In displacement pumps, the pumped fluid is delivered through solid volumes and can not flow freely through the pump. Displacement pumps provide an approximately constant flow rate at fixed speed, even with changes in backpressure.
The difference in performance depending on which of these pumps you are dealing with, a small change in the pump’s counterpressure results in differences in the flow. The flow of a centrifugal pump will change considerably, the flow of a displacement pump will change a little or will hardly change at all.
The pumps are typically designed with the finest tolerances possible to obtain the highest possible efficiency and suction capability. However, in some cases, it is necessary to increase the tolerances, for example when the pumps have to handle highly viscous liquids, liquids containing particles and liquids of high temperature.
Positive displacement pumps are pulsate, meaning that their volume flow within a cycle is not constant.
The variation in flow and speed leads to pressure fluctuations due to resistance in the pipe system and in valves.
Typical displacement pumps are: