A diaphragm pump (also known as a Membrane pump) is a positive displacement pump that uses a mixture of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either side of the diaphragm
A double diaphragm is a positive displacement pump which utilises two flexible diaphragms that reciprocate backwards and forwards, creating a temporary chamber, which both allures and expels liquid through the pump. … The two diaphragms that are linked by a shaft through the centre section where in fact the air valve is Air Diaphragm Pump located.
Though this will depend upon the pump and application, a positive 
displacement pump, is generally a self-priming pump. … For instance, air operated diaphragm pumps self-prime by creating a pressure differential in the diaphragm chamber. This draws in surroundings and pulls the fluid in to the suction port.