A diaphragm pump (also known as a Membrane pump) is a positive displacement pump that runs on the mixture of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either part of the diaphragm
A double 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 liquid through the pump. … Both diaphragms that are linked by a shaft through the centre 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 instance, air Air Diaphragm Pump operated diaphragm pumps self-prime by creating a pressure differential in the diaphragm chamber. This draws in air flow and pulls the liquid into the suction port.