Combination Valves
An umbrella can be combined with a duckbill to provide a three-port valve which has an inlet, an outlet, and a port which articulates with a common variable volume. This is a convenient way to make a pump. When the volume increases, fluid is drawn in through one valve (usually the umbrella) and then when the volume decreases, fluid is pushed out through the second valve (usually the duckbill). The umbrella can have an opening (or cracking) pressure which is chosen to prevent flow due to head height at the inlet.