Rubber Duckbill Valves, widely used in the demanding applications of many industries, are precision-molded, one-piece elastomeric valves that provide reliable backflow prevention at low pressure differentials. Rubber Duckbill valves allow free flow with positive differential pressure.  With negative differential pressure, backflow is checked. Pressure drop across the valve is a feature unique to Rubber Duckbill valves. They can be designed to operate in a few millimeters (inches) of water to several hundred kappas' (psi), depending on valve size, geometry and compound characteristics. Rubber Duckbill valves are also designed to start functioning at specific opening and closing pressure ranges, depending on your specifications.