Instantaneous Flow Characteristics of Metering Pumps and Their Effects

A metering pump delivers fluid through the reciprocating motion of its plunger, and this reciprocating motion is converted from the rotation of the motor. As a result, the instantaneous flow of a metering pump is not the steady flow of a centrifugal pump; instead it is a pulsating flow in which the pump draws in fluid during one half of the cycle and discharges it during the other half. The instantaneous flow of a metering pump can be seen in the flow curve below.

The instantaneous flow curve shows that the suction and discharge flow of a metering pump follow a sinusoidal characteristic, so the flow fluctuation during fluid delivery is very pronounced. This flow fluctuation causes pipe vibration and noise; for high-flow pumps in particular, the fluctuation can do significant harm to the pump body and the piping.

To mitigate the impact of flow fluctuation on equipment and piping, a pulsation dampener can be installed in the discharge line of the metering pump. The dampener smooths out the flow fluctuation in the pipe, thereby protecting both the piping and the equipment. A pressure gauge should also be installed in the pump discharge line to monitor pressure changes in the fluid.

The flow rate quoted in a metering pump's performance specification refers to the pump's average flow. As the flow curve shows, the maximum instantaneous flow of the pump is not its average flow but 3.14 times the average flow. For this reason, the maximum instantaneous flow should be used as the reference when sizing the metering pump's piping. If the pipe size is designed around the pump's average flow, the pump may experience insufficient suction at the inlet or pressure build-up at the outlet during operation, compromising its performance.