Plungers, Pistons, and Diaphragm Pumps

What is a Diaphragm Pump?
Simply speaking, a diaphragm is a separating membrane. In the water treatment industry, however, a diaphragm simply refers to the elastic membrane part on a pump or pressure gauge that is used to separate the mechanical section from the liquid.

A diaphragm pump is a type of volumetric pump. Part of the pump chamber comprises an elastic membrane (diaphragm) which is made to expand and contract by motor, solenoid, air, or hydraulic oil pressure action to adjust  the volume of the pump chamber. Two check valves are provided on diaphragm pumps to ensure that the liquid is fed out in a fixed direction. The check valve on the suction side allows liquid to flow only into the pump chamber while the check valve on the discharge side allows liquid to flow only to the discharge destination.

Advantages of a Diaphragm Pump
Liquid does not leak from diaphragm pumps as they do not have mechanical sliding sections at liquid contacting parts. This makes diaphragm pumps ideal for volatile liquids, liquids that cause hazards when leakage occurs, and liquids that solidify through contact with air.  Diaphragm pumps ensure feeding of liquids at constant amounts in such applications even if the pressure changes at the diaphragm.

Plungers and Pistons
Plunger type and piston type metering pumps are also available. These pumps are constructed very much like a syringe and comprise of a rod that moves reciprocally inside a cylinder. Seals (packings and O rings) are provided to prevent leakage from the piston. The moving seal provided on the rod side is the piston and fixed seal attached to the cylinder is the plunger. The pump in this structure leaks extremely small amounts of liquid to lubricate the seals. It is for this reason, plunger and piston type metering pumps are not suitable for toxic or corrosive liquids or liquids which crystallize in the air. Now that the PRTR Act is in force, everyone must pay more attention to liquids in use. Some say leakage from plungers cannot be helped. However, if the leak is large enough to be seen then the pump is malfunctioning.