Patent Publication Number: US-2023151888-A1

Title: Double eccentric butterfly valve and manufacturing method therefor

Description:
TECHNICAL FIELD 
     The present invention relates to a double eccentric butterfly valve, in particular, suitable for high-pressure fluids, and a manufacturing method thereof. 
     BACKGROUND ART 
     Conventionally, in particular, as valves suitable for high-pressure fluids, eccentric butterfly valves have been generally known. Of these, in particular, in a double eccentric butterfly valve, the rotation axis (valve axis) of the valve disk is eccentric from the center line of the flow path and, furthermore, the rotation axis is eccentric from the sealing surface of the valve disk to a flow path direction, Thus, favorable sealing performance by the seat ring is ensured even at the time of high pressure and, simultaneously, abrasion of the seal surface of the seat ring is prevented. 
     In the valve of this type, to enhance resistance to high temperatures, the seat ring is often formed of fluororesin. This seat ring is arranged at the above-described double eccentric position, in a state of being interposed between the seat retainer and the body. With this double eccentric structure, in various sizes from a small diameter to a large diameter, it is possible to prevent seal leakage with high accuracy at the time of valve closing in both flow directions, positive pressure and counter pressure. In addition to this, in recent years, it is desired to also inhibit so-called “back leakage”. “Back leakage” refers to a phenomenon in which, due to a gap between the body and the seat ring, leakage occurs through the back side of the seat ring. 
     As a valve taking back leakage measures, the Applicant has suggested the eccentric butterfly valve of PTL 1. In this eccentric butterfly valve, the seat ring has a seat ring main body and a gasket portion integrally provided on the outer circumferential side of this seat ring main body to extend through an edge cutting portion. The edge cutting portion is provided with an engaging portion which prevents back leakage of this gasket portion in a state of engagement between the body and the gasket portion. 
     The engaging portion is a portion where a tapered step portion surface formed on the gasket portion and a tapered surface formed on the body are engaged. These tapered step portion surface and tapered surface are both formed of tilted. surfaces at the same tilt angle. After the seat ring is attached between the body and the seat retainer, the tapered step portion surface and the tapered surface make a surface contact over their entire surfaces to exert a strong seal surface force, allowing prevention of back leakage. 
     Meanwhile, in the double eccentric butterfly valve, normally, an annular clearance is provided between the outer circumferential side of the seat retainer and the inner circumferential side of the body opposed to this outer circumferential side of the seat retainer. When the valve is assembled, the seat retainer is moved so as to align with the position of the valve disk through the clearance. By aligning the seat ring attached to this seat retainer with the position of the valve disk, even if a dimensional error among components occurs, integral incorporation can be made while a positional discrepancy between sealing surfaces of the valve disk and the seat ring at the time of valve closing is prevented. 
     CITATION LIST 
     Patent Literatures 
     PTL 1: Japanese Patent Application Laid-Open No. 2017-180742 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the double eccentric butterfly valve, for example, when the valve disk and the valve stem are assembled to the body, if a dimensional error occurs between the stem hole of each of these valve disk and the body and the center line of the flow path, the valve disk is arranged as being deviated from the body by that dimensional error from a normal position. In this case, when the seat ring is positionally aligned with the valve disk, a deviation occurs also to the seat retainer with respect to the body by this dimensional error. 
     When a dimensional error among components occurs in this manner, in PTL 1, the tapered step portion surface of the seat ring and the tapered surface of the body are not able to engage in a predetermined surface contact state. In a circumferential direction in which these tapered step portion surface and the tapered surface are superposed each other, a portion where the tapered step portion surface overrides the tapered surface and a portion where a gap occurs between the tapered step portion surface and the tapered surface may occur. In these switching portions, the surface pressure between the tapered step portion surface and the tapered surface is partially interrupted and, when a high-pressure fluid flows or the like, there is a possibility that back leakage occurs from these portions where the surface pressure is interrupted. 
     Moreover, in the portion where the tapered step portion surface overrides the tapered surface, for example, the seal surface pressure becomes larger than the portion where a gap occurs between these, and this makes it difficult to uniformly form an annular seal portion of the seat ring by the seat retainer and the body. These phenomena occur when, for example, a material resistant to crushing and deformation (such as filler-containing PTFE) is used for the seat ring or the seat is incapable of being sufficiently fastened with the seat retainer. That is, with the seat ring where crushing or deformation easily occurs (such as PTFE not containing a filler), the seat ring can be deformed just right to fill the gap. Also, even if the seat ring is fastened with an excessive force, the gap of the seat ring can be crushed out, and therefore the problem as described above is difficult to occur. However, for example, to seal a fluid with a pressure higher than normal or to seal it in both directions, positive pressure and counter pressure, it is required to use a seat ring with high stiffness, which is unable to be easily deformed, thereby also making it difficult to increase the fastening force. 
     From these, in PTL 1, to cause the tapered step portion surface and the tapered surface to make a surface contact in a predetermined state, dimensional accuracy at a level where no dimensional error occurs to each component is desired. At the time of assembling, the seat retainer with the seat ring attached thereto and the body are also required to be integrated in a state in which the former is positionally aligned accurately with the latter. Moreover, it is difficult to satisfy both of this high dimensional accuracy and alignment between the seat ring and the valve disk. In actual alignment, the positions of the tapered step portion surface and the tapered surface are shifted, and there is also a possibility that the above-described problems occur. 
     Thus, it has been desired to develop a double eccentric butterfly valve in which, even if a dimensional error among components such as the valve disk and the valve stem hole in the body is large, the seat ring and the body are positionally aligned to inhibit back leakage and the seat ring and the valve disk are aligned to allow sealability to be ensured. 
     The present invention was developed to solve the conventional problem, and has an object of providing an eccentric butterfly valve, in particular, suitable for high-pressure fluids, in which: even if a dimensional error among components is large, the seat ring is aligned with the disk while sealability between the seat ring and the body is ensured; for either pressure, positive pressure or counter pressure, the seal surface pressure between the disk and the seat ring is ensured to achieve sealability; and a continuous seal portion is ensured between the body and the seat ring, thereby reliably inhibiting fluid leakage, and a method of manufacturing the eccentric butterfly valve. 
     Solution to Problem 
     To achieve the above-described object, an invention according to claim  1  is directed to a double eccentric butterfly valve in which a disk is rotatably and axially supported via a stern inside a cylindrically-shaped body at a position eccentric from each of a center of a flow path and a sealing surface and this disk is provided to be able to seal in a hermetically-sealing state by a seat ring fixed with a seat retainer inside the body and made of an elastic material, wherein a tapered surface formed on the seat ring and a step portion formed on the body are opposed to each other and, with a corner portion of the step portion on the tapered surface side digging into the tapered surface, an annular seal portion is formed on an opposed plane between the body and the seat ring. 
     An invention according to claim  2  is directed to the double eccentric butterfly valve, in which a protruding portion protruding in vicinity of an opposed portion between the tapered surface and the step portion is formed on the seat retainer, and this protruding portion is partially or entirely provided at a position of being superposed on the step portion in the flow path direction. 
     An invention according to claim  3  is directed to the double eccentric butterfly valve, in which the seat ring is formed of a seat ring main body and a gasket portion integrally provided to extend on an outer circumferential side of this seat ring main body, the tapered surface of the seat ring and the step portion of the body are opposed to each other in vicinity of a boundary portion between the seat ring main body and the gasket portion. 
     An invention according to claim  4  is directed to the double eccentric butterfly valve, in which in an opposed region between the tapered surface and the step portion, a volume of a region crushed by the step portion digging into the tapered surface and a volume of a space region where the step portion does not dig into the tapered surface have a substantially equivalent size. 
     An invention according to claim  5  is directed to a manufacturing method of a double eccentric butterfly valve in which a disk is rotatably and axially supported via a stem inside a cylindrically-shaped body at a position eccentric from each of a center of a flow path and a sealing surface and this disk is provided to be able to seal in a hermetically-sealing state by a seat ring fixed with a seat retainer inside the body and made of an elastic material, wherein the disk is caused to be in a closed state and the seat retainer is accommodated in an annular attachment concave portion formed in the body; the seat ring is aligned with a position of the disk in a range of a clearance formed between this attachment concave portion and an outer circumference of the seat retainer; a tapered surface formed on the seat ring and a step portion formed on the body are caused to be opposed to each other; with a corner portion of the step portion on the tapered surface side digging into the tapered surface, an annular seal portion is formed on an opposed plane between the body and the seat ring; and, in a state in which a seal surface of the seat ring on a tip side and a sealing surface of the disk on a tip side can be annularly sealed, the seat retainer and the body are fixed together. 
     An invention according to claim  6  is directed to the manufacturing method of the double eccentric butterfly valve, in which the seat ring is formed of a seat ring main body and a gasket portion integrally provided to extend on an outer circumferential side of this seat ring main body and, in vicinity of a boundary portion between the seat ring main body and the gasket portion, the tapered surface of the seat ring and the step portion of the body are opposed to each other. 
     An invention according to claim  7  is directed to the manufacturing method of the double eccentric butterfly valve, in which a length of the clearance in a radial direction is shorter than a width of the tapered surface in a radial direction. 
     Advantageous Effects of Invention 
     From the invention according to claim I, because of the double eccentric type, the invention is, in particular, suitable for high-pressure fluids. The annular seal portion is formed of the tapered surface formed on a seat ring side and the step portion formed on a body side. Thus, even if a dimensional error among components is large, with positional alignment within the width of the tapered surface, the corner portion of the step portion reliably digs to ensure sealability between the seat ring and the body. Also, the seat ring is integrated with the disk in a state of being aligned therewith. In either case, positive pressure or counter pressure, the seal surface pressure between the disk and the seat ring at the time of valve closing is ensured to allow sealability to be exerted. A continuous seal portion is ensured by the body and the seat ring in the circumferential direction, thereby allowing leakage from a portion therebetween to be reliably inhibited. When the seat ring is combined with the body, the tapered surface and the step portion are caused to reliably abut on each other in a state in which at least a linear contact is made therebetween in the circumferential direction. From this state, the seat ring and the body are integrated to form an annular seal portion. Thus, with this seal portion, a sealing force in the circumferential direction is ensured to be constant, and fluid leakage can be reliably inhibited. 
     From the invention according to claim  2 , by pressurizing the tapered surface by the protruding portion to a step portion side, a reliable locally-pressurized portion can be generated. With an increase in surface pressure force of the seal portion including this locally-pressurized portion, leakage from a portion between the seat ring and the body is reliably inhibited. 
     From the invention according to claim  3 , since the seat ring is provided to have the seat ring main body and the gasket portion, even if the fluid flows in either direction, positive pressure or counter pressure, while the seat ring main body Ilexes at the time of valve closing to ensure sealability with respect to the disk, back leakage can be inhibited by the gasket portion. Here, with the tapered surface and the step portion opposed to each other in the vicinity of the boundary portion between the seat ring main body and the gasket portion, the gasket portion is functionally separated from the seat ring main body, and this gasket portion less tends to receive the influence of deformation of the seat ring main body. Thus, sealability by the gasket portion is enhanced to further improve leakage prevention performance. 
     From the invention according to claim  4 , in the opposed region between the tapered surface and the step portion, the volume of the crushed region of the tapered surface and the volume of the other space region have a substantially equivalent size. Thus, the seat ring can be mounted while the filling ratio by the seat ring pushed from the crushed region to the space region is approximately 100% or more. Thus, excessive filling is prevented, a non-filled region is prevented from being left, and the surface pressure force of the seal portion is substantially uniformly ensured in the circumferential direction to allow an improvement in sealability. 
     From the invention according to claim  5 , even if a dimensional error among components is large, while uniform sealability between the seat ring and the body is ensured by the annular seal portion, the seal surface of the seat ring and the sealing surface of the disk can be caused to be in a state of being annularly sealed. In a state in which sealability between the seat ring and the body and sealability between the seat ring and the disk are both satisfied, the seat retainer and the body can be assembled. After assembling, in either case, positive pressure or counter pressure, sealability between the disk and the seat ring at the time of valve closing is ensured, and fluid leakage from a portion between the body and the seat ring can be reliably inhibited. 
     In particular, when the seat retainer is accommodated in the body, the tapered surface and the step portion are caused to reliably abut on each other in a state in which at least a linear contact is made therebetween in the circumferential direction. From this state, the seat ring and the body are integrated to form an annular seal portion. With this seal portion, a sealing force in the circumferential direction is continuously ensured, and fluid leakage can be inhibited. 
     From the invention according to claim  6 , even if the fluid flows in either direction, while positive pressure or counter pressure, while the seat ring main body flexes at the time of valve closing to ensure sealability with respect to the disk, back leakage can be inhibited by the gasket portion. Furthermore, with the tapered surface and the step portion opposed to each other in the vicinity of the boundary portion between the seat ring main body and the gasket portion, the gasket portion is functionally separated from the seat ring main body, and this gasket portion less tends to receive the influence of deformation of the seat ring main body. Thus, sealability by the gasket portion is enhanced to further improve leakage prevention performance. 
     From the invention according to claim  7 , when the seat retainer is accommodated in the body, the step portion is not removed from the tapered surface. From a state in which the step portion is reliably opposed to the tapered surface and they are caused to at least make a linear contact therebetween, an annular seal portion is configured, thereby allowing sufficient sealability to be ensured. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a longitudinal sectional view depicting an embodiment of a double eccentric butterfly valve of the present invention. 
         FIG.  2    is a front view of the double eccentric butterfly valve of  FIG.  1   . 
         FIG.  3    is an enlarged schematic view of main parts of  FIG.  1   . 
         FIG.  4    is an enlarged schematic view of an A portion of  FIG.  3   . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, embodiments of a double eccentric butterfly valve and a manufacturing method thereof in the present invention are described based on the drawings. In  FIG.  1    and  FIG.  2   , an embodiment of the double eccentric butterfly valve of the present invention is depicted. In  FIG.  3   , an enlarged schematic view of main parts of  FIG.  1    is depicted. 
     A double eccentric butterfly valve (hereinafter referred to as a valve main body  1 ) depicted in  FIG.  1    includes a body  2 , a stem  3 , a disk  4 , a seat retainer  5 , and a seat ring  6 . Of these, the body  2 , the stem  3 , the disk  4 , and the seat retainer  5  are each molded of a metal material such as stainless steel. 
     In the valve main body  1 , the body  2  is formed in a cylindrical shape. At each of upper and lower portions of this body  2 , a shaft-mounting portion  10  for mounting the stem  3  is provided. On one side of the body  2 , an annular attachment concave portion  11  is formed. This attachment concave portion  11  is provided to have an inner diameter which can accommodate the seat retainer  5  to which the seat ring  6  is attached. A bottom surface  11   a  of this attachment concave portion  11  is provided so as to be parallel to a rotation axis of the stem  3 . 
     The disk  4  is formed in a substantially disk shape. On an outer circumferential surface of this disk  4 , a sealing surface  12  is provided. Also, on one surface side, a boss portion  13  is formed so as to be provided as protruding. In this boss portion  13 , a hole portion  14  for mounting the stem  3  is formed at a position that is eccentric from the sealing surface  12  as a seal position. Inside the valve main body  1  of  FIG.  1   , a flow path  15  is provided. In the case of a flow in a position direction, the right side of the valve main body  1  serves as a primary side of the flow path  15 , and the left side thereof serves as a secondary side, and a fluid flows from the right side to the left side inside the valve main body  1 . 
     The disk  4  is integrally fixed to the stem  3  with tapered pins  16 , and is rotatably and axially supported via the stem  3  at a position eccentric from each of the center of the flow path  15  and the sealing surface  12 . With the disk  4  attached in a double eccentric structure in this manner, the disk  4  is provided so as to be able to seal at the time of valve closing in a hermetically-sealing state by the seat ring  6  fixed with the seat retainer  5  inside the body  2 . 
     In  FIG.  2    and  FIG.  3    the seat ring  6  is annularly molded of an elastic material and, in the present embodiment, for example, is formed of a resin material such as PTFE (polytetrafluoroethylene) containing a filler such as carbon. This seat ring  6  has a seat ring main body  20  and a gasket portion  21  integrally provided to extend on an outer circumferential side of this seat ring main body  20 . 
     The seat ring main body  20  is provided so as to have a portion in the vicinity of the outer diameter fixed between the body  2  and the seat retainer  5  and, on the other hand, have a portion in the vicinity of the inner circumference capable of flexing. An end face on its inner circumferential side (inner circumferential tip side) is provided to serve as a seal surface and abut on the sealing surface  12  of the disk  4  at the time of valve closing to be capable of annular sealing. 
     In the seat ring main body  20 , a flexible portion  22  is provided on its inner diameter side, and a fixing portion  23  is provided on an outer diameter side with respect to this flexible portion  22 . Between these flexible portion  22  and the fixing portion  23 , a space portion  24  is provided. The flexible portion  22  has a seal surface  25  and a tilted surface  26 . The seal surface  25  is annularly provided on an inner diameter end portion side of the flexible portion  22 , having a cross section in a C-surface shape or a cross section in an It-surface shape and a predetermined seal width. Continuously to this seal contact portion  25 , the tilted surface  26  is provided in a mildly tilted shape. 
       FIG.  4 ( a )  is an enlarged schematic view of an A portion of  FIG.  3   , and  FIG.  4 ( b )  represents a state of the body  2  and the seat ring  6  before attachment. In the vicinity of a boundary portion  27  between the seat ring main body  20  and the gasket portion  21 , a step surface  30  forming a step in a diameter direction is formed between the body  2  and the seat ring  6  as an opposed plane in a circumferential direction. On the step surface  30 , a tapered surface  31  formed on a seat ring  6  side and a step portion  32  formed on a body  2  side are opposed to each other. 
     Of these, the tapered surface  31  is provided so as to have a size of a width W 1  in a radial direction by a mild R surface connecting the seat ring main body  20  and the gasket portion  21  in a sectional direction, and is tilted so as to go toward a body  2  side from an inner diameter side to an outer diameter side. 
     The step portion  32  is provided at a position opposed to the tapered surface  31  so as to have a surface (tilted surface  36 ) tilted at an angle on the order of substantially 45 degrees in cross section from a bottom surface  11   a  of the attachment concave portion  11 . The step portion  32  has a section in an uneven shape in which an inner diameter side protrudes to a seat ring  6  side with this tilted surface  36  taken as a boundary. The step portion  32  has a corner portion  35  protruding to a seat ring  6  side and positioned. inside the width W 1  of the tapered surface  31  in the radial direction. The tilted surface  36  of this step portion  32  has a width W 2  which allows arrangement inside the width W 1  of the tapered surface  31  in the radial direction, and is formed so that the tilt in the radial direction is steeper than the tapered surface  31 . In this manner, the tilted surface  36  of the step portion  32  and the tapered surface  31  are opposed so as to cross each other. Also, since the height (width in a flow path direction) of the step portion  32  of the body  2  and the height of a step interposing the tapered surface  31  of the seat ring  6  are substantially equal. Thus, opposed surfaces of the seat ring  6  and the body  2  can just abut on each other except for the step surface  30 . 
     From the shape described above, when the seat ring  6  is pressed onto the body  2 , the corner portion  35  of the step portion  32  on a tapered surface  31  side digs into the tapered surface  31 , and the tapered surface  31  side goes along the shape of the step portion  32  to cause the seat ring  6  to be elastically deformed. With the tapered surface  31  and the corner portion  35 , an annular seal portion  33  is formed on the step surface  30 . This seal portion  33  allows the seat ring  6  and the body  2  to be uniformly sealed. 
     In  FIG.  4 ( a ) , before deformation of the seat ring  6 , a region RI (a region indicated by solid-line hatching) crushed  1   w  the step portion  32  on the tapered surface  31  and a space region R 2  (a region indicated by broken-line hatching) where the step portion  32  in which the seat ring  6  in this region R 1  is pushed out and accommodated does not dig into the tapered surface  31  are provided so as to have a substantially equal size. By deformation of the seat ring  6 , the region R 1  crushed by the step portion  32  is provided so as to be deformable inside the space region R 2 . 
     With this, in an opposed region between the tapered surface  31  and the step portion  32 , the volume of the region R 1  crushed by the step portion  32  digging into the tapered surface  31  and the volume of the space region R 2  where the step portion  32  does not dig into the tapered surface  31  have a substantially equivalent size, approximately 100% of a portion of the seat ring  6  having the amount of the former region R 1  is pushed out by the space region R 2  and the filling ratio in the vicinity of this step surface  30  is approximately 100%. And, with pressurization by a protruding portion  42  described below, this filling ratio becomes equal to or more than 100%, and the seal surface pressure of the seal portion  33  is highly maintained. If this filling ratio is smaller than 100%, the seal surface pressure of the seal part  33  extremely decreases. On the other hand, if the filling ratio is too high, the fastening force becomes excessive. Thus, it is desired to configure the tapered surface  31  and the step portion  32  so that the above-described region R 1  and space region R 2  are approximately equal so that the filling ratio reliably exceeds 100% and is not excessive too much. 
     The fixing portion  23  is tightly interposed between the body  2  and the seat retainer  5  in a state in which the flexible portion  22  flexes and, in this state, the seat ring main body  20  is fixed. The space portion  24  is formed so as to be cut out in a groove shape along a circumferential direction. Via this space portion  24 , the flexible portion  22  flexes to a positive pressure direction or counter pressure direction by a surface pressure from the disk  4  and a fluid pressure, and the seal surface  25  is provided so as to be able to abut on and seal the sealing surface  12  of the disk  4 . 
     A solid line indicating the seat ring main body  20  in  FIG.  3    indicates a shape when the seal surface  25  receives a positive pressure at the time of valve closing to abut on the disk  4 , and a broken line indicates a shape when the disk  4  receives a counter pressure at the time of valve dosing to be deformed. A one-dot-chain line indicates a shape of a section of the seat ring main body  20  before attachment to the seat retainer  5 . 
     The gasket portion  21  is integrally provided by a thin-walled portion  34  to extend on the outer circumferential side of the fixing portion  23 . This thin-walled portion  34  is formed to have a thickness which can inhibit an outflow of a superfluous portion by thermal expansion, and has a sufficient thickness ensured together with the gasket portion  21 . 
     The seat retainer  5  is substantially annularly provided, and has formed on a side TO opposed to the body  2  a protrusively-provided portion  40 , a concave-shaped groove  41 , the protruding portion  42 , and an attachment concave groove  43 . The protrusively-provided portion  40  is formed at a position opposed to the space portion  24  of the seat ring main body  20 , and has a length shorter than the depth of the space portion  24  in a perpendicular direction with respect to the stem  3 . 
     The concave-shaped groove  41  is formed on an outer diameter side of the protrusively-provided portion  40 , and has a width approximately equal to or slightly longer than the length of the fixing portion  23  in the perpendicular direction with respect to the stem  3  and longer than the fixing portion  23  in the radial direction. With this, before fastening of the seat retainer  5 , a gap portion G is provided between the outer circumferential surface of the fixing portion  23  and the inner circumferential surface of the seat retainer  5 . The capacity of the gap portion G is set so as to be larger than the volume of the fixing portion  23  even with fastening with retainer bolts  50  for fixedly attaching the seat retainer  5  and if the fixing portion  23  is crushed to be deformed to an outer diameter side. With this structure, after fastening of the seat retainer  5 , the fixing portion  23  is fixed inside the concave-shaped groove  41 . 
     Also, the fixing portion  23  is configured so as to hardly crush the concave-shaped groove  41 , thereby easily causing pressing and crushing by the protruding portion  42  of the thin-walled portion  34  and pressing and crushing by the attachment concave groove  43  of the gasket portion  21 . 
     The protruding portion  42  is formed to protrude with a width narrower than the concave-shaped groove  41  to an outer diameter side in the perpendicular direction with respect to the stem  3  in the vicinity of the boundary portion  27  between the seat ring main body  20  and the gasket portion  21 , and is provided at a position where this protruding portion  42  is partially or entirely superposed on the step portion  32  in a flow path direction, that is, an opposed portion between the tapered surface  31  and the step portion  32 . In this case, a gap D is provided to have a width shorter than the protruding portion of the gasket portion  21  and thinner than the thin-walled portion  34  in a radial direction and is provided between an outer circumferential side of the fixing portion  23  and an inner circumferential side of the protruding portion  42 . 
     The protruding portion  42  is inserted from an insertion space  51  having a narrow width provided between the fixing portion  23  of the seat ring main body  20  and the gasket portion  21 , and the thin-walled portion  34  is pressurized by a tip of the protruding portion  42  to a body  2  side with a high surface pressure. When thermal expansion occurs in the flexible portion  22  and the fixing portion  23  by a heat cycle, a superfluous portion by thermal expansion is absorbed by the gap D and does not influence the seal surface  25  and the gasket portion  21 . 
     The attachment concave groove  43  is formed on the outer circumferential side of the protruding portion  42  to be longer than a tip side of the gasket portion  21  in the perpendicular direction with respect to the stem  3  and have a long width in the radial direction. With this structure, between the gasket portion  21  and the seat retainer  5 , a gap portion is formed in each of the radial direction and the perpendicular direction with respect to the stem  3 . Via these gap portions, the gasket portion  21  is attached in a filling state. 
     The seat retainer  5  is provided so as to have an outer diameter slightly smaller than the attachment concave portion  11 , and an annular clearance C is formed between these attachment concave portion  11  and the outer circumference of the seat retainer  5 . In the range of this clearance C, the seat retainer  5  can move with respect to the body  2 , In this range, the seat ring  6  can be aligned with the position of the disk  4 . With this, in a state in which the seal surface  25  of the seat ring  6  and the sealing surface  12  of the disk  4  are annularly sealed, the seat retainer  5  and the body  2  can be assembled. 
     The seat retainer  5  is formed so as to have an outer diameter so that a length L of the clearance C in the radial direction is shorter than the width W 1  of the tapered surface  31  in the radial direction. From this, when the seat retainer  5  is accommodated in the attachment concave portion  11 , even if the seat retainer  5  is put to one side at the maximum inside this attachment concave portion  11 , the seat retainer  5  and the body  2  can be fixed while the state is maintained in which the tapered surface  31  is superposed on the step portion  32  (in particular, in the vicinity of the corner portion). 
     As depicted in  FIG.  2    and  FIG.  3   , in the seat retainer  5 , stepped fastening holes  52  are equidistantly formed at eight locations. On a body  2  side corresponding these fastening holes  52 , female screws  54  where male screws  53  of the retainer bolts  50  are screwable are provided. The body  2  and the seat retainer  5  are provided so as to be able to be fastened, in a state in which the seat ring main body  20  is attached therebetween, with the retainer bolts  50 . With their fastening forces, the gasket portion  21  is interposed between the body  2  and the seat retainer  5 . 
     In this case, the positions of the fastening holes  52  and the female screws  54  are set so that the gasket portion  21  is arranged at positions where head portions  55  of the retainer bolts  50  are opposed. Thus, the force of interposing the gasket portion  21  effectively occurs. 
     After fastening of the retainer bolts  50 , the valve main body  1  is interposed by a piping flange not depicted. With the outer circumferential side jointed with piping bolts  56  depicted in  FIG.  2   , the seat ring main body  20  becomes in a state of being more firmly mounted. 
     After attachment of the seat retainer  5 , as depicted in  FIG.  3   , a space S 1  is provided between the body  2  and the flexible portion  22  and a space S 2  is provided between the seat retainer  5  and the flexible portion  22 . With these spaces S 1  and S 2 , the flexible portion  22  is flexibly deformed in a direction of the seat retainer  5  and/or the body  2 . 
     Note that while the tapered surface  31  is formed to have a mild R surface in the above-described embodiment, this tapered surface may be formed of a linear tilted surface or a mild convex curved surface. On the other hand, the tilted surface  36  of the step portion  32  is formed at an angle of substantially 45 degrees. Although this angle may be other than 45 degrees, the tilted surface  36  is desired to be steeper than at least the tapered surface  31 . This facilitate digging of the corner portion  35  of the step portion  32  into the tapered surface  31 . 
     Also, a tapered surface may be formed on a body  2  side, and a step portion may be formed on a seat ring  6  side. Furthermore, in either case, with the step portion as a boundary, an uneven shape with its outer diameter side deformed to a primary side and its inner diameter side deformed to a secondary side may be provided in the vicinity of the boundary portion. 
     Next, a manufacturing method of the above-described double eccentric butterfly valve and its action are described. 
     First, the stem  3  is inserted through the shaft-mounting portion  10  of the body  2  and the hole portion  14  of the disk  4 , the disk  4  is fixed to the stern  3  with the tapered pins  16 , and the disk  4  is mounted to the body  2  so as to be in a state of being rotatable by the stem  3 . 
     From this state, the stem  3  is operated to rotate to a closing direction to cause the disk  4  to be in a closed state. Then, the seat retainer  5  with the seat ring  6  attached thereon is accommodated in the attachment concave portion  11  of the body  2 . Here, between the outer circumference of the seat ring  6  and the inner circumference of the attachment concave portion  11 , the annular clearance C is formed. The seat ring  6  can move in the radial direction by this clearance C inside the attachment concave portion  11 . 
     Subsequently, the retainer bolts  50  are inserted into the fastening holes  52  for fastening. With these retainer bolts  50 , the seat retainer  5  and the body  2  are fixed together. Also, between these retainer bolts  50  and the fastening holes  52  (including the head portions  55  and the surroundings), a clearance is provided, allowing alignment described below. 
     Here, with fastening of the retainer bolts  50 , the seal surface  25  is guided along the tilt of the sealing surface  12 . With this guide, the seat ring  6  (seat retainer  5 ) moves the attachment concave portion  11  in the radial direction in the range of the clearance C, and a contact between the seal surface  25  and the sealing surface  12  is appropriately adjusted to become in a state of allowing annual abutting and sealing. In this manner, with fastening of the retainer bolts  50 , an alignment action of aligning the seat ring  6  with the position of the disk  4  is exerted on the seat retainer  5 . 
     In this case, the tapered surface  31  and the step portion  32  are opposed to each other, the tapered surface  31  abuts on the step portion  32 , and the tapered surface  31  is guided so as to slide in the vicinity of the corner portion of the step portion  32 . Thus, the seat ring  6  is smoothly aligned. Simultaneously, with fastening of the retainer bolts  50 , the corner portion  35  of the step portion  32  digs into the tapered surface  31 , and the tapered surface  31  is elastically deformed along the step portion  32 . 
     At that time, while the state of the tapered surface  31  abutting on the corner portion of the step portion  32  is maintained, the tapered surface  31  is gradually deformed along the uneven shape of the step portion  32 . Thus, in a state in which while a linear contact between the corner portion  35  of the step portion  32  and the tapered surface  31  causes the annual seal portion  33  to be formed on an opposed plane between the body  2  and the seat ring  6 , the seal surface  25  and the sealing surface  12  can be annularly sealed, the seat retainer  5  and the body  2  are fixed together. Thus, while the contact area TO gradually increases from a state in which at least a linear contact is maintained, a surface contact is made to allow the seal portion  33  to be configured. 
     In the valve main body  1  in the above-described embodiment of the present invention, the tapered surface  31  is formed to have the width W 1  by a mild R surface; while the step portion  32  is formed so as to have the tilted surface  36  at an angle of substantially 45 degrees from the bottom surface  11   a  of the attachment concave portion, its width W 2  is provided so as to have a size allowing arrangement inside the width W 1  of the tapered surface  31 ; and the tilt of the step portion  32  in the radial direction is provided so as to be steeper than the tapered surface  31 . With this, as described above, at the time of assembling, in a state of a contact by the annular seal portion  33  occurring from the tapered surface  31  and the corner portion  35  of the step portion  32 , the seat retainer  5  moves in the range of the clearance C to allow the seat ring  6  to be aligned with the disk  4 . Thus, in a state in which sealability of the seal portion  33  in the circumferential direction is uniformly ensured, a seal surface pressure between the disk  4  and the seat ring  6  can be improved. 
     In this case, since the length L of the clearance C in the radial direction is shorter than the width W 1  of the tapered surface  31  in the radial direction, when the seat retainer  5  is accommodated in the attachment concave portion  11 , even if this seat retainer  5  moves at the maximum inside the clearance C, the state in which the tapered surface  31  is partially superposed on the step portion  32  in the flow path  15  direction can be maintained. Thus, from the state in which at least a linear contact is made between the tapered surface  31  and the corner portion  35  of the step portion  32 , the annular seal portion  33  can be reliably formed. If the width W 1  is four to six times as long as the length L of the clearance, the annular seal portion  33  can be reliably formed, allowing also a further higher seal surface pressure to be obtained. 
     After assembling, it is possible to support a case in which a fluid pressure is applied to the disk  4  at the time of valve closing to either direction, positive pressure or counter pressure. 
     When a positive pressure is applied to the disk  4 , while the flexible portion  22  is tilted to left by the fluid pressure, the state of abutting on the disk  4  is ensured to maintain the seal state. Even when a further high pressure is applied from this state, the flexible portion  22  is elastically deformed to a disk  4  side to fall down, thereby causing the seal surface  25  to strongly make pressure contact with the disk  4 . Moreover, since so-called a self-seal function is exerted, in which the fluid enters the space portion from the space S 2  to pressurize to a downstream side, the seal surface pressure at the seal portion increases so as to be proportional to an increase in pressure. 
     On the other hand, when a counter pressure is applied to the disk  4 , the flexible portion  22  is tilted to right by the fluid pressure, thereby improving the seal surface pressure while the state of abutting on the disk  4  is ensured. When a further high pressure is applied from this state, the flexible portion  22  abuts on the seat retainer  5  to prevent an increase in internal stress of the seat ring main body  20  and maintain elasticity of the material, and sealability can be ensured. 
     In either case, positive pressure and counter pressure, the sealing surface  12  and the seal surface  25  are arranged in an aligned state. Thus, a uniform seal surface pressure is exerted between the disk  4  and the seat ring  6  in the circumferential direction, and leakage from a portion between these disk  4  and seat ring  6  is reliably prevented. 
     In this case, in particular, at the time of counter pressure, a force F in a direction indicated by an arrow in  FIG.  3    is exerted on the flexible portion  22 . This force F increases more since the movement of the disk  4  increases as the diameter of the valve main body  1  is larger, such as  250 A or  300 A. When the force F increase, with that increase, a tensile force F 1  to the inner diameter direction with the tilt of the flexible portion  22  also increases, and a profound pulling force is applied in the vicinity of the gasket portion  21 . 
     By contrast, since the seal portion  33  including the tapered surface  31  and the step portion  32  is provided in the vicinity of the step surface  30 , a seal force in the circumferential direction is exerted by this seal portion  33  to reliably inhibit back leakage and also inhibit the influence of the tilt of the seat ring main body  20  due to the flow of the fluid over the gasket portion  21 . 
     Since the region R 1  crushed by the step portion  32  and the space region R 2  as a space with respect to the step portion  32 , the space where this region R 1  is accommodated, have a substantially equivalent size, when the tapered surface  31  is deformed, while the region R 1  crushed by the step portion  32  is deformed so as to escape to the space region R 2 , the seal portion  33  is provided in the vicinity of the step surface  30 . With this seal portion  33 , sealing is made while the surface pressure between the tapered surface  31  and the step portion  32  is continuously and substantially uniformly exerted without interruption in the circumferential direction. Thus, application of excessive pressure from the step portion  32  onto part of the tapered surface  31  and an occurrence of a gap between the tapered surface  31  and the step portion  32  can be prevented, and excellent sealability can be exerted by the seal portion  33 . 
     On the seat retainer  5 , the protruding portion  42  protruding in the vicinity of the boundary portion  27  is formed, and this protruding portion  42  is partially or entirely provided at a position of being superposed on the step portion  32  in the flow path  15  direction. Thus, when the seat retainer  5  is fixed to the body  2 , the tapered surface  31  is pressurized onto the step portion  32  by the protruding portion  42  via the thin-walled portion  34  and these are strongly pressed to deform the tapered surface  31  along the shape of the step portion  32 , and the seal portion  33  exerting a uniform surface pressure can be configured. 
     The body  2  and the seat retainer  5  are fastened with the retainer bolts  50 , and the gasket portion  21  is interposed by the fastening force of these retainer bolts  50 . Thus, in particular, in the case of a large diameter, the fastening force to the gasket portion  21 , which tends to run short, is increased by the piping bolts  56  to enhance the sealing force by this gasket portion  21 , and a back-leakage preventing function can be thereby improved. 
     in the foregoing, while the embodiments of the present invention have been described in detail, the present invention is not limited to the embodiments described above and can be variously changed in a scope not deviating from the spirit of the invention described in the claims of the present invention. 
     REFERENCE SIGNS LIST 
       1  valve main body 
       2  body 
       3  stem 
       4  disk 
       5  seat retainer 
       6  seat ring 
       12  sealing surface 
       15  flow path 
       20  seat ring main body 
       21  gasket portion 
       25  seal surface 
       31  tapered surface 
       32  step portion 
       33  seal portion 
       35  corner portion 
       42  protruding portion 
       43  attachment concave portion 
     C clearance 
     L length of clearance in a radial direction 
     R 1  crushed region 
     R 2  space region 
     W 1  width of the tapered surface in a radial direction 
     W 2  width