The working principle of a Verderair double diaphragm pump

The internal mechanisms of a double diaphragm pump can vary subtly between brands, however there is a common working principle applicable to all models.  

An air operated double diaphragm pump has two diaphragms. These diaphragms are connected by a shaft in the center section. The diaphragms are working as separation wall between the air and the liquid. The air valve is located In the center section of the diaphragm pump.  

The air valve directs the compressed air to the back of diaphragm number one. This way, diaphragm number one moves away from the center section. This diaphragm causes a press stroke moving liquid out of the pump. At the same time diaphragm number two is performing a suction stroke. The air behind diaphragm number two is being pushed out to the atmosphere. Atmospheric pressure pushes the liquid to the suction side. The suction ball valve is being pushed away off its seat. This allows the fluid to flow along the ball valve into the liquid chamber.

When the pressurized diaphragm number one has reached the end of its stroke, the movement of the air is switched from diaphragm number one to the back of diaphragm number two by the air valve. The compressed air pushes diaphragm number two away from the center block. Doing so, diaphragm number one is pulled toward the center block. In pump chamber number two the discharge ball valve is pushed off its seat. In pump chamber number one the opposite occurs. Upon completion of the stroke the air valve leads the air again to the back of diaphragm number one and restarts the cycle as described above. 

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