Which pump type is unable to pump air and is not self-priming?

• A. Diaphragm pump
• B. Centrifugal pump
• C. Rotary pump
• D. Positive displacement

Answer: (B)

Centrifugal pumps rely on a continuous column of liquid to create flow. The impeller throws liquid outward, and that movement is converted into pressure that draws more liquid in. If air fills the suction line, there’s no liquid to develop suction, so the pump loses prime and stops delivering flow. Because it cannot prime itself in the presence of air, it is not self-priming. In contrast, diaphragm and other positive-displacement pumps move gas and liquid by trapping a fixed volume and are typically able to prime themselves or operate with air in the line more effectively. So the centrifugal pump is the one that cannot pump air and is not self-priming, which is why priming the suction line is essential before using it.