Which of the following is NOT a type of positive displacement pump?

The correct answer is Axial flow. Axial flow pumps operate by moving fluid along the axis of the pump's impeller, utilizing a rotor to create a flow of liquid. This design relies on kinetic energy to move the fluid rather than displacing a fixed volume of liquid with each cycle, which is the hallmark of positive displacement pumps.

In contrast, peristaltic, piston, and screw pumps are all classified as positive displacement pumps because they work by trapping a specific volume of fluid and moving it through the system, effectively displacing the liquid based on the pump's capacity during each cycle. This characteristic allows for consistent flow levels regardless of the pressure, which is a key feature of positive displacement technology.

Therefore, recognizing the operational principles and characteristics associated with different pump types helps to clarify why axial flow does not fit into the positive displacement category.