A diaphragm pump (also called a Membrane pump) is a positive displacement pump that uses a combination of the reciprocating action 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 diaphragms that reciprocate backwards and forwards, creating a short-term chamber, which both draws in 
and expels fluid through the pump. … The two diaphragms that are linked by a shaft through the center section where in fact the air valve is situated.
Though this will depend upon the pump and application, a positive displacement pump, is normally a self-priming pump. … For Air Diaphragm Pump example, air operated diaphragm pumps self-prime by creating a pressure differential in the diaphragm chamber. This allures air flow and pulls the fluid in to the suction port.