  71  vents from lower portion  49  into bore  55  and through venting port  75 . 
   Venting the gas from lower portion  47  of chamber  43  advantageously discontinues the delay forces experienced by piston  45 . The biasing force on the upper side of piston  45  can more easily force piston  45  and valve member  25  to return fully, or land completely in valve seat  23 , after the pressure of system fluid  21  returns to below the predetermined value. The problem involving chattering is still reduced because control fluid  71  prevents valve member  25  from rapidly closing after each opening while valve member port  73  is above seal  59  and transmitting control fluid  71  into lower portion  49 . 
   It should be apparent from the foregoing that an invention having significant advantages has been provided. While the invention is shown in only one of its forms, it is not just limited but is susceptible to various changes and modifications without departing from the spirit thereof.