Which of the following is NOT an example of a positive displacement pump?

A centrifugal pump is categorized as a non-positive displacement pump because it operates on a different principle compared to positive displacement pumps. While positive displacement pumps work by moving a specific volume of fluid with each cycle of operation (either by displacement from a chamber or by rotating gears or vanes), centrifugal pumps rely on kinetic energy and the principle of centrifugal force.

In a centrifugal pump, a rotating impeller transfers energy to the fluid, increasing its velocity and generating flow. This process does not involve the direct displacement of a fixed amount of fluid per cycle, which is the hallmark of positive displacement pumps. As a result, while the other types listed — moving vane, rotary, and reciprocating — all involve mechanisms that displace a defined volume of fluid, the centrifugal pump stands apart in its operational principle. This distinction is critical in understanding the various types of pumps and their applications in engineering contexts.