Abstract: A check valve adapted for mounting between any flanges and specifically between sanitary flanges in a piping system, the check valve comprising a stationary portion and a flexible portion; the single-piece rigid stationary portion including and supporting a valve seat; the single-piece molded elastomeric flexible portion including a flange gasket, a valve plug, flexible retaining arms for guiding and securing the valve plug movement with respect to the valve seat, and a visual means to indicate that the check valve is installed, permits flow in the desired direction, and is drainable; the check valve being fully cleanable and sterilizable in situ by flow-through methods and having at least one embodiment that is fully drainable when mounted in any orientation.