cooperate to provide a limited vertical range of travel for ring member 38 inside annulus 32 between valve seat sleeve member 24 and body section 14 the upward range of travel of ring member 38 is also limited by headspace 49 between flange 26 although only one pin 45 and slot 47 are shown in fig1 and 2 it is understood that a plurality of circumferentially spaced pins and slots can be used if desired also while ring member 38 is shown as a complete annular ring in fig1 and 2 it will be appreciated that a semi circular ring section or other functionally equivalent structure can likewise be used within the scope of the invention to provide vertical translational movement of hinge pin 40 relative to valve seat sleeve member 24 and body section 14 thereby creating a floating hinge for flapper closure member 36 the lower end of valve seat sleeve member 24 preferably comprises a machined section 46 terminating in seating surface 43 formed at some angle preferably 45 degrees with the longitudinal axis of sleeve member 24 the preferred flapper 36 for use in valve assembly 10 of the invention is a section of a cylinder having a constant radius of curvature that is substantially the same as the radius of valve seat sleeve member 24 both the.