When the plunger reaches the end of its suction stroke, the pump mechanism pushes it to the left on a discharge stroke. As the plunger moves, it displaces liquid that again tries to flow out in all directions. This time, the liquid trying to move back out the suction seats and seals the suction check valve, and it lifts and flows around the discharge check valve ball. So during the discharge stroke, there is flow in the discharge pipe, and no flow in the suction pipe. Notice that when the plunger moves, it doesn't just snap forward and back.
Most metering pump stroking mechanisms transmit a nearly sinusoidal velocity to the plunger stroke.
This motion is transferred to the liquid in the suction and discharge pipes. The peak flow rate is significantly greater than the average flow rate. This has a serious effect on the friction pressure drop encountered in the pipes. Widely accepted equations used to predict frictional pressure drop in pipelines indicate that the pressure drop caused by friction does change proportionally with the flow rate. It follows, then, that if the peak flow rate from a metering pump is 3.14 times greater than average flow rate, then the peak pressure drop will be 3.14 times greater than the average pressure drop.