How do positive displacement pumps differ from centrifugal pumps?

• A. Centrifugal pumps move a fixed volume per cycle; positive displacement pumps rely on impeller acceleration to create flow.
• B. Positive displacement pumps move a fixed volume per cycle; centrifugal pumps rely on impeller acceleration to create flow.
• C. Positive displacement pumps rely on pressure differences to move liquid.
• D. Centrifugal pumps maintain constant flow regardless of speed.

Answer: (B)

Positive displacement pumps move a fixed amount of fluid with each cycle because they trap a specific volume and push it out with every stroke. Centrifugal pumps create flow by accelerating the fluid with an impeller and then converting that velocity into pressure; the amount of liquid they move depends on the system pressure (head) and the pump speed, not a fixed per-cycle volume. So the statement that positive displacement pumps move a fixed volume per cycle while centrifugal pumps rely on impeller acceleration to create flow captures the fundamental difference. The other options either reverse the roles, imply an incomplete mechanism, or claim constant flow regardless of conditions, which isn’t how centrifugal pumps behave.