Abstract: A non return valve (10) adapted to be supported between two pipe end flanges (20) has a tubular body (11) with an internal flow passage (12) and a valve seat (15) around the inner end of the inlet opening (13), which is a major segment of a circle, and a flat circular elastomeric valve member (30) having a flexible segment (32) fixed to the seat and an internally reinforced rigid segment (31) located over the seat and normally covering the inlet opening. The rigid part hinges inwardly under upstream fluid pressure to uncover the inner end of the inlet opening but reseats itself when fluid pressure is reversed.