each containing within a central circular hole 11 occupied by a valve shaft 12 which supports the tiltingly arranged disc 5 the latter is composed of two partially flat wall portions the upper one being labeled 13 and the lower one being labeled 14 the upper semi circular portion 13 is arranged to have a planary axis that is essentially vertical when the vane is closed the lower semi circular wall portion 14 is tilted at an angle u03b1 of typically 30 u00b0 35 u00b0 in respect to wall portion 13 both wall portions 13 and 14 are interjoined at the outer periphery to form a common rim 15 which has a flat and essentially rectangular terminating surface one of whose corners 9 sealingly compress as elastomeric liner 17 which may be part of butterfly valve body 7 as shown in fig1 both wall portions 13 and 14 have a nearly constant wall thickness throughout and each section resembles a semi circular flat plate having rounded edges at portions of their outer periphery to join in a rim 15 as more clearly shown in fig4 fig1 also shows that the intersection or joint of both wall portions 13 and 14 near the horizontal axis of bore 8 is located exterior of planary disc axis 6 and bore 11 thus generating an overall cup shaped configuration of disc 5 fig2 shows an exterior view of vane 5 with the butterfly valve body 7 removed for clarity as can.