98 where significant throttling of fluid flow occurs the high magnitude reaction forces may distort the operator tube 82 flapper closure plate 86 or pivot pin 100 moreover the alignment of flapper plate 86 relative to valve seat 92 may be disturbed in response to slamming impact of flapper closure plate 86 against valve seat 92 referring now to fig4 a valve is depicted and generally designated 120 valve 120 includes a valve closure mechanism which is depicted as flapper closure plate 122 valve 120 also includes a valve seat 124 in the illustrated embodiment the sealing surfaces of flapper closure plate 122 and valve seat 124 have mating segments which are matched in curvature to provide a metal to metal seal sealing surface 126 of valve seat 124 is a concave spherical segment sealing surface 128 of flapper closure plate 122 is a convex spherical segment convex flapper closure plate sealing surface 128 and concave valve seat sealing surface 126 are both generally a surface of revolution produced by revolving a semi circular arc having an arc length 130 and radius of curvature 132 as shown in fig4 the radius of curvature of convex flapper closure plate sealing surface 128 is substantially equal to the radius of curvature of concave valve seat sealing surface 126 specifically the spherical radius of curvature of the concave valve seat sealing surface 126 is matched with the spherical radius of curvature of the convex flapper closure plate sealing surface 128 as used herein u201c matched radius of curvature u201d means that the radius of curvature of the flapper plate convex sealing surface 128 is substantially the same as but not greater than the radius of curvature of the concave valve seat sealing surface 126 preferably the convex and concave surfaces are matched in.