Construction and Working Principle of Metering Pumps

A metering pump is a reciprocating positive-displacement pump whose flow rate can be continuously adjusted between 0 and 100%. Based on their drive method, metering pumps can be classified as motor-driven metering pumps, solenoid-driven metering pumps, and pneumatic metering pumps. This article focuses on explaining the construction and working principle of the motor-driven metering pump.

Working principle of the motor-driven metering pump: the rotational motion of the motor drives the metering pump's worm to rotate through the motor output shaft; the worm meshes with the worm wheel to reduce the speed, and the eccentric is driven to rotate by the worm wheel. The connecting rod, engaging with the outer edge of the eccentric, converts the rotation of the eccentric into reciprocating motion. The connecting rod pushes the piston so that the piston reciprocates within the piston chamber of the metering pump. At the same time, the metering pump must be equipped with an inlet check valve and an outlet check valve that direct the liquid (as shown below).

When the piston moves to the left, a low pressure forms in the pump chamber; under this pressure the suction valve opens and the discharge valve closes, and liquid enters the pump chamber. When the piston moves to the right, a high pressure forms in the pump chamber; under this pressure the suction valve closes and the discharge valve opens, and liquid is discharged through the discharge valve, completing one suction and discharge stroke of the metering pump. The delivery of liquid is thereby achieved.

The flow rate of a metering pump is linearly related to the stroke length and stroke frequency of the plunger, so the flow can be adjusted by changing the stroke length and stroke frequency of the piston's reciprocating motion. Stroke-length adjustment is performed using an adjustable eccentric mechanism, an adjustable-inclination crank-and-connecting-rod mechanism, or an adjustable bypass-port structure. The stroke frequency of the plunger can be changed by using a variable-frequency drive to vary the motor speed.