Abstract:
A fail-safe gate valve for sub-sea use features a floating, pressure biased compensating piston whose movement prevents internal pressure buildup from opening movement of the gate. A pre-charged fluid chamber provides the bias on the balancing piston. Using unequal piston diameters reduces the charge pressure. The balancing piston is not connected to the gate so that internal pressures can be employed to act on a net area, which biases the gate toward its fail-safe position.

Description:
FIELD OF THE INVENTION 
     The field of the invention is valves with a failsafe mode of closure for oilfield use, primarily in sub-sea applications and more particularly, in the preferred embodiment, which compensate for a rise in internal pressure around the gate when opening and allow internal line pressure to assist in valve closure. 
     BACKGROUND OF THE INVENTION 
     Valves used in sub-sea drilling applications have had actuators with fail-safe closure provisions. Generally, the force required to return the actuator piston and the valve to a fail safe position, which, in most cases were the fail closed position is from the spring force and the actuator stem force. The spring force is normally relatively low in comparison to the total force required for fail-safe operation. The actuator stem force is a primary fail-safe force presented a net area of the stem cross-sectional area that was exposed internally to the valve body. Generally a spring or springs were used to return an actuating piston and the valve gate to a fail-safe position, which, in most cases was the closed position. In some designs, the valve actuator stem presented a net area exposed to internal valve pressure, which, in the absence of hydraulic pressure on the actuating piston provided a net force to move the gate to its fail-safe position. These large unbalanced forces were needed to overcome gate drag due to internal pressures in the valve body forcing the gate laterally. The return spring would also act on the actuating piston to urge the gate to the fail-safe position. 
     In drilling applications a condition could exist where the valve body is full of an incompressible fluid like drilling mud. When trying to stroke the gate from a closed to an open position, the stem connecting the gate and the actuating piston would enter the valve body. If the valve body was full of an incompressible fluid, the internal pressure could rise to the point that the maximum working pressure of the valve body could be exceeded. Additionally, further movement of the gate could be stalled as the pressure buildup around the gate could rise to the level where the hydraulic system acting on the actuating piston could not overcome the built up internal pressure from the surrounding incompressible fluid. To compensate for this effect, a balancing stem was attached to the lower end of the gate, to minimize or eliminate this pressure buildup that would otherwise occur as the valve is actuated to open. However, the addition of the balancing stem attached to the gate solved one problem but created another. Since the gate was essentially in pressure balance from internal valve pressure a net unbalanced force was no longer available to overcome gate drag when a fail-safe operation was required. Normally, the return springs could only put out a few thousand pounds of force to assist in the fail-safe movement, but to overcome gate drag forces well in excess of 25,000 pounds would be needed. The solution to the problem was to design an auxiliary pressurized accumulator, which could take the place of the force formerly provided by internal pressure acting on a net area of the gate assembly to drive it to the fail-safe position. The accumulators were large and heavy and their required size and weight increased with the sub-sea depth of the application. They also presented safety concerns in that their pressure had to be released to equalize with the sub-sea pressure before being brought to the surface. They also presented safety concerns in that their pressure had to be vented prior to actuator disassembly to avoid injury to maintenance personnel. 
     Various designs of sub-sea drilling gate valves have been attempted, some with the pressure balanced feature, as shown in U.S. Pat. Nos. 4,809,733; 4,311,297; 4,230,299; 4,489,918; U.S. Pat. No. Re 29,322; U.S. Pat. Nos. 4,281,819; 6,125,874; and U.S. Pat. No. Re 30,115. Of these, the latter two are of most interest as they provide a way to use the surrounding seabed pressure to urge a balancing piston against the gate to make the valve fail-safe. However, even these two latter references do not provide the ability to compensate for a buildup in internal pressure around the gate during opening while at the same time having a provision to allow a net internal pressure to act on an unbalanced gate to achieve a fail-safe position. In the present invention large accumulators are eliminated or minimized. A compensating piston, which is biased toward the gate but not connected to it, is used in the preferred embodiment. A self contained, charged, pressure chamber acts on the compensating piston. An easy retrofit of existing valves is also possible. These and other advantages of the present invention as well as additional features will be more readily appreciated by those skilled in the art from reading the description of the preferred embodiment, which appears below. 
     SUMMARY OF THE INVENTION 
     A fail-safe gate valve for sub-sea use features a floating, pressure biased compensating piston whose movement prevents internal pressure buildup from opening movement of the gate. A pre-charged fluid chamber provides the bias on the balancing piston. Using unequal piston diameters reduces the charge pressure. The balancing piston is not connected to the gate so that internal pressures can be employed to act on a net area, which biases the gate toward its fail-safe position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a prior art valve using a balancing stem attached to the gate and a single control line to the surface; 
     FIG. 2 is a sectional view of the valve of FIG. 1, using a dual control line system; 
     FIG. 3 is a sectional view of the valve of the present invention, in the closed position; 
     FIG. 4 is the view of FIG. 3 showing downward gate movement prior to the onset of flow through the valve; 
     FIG. 5 is the view of FIG. 4 with flow through the valve just beginning; 
     FIG. 6 is the view of FIG. 5 with the valve fully open; and 
     FIG. 7 is an alternative embodiment as to the placement of the compensating piston. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 show, respectively, a single and dual control line actuation system for a sub-sea gate valve. The valve  10  has a body  12  and an inlet  14  and an outlet  16 . Located in cavity  17  are inlet seat assembly  18  and outlet seat assembly  20 , respectively surrounding inlet  14  and outlet  16 . A gate  22  is moved between the seat assemblies  18  and  20  so as to isolate with seats, the cavity  17  from passages  16  and  19  in gate  22 . An actuator rod  24  is connected to the gate  22  and has a piston  26  near its top end. Piston  26  is sealed at its periphery where it slides against housing  28 . An actuation system comprises an accumulator  30  connected to a diverter valve  32  through control line  40 . Control line  34  runs from the surface to the sub-sea location of diverter valve  32 . Control line  36  runs from housing  28  above piston  26  to control line  40  and to diverter valve  32 . Control line  38  runs from housing  28  below piston  26  to diverter valve  32 . A balance stem  42  is sealed where it extends through opening  44 . 
     In operation, pressure from control line  34  is directed to control line  36  via diverter  32  and line  40  while control line  38  is aligned through the diverter valve  32  to dump fluid to the surrounding seawater. The accumulator  30  is pressurized from line  34 , at this time. Piston  26 , actuator rod  24 , gate  22  and balance stem  42  all move in tandem to open the valve  10 . Because of the presence of the connected balance stem there is no internal pressure buildup in the cavity  17  as the valve opens. At the same time because of the balance stem  42 , internal pressure in cavity  17  does not apply a force that will urge the gate  22  in the opposite and fail-safe direction. Upon failure of hydraulic pressure to diverter valve  32  it assumes a position where pressure from control line  38 , coming from the gas charged accumulator  30 , moves the piston  26  upwardly as flow from line  36  is directed through diverter  32  and back to the surface through line  34 . At the time of failure, there is no pressure beyond hydrostatic in line  34 . 
     FIG. 2 illustrates the use of two control lines, which can be alternatively pressurized or vented to urge the gate  22  up or down. The equipment to do that is at the surface. FIG. 2 has the disadvantage of having to run double the amount of control lines potentially thousands of feet sub-sea. The design of FIG. 1 has the disadvantage of large and heavy equipment, which may not fit in confined areas sub-sea or may be difficult to access or to deliver to the location. The cost factor can become significant due to the high pressure ratings involved for the components, such as the accumulator  30 . 
     The present invention, in the preferred mode, is illustrated in FIGS. 3-7. The parts that are the same as in FIGS. 1-2 will be identically numbered. The differences are the use of a balancing piston  50 , which has a large area  52  in chamber  54  and a small area  56  exposed to cavity  17 . While piston  50  is shown to be solid it can take many shapes. Area  56  can be recessed to create an upwardly facing receptacle to overly a tab (not shown) at the base of gate  22  to guide gate  22  while still performing the same pressure compensation feature and allowing internal pressure to exert an unbalanced force on the gate  22  to urge it to its fail-safe position. Gate  22  is not attached to piston  50  and is not intended to contact piston  50 . As shown in FIG. 3 the piston  50  is in alignment with gate  22 , but such alignment is optional, as shown in FIG.  7 . There a passage  58  communicates to chamber  54  and piston  50  is offset and parallel to gate  22 . Chamber  54  has a variable volume cavity  60 , which connects to a reservoir  62  through line  64 . Reservoir  62  has a movable piston  68 , above which is a pre-charge of pressure, preferably nitrogen. The area  52  being larger than the area  56  allows the use of lower pressure in reservoir  62 . Thus, for example if the maximum desired pressure in cavity  17  is 15,000 pounds per square inch (PSI) and the area ratio of areas  52  to  56  is 3 to 1, then the required nitrogen pressure in reservoir  62  is only 5,000 PSI. Piston  50  is biased by the nitrogen against a travel stop and in FIG. 3 is in its uppermost position. Conversely, because piston  50  is inverted in FIG. 7, it is in its lowermost position, as the valve  10  is getting ready to open. FIG. 7 shows a split view of piston  50  in the extremes of its range of motion. 
     Comparing FIGS. 3 and 4 it can be seen that as the gate moves downwardly tending to raise the pressure in cavity  17 , the piston  50  moves in a direction to decrease the volume of variable volume cavity  60 , which at the same time increases the volume of cavity  17  to avoid pressure buildup. There is as yet no flow in the FIG. 4 position. The only thing that has occurred is the gate moving down as well as piston  50  so as to avoid pressure buildup beyond the desired pressure in cavity  17 . That target pressure in cavity  17  is based on the area ratios of areas  52  and  56  and the nitrogen pressure initially charged in reservoir  62 . Since the piston  50  is not linked to gate  22 , when it comes time to go to the fail-safe position, there is an unbalanced force on the gate  22  from internal pressure in valve  10 . This force is enhanced by closure spring  66 . Unlike the FIG. 1 design, an accumulator  30  is not needed in the control system. In the event there is low or no pressure in valve  10  when it needs to go into the fail-safe mode, the force of spring  66  is sufficient because there is little or no gate drag force to overcome. 
     FIG. 5 shows the onset of flow through the valve  10 , at which point further displacement of gate  22  does not tend to further raise the pressure in cavity  17  and there is no further displacement of piston  50  into chamber  54 . FIG. 6 shows the wide open position. A variety of control systems, hooked up to actuator housing  28  to make the piston  26  travel down or allow it to be driven up for the fail-safe mode can be used without departing from the invention. Reservoir  62  can be made integral with chamber  54  such as by placing barrier piston  68  in cavity  60  with the nitrogen pressure on the opposite side from piston  50 . The configuration of FIG. 3 is readily amenable to a retrofit on existing valves so as to simplify the attendant control system by elimination of an accumulator  30  and some of the associated control lines. The control system can be no more complicated than a single control line  70 , which can equalize with line  72  for closure of the gate  22 . Normal operation can be nothing more than applying or removing a pressure in line  70 . Provisions can be made in the control system so that spring  66  does not have to close against hydrostatic pressure in line  70 . While those skilled in control system design will appreciate the variety of systems that can be implemented, the system simplification as compared to FIGS. 1 and 2 is due to the piston  50  not being attached to the gate  22 , which lets an unbalanced force act to close the valve from within using internal pressure. Spring  66  also provides an assist to reach the fail-safe condition. If the valve has no internal pressure when the fail-safe position is needed, the spring  66  can push the piston  26  against the minimal gate drag present with no internal pressure. The accumulator of FIG. 1 is no longer needed. For opening, the use of piston  50  biased with nitrogen or other type of pressure from reservoir  62 , if separate or from chamber  54  if reservoir  62  is integral with it, prevents housing overpressure or stalling of gate  22  during the opening procedure. 
     In FIG. 1 item  14  is the inlet and  16  is the outlet. This valve is unidirectional, where  14  and  16  cannot be reversed and bidirectional, where  14  and  16  can be reversed. One reservoir  62  can be used to control the cavity  60  pressure to two or more valves. Line  64  would tee or branch to the individual valves, each having its own chamber  54 . The reservoir  62  would be sized with capacity to control any valve individually or to control all valves, if actuated simultaneously. Chamber  54  can be mounted remotely from the individual valve. Separate chambers or one larger common chamber  54  would service all valves. A line could be run from the individual cavities  17  to the common chamber  54 . Chamber  54  and reservoir  62  could be a combined unit or separate structures. 
     The above description is illustrative of the preferred embodiment and various alternatives and is not intended to embody the broadest scope of the invention, which is determined from the claims appended below, and properly given their full scope literally and equivalently.