Expansion tanks are known for use in flow systems for controlling flow of liquid under varying pressures.  Generally, expansion tanks comprise an essentially cylindrical or spherical housing containing a bladder-type diaphragm that divides areas of a liquid and a pressurized gas.  For a general discussion of expansion tanks and bladder-type diaphragms, see U.S. Pat.  No.  4,784,181 to Hilverdink entitled “Expansion Tank with a Bladder-Type Diaphragm”. 
An expansion tank often has one nozzle through which liquid flows in and out depending on the pressure level controlled by the contraction and expansion of the interior diaphragm.  Many municipalities require that expansion tanks for use with potable water include devices to ensure that water circulates through the tank and that there is not actually stagnant water in the tank when the tank is “empty”.