Pressure compensation/control for fail-safe gate valve

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.