What are the primary characteristics of a positive displacement pump?

• A. It moves a constant amount of liquid regardless of pressure
• B. It adjusts flow to match pressure
• C. It only works with gases
• D. It has no moving parts

Answer: (A)

The key idea is that positive displacement pumps move a fixed amount of liquid with each cycle. Because they trap a precise volume and physically push it to the discharge side, the amount pumped per stroke is essentially set by the pump’s displacement and speed. That means the flow rate remains nearly constant as you change the discharge pressure, up to the pump’s operating limit. When system backpressure goes up, the pump develops higher head and the pressure at the discharge increases, but the volume moved per cycle doesn’t change. This constant-volume behavior is what defines their primary characteristic.

That’s why this option is best: it emphasizes delivering a constant amount of liquid per cycle regardless of pressure. In contrast, centrifugal pumps rely on pressure to affect flow and typically deliver less flow as backpressure increases, and they don’t provide a fixed volume per cycle. Positive displacement pumps do need moving parts (gears, pistons, diaphragms, etc.), and they are designed for pumping liquids rather than gases, so those other statements don’t fit.