A diaphragm pump (also known as a Membrane pump) is a positive displacement pump that runs on the combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either side of the diaphragm
A dual diaphragm 
is a positive displacement pump which utilises two flexible diaphragms that reciprocate back and forth, creating a temporary chamber, which both draws in and expels fluid through the pump. … Both diaphragms that are linked by a shaft through the center section where in fact the air valve is situated.
Though it depends upon the pump and application, a positive displacement pump, is normally a self-priming pump. … For example, air operated diaphragm pumps Air Diaphragm Pump self-primary by creating a pressure differential in the diaphragm chamber. This allures surroundings and pulls the liquid in to the suction port.