It's a pump used to deliver liquids at a predictable, stable flow rate, relatively independent of system pressure changes. Metering pumps are also known as dosing pumps, controlled volume pumps and chemical feed pumps. This discussion relates only to reciprocating piston metering pumps. Oil field, petroleum refining, textile, public and industrial water treatment, electric power generation and food processing are just a few industries served. The applications are widespread and varied.
Diaphragm pumps are simply a variation of plunger pumps.
Both designs use a reciprocating plunger to generate the pumping action. As the plunger moves to the right on a suction stroke, liquid tries to follow it from all directions. Liquid from the discharge pipe, trying to flow in, seats and seals the discharge check valve. No flow can enter to follow the plunger from that source. On the suction side, however, liquid can move up and lift the suction check valve ball, which admits flow from that direction.