______ are positive displacement pumps that can pump air as they operate.

• A. Priming pump
• B. Centrifugal pump
• C. Diaphragm pump
• D. Gear pump

Answer: (A)

The main idea here is that a positive displacement pump moves a fixed amount of fluid (or gas) with each cycle, which allows it to handle air moving through the system. A priming pump is a small positive displacement device used specifically to remove air from the suction line and to prime the main pump. As it runs, it traps a set volume of air and pushes it out through the discharge, effectively pumping air while it operates. This is exactly what priming is for: clearing the air so the main pump can develop prime and draw water.

Centrifugal pumps rely on imparting velocity to the liquid with an impeller; when air is present, they can lose prime and won’t reliably move air. A diaphragm pump is a positive displacement pump and can move gases in principle, but in this context the priming pump is the intended device for removing air from the system. Gear pumps are designed for liquids and aren’t used to prime by moving air through the suction line.