Abstract:
A flapper valve having a curved flapper and a seat, the flapper and seat each having complementary undulating and tapered perimeters, with sealing surfaces designed to maintain seal integrity under higher pressure, for a given outer diameter to inner diameter ratio, compared to prior art flapper valves.

Description:
This application claim the benefit of U.S. Provisional Application No. 60/392,673 filed Jun. 27, 2002. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention pertains to flapper valves, and particularly to flapper valves used for well completions. 
     2. Related Art 
     Flapper valves are often used in subsurface safety valves. Subsurface safety valves are used in wells to contain wellbore fluids, particularly in the event of emergency situations in which there is potential danger to personnel, equipment, or the environment. 
     SUMMARY OF THE INVENTION 
     The present invention improves flapper valves by providing a curved flapper and a seat, the flapper and seat each having complementary undulating and tapered perimeters, with sealing surfaces designed to maintain seal integrity under higher pressure, for a given outer diameter to inner diameter ratio, compared to prior art flapper valves. 
    
    
     DESCRIPTION OF FIGURES 
     FIG. 1 is a perspective drawing of a flapper valve subassembly constructed in accordance with the present invention. 
     FIG. 2A is a schematic drawing of prior art showing one orientation of sealing surfaces relative to externally applied pressure. 
     FIG. 2B is a schematic drawing of prior art showing another orientation of sealing surfaces relative to externally applied pressure. 
     FIG. 2C is a schematic drawing showing an orientation of sealing surfaces relative to externally applied pressure in accordance with the present invention. 
     FIG. 3 is a perspective drawing of a flapper valve constructed in accordance with the present invention. 
     FIG. 4 is a perspective drawing of a flapper valve subassembly constructed in accordance with the present invention. 
     FIG. 5 is a perspective drawing showing a flapper valve constructed in accordance with this invention as an integral part of a completion assembly. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 3 show one embodiment of a flapper valve  10 . In this embodiment, flapper valve  10  comprises a housing  12 , a flapper  14 , and a seat  16 . Housing  12  has a bore  18  providing a longitudinal passageway therethrough. Flapper valve  10  is generally installed some desired distance below ground as part of a tubing string (FIG. 5) used to convey fluids between a wellbore and the earth&#39;s surface. Bore  18  is in fluid communication with the tubing&#39;s interior passageway such that the fluids entering one end of the tubing must pass through bore  18  before exiting the opposite end of the tubing. 
     Flapper  14 , in the embodiment of FIG. 1, is a curved member having a high pressure surface  20  and a low pressure surface  22 . The terms “high” and “low” are meant to connote the pressure differential across flapper  14  when flapper  14  is in a closed state, blocking fluid flow from the wellbore to the surface. Flapper  14  has a hinge  24  by which it rotatably mounts to housing  12 . Hinge  24  allows flapper  14  to rotate between an open state and the closed state. Flapper  14  also has an orienting finger  26  extending radially outward opposite hinge  24 . High pressure surface  20  is curved to conform with the tubing curvature when flapper  14  is in the open state. 
     Extending between high pressure surface  20  and low pressure surface  22  is a transitional sealing surface  28 . Sealing surface  28  can taper radially inward or outward from high pressure surface  20  to low pressure surface  22 . In certain embodiments, such as in FIGS. 1,  3  and  4 , the taper angle can vary along the perimeter of flapper  14 . Flapper  14  has an undulating perimeter. 
     Seat  16  extends from within housing  12  such that it aligns and mates with flapper  14  when flapper  14  is in the closed state. Seat  16  has a mating sealing surface  30  that conforms to the slope of sealing surface  28  everywhere along sealing surface  30 . Seat  16  has an undulating perimeter to conform with that of flapper  14 . Thus, seat  16  has crests  32  and valleys  34 . 
     In operation, flapper valve  10  is usually set in either the open or the closed state. When flapper valve  10  is set in the open state, flapper  14  lays adjacent an inner wall of the tubing. Because the curvature of high pressure surface  20  matches the curvature of the tubing, bore  18  is largely unobstructed by flapper  14 . This is particularly true when flapper  14  is held against the tubing by a flow tube (not shown), as is well understood in the art. 
     When flapper valve  10  is set in the closed state, normally by moving the flow tube and allowing a biasing spring (not shown) to act on flapper  14  (all of which is well understood in the art), flapper  14  is rotated to contact seat  16 , sealing flapper valve  10  along sealing surfaces  28 ,  30  and effectively blocking flow through bore  18 . Orienting finger  26  engages a slot  31  (FIG. 3) in housing  12  to help align flapper  14  onto seat  16 . 
     In the closed state, pressure from wellbore fluids act on flapper  14  and seat  16 . In certain flapper valves  10 , flapper  14  may have greater structural strength than seat  16 . In other flapper valves  10 , seat  16  may have greater structural strength than flapper  14 . In still other flapper valves  10 , flapper  14  and seat  16  may have comparable structural strengths. 
     For those cases in which seat  16  is relatively weak with respect to the flapper, the pressure has the most effect near crests  32  of seat  16 , inducing them to deflect radialy inward. As used herein, the term “collapse force” refers to the force applied to seat  16  or flapper  14  causing the relevant component to move radially ward. The pressure also applies a net force on flapper  14 , driving flapper  14  into seat  16 . Tapered sealing surfaces  28 ,  30  react against each other. If sealing surface  30  slopes radially inward, as shown in FIG. 2C, the net force applied to flapper  14  by the wellbore fluids is transferred across sealing surfaces  28 ,  30  such that there is a radially outward component applied to seat  16  by flapper  14 . Thus, flapper  14  opposes the radially inward deflection of the crests  32  of seat  16 . That keeps sealing surfaces  28 ,  30  properly aligned and mated to maintain an effective seal. 
     For those cases in which flapper  14  is relatively weak with respect to the seat, the pressure has the most effect on the portions of flapper  14  near valleys  34  of seat  16 , inducing flapper  14  to deflect radially inward. As before, the pressure also applies a net force on flapper  14 , driving flapper  14  into seat  16 . Tapered sealing surfaces  28 ,  30  react against each other. If sealing surface  30  slopes radially outward, as shown in FIG.  4  in the vicinity of valleys  34 , the radially inward force applied to flapper  14  by the wellbore fluids is opposed by seat  16 . Thus, seat  16  opposes the radially inward deflection of flapper  14  in the vicinity of valleys  34  of seat  16 . That keeps sealing surfaces  28 ,  30  properly aligned and mated to maintain an effective seal. 
     Similarly, the pressure may also tend to deflect flapper  14  radially inward near crests  32  of seat  16 . Thus, in some embodiments, it may be desirable for seat  16  to have an outward taper at crests  32  so seat  16  can support flapper  14  at crests  32 . 
     For those cases in which flapper  14  and seat  16  are of comparable structural strength, the pressure has the most effect near crests  32  of seat  16 , inducing them to deflect radially inward, and on those portions of flapper  14  near valleys  34  of seat  16 , inducing flapper  14  to deflect radially inward. The pressure also applies a net force on flapper  14 , driving flapper  14  into seat  16 . Tapered sealing surfaces  28 ,  30  react against each other and flapper  14  and seat  16  reciprocally support each other against the pressure. Specifically, if sealing surface  30  slopes radially inward in the vicinity of crests  32  and radially outward in the vicinity of valleys  34 , seat  16  in the vicinity of crests  32  is supported by flapper  14  and flapper  14  in the vicinity of valleys  34  is supported by seat  16 . That keeps sealing surfaces  28 ,  30  properly aligned and mated to maintain an effective seal. 
     Note that in some embodiments the flapper may be relatively weaker in some portions of the circumference and the seat in other portions. Other factors may also affect the taper of the sealing surfaces. Accordingly, many other embodiments are possible. For example, in one embodiment, the seat supports one portion of the flapper (e.g., a portion that is especially sensitive to radial deflection). In another example, the seat supports the flapper in one portion of the circumference and the flapper supports the seat in another portion. 
     In FIGS. 2A,  2 B, and  2 C, the arrows represent the pressure applied by wellbore fluids. In FIG. 2C, the sealing surfaces taper radially inward from the high pressure side of the flapper to the low pressure side. Thus, the flapper and seat reciprocally oppose deformation by the other. 
     FIG. 2B shows a neutral flapper/seat orientation. In this case, the forces transferred between the elements are all in the longitudinal direction. Thus, no lateral support is provided between the elements, for example at the crests of typical flapper valves. 
     In FIG. 2A, the sealing surfaces taper radially outward from the high pressure side of the flapper to the low pressure side. Thus, the force from the flapper tends to further deform the seat in the same direction as the pressure, contributing to the seat&#39;s collapse in the case of a relatively weak seat  16 . 
     FIG. 4 shows an embodiment of a flapper valve subassembly in which flapper  14  and seat  16  have sealing surfaces  28 ,  30  designed to mutually and reciprocally support each other against collapse forces applied by wellbore fluids onto the flapper  14  and seat  16 . The taper angle can vary from an outward angle, meaning the taper extends radially outward from the high pressure surface  20  to the low pressure surface  22  at valleys  34 , to an inward angle, meaning the taper extends radially inward from the high pressure surface  20  to the low pressure surface  22  at the crests  32 . In other embodiments, the taper angle may vary from an inward angle at valleys  34  to an outward angle at crests  32 . These angles are illustrated in FIGS. 2A and 2C. FIG. 2C shows an inward angle “A” measured form a horizontal or radial reference. FIG. 2A shows an outward angle “B”, also measured from a horizontal or radial reference. 
     Depending on the relative strengths of materials and other design characteristics, some embodiments have shown beneficial results if the taper angle at crest  32  varies between an outward angle of five degrees to an inward angle of sixty degrees, and the taper angle at valley  34  varies between an outward angle of thirty degrees to an inward angle of sixty degrees. The taper angles of each embodiment are selected in light of the preceding discussion. 
     The flapper and seat can be formed using a wire electrical discharge machining process, a ram or plunge electrical discharge machining process, by milling, or by a combination of those techniques. 
     Although only a few example embodiments of the present invention are described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.