A diaphragm pump (also called a Membrane pump) is a positive displacement pump that uses a combination of the reciprocating actions of a rubber, thermoplastic or teflon diaphragm and suitable valves on either part of the diaphragm
A dual diaphragm is a positive displacement pump which utilises two flexible Air Diaphragm Pump diaphragms that reciprocate backwards and forwards, creating a temporary chamber, which both draws in and expels fluid through the pump. … The two diaphragms that are connected by a shaft through the center section where the air valve is located.
Though this will depend upon the pump and application, a positive displacement pump, is normally a self-priming pump. … For example, air operated diaphragm pumps self-prime by creating a pressure differential in 
the diaphragm chamber. This draws in air and pulls the liquid into the suction port.