Which of the following is not a type of fixed displacement pump?

• A. Centrifugal
• B. Vane
• C. Gear
• D. Piston

Answer: (A)

The main idea is the difference between fixed-displacement (positive-displacement) pumps and dynamic (centrifugal) pumps. Fixed-displacement pumps deliver a constant volume per revolution, so their output is mainly set by speed and not by the pressure they are pumping against. Centrifugal pumps are dynamic: they move fluid by imparting velocity with an impeller, and the actual flow depends on the system head. As head increases, the flow decreases, and the displacement per revolution isn’t fixed.

So the centrifugal pump is not a fixed-displacement device, which is why it’s the correct choice. The other options—gear, vane, and piston pumps—are all positive-displacement types, moving a near-constant volume with each revolution, giving a fixed displacement at a given speed.