Abstract: A device for controlling liquid flow in a conduit which is of a hat-like configuration defined by a cylindrical base part having a flange part at one end and a closure part at another end joined thereto by an integral spring which bends to permit through-flow of liquid through a peripheral gap between the body part and the closure part. The gap is of a stepwise progression including either an abrupt or curved step. The gap has a medial gap portion and opposite gap portion ends which are relatively sized to effect the stepwise progression.